Organization of Cells

- Problems - Comments and Questions - Back to Course Syllabus

Cells
The cell is basic unit of living organisms. Cells have a plasma membrane, composed of a lipid bilayer , which acts as a barrier to the outside, controlling the flow of substances in and out of the cell. Everything enclosed by the plasma membrane is the cytoplasm , with the exception of the DNA containing nucleus or the nucleoid . Single cells have dimensions which range from less than 1 um in diameter to huge sizes as the yolks of bird eggs. However, most cells are very small. Cells have to exchange nutrients continuously. Very large cells would have the problem of moving metabolites (or wastes) and nutrients through a large volume of cytoplasm. Since molecules move only randomly at very slow diffusion rates, it would take to much time to distribute substances efficiently.

Prokaryotic Cells
There are two major cell types, prokaryotic and eukaryotic cells.
Prokaryotes (or bacteria) are very small, their cells are between 1 and 5 um in diameter. Bacteria appear in a variety of different shapes: Spherical (coccus), rodlike (bacillus), curved rod (vibrios) and spiral (spirilli).

In addition there are flat, triangular, starlike and square shapes, which are however, less frequent. Bacterial cells are surrounded by a semirigid cell wall , which consists of a polysaccharide matrix, crosslinked by polypeptide chains. Based on their cell wall composition bacteria can be divided into two groups: Gram positive and Gram negative.

Gram positive bacteria
Gram positive organisms react with the so called Gram stain to appear purple, whereas Gram negative bacteria don't react with the stain. The cell wall of gram positive bacteria consists of several layer of peptidoglycan a polymer made up of N-acetylglucosamine chains crosslinked by short peptide chain which contain D-amino acids. In addition, the cell wall of gram positive bacteria contains teichoic acid , which consists of glycerol or ribitol and phosphate. A schematic drawing of the cell wall of gram positive organism is shown to the left.
On the external side of the cell wall, prokaryotes have the glycocalyx a gelatinous polymer, consisting of polysaccharide chains. In addition there are extensions protruding from the cell wall such as flagella , fimbriae and pili. Underneath the cell wall gram positive bacteria have a plasma membrane, which controls the flow of metabolites and nutrients in and out of the cell.

Gram negative bacteria
Gram negative bacteria have a much more complex cell wall. On the cytoplasmic side the plasma membrane is connected to a single layer of peptidoglycan , which in turn is connected to a lipoprotein unit. The lipoprotein layer connects to an outer membrane which contains a lipopolysaccharide that is made up of a Lipid A portion (two sugar units connected to a hydroxy fatty acid) and an O-antigen subunit.
The Lipid A portion is also referred as an endotoxin since it is toxic in a host's bloodstream or gastrointestinal tract. Symptoms are shock and fever. The O-antigen portion consists of sugar molecules and it is the primary site of gram negative bacteria, recognized by antibodies . The variability of the O-antigen chain can cause problems with the immune response. For example the gram negative E. coli is a harmless bacterium present in the human large intestine. It can become pathogenic in other parts of the body, especially because of mutant O-antigen chains, which are not recognized by previously developed antibodies.

The inside of prokaryotic cells (i.e. of both, gram positive and gram negative bacteria) consists of one single compartment called cytoplasm. It contains the chromosomal DNA and ribosomes , which are the site of protein synthesis. Some bacteria contain also plasmids, which are small circular fragments of DNA that replicate independently. Drug resistance of bacteria is generally associated with plasmids.
These are the only macro-structures in the bacterial cell. They are suspended in an aqueous medium, called cytosol, which contains also all enzymes and nutrients necessary for metabolism and reproduction.

Eukaryotic Cells

Eukaryotic cells are much more complex than prokaryotic cells. Animals cells range from 10 to 30 um in diameter and plant cells about 10um. Eukaryotic cell may be spherical, flat or extremely elongated (nerve cells).

Eukaryotic cells have two distinct compartments, the nucleus and the cytoplasm . The nucleus which contains the chromosomal DNA is surrounded by two membranes, called the nuclear envelope. The cytoplasm consists of a number of membrane bound structures collectively called organelles. They carry out very specialized functions. In addition to organelles, ribosomes are suspended in the cytoplasm or are attached to membranous sacs, called rough endoplasmic reticulum or RER. The RER is the site of the synthesis of enzymes destined for the degradation of certain nutrients, metabolites or spent organelles, protein destined for export from the cell (secretion), or proteins be to incorporated into the plasma membrane. Before the proteins reach their final destination, they are structurally modified in a neighboring membrane bound organelle, called Golgi complex. From there certain hydrolytic enzymes are packaged into lysosomes , which are capable of breaking down biological molecules. Mitochondrion , an organelle surrounded by two membranes is the cell's powerhouse, i.e. it is continuously producing the high energy molecule ATP. In animal cells peroxisomes carry out special oxidation reactions in which hydrogen peroxide is produced and neutralized. In plants the conversion of lipids into sugar is carried out in Glyoxisomes. Chloroplasts in green plants are the site of photosynthesis, producing glucose and glyceraldyde. Animal cells and plant cells differ from each other in several components and assembly, check your textbook for details. In addition the eukaryotic cell contains a network of filaments and microtubules known as cytoskeleton, which provides mechanical support and helps in the guided transport of small vesicles and organelles.

Problems: (cell organization, plasma membrane)

1. Explain how bacterial cells are divided into two groups
2. What are the principal differences between eukaryotic and prokaryotic cells
3. What is the function of the mitochondria?
4. What is the function of microfilaments ?