Cell Structure and Function

Processes of Life

All living organisms share several fundamental processes that define life. These processes are essential for the survival, growth, and reproduction of cells.

• Growth: An increase in size of an organism or cell.

• Reproduction: An increase in number, either by cell division or production of offspring.

• Responsiveness: The ability to sense and respond to environmental stimuli.

• Metabolism: Controlled chemical reactions that provide energy and build cellular components.

Example: Bacteria grow by binary fission, respond to nutrients or toxins, and metabolize sugars for energy.

Characteristics of Life in Microbes vs. Viruses

Microbial life (bacteria, archaea, eukaryotes) and viruses differ in their characteristics of life. The table below summarizes these differences:

Example: Coronavirus replication depends on the host cell's machinery, illustrating why viruses are not considered fully alive by all definitions.

Prokaryotic and Eukaryotic Cells: An Overview

Prokaryotes

Prokaryotes include Bacteria and Archaea. They are characterized by their simple structure and lack of a nucleus.

• Typically 0.2–2.0 μm in diameter

• No membrane-bound nucleus; DNA is in a nucleoid region

• Transcription and translation occur simultaneously

• Lack membrane-bound organelles

Example: Escherichia coli is a common prokaryotic bacterium.

Eukaryotes

Eukaryotes include algae, protozoa, fungi, animals, and plants. They have a more complex cell structure.

• Typically 10–100 μm in diameter

• Have a true nucleus surrounded by a nuclear envelope

• Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum)

• Complex internal organization

Example: Paramecium is a eukaryotic protozoan.

Relative Size of Microorganisms

• Viruses: ~0.03–0.3 μm (30–300 nm)

• Bacteria: ~1 μm

• Eukaryotic cells: 10–100 μm

• Chicken egg (for scale): ~47,000 μm

Additional info: The size difference between viruses, bacteria, and eukaryotic cells is significant and affects their biological properties and methods of study.

External Structures of Bacterial Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells, composed of polysaccharides, polypeptides, or both.

• Capsule: Firmly attached, composed of repeating organic units, helps evade host immune system.

• Slime layer: Loosely attached, water-soluble, aids in attachment to surfaces (biofilms).

Example: Streptococcus pneumoniae forms a capsule that increases its virulence.

Flagella

Flagella are long, whip-like structures responsible for bacterial motility.

• Composed of filament, hook, and basal body

• Arrangements: monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over)

• Movement: rotation propels bacteria; direction can be clockwise or counterclockwise

• Taxis: Movement in response to stimuli (e.g., chemotaxis, phototaxis)

Example: Escherichia coli uses peritrichous flagella for movement toward nutrients.

Fimbriae and Pili

• Fimbriae: Short, bristlelike projections for attachment to surfaces and other cells; important in biofilm formation.

• Pili (conjugation pili): Longer than fimbriae, shorter than flagella; used for DNA transfer between cells (conjugation).

Example: Neisseria gonorrhoeae uses fimbriae to attach to host tissues.

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structure, shape, and protection from osmotic pressure. They are primarily composed of peptidoglycan, a mesh-like polymer of sugars and amino acids.

• Peptidoglycan: Consists of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars, cross-linked by short peptide chains.

• Functions: Maintains cell shape, prevents lysis, and can be a target for antibiotics.

Gram-Positive vs. Gram-Negative Cell Walls

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Cells Without Cell Walls

• Some bacteria (e.g., Mycoplasma) lack cell walls but have sterols in their membranes for stability.

• These can be mistaken for viruses due to their small size.

Bacterial Cytoplasmic Membranes

Structure

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model.

• Integral and peripheral proteins serve as channels, carriers, or receptors.

• Functions: energy storage, selective permeability, maintaining gradients.

Transport Processes

• Passive Transport: Does not require energy. Includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (usually ATP). Includes uniport, antiport, and coupled transport mechanisms.

Osmosis: Diffusion of water across a semipermeable membrane.

Effects of Solutions:

• Isotonic: No net water movement; cell remains stable.

• Hypertonic: Water leaves the cell; cell shrinks (plasmolysis).

• Hypotonic: Water enters the cell; cell may burst (lysis).

Cytoplasm of Bacteria

• Mostly water, contains nucleoid (DNA region), ribosomes (70S), and inclusions (storage granules).

• Endospores: Highly resistant structures formed by some bacteria (e.g., Bacillus, Clostridium) for survival in harsh conditions.

• Cytoskeleton: Protein fibers for cell shape, division, and movement.

Archaeal Cell Structure

External Structures

• Glycocalyces: Involved in biofilm formation and adherence.

• Flagella: Structurally different from bacterial flagella; powered by ATP.

• Fimbriae and Hami: Hami are unique, hook-like structures for attachment.

Cell Walls and Membranes

• Cell walls lack peptidoglycan; composed of specialized polysaccharides and proteins.

• Cytoplasmic membranes maintain gradients and control transport.

Similarities to Bacteria: 70S ribosomes, circular DNA, cytoskeleton.

Differences: Unique ribosomal proteins, metabolic enzymes, and genetic code more similar to eukaryotes.

Eukaryotic Cell Structure

External Structures

• Glycocalyces: Less organized than prokaryotic capsules; aid in cell recognition and protection.

Cell Walls and Cytoplasmic Membranes

• Cell walls (in plants, algae, fungi): Composed of cellulose, chitin, or other polysaccharides.

• Cytoplasmic membrane: Phospholipid bilayer with proteins and sterols; controls movement and maintains fluidity.

Transport Processes Unique to Eukaryotes

• Endocytosis: Uptake of materials via membrane invagination (includes phagocytosis and pinocytosis).

• Exocytosis: Release of materials from the cell via vesicle fusion with the membrane.

Flagella and Cilia

• Flagella: Structurally distinct from prokaryotic flagella; composed of microtubules, move in a whip-like fashion.

• Cilia: Shorter and more numerous than flagella; coordinated beating moves substances or the cell itself.

Ribosomes and Cytoskeleton

• Ribosomes: Larger (80S) than prokaryotic (70S); composed of 60S and 40S subunits.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for shape, movement, and organelle positioning.

Organelles

• Nucleus: Contains DNA, surrounded by nuclear envelope with pores.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; Smooth ER synthesizes lipids.

• Golgi Body: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes, Peroxisomes, Vacuoles, Vesicles: Involved in storage, digestion, and detoxification.

• Mitochondria: Site of ATP production; contains its own DNA and 70S ribosomes.

• Chloroplasts: Found in photosynthetic eukaryotes; site of photosynthesis, contains DNA and 70S ribosomes.

Endosymbiotic Theory

This theory proposes that eukaryotic organelles such as mitochondria and chloroplasts originated from symbiotic relationships between ancestral prokaryotic cells.

• Evidence: Double membranes, own DNA, 70S ribosomes, and similarities to certain bacteria.

Additional info: The endosymbiotic theory is widely accepted but not without debate in the scientific community.

Comparison of Archaea, Bacteria, and Eukaryotic Cells

The following table summarizes key differences and similarities among the three domains of life: