Membrane Transport (College Board AP® Biology): Study Guide

Introduction to membrane transport

• There are several processes by which ions and molecules can cross cell membranes; the mechanism required will depend on factors such as:

  • the substance to be transported, e.g. whether it is small, large, polar or nonpolar

  • the concentration gradient across the membrane

• Mechanisms of membrane transport include:

  • passive transport, e.g.

    • diffusion

    • osmosis

  • active transport, including endocytosis & exocytosis

Passive transport

• Passive transport can be described as:

the net movement of molecules from high concentration to low concentration without the direct input of metabolic energy

• Examples of passive transport include:

  • simple diffusion: the movement of molecules, down a concentration gradient, directly across the phospholipid bilayer

  • facilitated diffusion: the movement of molecules, down a concentration gradient, via specialized transport proteins

  • osmosis: the movement of water, down a water potential gradient, either between the phospholipids or via aquaporins

• Passive transport plays a primary role in the import of useful materials and the export of metabolic waste from living organisms

  • E.g. gas exchange in mammalian lungs involves the passive uptake of oxygen and removal of carbon dioxide by diffusion

Active transport

• Active transport is:

the direct input of energy to move molecules from regions of low concentration to regions of high concentration

• Energy from respiration, in the form of ATP, is required for active transport

• Active transport always occurs across a membrane, and involves the use of carrier proteins

Establishing and maintaining concentration gradients: Na⁺/K⁺ ATPase

• Active transport of molecules and/or ions across membranes allows the establishment and maintenance of concentration gradients; Na⁺/K⁺ ATPase is an example of a transport protein with this role

• Na⁺/K⁺ ATPase uses energy from ATP to pump sodium and potassium ions across the membranes of nerve cells; this contributes to the maintenance of membrane potential in neurones

  • Generation and transmission of nerve impulses occurs due to changes in neurone membrane potential

• Na⁺/K⁺ ATPase pumps sodium ions out of nerve cell axons and potassium ions in via active transport:

  1. 3 sodium ions from the inside of the axon bind to the pump

  2. ATP attaches to the pump and transfers a phosphate to the pump, causing it to change shape and resulting in the pump opening to the outside of the axon

  3. The 3 sodium ions are released out of the axon

  4. 2 potassium ions from outside the axon bind to their binding sites

  5. The attached phosphate is released, altering the shape of the pump again

  6. The change in shape causes the potassium ions to be released inside the axon