Cell Biology: Key Concepts and Processes

Topic 7: Cells and Organelles

Cell Size, Limitations, and Types

Cells are the fundamental units of life, and their size and structure impose certain limitations on their function. Eukaryotic cells have evolved mechanisms to overcome these limitations.

• Cell Size Limitations: The surface area-to-volume ratio limits cell size, affecting nutrient uptake and waste removal.

• Prokaryotic vs. Eukaryotic Cells:

  • Prokaryotes lack a nucleus and most intracellular organelles.

  • Eukaryotes possess a nucleus and various organelles.

  • Presence or absence of a cell wall distinguishes many cell types.

Major Cell Structures and Their Functions

Each cell structure plays a specific role in maintaining cellular function and organization.

• Plasma Membrane: Controls entry and exit of substances; maintains homeostasis.

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Nucleus: Stores genetic material and coordinates cellular activities.

• Endomembrane System (ER, Golgi, Lysosomes): Involved in synthesis, modification, and transport of proteins and lipids.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Vacuoles: Store nutrients and waste products; prominent in plant cells.

• Ribosomes: Synthesize proteins.

• Mitochondria: Generate ATP through cellular respiration.

Topic 8: Membranes: Structure, Function, and Chemistry

Membrane Composition and Function

Cell membranes are dynamic structures composed of lipids, proteins, and carbohydrates, crucial for compartmentalization and communication.

• Major Functions: Barrier, transport, signaling, energy transduction, and cell recognition.

• Phospholipids, Glycolipids, Sterols: Phospholipids are most abundant; glycolipids and sterols (e.g., cholesterol) modulate fluidity and stability.

• Asymmetry: Lipid and protein distribution differs between the two leaflets of the bilayer.

Lipid Movement and Membrane Proteins

• Lipid Movement:

  • Rotation

  • Lateral diffusion

  • Transverse diffusion (flip-flop, energetically unfavorable; requires flippases)

• Temperature and Fatty Acid Structure: Affect membrane fluidity; cholesterol stabilizes membranes.

• Membrane Proteins: Classified by structure and function; many are glycosylated.

Topic 9: Transport Across Membranes: Active and Passive

Types of Membrane Transport

Cells transport molecules across membranes using passive and active mechanisms, each with distinct energy requirements and specificity.

• Passive Transport: Includes simple diffusion, facilitated diffusion, and osmosis; does not require energy.

• Active Transport: Requires energy (often ATP) to move substances against their concentration gradient.

• Factors Affecting Transport: Solute size, charge, and polarity.

• Osmosis: Movement of water across membranes; cells respond to hypotonic and hypertonic environments.

• Transport Proteins:

  • Uniport: Moves one type of molecule.

  • Symport: Moves two molecules in the same direction.

  • Antiport: Moves two molecules in opposite directions.

Topic 10: Active Transport (cont.) and the Endomembrane System

Endomembrane System Components and Functions

The endomembrane system coordinates the synthesis, modification, and transport of cellular materials.

• Components:

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

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

  • Endosomes: Involved in sorting and trafficking of internalized materials.

  • Lysosomes: Digest cellular waste and macromolecules.

• Material Trafficking: Movement of vesicles between ER, Golgi, and other organelles.

• Flippases: Enzymes that facilitate transverse diffusion of lipids.

• Golgi Network: Distinction between cis (entry) and trans (exit) faces; retrograde and anterograde transport.

• Protein Processing: Glycosylation and other modifications occur during trafficking.

Topic 11: Protein Sorting

Mechanisms of Protein Targeting and Sorting

Proteins are directed to specific cellular locations by sorting signals and trafficking pathways.

• Sorting Signals: Short amino acid sequences ("tags") that direct proteins to their destinations.

• Endomembrane System Role: Recognizes sorting signals and facilitates proper localization.

• Protein Trafficking: Involves retention, retrieval, and vesicular transport.

• Endocytosis and Exocytosis: General processes for importing and exporting materials.

• Vesicle Coats:

  • Clathrin: Endocytosis

  • COPI: Retrograde transport (Golgi to ER)

  • COPII: Anterograde transport (ER to Golgi)

• Vesicle Fusion: SNARE proteins mediate fusion of vesicles with target membranes.

Topic 12: Electrical Communication: Membrane Potentials and Synaptic Transmission

Membrane Potentials and Action Potentials

Cells use electrical signals to communicate, especially in nervous tissue. Membrane potentials arise from differences in ion concentrations across the plasma membrane.

• Resting Membrane Potential: Established by ion gradients and selective permeability.

• Action Potential: Rapid change in membrane voltage due to ion channel activity.

• Propagation: Action potentials travel along axons, triggering neurotransmitter release at synapses.

• Synaptic Transmission: Neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic cell.

Key Equations

• Nernst Equation: Calculates equilibrium potential for a particular ion: $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$

• Action Potential: Involves sequential opening and closing of voltage-gated Na+ and K+ channels.

Topic 13: Cellular Communication: Chemical Signaling

Signal Transduction Pathways

Cells communicate using chemical signals that bind to specific receptors, initiating intracellular signaling cascades.

• Receptor Types:

  • Ligand-gated ion channels

  • G-protein coupled receptors (GPCRs)

  • Receptor kinases

• Signal Transduction: Binding of ligand activates receptor, triggering downstream effects (e.g., second messengers like cAMP).

• Integration: Cells integrate multiple signals for appropriate responses.

• Agonists and Antagonists: Agonists activate receptors; antagonists block activation.

Key Equations

• cAMP Production: GPCR activation leads to increased cAMP: $\text{ATP} \xrightarrow{\text{Adenylyl cyclase}} \text{cAMP} + \text{PP}_i$

Example Table: Membrane Transport Comparison

Transport Type	Energy Requirement	Direction	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump