Cell Biology Exam 2 Study Guide: Membranes, Transport, Endomembrane System, and Cytoskeleton

Chapter 7: Membranes – Structure, Function, and Chemistry

Functions and Composition of Cellular Membranes

Cellular membranes are essential for compartmentalization, protection, and regulation of cellular processes. While all membranes share a phospholipid bilayer structure, their protein and lipid composition varies according to organelle function.

• Phospholipid Bilayer: The fundamental structure of all cellular membranes, composed of hydrophilic heads and hydrophobic fatty acid tails.

• Organelle Membrane Specialization:

  • Mitochondrial membrane: Contains enzymes for energy production.

  • Lysosomal membrane: Maintains an acidic environment for degradation.

  • Nucleus and mitochondria: Possess double membranes, unlike the single plasma membrane.

• Membrane Asymmetry: Lipids and proteins are distributed unequally between the two leaflets, affecting function and signaling.

Phospholipids and Membrane Structure

Phospholipids are amphipathic molecules forming the bilayer, with distinct regions contributing to membrane properties.

• Structure: Composed of a glycerol backbone, phosphate group (hydrophilic head), and two fatty acid tails (hydrophobic).

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

• Glycolipids: Lipids with carbohydrate groups, important for cell recognition.

• Bonds: Ester bonds connect fatty acid tails to the glycerol backbone.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds, straight tails, less fluid.

  • Unsaturated: One or more double bonds, kinked tails, more fluid.

Membrane Fluidity and Factors Affecting It

Membrane fluidity is crucial for function, influenced by several factors:

• Temperature: Higher temperature increases fluidity.

• Pressure: Can affect packing of lipids.

• Type of Fatty Acid Tail: Unsaturated tails increase fluidity; longer tails decrease fluidity.

• Protein Concentration: Integral and peripheral proteins affect membrane dynamics.

Membrane Proteins and Junctions

Proteins embedded in membranes serve various functions, including transport, signaling, and structural support.

• Integral Proteins: Span the membrane (single pass, multipass).

• Peripheral Proteins: Attach to membrane surface.

• Lipid-Anchored Proteins: Covalently attached to lipids.

• Glycosylation: Addition of sugars to proteins, important for recognition and stability.

• Cell Junctions: Structures connecting cells, including tight junctions, gap junctions, and desmosomes.

Chapter 8: Transport Across Membranes – Overcoming the Permeability Barrier

Types of Membrane Transport

Transport across membranes is essential for nutrient uptake, waste removal, and signaling. It can be passive or active.

• Passive Transport: No energy required; includes simple diffusion, osmosis, and facilitated diffusion.

• Active Transport: Requires energy (usually ATP); includes pumps and transporters.

• Endocytosis/Exocytosis: Bulk transport mechanisms for large molecules.

Osmosis and Tonicity

Osmosis is the movement of water across membranes, influenced by solute concentration.

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

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

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

• Turgor Pressure: Pressure exerted by water inside plant cells, maintaining rigidity.

Facilitated Diffusion and Channels

Facilitated diffusion uses proteins to move substances down their concentration gradient.

• Carrier Proteins: Bind and transport specific molecules.

• Channel Proteins: Form pores for ions and small molecules.

• Types of Channels: Voltage-gated, ligand-gated, and mechanically-gated.

• Gates: Control channel opening/closing.

Active Transport and ATPases

Active transport moves substances against their gradient using energy.

• Na/K Pump: Maintains ion gradients; pumps 3 Na+ out, 2 K+ in per ATP.

• Na/Glucose Transporter: Uses Na+ gradient to import glucose.

• ATPases: Four types, located in various membranes, responsible for ion transport.

Transport Mechanisms Comparison Table

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT
Active Transport	Yes (ATP)	Against gradient	Na/K pump
Endocytosis	Yes	Bulk import	Phagocytosis
Exocytosis	Yes	Bulk export	Neurotransmitter release

Endomembrane System

Major Regions and Functions

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

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, plasma membrane.

