Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all cells, serving as a selective barrier that regulates the movement of substances into and out of the cell. Its structure and function are essential for maintaining cellular homeostasis and enabling communication with the environment.

Cellular Membranes: Structure and Components

Phospholipids and the Fluid Mosaic Model

• Phospholipids are the main structural components of cell membranes. They are amphipathic, meaning each molecule has both a hydrophilic ("water-loving") head and hydrophobic ("water-fearing") tails.

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

• The fluid mosaic model describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Membrane proteins are also amphipathic, with hydrophilic regions exposed to water and hydrophobic regions embedded in the bilayer.

Example: The plasma membrane of animal cells contains both phospholipids and proteins, allowing for selective permeability and various cellular functions.

Membrane Fluidity

• Membranes are held together mainly by weak hydrophobic interactions, allowing most lipids and some proteins to move laterally within the layer.

• Membrane fluidity is affected by temperature and the types of lipids present:

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

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

• Organisms can adjust membrane lipid composition in response to environmental conditions (e.g., winter wheat increases unsaturated phospholipids in cold weather).

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Integral proteins penetrate the hydrophobic core of the bilayer; some span the membrane (transmembrane proteins).

• Peripheral proteins are loosely bound to the membrane surface.

Functions of Membrane Proteins

• Transport: Move substances across the membrane.

• Enzymatic activity: Catalyze specific reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Allow cells to identify each other.

• Intercellular joining: Connect adjacent cells.

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

Example: HIV enters immune cells by binding to specific cell-surface proteins (CD4 and CCR5). Individuals lacking CCR5 are resistant to HIV infection.

Membrane Carbohydrates and Cell Recognition

• Carbohydrates on the cell surface are bonded to lipids (glycolipids) or proteins (glycoproteins).

• These molecules function as markers for cell identification, enabling cells to recognize each other.

Membrane Synthesis and Sidedness

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

• This sidedness is determined during membrane synthesis in the endoplasmic reticulum and Golgi apparatus.

Selective Permeability of Membranes

Permeability to Different Substances

• Membranes are selectively permeable, allowing some substances to cross more easily than others.

• Hydrophobic (nonpolar) molecules (e.g., O2, CO2) pass through the lipid bilayer rapidly.

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

Transport Proteins

• Channel proteins provide hydrophilic tunnels for specific molecules or ions.

• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.

• Aquaporins are channel proteins that facilitate rapid water transport.

Example: Glucose carrier proteins transport only glucose, not its isomer fructose.

Passive Transport

Diffusion

• Diffusion is the movement of particles from an area of higher concentration to lower concentration (down their concentration gradient).

• Each substance diffuses independently of others.

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

Osmosis

• Osmosis is the diffusion of free water across a selectively permeable membrane.

• Water moves from regions of lower solute concentration to higher solute concentration until equilibrium is reached.

Tonicity and Water Balance in Cells Without Cell Walls

• Tonicity is the ability of a solution to cause a cell to gain or lose water.

• Isotonic solution: Solute concentration is equal inside and outside the cell; 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 (lyse).

Osmoregulation

• Cells without walls must regulate water balance to prevent excessive water loss or gain.

• Some protists (e.g., Paramecium) use contractile vacuoles to expel excess water in hypotonic environments.

• Bacteria and archaea in hypersaline environments have adaptations to prevent water loss.

Water Balance in Cells with Cell Walls

• Plant, prokaryotic, fungal, and some protist cells have cell walls that help maintain water balance.

• In a hypotonic solution, plant cells become turgid (firm), which is healthy for most plants.

• In an isotonic solution, plant cells become flaccid (limp), and the plant may wilt.

• In a hypertonic solution, plant cells undergo plasmolysis (membrane pulls away from the wall), leading to wilting and possible death.

Facilitated Diffusion

• Facilitated diffusion is passive transport aided by proteins (channel or carrier proteins).

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

• Some ion channels are gated, opening or closing in response to stimuli (e.g., electrical or chemical signals).

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

Active Transport

Mechanism and Importance

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

• Allows cells to maintain internal concentrations of small molecules that differ from those in the environment.

• Transport proteins involved in active transport are always carrier proteins.

Sodium-Potassium Pump

• The sodium-potassium pump uses ATP to transport Na+ out of the cell and K+ into the cell, maintaining electrochemical gradients essential for cell function.

Membrane Potential and Electrochemical Gradients

• Membrane potential is the voltage across a membrane, created by differences in ion distribution.

• The electrochemical gradient combines the chemical (concentration) and electrical (charge) forces driving ion movement.

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants/fungi/bacteria) generate membrane potential and store energy for cellular work.

Cotransport

• Cotransport is a form of active transport where the downhill diffusion of one solute drives the uphill transport of another.

• In plants, proton pumps generate H+ gradients used to drive sucrose uptake via cotransporters.

• In animals, Na+/glucose cotransporters in the intestine enable glucose absorption, especially important during diarrhea when sodium reabsorption is compromised.

Bulk Transport Across the Plasma Membrane

Exocytosis

• Exocytosis is the process by which cells export large molecules by fusing vesicles with the plasma membrane, releasing contents outside the cell.

• Example: Pancreatic cells secrete insulin via exocytosis.

Endocytosis

• Endocytosis is the process of taking in large molecules by forming vesicles from the plasma membrane.

• Types of endocytosis:

  • Phagocytosis ("cellular eating"): Cell engulfs particles into food vacuoles, which fuse with lysosomes for digestion.

  • Pinocytosis ("cellular drinking"): Cell "gulps" extracellular fluid into tiny vesicles; nonspecific for solutes.

  • Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

• Example: Human cells use receptor-mediated endocytosis to take in cholesterol via LDL particles. Defects in LDL receptors can lead to high blood cholesterol and cardiovascular disease.

Summary Table: Types of Membrane Transport

Key Equations

• Electrochemical Gradient:

  • The net movement of ions is determined by both the concentration gradient and the electrical potential across the membrane.

• Osmotic Pressure (π):

• Where is the van 't Hoff factor, is molarity, is the gas constant, and is temperature in Kelvin.

Additional info: These notes are based on Chapter 7 of Campbell Biology, covering the structure and function of biological membranes, including transport mechanisms and the role of membrane proteins and carbohydrates.