Cell Structure, Function, and Diversity: General Biology Study Notes

Three Domains of Life

Overview of Cellular Domains

The three domains of life represent the major evolutionary lineages of cellular organisms. Each domain is defined by unique structural and genetic characteristics.

• Bacteria: Prokaryotic, unicellular, no nucleus, cell wall contains peptidoglycan.

• Archaea: Prokaryotic, unicellular, no nucleus, cell wall lacks peptidoglycan, unique membrane lipids.

• Eukarya: Eukaryotic, nucleus present, membrane-bound organelles.

Example: Escherichia coli (Bacteria), Halobacterium (Archaea), Homo sapiens (Eukarya)

Common Components & Characteristics of All Cells

Universal Cellular Structures

Despite diversity, all cells share several fundamental components necessary for life.

• DNA: Genetic material for inheritance and function.

• Ribosomes: Sites of protein synthesis.

• Plasma Membrane: Selective barrier for transport and communication.

• Proteins: Structural and functional molecules.

Proteins:

• Primary structure: Linear sequence of amino acids (determined by DNA sequence).

• Secondary structure: Local folding (e.g., α-helix, β-pleated sheet) via hydrogen bonds.

• Tertiary structure: 3D folding stabilized by interactions among R-groups.

• Quaternary structure: Multiple polypeptides assembled together.

Example: Hemoglobin (quaternary structure, oxygen transport)

Macromolecules in Cells

Types and Functions

Cells contain four major types of macromolecules, each with distinct roles.

• Carbohydrates: Energy storage, structural support.

• Proteins: Enzymes, structural components, signaling.

• Lipids: Membrane structure, energy storage, signaling.

• Nucleic acids: Genetic information (DNA, RNA).

Cell Theory & Evolution

Foundational Biological Principles

Cell theory and evolutionary theory are central to understanding biology.

• Cell Theory: All living things are composed of cells; cells are the basic unit of life; all cells arise from pre-existing cells.

• Evolution: Change in characteristics of a population over time; driven by natural selection.

Energy & Metabolism

Cellular Energy Requirements

All cells require energy and carbon sources to sustain life and build macromolecules. ATP is the universal energy currency.

• ATP: Adenosine triphosphate, provides energy for cellular processes.

• Energy sources:

  • Chemoautotrophs: Use energy from breaking down molecules; use CO2 as carbon source (e.g., bacteria, archaea).

  • Photoautotrophs: Use energy from light; use CO2 as carbon source (e.g., plants, photosynthetic bacteria).

  • Chemoheterotrophs: Use organic molecules as energy and carbon source (e.g., animals, fungi, bacteria).

  • Photoheterotrophs: Use light for energy, organic molecules for carbon (e.g., some bacteria).

Cell Wall Composition & Gram Staining

Bacterial Cell Walls and Classification

Bacterial cell walls are classified based on their structure and response to Gram staining.

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

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

Application: Gram staining helps identify bacterial species and informs antibiotic treatment.

Cell Surface Structures

Motility and Attachment

Cells possess specialized structures for movement and attachment.

• Flagella: Long, whip-like structures for motility; made of rigid helical proteins.

• Fimbriae & Pili: Shorter, used for attachment; pili can mediate DNA transfer (conjugation).

Example: Fimbriae help bacteria adhere to surfaces, such as teeth.

Archaea: Unique Features

Adaptations and Structure

Archaea are adapted to extreme environments and differ from bacteria and eukaryotes in cell wall and membrane composition.

• Extreme conditions: High temperature, salinity, acidity.

• Unique lipids: Ether-linked membrane lipids.

• Genome organization: Circular chromosomes, plasmids.

Cell Size and Volume

Determinants and Implications

Cell size is determined by surface area-to-volume ratio, which affects nutrient uptake and waste removal.

• Smaller cells have higher surface area-to-volume ratio, facilitating efficient exchange.

• Larger cells may require specialized structures to maintain function.

Structural Features of Eukaryotic Cells

Compartmentalization and Organelles

Eukaryotic cells contain membrane-bound organelles that compartmentalize functions.

• Compartmentalization: Internal membranes (endomembrane system) allow efficient use of space and specialization.

• Organelles: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts (in plants).

• ATP generation: Mitochondria (all eukaryotes), chloroplasts (plants and algae).

Chromosomal vs Plasmid DNA

Genetic Material in Prokaryotes

Prokaryotes possess both chromosomal and plasmid DNA, each serving different functions.

• Chromosomal DNA: Single, circular, contains essential genes.

• Plasmid DNA: Small, circular, non-essential genes, transferable between cells.

Endomembrane System

Structure and Function

The endomembrane system is a network of membranes within eukaryotic cells that coordinates protein and lipid transport.

• Components: Rough and smooth endoplasmic reticulum (RER/SER), Golgi apparatus, lysosomes, plasma membrane.

• Function: Protein synthesis, modification, sorting, and transport.

Lysosomes & Vacuoles

Digestion and Recycling

Lysosomes and vacuoles are involved in digestion and recycling of cellular materials.

• Lysosomes: Contain hydrolytic enzymes, digest macromolecules, originate from Golgi.

• Vacuoles: Large membrane-bound structures in plants and fungi; storage, digestion, recycling.

Three pathways to lysosome:

• Endocytosis: Cell engulfs molecules from outside.

• Phagocytosis: Cell engulfs large particles or cells.

• Autophagy: Cell digests its own damaged organelles.

Origin of Eukaryotic Cells

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Mitochondria: Originated from aerobic bacteria.

• Chloroplasts: Originated from cyanobacteria.

• Evidence: Double membranes, own DNA, ribosomes similar to prokaryotes.

Transport Into/Out of Nucleus

Nuclear Pore Complex and Protein Targeting

Transport of molecules into and out of the nucleus is regulated by nuclear pore complexes (NPCs) and signal sequences.

• Nuclear Localization Signal (NLS): Directs proteins into the nucleus.

• Protein targeting to RER: Signal sequences direct proteins for synthesis and folding.

Cytoskeleton & Motor Proteins

Cell Shape, Movement, and Division

The cytoskeleton provides structural support, enables movement, and organizes cell division.

• Microfilaments (actin filaments): Maintain cell shape, enable movement, division.

• Intermediate filaments: Provide mechanical strength, support organelles.

• Microtubules: Resist compression, facilitate vesicle transport, chromosome movement.

• Motor proteins: Myosin (actin-based movement), dynein and kinesin (microtubule-based movement).

Functions of actin filaments:

• Resist tension (pulling forces).

• Enable muscle contraction, cell movement.

• Facilitate cytokinesis (cell division).

• Support cytoplasmic streaming (movement of organelles).

Comparison of Plant and Animal Cells

Structural Differences

Plant and animal cells differ in several key structural features.

• Plant cells: Cell wall, chloroplasts, large central vacuole.

• Animal cells: No cell wall, no chloroplasts, smaller vacuoles.

Comparison of Bacteria, Archaea, and Eukaryotes

Similarities and Differences

All three domains share some cellular features but differ in cell wall composition, membrane lipids, and organelle presence.

• Similarities: DNA, ribosomes, plasma membrane, cytoplasm.

• Differences:

  • Eukaryotes: Nucleus, organelles, larger size.

  • Bacteria vs Archaea: Cell wall composition, membrane lipids.

Protein Structure and R-Groups

Types and Properties of Amino Acid Side Chains

Amino acid side chains (R-groups) determine protein folding and function.

• Nonpolar R-groups: Hydrophobic, avoid water, drive folding.

• Polar R-groups: Hydrophilic, interact with water.

• Ionic R-groups: Carry full charge, participate in ionic bonds.

Key Table: Comparison of Cell Types

Major Features of Bacteria, Archaea, and Eukaryotes

Feature	Bacteria	Archaea	Eukaryotes
Nucleus	No	No	Yes
Cell Wall	Peptidoglycan	No peptidoglycan, unique lipids	Cellulose (plants), chitin (fungi), none (animals)
Membrane-bound Organelles	No	No	Yes
Chromosome	Circular	Circular	Linear
Ribosomes	70S	70S	80S (cytoplasm), 70S (organelles)

Key Table: Cytoskeletal Elements

Functions and Properties

Element	Main Function	Example
Microfilaments (Actin)	Resist tension, cell movement, division	Muscle contraction, cytokinesis
Intermediate Filaments	Mechanical strength, support	Keratin (skin), nuclear lamins
Microtubules	Resist compression, vesicle/chromosome transport	Mitotic spindle, cilia, flagella

Key Equations

Surface Area to Volume Ratio

The surface area to volume ratio is critical for cell function:

• For a sphere: $\text{Surface Area} = 4\pi r^2$ $\text{Volume} = \frac{4}{3}\pi r^3$ $\text{SA:V Ratio} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$

Application: As cell size increases, SA:V ratio decreases, limiting exchange efficiency.

Summary

These notes cover the fundamental aspects of cell structure, function, and diversity, providing a comprehensive overview for General Biology students.