Biology Chapter 5: Membrane Transport

Chapter 5: Membrane transport and cell signaling

• Plasma membrane: A boundary separating the living cell from its external environment. It exhibits selective permeability, allowing some substances to cross more easily than others.
• Selective permeability: Crucial for maintaining cellular homeostasis. Small nonpolar molecules cross more easily than large polar molecules or ions.
• Plasma membrane function: Controls material exchange, which is essential for cellular homeostasis.

Concept 5.1: Cellular Membranes Are Fluid Mosaics of Lipids and Proteins

Phospholipid Structure and Membrane Composition

• Phospholipids: The most abundant lipids in membranes.
• Amphipathic properties: Phospholipids have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
  • Hydrophilic head: Composed of a phosphate group, facing the aqueous external and internal environments.
  • Hydrophobic tail: Composed of fatty acids, facing inward and shielded from water.
• Phospholipid bilayer: Forms a stable barrier between aqueous regions, like the cytoplasm and extracellular fluid.

Membrane Proteins

• Amphipathic: Most membrane proteins are amphipathic, with hydrophilic portions exposed to water and hydrophobic regions interacting with the lipid core.
• Fluid mosaic model: Membranes are fluid structures with embedded or attached proteins in a phospholipid bilayer.
• Lateral movement: Proteins and lipids can move laterally within the membrane layer.
• Specialized regions/rafts: Some proteins and lipids associate in specific regions or rafts, creating specialized functional patches.

Fluidity of Membranes

• Lateral movement: Lipids and proteins move rapidly (lipids) to more slowly (proteins). Some proteins are anchored in place by cytoskeletal attachments.
• Temperature effects: Temperature affects membrane fluidity, leading to a solid state at lower temperatures (saturated fatty acid tails pack tightly) and a more fluid state at higher temperatures (unsaturated fatty acid tails create kinks that prevent tight packing)
• Cholesterol's role: Cholesterol regulates membrane fluidity; it prevents close packing at low temperatures and restricts movement at high temperatures.

Membrane Proteins and Their Functions

• Variety of functions: Membrane proteins perform a variety of functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).
• Integral proteins: Span the entire lipid bilayer, often acting as channels or carriers for substances to pass across.
• Peripheral proteins: Bound loosely to the surface of the membrane, often attached to integral proteins, cytoskeleton or ECM.

Six major functions of membrane proteins

• Transport: Channel and carrier proteins allow certain molecules and ions to pass through.
• Enzymatic activity: Membrane-bound enzymes catalyze reactions in the membrane environment.
• Signal transduction: Specific signal molecules bind to protein receptors, initiating a signaling pathway within the cell.
• Cell-cell recognition: Glycoproteins serve as identification tags.
• Intercellular joining: Membrane proteins form connections, such as gap junctions or tight junctions.
• Attachment to the cytoskeleton and ECM: Linking the membrane protein to internal support or the extracellular environment.

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Carbohydrates attached to proteins and lipids: Form glycoproteins and glycolipids, which influence cell recognition and interactions.
• Varied characteristics: Carbohydrate structures vary between species, individuals, even cell types, giving unique surfaces.

Concept 5.2: Membrane Structure and Selective Permeability

• Selective permeability: Membranes control molecule and ion passage for maintaining proper internal conditions.
• Lipid bilayer permeability: Hydrophobic molecules (hydrocarbons, CO2, O2) cross easily, while hydrophilic molecules (ions, polar molecules) cross less readily.
• Transport Proteins: Allow hydrophilic substances to cross, including channel and carrier proteins

Transport Proteins

• Channel proteins: Form hydrophilic tunnels enabling some molecules and ions to cross.
• Aquaporins: Specialized channel proteins facilitating water movement.
• Carrier proteins: Change shape to transport molecules across the membrane.

Concept 5.3: Passive Transport/ Concept 5.4: Active transport

• Concentration gradient: Difference in concentration between regions.
• Passive transport: No energy needed; substances move down their concentration gradient (high to low)
  • Diffusion Molecules spread out evenly; move from high to low concentration via random motion.
  • Osmosis Water moves from low solute concentration (high water concentration) to high solute concentration (low water concentration).
• Active transport: Requires energy (ATP); substances move against their concentration gradient (low to high).
• Sodium-Potassium Pump: Example of active transport; maintains concentration differences vital for cell functions (three sodium ions pumped out, two potassium ions pumped in).

Transport of Large Particles

• Endocytosis: Engulfs particles from the outside into a vacuole within the cell. Types include phagocytosis, pinocytosis and receptor-mediated endocytosis.
• Exocytosis: Vesicles fuse with the plasma membrane to expel contents out of the cell.

Chapter 7: Cellular Respiration & Fermentation

• Cellular respiration: A series of metabolic reactions converting biochemical energy from nutrients to ATP, releasing CO2 and H2O.
• General Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (36-38 ATP)
• Types of Cellular Respiration:
  • Aerobic respiration: Requires oxygen; produces 36-38 ATP per glucose molecule.
  • Anaerobic respiration (fermentation): Occurs without oxygen; produces 2 ATP per glucose. Examples include alcohol fermentation and lactic acid fermentation.

Stages of Cellular Respiration

• Glycolysis: The first step in breaking down glucose into two pyruvate molecules in the cytoplasm.
• Pyruvate oxidation: Converts pyruvate to acetyl-CoA in the mitochondrial matrix.
• Citric acid cycle (Krebs cycle): Further breaks down acetyl-CoA, generating NADH, FADH2, and ATP in the mitochondrial matrix.
• Oxidative phosphorylation (Electron transport chain): Electrons from NADH and FADH2 are passed down an electron transport chain generating ATP and oxygen acts as the final electron acceptor; in the inner mitochondrial membrane.
• ATP synthase: Enzyme that synthesizes ATP during oxidative phosphorylation (using a proton gradient).

Why Do You Need to Breathe?

• Breathing is required to supply oxygen for aerobic cellular respiration.
• Without oxygen, cells must use fermentation, which produces less ATP and can have negative consequences for the body.

Enzymes

• Enzymes are proteins that act as biological catalysts, speeding up chemical reactions by lowering the activation energy needed for them to proceed.

• Factors Affecting Enzyme Activity include temperature, pH, ionic concentration and presence of cofactors or inhibitors (like competitive or noncompetitive ones)

• Enzyme Mechanism Enzyme-substrate interaction, Activation Energy reduction by catalysis, Product formation.

• Important Graphs Showing Enzyme-Substrate Interaction, Graphs Showing the effects of temperature and pH on enzyme activity.