Cell Structure and Physiology in Microbiology ~ Chp 4 - 2

Cell Structure and Physiology in Microbiology

I. Cell Types

A. Prokaryotes (Bacteria)

Prokaryotes are unicellular organisms that lack membrane-bound organelles and a true nucleus. Bacteria are the primary example of prokaryotes and possess unique structural and physiological features.

• No membrane-enclosed organelles: Prokaryotic cells do not have a nucleus, lysosomes, mitochondria, or endoplasmic reticulum.

• Cell wall (CW): Most bacteria have a cell wall composed of peptidoglycan, which provides structural support and protection.

• Ribosomes: Bacterial ribosomes are 70S (Svedberg units), which are smaller than those found in eukaryotes.

B. Eukaryotes (Everything Except Bacteria and Viruses)

Eukaryotic cells are more complex than prokaryotes, containing numerous membrane-bound organelles and a true nucleus. This group includes animals, plants, fungi, protozoa, and algae.

• Many membrane-enclosed organelles: Eukaryotes possess organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Cell wall: Most eukaryotes have a cell wall made of cellulose or chitin. Note: Not all eukaryotes have a cell wall; animal cells, protozoa, and the bacteria genus Mycoplasma do not.

• Ribosomes: Eukaryotic ribosomes are 80S, larger than those in prokaryotes.

II. Physiology of Prokaryotes

Prokaryotes exhibit diverse physiological traits that contribute to their survival and adaptability in various environments.

• Cell shapes and arrangements:

  • Cocci: Spherical bacteria; can be single, in pairs (diplococci), chains (streptococci), or clusters (staphylococci).

  • Bacilli: Rod-shaped bacteria; can be single or in pairs (diplobacilli).

  • Spirillum: Spiral-shaped, rigid bacteria.

  • Spirochetes: Helical, flexible bacteria with axial filaments (e.g., Treponema pallidum, Borrelia burgdorferi).

• Cell surface structures:

  • Glycocalyx: A polysaccharide layer outside the cell wall, which can be a capsule (organized, protective) or slime layer (loose, unorganized).

  • Flagella: Long, whip-like appendages used for motility.

  • Fimbriae: Short, hair-like structures for attachment to surfaces.

  • Pili: Longer than fimbriae; involved in conjugation (transfer of DNA between bacteria) and sometimes motility (twitching).

• Cell wall protection: The cell wall protects against osmotic lysis and acts as a permeability barrier, especially in Gram-negative bacteria.

Table: Bacterial Cell Shapes and Arrangements

Shape	Arrangement	Example
Coccus	Single, diplo-, strepto-, staphylo-	Streptococcus, Staphylococcus
Bacillus	Single, diplo-	Bacillus subtilis
Spirillum	Single	Spirillum volutans
Spirochete	Flexible, axial filaments	Treponema pallidum, Borrelia burgdorferi

Lipopolysaccharide (LPS) in Gram-Negative Bacteria

LPS is a major component of the outer membrane of Gram-negative bacteria. It consists of:

• O polysaccharide: Variable and antigenic, used for typing/identification (e.g., O157 in E. coli O157:H7).

• Lipid A: Toxic component that can cause severe symptoms during systemic infection.

Example: E. coli O157:H7 is identified by its O157 type polysaccharide. Lipid A can cause symptoms such as fever and shock when released into the bloodstream.

Plasmids

Plasmids are autonomously replicating, circular DNA molecules found in bacteria. They often carry non-essential genes, such as those for antibiotic resistance.

III. Control of Water and Osmotic Balance

Cells must regulate water movement to maintain homeostasis and prevent damage from osmotic stress. This is especially important in microbial environments.

• Osmosis: Movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water moves out, causing cell shrinkage.

• Hypotonic: Lower solute concentration outside the cell; water moves in, risking cell lysis.

• Lysis: Destruction of the cell due to excessive water intake.

• Plasmolysis: Shrinking of the cell membrane away from the cell wall due to water loss.

Transport Mechanisms

• Simple diffusion: Passive movement of molecules from high to low concentration.

• Facilitated diffusion: Passive movement via membrane proteins.

• Active transport: Energy-dependent movement against a concentration gradient.

• Group translocation: A type of active transport in bacteria where the transported molecule is chemically modified during passage (e.g., phosphorylation of glucose). This prevents the molecule from diffusing back out of the cell.

Equation for Osmosis:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

IV. Survival in Hypotonic Environments

A. Bacterial Cells

Bacteria with cell walls are protected from lysis in hypotonic environments. The rigid cell wall prevents the cell from bursting as water enters.

• Example: Most bacterial cells (except Mycoplasma) have a cell wall that acts like a baseball bat, providing rigidity and protection.

B. Protozoa

Protozoa lack a rigid cell wall and must actively expel water to survive in hypotonic environments.

• Contractile vacuole: Specialized organelle that pumps excess water out of the cell, requiring cellular energy.

• Food ingestion: Protozoa can ingest large particles, including whole cells, due to the flexibility of their cell surface.

• Comparison: Cells with a cell wall are restricted to absorbing small molecules, while protozoa and other protists can ingest larger particles.

Example: Many protozoa can consume whole bacteria or other cells, compensating for water influx by expelling water and ingesting food.

Additional info: The contractile vacuole is a key adaptation for protozoa living in freshwater environments, where osmotic pressure is high.