Plant Structures and their Functions

CB6a
1. All organisms need energy
  1. Plants and algae (a type of protist) can trap energy transferred by light from the Sun
    1. This energy is then transferred to molecules of a sugar called glucose in a process called photosynthesis
      1. Glucose and substances made from glucose are stores of energy
        As glucose molecules are made, they are linked together to form a polymer called starch
        This stays in the chloroplasts until photosynthesis stops
        The starch is then broken down into simpler substances, which are moved into the cytoplasm and used to make sucrose
        Sucrose is moved around the plant and may be used to make: starch (in a storage organ such as potato), other molecules for the plant (such as cellulose, lipids or proteins) or glucose for respiration (to release energy)
      2. When animals eat plants, they get the energy from these stores
2. The materials in an organism are its biomass
  1. Plants and algae produce their own biomass and so produce the food for almost all other life on Earth
    1. They are the producers in food chains
3. Photosynthesis is a series of chemical reactions, catalysed (speeded up) by enzymes
  1. We can model the overall process using a word equation: carbon dioxide + water --> glucose + oxygen
  2. Photosynthesis occurs in chloroplasts, which contain a green substance called chlorophyll that traps energy transferred by light
    1. Since energy enters from the surroundings, the products of photosynthesis have more energy than the reactants and so this is an endothermic reaction
4. Leaves are often broad and flat, giving them a large surface area
  1. The palisade cells near the top of the leaf are packed with chloroplasts
    1. These adaptations allow a leaf to absorb a great deal of light
5. Carbon dioxide for photosynthesis comes from the air
  1. Leaves contain microscopic pores called stomata which allow carbon dioxide to diffuse into the leaf
    1. The stoma are opened and closed by specialised guard cells
    2. In the light, water flows into pairs of guard cells making them rigid, this opens the stomata, at night water flows out of the guard cells, they loose their rigidity and the stomata shuts
  2. Leaves are thin, carbon dioxide does not have far to diffuse into the leaf before reaching the cells that need it
    1. Stomata also allow the oxygen produced by photosynthesis to escape into the air, as wells as water vapour
      1. The flow of different substances in and out of a leaf is an example of gas exchange
CB6b
1. There are fewer molecules in each cubic centimetre of air at the top of a mountain than at the bottom
  1. This reduced concentration of air molecules causes a lower rate (speed) of photosynthesis in high mountains compared with sea level
2. Reactions in photosynthesis are catalysed by enzymes that work better at warmer temperatures
  1. High mountain areas are cold, which is another reason why photosynthesis is slower at the top of a mountain
3. A factor that prevents a rate increasing is called a limiting factor
  1. Limiting factors for photosynthesis can be carbon dioxide concentration, temperature and light intensity
4. The maximum rate of photosynthesis is controlled by the factor in shortest supply
CB6c
1. Some trees like buttress trees are adapted to living in the rain forest because they absorb water and dissolved mineral ions from the soil like most roots, but they also help stop the tree from falling and trap leaves and other dead vegetation which then provides more minerals for the tree
2. The water absorbed by plant roots is used for carrying dissolved mineral ions, keeping cells rigid (otherwise the plants wilt), cooling the leaves (when it evaporates) and photosynthesis
3. The outer surfaces of many roots are covered with root hair cells, these are extensions of the cell that provide a large surface area so water and mineral ions can be absorbed quicker
  1. The cells also have thin cell walls so that the flow of water into the cells is not slowed down
4. A certain volume containing more molecules of a substance than another identical volume has a greater concentration of the molecules
  1. If the two spaces are connected, there will be a concentration gradient from higher concentration to lower concentration
5. Particles constantly move in random directions and so particles in a fluid spread down a concentration gradient, this is diffusion
  1. Inside plant roots, the cell walls have an open structure allowing water particles to diffuse towards the middle of the root (from where there are more of them to fewer)
6. Osmosis is when solvent molecules (such as water) diffuse through a semi-permeable membrane
  1. They diffuse from where there are more of them (a dilute solution of solutes) to where there are fewer (a more concentrated solution)
    1. Cell membranes are semi-permeable and so water passes into the cytoplasm of root hair cells by osmosis
7. Mineral salts are naturally occurring ionic compounds, plants need ions from these compounds to produce new substances
  1. For example, nitrate ions are needed to make proteins
  2. The concentration of ions inside a root hair cell is greater than in the soil
    1. Mineral ions cannot diffuse against this concentration gradient
      1. So, proteins in the cell membrane pump the ions into the cell, this is an example of active transport
CB6d
1. The evaporation of water from leaves keeps them cool and helps move water (and dissolved mineral ions) up the plant
  1. The flow of water into a root, up the stem and out of the leaves is called transpiration
2. Xylem vessels form tiny continuous pipes leading from a plant's roots up into it leaves
  1. Inside the vessels is an unbroken chain of water, due to the weak forces of attraction between water molecules
    1. Water is pulled up the xylem vessel in the stem as water evaporates from the xylem vessels in the leaves
      1. As the water vapour diffuses out of a leaf, more water evaporates from the xylem inside the leaf
3. The concentration of water vapour in the air spaces inside a leaf is greater than outside it, so, water molecules diffuse down the concentration gradient, out of the leaf
  1. A bigger difference between the concentrations makes the gradient steeper, which makes diffusion faster
4. During their development, xylem cells die and their top and bottom cell walls disintegrate
  1. This creates long empty vessels (tubes) through which water can move easily
    1. Xylem vessels are rigid because they have thick side walls and rings of hard lignin, and so water pressure inside the vessels does not burst or collapse them
      1. The rigid xylem vessels also helps to support the plants
5. Plants make sucrose from the glucose and starch made by photosynthesis
  1. Sucrose is translocated (transported) in the sieve tube of the phloem tissue

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	Created by Emily Chase about 5 years ago