Biology

• Mitochondria produce energy through aerobic respiration.
• The nucleus is the control centre of the cell, containing DNA.
• Organisms

  • Must take in food, oxygen and water, and other essential substances, from the environment
  • Plants also need carbon dioxide for photosynthesis
  • Organisms also need to remove waste substances
• Exchange of essential and waste substances

  1. Small organisms exchange these substances between themselves and the environment

  2. They do this over their body surface

  3. Simple chemical substances can diffuse in and out of their bodies
• Inside their bodies, in small organisms, substances don't have to move far
• Volume

  Defines how much of these substances they need
• In multicellular organisms, surfaces and body organs are specialised for exchanging materials.

  The effectiveness of exchange surfaces in plants and animals is increased by having:

  A large surface area:

  • the flattened shape of structures such as leaves
  • the alveoli in the respiratory system
  • the villi in the digestive system

  A short distance required for diffusion:

  • the membranes of cells
  • the flattened shape of structures such as leaves
  • the walls of blood capillaries are one cell thick
  • the epithelia of alveoli in the respiratory system and the villi in the small intestine are only one cell thick
• Spongy mesophyll -The plant tissue in a leaf which has loosely packed cells and air spaces between them to allow gas exchange.
• Palisade mesophyll tissue- Plant tissue containing closely packed cells in the upper layer of a leaf.
• When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyll and palisade mesophyll evaporates and diffuses out of the leaf. This process is called transpiration
• Water transport in plants


  1. Water drawn from cells in xylem to replace that lost from leaves

  2. Water molecules in xylem strongly attracted to each other due to hydrogen bonding, creating a continuous column pulled up by evaporation from leaves

  3. Water taken up by roots replaces water travelling through xylem in stem and leaf
• Transpiration


  • Unavoidable consequence of photosynthesis
  • Only 5% of water taken up used for photosynthesis
  • Provides water for photosynthesis
  • Transports mineral ions
  • Cools the leaf as water evaporates
  • Provides water to keep cells turgid, supporting herbaceous plants