Movement of Substances into and out of Cells

Movement of Substances into and out of Cells


  • Diffusion is a method of transporting materials in and out of cells and it is driven by a concentration gradient.
  • It is a passive process, which means it does not require energy from the cell.
  • It involves the movement of particles from an area of higher concentration to an area of lower concentration until they are evenly distributed.
  • Oxygen and carbon dioxide are often transported by diffusion.


  • Osmosis is a special type of diffusion involving the movement of water molecules across a selectively permeable membrane.
  • Like diffusion, it is a passive process and doesn’t require energy, instead it relies on differences in water concentration.
  • Water moves from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration) through the cell membrane.
  • Osmosis is critical for the absorption of water in plant roots and the absorption and release of water in all cells.

Active Transport

  • In contrast to diffusion and osmosis, active transport does require energy to move substances across cell membranes.
  • This energy is usually sourced from ATP, the cell’s energy storage molecule.
  • Active transport mechanisms move substances from an area of lower concentration to an area of higher concentration, working against the concentration gradient.
  • Active transport is necessary when the cell needs substances that are not in abundance in the environment, such as glucose or ions.

Factors Affecting the Rate of Transport

  • The rate of substance transport into and out of cells is influenced by a number of factors: temperature, surface area, length of diffusion paths and concentration gradients.
  • Higher temperatures increase the rate because the particles have more energy and move more quickly.
  • Greater surface area of the cell membrane also increases the rate as there is more area for substances to move across.
  • Shorter diffusion paths increase the rate because the substances have less distance to travel.
  • A steeper concentration gradient increases the rate because the difference between concentrations inside and outside the cell is greater, driving faster movement.