Lecture 7: Membranes and Transport

Membrane Composition and Properties

Biological membranes are composed of a lipid bilayer with embedded proteins, and their properties depend on the types of lipids present.

• Saturated fatty acids increase membrane viscosity, making the membrane less fluid.

• Fluidity is essential for membrane function, affecting permeability and protein mobility.

• Viscous membranes are less permeable and more rigid.

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane, influenced by the tonicity of the surrounding solution.

• Isotonic solution: No net movement of water; cell volume remains constant.

• Hypotonic solution: Water enters the cell, causing it to swell and potentially lyse.

• Hypertonic solution: Water leaves the cell, causing it to shrink.

• Example: Animal cells placed in a hypotonic solution will lyse due to water influx.

Transport Across Membranes

Cells use various mechanisms to move substances across membranes, either down or against concentration gradients.

• Passive transport: Movement down a concentration gradient without energy input (e.g., diffusion, facilitated diffusion).

• Active transport: Movement against a concentration gradient, requiring energy (often ATP).

• Concentration gradient can be used as an energy source for cotransporters.

• CFTR gene: Encodes a transmembrane protein involved in passive transport (chloride ion channel).

Comparison Table: Passive vs Active Transport

Endocytosis

Cells ingest large molecules via specialized processes.

• Pinocytosis: Uptake of fluids and small molecules.

• Phagocytosis: Uptake of large particles or cells.

• Receptor-mediated endocytosis: Specific uptake via receptor binding.

Lecture 8: Cell Communication

Phosphorylation Cascades

Cell signaling often involves phosphorylation cascades, where proteins are activated by the addition of phosphate groups.

• Kinases: Enzymes that phosphorylate other proteins, transferring a phosphate group from ATP.

• Receptors: Proteins that receive signals and may initiate phosphorylation cascades.

G-Protein Coupled Receptors (GPCRs)

GPCRs are a large family of membrane receptors involved in signal transduction.

• Relatively nonpolar molecules can cross membranes and bind intracellular receptors.

• Epigenphrine signaling: Activates GPCRs, which in turn activate adenylyl cyclase to produce cAMP.

• Examples of GPCR genes: OPN1LW, CFTR, TAS2R3 (among others).

Lecture 9: Endergonic vs Exergonic Reactions and ATP

Thermodynamics of Cellular Reactions

Cellular reactions are classified based on energy changes.

• Exergonic reactions: Release energy; spontaneous ().

• Endergonic reactions: Require energy input; non-spontaneous ().

• ATP: Universal energy currency; cells use ATP to drive endergonic reactions.

• Coupling reactions: Cells can couple endergonic reactions to exergonic ones to make them proceed.

Equation:

• Cells can drive reactions forward by increasing reactant concentration, decreasing product concentration, or coupling to exergonic reactions.

Lecture 10: Enzymes

Enzyme Function and Activation Energy

Enzymes are biological catalysts that speed up reactions by lowering the activation energy.

• Activation energy (): The energy required to start a reaction.

• Reactions with lower activation energy proceed faster.

• Enzymes do not increase the energy of the transition state; they lower it.

Graphical Representation:

Enzyme-catalyzed reactions have a lower peak (activation energy) compared to uncatalyzed reactions.

Equation:

(activation energy) is lower in the presence of an enzyme.

• Competitive inhibitors: Molecules like ibuprofen can inhibit enzymes by binding to the active site.

Lecture 11: Redox Reactions, Glycolysis, Pyruvate Decarboxylation, Citric Acid Cycle

Glycolysis and Pyruvate Decarboxylation

Glucose metabolism involves several steps, each producing energy and intermediates.

• Glycolysis: Occurs in the cytosol; produces ATP and NADH.

• Pyruvate decarboxylation: Converts pyruvate to acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle: Acetyl-CoA enters the cycle, generating ATP, NADH, FADH2, and CO2.

• Substrate-level phosphorylation: Direct generation of ATP during glycolysis.

Summary Table: Carbon and Energy Yield

• Acetyl-CoA: Formed from pyruvate; enters the citric acid cycle.

• ATP synthase: Produces ATP using the proton motive force.

• Alcoholic fermentation: NADH is oxidized by acetaldehyde.

Lecture 12: Oxidative Phosphorylation and Fermentation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, generating the majority of ATP.

• Electron transport chain: Transfers electrons from NADH and FADH2 to oxygen, pumping protons to create a gradient.

• Proton motive force: Drives ATP synthesis via ATP synthase.

• Fermentation: Occurs when oxygen is absent; regenerates NAD+ for glycolysis.

Equation:

• After the citric acid cycle, the bulk of energy is stored in NADH and FADH2 before entering the electron transport chain.

Additional info:

• Some gene names and signaling pathways were inferred from context and standard biology knowledge.

• Tables and diagrams were recreated and expanded for clarity.