Membrane Structure and Function: Transport Across Cell Membranes

Membrane Structure and Function

Introduction

The plasma membrane is a dynamic structure composed primarily of lipids and proteins. It serves as a selective barrier, regulating the movement of substances into and out of the cell, and is essential for maintaining cellular homeostasis.

Key Concepts

• Cellular membranes are fluid mosaics of lipids and proteins.

• Membrane structure results in selective permeability, allowing some substances to cross more easily than others.

• Passive transport is the diffusion of a substance across a membrane without energy input.

• Active transport uses energy to move solutes against their gradients.

• Bulk transport moves large substances across the plasma membrane via exocytosis and endocytosis.

Membrane Structure

Lipids and Proteins in Membranes

• Phospholipids are the most abundant membrane lipids.

• A phospholipid is amphipathic: it has both a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

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

• Proteins determine most of the membrane's specific functions.

Phospholipid Bilayer

• Phospholipids form a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward.

• This arrangement creates a semi-permeable barrier.

Membrane Proteins

• Integral proteins penetrate the hydrophobic core of the bilayer; many are transmembrane proteins.

• Peripheral proteins are loosely bound to the membrane surface.

Six Major Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix (ECM)

Carbohydrates in Membranes

• Glycoproteins and glycolipids are involved in cell-cell recognition.

Selective Permeability of Membranes

What Can Cross the Membrane?

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

• Large polar molecules and ions require transport proteins.

Factors Affecting Membrane Permeability

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Cholesterol: Reduces membrane fluidity at high temperatures, prevents tight packing at low temperatures.

• Length and saturation of fatty acid tails: Unsaturated tails (with double bonds) increase fluidity; saturated tails decrease it.

Key Vocabulary

• Solvent: The liquid in which molecules are dissolved.

• Solute: The molecules dissolved in the solvent.

• Concentration: The amount of a given solute per volume of solvent.

• Gradient: A difference in concentration across the membrane.

Transport Across Membranes

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradients.

• Simple diffusion: Movement of molecules from high to low concentration until equilibrium is reached.

• Facilitated diffusion: Transport proteins (channels or carriers) help polar molecules and ions cross the membrane.

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

Osmosis and Tonicity

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

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, causing it to swell and possibly burst.

Water Balance in Cells

• Cells without cell walls (e.g., animal cells) are sensitive to tonicity and may burst or shrivel.

• Cells with cell walls (e.g., plant cells) become turgid in hypotonic solutions, flaccid in isotonic solutions, and plasmolyzed in hypertonic solutions.

Facilitated Diffusion: Role of Proteins

• Channel proteins: Provide corridors for specific molecules or ions to cross (e.g., aquaporins for water).

• Carrier proteins: Undergo shape changes to move molecules across the membrane.

Active Transport

Mechanism

Active transport requires energy (usually from ATP) to move substances against their concentration gradients.

• Pumps: Protein complexes that transport ions or molecules (e.g., sodium-potassium pump).

• Electrochemical gradient: Combination of concentration and electrical gradients across the membrane.

Examples

• Sodium-potassium pump: Exchanges Na+ out of the cell for K+ into the cell, maintaining membrane potential.

• Proton pump: Moves H+ ions across membranes, important in plants, fungi, and bacteria.

Bulk Transport

Endocytosis and Exocytosis

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

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

• Types of endocytosis:

  • Phagocytosis: "Cell eating"; engulfing large particles.

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

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

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions via channels
Osmosis	No	Down water potential gradient	Water via aquaporins
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	In or out (via vesicles)	Endocytosis, exocytosis

Key Equations

• Fick's Law of Diffusion: $J = -D \frac{dC}{dx}$ Where $J$ is the rate of diffusion, $D$ is the diffusion coefficient, $\frac{dC}{dx}$ is the concentration gradient.

Additional Info

• Osmoregulation is the control of water balance, crucial for cells in hypotonic or hypertonic environments.

• Membrane fluidity is essential for proper function; cholesterol and unsaturated fatty acids help maintain fluidity at various temperatures.