Functional Anatomy of Bacteria: Cell Size, Shape, and Plasma Membrane

Functional Anatomy of Bacteria

Lecture Outline

This section introduces the major structural and functional features of bacterial cells, focusing on cell size, shape, and the plasma membrane. Understanding these features is essential for studying bacterial physiology and classification.

• Bacterial cell size and shape

• Cell membrane

• Cell wall

• Gram positive, Gram negative, Mycobacteria

• Capsule

• External structures (flagella, axial filaments, pili, fimbriae)

• Cytoplasm (cytoskeleton, genome, ribosome, inclusions, endospore)

Bacterial Cell Size

Average Size and Measurement

Bacterial cells are typically much smaller than eukaryotic cells, and their size is measured in micrometers (μm). The size of bacteria affects their physiology, nutrient uptake, and ability to evade host defenses.

• Average size: 0.2 – 1.0 μm wide × 2 – 8 μm long

• Cellular organisms <0.15 μm in diameter are unlikely

• Metric units of length: Used to describe cell size, ranging from millimeters (mm) to nanometers (nm)

Metric Unit	Prefix	Equivalent
Millimeter (mm)	10-3 m	0.001 m
Micrometer (μm)	10-6 m	0.000001 m
Nanometer (nm)	10-9 m	0.000000001 m

Microscopy range: Most bacteria are visible under light microscopy (0.2–10 μm).

Bacterial Cell Shape

Classification of Shapes

Bacteria exhibit a variety of shapes, which are important for identification and classification. Most bacteria are monomorphic (single shape), but some are pleomorphic (variable shapes).

• Bacillus: Rod-shaped (e.g., Escherichia coli)

• Coccus: Spherical-shaped (e.g., Streptococcus pneumoniae)

• Spiral: Includes vibrio, spirillum, and spirochete forms

• Other shapes: Star-shaped, rectangular

• Pleomorphic: Bacteria with many shapes

Bacillus Arrangements

• Single: e.g., E. coli

• Diplo: e.g., E. coli

• Chains (streptobacilli): e.g., Bacillus subtilis

• Coccobacilli: e.g., Proteus sp.

Coccus Arrangements

• Diplococci or streptococci: e.g., Streptococcus pneumoniae

• Tetrads: e.g., Micrococcus luteus

• Sarcina: e.g., Sarcina sp.

• Staphylococci: e.g., Staphylococcus aureus

Spiral Arrangements

• Vibrio: e.g., Vibrio cholerae

• Spirillum: e.g., Spirillum sp.

• Spirochete: e.g., Treponema pallidum

Structure of a Bacterial Cell

Overview

Bacterial cells have a complex structure that includes the cell wall, plasma membrane, cytoplasm, and various external appendages. Each structure contributes to the cell's function and survival.

• Capsule

• Cell wall

• Plasma membrane

• Pili and flagella

• Cytoplasm

Structure determines function: The arrangement and composition of cellular structures dictate their roles in protection, transport, motility, and metabolism.

Bacterial Plasma Membrane (Cell Membrane)

Structure of Plasma Membrane

The plasma membrane is a selectively permeable barrier that separates the interior of the cell from its environment. It is primarily composed of a phospholipid bilayer with embedded proteins.

• Phospholipid bilayer: Amphipathic molecules with hydrophilic heads and hydrophobic tails

• Fluid mosaic model: Proteins and lipids move freely within the membrane

• Peripheral proteins: Located on the membrane surface

• Integral and transmembrane proteins: Penetrate the membrane

• Flexible and self-sealing

• Selectively permeable

Membrane viscosity: The membrane is as viscous as olive oil, allowing for dynamic movement of components.

Role of Sterols and Hopanoids

While animal and plant cell membranes contain sterols (e.g., cholesterol) to regulate fluidity and permeability, bacterial membranes typically contain hopanoids, which serve a similar function.

• Hopanoids: Stabilize the membrane and decrease permeability

• Cholesterol: Not present in most bacteria, but found in eukaryotes

Component	Function
Phospholipids	Form bilayer, barrier to diffusion
Proteins	Transport, signaling, enzymatic activity
Hopanoids	Membrane stability
Cholesterol (eukaryotes)	Membrane fluidity and permeability

Functions of Plasma Membrane

The plasma membrane is essential for maintaining cellular integrity and mediating interactions with the environment.

1. Encloses the cytoplasm: Protects cell contents; damage by chemicals can cause leakage

2. Selective permeability: Allows passage of some molecules, but not others

3. Contains enzymes for ATP production: Electron transport chain is located here

4. Photosynthetic pigments: Some bacteria have chromatophores on membrane foldings

5. Anchors flagella, pili, and fimbriae: Essential for motility and attachment

Key Terms

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions

• Fluid mosaic model: Describes the dynamic nature of the membrane

• Selective permeability: Ability to allow certain substances to pass while blocking others

• Hopanoids: Sterol-like molecules in bacterial membranes

Example: Electron Transport Chain in Bacteria

In bacteria, the electron transport chain for ATP production is located in the plasma membrane, unlike eukaryotes where it is found in mitochondria.

Equations

Surface area to volume ratio is important for nutrient uptake:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Additional info: The notes focus on the foundational aspects of bacterial cell anatomy, which are critical for understanding microbial physiology, taxonomy, and the mechanisms of antibiotic action.