Membrane Function and Structure

Membrane Function

• Plasma membranes separate living cells from their nonliving surroundings.
• Plasma membranes control the movement of substances in and out of cells.
• Plasma membranes interact with the external environment.
• Plasma membranes are involved in cell identity and interactions with other cells.
• Intracellular membranes maintain the structural integrity of cellular organelles.
• Intracellular membranes divide the cytoplasm into distinct chemical regions.
• Intracellular membranes provide surface area for enzyme attachment.
• Intracellular membranes facilitate transport and secretion via transport vesicles.
• The fluid mosaic model describes the current understanding of membrane structure.
• Phospholipids are the primary structural component, forming bilayers in water.
• Glycolipids and sterols are other important membrane lipids.
• Embedded proteins perform functions like transport, cell identity, recognition, and reception.

Lipid Bilayer Properties

• Phospholipids have hydrophobic fatty acid tails and hydrophilic phosphate/choline regions
• Phospholipids are amphipathic, possessing both non-polar and polar regions.
• Glycolipids have a non-polar lipid region and a polar region of sugars.
• Sterols (cholesterol in animals, phytosterols in plants) are mostly non-polar with a polar functional group.
• Phospholipids form bilayers due to water repelling their fatty acid tails.
• Phosphate heads attract water, forming the exposed surfaces of the bilayer.
• Phospholipids move laterally within the membrane, maintaining fluidity.
• Sterols, like cholesterol, are located between phospholipid tails.
• Sterols prevent lipids from packing too tightly or pulling apart, maintaining fluidity.
• Glycolipids expose their carbohydrate regions at the cell surface.
• Glycolipids are involved in cell recognition and identity.

Membrane Proteins – Location and Function

• Integral proteins are embedded within or span the entire lipid bilayer
• Integral proteins are exposed on one or both membrane surfaces.
• Peripheral proteins are located on the membrane surface.
• Peripheral proteins are loosely associated with integral proteins.
• Transport proteins facilitate the movement of substances across the membrane.
• Carriers bind to specific molecules, changing shape to move them across.
• Channel proteins create pores for ions or small polar molecules to move.
• Regulated channel proteins open or close via a molecular gate.
• Non-regulated channel proteins are always open.
• Cell-to-cell recognition proteins have oligosaccharides on the outer surface, acting as cellular fingerprints.
• Enzymes catalyze chemical reactions.
• Receptor proteins bind to hormones or other substances, triggering cellular responses.
• Attachment proteins link the extracellular matrix to the cytoskeleton.
• Intercellular joining proteins form gap junctions and desmosomes.

Membrane Permeability

• Permeable membranes allow substances to cross.
• Impermeable membranes prevent substances from crossing.
• Semipermeable membranes allow some substances to cross partially.
• Semipermeable membranes are selectively permeable or differentially permeable.

Physical or Passive Methods for Crossing the Membrane

• Diffusion: movement of particles from high to low concentration
• Diffusion results from random molecular motion.
• Diffusion occurs in gases, liquids, and solids, fastest in gases, slowest in solids.
• Diffusion happens quicker at high temperatures than low temperatures.
• Osmosis: diffusion of a solvent (usually water) across a semipermeable membrane.
• Dialysis: diffusion of a solute across a semipermeable membrane.

Isotonic, Hypertonic, and Hypotonic Solutions

• Isotonic solution: equal concentration of solvent and solute compared to another solution.
• Hypertonic solution: higher solute, lower solvent concentration compared to another solution.
• Hypotonic solution: higher solvent, lower solute concentration compared to another solution.
• Turgor: internal water pressure in plant cells from water diffusing into the vacuole.
• Turgor pushes outward against the cell wall, causing it to stretch and become rigid.
• Turgor happens because plant roots are in contact with soil water, which is hypotonic to plant cells.
• Water diffuses into plant cells until internal pressure equals the osmotic tendency to diffuse in.
• Turgor supports and shapes soft plant tissues.

Physiological (Active) vs. Passive Transport

• Physiological transport (active transport) requires energy from the cell.
• Passive transport does not require energy.

Carrier Mediated Transport

• Carrier mediated transport uses a protein carrier to move a substance across the membrane.
• Facilitated diffusion: passive process with the substance moving down its concentration gradient.
• The protein carrier changes shape upon binding, allowing the substance to cross.
• Energy for facilitated diffusion is derived from the diffusion gradient.
• Active transport: substance moves against its concentration gradient.
• Active transport requires cellular energy (ATP) to trigger a conformational change in the carrier.

