Cell Structure and Function

Processes of Life

All living cells exhibit fundamental processes that define life. These include growth, reproduction, responsiveness, and metabolism. Understanding these processes is essential for studying microbiology.

• Growth: Increase in size and mass of a cell.

• Reproduction: Ability to produce new cells, either sexually or asexually.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Sum of all chemical reactions within a cell, including energy production and biosynthesis.

Types of Cells

Prokaryotic and Eukaryotic Cells: An Overview

Cells are classified into two main types: prokaryotic and eukaryotic. This classification is based on structural differences, particularly the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotes: Lack a nucleus and internal membrane-bound organelles. They are generally smaller (~1.0 µm in diameter) and have a simple structure. Examples include bacteria and archaea.

• Eukaryotes: Possess a nucleus and internal membrane-bound organelles. They are larger (10–100 µm in diameter) and structurally more complex. Examples include algae, protozoa, fungi, animals, and plants.

Typical Prokaryotic Cell Structure

Prokaryotic cells have a simple internal structure, lacking membrane-bound organelles. Key components include the cytoplasmic membrane, cell wall, cytoplasm, ribosomes, nucleoid, glycocalyx, flagellum, and inclusions.

• Cytoplasmic membrane: Phospholipid bilayer controlling substance movement.

• Cell wall: Provides shape and protection.

• Nucleoid: Region containing DNA.

• Glycocalyx: External layer for protection and attachment.

• Flagellum: Structure for motility.

• Ribosomes: Sites of protein synthesis.

• Inclusions: Storage granules.

Typical Eukaryotic Cell Structure

Eukaryotic cells contain numerous membrane-bound organelles, each with specialized functions. These include the nucleus, mitochondria, endoplasmic reticulum, Golgi body, lysosomes, and cytoskeleton.

• Nucleus: Contains genetic material (DNA).

• Mitochondria: Site of ATP production.

• Endoplasmic reticulum (ER): Smooth and rough ER for lipid and protein synthesis.

• Golgi body: Processes and packages proteins.

• Lysosomes: Contain digestive enzymes.

• Cytoskeleton: Provides structural support.

• Cilia and flagella: Structures for movement.

Approximate Size of Various Types of Cells

Microbial cells vary greatly in size, from viruses (0.3 µm) to bacteria (1 µm) and protozoa (up to 14 µm). Eukaryotic cells are generally larger than prokaryotic cells.

• Viruses: Smallest, often less than 1 µm.

• Bacteria: Typically 1–5 µm.

• Protozoa: Can be up to 14 µm or more.

• Chicken egg: Example of a large cell (4.7 cm diameter).

External Structures of Bacterial Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the outside of the cell, composed of polysaccharides, polypeptides, or both. It plays a role in protection and attachment.

• Capsule: Organized, firmly attached, may prevent recognition by host immune system.

• Slime layer: Loosely attached, water soluble, aids in surface attachment.

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. They consist of a filament, hook, and basal body, which anchors the flagellum to the cell wall and membrane.

• Structure: Filament, hook, basal body (with rings and rods).

• Function: Rotation propels bacteria; movement can be toward (runs) or away (tumbles) from stimuli (taxis).

Basic Arrangements of Bacterial Flagella

Bacterial flagella can be arranged in several ways, affecting motility and behavior.

• Monotrichous: Single flagellum.

• Lophotrichous: Cluster of flagella at one end.

• Amphitrichous: Flagella at both ends.

• Peritrichous: Flagella distributed over entire surface.

Axial Filament (Spirochetes)

Spirochetes possess axial filaments (endoflagella) that allow corkscrew movement, enabling them to move through viscous environments.

• Axial filament: Located between cell membrane and outer membrane.

• Function: Rotation causes cell to move in a corkscrew fashion.

Fimbriae and Pili

Fimbriae and pili are proteinaceous extensions used for attachment and genetic exchange.

• Fimbriae: Short, bristle-like, used for adherence and biofilm formation.

• Pili (conjugation pili): Longer, used for DNA transfer between cells (conjugation).

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structural support, shape, and protection from osmotic forces. They are primarily composed of peptidoglycan, which is a polymer of sugars and amino acids.

• Peptidoglycan: Composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Shape: Determines characteristic bacterial shapes (cocci, bacilli, etc.).

• Antibiotic target: Many antibiotics disrupt cell wall synthesis.

Gram-Positive vs. Gram-Negative Cell Walls

Bacterial cell walls are classified as Gram-positive or Gram-negative based on their structure and staining properties.

