Transport Into & Out Of Cells (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Transport: Basics

• In order for any organism to function properly, it needs to exchange substances such as food molecules and waste products between itself and its environment

• This exchange of substances occurs across the cell membrane

• There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport

Diffusion

What is Diffusion?

• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration

• This means:

  • Particles in a solution or a gas are always moving about randomly

  • If there happen to be more particles in one area/region, then there tends to be an overall (net) movement of particles from this region of higher concentration to a region where there are fewer particles (an area of lower concentration)

Diffusion of perfume particles through a room is one of the simplest examples of diffusion

• Diffusion is an entirely passive process; the movement of particles in a fluid is dependent on how much kinetic energy they have

  • The higher the temperature, the more kinetic energy particles have and therefore the faster they can move by diffusion

Factors Affecting Diffusion

• You need to be able to explain how the following factors affect the rate of diffusion:

Diffusion factors

Osmosis

The Process of Osmosis

Osmosis occurs when two solutions are separated by a partially permeable membrane 

• The movement of water into and out of cells occurs by osmosis

• Osmosis in biology is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane

Active Transport

• Diffusion and osmosis rely upon the passive transport of substances down concentration gradients, relying on the random movement of particles

• However, cells also need to be able to transport substances across the cell membrane against a concentration gradient. This requires energy released by respiration

  • The energy from respiration is used by protein transport molecules embedded in the cell membrane to move substances into or out of the cell

• Active transport moves substances from a more dilute solution to a more concentrated solution

Active transport across the cell membrane involves protein carrier molecules embedded in the cell membrane

Active Transport: Examples

Plants

• Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)

• Mineral ions are needed by plants to function healthily

  • Magnesium ions are needed to make chlorophyll

  • Nitrate ions are needed to make amino acids and therefore for protein synthesis (and subsequently growth)

Animals

• Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream

• Active transport allows sugar such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher

  • Sugar molecules are used in respiration to release energy for cells to function