Chapter 7: Membrane Structure and Function

Membrane Composition and Structure

The plasma membrane is a dynamic structure composed primarily of lipids, proteins, and carbohydrates. Its composition and properties can vary depending on cell type and environmental conditions.

• Lipid Bilayer: The fundamental structure is a double layer of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Membrane Proteins: Embedded or associated proteins perform various functions, including transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), contributing to cell recognition and signaling.

• Variability: The ratio of lipids to proteins and the types of lipids (e.g., cholesterol, saturated/unsaturated fatty acids) differ among cell types and organelles.

Role of Fatty Acids and Cholesterol

• Saturated Fatty Acids: Pack tightly, making membranes less fluid and more rigid.

• Unsaturated Fatty Acids: Contain double bonds, creating kinks that increase membrane fluidity.

• Cholesterol: Modulates membrane fluidity and stability, preventing extremes of rigidity or fluidity.

Membrane Functions

• Selective permeability barrier

• Compartmentalization of cellular processes

• Signal transduction

• Cell-cell recognition and adhesion

• Anchoring of the cytoskeleton

Membrane Response to Environmental Changes

• Temperature Effects: At low temperatures, membranes become more rigid; at high temperatures, more fluid.

• Adaptive Changes: Cells may alter fatty acid composition (more unsaturated at low temps, more saturated at high temps) to maintain optimal fluidity.

Phase Transition and Transition Temperature

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

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

Membrane Proteins: Types and Functions

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

• Peripheral Proteins: Loosely attached to the membrane surface; involved in signaling or structural support.

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

• Transmembrane Regions: Typically composed of hydrophobic amino acids that interact with the lipid bilayer.

• Removal: Peripheral proteins can be removed by changes in pH or ionic strength; integral proteins require detergents.

Glycosylation of Membrane Proteins

• Definition: Addition of carbohydrate groups to proteins.

• Functions: Aids in protein folding, stability, and cell recognition.

Chapter 8: Transport Across Membranes

Membrane 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.

• Facilitated Diffusion: Movement of larger or polar molecules via specific transport proteins; does not require energy.

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

Properties Affecting Diffusion

• Size: Smaller molecules diffuse more easily.

• Polarity: Nonpolar molecules cross more readily than polar or charged molecules.

• Partition Coefficient: Ratio of solubility in lipid vs. water; higher values indicate easier membrane passage.

Ion Transport and Electrochemical Gradients

• Ion Channels: Allow specific ions to pass through the membrane.

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

• Membrane Potential (Vm): The voltage difference across the membrane.

Kinetics of Transport

• Simple Diffusion: Linear relationship between rate and concentration difference.

• Facilitated Diffusion: Saturates at high substrate concentrations (shows Michaelis-Menten kinetics).

Types of Membrane Transport Proteins

• Carrier Proteins: Bind and transport specific molecules; undergo conformational changes.

• Channel Proteins: Form pores for passive movement of ions or water.

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

Glucose Transport and Modification

• GLUT Transporters: Facilitate glucose entry into cells.

• Modification: Glucose is phosphorylated to glucose-6-phosphate upon entry, trapping it inside the cell.

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 vs. Animal Cells: Plant cells resist bursting due to cell wall; animal cells may lyse in hypotonic solutions.

Pumps and Active Transport

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

• Na+/K+ ATPase: Exchanges 3 Na+ out for 2 K+ in, using ATP.

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

• ABC Transporters: Use ATP to transport various molecules.

• Directionality: Active transport is unidirectional.

• Coupling: Direct (uses ATP) vs. indirect (uses gradient of another molecule).

Chapter 12: The Endomembrane System and Protein Sorting

Endoplasmic Reticulum (ER)

• Structure: Network of membranes; two types: rough (RER) and smooth (SER).

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

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

Lipoproteins

• Definition: Complexes of lipids and proteins for lipid transport in blood.

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

Protein Glycosylation

• Process: Addition of carbohydrate groups to proteins, mainly in ER and Golgi.

• Functions: Protein folding, stability, and cell signaling.

Protein Quality Control

• Chaperones: Assist in proper folding.

• Quality Control Mechanisms: Misfolded proteins are retained in ER or targeted for degradation.

Golgi Apparatus

• Structure: Stacks of flattened membranes (cisternae).

• Functions: Modifies, sorts, and packages proteins and lipids.

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

• Retrograde Transport: Movement from Golgi back to ER.

Protein Targeting and Sorting

• Signal Sequences: Direct proteins to correct destinations.

• Signal Recognition Particle (SRP): Binds signal sequence and directs ribosome to ER membrane.

• Fusion/Chimeric Proteins: Engineered proteins combining domains from different sources.

Protein Secretion and Cytoskeleton Role

• Secretion: Proteins can be secreted constitutively or in a regulated manner.

• Cytoskeleton: Provides tracks for vesicle movement.

Endocytosis

• Specific Endocytosis: Receptor-mediated (e.g., uptake of LDL).

• Nonspecific Endocytosis: Fluid-phase endocytosis (pinocytosis).

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

Chapter 23: Signal Transduction Mechanisms

Types of Cell Signaling

• Endocrine: Signals (hormones) travel through bloodstream to distant cells.

• Paracrine: Signals affect nearby cells.

• Autocrine: Signals affect the same cell that released them.

• Juxtacrine: Direct contact between neighboring cells.

Receptors and Ligands

• Receptors: Proteins that bind signaling molecules (ligands).

• Ligands: Molecules that bind to receptors to initiate a response.

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

• Kd (Dissociation Constant): Lower Kd indicates higher affinity.

• Receptor Turn-Off: Via internalization, degradation, or modification.

• Coreceptors: Assist main receptors in ligand binding or signaling.

Types of Signaling Pathways

• Ligand-Gated Channels: Open in response to ligand binding.

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

• Enzyme-Coupled Receptors: Have intrinsic enzymatic activity (e.g., RTKs, serine-threonine kinases).

• Nuclear Receptors: Bind ligands and act as transcription factors.

Signal Amplification and Integration

• Amplification: One ligand can activate many downstream molecules.

• Integration: Multiple signals can converge on the same pathway.

Second Messengers and Key Molecules

• cAMP: Produced from ATP by adenylyl cyclase; activates protein kinase A.

• IP3 and DAG: Produced by phospholipase C acting on PIP2; IP3 releases Ca2+ from ER, DAG activates protein kinase C.

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

Enzyme-Coupled Receptors and Growth Factors

• Receptor Tyrosine Kinases (RTKs): Phosphorylate tyrosine residues on themselves and other proteins.

• Serine-Threonine Kinases: Phosphorylate serine/threonine residues.

• Growth Factors: Stimulate cell growth, proliferation, and differentiation.

Mutant Receptors and Signaling Studies

• Mutant Receptors: Used to dissect signaling pathways by altering or abolishing function.

• Scaffold Proteins: Organize signaling complexes for efficiency and specificity.

• Crosstalk: Interaction between different signaling pathways.

Endocrine Hormones and Classification

• Endocrine Hormones: Secreted into blood, act on distant targets.

• Classification: Peptide hormones, steroid hormones, amino acid derivatives (see Table 23-4).

Blood Glucose Control

• Insulin: Lowers blood glucose by promoting uptake and storage.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown.