A cell is the smallest unit of a living organism, capable of self-reproduction. A bacterium comprises a single cell which can reproduce by itself.
However, the virus does not contain sufficient materials to constitute as a cell. It may replicate only within its host cell.
Although a nerve cell looks entirely different from a red blood cell (Figure 1-A-1), their organizations are essentially the same.
Even plant cells and animal cells share significant similarity in the overall organization.
Figure 1-A-1. Comparison between a nerve cell and a red blood cell.
Classification of cells and organisms
All cells are divided into two categories: prokaryotic cells and eukaryotic cells:
The prokaryotic cell does not have a nucleus.
The eukaryotic cell contains a nucleus.
Eukaryotes are the organisms made up of eukaryotic cells. They include
protista, fungi, animals and plants. Prokaryotes are single-cell organisms including bacteria and archaea.
Archaea lives in extreme environments.
Methanogens live in anaerobic environment such as swamps. They produce methane and cannot tolerate exposure to oxygen.
Extreme halophiles live in very high concentrations of salt (NaCl), e.g., the Dead Sea and the Great Salt Lake.
Extreme thermophiles live in hot, sulfur rich and low pH environment, such as hot springs, geysers and fumaroles in the Yellowstone National Park.
Basic cellular components
All cells share four common components:
Plasma membrane is the cell membrane separating the cell's interior from its surrounding environment. It consists of a phospholipid
bilayer, proteins and carbohydrates. The phospholipid bilayer is also the basic component of other biomembranes.
Cytoplasm includes all components inside the plasma membrane excluding the nucleus. The word cytosol refers to the cytoplasm excluding
all organelles (the membrane-bound structures inside the cell).
DNA (deoxyribonucleic acid) is the genetic material. An eukaryotic cell contains several DNA molecules, located
in the nucleus and mitochondria which are membrane-bound organelles. A prokaryotic cell
contains a single DNA molecule, which has no specific boundary with the cytoplasm.
Ribosomes are the sites of synthesize proteins.