Plant Structures & their Functions

• What is the equation for Photosynthesis ?

  carbon dioxide + water -> glucose + oxygen
• Photosynthesis is an endothermic reaction.
• The rate of photosynthesis is affected by light intensity , the concentration of CO2 and the temperature. Any of these three factors can become a limiting factor.
• Canadian pondweed can be used to measure the effect of light intensity on the rate of photosynthesis. The rate at which the pondweed produces oxygen corresponds to the rate at which it is photosynthesising - the faster the rate of oxygen production, the faster the rate of photosynthesis.
• Method to investigate the effect of light intensity on the rate of photosynthesis:

  • A source of white light is placed at a specific distance from the pondweed in the water.
  • The pondweed is left to photosynthesise for a set amount of time.
  • As it photosynthesises, the oxygen released will collect in the gas syringe. This allows you to accurately measure the volume of oxygen produced.
  • The whole experiment is repeated with the light source at different distances from the pondweed. The rate of oxygen production at each distance can then be calculated (volume produced / time taken).
• What are the variables for the investigation of the effect of light intensity on the rate of photosynthesis?

  temperature and the carbon dioxide concentration must be controlled
• Not enough light slows down the rate of photosynthesis.
• Too little carbon dioxide slows the rate of photosynthesis down.
• The temperature must be kept constant to keep the rate of photosynthesis constant.
• Root hairs take in minerals and water.

  The concentration of mineral ions is usually higher in the root hair cells than in the soil around them, so mineral ions are absorbed by active transport. Water is absorbed by osmosis.
• Phloem tubes are made of columns of elongated living cells with small pores in the end walls to allow things to flow through. They transport food substances made in the leaves to the rest of the plant for immediate use or for storage.

  This process is called translocation and it requires energy from respiration. The transport goes in both directions.
• Xylem tubes are made of dead cells joined end to end with no end walls between them and a hole down the middle. They are strengthened with a material called lignin.

  They carry water and mineral ions from the roots to the stem and leaves.

  The movement of water from the roots, through the Xylem and out of the leaves is called the transpiration stream.
• Transpiration is the loss of water from the plant.

  It is caused by the evaporation and diffusion of water from a plant's surface. Most transpiration happens in the leaves.
• Stomata are tiny pores on the surface of a plant. They are mostly found on the lower surface of the leaves.

  Stomata allow carbon dioxide and oxygen to diffuse in and out of a leaf. They also allow water vapour to escape during transpiration.
• Stomata are surrounded by guard cells, which change shape to control the size of the pore - when the guard cells are swollen with water, the stomata are open and when the guard cells are low on water, the stomata are closed.
• Light Intensity - the brighter the light, the greater the transpiration rate.
• Temperature - the warmer it is, the faster transpiration happens.
• Air Flow - the better the air flow around a leaf, the greater the transpiration rate.
• Leaves are broad, so there is a large surface area exposed to light, which is needed for photosynthesis.
• The palisade layer has lots of chloroplasts. This means that they are near the top of the leaf where they can get the most light.
• The upper epidermis is transparent so that light can pass through it to the palisade layer.
• The Xylem and Phloem provide a leaf with water for photosynthesis and take away the glucose produced. They also help support the structure.
• The epidermis tissues are covered with a waxy cuticle, which helps to reduce water loss by evaporation.
• The tissues of leaves are also adapted for efficient gas exchange.
• The spongy mesophyll tissue contains air spaces which increase the rate of diffusion of gases into and out of the leaf's cells.
• Small leaves, or spines instead of leaves - This reduces the surface area for water loss by evaporation. Spines also help to stop animals eating the plant to get water.
• Curled leaves, or hairs on the surface of leaves - this reduces air flow close to the leaf, trapping wate vapour near the surface and reducing diffusion form the leaf to the air.
• Thick waxy cuticles - to reduce water loss by evaporation.
• A thick fleshy stem - stores water.
• Stomata sunken in pits - this makes the stomata lower than the surface of the leaf, which reduces air flow close to the stomata, also reducing water loss.
• Auxins are plant hormones which control growth at the tips of shoots and roots. They move through the plant in solution.
• Shoots are positively phototropic (grows towards the light).
• Shoots are negatively gravitropic (grow away from gravity).
• Roots are positively gravitropic (grow towards gravity).
• Roots are negatively phototropic (grow away from light).
• What are the 6 different commercial uses of plant hormones ?

  selective weedkillers

  growing from cuttings with rooting powder

  controlling flower and fruit formation

  producing seedless fruit

  controlling the ripening of fruits

  controlling seed germination