COMPOSITION OF EUKARYOTE CELLS
Eukaryotic organisms include algae, Protozoa, fungi, plants, and animals.
1. FLAGELLA AND CILIA
These are used for cellular locomotion or for moving substances along the surface of the cell. Flagella are few and long in relation to the size of the cell. Cilia are more numerous and shorter. Algae of the genus Euglena use flagella for locomotion, whereas Protozoa use cilia for locomotion. Both flagella and cilia are anchored to the plasma membrane by a basal body, which consists of nine pairs of microtubules arranged in a ring, plus another two microtubules in the center of the ring, an arrangement called a 9 + 2 array. Microtubules are made up of a protein called tubulin. Prokaryotic flagella rotate, but the eukaryotic flagellum moves in a wavelike manner. Ciliated cells of the human respiratory system move mucous and debris along the surface of the cells in the bronchial tubes and trachea to clear the lungs.
2. CELL WALL and GLYCOCALYX
Most eukaryotic cells have cell walls, although they are much simple or than those of prokaryotic cells. Algae and fungi have cellulose in their cell walls, as do all plants. Eukaryotic cells that lack a cell wall and have direct contact with the environment may have a coating outside the plasma membrane. In animals, the plasma membrane is covered by the glycocalyx, which is a sticky carbohydrate. The glycolcalyx strengthens the cell surface, helps attach cells together, and contributes to cell-cell recognition. Eukaryotic cells do not contain peptidoglycan. This is medically significant because antibiotics such as penicillins and cephalosporins only act against peptidoglycan and therefore do not affect human eukaryotic cells.
3. PLASMA MEMBRANE
The plasma membrane of eukaryotic and prokaryotic cells is very similar in function and structure. There are, however, differences in the types of proteins found in the membranes. Eukaryotic membranes also contain carbohydrates, which serve in cell to cell recognition. Bacteria take advantage of these sites and attach there. Eukaryotic plasma membranes also contain sterols, which are complex lipids not found in prokayriotic plasma membranes (with the exception of Mycoplasma). Sterols help the membranes resist lysis from increased osmotic pressure.
Substances can cross the plasma membrane by diffusion or active transport, or a mechanism called endocytosis. This occurs when a segment of the plasma membrane surrounds a particle, encloses it, and brings it into the cell. This process is called phagocytosis when the plasma membrane projects a pseudopod (false foot), engulfs the particle and brings it the cell. Phagocytosis is used by white blood cells to destroy bacteria and foreign substances.
4. CYTOPLASM
The cytoplasm of eukaryotic cells is the watery substance inside the plasma membrane and outside the nucleus. Cytosol is the fluid portion of the cytoplasm. Unlike prokaryotic cytoplasm, eukaryotic cytoplasm has a complex internal structure, consisting of very small rods and cylinders called microfiaments and microtubules. Together, they form the cytoskeleton. This provides support and shape, and also assists in transporting substances through the cell. They can also move the entire cell, as in phagocytosis. The movement of cytoplasm from one part of the cell to another to distribute nutrients is called cytoplasmic streaming.
Another difference between prokaryotic and eukaryotic cytoplasm is that many of the important enzymes found in the cytoplasmic fluid of prokaryotes is contained within organelles of eukaryotes. Some organelles are bound by a membrane and other organelles are non-membrane bound.
5. MEMBRANE-BOUND ORGANELLES
a. NUCLEUS: this is usually the largest structure in the cell, and contains almost all of the cell’s hereditary information (DNA). Some DNA is also found in mitochondria and in the chloroplasts of photosynthetic organisms. The nucleus is surrounded by a double membrane called the nuclear envelope. Tiny channels in the membrane called the nuclear pores allow the nucleus to communicate with the cytoplasm. Within the nucleus are one or more spherical bodies called nucleoli, which are condensed regions of chromosomes where ribosomal RNA is being synthesized. The nucleus also contains some proteins called histones, which wrap around the DNA and organize it.
