Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient.Particles in liquids and gases move randomly because of their kinetic energy. More particles move from the concentrated region to the dilute region than the other way, so there is a net movement down the gradient. Diffusion is passive: the cell uses no ATP.When the concentrations become equal, particles still move but in equal numbers in both directions, so there is no net movement. This is equilibrium.Across a cell membrane, simple diffusion occurs through the phospholipid bilayer. It works for:
• small molecules, such as oxygen and carbon dioxide;
• non-polar and lipid-soluble molecules, which dissolve in the hydrophobic fatty acid tails of the phospholipids.Charged ions and large polar molecules cannot diffuse through the bilayer.
The rate of diffusion across a membrane depends on:
• the concentration gradient: steeper gradient, faster diffusion;
• temperature: higher temperature gives particles more kinetic energy, so the rate increases;
• surface area of the membrane: larger area, faster diffusion;
• diffusion distance: thinner membrane, faster diffusion;
• size and lipid solubility of the particle: small, lipid-soluble particles diffuse fastest.Cells keep gradients steep by using or removing substances. For example, respiration uses oxygen, keeping the oxygen concentration inside the cell low, so oxygen keeps diffusing in.
• small molecules, such as oxygen and carbon dioxide;
• non-polar and lipid-soluble molecules, which dissolve in the hydrophobic fatty acid tails of the phospholipids.Charged ions and large polar molecules cannot diffuse through the bilayer.
The rate of diffusion across a membrane depends on:• the concentration gradient: steeper gradient, faster diffusion;
• temperature: higher temperature gives particles more kinetic energy, so the rate increases;
• surface area of the membrane: larger area, faster diffusion;
• diffusion distance: thinner membrane, faster diffusion;
• size and lipid solubility of the particle: small, lipid-soluble particles diffuse fastest.Cells keep gradients steep by using or removing substances. For example, respiration uses oxygen, keeping the oxygen concentration inside the cell low, so oxygen keeps diffusing in.