Membrane Structure and Function – Study Notes (Campbell Biology, Ch. 7)

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 organization enable selective permeability, communication, and interaction with the environment.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Membrane Composition

• Phospholipids are the main component, forming a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins are embedded within or attached to the bilayer, contributing to membrane function.

• Carbohydrates are present on the extracellular surface, often attached to lipids (glycolipids) or proteins (glycoproteins).

Phospholipids are amphipathic, containing both hydrophobic and hydrophilic regions, which drives bilayer formation.

The Fluid Mosaic Model

The fluid mosaic model describes the membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer. Proteins are not randomly distributed but often form functional groups.

Membrane Fluidity

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

• Fluidity is affected by temperature and lipid composition:

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

  • 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 phospholipids in cold environments).

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral proteins: Loosely bound to the membrane surface.

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

• Hydrophobic regions of integral proteins are typically composed of nonpolar amino acids in α helices.

Protein Functions

• Transport: Move substances across the membrane.

• Enzymatic activity: Catalyze reactions at the membrane surface.

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

• Cell-cell recognition: Identify and interact with other cells.

• Intercellular joining: Connect adjacent cells.

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

Example: HIV enters immune cells by binding to the CD4 protein and the CCR5 co-receptor. Individuals lacking CCR5 are resistant to HIV infection.

The Role of Membrane Carbohydrates in Cell-Cell Recognition

Carbohydrates on the cell surface function as markers for cell identification. They are attached to lipids (glycolipids) or proteins (glycoproteins), and their diversity enables specific recognition among cells.

Synthesis and Sidedness of Membranes

Membranes are asymmetrical; the composition and distribution of proteins, lipids, and carbohydrates differ between the inner and outer faces, reflecting their distinct functions.

Selective Permeability of the Membrane

Lipid Bilayer Permeability

• Hydrophobic (nonpolar) molecules (e.g., hydrocarbons) pass through the membrane easily.

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

Transport Proteins

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

• Carrier proteins: Bind and transport specific molecules by changing shape.

• Transport proteins are selective for the substances they move (e.g., glucose carriers transport only glucose).

Passive Transport: Diffusion and Facilitated Diffusion

Simple Diffusion

Movement of molecules from high to low concentration, driven by the concentration gradient, without energy input.

Facilitated Diffusion

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

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

• Gated channels open or close in response to stimuli (electrical or chemical).

• Carrier proteins undergo conformational changes to move solutes down their concentration gradients.

Active Transport

Mechanism and Importance

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

• All active transport proteins are carrier proteins.

• Example: The sodium-potassium pump maintains high K+ and low Na+ inside animal cells by pumping Na+ out and K+ in, using ATP.

$\text{ATP} + \text{Na}^+_{\text{in}} + \text{K}^+_{\text{out}} \rightarrow \text{ADP} + \text{Na}^+_{\text{out}} + \text{K}^+_{\text{in}}$

Membrane Potential and Ion Pumps

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

• The electrochemical gradient combines the chemical gradient (concentration) and electrical gradient (charge).

• Electrogenic pumps generate membrane potential (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

Cotransport

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

• Example: In plants, a proton pump creates a gradient used to drive sucrose uptake via a cotransporter.

• In animals, sodium-glucose cotransporters in the intestine enable glucose absorption coupled to sodium reuptake.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

Transport vesicles fuse with the plasma membrane to release large molecules (e.g., proteins, polysaccharides) outside the cell. Example: Pancreatic cells secrete insulin via exocytosis.

Endocytosis

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

• Pinocytosis ("cellular drinking"): Cell takes in extracellular fluid and dissolved solutes via small vesicles; nonspecific process.

• Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation; allows selective uptake (e.g., cholesterol via LDL receptors).

Defects in receptor-mediated endocytosis (e.g., missing LDL receptors) can lead to diseases such as familial hypercholesterolemia.

Additional info: This guide covers the main concepts of membrane structure and function, including the molecular composition, fluid mosaic model, membrane protein roles, selective permeability, passive and active transport, and bulk transport mechanisms. These topics are essential for understanding cellular physiology and are foundational for further studies in cell biology, physiology, and biochemistry.