Membrane Structure and Function

Introduction

The plasma membrane is a fundamental cellular structure that regulates the movement of substances into and out of the cell. Its unique composition and properties enable cells to maintain homeostasis, communicate, and interact with their environment.

How Does the Plasma Membrane Regulate Inbound and Outbound Traffic?

• Passive Transport: Small molecules move across the membrane without energy input, sometimes requiring transport proteins.

• Active Transport: Small molecules are transported against their concentration gradient, requiring both energy (usually from ATP) and a transport protein.

• Bulk Transport: Large molecules are moved via vesicles in processes called exocytosis (out of the cell) and endocytosis (into the cell).

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Membrane Composition

• Lipids and Proteins: The main components of membranes; carbohydrates are also present and play important roles.

• Phospholipids: Amphipathic molecules with hydrophobic ("water-fearing") tails and hydrophilic ("water-loving") heads.

• Bilayer Structure: Phospholipids form a bilayer with hydrophobic tails inside and hydrophilic heads exposed to water.

• Membrane Proteins: Most are also amphipathic, with hydrophilic regions facing the aqueous environment and hydrophobic regions embedded in the bilayer.

The Fluid Mosaic Model

• The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Proteins are not randomly distributed; they often form groups for specific functions.

The Fluidity of Membranes

Factors Affecting Fluidity

• Membranes are held together by weak hydrophobic interactions, allowing lateral movement of lipids and some proteins.

• As temperature decreases, membranes become less fluid and may solidify; the temperature at which this occurs depends on lipid composition.

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.

Role of Cholesterol

• Cholesterol acts as a "fluidity buffer" in animal cell membranes.

• At high temperatures, cholesterol restrains phospholipid movement; at low temperatures, it prevents tight packing and solidification.

Adaptations in Membrane Lipid Composition

• Organisms in extreme temperatures adjust membrane lipid composition for optimal fluidity (e.g., more unsaturated fatty acids in cold environments).

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral Proteins: Bound to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; transmembrane proteins span the membrane.

• Hydrophobic regions of integral proteins often form α helices.

Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix (ECM)

Medical Relevance

• Cell-surface proteins are critical in medicine (e.g., HIV entry into immune cells via CD4 and CCR5 receptors).

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates bonded to lipids (glycolipids) or proteins (glycoproteins).

• The diversity of surface carbohydrates allows for cell identification and communication.

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces, with asymmetrical distribution of proteins, lipids, and carbohydrates.

Membrane Structure and Selective Permeability

• The plasma membrane controls material exchange and exhibits selective permeability—some substances cross more easily than others.

• Hydrophobic molecules (e.g., hydrocarbons, O2, CO2) pass rapidly; hydrophilic molecules (e.g., sugars, ions) pass slowly or require transport proteins.

Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions.

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

• Aquaporins: Channel proteins that facilitate rapid water transport.

Passive Transport: Diffusion and Osmosis

Diffusion

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

• At dynamic equilibrium, movement occurs equally in both directions.

Osmosis

• Diffusion of free water across a selectively permeable membrane from lower to higher solute concentration.

Water Balance of Cells

Cells Without Cell Walls

• Isotonic Solution: Equal solute concentration; no net water movement.

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

• Hypotonic Solution: Lower solute concentration outside; cell gains water, swells, and may burst.

Osmoregulation

• Organisms in extreme environments regulate solute and water balance (e.g., Paramecium uses a contractile vacuole).

Cells With Cell Walls

• Turgid: Plant cell in hypotonic solution; healthy state due to turgor pressure.

• Flaccid: Plant cell in isotonic solution; limp and wilted.

• Plasmolysis: Plant cell in hypertonic solution; membrane pulls away from cell wall, causing wilting.

Facilitated Diffusion

• Transport proteins (channel and carrier) speed passive movement of molecules across the membrane.

• Channel proteins provide corridors; carrier proteins change shape to move substances.

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

Active Transport

• Moves substances against their concentration gradients using energy (usually ATP).

• All active transport proteins are carrier proteins.

• Example: Sodium-potassium pump maintains high K+ and low Na+ inside animal cells.

Ion Pumps and Membrane Potential

• Membrane Potential: Voltage across a membrane due to ion distribution.

• Electrochemical Gradient: Combination of chemical and electrical forces driving ion diffusion.

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

Cotransport

• Active transport of one solute indirectly drives transport of another (e.g., H+/sucrose cotransport in plants, Na+/glucose cotransport in animals).

Bulk Transport: Exocytosis and Endocytosis

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell (e.g., insulin secretion).

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

• Types of endocytosis:

  • Phagocytosis: "Cellular eating"—engulfing particles into food vacuoles.

  • Pinocytosis: "Cellular drinking"—ingesting extracellular fluid and solutes.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

Key Equations

• Osmosis (Water Potential): Where is water potential, is solute potential, and is pressure potential.

• Membrane Potential (Nernst Equation): Where is equilibrium potential, is the gas constant, is temperature, is ion charge, and is Faraday's constant.

Additional info: This summary integrates and expands upon the provided textbook slides, ensuring a comprehensive, exam-ready overview of membrane structure and function for General Biology students.