Checkpoints for Chapter 4

Chapter Checkpoints
These are some important ideas you are learning in Chapter 4:

The Structure of a Generalized Procaryotic Cell
Bacteria are the oldest form of cellular life. They are also the most widely dispersed, occupying every conceivable microclimate on the planet.

The appendages of bacteria provide motility (flagella), attachment (pili and fimbriae), and, in some bacteria, a means of DNA transfer (sex pili).

Flagella vary in number and arrangement, as well as in the type and rate of motion they produce.

The cell envelope is the outermost covering of bacteria. It consists of three basic layers: (1) the glycocalyx, (2) the cell wall, (3) the cell membrane.

The composition of the procaryote cell wall is used to classify bacteria into four major divisions: Gram Positive, Gram Negative, bacteria with no cell walls, and bacteria with chemically unique cell walls.

Gram-positive bacteria retain the crystal violet and stain purple. Gram-negative bacteria lose the crystal violet and stain red form the safranin counterstain.

Gram-positive bacteria have thick cell walls of peptidoglycan and acidic polysaccharides such as techoic acid, and have a thin peripalsmic space. The cell walls of gram-negative bacteria are thinner but structurally more complex and have a wide periplasmic space.

The bacterial cell membrane is typically composed of phospholipids and proteins, and performs many metabolic functions.

The mesosome is an extension of the cell membrane. It is thought to participate in cell wall synthesis and cell division.

Bacterial Form and Function: Internal Structure
The protoplasm of bacterial cells serves as a solvent for materials (the cell pool) used in all cell functions.

The genetic material of bacteria is DNA. Genes are arranged in a circular chromosome. Additional genes are carried on plasmids.

Bacterial ribosomes are dispersed in the cytoplasm in chains (polysomes) and are embedded in both the cell membrane and the mesosome.

Bacteria store nutrients in their cytoplasm in structures called inclusions. Inclusions vary in structure and the materials that are stored.

A few families of bacteria produce dormant bodies called endospores, which are the hardiest of all life forms, surviving for centuries.

The genera Bacillus and Clostridium are both sporeformers and both contain deadly pathogens.

Bacterial Shapes, Arrangements, and Sizes
Most bacteria have one of three general shapes: coccus (round), bacillus (rod), or spiral, based on the configuration of the cell wall. Two types of spiral cells are spirochetus and spirilla.

Shape and arrangement of cells are key descriptors of bacteria. Arrangements of cells are based on the number of planes in which a given species divides.

Cocci can divide in many planes to form pairs, chains, packets, or clumps. Bacilli divide only in the transverse plane. If they remain attached, they form chains or palisades. Spirilla can form short chains, but spirochetes form only single cells.

Bacteria range in size depending on their shape.

Bacterial Identification and Classification Systems
In order to properly identify a bacterial species, it must be grown in pure culture; that is in isolation from all other forms of life.

Key traits which are used to identify a bacterial species include: 1. morphology, 2. Gram stain or other stain characteristics, 3. presence of specialized structures, 4. macroscopic appearance of colonies, 5. biochemical reactions, and 6. nucleotide composition of both DNA and rRNA.

Bacteria are formally classified by phylogenetic relationships and phenotypic characteristics.

Medical identification of pathogens uses a more informal system of classification based on Gram stain, morphology, biochemical reactions, and metabolic requirements.

A bacterial species is loosely defined a s a collection of bacterial cells which shares an overall similar pattern of traits different from other groups of bacteria.

Variant forms within a species are termed subspecies.

Survey of Bacterial Groups with Unusual Characteristics
The Rickettsiae are a group of bacteria which are intracellular parasites, dependent on their eucaryote host for energy and nutrients. Most are pathogens which alternate between arthropods and mammalian hosts.

The Chlamydiae are also small, intracellular parasites which infect humans, mammals, and birds. They do not require arthropod vectors.

The Mycoplasmas are bacteria which lack cell walls and therefore lack a definite cell shape. Although they are not obligate parasites, many require a eucaryote host and have specialized growth requirements when grown in culture.

Many bacteria are free living, rather than parasites. The photosynthetic bacteria, gliding bacteria, appendaged bacteria, and Archaebacteria encompass many subgroups which colonize specialized habitats, not other living organisms.

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