Chapter 7: Membrane Structure and Function

Membrane Structure and Composition

Biological membranes are dynamic structures composed primarily of lipids, proteins, and carbohydrates. Their composition and properties can vary depending on cell type and environmental conditions.

• Lipid Bilayer: The fundamental structure is a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane Composition: Includes phospholipids, cholesterol, glycolipids, and proteins. The ratio of these components can differ between cell types and organelles.

• Variability: Membrane composition adapts to environmental changes (e.g., temperature shifts) to maintain fluidity and function.

Fatty Acids and Cholesterol in Membranes

• Saturated Fatty Acids: Pack tightly, decreasing membrane fluidity.

• Unsaturated Fatty Acids: Have kinks that prevent tight packing, increasing fluidity.

• Cholesterol: Modulates fluidity by preventing extremes—stabilizes membranes at high temperatures and prevents rigidity at low temperatures.

Membrane Functions

• Selective permeability

• Compartmentalization

• Signal transduction

• Cell recognition and adhesion

• Energy transduction

Membrane Response to Environmental Changes

• Cells adjust fatty acid composition to maintain optimal fluidity as temperatures change.

• At low temperatures, more unsaturated fatty acids are incorporated; at high temperatures, more saturated fatty acids and cholesterol are present.

Phase Transition and Transition Temperature

• Phase Transition: The shift from a gel-like (ordered) to a fluid (disordered) state.

• Transition Temperature (Tm): The temperature at which this transition occurs; influenced by lipid composition.

Membrane Proteins

• Integral Proteins: Span the membrane; often function as transporters or receptors.

• Peripheral Proteins: Loosely attached to the membrane surface; can be removed by mild treatments (e.g., changes in pH or ionic strength).

• Lipid-Anchored Proteins: Covalently attached to lipids within the bilayer.

• Transmembrane: Refers to proteins that cross the entire membrane; typically contain hydrophobic amino acids in the membrane-spanning regions.

Glycosylation of Membrane Proteins

• Glycosylation: Addition of carbohydrate groups to proteins; affects folding, stability, and cell recognition.

• Glycosylated proteins play roles in immune response and cell signaling.

Chapter 8: Membrane Transport and Energetics

Transport Mechanisms

Cells use various mechanisms to move substances across membranes, maintaining homeostasis and enabling communication.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient without energy input.

• Facilitated Diffusion: Movement of molecules via specific transport proteins (channels or carriers); no energy required.

• Active Transport: Movement against a concentration gradient, requiring energy (usually ATP).

Comparison of Transport Mechanisms

Properties Affecting Diffusion

• Size, polarity, and charge of molecules affect their ability to diffuse across membranes.

• Partition coefficient: Ratio of a substance's solubility in lipid vs. water; higher values favor membrane diffusion.

Ion Transport and Electrochemical Gradients

• Ions traverse membranes via channels and transporters due to their charge and polarity.

• Electrochemical Gradient: Combination of concentration and electrical gradients across the membrane.

• Membrane Potential (Vm): The voltage difference across a membrane, crucial for nerve and muscle function.

Membrane Transport Proteins

• Channel Proteins: Form pores for passive movement of ions or water (e.g., ion channels, aquaporins).

• Carrier Proteins: Bind and transport specific molecules; exhibit saturation kinetics.

• Porins: Large, less selective channels found in outer membranes of bacteria and mitochondria.

Osmosis and Tonicity

• Osmosis: Diffusion of water across a semipermeable membrane.

• Hypertonic Solution: Higher solute concentration outside; cell shrinks.

• Hypotonic Solution: Lower solute concentration outside; cell swells.

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

• Plant cells become turgid in hypotonic solutions; animal cells may lyse.

Pumps and Active Transport

• Pumps: Proteins that use energy to move substances against gradients (e.g., Na+/K+ ATPase).

• Na+/K+ ATPase: Moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• ΔG (Gibbs Free Energy): Determines spontaneity of transport; calculated as:

• Where R is the gas constant, T is temperature, z is ion charge, F is Faraday's constant, and ΔV is membrane potential.

Types of Active Transport

• Primary Active Transport: Direct use of ATP (e.g., Na+/K+ ATPase, ABC transporters).

• Secondary Active Transport: Indirect use of energy via coupling to another gradient (e.g., glucose/Na+ symporter).

• Directionality: Refers to the one-way movement of substances by transporters.

