Cells
Cells are the basic unit of life. There is both unity and diversity in
cells.
Prokaryotic cells do not have a nucleus.
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Figure
1 illustrates Lactobacillus the bacteria that is found in
yogurt. Note the rod shape of these bacteria and their small size (2 mm
x 0.5mm).
The human squamous epithelial cells in contrast are eukaryotic cells. Note
the oval shaped nucleus. These cells which are thin and flat about 150mm
in diameter.
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Figure 2 is Streptococcus pneumonia
(pneumococcus) the bacteria which causes some cases of pneumonia. This
micrograph is taken with dark-field microscopy.
The prokaryotes belong to the Kingdom Monera and
the human epithelial cells belong to the Kingdom
Animalia.
Yeast (Figure 3)belong to the Kingdom
Fungi. Fungi are eukaryotes. Note the nucleus and the clear nuclear
membrane. Fungi have cells wall made of chitin.
Bacteria have cell walls composed of peptidoglycans and plants have cell
walls which contain cellulose and other complex carbohydrates. Yeast like
plants have cells with large central
vacuoles. All cells both eukaryotes and prokaryotes have plasma
membranes that surround the cytoplasm. All cells
have ribosomes
and cytoplasm, and genetic material (DNA).
Mitochondria
are double membrane bound structures found in eukaryotes which are the site of
aerobic ATP synthesis. Note in
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Figure 3 the many mitochondria just inside the plasma membrane. The image of the
yeast cell is taken with a transmission electron microscope (TEM see
figure 7)
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Figure 4 is an amoeba which is a protists
(Kingdom Protista). Protists are single cell
eukaryotes found in aquatic environments. Note that in this image a clear
nucleoli in the center of the nucleus is seen. The nucleolus
is the site of ribonucleic acid synthesis. This cell also shows vacuoles
and has a coccus shaped bacteria (dark spot at top) in a phagosome.
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Figure 5 is a scanning electron micrograph
(SEM figure 8) of red blood cells (RBC) that have been colorized
(red). Also in the corner is a colorized (purple) platelet. RBC transport
gases to and from tissues.
Cells come in different shapes. Shapes reflex the function of the cells.
RBC (Figure 5) are biconcave not spherical.
This is the optimal shape for diffusion of gases. A primary function of
RBC is to transport oxygen to the tissues where oxygen diffuses out of
the RBC and into the tissues. It also picks up carbon dioxide also by diffusion
and transports this waste gas back to the lungs. The function of platelets
is in blood coagulation.
Figure 6 is an image of Elodea a common
aquarium plant (Kingdom Plantae). Note the
rectangle shaped cells. The outer boundary of these cells is the cell wall.
Plant cell walls contain the polysaccharide cellulose. The green oval shaped
structures in the cytoplasm are chloroplasts
the site of photosynthesis.