Membrane Structure and Function: Chapter 7 Study Guide

Membrane Structure and Function

Membrane Properties

The plasma membrane is a fundamental structure in all cells, separating the internal environment from the external surroundings and compartmentalizing organelles. Its unique composition allows for selective transport and communication.

• Plasma Membrane: A boundary common to all cells, composed primarily of a phospholipid bilayer.

• Phospholipid Bilayer: Consists of two layers of phospholipids, each with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Amphipathic: Phospholipids possess both hydrophilic and hydrophobic regions, enabling spontaneous bilayer formation in water.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins, cholesterol, glycolipids, and glycoproteins embedded within the lipid bilayer. Components can move laterally, contributing to fluidity.

• Cholesterol: Acts as a "fluidity buffer," restraining movement at high temperatures and preventing tight packing at low temperatures.

• Peripheral Proteins: Attached to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; those spanning the membrane are called transmembrane proteins.

• Glycolipids & Glycoproteins: Lipids or proteins with covalently attached carbohydrates, important for cell recognition.

Example: The plasma membrane of a neuron contains specific proteins for signal transmission and cholesterol for fluidity regulation.

Membrane Fluidity

• Fluidity is essential for membrane function; most lipids and some proteins drift laterally.

• Temperature affects fluidity: cooling causes membranes to solidify.

• Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

• Cholesterol moderates fluidity changes due to temperature.

Membrane Protein Functions

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to cytoskeleton and extracellular matrix (ECM)

Cell "Fingerprint"

• Glycoproteins and glycolipids serve as cellular "fingerprints," enabling cell-to-cell recognition.

• Carbohydrate patterns on the membrane surface vary among species and cell types.

Membrane Selectivity

The phospholipid bilayer's chemical nature makes the membrane selectively permeable, allowing certain substances to cross while blocking others.

• Selectively Permeable: Only specific molecules can pass through unaided.

• Small, non-charged molecules (e.g., CO2, O2, glycerol, alcohol, H2O) can diffuse freely.

• Large, charged, or polar molecules require transport proteins to cross.

• Transport Proteins: Facilitate movement of hydrophilic substances; include channel proteins (e.g., aquaporins for water) and carrier proteins.

Passive Transport

Passive transport involves the movement of substances across the membrane without energy input, primarily through diffusion and osmosis.

• Concentration Gradient: Difference in concentration across a space.

• Diffusion: Movement of molecules from high to low concentration, down their gradient.

• Dynamic Equilibrium: Achieved when net movement stops and concentrations are uniform.

• Facilitated Diffusion: Passive movement of molecules via transport proteins; no energy required.m

• Osmosis: Diffusion of water across a selectively permeable membrane.

Example: Oxygen diffuses into cells from the bloodstream without energy input.

Tonicity and Water Balance

• Tonicity: Ability of a solution to cause a cell to gain or lose water.

• Isotonic Solution: Equal solute concentration; cell volume remains constant.

• Hypotonic Solution: Lower solute concentration outside; cell gains water, swells (animal cells may burst, plant cells become turgid).

• Hypertonic Solution: Higher solute concentration outside; cell loses water, shrivels (plant cells undergo plasmolysis).

• Plasmolysis: Plant cell membrane pulls away from cell wall due to water loss.

• Osmoregulation: Regulation of water and solute balance; contractile vacuole in freshwater protists pumps out excess water.

Table: Effects of Tonicity on Animal and Plant Cells

Solution Type	Animal Cell	Plant Cell
Isotonic	Volume constant	Flaccid (wilts)
Hypotonic	Swells, may burst (lyse)	Turgid (firm)
Hypertonic	Shrivels	Plasmolysis (membrane pulls away)

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually ATP).

• Active Transport: Movement from low to high concentration via transport proteins; requires ATP.

• Membrane Potential: Voltage difference across the membrane due to unequal ion distribution.

• Electrochemical Gradient: Combined effect of concentration gradient and membrane potential.

• Electrogenic Pump: Transport protein that generates voltage across the membrane (e.g., sodium-potassium pump in animals, proton pump in plants).

• Cotransport: Active transport of one solute indirectly drives transport of another.

Example: The sodium-potassium pump maintains nerve cell function by creating an electrochemical gradient.

Key Equations

• Membrane Potential: $V = Q / C$ (where V is voltage, Q is charge, C is capacitance)

• Electrochemical Gradient: $\Delta G = RT \ln \left( \frac{[C]_\text{inside}}{[C]_\text{outside}} \right) + zF\Delta \Psi$

• Osmosis: $\Pi = iMRT$ (osmotic pressure, where i is ionization constant, M is molarity, R is gas constant, T is temperature)

Bulk Transport

Bulk transport moves large molecules across the membrane via vesicles, requiring energy.

• Bulk Transport: Movement of large molecules (e.g., proteins, polysaccharides) via vesicles.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

• Endocytosis: Cell takes in macromolecules by forming vesicles from the plasma membrane.

• Three types of endocytosis:

  • Phagocytosis: "Cellular eating"; uptake of large particles.

  • Pinocytosis: "Cellular drinking"; uptake of fluids and small particles.

  • Receptor-mediated Endocytosis: Specific uptake triggered by ligand binding to receptors.

• Bulk transport requires energy input.

Table: Types of Bulk Transport

Type	Direction	Function
Exocytosis	Out of cell	Secretion of products (e.g., hormones, neurotransmitters)
Phagocytosis	Into cell	Uptake of large particles (e.g., bacteria)
Pinocytosis	Into cell	Uptake of fluids and small molecules
Receptor-mediated Endocytosis	Into cell	Specific uptake of substances via receptor binding

Example: White blood cells use phagocytosis to engulf pathogens.

Additional info: Academic context was added to clarify definitions, provide examples, and include relevant equations for membrane potential, osmosis, and electrochemical gradients. Tables were inferred and expanded for clarity.