3.1: Module 12 Circles
12
Module 12 Circles
Wally Baumback
Learning Outcomes
When you have completed this module, you will be able to:
- Describe construction objects and their use in a 2D sketch.
- Describe drawing circles and offsets in a 2D sketch.
- Apply the CENTER POINT CIRCLE and OFFSET commands to complete 2D sketches.
Geometry Lesson: Circles
A circle is defined as a closed curve in which all points are the same distance from its centre point. The centre point is a single XY coordinate. A circle is 360 degrees and can be divided into four quadrants . All points on a circle are at a given distance from is centre point. The distance between any of the points and the centre is called the radius .
Study the drawings in Figure 12-1, 12-2, 12-3, and 12-4 for a description of the geometry of a circle.
Drafting Lesson: Reading Dimensions for Circles and Arcs
When reading the dimensions for circles and arcs, consider the following: Circles are dimensioned as diameters. For example: 2.0 DIA.
Arcs are dimensioned as radii. For example: 1.5 R
When there is more than one circle of the same diameter, they are only dimensioned once. For example: 0.5 DIA., 4 PLACES
Sometimes multiple arcs are dimensioned as typical (TYP.). For example: 2.0 R TYP. This simply means that there is at least one additional arc of the same size, See Figure 12-5.
Inventor Command: CENTER POINT CIRCLE
The CENTER POINT CIRCLE command is used to draw a circle by entering the location of its centre point and its radius.
Shortcut: C
Inventor Command: OFFSET
The OFFSET command is used to draw an object parallel to an existing object.
Shortcut: O
Construction objects are objects that are drawn in the sketch to assist you to complete the sketch but will be ignored by Inventor when the sketch is extruded or revolved to create the Base model. Geometrical and dimensional constraints can be applied to construction objects the same as they are to drawing objects. Construction objects and drawing objects only differ in the properties of the objects. They are both drawn and manipulated the same way in a 2D sketch.
There are two methods of creating construction objects as follows:
Method 1
Enable the Construction icon, see Figure 12-6, and then draw the objects. All objects drawn while the Construction icon is enabled, will be created as construction objects.
Method 2
Draw the object with the Construction icon disabled. After the drawing object(s) is created, select it. While it is selected, click the Construction icon. Construction objects will appear as a dashed lines as shown in Figure 12-7.
USER TIP: Some commands require you to select one or more drawing objects when there is no command active. For example, the DELETE command. The Status bar will display Ready when there is no command active. To select one object at a time, select the object by clicking it with the left mouse button. The selected object will highlight and change colour. To select more then one object at time, hold down either the CTRL or SHIFT key while selecting the objects. When all objects have been selected, click the command icon or enter the command on the command line.
WORK ALONG: Drawing Circles and Cylindrical Models
Step 1
Check the default project and if necessary, set it to Inventor Course .
Step 2
Enter the NEW command to start a new part file using the template: English-Modules Part (in).ipt .
Step 3
Save the file with the name: Inventor Workalong 12-1 . (Figure Step 3A and 3B)
Step 4
Draw the Base sketch on the Top view. Since this is the XY plane (the default plane), use Sketch1.
Step 5
Project the Center Point onto the sketching plane.
Step 6
Enter the CENTER POINT CIRCLE command and draw a 4.0 inch diameter circle. Watch the Status bar. The first prompt will be to select the centre point for the circle. For the centre point location, snap to the Center Point that was projected onto the sketching plane. Select the radius by guessing 2 inches from the centre. (Figure Step 6)
Step 7
Click the OFFSET command and when prompted, select the circle. Right-click the mouse. In the Right-click menu, ensure that Loop Select is enabled. Move the mouse towards the centre, guess at the diameter size of the 1.0 inch. (Figure Step 7A, 7B, and 7C)
Step 8
Click the Construction icon to enable it. (Figure Step 8)
Step 9
Using what you just learned, enter the OFFSET command and offset the large circle to draw the 3 inch diameter construction circle. (Figure Step 9)
Step 10
Click the Tools menu. Click the Applications Options icon to open the Applications Options dialogue box. Enable the Sketch tab. In the Display area, disable Axes. Click OK to close the dialogue box. (Figure Step 10A and 10B)
Step 11
Dimension the three circles. Press F8 to display the geometrical constraint icons. (Figure Step 11)
Step 12
Using the LINE command, draw a line by first snapping to the Center Point and ending it by snapping onto the circle. Make sure that the Horizontal constraint icon displays when you are drawing the line. The constraint icons should appear similar to the figure. (Figure Step 12)
Step 13
Using the LINE command, draw three additional construction lines by first snapping to the Center Point and ending them by snapping onto the circle. The constraint icons should appear similar to the figure. (Figure Step 13)
Step 14
Press F8 to display the constraint icons. (Figure Step 14)
Step 15
Click the Construction icon to disable it.
Step 16
Enter the CENTER POINT CIRCLE command. With the cursor in the sketching window, right-click the mouse. In the Right-click menu, select Intersection. Select the construction circle and then the construction line. Notice how the cursor will display a plus sign beside it. When you select the line, the green snap indicator will display. Select a location for the radius of the circle by guessing at its size. (Figure Step 16A, 16B, 16C, and 16D)
Step 17
Using what you just learned in Step 14, insert the 3 additional circles. For now, the diameter of the circles is not important. (Figure Step 17)
Step 18
Dimension one circle only. (Figure Step 18)
Step 19
Ensure that there is no active command and select the Equal constrain icon. (Figure Step 19)
Step 20
For the first circle, select the top (dimensioned circle) and for the second circle select one of the other circles. It will constrain the second circle as equal in size to the dimensioned circle. (Figure Step 20A and 20B)
Step 21
Using what you learned in Step 20, constrain the additional 2 circles with the Equal constraint to equal the dimensioned circle. The constraint icons should appear similar to the figure. (Figure Step 21)
Step 22
Change the view to the Home view. The sketch should be fully constrained and display purple. (Figure Step 22)
Step 23
Extrude the sketch as shown in the figure. (Figure Step 23A and 23B)
Step 24
Start a new sketch on the top plane of the model. Using the OFFSET command, offset the outside diameter of the object to insert a 1.5 inch circle. Add the diameter dimension as shown in figure. (Figure Step 24)
Step 25
Extrude the sketch as shown in figure. (Figure Step 25)
Step 26
Change the colour to: Chrome -Polished Black . (Figure Step 26)
Step 27
Save and close the file.
Key Principles
Key Principles in Module 12
- Construction objects are objects that are drawn in the sketch to assist you in completing the sketch but will be ignored by Inventor when the sketch is extruded or revolved. Geometrical and dimensional constraints can be applied to construction objects the same as they are to drawing Construction objects and drawing objects only differ in the properties of the objects. They are both drawn and manipulated the same way in a 2D sketch.
- To select more then one object at time, hold down either the CTRL or SHIFT key while selecting the objects.
Lab Exercise 12-1
Time allowed: 45 minutes.
| Part Name | Project | Units | Template | Color | Material |
| Inventor Lab Lab 12-1 | Inventor Course | Millimeters | Metric-Modules Part (mm).ipt | Aluminum – Flat | N/A |
Step 1
Project the Center Point onto the Base plane.
Step 2
Note the location of X0Y0Z0. Draw the necessary sketches and extrude them to produced the solid model shown below. Apply all of the necessary geometrical and dimensional constraints to fully constrain all sketches. (Figure Step 2A and 2B)
Step 3
Apply the colour shown above. (Figure Step 3A and 3B)
Hint:
The small holes do NOT go all the way through the flange.
Lab Exercise 12-2
Time allowed: 45 minutes.
| Part Name | Project | Units | Template | Color | Material |
| Inventor Lab Lab 12-2 | Inventor Course | Inches | English-Modules Part (in).ipt | White | N/A |
Step 1
Project the Center Point onto the base plane.
Step 2
Note the location of X0Y0Z0. Draw the necessary sketches and extrude them to produced the solid model shown below. Apply all of the necessary geometrical and dimensional constraints to fully constrain all sketches. (Figure Step 2A and 2B)
Hint
Draw the base sketch on the Front view or XZ plane.
