Cell Biology Study Guide: Membranes, Transport, Endomembrane System, and Signal Transduction

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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).

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT
Active Transport	Yes	Against gradient	Na+/K+ ATPase

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).

Hormone Type	Example	Solubility	Receptor Location
Peptide	Insulin	Water-soluble	Cell surface
Steroid	Cortisol	Lipid-soluble	Intracellular
Amino Acid Derivative	Epinephrine	Water-soluble	Cell surface

Blood Glucose Control

Insulin: Lowers blood glucose by promoting uptake and storage.
Glucagon: Raises blood glucose by stimulating glycogen breakdown.