The cell membrane consists of

• phospholipids
• and proteins

and is selectively permeable.

Transport

• Passive transport occurs down a concentration gradient and does not require energy.
• Examples of passive transport are diffusion and osmosis.
• Different concentrations of substances exist between cells and their environment.

• Diffusion is the movement of molecules down a concentration gradient from a higher to a lower concentration.
• Molecules such as glucose, oxygen and carbon dioxide move into or out of cells via the cell membrane.

• Osmosis is the movement of water molecules from a higher water concentration to a lower water concentration through a selectively permeable membrane.
• The concentration gradient is the difference between the concentration inside a cell compared with outside a cell.
• Animal cells can burst or shrink and plant cells can become turgid or plasmolysed.

Active transport requires energy for membrane proteins to move molecules and ions against the concentration gradient (from lower to higher concentration).

Reminder:

Osmosis and Diffusion are passive transport and occur down a concentration gradient and do not require energy.

Topic Summary