Chapter 6

• 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


  The organelles in leaves where light energy is transferred to
• 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 (such as light intensity) which, in low levels, restricts any increase in the rate of photosynthesis

  
• Measuring oxygen production to calculate rate of photosynthesis

  1. Pondweed in sealed test tube with capillary tube

  2. Lamp at measured distance

  3. Bubble movement in capillary tube measured to calculate oxygen produced

  
• Controlling all factors except the independent variable is important for a valid experiment
• Light intensity is the limiting factor


  The rate of photosynthesis levels off at higher light intensities
• Temperature is the limiting factor


  The rate of photosynthesis levels off at higher temperatures

  
• Farmers can use knowledge of limiting factors to enhance greenhouse conditions for greater photosynthesis and growth
• Core practical: Measuring rate of photosynthesis with light intensity


  1. Use sealed flask with pondweed, gas syringe, and lamp

  2. Measure gas volume change at different lamp distances

  
• 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


  The loss of water vapour from leaves and stems
• Guard cells


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


  The movement of food substances like sucrose made in the leaves up or down 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 on leaves, 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