Step 3
Apply the colour shown above. (Figure Step 3A and 3B)


14.4: Prokaryotic Replication
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Prokaryotes such as bacteria propagate by binary fission. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms, daughter cells are individuals.
To achieve the outcome of identical daughter cells, some steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells; the cytoplasmic contents must also be divided to give both new cells the machinery to sustain life. In bacterial cells, the genome consists of a single, circular DNA chromosome; therefore, the process of cell division is simplified. Mitosis is unnecessary because there is no nucleus or multiple chromosomes. This type of cell division is called binary fission.
Binary Fission
The cell division process of prokaryotes, called binary fission, is a less complicated and much quicker process than cell division in eukaryotes. Because of the speed of bacterial cell division, populations of bacteria can grow very rapidly. The single, circular DNA chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell. As in eukaryotes, the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size. The packing proteins of bacteria are, however, related to some of the proteins involved in the chromosome compaction of eukaryotes.
The starting point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane (Figure \(\PageIndex{1}\)). Replication of the DNA is bidirectional—moving away from the origin on both strands of the DNA loop simultaneously. As the new double strands are formed, each origin point moves away from the cell-wall attachment toward opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. A septum is formed between the nucleoids from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.
EVOLUTION IN ACTION: Mitotic Spindle Apparatus
The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo mitosis and therefore have no need for a mitotic spindle. However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally and functionally very similar to tubulin, the building block of the microtubules that make up the mitotic spindle fibers that are necessary for eukaryotes. The formation of a ring composed of repeating units of a protein called FtsZ directs the partition between the nucleoids in prokaryotes. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell-wall materials to the site. FtsZ proteins can form filaments, rings, and other three-dimensional structures resembling the way tubulin forms microtubules, centrioles, and various cytoskeleton components. In addition, both FtsZ and tubulin employ the same energy source, GTP (guanosine triphosphate), to rapidly assemble and disassemble complex structures.
FtsZ and tubulin are an example of homology, structures derived from the same evolutionary origins. In this example, FtsZ is presumed to be similar to the ancestor protein to both the modern FtsZ and tubulin. While both proteins are found in extant organisms, tubulin function has evolved and diversified tremendously since the evolution from its FtsZ-like prokaryotic origin. A survey of cell-division machinery in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex mitotic machinery of multicellular eukaryotes (Table \(\PageIndex{1}\)).
| Structure of genetic material | Division of nuclear material | Separation of daughter cells | |
|---|---|---|---|
| Prokaryotes | There is no nucleus. The single, circular chromosome exists in a region of cytoplasm called the nucleoid. | Occurs through binary fission. As the chromosome is replicated, the two copies move to opposite ends of the cell by an unknown mechanism. | FtsZ proteins assemble into a ring that pinches the cell in two. |
| Some protists | Linear chromosomes exist in the nucleus. | Chromosomes attach to the nuclear envelope, which remains intact. The mitotic spindle passes through the envelope and elongates the cell. No centrioles exist. | Microfilaments form a cleavage furrow that pinches the cell in two. |
| Other protists | Linear chromosomes exist in the nucleus. | A mitotic spindle forms from the centrioles and passes through the nuclear membrane, which remains intact. Chromosomes attach to the mitotic spindle. The mitotic spindle separates the chromosomes and elongates the cell. | Microfilaments form a cleavage furrow that pinches the cell in two. |
| Animal cells | Linear chromosomes exist in the nucleus. | A mitotic spindle forms from the centrioles. The nuclear envelope dissolves. Chromosomes attach to the mitotic spindle, which separates them and elongates the cell. | Microfilaments form a cleavage furrow that pinches the cell in two. |
Summary
In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and each copy is allocated into a daughter cell. The cytoplasmic contents are also divided evenly to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome and no nucleus. Therefore, mitosis is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell-wall material from the periphery of the cells results in a septum that eventually forms the separate cell walls of the daughter cells.
Glossary
- binary fission
- the process of prokaryotic cell division
- FtsZ
- a tubulin-like protein component of the prokaryotic cytoskeleton that is important in prokaryotic cytokinesis (name origin: F ilamenting t emperature- s ensitive mutant Z )
- origin
- the region of the prokaryotic chromosome at which replication begins
- septum
- a wall formed between bacterial daughter cells as a precursor to cell separation
Contributors and Attributions
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Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4-e119a8aafbdd ).
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- 14.4.1: DNA Replication in Prokaryotes
- DNA replication has been extremely well studied in prokaryotes primarily because of the small size of the genome and the mutants that are available. E. coli has 4.6 million base pairs in a single circular chromosome and all of it gets replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the circle in both directions. This means that approximately 1000 nucleotides are added per second. The process is quite rapid and occurs without many mistake


18.3: The Molecular Structure and Sub-Cellular Organization of Cytoskeletal Components
Of the three main cytoskeletal fibers, intermediate filaments serve a mainly structural role in cells. Microtubules and microfilaments have dual functions, dynamically maintaining cell shape and enabling cell motility. For example, when attached to the plasma membrane, microfilaments maintain cell shape. However, by interacting with motor proteins (e.g., myosin ), they can pull or push against a muscle cell membrane, changing the shape of the cell. Likewise, motor proteins such as dynein and kinesin can move ‘cargo’ to and fro along microtubule tracks from one point to another in the cell. We will look at how motor proteins interact with microtubules and microfilaments shortly. At this point, let’s take another look at the drawings and micrographs of the three main cytoskeletal filaments of eukaryotic cells (below) that we saw earlier in the text.
321 Introduction to the Cytoskeleton
The location and general functions of microtubules, microfilaments and intermediate filaments were demonstrated by immunofluorescence microscopy. After exposing cells to fluorophore-tagged antibodies against either microtubule, microfilament (actin) or intermediate filament proteins, fluorescence micrographs of the stained cells revealed the different locations of the fibers in cells. The typical localization of the different cytoskeletal fibers is shown below.
These localizations are consistent with known functions of the major cytoskeletal component filaments in cell structure and motility. Despite the small size of prokaryotic cells, they too were recently found have previously unsuspected cytoplasmic structures that could serve as a cytoskeleton ( ncbi-A Prokaryotic Cytoskeleton? ). So perhaps all (not just eukaryotic) cells are more than an unorganized bag of fluid sap! Next, we consider specific roles of microtubules, microfilaments, intermediate filaments and related proteins in the eukaryotic cytoskeleton.
322 Microtubules, Microfilaments, and Intermediate Filaments in Cells
A. Microtubules - an Overview
Microtubules assemble from dimers of \(\alpha \)-tubulin and \(\beta \)-tubulin monomers. After formation, \(\alpha \)/\(\beta \) -tubulin dimers add to a growing, or plus end ( +end ), fueled by GTP hydrolysis. Disassembly at the -end of microtubules powers changing the shape of cells or the separation and movement of chromatids to opposite poles of cells during cell division (i.e., mitosis or meiosis). Isolated single microtubules were shown to grow by addition to one end and to disassemble at the opposite end, thus distinguishing the +ends and -ends . A summary of this experiment demonstrating microtubule polarity is in the link below.
323 Demonstration of the Polarity and Dynamics of Microtubules
Microtubules in most cells can seem disordered. In interphase, they tend to radiate from centrioles in non-dividing animal cells, without forming discrete structures. However, in the run-up to cell division, microtubules reorganize to form spindle fibers. This reorganization is nucleated from centrioles in animal cells and from a more amorphous microtubule organizing center ( MTOC ) in plant cells. A typical centriole (or basal body ) in has a ‘ 9 triplet ’ microtubule array as seen in the electron micrograph cross section (below).
