Membranes, Cell Structure, and Cell-Cell Interactions: Study Guide for Biology 161 Test 2

Membranes

Fluid Mosaic Model

The fluid mosaic model describes the structure of biological membranes as a dynamic combination of lipids, proteins, and carbohydrates. The membrane is flexible, with proteins embedded or attached to a bilayer of phospholipids.

• Phospholipid bilayer: Provides the basic structural framework.

• Proteins: Integral and peripheral proteins serve functions such as transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins or lipids, important for cell recognition.

Example: The plasma membrane of animal cells contains cholesterol, which modulates fluidity.

Permeability and Van der Waals Forces

Membrane permeability is influenced by the length and saturation of phospholipid tails and the presence of cholesterol.

• Longer tails: Increase van der Waals forces, decreasing fluidity and permeability.

• Saturated tails: Pack tightly, reducing fluidity.

• Cholesterol: Acts as a fluidity buffer, stabilizing membranes at different temperatures.

Additional info: Unsaturated tails (with double bonds) create kinks, increasing fluidity and permeability.

Tonicity, Osmosis, and Selective Permeability

Tonicity refers to the relative concentration of solutes outside versus inside the cell, affecting water movement by osmosis.

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute outside; water leaves the cell.

• Hypotonic: Lower solute outside; water enters the cell.

• Selective permeability: Membranes allow some substances to cross more easily than others.

Passive and Active Transport

Transport across membranes can be passive (no energy required) or active (requires energy).

• Passive transport: Includes simple diffusion, facilitated diffusion (via channels or carriers).

• Active transport: Uses energy (usually ATP) to move substances against their concentration gradient.

• Channels: Often gated; open or close in response to signals. Usually do not undergo conformational change.

• Transporters: Undergo conformational change to move molecules across the membrane.

• Pumps: E.g., Na+/K+ pump moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Electrochemical gradients drive movement of ions, combining concentration and electrical differences.

Equation:

$\Delta G = RT \ln \left( \frac{[C]_\text{in}}{[C]_\text{out}} \right) + zF\Delta \Psi$

Example: The Na+/K+ pump maintains membrane potential and ion gradients in animal cells.

Cell Structures

Cell Size

Cell size is limited by surface area-to-volume ratio, affecting transport and metabolic efficiency.

• Prokaryotes: Generally smaller, lack membrane-bound organelles.

• Eukaryotes: Larger, contain organelles for compartmentalized functions.

Prokaryotic vs. Eukaryotic Cells

Both cell types share basic features but differ in complexity.

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

• Differences:

  • Prokaryotes: No nucleus, no organelles, circular DNA.

  • Eukaryotes: Nucleus, organelles (mitochondria, ER, Golgi, etc.), linear DNA.

Structure and Function of Eukaryotic Organelles

• Nucleus: Stores genetic material, site of transcription.

• Mitochondria: ATP production via cellular respiration.

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

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

• Lysosomes: Digestive enzymes for breakdown of macromolecules.

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

• Vacuoles: Storage and structural support in plant cells.

Endomembrane System

The endomembrane system coordinates synthesis, modification, and transport of proteins and lipids.

• Components: Nuclear envelope, ER, Golgi, lysosomes, vesicles, plasma membrane.

• Mechanisms: Vesicle transport, fusion, and sorting.

Example: Proteins synthesized in the rough ER are modified in the Golgi and sent to their destination.

Cytoskeleton Components and Functions

• Microfilaments (actin): Cell shape, movement, muscle contraction.

• Intermediate filaments: Structural support.

• Microtubules: Vesicle transport, flagellar motion, mitosis.

Extracellular Structures and Functions

• Cell wall: Provides rigidity in plants, fungi, and some prokaryotes.

• Extracellular matrix (ECM): Animal cells; structural support, cell signaling, adhesion.

Exocytosis, Endocytosis, Phagocytosis

• Exocytosis: Release of substances from cells via vesicle fusion.

• Endocytosis: Uptake of substances into cells via vesicle formation.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Receptor-mediated endocytosis: Specific uptake via receptors.

Cell-Cell Interactions

Extracellular Matrix and Cell Wall

The ECM in animal cells and cell wall in plant cells provide structural support and mediate cell interactions.

• ECM: Composed of proteins (collagen, elastin) and polysaccharides.

• Cell wall: Cellulose in plants; provides rigidity and protection.

Cell-Cell Attachments

• Tight junctions: Seal cells together, prevent leakage.

• Gap junctions: Allow direct communication between animal cells; present in most tissues except skeletal muscle.

• Desmosomes: Provide mechanical strength; connect intermediate filaments.

• Plasmodesmata: Channels between plant cells for communication.

Selective Adhesion and Cadherins

Animal cells use cadherins for selective adhesion, important in tissue formation and embryonic development.

• Cadherins: Calcium-dependent adhesion proteins.

• Role: Recognition, tissue cohesiveness, development.

Cell Signaling

Cell signaling involves reception, processing, response, and deactivation of signals.

• Signal reception: Binding of signaling molecule to receptor.

• Signal processing/transduction: Conversion of signal via cascades or second messengers.

• Signal response: Cellular change (gene expression, metabolism, movement).

• Signal deactivation: Termination of signal.

Signal amplification: One signal can trigger many downstream molecules.

Phosphorylation cascades: Sequential activation of kinases.

Second messengers: Small molecules (e.g., cAMP, Ca2+) that relay signals.

Enzyme-linked receptors: Activate intracellular enzymes upon ligand binding.

G-protein-coupled receptors: Activate G-proteins, which trigger downstream effects.

Higher Order Thinking Questions

• Selective permeability: Membranes are selectively permeable due to lipid composition, protein channels, and transporters.

• Fluidity and permeability: Affected by tail length, saturation, cholesterol, and temperature.

• Transport mechanisms: Simple diffusion (no proteins), facilitated diffusion (channels/carriers), active transport (pumps).

• Osmosis: Water movement; aquaporins facilitate rapid water transport.

• GLUT-1: Carrier protein for glucose; undergoes conformational change.

• Na+/K+ pump: Maintains ion gradients; crucial for cell function.

• Cellular organelles: Each has a specific function contributing to cell survival and specialization.

• Protein targeting: Signal sequences direct proteins to correct compartments.

• Endomembrane system: Sequential modification and sorting of substances.

• Cell surface and attachments: ECM/cell wall and junctions affect cell interactions and tissue integrity.

• Cell signaling steps: Reception, transduction, response, deactivation.

Example Questions

• Which organelle breaks down long fatty acid chains? Answer: Peroxisome

• Signal transduction involving a receptor: Answer: The receptor changes shape due to the binding of the signaling molecule.

Membrane Transport Example

Given concentrations:

• Inside: 1 mM glucose, 100 mM Na+, 100 mM Ca2+, 400 ppm CO2

• Outside: 10 mM glucose, 10 mM Na+, 1000 mM Ca2+, 300 ppm CO2

Molecule	Net Movement	Transport Protein
Glucose	Into cell (higher outside)	GLUT-1 carrier
Na+	Out of cell (higher inside)	Na+ channel, Na+/K+ pump
Ca2+	Out of cell (higher outside)	Ca2+ channel
CO2	Out of cell (higher inside)	Diffusion

Additional info: The direction of net movement is determined by concentration gradients and the presence of specific transport proteins.