Diffusion, Osmosis & Active Transport (Edexcel IGCSE Biology: Double Science)

• Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration
  • Molecules are said to move down a concentration gradient 
• Note that the movement of molecules is random, but the result of this random movement is the spreading out of molecules until they are at even concentration throughout the available space

Diffusion in living organisms

• Molecules can move into or out of living cells by diffusion; this involves molecules moving across the cell membrane
• The cell membrane is a partially permeable membrane, meaning that it allows some molecules to cross easily, but others with difficulty or not at all
  • E.g. smaller molecules can diffuse across the membrane but larger molecules cannot
• Diffusion helps living organisms to:
  • Gain nutrients and oxygen
  • Remove waste products

Diffusion into living cells occurs across the cell membrane

Examples of diffusion in living organisms table

• Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane
  • Osmosis is the diffusion of water, as the water is moving down its concentration gradient
• Partially permeable membranes prevent the movement of larger molecules, e.g. sugars, but allow the movement of small water molecules

Osmosis is the diffusion of water molecules across a partially permeable membrane

• Animal cells lose and gain water as a result of osmosis
• As animal cells do not have a supporting cell wall the results of osmosis can be severe:
  • If an animal cell is placed into a strong sugar solution (with a lower water concentration than the cell), it will lose water by osmosis and become crenated (shrivelled up)
  • If an animal cell is placed into distilled water (with a higher water concentration than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure
    • It will continue to gain water until the cell membrane is stretched too far and it bursts

Effect of osmosis on animal cells

• Plant cells lose or gain water as a result of osmosis
• As plant cells have a supporting cell wall, they are protected from cell lysis:
  • If a plant cell is placed into a strong sugar solution (with a lower water concentration than the cell), it will lose water by osmosis
    • The vacuole gets smaller and the cell membrane shrinks away from the cell wall
    • The cell becomes flaccid or plasmolysed
  • If a plant cell is placed into distilled water (with a higher water concentration than the cell), it will gain water by osmosis
    • The vacuole gets bigger, pushing the cell membrane against the cell wall
    • The plant cell is described as being turgid
    • Turgid cells provide structural support for plants, preventing them from wilting

The effect of osmosis on plant cells

• Active transport can be defined as:

The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration

• Energy is needed for active transport because particles are being moved against a concentration gradient
  • Energy is released during cellular respiration
• Active transport across the cell membrane involves protein carrier molecules embedded in the cell membrane

Active transport involves movement against a concentration gradient, and so requires energy

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 molecules 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
• The active uptake of glucose by epithelial cells in kidney tubules in the kidney nephron allows for the reabsorption of glucose back into the blood so that none is lost in the urine
• Sugar molecules are used in respiration to release energy for cells to function

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