Eukaryotes: Plant Cells

How do the Cells of Plants Differ from those of Animals?

Cellulose Fibrils
Plant cell walls are composed mainly of cellulose as shown in this diagram.  A second type of polysaccharide, called pectin, is also present in small amounts.  The middle lamella is a layer of sticky material, also containing pectin, which serves as “glue” to hold adjacent cells together.  Pectin is an interesting plant product that is used commercially as a thickening for jams and jellies.

Adjacent cells within a plant are connected to one another by strands of cytoplasm called plasmodesmata.  This electron micrograph shows the cell walls of two adjacent cells.  Small openings with the cell walls provide spacesfor “bridges” between the cells.  The plasma membranes as well as cytoplasm of the cells pass through these openings.  Most cells within a plant are attached in this way, allowing molecules to pass freely from one cell to another.  Even macromolecules such as proteins and RNA have been seen to pass through the plasmodesmata.

Turgor Pressure
Because the central vacuole contains stored molecules, water flows into it by osmosis.  In other words, water outside of the cell has a lower concentration of solutes than water within the vacuole, so moves across both the plasma membrane and the vacuole membrane.  This causes the vacuole to swell and exerts pressure against the cell wall called turgor pressure.  If water is lost from the vacuole, it shrinks and the cell becomes flaccid.  This is why plants die if salt is added to the soil in which they grow.

 This video shows the vacuole shrinking when a high concentration of sugar is placed in the water outside of the cells.  As the vacuole loses water and shrinks, the living parts of the cell contract and we see the cytoplasm and chloroplasts moving to the center of the cells.  The plasma membrane surrounds the centrally located cytoplasm and only the rigid cell walls maintain the shape of these cells.  Plant cells in this condition will die unless the external solution is replaced with pure water.

Plastids are large organelles surrounded by two membranes.  A proplastid has little internal structure and can develop into any of the other types of plastid. 
Chromoplasts containing pigment granules and amyloplasts, which contain stored starch can form from the proplastids.  Chloroplasts are the most complex plastid.  They contain the pigment chlorophyll and have precisely arranged stacks of internal membranes called thylakoids.  Watch the following animation which shows a proplastid developing into a chloroplast.

These diagrams show the complex internal structure of a chloroplast.  Membrane-bound structures called thylakoids are arranged in stacks throughout the chloroplast.  The stacks are called grana (granum is singular).  Between the grana lies a fluid-filled space containing enzymes.  This is called the stroma.  Note that each thylakoid has an internal space surrounded by a membrane.  This structure is crucial for the conversion of light energy into chemical energy, as you will learn shortly.

A second type of plastid, called a chromoplast, contains yellow and orange pigments.  This gives color to many fruits and flowers.  The green pepper shown here has chloroplasts within its cells, whereas the yellow and orange peppers have mainly chromoplasts.

The numerous chloroplasts or chromoplasts are visible within these pepper cells.

Here we see cells of a daffodil.  The tightly packed chromoplasts give the flower its yellow color.

The roots and tubers of many plants store food in the form of starch.  Here we see potato cells at high magnification.  The large oval structures within the cells are amyloplasts.  They contain the starch.  The preparation on the left is unstained, whereas the one on the right has been stained with iodine.  Since iodine reacts with starch to give a purple color, the amyloplasts within these cells are purple.

What is Photosynthesis and How Does it Work?

Evolution of Life
As you just learned, photosynthesis was a crucial development in early life forms, allowing them to make food (organic molecules) from carbon dioxide and water.  This allowed cells to survive as the supply of organic molecules in the primordial soup was depleted.  But equally important was the generation of oxygen as a byproduct of photosynthesis.  Since the atmosphere of early earth had no oxygen, metabolic processes that require oxygen, such as cellular respiration, could not occur.  For hundreds of millions of years photosynthetic prokaryotes, the first eukaryotic cells, and finally plants produced oxygen and slowly changed the composition of the atmosphere to the nitrogen-oxygen mixture that we see today.  It was only when atmospheric oxygen reached a relatively high level that organisms using cellular respiration and eventually animals were able to evolve.

Remember the endosymbiotic theory?  As you learned earlier, it is thought that heterotrophic prokaryotes with aerobic metabolism  were engulfed by early eukaryotic cells giving rise to mitochondria.  Likewise, it seems that certain photosynthetic bacteria entered eukaryotic cells and became chloroplasts.  The presence of circular DNA and ribosomes within chloroplasts supports this idea.

Sulfur Bacteria
The oxygen released during photosynthesis usually comes from the splitting of water.  The hydrogen component of water is utilized in making sugar, and the oxygen is a byproduct.  While most photosynthetic organisms utilize water as the source of hydrogen, the sulfur bacteria use hydrogen sulfide.  When this compound is split, hydrogen is used for photosynthesis and the resulting sulfur is stored within the cell.  Yellow sulfur granules can be seen within these bacterial cells.

Cellular Respiration
Here again is the overall equation for photosynthesis.  Note that it is the opposite of the equation for cellular respiration.  Of course, photosynthesis is an endergonic process requiring energy input, whereas respiration is exergonic and results in ATP production.

The vast majority of eukaryotic cells alive today utilize mitochondria to produce ATP by aerobic respiration.  This includes plant cells.  So within a plant cell both photosynthesis and respiration are occurring.  In fact, approximately 50% of the sugars made by photosynthesis are used to produce the ATP that powers the cell’s metabolic reactions and other energy needs.  Some of the remaining sugars are utilized as carbon skeletons in the synthesis of other organic molecules and the rest are stored for future needs such as seed and fruit production.  Note that when sugars are utilized in respiration, the end products carbon dioxide and water can be cycled back to the chloroplasts and used to make more sugar.

Before we tackle the details of photosynthesis, we will look at some general aspects of the process.  Photosynthesis consists of two parts.  First, there is the “photo” part that requires light energy.  Water is split, oxygen is given off, and the light energy is stored in the form of ATP plus the hydrogen-containing molecule NADPH. These processes take place within the grana of the chloroplast and are called the light reactions.  The other half of photosynthesis is the “synthesis” part.  This cyclical series of reactions utilizes the ATP and NADPH to “fix” carbon dioxide into organic molecules resulting in sugar.  The synthesis steps take place in the stroma of the chloroplast and do not require light.  Thus, they may be called the dark reactions.

How Sugar is Made by Plants
Now let’s consider where the atoms of a sugar molecule come from during photosynthesis.  In an overview of the process, we can say that all of the carbon and oxygen atoms in sugar originate from carbon dioxide, whereas all of the hydrogen comes from water.  And of course the oxygen component of water is released as a byproduct.  The water shown here as a product is formed from the “extra” hydrogen and oxygen atoms.

Electromagnetic Spectrum
Before we proceed any further, we must briefly consider the nature of light.  Visible light is a small part of the electromagnetic spectrum, the part that our eyes can perceive.  Light is composed of different wave-lengths, resulting in colors ranging from red to purple.  We can see this spectrum of colors when the wave-lengths are separated in a rainbow.

The upper part of this diagram shows the electromagnetic spectrum from the very short wave-lengths of gamma rays and X-rays up to the very long wave-length of radio waves.  Note that the shorter the wave-length, the higher its energy.  Visible light consists of a small region within the middle of this spectrum.  Note that blue and purple light have the shortest wavelengths and thus the highest energy of visible light.

Photosystems are collections of pigment molecules, chlorophyll and carotenoids, that absorb solar energy and transform it into chemical energy.  They are embedded in the thylakoid membranes.  Each photosystem consists of a few hundred pigment molecules that absorb sunlight.  These molecules serve as “collecting antennae” that absorb energy from the sun and pass it on.  At the center of the antennae system is a reaction center composed of two special molecules of chlorophyll a.   The reaction center receives energy from the other pigments and uses it to move an electron to a higher energy state.  This energized electron is a form of chemical potential energy.

Light Reactions
This diagram is a cartoon depicting the key steps during the light reactions of photosynthesis.  Two photosystems are involved.  In the first step, a photosystem absorbs a photon of light.  This energizes an electron within the reaction center.  The potential energy of the energized electron is used to make ATP as the electron is moved to a second photosystem.  Now the electron has lost most of its energy, but it is energized  again to an even higher level as the second reaction center absorbs a photon.  The electron’s potential energy is now utilized to produce NADPH which captures the electron in the process.  NADPH is quite similar to the NADH of cellular respiration.  It carries the electron to the next phase of photosynthesis.

