Transport Across Cell Membranes: Diffusion in a Snap! Unlock the full A-level Biology course at [ Ссылка ] created by Adam Tildesley, Biology expert at SnapRevise and graduate of Cambridge University.
The key points covered of this video include:
1. Passive Transport
2. Simple Diffusion
3. Diffusion across Cell Membranes
Passive Transport
In order to sustain life, there must be exchange of substances between cells and the environment. Some substances can be exchanged across cell membranes without requiring metabolic energy from the cell - this is passive transport. These substances are able to move across the cell membranes due to the natural motion of particles - they only use kinetic energy.
Simple Diffusion
One type of passive transport is simple diffusion. Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Diffusion occurs because all particles have kinetic energy - this means they can move freely, randomly changing direction when they collide. In order for diffusion to occur, there must be a concentration gradient of that substance. Where there is a high concentration of particles, the random movement of particles will result in the particles colliding and moving away from each other. This eventually results in a net movement of particles from high to low concentration until they are evenly dispersed - this is equilibrium. At equilibrium, particles are still moving freely and randomly but there is no net movement - there is equal movement in all directions.
Simple Diffusion Across Membranes
Simple diffusion across cell membranes requires particles to pass through the phospholipid bilayer. Equilibrium is reached when the concentration of particles is equal on both sides of the cell membrane so there is no net movement. Only specific molecules are able to easily do this - these molecules must be small and non-polar. Small polar molecules with small differences in charge are able to slowly diffuse across the phospholipid bilayer. Charged particles (ions) and larger molecules are not able to pass through the phospholipid bilayer. The cell membrane is therefore described as partially permeable.
Summary
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration
Diffusion ends when the concentrations become equal - this is called equilibrium and there is no net movement
Diffusion is a passive process and does not require any metabolic energy from a cell - it only uses kinetic energy
Only small, non-polar molecules are able to cross a cell membrane by simple diffusion
