Cellular Organization

Cells are the smallest structural unit of living organisms, capable of maintaining life and reproducing.  Viruses are not cells because they cannot maintain life and reproduce by themselves.

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.  Schematic drawing of a nerve cell and a red blood cell.

Classification of cells and organisms

All cells are divided into two types: 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 include archaebacteria and eubacteria.  They are single-cell organisms.

More recently, "archaebacteria" have been placed in a category outside "bacteria", because they are quite different from the ordinary bacteria.  According to the new classification, prokaryotes are divided into archaea and bacteria,  where "archaea" is equivalent to "archaebacteria", and "bacteria" is the same as  "eubacteria".  

Archaea live in extreme environments.  They may be organized into three groups:

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 contain cytoplasm, plasma membrane, and DNA.

Cytoplasm is the viscous contents of a cell, including proteins, ribosomes, metabolites and ions.  Ribosomes are the sites of protein synthesis.

Plasma membrane is the cell membrane surrounding cytoplasm.  It consists of phospholipid bilayer, associated proteins and carbohydrates.  Phospholipid bilayer is also the basic constituent of other biomembranes.

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.