When the cell is not reproducing, the DNA appears as a thread-like mass called chromatin. During nuclear division, the chromatin becomes shorter and thicker, and they are now called chromosomes. Prokaryotic chromosomes do not undergo this process, do not have histones, and are not enclosed in a nuclear envelope. Eukaryotic cells divide by mitosis and meiosis; these processes do not occur in prokaryotic cells.
b. ER: within the cytoplasm of eukaryotic cells is the endoplasmic reticulum, an extensive network of channels which are continuous with the nuclear envelope. Most eukaryotic cells contain two distinct forms of ER that differ and structure and function. The rough ER is studded with ribosomes, the sites of protein synthesis. Proteins synthesized by ribosomes that are attached to rough ER enter the channels within the ER to be processed and sorted. These proteins may be incorporated into the cell’s organelles or plasma membrane. Thus, rough ER are protein factories.
Smooth ER extends from the rough ER to form a separate network. Smooth ER does not have any ribosomes. However smooth ER contains unique enzymes; it synthesizes phospholipids, fats, and steroids such as estrogen and testosterone. In liver cells, the enzymes of smooth ER detoxify drugs.
c. GOLGI COMPLEX: most of the proteins synthesized by ribosomes from rough ER are transported to other regions of the cell. The first step in the transport pathway is through an organelle called the Golgi complex. Proteins synthesized by ribosomes on the rough ER are surrounded by a portion of the ER membrane, which eventually buds from the membrane surface to form a transport vesicle. This transport vesicle fuses with the Golgi complex, releasing the proteins into the channels of the Golgi complex. Within the Golgi complex, the proteins are modified. For instance, enzymes in the Golgi complex can modify proteins to form glycoproteins, glycolipids, and lipoproteins. Some of the processed proteins leave the Golgi complex in secretory vesicles, which detach from the Golgi membrane and deliver the proteins to the plasma membrane, where they are discharged from the cell. Some of the processed proteins leave the Golgi complex in vesicles that are called storage vesicles. The major storage vesicle is a lysosome.
d. LYSOSOMES: lysosomes are formed from the Golgi complexes and look like membrane-enclosed spheres. Unlike mitochondria, lysosomes have only one membrane and lack internal structure. They contain as many as 40 different kinds of powerful digestive enzymes capable of breaking down various molecules. They can also digest bacteria that enter the cell. Human white blood cells, which use phagocytosis to ingest bacteria, contain large numbers of lysosomes.
e. VACUOLES: a vacuole is a space or cavity in the cytoplasm that is enclosed by a membrane. In plant cells, vacuoles may occupy 5 to 90% of the cell volume. Vacuoles are made by the Golgi complex and have several functions. Some vacuoles serve as temporary storage organelles for proteins, sugars, etc. Some plant cells also store wastes and poisons to prevent toxicity to the cytoplasm. Vacuoles may also take up water, enabling plant cells to increase in size and also provide rigidity to leaves and stems.
f. MITOCHONDRIA: these are rod-shaped organelles which appear throughout the cytoplasm of most eukaryotic cells. There can be as many as 2000 mitochondria and one cell. Mitochondria have a double membrane; the outer membrane is smooth but the inner membrane is arranged in a series of folds called cristae. The center of the mitochondrion is a semi-fluid substance called the matrix. The convolutions of the cristae provide an enormous surface area on which chemical reactions can occur. Some proteins that function in cellular respiration, including the enzyme that makes ATP, are located on the cristae, and many of the metabolic steps involved in cellular respiration occur in the matrix. Mitochondria are called the powerhouses of the cell because of their central role in ATP production. Mitochondria contain their own ribosomes and DNA and are able to replicate themselves and make their own proteins.
