Membrane Structure and Function

Introduction

The plasma membrane is a fundamental structure in all living cells, responsible for regulating the movement of substances into and out of the cell. Its unique composition and organization enable selective permeability, communication, and compartmentalization essential for cellular function.

Regulation of Membrane Traffic

Mechanisms of Transport

• Passive Transport: Movement of small molecules across the membrane without energy input. May involve transport proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring both energy (usually ATP) and a transport protein.

• Bulk Transport: Large molecules (e.g., proteins, polysaccharides) are transported via exocytosis (out of the cell) or endocytosis (into the cell).

Example: Glucose uptake by cells can occur via facilitated diffusion (passive) or active transport, depending on cellular needs.

Fluid Mosaic Model of Membrane Structure

Components of the Membrane

• Lipids: Primarily phospholipids, which are amphipathic (having both hydrophobic and hydrophilic regions).

• Proteins: Embedded within or attached to the lipid bilayer; responsible for most membrane functions.

• Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins); play roles in cell recognition.

Phospholipid Bilayer

• Phospholipids arrange themselves into a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward toward aqueous environments.

• This arrangement forms the basic structure of the membrane, providing a semi-permeable barrier.

Membrane Proteins

• Integral Proteins: Span the membrane and are involved in transport and signaling.

• Peripheral Proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

• Many proteins are also amphipathic, with hydrophilic regions exposed to the cytosol or extracellular fluid and hydrophobic regions embedded in the bilayer.

Fluid Mosaic Model

• The membrane is described as a "fluid mosaic" because proteins float in or on the fluid lipid bilayer like boats on a pond.

• Proteins are not randomly distributed; they often form functional groups.

Membrane Fluidity

Factors Affecting Fluidity

• Membranes are held together by weak hydrophobic interactions, allowing lipids and some proteins to move laterally.

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

• Cholesterol acts as a "fluidity buffer" in animal cells, restraining movement at high temperatures and preventing tight packing at low temperatures.

• Organisms adapt membrane lipid composition to environmental conditions (e.g., more unsaturated fats in cold environments).

Membrane Proteins and Their Functions

Types and Roles

• Transport: Channel and carrier proteins facilitate movement of substances.

• Enzymatic Activity: Some proteins catalyze reactions at the membrane surface.

• Signal Transduction: Receptors transmit signals from outside to inside the cell.

• Cell-Cell Recognition: Glycoproteins serve as identification tags.

• Intercellular Joining: Proteins form junctions between cells.

• Attachment: Proteins anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

Membrane Carbohydrates and Cell Recognition

• Carbohydrates on the cell surface are crucial for cell-cell recognition.

• Glycolipids and glycoproteins act as markers for cellular identification.

Selective Permeability of Membranes

Lipid Bilayer Permeability

• Hydrophobic (nonpolar) molecules (e.g., O2, CO2) pass through easily.

• Hydrophilic (polar) molecules (e.g., sugars, ions) cross slowly or require transport proteins.

Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Transport proteins are specific for the substances they move.

Passive Transport

Diffusion

• Movement of molecules from high to low concentration (down the concentration gradient).

• No energy input required; driven by the potential energy of the gradient.

• At dynamic equilibrium, movement continues but there is no net change in concentration.

Osmosis

• Diffusion of free water across a selectively permeable membrane.

• Water moves from regions of lower solute concentration to higher solute concentration.

Tonicity and Water Balance

• Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside; cell loses water and shrivels.

• Hypotonic: Lower solute concentration outside; cell gains water and may burst (lyse).

Table: Effects of Tonicity on Animal and Plant Cells

Osmoregulation

• Organisms in hypotonic or hypertonic environments must regulate water balance (e.g., Paramecium uses a contractile vacuole).

Facilitated Diffusion

• Transport proteins (channels and carriers) speed up passive movement of molecules across the membrane.

• Channel proteins provide corridors for specific molecules or ions (e.g., aquaporins for water, ion channels for ions).

• Gated channels open or close in response to stimuli (e.g., electrical or chemical signals).

• Carrier proteins undergo shape changes to move substances down their concentration gradients.

Active Transport

• Requires energy (usually ATP) to move substances against their concentration gradients.

• Carrier proteins are involved in active transport.

• Enables cells to maintain internal concentrations of ions and molecules different from the environment.

• Example: The sodium-potassium pump moves Na+ out and K+ into animal cells.

Sodium-Potassium Pump Equation

Membrane Potential and Electrochemical Gradients

• Membrane potential: Voltage across a membrane due to differences in ion distribution.

• Electrochemical gradient: Combined effect of concentration gradient and electrical gradient on ion movement.

• Electrogenic pumps: Transport proteins that generate voltage (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

Cotransport (Coupled Transport)

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

• Example: In plants, proton pumps create an H+ gradient used to drive sucrose uptake via a cotransporter.

• In animals, Na+/glucose cotransporters in the intestine enable glucose absorption.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

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

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

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

• Three types:

  • Phagocytosis: "Cellular eating"; cell engulfs particles into a food vacuole.

  • Pinocytosis: "Cellular drinking"; cell takes in extracellular fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

• Example: Uptake of cholesterol via LDL receptors.

Additional info: Individuals with defective LDL receptors develop familial hypercholesterolemia, leading to high blood cholesterol and increased risk of heart disease.