Exocytosis and Endocytosis

• Exocytosis: vesicles transport substances to the cell surface, fuse, and release contents outside.
• Exocytosis transports large molecules too big for diffusion or carrier proteins.
• Endocytosis: vesicles form from the cell surface, bringing substances inside the cell.
• Phagocytosis: engulfing large particles via cytoplasmic extensions, forming a vacuole.
• Phagocytosis is also known as "cell eating."
• Pinocytosis: membrane depression draws in liquid molecules, forming vesicles.
• Pinocytosis is also known as "cell drinking."
• Receptor mediated endocytosis: molecules bind to surface receptors.
• Receptor binding triggers depression formation and vesicle pinching, bringing the molecule inside.

Cotransport

• Cotransport involves active transport of a particle (e.g., H+) to create a potential across the membrane.
• Another particle is cotransported in the opposite direction as the original particle moves back across the membrane.
• Movement is through a carrier, along its concentration gradient.

Ground Rules of Metabolism

• Metabolism: the sum of chemical activities in an organism.
• Metabolic pathway: a series of reactions from reactants to a final product.
• Anabolism: synthesis of complex products from simpler substrates.
• Catabolism: breakdown of complex molecules into simpler ones.
• Energy: the capacity to do work or cause change.
• Matter: anything that has mass and occupies space.
• Potential energy: stored energy due to position or state.
• Kinetic energy: energy in motion.
• First Law of Thermodynamics: energy is conserved, neither created nor destroyed.
• Second Law of Thermodynamics: systems tend toward disorder (entropy).
• Entropy (S): a measure of randomness or disorder.
• Free energy (G): energy available to do work.
• Enthalpy (H): total heat content or potential energy of a system.
• Free energy and entropy have an inverse relationship: as entropy increases, free energy decreases.

Chemical Reactions

• Reactants: substances that enter a chemical reaction.
• Products: substances present at the end of a reaction.
• Transition state: a complex of reactants that will proceed to the product.
• Activation energy: energy needed to raise reactants to the transition state.
• Exergonic reaction: releases energy to the environment.
• Products contain less energy than reactants in exergonic reaction.
• Endergonic reaction: absorbs energy from the environment.
• Products contain more energy than reactants in endergonic reaction.
• Reversible chemical reaction: proceeds in either direction.
• Direction depends on reactant and product concentrations.
• Chemical equilibrium: rate of change is equal in both directions in a reaction.
• Factors influencing reaction rate:
  • Temperature.
  • Pressure.
  • Concentration.
  • Catalysts/enzymes.
• Coupled reactions: exergonic reactions drive endergonic reactions in metabolic processes.

Enzymes and Catalysts

• Catalyst: a substance that promotes a chemical reaction.
• Inorganic catalysts are usually not very reaction-specific.
• Organic catalysts (enzymes) are usually very specific.
• Enzyme: a biological catalyst.
• Almost all enzymes are proteins.
• Induced Fit Hypothesis: substrate binding causes enzyme shape change, bringing substrates together to react.
• Enzymes speed up reactions without being altered or used up.
• Enzymes are highly selective as to substrates.
• Enzymes recognize both reactants and products as substrates.
• Intermediates: compounds formed between the start and end of a metabolic pathway.
• Cofactor: a metal ion required for enzyme function.
• Coenzyme: a non-protein organic molecule required for enzyme function.
• End products: substances at the end of a metabolic pathway.

Enzyme Regulation

• Feedback control: product or intermediate inhibits the pathway by interacting with an enzyme.
• Competitive inhibition: inhibitor binds to and blocks the active site of an enzyme.
• Allosteric or Noncompetitive Inhibition: inhibitor binds to a second site (allosteric site).
• Allosteric/Noncompetitive Inhibition changing the shape of the active site and blocking enzyme activity.
• Reversible inhibition: enzyme activity is alternately active or blocked by an inhibitor.
• Nonreversible inhibition: inhibitor permanently blocks enzyme activity.
• Temperature/pH changes can alter enzyme shape and reduce activity.
• Ribozymes: RNA molecules that act as enzymes.
• ATP: nucleotide phosphate, energy transfer molecule involved in biochemical reactions.
• ATP transfers energy by transferring a terminal phosphate to a substrate.