• Coupling: Linking the movement of one molecule to another; can be direct (ATP hydrolysis) or indirect (using ion gradients).

Chapter 12: The Endomembrane System

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes membrane and secretory proteins.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Both types play roles in cell metabolism and the production of lipoproteins.

Lipoproteins

• Complexes of lipids and proteins; transport lipids in the bloodstream.

• Uptake involves receptor-mediated endocytosis (e.g., LDL uptake).

Protein Glycosylation and Quality Control

• Glycosylation: Addition of carbohydrate groups to proteins, mainly in the ER and Golgi apparatus.

• Ensures proper folding, stability, and targeting of proteins.

• Quality-control mechanisms (e.g., chaperones) ensure only properly folded proteins proceed through the secretory pathway.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for delivery to various destinations.

• Anterograde Transport: Movement from ER to Golgi to plasma membrane.

• Retrograde Transport: Movement from Golgi back to ER.

Protein Targeting and Signal Recognition

• Proteins contain signal sequences that direct them to correct cellular locations.

• Signal Recognition Particle (SRP): Binds to signal sequences and ribosomes, targeting them to the ER membrane.

Fusion (Chimeric) Proteins

• Proteins engineered by fusing sequences from different sources; used to study protein localization and function.

Protein Secretion and the Cytoskeleton

• Proteins are secreted via exocytosis; cytoskeleton (microtubules, actin) guides vesicle movement.

Endocytosis

• Specific Endocytosis: Receptor-mediated; selective uptake of molecules.

• Nonspecific Endocytosis: Fluid-phase; bulk uptake of extracellular fluid.

• Desensitization: Decreased cellular response due to receptor internalization or modification.

Chapter 23: Cell Communication and Signaling

Types of Cell Signaling

• Endocrine: Signals (hormones) travel long distances via the bloodstream.

• Paracrine: Signals act on nearby cells.

• Autocrine: Cells respond to signals they produce themselves.

• Juxtacrine: Direct contact between signaling and target cells.

Receptors and Ligands

• Receptors: Proteins that bind signaling molecules (ligands) and initiate cellular responses.

• Ligands: Molecules that bind to receptors (e.g., hormones, neurotransmitters).

• Receptor Affinity: Strength of ligand binding; measured by dissociation constant (Kd).

• Lower Kd indicates higher affinity.

• Receptors can be turned off by ligand dissociation, internalization, or modification.

• Coreceptors: Assist primary receptors in ligand binding and signaling.

Types of Signaling Pathways

• Ligand-Gated Channels: Open in response to ligand binding (e.g., neurotransmitter receptors).

• G Protein-Coupled Receptors (GPCRs): Activate intracellular G proteins to relay signals.

• Enzyme-Coupled Receptors: Possess intrinsic enzymatic activity (e.g., receptor tyrosine kinases, RTKs; serine-threonine kinases).

• Nuclear Receptors: Bind ligands and regulate gene expression directly.

Signal Amplification and Integration

• Signal transduction pathways amplify signals via cascades (e.g., kinase cascades).

• Integration occurs when multiple pathways converge or interact (crosstalk).

Second Messengers

• cAMP (cyclic AMP): Produced from ATP by adenylyl cyclase; activates protein kinase A (PKA).

• IP3 (Inositol trisphosphate): Releases Ca2+ from intracellular stores.

• PIP2 (Phosphatidylinositol 4,5-bisphosphate): Cleaved by phospholipase C to generate IP3 and DAG.

• DAG (Diacylglycerol): Activates protein kinase C (PKC).

• Calcium: Acts as a ubiquitous second messenger in many pathways.

Growth Factors and Mutant Receptors

• Growth Factors: Proteins that stimulate cell proliferation and differentiation.

• Mutant Receptors: Used experimentally to dissect signaling pathways.

Scaffolds and Crosstalk

• Scaffold Proteins: Organize signaling components to enhance specificity and efficiency.

• Crosstalk: Interaction between different signaling pathways, allowing integration of multiple signals.

Endocrine Hormones

• Classified by chemical structure (e.g., peptides, steroids, amino acid derivatives).

• Regulate physiological processes such as blood glucose control (e.g., insulin, glucagon).

Example: Insulin lowers blood glucose by promoting uptake in muscle and fat cells; glucagon raises blood glucose by stimulating glycogen breakdown in the liver.