1. The Two Kinds of Microtubules in Spindle Fibers
a) Kinetochore Microtubules
Duplicated chromosomes condense in prophase of mitosis and meiosis, forming visible paired chromatids attached at their centromeres . Specific proteins associate with centromeres to make a kinetochore during condensation. As the spindle apparatus forms, some spindle fibers attach to the kinetochore; these are the kinetochore microtubules. By metaphase , bundles of kinetochore microtubules stretch from the kinetochores at the cell center to the polar centrioles or MTOCs of the dividing cell, as drawn below.
We now know that the +ends of kinetochore microtubules are in fact at the kinetochores, where these fibers are assembled! At anaphase , forces generated when microtubules shorten at their –ends (disassembly ends) separate the chromatids. Microtubule disassembly at centrioles/MTOCs provides the force that draws daughter chromosomes to the opposite poles of the cell as cell division continues.
b) Polar Microtubules
The spindle fiber polar microtubules extend from centrioles/MTOCs at opposite poles of the cell. They do not bind to kinetochores of chromatids, but instead, overlap at the center of the dividing cells. As kinetochore microtubules pull at chromatids in anaphase, polar microtubules slide past one another in opposite directions, pushing apart the poles of the cell. In this case, dynein (a motor protein attached to microtubules) catalyzes ATP hydrolysis to power microtubule sliding. Dynein motors on the microtubules from one pole of the cell in effect, ‘walk’ along overlapping microtubules extending from the opposite pole. The role of microtubule disassembly at the centrioles (i.e., at the minus end) was demonstrated in a clever experiment in which a tiny laser beam was aimed into a cell at spindle fibers attached to the kinetochore of a pair of chromatids (see this animated at the link below).
2. Microtubules in Cilia and Flagella
The microtubules of cilia or flagella emerge from a basal body , shown at the left in the electron micrograph below
Basal bodies are structurally similar to centrioles, organized as a ring of nine microtubule triplets. Cilia and flagella formation begin at basal bodies but show a typical 9+2 arrangement (9 outer doublet plus 2 central microtubules) in cross section (shown in the micrograph, above right). After detergent treatment to remove the membranes of isolated cilia or flagella, the remaining axonemes preserve the 9+2 microtubule arrangement. The structural relationship between the axonemes of a cilium or flagellum and an individual microtubule are shown in the cross sections below.
It is possible to see the tubulin subunits that make up a microtubule polymer in cross section. Each tubule is made up of a ring of 13 tubulin subunits. The microtubules in the ‘doublets’ share tubulins, but are also composed of 13 tubulins. When fully formed, the 25 nm diameter microtubules appear to be a hollow cylinder. When microtubules are isolated, they typically come along with dynein motor proteins and other Microtubule-Associated Proteins ( MAPs ), some of which hold microtubules together in an axoneme.
3. Microtubule Motor Proteins Move Cargo from Place to Place in Cells
Motor proteins such as dynein and kinesin, are ATPases ; they use the free energy of ATP hydrolysis to power intracellular motility. Let’s take a closer look at how these two major motor proteins carry cargo from place to place inside of cells.  Examples include vesicles formed at the trans Golgi face containing secretory proteins, pigments or neurotransmitters. Secretory vesicles move along microtubule tracks to the plasma membrane for exocytosis . Vesicles containing neurotransmitters move from the cell body of neurons along microtubule tracks in the axons, reaching the nerve ending where they become synaptic vesicles . In a chameleon, pigment vesicles in skin cells disperse or aggregate along microtubule tracks to change skin color to match the background.
Motor proteins carry cargo vesicles in opposite directions. The transport of neurotransmitters is a well-understood example. Neurotransmitter vesicles arise from the endomembrane system in neuron cell bodies . Powered by ATP, kinesin powers anterograde vesicle movement from the cell body to nerve endings. In contrast, an ATP-dependent dynein motor, as part of a dynactin complex, powers retrograde movement of empty vesicles back to the cell body. Motor protein structure and action are below.
A fanciful (and not too inaccurate!) cartoon of a motor protein ‘walking along an axonal microtubule is animated at this link: Kinesin 'walking' an organelle along a microtubule .
325 Microtubule Motor Proteins
At this point, we can look at several specific kinds of cell motility involving microtubules and microfilaments.
4. The Motor Protein Dynein Enables Axonemes to Bend
Take a look at the cross-section of axonemes a few illustrations ago. In the 9+2 axoneme of cilia and flagella, dynein arms attached to the A tubules of the outer doublets walk along the B tubules of the adjacent doublet. If only the doublets on one side of an axoneme take a walk while those on the other side hold still, the microtubules will slide past one another and the axoneme (and therefore a cilium or flagellum) will bend. However, microtubule sliding is constrained by flexible nexin and radial spoke attachments. The movements of cilia and flagella are illustrated below.
The differences in flagellar motion (wave-like propeller) and ciliary motion (single plane, back and forth beat) result in part from which microtubules are sliding at a given moment and the nature of their restraint by axoneme proteins. Let’s look at some experiments that demonstrate these events. CC-BY 3.0; From: en.Wikipedia.org/wiki/File:Fl...um-beating.svg CMB3e 467.
Experiments on isolated axonemes demonstrate the sliding microtubule mechanism of ciliary and flagellar motility. In one experiment, isolated flagella and purified axonemes were both shown to ‘beat’ in the presence of added ATP (below).
Agitating sperm or ciliated cells in a high-speed blender for a few seconds will shear and detach flagella or cilia from the rest of the cell. Adding ATP to detached cilia or flagella will cause them to beat, a phenomenon easily seen in a light microscope. Axonemes isolated from detached cilia or flagella by detergent treatment (to disrupt membranes) retain their characteristic 9+2 microtubule arrangement as well as other ultrastructural features…, and will even ‘beat’ in the presence of ATP!
326 9+2 Microtubule Array in Axonemes that Beat
Additional detergent treatment removes radial spokes , nexin and other proteins from the axoneme, causing the microtubules to separate. Dissociated microtubule doublets and central ‘singlets’ can then be observed in the electron microscope.
When separated microtubules are dialyzed to remove the detergents, the doublet microtubules re-associate, forming sheets, as shown in the cartoon below.
ATP added to these ‘ reconstituted ’ microtubule doublets causes the microtubules to separate as the ATP is hydrolyzed. When such preparations are fixed for electron microscopy immediately after adding the ATP, they are caught in the act of sliding. See this animated in the first link below.
327 Proof of Sliding Microtubules During the Bending of Flagella and Cilia
328 Bacterial Flagella are Powered by a Proton Gradient
329 The Effects of Different Drugs on Microtubules and Cancer
C. Microfilaments - Structure and Role in Muscle Contraction
At 7 nm in diameter, microfilaments (actin filaments) are the thinnest cytoskeletal component. Globular actin ( G-actin ) monomers polymerize to form linear F-actin polymers. Two polymers then combine to form the twin-helical actin microfilament. As with microtubules, microfilaments have a +end to which new actin monomers are added to assemble F-actin , and a –end at which they disassemble when they are in a dynamic state, such as when a cell is changing shape. When one end of a microfilament is anchored to a cellular structure, for example to plaques in the cell membrane, motor proteins like myosin can use ATP to generate a force that deforms the plasma membrane and thus, the shape of the cell. One of the best-studied examples of myosin/actin interaction is in skeletal muscle where the sliding of highly organized thick myosin rods and the thin actin microfilaments results in muscle contraction.
1. Thick and Thin Filaments of Skeletal Muscle Contraction
Bundles of parallel muscle cells make up a skeletal muscle. Light microscopy of skeletal muscle thin sections show striated muscle cells ( myocytes , below).
The dark purplish structures surrounding the myocyte are mitochondria, which will provide the ATP to fuel contraction.  The bundled myocytes (also called myofibers ) are further organized into fascicles that are finally bundled into a muscle. The blowout illustration on the next page shows this anatomical organization and fine structure of a muscle (left panel).
High-resolution electron microscopy from the 1940s revealed the fine structure of skeletal muscle (right panel of the illustration), allowing characterization of the sarcomere . The dark bands of the striations in the light micrograph of myocytes are regions of aligned, adjacent sarcomeres. A pair of Z lines demarcate a sarcomere (Z for zwischen , German for between ). The I band is a relatively clear region of the sarcomere, largely made up of thin (actin) microfilaments. The A band at the center of the sarcomere consists of overlapping thin and thick (actin and myosin) filaments, while the H zone is a region where myosin does not overlap actin filaments. An M line lies at the center of the H zone. Multiple repeating sarcomeres of myocytes aligned in register in the fascicles give the appearance of striations in whole muscles.
2. The Sliding Filament Model of Skeletal Muscle Contraction
Electron microscopy of relaxed and contracted muscle shown below is consistent with the sliding of thick and thin filaments during contraction. Additional key structures of the sarcomere can be seen in the drawing at the right.
Note that in the sarcomeres of a contracted muscle cell, the H zone has almost disappeared. While the width of the A band has not changed after contraction, the width of the I bands has decreased and the Z-lines are closer in the contracted sarcomere. The best explanation here was the Sliding Filament Hypothesis ( model ) of skeletal muscle contraction.
330 The Sliding Filament Model of Skeletal Muscle Contraction
3. The Contraction Paradox: Contraction and Relaxation Require ATP
The role of ATP in fueling the movement of sliding filaments during skeletal muscle contraction was based in part on experiments with glycerinated fibers (muscle fibers soaked in glycerin to permeabilize the plasma membrane). The soluble cytoplasmic components leak out of glycerinated fibers, but leave the sarcomere structures intact, as visualized by electron microscopy. Investigators found that, if ATP and calcium were added back to glycerinated fibers, the ATP was hydrolyzed and the fiber could still contract… and even lift a weight!
The contraction of a glycerinated muscle fiber in the presence of ATP is illustrated below.
When assays showed that all of the added ATP had been hydrolyzed, the muscle remained contracted. It would not relax, even with the weight it had lifted still attached! Attempting to manually force the muscle back to its relaxed position didn’t work. But the fiber could be stretched when fresh ATP was added to the preparation! Moreover, if the experimenter let go immediately after stretching the fiber, it would again contract and lift the weight! A cycle of forced stretching and contraction could be repeated until all of the added ATP was hydrolyzed. At that point, the fiber would again no longer contract…, or if contracted, could no longer be stretched.
The contraction paradox then, was that ATP hydrolysis was required for muscle contraction as well as for relaxation (stretching). The paradox was resolved when the functions of the molecular actors in contraction were finally understood. Here we review some of the classic experiments that led to this understanding
4. Actin-Myosin Interactions In Vitro: Dissections and Reconstitutions
An early experiment hinted at the interaction of actin and myosin in contraction. Homogenates of skeletal muscle were viscous. The viscous component was isolated and shown to contain a substance that was called actomyosin ( acto , active; myosin , muscle substance). Under appropriate conditions, adding ATP to actomyosin preparations caused a decrease in viscosity. However, after the added ATP was hydrolyzed, the mixture became viscous again. Extraction of the nonviscous preparation (before it re-congealed and before the ATP was consumed) led to the biochemical separation of two the main substances we now recognize as the actin and myosin filaments of contraction. What’s more, adding these components back together reconstituted the viscous actomyosin extract (now referred to as actinomyosin to reflect its composition). And…, adding ATP to the reconstituted solution eliminated its viscosity. The ATP-dependent viscosity changes of actinomyosin solutions were consistent with an ATP-dependent separation of thick and thin filaments. Perhaps actin and myosin also separate in glycerinated muscles exposed to ATP, allowing them to stretch and relax.
The advent of electron microscopy provided further evidence of a role for ATP in both contraction and relaxation of skeletal muscle The purification of skeletal muscle actin (still attached to Z Lines) from myosin is cartooned below, showing what the separated components looked like in the electron microscope.
Next, when actin (still attached to Z-Lines) and myosin were mixed, electron microscopy of the resulting viscous material revealed thin filaments interdigitating with thick filaments. The result of this reconstitution experiment is shown below.
As expected, when ATP was added to these extracts, the solution viscosity dropped, and electron microscopy that the revealed thick (myosin) and thin (actin) filaments had again separated. The two components could again be isolated and separated by centrifugation.
In yet further experiments, actinomyosin preparations could be spread on over an aqueous surface, forming a film on the surface of the water. When ATP was added to the water, the film visibly “contracted”, pulling away from the edges of the vessel, reducing its surface area! Electron microscopy of the film revealed shortened sarcomere-like structures with closely spaced Z lines and short I bands…, further confirming the sliding filament model of muscle contraction.
332 In Vitro & Electron Microscope Evidence for a Sliding Filament Model
When actin and myosin were further purified from isolated actinomyosin, the thick myosin rods could be dissociated into large ~599Kd myosin monomers. Thus, thick filaments are massive polymers of myosin monomers! The molecular structure of myosin thick filaments is shown below.
An early observation of isolated actin filaments was that they had no ATPase activity. On the other hand, while isolated myosin preparations did have an ATPase activity, they would only catalyze ATP hydrolysis very slowly compared to intact muscle fibers. Faster ATP hydrolysis occurred only if myosin filaments were mixed with microfilaments (either on, or detached from Z-lines). In the electron microscope, isolated myosin protein monomers appeared to have a double-head and single tail regions. Biochemical analysis showed that the myosin monomers themselves were composed of the two heavy chain and two pairs of light chain polypeptides shown in the illustration above
High magnification, high resolution electron micrographs and the corresponding illustration below show the component structures of myosin monomers.
Proteolytic enzymes that hydrolyze peptide linkages only between specific amino acids, can ‘cut’ the heavy chains of myosin monomers into S1 (head) and tail fragments. Electron micrographs of these two fragments after separation by ultracentrifugation are shown above. S1 fragments were shown to have a slow ATPase activity, while the tails had none. The slow activity was not an artifact of isolation; mixing the S1 fraction with isolated actin filaments resulted in a higher rate of ATP hydrolysis. Clearly, myosin heads are ATPases that interact with actin microfilaments.
333 Thick Filament & Myosin Monomer Structure
The direct demonstration of an association of S1 myosin head fragments with rabbit smooth muscle actin microfilaments is shown below.
Just as for skeletal muscle, smooth muscle contraction is due to actin-myosin sliding, though smooth muscle is not striated and lacks sarcomere morphology; a white arrow in the micrograph points to one of several myosin (thick) filaments visible in the micrograph. The interaction of the S1 myosin heads with actin filaments dramatically alters their morphology. In this image, the diagonal stripes, or arrowhead -like appearance of the S1-actin binding all along actin filaments indicates that F-actin filaments are polar , with a plus (+) and a minus (–) end, as was expected.
The same “decoration” of microfilaments with arrowheads is seen when S1 heads (or even intact myosin monomers) bind to thin sections of skeletal muscle sarcomeres, preparations of actin still attached to the Z lines, and with isolated Factin preparations. These images are consistent with the requirement that myosin must bind to actin to achieve a maximum rate of ATPase activity during contraction.
The arrowheads on decorated actin still attached to Z lines always face in opposite directions, as shown below.
These opposing arrowheads, consistent with the sliding filament model of contraction in which bipolar thick filament pull actin filaments towards each other from opposite sides of the myosin rods, drawing the Z-lines closer together and shortening sarcomeres.
5. Allosteric Change and the Micro-Contraction Cycle
Whereas dynein and kinesin are motor proteins that ‘walk’ along microtubules, the myosin monomer is a motor protein that walks along microfilaments. In each case, these motor proteins are ATPases that use free energy of ATP hydrolysis to effect conformational changes that result in the walking, i.e., motility. In skeletal muscle, allosteric changes in myosin heads enable the myosin rods to do the walking along F-actin filaments
When placed in sequence such different myosin head conformations are likely the same as would occur during a micro-contraction cycle (illustrated below).
