Biology 102 - General Biology

Endosymbiosis: The origin of eukaryotic cells

Eukaryotic cells have evolved from prokaryotic cells by a process called endosymbiosis. Specifically, the mitochondria found in all eukaryotic cells and the chloroplasts found in photosynthetic eukaryotic cells are very similar to aerobic bacteria and blue-green algae (cyanobacteria) respectively. Both have a double membrane where the chemical composition of the outer membrane is more like the eukaryotic plasma membrane and the inner one is more like a prokaryotic cell membrane. It appears that the host cell engulfed the aerobic bacteria and/or the blue-green algae. Also, the two organelles have prokaryotic type ribosomes (smaller) and not eukaryotic ribosomes (larger) and both mitochondria and chloroplasts have their own DNA separate from the nuclear DNA of the cell. Their DNA is a single, circular, double stranded DNA molecule, similar to the prokaryotic cell chromosome. Another clue to the relationship between prokaryotic cells and our mitochondria is that mitochondria can be affected by the antibiotics aimed at bacteria in our body because these antibiotics interfere with prokaryotic ribosomal protein synthesis.

Eukaryotes

The Kingdoms Protista, Fungi, Plants and Animals are the other five kingdoms and they are composed solely of eukaryotic organisms. Eukaryotic cells have a much more complex architecture that was described only after the advent of the electron microscope. The transmission electron microscope looks at thin sections of tissues and cells while the scanning electron microscope looks at the surfaces of cells and molecules. The transmission electron microscope (TEM) is analogous to the compound light microscope and the scanning electron microscope (SEM) is analogous to the dissecting microscope you have used in lab.

The largest structure in the eukaryotic cell is the nucleus. It has a double membrane which has pores in it for molecules to come in and out. The outer membrane of the nucleus is continuous with the endoplasmic reticulum, a network of membranes in the cytoplasm. If the endoplasmic reticulum has ribosomes on it, it is referred to as rough endoplasmic reticulum (RER) if it does not, it is called smooth endoplasmic reticulum (SER). SER is where lipids are synthesized. RER is the site of protein synthesis for proteins which are for export out of the cell or for incorporation into the lysosomes. The proteins made in the RER is processed in the Golgi body and packaged there. Lysosomes are like garbage disposals and contain a wide variety of enzymes which degrade old, used molecules and organelles. This recycling is essential to keep the cell in good working order. Lysosomes are single membrane bound organelles.

The consequences of not getting rid of used molecules is seen in a group of human genetic disorders known as lysosomal storage diseases. One such disease is Tay-Sachs Disease (TSD) which has a higher incidence among Ashkenazi Jews, French Canadians, and Creoles. It is an autosomal recessive disease in which the parents are unaffected carriers of a mutant gene that does not produce its gene product, one of the lysosomal enzymes. This gene normally codes for an enzyme that degrades a lipid found in the brain. When it does not function, the lysosomes of the brain cells become engorged with the lipid substrate and the cell dies. The child with TSD is fine when s/he is born but within a few months the child becomes blind and deaf and dies before the age of three. In the Los Angeles Times there was a story of a family who had three boys aged 3, 4, and 6 years old with Sanfilippo syndrome. This syndrome is also due to the lack of a lysosomal enzyme which breaks down polysaccharides. Both parents carry the recessive gene for the syndrome and have a risk of 25% of having an affected child. The probability of them having three affected children was 1/4 X 1/4 X 1/4 = 1/64. They had unusually bad luck. The youngest child was treated with an experimental stem-cell transplant using umbilical cord blood in an effort to replace the enzyme. The older two were not treated because their damage was irreversible.

Like prokaryotic cells, all eukaryotic cells have a plasma membrane, cytoplasm with a multitude of enzymes, and ribosomes. However, eukaryotic ribosomes are larger than prokaryotic ribosomes. The eukaryotic cells also have mitochondria, a nucleus, and, usually, lysosomes, endoplasmic reticulum and Golgi. Free ribosomes in the cytoplasm make all the proteins for the nucleus, cytoplasm, peroxisomes, and some of the mitochondrial and chloroplast proteins. Plant cells have a central vacuole which contains water but the vacuole can also store ions, pigments, and other molecules. Plant cells also have a cell wall often composed of cellulose but it can contain other molecules. Plant cells that carry out photosynthesis also contain chloroplasts which are double membrane bound organelles.