• Smooth ER: Lipid synthesis, detoxification, carbohydrate metabolism.

• Rough ER: Protein synthesis and folding; ribosomes attached.

• Golgi Apparatus: Protein modification, sorting, and packaging.

• Lysosomes: Degradation of macromolecules; acidic environment.

Protein Targeting and Vesicular Transport

Proteins are directed to specific organelles via signal sequences and vesicle trafficking.

• ER Retention Signal: Sequence that retains proteins in the ER.

• Coat Proteins: Clathrin, COPI, COPII; mediate vesicle formation and directionality.

• Vesicle-Mediated Transport: Involves budding, movement, and fusion of vesicles.

• Constitutive vs. Regulated Secretion: Constitutive is continuous; regulated requires a signal (e.g., hormone release).

Endomembrane System Table

Organelle	Main Function	Key Features
Rough ER	Protein synthesis	Ribosomes attached
Smooth ER	Lipid synthesis, detox	No ribosomes
Golgi Apparatus	Protein modification/sorting	Cisternae stacks
Lysosome	Degradation	Acidic pH, hydrolases

Chapters 13 & 14: The Cytoskeleton

Main Types of Cytoskeletal Filaments

The cytoskeleton provides structural support, facilitates movement, and organizes cellular components. There are three main filament types:

• Microfilaments (Actin Filaments): Involved in cell shape, movement, and muscle contraction.

• Intermediate Filaments: Provide mechanical strength; examples include keratin, vimentin, neurofilaments.

• Microtubules: Hollow tubes for intracellular transport, cell division, and cilia/flagella movement.

Comparison of Cytoskeletal Filaments

Filament Type	Subunits	Function	Polarity
Microfilaments	Actin	Cell shape, movement	Yes
Intermediate Filaments	Various (e.g., keratin)	Mechanical strength	No
Microtubules	Tubulin dimers	Transport, division	Yes

Microtubules

• Structure: Composed of α- and β-tubulin dimers forming protofilaments.

• Polarity: Plus and minus ends; growth occurs at plus end.

• Dynamic Instability: Rapid growth and shrinkage; catastrophe and rescue events.

• MAPs (Microtubule-Associated Proteins): Tau, CLASP, Katanin, etc.

• Motor Proteins: Dynein (moves toward minus end), Kinesin (moves toward plus end).

• Centrosome (MTOC): Main microtubule organizing center.

Intermediate Filaments

• Structure: Coiled-coil proteins; no polarity.

• Types: Keratin, vimentin, neurofilaments.

• Function: Provide tensile strength; resist mechanical stress.

Microfilaments (Actin Filaments)

• Structure: Two intertwined actin strands; have polarity.

• Function: Cell movement, shape, muscle contraction.

• Drugs Affecting Actin: Cytochalasin (inhibits polymerization), phalloidin (stabilizes filaments).

• Binding Proteins: Thymosin, profilin, cofilin, Arp2/3, fimbrin, alpha-actinin, filamin.

• Special Structures: Microvilli (increase surface area), lamellipodia, filopodia (cell movement).

Cilia and Flagella

• Structure: 9+2 arrangement of microtubules.

• Function: Motility; organized by basal bodies.

• Motor Proteins: Dynein powers movement.

Cell Motility and Crawling

• Steps: Extension of membrane (lamellipodia/filopodia), adhesion, translocation, and de-adhesion.

• Actin-Myosin Interaction: Myosin moves along actin filaments using ATP.

• Calcium Regulation: Calcium ions regulate contraction and motility.

Key Equations

• Osmotic Pressure: $\Pi = iMRT$ where $\Pi$ is osmotic pressure, $i$ is van 't Hoff factor, $M$ is molarity, $R$ is gas constant, $T$ is temperature.

• Na/K Pump Stoichiometry: $3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Additional info: Some details (e.g., specific protein examples, mechanisms) were expanded for clarity and completeness based on standard cell biology curriculum.