Membrane Structure and Transport

Introduction

Cell membranes are essential for maintaining the internal environment of the cell by regulating the movement of substances in and out. This chapter explores the structure of biological membranes and the mechanisms by which materials are transported across them, focusing on both passive and active processes.

Passive Transport Across Membranes

Overview of Passive Transport

Passive transport is the movement of substances across a cell membrane without the expenditure of cellular energy. It relies on the natural motion of molecules and the presence of concentration gradients.

• Selective Permeability: Biological membranes are selectively permeable, allowing some substances to cross more easily than others.

• Concentration Gradient: The difference in concentration of a substance across a space or membrane. This gradient provides potential energy that drives diffusion.

• No Energy Required: Passive transport does not require input of cellular energy (ATP).

Simple Diffusion

Simple diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached.

• Definition: The tendency for molecules to spread out evenly into the available space.

• Direction: Movement is always down the concentration gradient.

• Example: The diffusion of dye in water until the color is evenly distributed.

Membrane Permeability

The structure of the cell membrane determines which substances can cross it easily.

• Hydrophobic Interior: The major barrier to crossing a biological membrane is the hydrophobic (nonpolar) interior of the phospholipid bilayer.

• Substances That Cannot Cross Easily:

  • Polar molecules (e.g., water, H2O)

  • Ions (e.g., Na+, Cl-)

  • Amino acids

  • Sugars (e.g., glucose)

• Substances That Can Cross Easily:

  • Nonpolar molecules (e.g., hydrocarbons, O2, CO2)

Facilitated Diffusion

Facilitated diffusion is a type of passive transport that uses membrane proteins to help certain molecules cross the membrane.

• Transport Proteins: Two main types are involved:

  • Channel Proteins: Form hydrophilic passageways for specific molecules or ions.

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

• Examples: Aquaporins (water channels), ion channels, glucose transporters.

• Requirement: There must be a concentration gradient for facilitated diffusion to occur.

Osmosis and Tonicity

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

• Solvent: Water is the solvent in biological systems.

• Solutes: Dissolved substances such as salts, sugars, and proteins.

• Aquaporins: Specialized channel proteins that facilitate the rapid movement of water across membranes.

Tonicity and Its Effects on Cells

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

Osmotic Pressure: The pressure required to stop the net flow of water across a membrane due to osmosis.

Osmoregulation

Osmoregulation is the control of water balance in organisms without rigid cell walls.

• Adaptations: Contractile vacuoles in protists (e.g., Paramecium), regulation of urine concentration in fish.

• Examples:

  • Freshwater fish excrete dilute urine and actively take up salts.

  • Saltwater fish excrete concentrated urine and actively expel salts.

Active Transport Across Membranes

Overview of Active Transport

Active transport is the movement of substances across a membrane against their concentration gradient, requiring energy input, usually from ATP.

• Carrier Proteins: All active transport mechanisms use carrier proteins.

• Types of Transporters:

  • Uniporters: Move one molecule at a time.

  • Symporters: Move two molecules in the same direction.

  • Antiporters: Move two molecules in opposite directions.

Sodium-Potassium Pump (Na+/K+ Pump)

The sodium-potassium pump is a classic example of active transport in animal cells.

• Function: Moves 3 Na+ ions out of the cell and 2 K+ ions into the cell against their concentration gradients.

• Importance: Maintains membrane potential and proper cell function.

Equation:

Membrane Potential

Membrane potential is the voltage difference across a membrane, created by the unequal distribution of ions.

• Cytoplasmic Side: Negative relative to the extracellular side.

• Role: Drives the passive transport of cations into and anions out of the cell.

Cotransport

Cotransport involves the coupling of the movement of one substance down its concentration gradient to the movement of another substance against its gradient.

• Example: Na+-glucose cotransporter in intestinal cells uses the Na+ gradient (established by the Na+/K+ pump) to drive glucose uptake.

Bulk Transport

Endocytosis

Endocytosis is the process by which cells take in large molecules or particles by engulfing them in vesicles.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

• Receptor-Mediated Endocytosis: Uptake of specific molecules after they bind to cell surface receptors (e.g., cholesterol uptake via LDL receptors).

Exocytosis

Exocytosis is the process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Functions: Secretion of hormones, neurotransmitters, digestive enzymes, and cell wall materials.

• Examples: Pancreatic cells secrete insulin; nerve cells release neurotransmitters.

Summary Table: Types of Membrane Transport