To help you follow the sequence, follow the small red dot on a single monomer in the actin filament. Here are the steps:
a. In the presence of Ca 2+ ions, myosin binding sites on actin are open (Ca 2+ - regulation of muscle contraction is discussed in more detail below).
b. Myosin heads with attached ADP and Pi bind to open sites on actin filaments.
c. The result of actin-myosin binding is an allosteric change in the myosin head, a bending of the hinge region, that pulls the attached microfilament (follow the red dot - it has moved from right to left!). This bit of micro-sliding of actin along myosin is the power stroke .
d. In its ‘bent’ conformation, the myosin head, still bound to an actin monomer in the F-actin, binds ATP, causing ADP and Pi to come off the myosin head and dissociating it from the actin.
e. Once dissociated from actin, myosin heads catalyze ATP hydrolysis, resulting in another conformational change. The head, still bound to ADP and Pi, has bent at its hinge, taking on a high-energy conformation that stores the energy of ATP hydrolysis.
f. The stored free energy is released during the power stroke. If Ca 2+ has been removed, the myosins remain in the high-energy conformation of step e, until a release of Ca 2+ again signals contraction.
Micro-contraction cycles of actin sliding along myosin continue as long as ATP is available. During repetitive micro-contraction cycles , myosin heads on the thick filaments pull actin filaments attached to Z-lines, bringing the Z lines closer to each other. The result is shortening of the sarcomere and ultimately, of muscle cells and the entire muscle. In the absence of ATP (as after the death of an organism), the micro-contraction cycle is interrupted. All myosin heads will remain bound to the actin filaments in the state of muscle contraction or relaxation (stretch) at the time of death. This is rigor mortis at the molecular level (see the illustration above). At the level of whole muscle, rigor mortis results in the inability to stretch or otherwise move body parts when ATP is, once and for all, depleted.
6. Resolving the Contraction Paradox
The myosin head micro-contraction cycle resolves the contraction paradox:
- ATP is necessary for muscle contraction : In step e in the illustration above, as ATP on myosin heads is hydrolyzed, the heads change from a low-energy to a high-energy conformation. The myosin heads can now bind to actin monomers (step b in the micro-contraction cycle). This results in of the power stroke (step c ), where free energy released by an allosteric change in myosin, pulls the actin along the myosin, in effect causing a micro-shortening of the sarcomere, in other words, contraction!
- ATP is necessary for muscle relaxation : At the end of step c , myosin remains bound to actin until ATP can again bind to the myosin head. Binding of ATP in step d displaces ADP and inorganic phosphate (Pi) and breaks actinmyosin cross-bridges. A removal of Ca 2+ from sarcomeres at the end of a contraction event blocks myosin binding sites on actin, while the rapid breakage of actin-myosin cross-bridges by ATP-myosin binding allows muscle relaxation and the sliding apart of the actin and myosin filaments (i.e., stretching). This leaves the myosin heads in the ‘cocked’ (high-energy) conformation, ready for the next round of contraction.
To summarize, ATP-myosin binding breaks actin-myosin cross-bridges. The muscle can then relax and stretch. Free energy of ATP hydrolysis, now stored in a high-energy myosin conformation, is released during the microcontraction power stroke
Electron microscopic examination of myosin monomer heads at different ionic strengths or when bound to antibodies (as shown below), provides visual evidence that myosin heads are flexible and can take on alternate stable conformations, as would be expected during the micro-contraction cycle.
The arrow heads point to bound antibody molecules (immunoglobulins). For a video of conformational change in myosin monomers at Myosin heads in Action .
335 An Actin-Myosin Contraction Cycle Resolves the Contraction Paradox
336 Binding and Hydrolysis of ATP Changes Myosin Head Conformation
7. Ca 2+ Ions Regulate Skeletal Muscle Contraction
Typically, the neurotransmitter acetylcholine released by a motor neuron binds to receptors on muscle cells to initiate contraction. In early experiments, Ca 2+ was required, along with ATP, to get glycerinated skeletal muscle to contract. It was later demonstrated that Ca 2+ ions were stored in the sarcoplasmic reticulum , the smooth endoplasmic reticulum of muscle cells. As we have seen, an action potential generated in the cell body of a neuron propagates along an axon to the nerve terminal, or synapse. In a similar fashion, an action potential generated at a neuromuscular junction travels along the sarcolemma (the muscle plasma membrane) to points where it is continuous with transverse tubules ( T-tubules ). The action potential then moves along the T-tubules and then along the membranes of the sarcoplasmic reticulum. This propagation of an action potential opens Ca 2+ channels in the sarcoplasmic reticulum. The Ca 2+ released bathes the sarcomeres of the myofibrils, allowing filaments to slide (i.e., contraction). The action potential at a neuromuscular junction that initiates contraction is summarized in the illustration below.
Ca 2+ ions released from the sarcoplasmic reticulum bathe the myofibrils, where they bind to one of three troponin molecules to regulate skeletal muscle contraction. The three troponins and a tropomyosin molecule are bound to actin filaments.. Experiments using anti-troponin and anti-tropomyosin antibodies localize these proteins on thin filaments spaced at regular intervals in electron micrographs. The drawing below models this association of the troponin subunits and tropomyosin with the thin filaments.
In resting muscle, tropomyosin (a fibrous protein) lies along the actin filament where it covers up the myosin binding sites of seven G-actin subunits in the microfilament. In this conformation, troponin T ( tropomyosin-binding troponin ) and troponin I ( inhibitory troponin ) hold the tropomyosin in place. The ‘cross-section illustration below illustrates the role conformational changes in troponin C upon binding Ca++ in regulating contraction.
8. Muscle Contraction Generates Force
Contraction by ATP-powered sliding of thin along thick filaments generates force on the Z-lines . In three dimensions, the Z-lines are actually Z-disks ) to which the actin thin filaments are attached. The protein \(\alpha \)-actinin in the Z-disks anchors the ends of the actin filaments to the disks so that when the filaments slide, the Z-disks are drawn closer, shortening the sarcomeres. Another Z-disk protein, desmin , is an intermediate filament organized around the periphery of Z-disks. Desmin connects multiple Z-disks in a myofibril. By keeping the Z-Disks in register, muscle cell, and ultimately, muscle contraction is coordinated. Finally, actin filaments at the ends of the cell must be connected to the cell membrane for a muscle cell to shorten during myofibril contraction. Several proteins, including syntrophins and dystrophin (another intermediate filament protein) anchor the free ends of microfilaments coming from Z-disks to the cell membrane. Still other proteins anchor the cell membrane in this region to the extracellular matrix (tendons) that are in turn, attached to bones! Force generated by myosin hydrolysis of ATP and the sliding of filaments in individual sarcomeres are thus transmitted to the ends of muscles to effect movement. If the name dystrophin sounds familiar, it should! The gene and its protein were named for a mutation that causes muscular dystrophy, resulting in a progressive muscle weakening.
338 Contraction Generates Force Against Z Disks and Cell Membranes
9. The Elastic Sarcomere: Do Myosin Rods Just Float in the Sarcomere?
In fact, myosin rods are anchored to proteins in the Z discs and M-lines . In 1954, R. Natori realized that when contracted muscle relaxes, it lengthens beyond its resting state, then shortening again to its resting length. He proposed that this elasticity must be due to a fiber in the sarcomere. Twenty-five years later, the elastic structure was identified as titin , a protein that holds several molecular records! The gene for titin contains the largest number of exons (363) of known proteins. After actin and myosin, titin is also the most abundant protein in muscle cells. At almost 4 x 106 Da, the aptly named titin is the also the largest known polypeptide. Extending from the Z discs to the M line of sarcomeres, titin coils around thick filaments along the way. Titin is anchored at Z-disks by \(\alpha \)-actinin and telethonin proteins. At the M-line , titin binds to myosin-binding protein C ( MYBPC3 ) and calmodulin , among others (e.g., myomesin , obscurin and skelamin ). Some if not all of these proteins must participate in keeping the myosin thick filaments positioned and in register in the sarcomere. This is similar to how Z-disks bind the ends of actin filaments to keep sarcomeres in register. The location of titin and several other sarcomere proteins is illustrated below.
