Movement across the plasma membrane:
- Osmosis: the movement of water molecules from high concentration to low concentration through a partially permeable membrane.
- Diffusion: the movement of molecules or ions from an area of their high concentration to an area of their low concentration.
- It will continue until the substance is evenly distributed throughout the whole volume.
- Small-uncharged molecules e.g. oxygen and carbon dioxide can diffuse across the cell membrane.
- Hydrophilic molecules and ions cannot penetrate the hydrophobic phospholipid tails.
- Diffusion is made easier, or facilitated, by proteins:
- Channel proteins span the membrane and have a specific shape to transport specific particles. Some are gated – they can be open or closed.
- Carrier proteins bind with the molecule or ion, change shape and transport the particle across the membrane. Movement can occur in either direction, depending on the concentration gradient.
- Diffusion, facilitated diffusion, and osmosis are passive – they do not require energy.
- Active transport: ATP supplies energy to change the shape of a carrier protein molecule when substances are moved against the concentration gradient i.e. from low to high concentration.
- Exocytosis involves the bulk transport of substances out of the cell e.g. insulin into the blood.
- Vesicles (little membrane sacs) fuse with the cell surface membrane and the contents are released.
- Endocytosis is the reverse: substances are taken into a cell by the creation of a vesicle.