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

Chapter 7: Membranes – Structure, Function, and Chemistry

Functions of Cellular Membranes

Cellular membranes are essential for compartmentalization, protection, and regulation of cellular processes. They differ among organelles and cell types, reflecting specialized functions.

• Compartmentalization: Membranes separate cellular regions, allowing distinct environments and processes.

• Asymmetry: Membranes are asymmetric; certain lipids and proteins are restricted to one side, influencing function and signaling.

• Lipid and Protein Distribution: Concentrations of lipids and proteins vary between membranes, affecting properties and activities.

• Fluid Mosaic Model: Describes membranes as dynamic structures with proteins and lipids moving laterally within the bilayer.

Phospholipids: Structure and Diversity

Phospholipids are the primary components of cellular membranes, providing both structural integrity and functional diversity.

• Structure: Composed of a head group, phosphate, backbone (glycerol or sphingosine), and fatty acid tails.

• Hydrophilic vs. Hydrophobic Regions: Head group and phosphate are hydrophilic; fatty acid tails are hydrophobic.

• Backbones: Two main types: glycerol (phosphoglycerides) and sphingosine (sphingolipids).

• Amphipathic Nature: Molecules have both hydrophilic and hydrophobic regions, enabling bilayer formation.

• Bonding: Ester or amide bonds connect fatty acid tails to the backbone.

• Saturated vs. Unsaturated Tails: Unsaturated tails introduce kinks, increasing fluidity; saturated tails pack tightly, reducing fluidity.

Factors Affecting Membrane Fluidity

Membrane fluidity is crucial for function, influenced by lipid composition and environmental conditions.

• Temperature: Higher temperatures increase fluidity.

• Pressure: Can affect packing of lipids.

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

• Protein Concentration: Integral and peripheral proteins can modulate fluidity.

Types of Junctions Between Cells

Cell junctions maintain tissue integrity and facilitate communication.

• Tight Junctions: Seal adjacent cells, preventing passage of molecules.

• Desmosomes: Anchor cells together via intermediate filaments.

• Gap Junctions: Allow direct communication through channels.

Glycosylation of Proteins

Glycosylation is the addition of sugar moieties to proteins, affecting stability, localization, and cell recognition.

• Functions: Protein folding, protection, and signaling.

Chapter 8: Transport Across Membranes – Overcoming the Permeability Barrier

Types of Transport

Transport across membranes is essential for nutrient uptake, waste removal, and signaling.

• Passive Transport: Does not require energy; includes simple diffusion and facilitated diffusion.

• Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient.

• Carrier vs. Channel: Carriers bind and transport specific molecules; channels form pores for selective passage.

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

Tonicity and Osmosis

Tonicity describes the effect of solute concentration on cell volume.

• 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.

Channels and Carriers

Membrane proteins facilitate transport of ions and molecules.

• Channel Types: Leak, voltage-gated, ligand-gated.

• Gating Mechanisms: Control opening/closing of channels.

• Carrier Proteins: Undergo conformational changes to transport substances.

• Competitive vs. Non-Competitive Inhibition: Inhibitors can block transport by binding to the active site or elsewhere.

• Symport, Antiport, Uniport: Modes of coupled transport.

Active Transport Mechanisms

Active transport maintains essential gradients.

• Na+/K+ Pump: Exchanges sodium and potassium ions across the membrane.

• Na+/Glucose Transporter: Couples sodium movement to glucose uptake.

• ATPases: Enzymes that hydrolyze ATP to drive transport; types include P-type, V-type, F-type, and ABC transporters.

Chapter 12: The Endomembrane System and Protein Sorting

Major Regions of the Endomembrane System

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

• Components: Endoplasmic reticulum (ER), Golgi apparatus, lysosomes, endosomes, plasma membrane.

• Ribosome Localization: Ribosomes on the cytosol or rough ER direct protein targeting.