• Gram-positive: Thick peptidoglycan layer, teichoic acids, stains purple.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink.

Bacterial Cytoplasmic Membranes

Structure

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. It controls the movement of substances into and out of the cell.

• Phospholipid bilayer: Hydrophilic heads and hydrophobic tails.

• Integral and peripheral proteins: Functions include transport, signaling, and structural support.

Function

The cytoplasmic membrane is involved in energy storage, selective permeability, and maintenance of concentration and electrical gradients.

• Energy storage: Some bacteria harvest light energy.

• Selective permeability: Only certain substances can cross.

• Electrical gradient: Maintains membrane potential.

Passive Processes

Passive transport processes do not require energy and include diffusion, facilitated diffusion, and osmosis.

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Movement via channel proteins.

• Osmosis: Movement of water across a semipermeable membrane.

Active Processes

Active transport requires energy (ATP) to move substances against their concentration gradient. Group translocation is a special form where the substance is chemically modified during transport.

• Active transport: Includes uniport, antiport, and symport mechanisms.

• Group translocation: Substance is modified as it enters the cell.

Cytoplasm of Bacteria

Cytosol, Inclusions, and Endospores

The cytoplasm contains the cytosol (liquid portion), inclusions (storage granules), and endospores (defensive structures formed under adverse conditions).

• Cytosol: Site of metabolic reactions.

• Inclusions: Storage of nutrients and chemicals.

• Endospores: Highly resistant structures for survival.

Nonmembranous Organelles

Bacterial cells contain ribosomes (sites of protein synthesis) and a cytoskeleton (provides shape).

• Ribosomes: 70S in prokaryotes.

• Cytoskeleton: Simple, helical structure.

External Structures of Archaea

Glycocalyces, Flagella, Fimbriae, and Hami

Archaea possess unique external structures, including glycocalyces, flagella, fimbriae, and hami. These structures facilitate biofilm formation and attachment.

• Glycocalyces: Aid in biofilm formation and cell adhesion.

• Flagella: Structurally different from bacterial flagella.

• Fimbriae and hami: Attachment structures.

Cell Walls and Cytoplasmic Membranes

Most archaea have cell walls composed of specialized polysaccharides and proteins, but lack peptidoglycan. Their cytoplasmic membranes maintain gradients and control substance movement.

Cytoplasm of Archaea

Archaeal cytoplasm is similar to bacterial cytoplasm but differs in ribosomal proteins, metabolic enzymes, and genetic code.

• Ribosomes: 70S, but with different proteins.

• Genetic code: More similar to eukaryotes.

External Structure of Eukaryotic Cells

Glycocalyces

Eukaryotic glycocalyces are less organized than prokaryotic capsules and function in cell anchoring, surface strengthening, protection, and communication.

Eukaryotic Cell Walls and Cytoplasmic Membranes

Cell walls are present in fungi, algae, plants, and some protozoa, composed of various polysaccharides. All eukaryotic cells have a cytoplasmic membrane with steroid lipids and membrane rafts.

• Cellulose: Plant cell walls.

• Chitin: Fungal cell walls.

• Glucomannan: Fungal cell walls.

• Membrane rafts: Regions of lipids and proteins.

Membranous Organelles

Eukaryotic cells contain numerous organelles, each with specialized functions.

• Nucleus: DNA storage and RNA synthesis.

• Endoplasmic reticulum: Protein and lipid synthesis.

• Golgi body: Processing and packaging of molecules.

• Lysosomes, peroxisomes, vacuoles, vesicles: Storage, digestion, and waste degradation.

• Mitochondria: ATP production.

• Chloroplasts: Photosynthesis in plants and algae.

Nonmembranous Organelles

These include ribosomes (80S in eukaryotes), cytoskeleton (tubulin, actin, intermediate filaments), centrioles, and centrosomes.

• Ribosomes: Larger than prokaryotic ribosomes.

• Cytoskeleton: Extensive network for shape and organelle anchoring.

• Centrioles: Involved in cell division and formation of cilia/flagella.

Endosymbiotic Theory

This theory proposes that eukaryotes originated from a symbiotic relationship between small aerobic prokaryotes and larger anaerobic prokaryotes, leading to the development of mitochondria and chloroplasts.

• Mitochondria: Derived from aerobic prokaryotes.

• Chloroplasts: Derived from photosynthetic prokaryotes.

Additional info: The endosymbiotic theory is supported by the presence of circular DNA and 70S ribosomes in mitochondria and chloroplasts, similar to prokaryotes.

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