CB6

• Photosynthesis


  The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen
• Plants need energy from light to carry out photosynthesis
• Limiting factors of photosynthesis


  • Carbon dioxide concentration
  • Temperature
  • Light intensity
• As light intensity increases


  The rate of photosynthesis increases
• Mountain plants like edelweiss are smaller because photosynthesis is slower at higher altitudes
• Rate of photosynthesis


  The speed at which photosynthesis occurs
• There are fewer air molecules per cubic centimetre at the top of a mountain compared to the bottom, reducing the rate of photosynthesis
• Enzymes in photosynthesis work better at warmer temperatures, so photosynthesis is slower at the colder temperatures found at higher altitudes
• Factors that affect the rate of photosynthesis


  1. Carbon dioxide concentration

  2. Temperature

  3. Light intensity
• Limiting factor


  A factor that prevents the rate of a process from increasing further
• The maximum rate of photosynthesis is controlled by the limiting factor in shortest supply
• Being grown in water ('hydroponically') means water is never a limiting factor for photosynthesis
• Being in a greenhouse allows control of temperature, light intensity and carbon dioxide concentration to maximise photosynthesis
• As light intensity increases


  The rate of photosynthesis increases until another factor becomes limiting
• Increasing the temperature


  Allows higher rates of photosynthesis if temperature is the limiting factor
• A straight line on a graph shows a linear relationship between two variables, and if the line goes through the origin it shows the variables are in direct proportion
• Inverse square law


  Light intensity is inversely proportional to the distance from the light source squared
• If the distance from a light source doubles


  The light intensity is reduced to a quarter of the original
• If the distance from a light source halves


  The light intensity is increased by a factor of 4
• In a commercial greenhouse, lights switch on at 17:00 in winter and heating comes on if the inside temperature is below 21°C
• Algae are single-celled protists and their cells contain chloroplasts, allowing them to photosynthesize in the same way as plants
• Change in pH/hour


  A measure of the rate of photosynthesis
• The dependent variable in the experiment is the rate of photosynthesis
• The independent variable in the experiment is the light intensity
• Temperature is a control variable in the experiment because it needs to be kept constant
• Two other control variables in the experiment are the amount of indicator solution and the number of algal balls
• The hydrogen carbonate indicator ensures there is plenty of carbon dioxide dissolved in the solution around the algal balls
• The tube wrapped in kitchen foil is a control to show the rate of photosynthesis in the dark
• The gas produced by the pondweed in the bubbles is oxygen
• Plants lose water through transpiration
• Osmosis


  The diffusion of water molecules through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration
• Water evaporates from the leaves of a plant, causing the flow of water up the plant known as transpiration
• A tree does not lose much water at night because transpiration is reduced when the stomata are closed
• Transpiration helps plants by moving water and dissolved minerals up the plant, and cooling the leaves
• Xylem vessels form continuous pipes leading from the roots to the leaves, allowing an unbroken chain of water to be pulled up by transpiration
• As water evaporates from the xylem in the leaves, more water is pulled up the xylem in the stem
• Sucrose is translocated around the plant in the phloem tissue
• Xylem vessels form tiny continuous pipes leading from a plant's roots up into its leaves. Inside the vessels is an unbroken chain of water, due to the weak forces of attraction between water molecules. Water is pulled up the xylem vessels in the stem as water evaporates from the xylem vessels in the leaves. As the water vapour diffuses out of a leaf, more water evaporates from the xylem inside the leaf.
• We can investigate the factors affecting transpiration using a potometer. The air bubble moves along the tube as the plant loses water. The speed of the bubble gives a measure of the rate of transpiration (e.g. in mm/min).
• 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. A bigger difference between the concentrations makes the gradient steeper, which makes diffusion faster. So, any factor that reduces the concentration of water molecules outside the stomata will increase transpiration. Factors include: wind - moves water molecules away from stomata, low humidity (little water vapour in the air).