Cell Structure and Function: Study Guide (Chapter 3)

Cell Structure and Function

Four Characteristics Shared by Living Things

All living organisms share several fundamental characteristics that distinguish them from non-living matter.

• Growth: Increase in size and/or number of cells.

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

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Collection of controlled chemical reactions that take place within the organism.

Example: Bacteria reproduce by binary fission, a form of asexual reproduction.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells:

  • Lack a membrane-bound nucleus

  • Generally smaller and simpler

  • DNA is located in a nucleoid region

  • Examples: Bacteria and Archaea

• Eukaryotic Cells:

  • Have a true nucleus surrounded by a nuclear envelope

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

  • Generally larger and more complex

  • Examples: Fungi, Protozoa, Plants, Animals

Similarities: Both have cell membranes, ribosomes, cytoplasm, and genetic material.

Domains and Categories of Cells

All life is classified into three domains:

• Bacteria: Prokaryotic

• Archaea: Prokaryotic

• Eukarya: Eukaryotic (includes protists, fungi, plants, and animals)

Glycocalyces in Prokaryotic Cells

Prokaryotic cells may produce a glycocalyx, a gelatinous, sticky substance surrounding the cell.

• Capsule: Organized, firmly attached to the cell wall; protects against phagocytosis.

• Slime Layer: Loosely attached, water-soluble; aids in adherence to surfaces.

Function: Protection, adherence, and formation of biofilms.

Biofilms

A biofilm is a complex community of microorganisms attached to a surface and embedded in a self-produced extracellular matrix.

• Formation: Initial attachment, microcolony formation, maturation, and dispersion.

• Purpose: Protection from environmental threats (e.g., antibiotics), enhanced nutrient acquisition, and survival in harsh conditions.

Example: Dental plaque is a biofilm formed by bacteria on teeth.

Flagella, Pili, and Fimbriae

These are external structures found in prokaryotes, each with distinct functions.

• Flagella: Long, whip-like structures for motility.

• Pili: Longer than fimbriae, usually one or a few per cell; involved in conjugation (DNA transfer).

• Fimbriae: Short, numerous, used for attachment to surfaces and other cells.

Gram-Positive vs. Gram-Negative Cell Walls

The cell wall structure differs significantly between Gram-positive and Gram-negative bacteria.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide, LPS)
Periplasmic Space	Small or absent	Present
Stain Color	Purple	Pink/Red

Function: Provides shape, protection, and influences susceptibility to antibiotics.

Gram-Negative Cell Wall: Structure and Function

• Structure: Thin peptidoglycan layer, outer membrane with LPS, periplasmic space.

• Function: Barrier to certain antibiotics, contains porins for nutrient passage, LPS can trigger immune responses.

• Metabolic Activity: Many enzymes and metabolic processes occur in the periplasmic space.

Clinical Relevance: Outer membrane can impede drug entry, making Gram-negative infections harder to treat.

Acid-Fast Organisms

Acid-fast bacteria (e.g., Mycobacterium) have unique cell walls rich in mycolic acids, making them resistant to decolorization by acids during staining.

• Characteristic: Waxy, lipid-rich cell wall; stains retain carbol fuchsin dye.

• Example: Mycobacterium tuberculosis

Cell (Plasma) Membrane: Structure and Function

The plasma membrane is a phospholipid bilayer with embedded proteins.

• Structure: Fluid mosaic model; hydrophilic heads and hydrophobic tails.

• Function: Selective barrier, transport, energy generation, communication.

Transport Across the Cell Membrane

Cells use various mechanisms to move substances across the membrane.

• Passive Transport: No energy required; includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (usually ATP); moves substances against concentration gradients.

Example: When E. coli is in a hypertonic solution, it uses active transport to import K+ ions, preventing water loss and plasmolysis.

Active vs. Passive Transport

Feature	Active Transport	Passive Transport
Energy Requirement	Yes (ATP or proton motive force)	No
Direction	Against gradient	Down gradient
Examples	Na+/K+ pump, group translocation	Osmosis, simple diffusion

Transport Unique to Eukaryotes

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

• Exocytosis: Release of materials by fusion of vesicles with the plasma membrane.

Note: Prokaryotes do not perform endocytosis or exocytosis.

Membrane-Bound Organelles in Eukaryotic Cells

• Nucleus: Contains genetic material (DNA).

• Mitochondria: Site of ATP production.

• Endoplasmic Reticulum (ER): Protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digestion of macromolecules.

• Chloroplasts: Photosynthesis (in plants and algae).

Ribosomes in Prokaryotic and Eukaryotic Cells

Ribosomes are the sites of protein synthesis and are found in both cell types.

• Prokaryotic Ribosomes: 70S (composed of 50S and 30S subunits)

• Eukaryotic Ribosomes: 80S (composed of 60S and 40S subunits)

Reason: All cells require protein synthesis for survival.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Concept: Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

Evidence for Endosymbiotic Theory

• Both organelles have their own circular DNA, similar to bacteria.

• They have double membranes.

• They reproduce independently by binary fission.

• Ribosomes resemble those of prokaryotes (70S).

Significance: These features suggest a prokaryotic origin.

Endospore Formation (Sporulation)

Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Steps:

  1. DNA replication

  2. Septum formation

  3. Forespore development

  4. Cortex formation

  5. Spore coat synthesis

  6. Maturation

  7. Release of endospore

Purpose: Survival under harsh conditions (heat, desiccation, chemicals, radiation).