Topic 6 ~ Plant Structures and their Functions

• Producers


  Plants and algae are the main producers of food, which they synthesise from sunlight in a process called photosynthesis
• Photosynthesis


  The process where plants and algae synthesise food from sunlight
• Photosynthesis is an endothermic reaction, meaning that it takes in more energy than it releases
• Chloroplasts


  Organelles in leaves where light energy is transferred in photosynthesis
• Photosynthesis


  Carbon dioxide + water → glucose + oxygen
• Chemical symbols


  • Carbon dioxide: CO2
  • Water: H2O
  • Oxygen: O2
  • Glucose: C6H12O6
• Factors affecting rate of photosynthesis

  • Temperature
  • Light intensity
  • Carbon dioxide concentration
• Limiting factor


  An environmental condition that restricts any increase in the rate of photosynthesis, despite increases in other factors
• Measuring rate of photosynthesis using pondweed

  1. Pondweed in sealed test tube

  2. Capillary tube and syringe to measure oxygen production

  3. Variables that can be changed: temperature, time, light intensity
• Light intensity and rate of photosynthesis

  As light intensity increases, rate of photosynthesis increases
• Inverse square law


  Light intensity is inversely proportional to the square of the distance from the light source
• Root hair cells


  • Large surface area
  • Large permanent vacuole
  • Mitochondria for active transport
• Xylem cells


  • Lignified and hollow to transport water and minerals
  • Lignin deposited in spirals to withstand pressure
• Phloem cells


  • Sieve plates allow movement of substances
  • Alive with mitochondria in companion cells to provide energy for sucrose transport
• Transpiration


  Loss of water vapour from leaves and stems
• Guard cells


  • Kidney-shaped
  • Thin outer walls, thick inner walls
  • Open and close stomata
• Translocation


  Movement of food substances like sucrose in the phloem
• Leaf surfaces


  • Water molecules are attracted to each other, when some molecules leave the plant the rest are pulled up through the xylem
  • This results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant
• Guard cells


  • They close and open stomata
  • They are kidney shaped
  • They have thin outer walls and thick inner walls
  • When lots of water is available to the plant, the cells fill and change shape, opening stomata (they are also light sensitive)
  • This allows gases to be exchanged and more water to leave the plant via evaporation
  • More stomata are found on the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler
• Translocation


  The movement of food substances (such as sucrose) made in the leaves up or down the phloem, for use immediately or storage
• Translocation only occurs in the phloem, not the xylem or any other tissues in the plant
• Sources


  The places where sucrose is made
• Sinks


  The places where sucrose is used or stored
• The location of the sources and sinks can depend on the season
• Adaptations of the leaf

  • Stomata can close to minimise water loss and open to increase evaporation and transpiration, allowing gas exchange
  • Chlorophyll is green, which is the most efficient colour for absorbing light
  • Leaves are very thin, meaning that carbon dioxide only has a short distance to travel to enter the leaf (and work in photosynthesis) and oxygen only has a short distance to diffuse out
  • Having a large surface area means that the leaf can absorb more light at once, maximising the rate of photosynthesis
• Relative humidity


  The measure of the concentration of water vapour in the air in comparison to the total concentration of water that air can hold
• Increase in relative humidity

  Reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion, decreasing the rate of transpiration
• Increased air movement (wind)


  More air is moving away from the leaf, the concentration of water vapour surrounding the leaf will be lower, resulting in a steeper concentration gradient and faster diffusion, increasing the rate of transpiration
• Increase in light intensity

  Increased rate of photosynthesis, more stomata open to allow gaseous exchange, more water can evaporate, leading to an increased rate of transpiration
• Potometer


  Used to measure the uptake of water by a plant, giving an indication of the rate of transpiration
• Adaptations of plants to extreme environments

  • Leaf shape and size - many desert plants do not have leaves, or have very small leaves, reducing water loss from transpiration
  • Presence of a waxy cuticle - many leaves have a waxy cuticle on top, preventing evaporation of water
  • Stomata can be closed to prevent evaporation of water and opened when carbon dioxide is needed for photosynthesis
• Phototropism


  The response to light
• Gravitropism/Geotropism


  The response to gravity
• Positive phototropism


  Auxin, a growth hormone, moves to the shaded side of the shoot, stimulating cells to grow more there, causing the shoot to bend towards the light
• Negative gravitropism in shoots

  Auxin moves to the lower side, stimulating cells to grow more on that side, causing the shoot to bend and grow away from the ground
• Positive gravitropism in roots

  Auxin moves to the lower side, stimulating cells to grow more on the upper side, causing the root to bend and grow downwards
• Auxin


  Used as weed killers, rooting powders, and to promote growth in tissue culture
• Gibberellins


  Used to allow seed germination by breaking seed dormancy, allow fruits to grow heavier and larger, and encourage flowering plants to flower at a faster rate
• Ethene


  Controls ripening, used in the food industry to ripen fruit
• Producers


  Plants and algae are the main producers of food, which they synthesise from sunlight in a process called photosynthesis