4.1

• Photosynthesis is the process by which plants synthesise glucose using light energy from the Sun. Light energy is converted into chemical energy
• Photosynthesis takes place within chloroplasts in leaf palisade cells. They contain chlorophyll, a pigment which absorbs light energy
• Equations for photosynthesis:

  • Word: carbon dioxide + water → glucose + oxygen
  • Symbol: 6CO2 + 6H2O → C6H12O6 + 6O2 light
• Photosynthesis is an endothermic reaction because energy is transferred from the environment to chloroplasts by light
• To show that a plant gives off oxygen during photosynthesis, use a water plant (eg. Elodea) to collect gas bubbles produced. The gas will relight a glowing splint as it contains oxygen
• Leaf adaptations that maximise the rate of photosynthesis:

  • Broad leaves to maximise surface area
  • Thin leaves for a short diffusion distance
  • Chlorophyll present to trap light energy
  • Veins to transport water to leaves via xylem and remove photosynthesis products via phloem
  • Air spaces to allow CO2 to enter and O2 to leave
  • Guard cells to control opening of stomata for gaseous exchange and prevent water loss
• Four main factors that affect the rate of photosynthesis:

  • Temperature
  • Light intensity
  • Carbon dioxide concentration
  • Amount of chlorophyll
• Temperature affects the rate of photosynthesis by increasing it, which increases the rate until enzymes become denatured past a certain temperature
• Light intensity affects the rate of photosynthesis by increasing it, which increases the rate until another factor becomes limiting
• Carbon dioxide concentration affects the rate of photosynthesis by increasing it, which increases the rate until another factor becomes limiting as CO2 is required to make glucose
• The amount of chlorophyll affects the rate of photosynthesis by decreasing it, which decreases the rate as chlorophyll is required to absorb light energy
• A limiting factor is an environmental factor that can restrict the rate of photosynthesis, such as light intensity
• To calculate the rate of photosynthesis by measuring oxygen production, set up a bubble potometer apparatus with pondweed in a sealed tube of water, attached to a capillary tube and a gas syringe. Oxygen gas produced causes the bubble in the capillary tube to move, and the distance moved by the bubble is used to calculate the volume of oxygen produced
• Farmers can increase their profits by controlling temperature, light intensity, and CO2 concentration to achieve the fastest possible rate of photosynthesis, leading to a greater yield
• The law that describes the relationship between the distance of a light source from a plant and light intensity is the Inverse square law - light intensity ∝ 1 / distance^2
• Five uses of the glucose produced during photosynthesis:

  • Respiration
  • Starch for storage
  • Cellulose for strength
  • Amino acid and protein synthesis (combined with nitrates)
  • Lipids for energy storage in seeds