Cellular Respiration

• Cellular respiration breaks down biomolecules (ideally glucose) to release energy for ATP production.
• Cellular respiration can be aerobic (requires oxygen) or anaerobic (doesn't require oxygen).
• Aerobic respiration uses oxygen as the final hydrogen acceptor.
• Aerobic respiration produces 18 times more ATP than anaerobic respiration.
• Anaerobic respiration occurs as alcoholic fermentation or lactic acid fermentation.
• Anaerobic respiration occurs in the cytoplasm.
• Aerobic respiration is centered in the mitochondrion.
• Mitochondrion: double membrane bound organelle with a smooth outer membrane.
• Mitochondrion has a highly folded inner membrane.
• Cristae: folds of the inner membrane, increasing surface area.
• The electron transport compounds and ATP synthase are associated with the inner membrane.
• Matrix: the innermost compartment enclosed by the inner membrane.
• Contains enzymes of the citric acid cycle.
• Intermembrane compartment: the region between the inner and outer membranes.
• Hydrogen ions are concentrated in intermembrane compartment; necessary for ATP production through chemiosmosis.
• ATP (adenine nucleotide triphosphate): the primary energy transfer compound in living organisms.
• Energy is transferred by moving the terminal phosphate to another compound.
• ATP is produced by the combination of ADP and Pi.
• Oxidation: partial or complete loss of electrons.
• Reduction: partial or complete gain of electrons.
• Redox: reactions that involve the transfer of electrons from one reactant to another.
• Redox reactions occur in electron transport systems of cellular respiration (and photosynthesis).
• Redox reactions result in ATP production.
• In aerobic cellular respiration, ATP is produced by free energy released from the electron transport system.
• Free energy released from the electron transport system pumps H+ from the mitochondrial matrix to the intermembrane space.
• This creates an electrochemical gradient across the membrane.
• The energy potential of the electrochemical gradient is used to produce ATP through chemiosmosis.
• NAD+ and FAD+ are nucleotides that are hydrogen carriers that transfer H from glucose to the electron transport system.
• NAD+ is reduced to NADH + H+.
• FAD is reduced to FADH2.
• Each NADH + H+ usually results in the production of 2.5 ATP.
• Each FADH2 provides energy for the production of 1.5 ATP.
• Aerobic cellular respiration summary: Glucose + Oxygen → 6 Carbon Dioxide + 6 Water + 30-32 ATP.
• Stages of aerobic cellular respiration:
  • Glycolysis.
  • Acetyl CoA formation (intermediate step).
  • Citric Acid Cycle.
  • Electron Transport/Chemiosmosis.

Glycolysis and Fermentation

• Glycolysis: Glucose (6C) is broken to two Pyruvate(3C each).
• Two ATP are required as activation energy.
• Four ATP are produced.
• Net gain of 2 ATP from Glycolysis.
• Two molecules of NAD+ are reduced to NADH.
• Fermentation occurs if no oxygen is present.
• Electrons from NADH are used to reduce pyruvate during Fermentation.
• Production of NAD+ allows glycolysis to continue during Fermentation.
• Alcoholic fermentation: reduction of pyruvate produces ethyl alcohol (2C) and carbon dioxide (1C).
• Lactic acid fermentation: reduction of pyruvate produces lactic acid (3C), also called lactate.

Acetyl CoA Production and Citric Acid Cycle

• Acetyl CoA Production: Each pyruvate loses a carbon dioxide (CO2) to produce a 2C acetyl group.
• Each acetyl group combine with Coenzyme A to produce Acetyl CoA.
• Two Acetyl CoA molecules are produced from two pyruvate molecules.
• Two NADH are also produced during the break down of pyruvate.
• Citric Acid Cycle: Coenzyme A facilitates the transfer of 2C acetyl group to oxaloacetate.
• Oxaloacetate (4C compound), producing citrate molecule (6C).
• Citric acid cycle is cyclic; oxaloacetate is regenerated as carbons and hydrogens are removed.
• Carbons from acetyl groups are released as carbon dioxide (4).
• Hydrogen/electrons from carbon chain are accepted by NAD+ and FAD+.
• Six NADH and 2 FADH2 are produced during the citric acid cycle.

Electron Transport and Chemiosmosis

• During electron transport, electrons from NADH and FADH2 pass through electron acceptors.
• Electron acceptors is embedded in the inner mitochondrial membrane via redox reactions.
• Redox reactions release free energy.
• The released free energy pumps H+ from matrix to intermembrane space.
• This results in an electrochemical gradient across the inner membrane.
• Chemiosmosis: H+ moves back across the inner membrane from intermembrane space to matrix.
• Hydrogen ions pass through a channel protein coupled to ATP synthase.
• ATP synthase synthesizes ATP from ADP and Pi, powered by H+ movement.
• Chemiosmosis synthesizes 26-28 ATP from one molecule of glucose.
• Many biological compounds other than glucose can be used in cellular respiration.
• Proteins and lipids are broken down into 2 and 3C chains which are fed into Acetyl CoA or citric acid cycle.