• Smooth ER Functions: Lipid synthesis, detoxification (cytochrome P450), glycogen breakdown, calcium storage (sarcoplasmic reticulum).

• Rough ER Functions: Protein synthesis and initial glycosylation.

Golgi Apparatus

The Golgi modifies, sorts, and packages proteins and lipids for delivery.

• Cisternae: Flattened membrane sacs; modifications occur here.

• Modification Enzymes: Add or remove groups to regulate protein function.

• Vesicle Trafficking: Anterograde (ER to Golgi to plasma membrane) and retrograde (Golgi to ER) transport.

• Coat Proteins: COPI, COPII, clathrin; direct vesicle movement.

• ER Retention Signal: Sequence (e.g., KDEL) keeps proteins in the ER.

• Secretion Types: Constitutive (continuous) and regulated (stimulus-dependent).

Endocytosis and Lysosomes

Endocytosis imports molecules; lysosomes degrade and recycle cellular components.

• Endocytosis Types: Phagocytosis, pinocytosis, receptor-mediated endocytosis.

• Vesicle-Mediated Transport Steps: Involves dynamin (pinching off), snares (fusion), and adapter proteins (cargo selection).

• Lysosome Functions: Acidic environment, hydrolytic enzymes, autophagy (self-digestion).

• Endosome-Lysosome Pathway: Endosomes mature and deliver cargo to lysosomes for degradation.

Chapters 13 and 14: The Cytoskeleton

Cytoskeleton Families

The cytoskeleton provides structural support, motility, and intracellular transport.

• Microtubules: Tubulin dimers form hollow tubes; involved in cell shape, transport, and division.

• Microfilaments (Actin Filaments): Actin monomers form flexible fibers; support cell shape and movement.

• Intermediate Filaments: Diverse proteins (keratin, vimentin, neurofilaments) provide mechanical strength.

Intermediate Filaments

• Structure: Coiled coil; no polarity.

• Location: Varies by type; e.g., keratin in epithelial cells.

• Helper Proteins: May require chaperones for assembly.

Microtubules

• Polarity: Plus and minus ends; essential for directional transport.

• Structure: Tubulin dimers assemble into protofilaments and mature microtubules.

• Dynamic Instability: Rapid growth and shrinkage; regulated by GTP hydrolysis.

• Drugs: Taxol (stabilizes), colchicine/vinblastine (destabilize).

• MAPs (Microtubule-Associated Proteins): Include dynein, kinesin, tau, CLASP, katanin.

• MTOC (Microtubule Organizing Center): Site of microtubule nucleation (e.g., centrosome).

Microfilaments (Actin Filaments)

• Structure: Actin monomers; ATP-dependent assembly.

• Polarity: Plus and minus ends.

• Cytoskeleton Drugs: Cytochalasin (inhibits polymerization), phalloidin (stabilizes).

• Cell Protrusions: Filopodia (thin), lamellipodia (broad); involved in cell movement.

• Binding Proteins: Thymosin, profilin, cofilin (regulate actin dynamics).

Motility and Contractility

• Motility: Movement of cells or organelles (e.g., crawling, cilia/flagella beating).

• Contractility: Shortening of fibers (e.g., muscle contraction).

Motor Proteins

• Dynein: Moves toward microtubule minus end; important for cilia/flagella movement.

• Kinesin: Moves toward microtubule plus end; involved in vesicle transport.

• Myosin: Moves along actin filaments; multiple types for different functions.

• Attachment and Regulation: Myosin binds actin; regulated by calcium, troponin, and tropomyosin.

Cilia and Flagella

• Structure: 9+2 arrangement of microtubules; organized by basal bodies.

• Nexin: Links microtubules, allowing bending.

• Movement: Dynein-driven sliding of microtubules.

Cell Crawling

• Steps: Extension of protrusions (lamellipodia/filopodia), adhesion to substrate, translocation of cell body, detachment at rear.

Additional info: Where original notes were brief, standard textbook context and definitions have been added for completeness and clarity.