Coiled titin molecules (in red in the illustration) extend from the Z to M lines. The colorized electron micrograph of one extended titin molecule in the middle of the illustration above should convince you of the length (35,213 amino acids!) of this huge polypeptide! Titin’s elastic features are largely in the region labeled P in the micrograph, between Z discs and the myosin rods. The many domains of this P region are shown at the bottom of the illustration. With all the binding (and other) functions, you might expect that titin has many domains. It does! They include Ig (immunoglobulin) domains, fibronectin domains (not shown here), PEVK and N2A domains (that helps bind titin to \(\alpha \)-actinin in Z-disks). Which and how many Ig and/or PEVK domains are present in a particular muscle depends on which alternative splicing pathway is used to form a titin mRNA. Over a micron long, Titin functions as a molecular spring, as Natori predicted. Its coiled domains compress during contraction, passively storing some of the energy of contraction. When skeletal muscle relaxes, Ca 2+ is withdrawn from the sarcomere, ATP displaces ADP from myosin heads and actin and myosin dissociate. The muscle then stretches, typically under the influence of gravity or an opposing set of muscles. However, during contraction, 244 individually folded protein domains of titin were compressed, and during relaxation, these domains de-compress; the stored energy of compression also helps to power relaxation. At the same time, titin connections limit the stretch so that a potentially overstretched muscle can ‘bounce’ back to its normal relaxed length.
In a particularly elegant experiment, R. Linke et al. provided a visual demonstration of myofiber elasticity consistent with the coiled spring model of titin structure. They made antibodies to peptide domains on either side of the PEVK domain of titin ( N2A and I20-I22 ) and attached them to nanogold particles (which will appear as electron dense granules in transmission electron microscopy). Then individual myofibers were stretched to different lengths, fixed for electron microscopy and treated with the nanogold -linked antibodies. The antibodies localize to and define the boundaries of the titin PEVK domains. The image below does not show original immune-stained electron micrographs but show alternate sarcomere micrographs with simulated localization of nanogold particles, reflecting actual results.
In the experiment, increased stretch lengthened the I bands on either side of Z lines of sarcomeres (blue bars). Likewise, the titin PEVK domains also lengthened as is evident from the increased distance between the nanogold-linked N2A and 120/122 antibodies that bind on either side PEVK domains. This demonstration of titin (and therefore sarcomere) elasticity) is consistent with the storage of some of the free energy of contraction when the molecule is compressed, and the passive release of that energy during relaxation. Since titin tethers thick filaments to Zdisks and M-lines, it also limits the amount of sarcomere stretch during relaxation. An animation from Linke’s lab is at http://www.titin.info/ .
D. Non-muscle Microfilaments
Electron microscopy revealed that thin (~10 nm) filaments permeated the cytoskeleton of eukaryotic cells. These were suspected to be actin microfilaments. Microfilaments typically lie in the cortex of cells, just under the plasma membrane, where they support cell shape. These same microfilaments can also re-organize dynamically, allowing cells to change shape. A dramatic example of this occurs in dividing cells, during cytokinesis when the dividing cell forms a cleavage furrow in the middle of the cell. The cortical microfilaments slide past each other with the help of non-muscle myosin , progressively pinching the cell until it divides into two new cells.
To test whether these 10 nm ‘microfilaments’ were in fact actin, intact myosin monomers or S1 myosin head fragments were placed atop electron micrographs of many different cell types. When viewed in the electron microscope, such preparations always revealed that the 10 nm microfilaments were decorated with arrowheads , just like S1 fragment decorated muscle cell actin or Z line-bound actin! Clearly, these cytoplasmic microfilaments are a form of F-actin. In the example shown below, cells in cytokinesis were treated with S1 myosin head fragments.
See the role of cortical filaments in cytokinesis at Cortical Actin Filament Action in Cytokinesis. Of course, actin microfilaments are involved in all manner of cell motility in addition to their role in cell division. They enable cell movement and cytoplasmic streaming inside cells. And while they give intestinal microvilli strength, they even enable microvilli to move independent of the passive pressures of peristalsis.
Other examples of microfilaments in cell motility include the ability of amoeba and other phagocytic cells to extend pseudopodia to engulf food or foreign particles (e.g., bacteria), respectively. Similarly, when fibroblast cells move along surfaces, they extend thin filipodia into the direction of movement by assembling actin bundles along the axis of cell movement. Actin stress fibers that help to maintain cell shape fluorescence green in the immunofluorescence micrograph below (left panel). The dual roles of actin in fibroblast movement are also illustrated (below right).
As we saw for microtubule-mediated cell motility, some actin-mediated motility may be primarily based on actin assembly and disassembly, as in the extension of filipodia at the moving front of a fibroblast. As the fibroblast moves forward, a retraction fiber at the hind-end of the cell remains attached to the surface ( substratum ) along which it is migrating. Eventually however, actin-myosin interactions (in fact, sliding) causes retraction of most of this ‘fiber’ back into the body of the cell.
Movements mediated by stress fibers may also explain the cytoplasmic streaming that distributes cellular components and nutrients throughout a cell. The movements of both involve actin-myosin interactions. Studies of non-muscle cell motility suggest the structure and interacting molecular components of stress fibers. They reveal overlapping myosin and actin filaments that slide during movement, as illustrated below.
Filamin in this drawing is shown holding actin filaments together at an angle, while \(\alpha \)- actinin also helps to bundle the actin (thin) filaments. Titin (not shown) also seems to be associated with stress fibers. However, unlike highly organized skeletal muscle sarcomeres, the proteins and filaments in stress fibers are not part of Z- or M-line superstructures. Could such less-organized non-muscle stress fiber filament bundles be the evolutionary predecessor to sarcomeres in muscle cells?
E. Actins and Myosins are Encoded by Large Gene Families
Actins may be the most abundant protein in cells! At least six different actin isoforms encoded by a large actin gene family have nearly identical amino acid sequences, all of which are involved in cytoskeletal function. The \(\beta \)-actin isoform predominates. Genes for some isoforms are expressed in a cell-specific manner.
Are all actin isoforms functionally significant? Myosin monomers (or S1 heads) decorate virtually all actins. This makes one wonder if any one actin is an adaptation, however subtle, such that the absence of one isoform would pose a significant threat to the survival of an organism? Since amino acid sequence differences between actins would not predict dramatically different protein function, could they underlie some as yet unknown physiological advantage to different cells? In mice, the loss of a \(\gamma \)-actin gene has little effect on the organism, while loss of the \(\beta \)-actin gene in mice is lethal at embryonic stages. In contrast, studies show that a mutated \(\beta \)-actin gene in humans correlate with delayed development and later neurological problems (e.g., epilepsy), kidney and heart abnormalities, but is not lethal. In fact, people with such mutations can lead nearly normal, healthy lives (Beta-Actin Gene Mutations and Disease).
Like the actins, myosin genes encoding variant isoforms comprise a large eukaryotic gene family. All isoforms have ATPase activity and some are clearly involved in cell motility. Unique functions are not yet known for other isoforms, but different myosin monomers can decorate actin, and myosins from one species can decorate actin filaments of other species, even across wide phylogenetic distances.
F. Intermediate Filaments - an Overview
These 10 nm filaments are proteins with extended secondary structure that in fact, do not readily fold into tertiary structures, and they have no enzymatic activity. Recall their intercellular location in desmosomes where they firmly bind cells together to confer tensile strength to tissues. Within cells, intermediate filaments permeate the cells where they participate in regulating and maintaining cell shape. Recall their role in anchoring actin to either Z-disks or plasma membrane plaques in muscle cells, transmitting the forces of contraction to the shortening of the sarcomeres and then to the actual shortening of a muscle. The extracellular keratins that make up fur, hair, fingernails and toenails, are proteins related to intermediate filaments. Unlike intracellular intermediate filaments, keratins are bundles of rigid, insoluble extracellular proteins that combine to align to form stable, unchanging secondary structures. Finally, lamins are intermediate filaments that make up structural elements of the nuclear lamina , a kind of nucleoskeleton .
CMB3e 492 As we saw earlier, intermediate filament subunits have a common structure consisting of a pair of monomers, each with globular domains at their C- and N-terminal ends, separated by coiled rod regions. Monomers are non-polar; i.e., unlike microtubules and actin filaments, they do not have ‘plus’ and ‘minus’ ends. The basic unit of intermediate filament structure is a dimer of monomers. Dimers further aggregate to form tetramers and larger filament bundles. Like microtubules and actin filaments, intermediate filament bundles can disassemble and reassemble as needed when cells change shape. Unlike microtubules and actin, intermediate filaments can stretch, a property conferred by the coiled rod regions of the filaments. This should be reminiscent of titin molecules! The structural features and elasticity of intermediate filaments is illustrated in the cartoon below.
In the bundled intermediate filaments that permeate the cytoplasm of cells, the ability to stretch contributes to the viscosity of cytoplasm, and is even called viscoelasticity . This elastic property is thought to allow actins and microtubules a degree of freedom of movement of cells, and within the cytoplasm of cells.


4.5: Racialized, Gendered, and Sexualized Labor in the Global Economy
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- Miliann Kang, Donovan Lessard, Laura Heston, and Sonny Nordmarken
- University of Massachusetts Amherst via UMass Amherst Libraries
The structure of the global economy affects people differently not only by the economic situations of the nations in which they live, but also by gender and race. Predatory trade relationships between countries roughly reproduce the political situation of colonization in many nations of the Global South. This has led many to characterize neoliberal economic policies as a form of neocolonialism , or modern day colonization characterized by exploitation of a nation’s resources and people. Colonialism and neocolonialism are concepts that draw attention to the racialized global inequalities between white, affluent people of the Global North—historical colonizers—and people of color of the Global South—the historically colonized. Postcolonial theory emerged out of critiques of colonialism, empire, enslavement, and neocolonial racist-economic oppression more generally, which were advanced by scholars in the Asian and Middle Eastern diasporas. Postcolonial scholars primarily unpack and critique colonial discourses, depictions of colonized Others, and European scholars’ biased representations of those they colonized, which they figure as knowledge (for example, see Said 1995 and Spivak 1988). Decoloniality theoretical approaches, emerging chiefly in Latin America, illuminated how colonization invented the concepts of “the colonized,” “modernity” and “coloniality,” and disrupted the social arrangements, lives, gender relations, and understandings it invaded, imposing on the colonized European racialized conceptualizations of male and female (Quijano 2007; Lugones 2007).
Women of color of the Global South are disproportionately impacted by global economic policies. Not only are women in Asian and Latin American countries much more likely to work in low-wage factory jobs than men, women are also much more mobile in terms of immigration (Pessar 2005). Women have more labor-based mobility for low-income factory work in other countries as well as in domestic and sex work markets. When women immigrate to other nations they often sacrifice care of and contact with their own children in order to earn money caring for wealthier people’s children as domestic workers; this situation is known as transnational motherhood ( Parreñas 2001). Domestic work and sex work are two sectors of the service economy in which women immigrants participate. Immigrants, especially undocumented immigrants, have few options in terms of earning money, and economic circumstances are such that undocumented immigrants can make more money within illegal and unregulated markets in nations of the Global North, rather than regulated markets of the formal economy. Thus, it is not uncommon for women immigrants to participate in informal economies such as domestic work or sex work that employers and clients do not report in their taxes.
Women immigrants also participate in other parts of the service economy of the Global North. Miliann Kang (2010) has studied immigrant women who participate in beauty service work, particularly nail salons. This type of work does not require high amounts of skill or experience and can support women for whom English is a second language or those who may be undocumented. Like any service job, work in nail salons involves emotional labor. While clients may see the technician in the beauty salon as their confidant (like Queen Latifa’s character in Beauty Shop ), their relationship is primarily an unequal labor relationship in which one party is paid not only for the service they perform but also for their friendly personalities and listening skills. Kang (2010) refers to this type of labor involving both emotional and physical labor as body labor . To engage in both emotional and physical labor at work is exhausting. In addition, workers in nail and hair salons work with harsh chemicals that are ultimately toxic to their health and make them more susceptible to cancer than the general population.
Not only do gendered, racialized, and sexualized differences exist in the US domestic labor market, leading to differences in work and pay, these differences also characterize the globalized labor market. Trade relationships between countries and the ideology of neoliberalism that governs them have profound effects on the quality of life of people all over the world. Women bear the brunt of changes to the global marketplace as factory workers in some countries and domestic, sex, and beauty service workers in others. Fortunately, fair trade and anti-sweatshop movements as well as indigenous, decolonial, feminist and labor movements are fighting to change these conditions for the better in the face of well-funded and powerful multinational corporations and global trade organizations.


31.2: Astronomy Websites, Images, and Apps (Appendix B)
Throughout the textbook, we suggest useful resources for students on the specific topics in a given chapter. Here, we offer some websites for exploring astronomy in general, plus good sites for viewing and downloading the best astronomy images, and guides to astronomical apps for smartphones and tablets. This is not an exhaustive listing, but merely a series of suggestions to whet the appetite of those wanting to go beyond the textbook.
Websites for Exploring Astronomy in General
Astronomical Organizations
Amateur Astronomy Clubs. In most large cities and a number of rural areas, there are amateur astronomy clubs , where those interested in the hobby of astronomy gather to observe the sky, share telescopes, hear speakers, and help educate the public about the night sky. To find an astronomy club near you, you can try the following sites:
- Night Sky Network club finder: http://nightsky.jpl.nasa.gov/club-map.cfm .
- Sky & Telescope Magazine astronomy clubs and organizations: skyandtelescope.org/astronomy-clubs-organizations/.
- Astronomy Magazine club finder: www.astronomy.com/groups.aspx.
- Astronomical League astronomy clubs and societies: http://www.astroleague.org/societies/all .
- Go-Astronomy club search: http://www.go-astronomy.com/astro-club-search.htm .
American Astronomical Society: www.aas.org. Composed mainly of professional astronomers. They have an active education office and various materials for students and the public on the education pages of their website.
Astronomical League: http://www.astroleague.org . The league is the umbrella organization of American astronomy clubs. They offer a newsletter, national observing programs, and support for how to form and support a club.
Astronomical Society of the Pacific: http://www.astrosociety.org . Founded in 1889, this international society is devoted to astronomy education and outreach. They have programs, publications, and materials for families, teachers, amateur astronomers, museum guides, and anyone interested in astronomy.
European Space Agency (ESA): http://www.esa.int/ . Information on European space missions with an excellent gallery of images.
International Astronomical Union (IAU): http://www.iau.org/ . International organization for professional astronomers; see the menu choice “IAU for the Public” for information on naming astronomical objects and other topics of interest to students.
International Dark-Sky Association: www.darksky.org. Dedicated to combating light pollution, the encroachment of stray light that wastes energy and washes out the glories of the night sky.
NASA: http://www.nasa.gov . NASA has a wide range of information on its many websites; the trick is to find what you need. Most space missions and NASA centers have their own sites.
Planetary Society: http://www.planetary.org . Founded by the late Carl Sagan and others, this group works to encourage planetary exploration and the search for life elsewhere. While much of their work is advocacy, they have some educational outreach too.
Royal Astronomical Society of Canada: http://www.rasc.ca/ . Unites professional and amateur astronomers around Canada; has 28 centers with local activities, plus national magazines and meetings.
Some Astronomical Publications Students Can Read
Astronomy Now : www.astronomynow.com/. A colorful British monthly, with excellent articles about astronomy, the history of astronomy, and stargazing.
Astronomy : http://www.astronomy.com . Has the largest circulation of any magazine devoted to the universe and is designed especially for astronomy hobbyists and armchair astronomers.
Free Astronomy : http://www.astropublishing.com/ . A new web-based publication, with European roots.
Scientific American : www.sciam.com. Offers one astronomy article about every other issue. These articles, a number of which are reproduced on their website, are at a slightly higher level , but—often being written by the astronomers who have done the work—are authoritative and current.
