Written by Christopher Chow
Last Updated


BiologyArticle Free Pass
Alternate title: cell system
Written by Christopher Chow
Last Updated
Table of Contents

Secretory vesicles

The release of proteins or other molecules from a secretory vesicle is most often stimulated by a nervous or hormonal signal. For example, a nerve cell impulse triggers the fusion of secretory vesicles to the membrane at the nerve terminal, where the vesicles release neurotransmitters into the synaptic cleft (the gap between nerve endings). The action is one of exocytosis: the vesicle and the cell membrane fuse, allowing the proteins and glycoproteins in the vesicle to be released to the cell exterior.

As secretory vesicles fuse with the cell membrane, the area of the cell membrane increases. Normal size is regained by the reuptake of membrane components through endocytosis. Regions bud in from the cell membrane and then fuse with internal membranes to effect recycling.

Sorting of products by chemical receptors

Not all proteins synthesized on the ER are destined for export. Many, such as the hydrolases in lysosomes, remain inside the cell; others become anchored in the membrane of internal organelles or in the cell membrane. It is presumed that each protein has some type of marker that fits a specific location in the cell.


The nucleus

The nucleus is the information centre of the cell and is surrounded by a nuclear membrane in all eukaryotic organisms. It is separated from the cytoplasm by the nuclear envelope, and it houses the double-stranded, spiral-shaped deoxyribonucleic acid (DNA) molecules, which contain the genetic information necessary for the cell to retain its unique character as it grows and divides.

The presence of a nucleus distinguishes the eukaryotic cells of multicellular organisms from the prokaryotic, one-celled organisms such as bacteria. In contrast to the higher organisms, prokaryotes do not have nuclei, so their DNA is maintained in the same compartment as their other cellular components.



Structural organization of the nucleus

DNA packaging

The nucleus of the average human cell is only 6 micrometres (6 × 10−6 metre) in diameter, yet it contains about 1.8 metres of DNA. This is distributed among 46 chromosomes, each consisting of a single DNA molecule about 40 mm (1.5 inches) long. The extraordinary packaging problem this poses can be envisaged by a scale model enlarged a million times. On this scale a DNA molecule would be a thin string 2 mm thick, and the average chromosome would contain 40 km (25 miles) of DNA. With a diameter of only 6 metres, the nucleus would contain 1,800 km (1,118 miles) of DNA.


What made you want to look up cell?
Please select the sections you want to print
Select All
MLA style:
"cell". Encyclopædia Britannica. Encyclopædia Britannica Online.
APA style:
cell. (2014). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/101396/cell/37396/Secretory-vesicles
Harvard style:
Chicago Manual of Style:
Encyclopædia Britannica Online, s. v. "cell", accessed December 25, 2014, http://www.britannica.com/EBchecked/topic/101396/cell/37396/Secretory-vesicles.


We welcome suggested improvements to any of our articles.
(Please limit to 900 characters)

Or click Continue to submit anonymously: