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

Three Domains of Life

Overview of Cellular Classification

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, unique membrane lipids, cell wall lacks peptidoglycan.

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

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

Common Components & Characteristics of All Cells

Universal Cellular Features

All cells share several fundamental components that support life processes.

• 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.

Protein Structure

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

• Secondary structure: Local folding into α-helix or β-pleated sheet, stabilized by hydrogen bonds.

• Tertiary structure: 3D folding, determines protein function, stabilized by interactions among side chains.

• 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 essential for structure and function.

• Carbohydrates: Energy storage, structural support.

• Proteins: Enzymes, structural components, transport.

• Lipids: Membrane structure, energy storage, signaling.

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

Metabolic Diversity in Bacteria & Archaea

Energy & Carbon Acquisition

Prokaryotes exhibit diverse metabolic pathways to obtain energy and carbon.

• Chemoautotrophs: Use energy from breaking down molecules, CO2 as carbon source.

• Photoautotrophs: Use light energy, CO2 as carbon source.

• Chemoheterotrophs: Use organic molecules for energy and carbon.

• Photoheterotrophs: Use light for energy, organic molecules for carbon.

Equation:

$\text{ATP} = \text{energy currency of the cell}$

Cell Wall Composition and Gram Staining

Structural Differences in Bacteria

Bacterial cell walls differ in composition, affecting their response to Gram staining.

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

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

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

Chromosomal vs Plasmid DNA

Genetic Elements in Prokaryotes

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

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

Eukaryotic Cell & Organelles

Major Organelles and Their Functions

• Nucleus: Stores DNA, regulates transport via nuclear pores.

• Endomembrane system: Includes RER, Golgi apparatus, lysosomes, plasma membrane; coordinates protein/lipid transport.

• Ribosomes: Protein synthesis (free or attached to RER).

• Lysosomes: Digest macromolecules.

• Mitochondria: ATP production via cellular respiration.

• Chloroplasts: Photosynthesis in plants.

Transport Into/Out of Nucleus

Nuclear Pore Complex and Protein Targeting

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

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

Plant vs Animal Cells

Key Structural Differences

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

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

Endomembrane System Relationships

Dynamic Protein and Membrane Transport

• Proteins synthesized in RER, modified in Golgi, sent to lysosomes or secreted.

• Structures are physically and functionally connected for dynamic transport.

Cytoskeleton & Motor Proteins

Structural and Motility Elements

• Microfilaments (actin filaments): Maintain shape, cell movement, division, resits tensions cytoplasmic streaming

• Intermediate filaments: Structural support, anchor organelles.

• Microtubules: Maintain shape, resist compression, organelle/vesicle transport, chromosome movement.

• Motor proteins: Myosin (actin-based movement), Dynein (microtubule-based movement).

Example: Dynein powers cilia and flagella movement.

Cilia & Flagella

Motility Structures

• Made of microtubules in a 9+2 arrangement.

• Dynein arms "walk" along microtubules, causing bending and movement.

Comparison of Bacteria, Archaea, Eukaryotes

Similarities and Differences

Feature	Bacteria	Archaea	Eukaryotes
DNA	Yes	Yes	Yes
Ribosomes	Yes	Yes	Yes
Organelles	No	No	Yes
Cell Wall	Peptidoglycan	No peptidoglycan, unique lipids	Cellulose (plants), chitin (fungi), none (animals)

Origin of Eukaryotic Cells

Endomembrane System & Endosymbiotic Theory

• Endomembrane system: Invagination of plasma membrane.

• Endosymbiotic theory: Mitochondria and chloroplasts originated from prokaryotes; evidence includes double membranes and own DNA.

Example: Chloroplasts evolved from cyanobacteria.

R-group (Side Chain) Properties

Amino Acid Classification

• Nonpolar: Hydrophobic, avoid water, drive protein folding.

• Polar: Hydrophilic, interact with water.

• Ionic: Charged, participate in salt bridges.

Additional info: R-group properties influence protein structure and function.