Transport Across Biological Membranes

Introduction

Cells regulate the movement of substances across their plasma membranes to maintain homeostasis. This movement can occur through several mechanisms, including passive transport, facilitated diffusion, and active transport. Each mechanism differs in energy requirements, directionality, and the involvement of membrane proteins.

Passive Transport

Definition and Overview

• Passive transport is the movement of substances across a biological membrane without the expenditure of cellular energy (ATP).

• Substances move down their concentration gradient, from regions of higher concentration to regions of lower concentration.

• Examples include simple diffusion and osmosis.

Diffusion

• Diffusion is the net movement of molecules of any substance so that they spread out evenly into the available space.

• Each molecule moves randomly, but the overall movement is directional until equilibrium is reached.

• At dynamic equilibrium, molecules continue to move, but there is no net change in concentration on either side of the membrane.

• The concentration gradient is the region along which the density of a chemical substance increases or decreases.

• The rate of diffusion depends on factors such as the concentration difference, temperature, molecular size, and membrane permeability.

Example: Oxygen and carbon dioxide gases diffuse across cell membranes during respiration.

Osmosis

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

• Water moves from an area of lower solute concentration (more free water molecules) to an area of higher solute concentration (fewer free water molecules).

• Osmosis is crucial for maintaining cell turgor and volume.

Example: Plant cells absorb water from the soil by osmosis.

Tonicity and Water Balance

• Tonicity describes the ability of a surrounding 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: Solute concentration is higher outside the cell; cell loses water and may shrink (crenate in animal cells, plasmolyze in plant cells).

• Hypotonic solution: Solute concentration is lower outside the cell; cell gains water and may swell or burst (lyse in animal cells, become turgid in plant cells).

Facilitated Diffusion

Definition and Mechanism

• Facilitated diffusion is the passive movement of molecules across the membrane with the help of specific transport proteins.

• It allows substances that cannot directly diffuse through the lipid bilayer (such as ions and polar molecules) to move down their concentration gradient.

• No energy input is required.

Types of Transport Proteins

• Channel proteins: Provide hydrophilic pathways for specific molecules or ions to cross the membrane. Examples include aquaporins (for water) and ion channels (for Na+, K+, Cl-).

• Carrier proteins: Bind to specific molecules and undergo conformational changes to shuttle them across the membrane.

• Some channels are gated, opening or closing in response to stimuli (e.g., voltage-gated ion channels in neurons).

Example: Glucose transporters facilitate the movement of glucose into cells.

Active Transport

Definition and Overview

• Active transport is the movement of substances across a membrane against their concentration gradient (from low to high concentration), requiring energy input, usually from ATP.

• Active transport is essential for maintaining concentration differences of ions and other substances across membranes.

Mechanism of Active Transport

• Transport proteins called pumps are involved in active transport.

• The sodium-potassium pump (Na+/K+-ATPase) is a classic example, pumping 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.

• This process creates an electrochemical gradient and contributes to the membrane potential (voltage across the membrane).

Equation for ATP hydrolysis:

Types of Active Transport Proteins

Electrogenic Pumps and Membrane Potential

• An electrogenic pump is a transport protein that generates voltage across a membrane.

• In animal cells, the main electrogenic pump is the sodium-potassium pump.

• In plants, fungi, and bacteria, the proton pump (H+-ATPase) is the main electrogenic pump, moving protons out of the cell.

• The membrane potential is typically about -70 mV in animal cells, with the inside of the cell being negative relative to the outside.

Coupled Transport (Cotransport)

• Cotransport occurs when the active transport of one solute indirectly drives the transport of another solute.

• For example, the proton gradient created by the proton pump can drive the uptake of sucrose into plant cells via a sucrose-H+ cotransporter.

• Similarly, in animal intestines, the Na+-glucose symporter uses the Na+ gradient to import glucose against its concentration gradient.

Example: Sucrose loading into plant phloem cells for transport throughout the plant.

Summary Table: Types of Membrane Transport

Additional info: The notes also reference the importance of membrane permeability, the role of aquaporins in water transport, and the physiological significance of maintaining ion gradients for processes such as nerve impulse transmission and osmoregulation.