Carbon Fixation
We have come to the second phase of photosynthesis in which sugars are synthesized.  This phase is called the Calvin cycle and does not require light.  The diagram gives an overview of the Calvin cycle which occurs within the stroma of chloroplasts.  Note that the process is cyclical and incorporates 3 carbon dioxide molecules into organic compounds to produce sugar products.  This is called carbon fixation.  The energy and hydrogens required for reactions in the Calvin cycle are provided by the ATP and NADPH produced during the light reactions.

Importance of Glucose
The sugar glucose is the final product of photosynthesis.  Much of this glucose is broken down again by the process of cellular respiration to produce the ATP needed for work within the cell.  In growing cells, glucose monomers are joined together to make cellulose for cell walls.  Since plants contain a very large amount of cellulose, a significant portion of glucose is used for this purpose.  Glucose also serves as the carbon skeleton from which other organic molecules are synthesized.

Excess glucose is stored for later use.  The most efficient storage form is starch.  So some glucose monomers are joined together to make starch polymers.  This electron micrograph shows starch granules stored within chloroplasts.

Much glucose is transferred to other parts of the plant where it is need to produce ATP or to serve as the starting point for synthesis of other molecules.  Some parts of plants serve as storage sites for large amounts of glucose in the form of starch.  Here we see an electron micrograph of cells within a potato tuber.  As you learned earlier, these cells store starch in organelles called amyloplasts.  The roots and seeds of plants are other sites where starch is commonly stored.

What Are Some Additional Ways in which Plant Cells Differ from Other Cells?

Extracellular Matrix
Plants and animals are both multicellular organisms.   This means that their cells are joined together to form tissues.  Here we see many cells within a plant stem.

Animal cells also form tissues.  A single cell is outlined within the liver tissue shown here.  All tissues consist of cells plus an extracellular matrix.  All material that is external to the cell plasma membranes is defined as extracellular matrix.  The matrix plays the major role in holding the cells together within the tissue.

Plant Cell Walls
You have seen this diagram before.  It illustrates the structure of the plant cell wall.  The cell wall is usually considered to be part of the plant cell, but because it is external to the plasma membrane, it also forms most of the extracellular matrix.  Cell walls plus the sticky middle lamella between adjacent cells constitute the extracellular matrix of plant tissues.

Here is a cross section through the stem of a sunflower.  Note that the cells are held tightly together.  A relatively small amount of extracellular matrix between cells is typical of plant tissues.

Types of Extracellular Matrix
This diagram illustrates the complexity of the extracellular matrix found in animal tissues.  It is common to find long, glycoprotein chains extending from membrane proteins or intertwined among thick fibers external to the cell surface.  The large, complex glycoproteins attract water into the area, giving the extracellular matrix a gel-like consistency.  The large fibers are made of a protein called collagen.  They provide strength to the matrix. The extracellular matrix of animal cells can be very thick or extremely thin, as you will see shortly.

Extracellular Matrix in Animal Cells
Here we see an animal tissue that has a relatively small amount of extracellular matrix.  The matrix appears green in the micrograph.  The cells are dark with purple nuclei.  A small amount of extracellular matrix is typical of cells that are held tightly together within a tissue, such as liver or muscle.

This tissue has a large amount of extracellular matrix.  The cells are green with  dark nuclei and the fibers within the matrix are yellow-orange.  Note that the cells are not connected to one another, but are embedded with the watery gel of the extracellular matrix.  The deeper tissues of the skin are of this type.  In certain other animal tissues, the extracellular matrix contains minerals that make it hard.  Teeth, bones and shells are examples.

Cytoplasmic Streaming
Most plant cells are large, and rapid cytoplasmic movements are sometimes utilized to distribute molecules throughout the cell.  This video shows the cytoplasm streaming in a circular motion within the cells of an Elodea plant.  The chloroplasts are carried along, making the cytoplasmic motion easy to see.

The cytoplasm of animal cells does not usually move in such a directed way.  An exception is the amoeboid motion of some motile cells, such as the white blood cells that you have recently seen.  In both the motile animal cell and the large plant cell, actin microfilaments interact with cytoplasmic proteins to generate the cytoplasmic streaming.