Sky & Telescope : skyandtelescope.com. An older and somewhat higher-level magazine for astronomy hobbyists. Many noted astronomers write for this publication.
Sky News : www.skynews.ca/. A Canadian publication, featuring both astronomy and stargazing information. It also lists Canadian events for hobbyists.
StarDate : https://stardate.org/ . Magazine that accompanies the brief radio program, with a useful website for beginners.
Sites that Cover Astronomy News
Portal to the Universe: http://www.portaltotheuniverse.org/ . A site that gathers online astronomy and space news items, blogs, and pictures.
Science@NASA news stories and newscasts: http://science.nasa.gov/science-news/ . Well-written stories with, of course, a NASA focus.
Space.com: http://www.space.com/news/ . A commercial site, but with wide coverage of space and astronomy news.
Universe Today: http://www.universetoday.com/ . Another commercial site, with good articles by science journalists, but a lot of ads.
Sites for Answering Astronomical Questions
Ask an Astrobiologist: http://astrobiology.nasa.gov/ask-an-astrobiologist/ . On this site from the National Astrobiology Institute at NASA, astronomer David Morrison answered questions about the search for life on other planets, the origin of life on Earth, and many other topics.
Ask an Astronomer at Lick Observatory: http://www.ucolick.org/~mountain/AAA/ . Graduate students and staff members at this California observatory answered selected astronomy questions, particularly from high school students.
Ask an Astrophysicist: http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html . Questions and answers at NASA’s Laboratory for High-Energy Astrophysics focus on X-ray and gamma-ray astronomy, and such objects as black holes, quasars, and supernovae.
Ask an Infrared Astronomer: https://coolcosmos.ipac.caltech.edu/asks . A site from the California Institute of Technology, with an archive focusing on infrared (heat-ray) astronomy and the discoveries it makes about cool objects in the universe. No longer taking new questions.
Ask the Astronomer: http://www.astronomycafe.net/qadir/qanda.html . This site, run by astronomer Sten Odenwald, is no longer active, but lists 3001 answers to questions asked in the mid-1990s. They are nicely organized by topic.
Ask the Experts at PhysLink: http://www.physlink.com/Education/AskExperts/index.cfm . Lots of physics questions answered, with some astronomy as well, at this physics education site. Most answers are by physics teachers, not astronomers. Still taking new questions.
Ask the Space Scientist: image.gsfc.nasa.gov/poetry/ask/askmag.html. An archive of questions about the Sun and its interactions with Earth, answered by astronomer Sten Odenwald. Not accepting new questions.
Curious about Astronomy?: curious.astro.cornell.edu. An ask-an-astronomer site run by graduate students and professors of astronomy at Cornell University. Has searchable archives and is still answering new questions.
Miscellaneous Sites of Interest
A Guide to Careers in Astronomy: http://aas.org/files/resources/Careers-in-Astronomy.pdf . From the American Astronomical Society.
Astronomical Pseudo-Science: A Skeptic’s Resource List: http://bit.ly/pseudoastro . Readings and websites that analyze such claims as astrology, UFOs, moon-landing denial, creationism, human faces on other worlds, astronomical disasters, and more.
Astronomy for Beginners: www.skyandtelescope.com/astronomy-information/. A page to find resources for getting into amateur astronomy.
Black Lives in Astronomy: http://bit.ly/blackastro .
Contributions of Women to Astronomy: http://bit.ly/astronomywomen .
Music Inspired by Astronomy: http://bit.ly/astronomymusic .
Science Fiction Stories with Good Astronomy and Physics: bit.ly/astroscifi.
Astronomy Calendar: bit.ly/astrodates. Entitled “This Day in Astronomical History,” this list provides 158 astronomical anniversaries or events that you can celebrate.
Unheard Voices: The Astronomy of Many Cultures: https://astrosociety.org/education-outreach/resource-guides/multicultural-astronomy.html . A guide to resources about the astronomy of native, African, Asian, and other non-Western groups.
Videos To Go with Each Chapter of this Textbook: http://bit.ly/shortastronomyvideos .
Selected Websites for Viewing and Downloading Astronomical Images
The Top Image Sites
Astronomy Picture of the Day: http://antwrp.gsfc.nasa.gov/apod/lib/aptree.html . Two space scientists scour the internet and feature one interesting astronomy image each day.
European Southern Observatory Photo Gallery: http://www.eso.org/public/images/ . Magnificent color images from ESO’s largest telescopes. See the topical menu at the top.
Hubble Space Telescope Images: hubblesite.org/images/gallery. Starting at this page, you can select from among many hundreds of Hubble pictures by subject. Other ways to approach these images are through the more public-oriented Hubble Gallery (hubblesite.org/gallery/) or the European ESA site ( http://www.spacetelescope.org/images/ ).
National Optical Astronomy Observatories Image Gallery: www.noao.edu/image_gallery/. Growing archive of images from the many telescopes that are at the United States’ National Observatories.
Planetary Photojournal: http://photojournal.jpl.nasa.gov/index.html . Features thousands of images from NASA’s extensive set of planetary exploration missions with a good search menu. Does not include most of the missions from other countries.
The World at Night: http://www.twanight.org/newTWAN/index.asp . Dramatic night-sky images by professional photographers who are amateur astronomers. Note that while many of the astronomy sites allow free use of their images, these are copyrighted by photographers who make their living selling them.
Other Useful General Galleries
Anglo-Australian Observatory: images.datacentral.org.au/. Great copyrighted color images by leading astro-photographer David Malin and others.
Canada-France-Hawaii Telescope: www.cfht.hawaii.edu/HawaiianStarlight/images.html. Remarkable color images from a major telescope on top of the Maunakea peak in Hawaii.
European Space Agency Gallery: http://www.esa.int/spaceinimages/Images . Access images from such missions as Mars Express, Rosetta, and Herschel.
Gemini Observatory Images: www.gemini.edu/index.php?option=com_gallery. Images from a pair of large telescopes in the northern and the southern hemispheres.
Isaac Newton Group of Telescopes Image Gallery: http://www.ing.iac.es/PR/images_index.html . Beautiful images from the Herschel, Newton, and Kapteyn telescopes on La Palma.
National Radio Astronomy Observatory Image Gallery: https://public.nrao.edu/gallery/ . Organized by topic, the images show objects and processes that give off radio waves.
Our Infrared World Gallery: https://coolcosmos.ipac.caltech.edu/infrared_galleries . Images from a variety of infrared astronomy telescopes and missions. See also their “Cool Cosmos” site for the public: http://coolcosmos.ipac.caltech.edu/ .
Some Galleries on Specific Subjects
Astronaut Photography of Earth: http://eol.jsc.nasa.gov/ .
Chandra X-Ray Observatory Images: http://chandra.harvard.edu/photo/category.html .
NASA Human Spaceflight Gallery: https://www.flickr.com/photos/nasa2explore or spaceflight.nasa.gov/gallery/index.html. Astronaut images.
Robert Gendler: http://www.robgendlerastropics.com/ . One of the amateur astro-photographers who comes closest to being professional.
Sloan Digital Sky Survey Images: www.sdss.org/gallery/.
Solar Dynamics Observatory Gallery: http://sdo.gsfc.nasa.gov/gallery/main . Sun images.
Spitzer Infrared Telescope Images: http://www.spitzer.caltech.edu/images .
Astronomy Apps for Smartphones and Tablets
The Best Astronomy Apps for IOS and Android: https://www.digitaltrends.com/mobile/best-astronomy-apps/ .
The Best Stargazing Apps for Looking at the Night Sky: https://www.tomsguide.com/round-up/best-stargazing-apps .
The 19 Best Astronomy Apps for Stargazing: https://astrobackyard.com/astronomy-apps-for-stargazing/ .
The 10 Best Astronomy Apps for Enjoying the Night Sky: www.makeuseof.com/tag/great-android-astronomy-apps/.
The Best Astronomy Apps for Smartphones, Tablets, and Computers: https://www.thoughtco.com/best-astronomy-apps-4160999 .