It is theorized in the endosymbiotic theory that eukaryotic cells evolved from bacteria millions of years ago. According to this theory, larger bacterial cells lost their cell walls and engulfed smaller bacterial cells. Then the larger bacterial cell evolved into a eukaryotic cell as its plasma membrane folded over the DNA and created a nucleus. As the nucleus was formed, it also engulfed aerobic bacteria (require air). The host cell supplied nutrients to the ingested bacteria, which in turn supplied energy to the cell. These aerobic bacteria are now known as mitochondria. There is current research on mitochondrial DNA (mtDNA), which determines the distant ancestry of each human being. The nuclear DNA is from both parents, but all the mitochondrial DNA is inherited by the mother.
Unlike nuclear DNA, whose genes are rearranged in the process of recombination, there is usually no change in mDNA from parent to offspring. Because of this, mtDNA is a powerful tool for tracking matrilineage (lineage of the mother), and can track the female ancestry of many species back hundreds of generations. This process was used to prove that all domestic dogs are descendant from wolves.
The concept of the Mitochondrial Eve is based on the same type of analysis, attempting to discover the origin of humanity by tracking the lineage back in time. Mitochondrial Eve is the name given by researchers to the woman who is the matrilineal most recent common ancestor (MRCA) for all living humans. Passed down from mothers to offspring for over a hundred thousand years, her mitochondrial DNA (mtDNA) is now found in all living humans: every mtDNA in every living person is derived from hers. She is believed to have lived about 140,000 years ago in what is now Ethiopia, Kenya or Tanzania. The time she lived is calculated based on the molecular clock technique of correlating elapsed time with observed genetic drift.
g. CHLOROPLASTS: only algae and green plants contain this unique organelle. It contains the pigment chlorophyll plus enzymes required for photosynthesis. Like mitochondria, chloroplasts contain ribosomes, DNA, and enzymes involved in protein synthesis. They are capable of multiplying on their own within the cell. Again, chloroplasts and mitochondria are strikingly similar to bacteria in their method of multiplication.
h. PEROXISOMES: these organelles are similar to lysosomes but they are smaller. They contain one or more enzymes that can oxidize various substances. For example, amino acids are oxidized in peroxisomes as part of normal metabolism. In addition, enzymes in peroxisomes oxidize toxic substances such as alcohol. The end product of the oxidation reaction is hydrogen peroxide, which is a very toxic compound. However peroxisomes also contain the enzyme catalase, which decomposes hydrogen peroxide, so it is safe within the cell. These peroxisomes can also be used to digest bacteria that have invaded the cell.
6. NON-MEMBRANE-BOUND ORGANELLES
a. RIBOSOMES: attached to the outer surface of rough ER are ribosomes; they are also found floating free in the cytoplasm. They are the sites of protein synthesis in the cell. They are larger (80S instead of 70S) and denser than the ribosomes of prokaryotic cells. The free ribosomes synthesize proteins which are used inside the cell, and they do not have a membrane. The membrane-bound ribosomes are the ones attached to the rough ER. These ribosomes synthesize proteins destined for insertion in the plasma membrane or for export from the cell. Ribosomes within the mitochondria synthesize special mitochondrial proteins.
a. CENTROSOME: the centrosome is located near the nucleus. It consists of proteins fibers and two centrioles, which are small fibers. This area organizes the spindles that appear during mitosis to help the duplicated chromosomes move towards opposite ends of the cell. Therefore, the centrosome plays a critical role in cell division. Each of the two centrioles in the centrosome is arranged so that the long axis of one centriole is at a right angle to the long axis of the other.
EUKARYOTIC / PROKARYOTICOne circular chromosome, not membrane-bound / Paired chromosomes, membrane-bound
No histones / Histones present
No organelles / Organelles present: Golgi complex, ER, mitochondria, chloroplasts
Peptidoglycan cell walls / Polysaccharide cell walls
Reproduce by binary fission / Reproduce by mitosis
No true nucleus; no nuclear membrane / True nucleus; nuclear membrane; also has nucleoli
Glycocalyx present as capsule or slime layer / Present in some cells that lack a cell wall
Plasma membrane has no carbohydrates and lack sterols / Plasma membrane has carbohydrates and sterols
No cytoskeleton / Has a cytoskeleton
Ribosomes are small (70S) / Ribosomes are large (80S)