Cells are the smallest individual structures that possess all of the qualities formally associated with being alive. In fact, a significant fraction of the world's living things, the prokaryotes (the Bacteria and Archaea domains), consist of only a single cell. This implies that even the most basic cells must have a few specialized functions.
The cells of the Eukaryota domain, which include animals, plants, protists and fungi, have almost everything prokaryotic cells have and then some (cell walls are usually an exception, although plant cells share this feature with bacteria and some fungal cells). Eukaryotic cells have a number of exquisitely specialized internal structures, with the endomembrane system, including the membrane sacs known as vesicles, being among the most prominent.
Structure of Cells: Prokaryotic vs. Eukaryotic
Prokaryotes are organisms that have cells lacking internal membrane-bound structures. They do possess the four features common to all cells:
- Deoxyribonucleic acid (DNA): The nucleic acid that serves as the genetic material of life on earth.
- Ribosomes: The sites of protein synthesis.
- Cell membrane: A phospholipid bilayer around the outside of the cell.
- Cytoplasm: The gel-like substance that fills the space inside cells and serves as a place for reactions and other processes to occur.
Prokaryotic cells make only a limited number of proteins and have no compelling need for the endomembrane system of eukaryotes, which is necessary to process proteins manufactured in the cell.
Organelles are elements inside a cell that possess a double plasma membrane like the one surrounding the cell as a whole. The more notorious membrane-bound organelles include:
- Nucleus: This contains the cell's DNA. The nucleus is often excluded from discussions of "organelles" because of its stand-alone importance, but it is surrounded by a nuclear membrane, or nuclear envelope, so it certainly qualifies as one.
- Mitochondria: The sites of the Krebs cycle and the electron transport chain of aerobic respiration.
- Endoplasmic reticulum (ER): A sort of membranous "highway" continuous with the nucleus and extending into the cytoplasm, and sometimes to the cell membrane. Smooth ER does not have ribosomes attached; rough ER does, giving it both its "studded" appearance and its name. Smooth ER synthesizes lipids, while rough ER contains mostly proteins that have not yet been completely processed.
- Golgi bodies: These are like tiny stacks of pancakes. These bud off from the ER and are responsible for tagging and processing proteins and lipids before toting them to their ultimate destinations.
- Vesicles: These supplement the function of the ER and the Golgi bodies by transporting material from the former to the latter.
- Vacuoles: These are really just large vesicles and are described in their own section.
- Lysosomes: These contain digestive enzymes that break down cellular waste products.
- Peroxisomes: These resemble lysosomes but contain specific enzymes that move hydrogen atoms from carbon atoms to oxygen atoms.
- Chloroplasts and thylakoids: These are components of plant cells that take part in photosynthesis. Thylakoids are membrane sacs that contain chlorophyll, which is needed for the photosynthesis that occurs in chloroplasts.
Preeminent among the transport organelles, a vacuole is a membrane-bound, fluid-filled sac that has many functions. Vacuoles are especially important in plants, which have a large, multipurpose central vacuole. This body houses salts, minerals, nutrients, proteins and pigments, assisting in plant growth and giving the plant solidity.
As a plant stores more water in its central vacuoles, it becoming more turgid, or swollen. When the plant is short on water and the vacuoles shrink, the plant wilts.
Other organisms, like animal cells, also have vacuoles. However, animal cell vacuoles are more abundant and much smaller in size compared to the single large central vacuole found in plant cells.
Which Is Not a Function of the Endomembrane System of the Cell?
Based on what you've just learned, which of the following is not among the various jobs of the components of the endomembrane system?
- Moving proteins.
- Processing biomolecules.
- Breaking down waste.
- Contributing to genetic diversity.
- Offering structural support.
The answer is 4. The endomembrane system is critical and diverse, but it has no role in the genetics of the organism.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.