Prokaryotes have some common characteristics. Compared to eukaryotic cells, they are small. Prokaryotic cells are usually between 1 and 10 microns, while eukaryotic cells are minimally 10 microns and usually larger. It was not until the invention of the electron microscope that the essential difference between prokaryotic cells and eukaryotic cells was appreciated. That is because the wavelengths of light used in the light microscope cannot resolve structures as small as bacteria. The electron microscope uses electrons instead of light and electrons have much smaller wavelengths which can allow us to see the details of cells. It was only when we had the electron microscope that we could clearly see the difference between the two cell types.

Prokaryotic cells do not have membrane bound organelles within them. For example, they do not have a nucleus. They have a single, circular molecule of naked DNA instead of the more complex chromosomes found in eucalypts. They usually divide by a simple process known as fission where the progeny is identical (except for mutations) to the parent. Like the eukaryotic cell, they have a cell membrane (plasma membrane) and cytoplasm with ribosomes (protein manufacturing sites) and enzymes to carry out all the metabolic reactions. Prokaryotes also usually have a special type of cell wall exterior to the cell membrane. This cell wall is composed of a peptidoglycan molecule which is like a hair net that envelopes the cell. Penicillin, the first antibiotic, interferes with the synthesis of new cell walls. A few bacteria, such as the mycoplasma, do not have this cell wall and live within host cells. In this sense the mycoplasma are like viruses which are always obligate intracellular parasites.

The Kingdom Monera contains all the prokaryotic organisms, the Eubacteria and the Archaebacteria. This kingdom contains both autotrophs (self feeders) and heterotrophs (other feeders) and the metabolic diversity among the members of this kingdom is greater than in any other. Bacteria are the best known of the prokaryotes. While some bacteria are harmful, many are beneficial. Bacteria and fungi are the great recyclers. If it were not for them, the earth would be full of dead bodies.

Antibiotics are aimed at interfering with metabolic processes specific to prokaryotes. For example, some interfere specifically with prokaryotic protein synthesis. This is possible because prokaryotic ribosomes are different from eukaryotic ribosomes. Bacteria can become resistant to antibiotic action by acquiring changes in their DNA or by acquiring plasmids (rather like viruses) which carry antibiotic resistance genes. Plasmids are small circular DNA molecules in the cytoplasm of the cell that get replicated along with the cell's DNA. Plasmids can be transferred between bacterial 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 membrane. It appears that the host cell engulfed the aerobic bacteria and the blue-green algae. Also the two organelles have prokaryotic type ribosomes and not eukaryotic ribosomes and they both have their own DNA which is a single, circular, double stranded DNA molecule. Mitochondria can be affected by the antibiotics that interfere with prokaryotic ribosomal protein synthesis.
 

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 which 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 is continuous with the endoplasmic reticulum which is 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 gene which does not code for one of the lysosomal enzymes. The particular enzyme degrades a lipid found in the brain and when it does not function, the brain cells become engorged with lysosomes containing the lipid substrate. This kills the brain cells. The child with TSD is fine when s/he is born but in a few months the child becomes blind and deaf and dies before the age of three.

All eukaryotic cells have a plasma membrane, cytoplasm with a multitude of enzymes, ribosomes, mitochondria, nucleus, and usually lysosomes, ER 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 it can also store ions 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.

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