Photosynthesis and leaf structure

• Photosynthesis


  The process by which plants manufacture carbohydrates from raw materials using energy from light
• Photosynthesis


  Carbon dioxide + Water → Carbohydrate + Oxygen
• Chlorophyll


  A green pigment found in chloroplasts within plant cells that absorbs light energy and transfers it into energy in chemicals for the synthesis of carbohydrates
• Uses of carbohydrates produced by photosynthesis

  • Converted into starch for energy storage
  • Converted into cellulose to build cell walls
  • Used in respiration to provide energy
  • Converted to sucrose for transport
  • Used as nectar to attract insects for pollination
  • Converted into lipids and amino acids
• Minerals in plants


  • Plants obtain mineral ions from the soil to make substances like proteins, lipids and nucleic acids that they cannot obtain by eating
  • Mineral deficiencies can prevent proper growth and photosynthesis
• Testing for chlorophyll, light and carbon dioxide

  1. Destarve plant

  2. Cover leaf with foil or place in CO2-free environment

  3. Test leaf for starch using iodine
• Investigating rate of photosynthesis

  1. Use pondweed

  2. Measure oxygen bubbles released

  3. Vary light, temperature or CO2
• Care must be taken to keep variables constant when investigating photosynthesis rate
• Investigating the effect of changing temperature on the rate of photosynthesis

  Dissolving different amounts of sodium hydrogen carbonate in the water in the beaker
• Investigating the effect of changing carbon dioxide concentration on the rate of photosynthesis

  Dissolving different amounts of sodium hydrogen carbonate in the water in the beaker
• Care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test
• Investigating changing light intensity

  Place a glass tank in between the lamp and the beaker to absorb heat from the lamp and so avoid changing the temperature of the water as well as the light intensity
• Alternative ways of measuring the gas (oxygen) given off in these experiments


  • Measure the volume of gas produced using an inverted measuring cylinder with graduations filled with water that readings can be taken from as the water is displaced by the gas
  • Use a syringe attached by a delivery tube to the funnel
• Plants are respiring all the time and so plant cells are taking in oxygen and releasing carbon dioxide as a result of aerobic respiration
• Plants also photosynthesise during daylight hours, for which they need to take in carbon dioxide and release the oxygen made in photosynthesis
• At night, plants do not photosynthesise but they continue to respire, meaning they take in oxygen and give out carbon dioxide
• During the day, especially when the sun is bright, plants are photosynthesising at a faster rate than they are respiring, so there is a net intake of carbon dioxide and a net output of oxygen
• Investigating the effect of light on the net gas exchange in an aquatic plant using a pH indicator such as hydrogencarbonate indicator

  1. Place several leaves from the same plant in stoppered boiling tubes containing some hydrogencarbonate indicator

  2. Investigate the effect of light over a period of a few hours
• Hydrogencarbonate indicator


  Shows the carbon dioxide concentration in solution
• Colour of hydrogencarbonate indicator at different levels of carbon dioxide concentration

  • Colour 1
  • Colour 2
  • Colour 3
  • Colour 4
  • Colour 5
• The balanced chemical equation for photosynthesis is the exact reverse of the aerobic respiration equation
• Light energy


  Converted into chemical energy in the bonds holding the atoms in the glucose molecules together
• If a plant is given unlimited sunlight, carbon dioxide and water and is at a warm temperature, the limit on the rate (speed) at which it can photosynthesise is its own ability to absorb these materials and make them react
• Limiting factor


  Something present in the environment in such short supply that it restricts life processes
• Main factors which limit the rate of photosynthesis

  • Temperature
  • Light intensity
  • Carbon dioxide concentration
• Water is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis
• As temperature increases


  The rate of photosynthesis increases as the reaction is controlled by enzymes
• As the reaction is controlled by enzymes, the trend of increasing rate with temperature only continues up to a certain temperature beyond which the enzymes begin to denature and the rate of reaction decreases
• The more light a plant receives


  The faster the rate of photosynthesis
• At a certain point, increasing the light intensity stops increasing the rate. The rate becomes constant regardless of how much light intensity increases as something else is limiting the rate
• The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough carbon dioxide
• The more carbon dioxide that is present


  The faster the photosynthesis reaction can occur
• The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough light
• Leaf structures you will be expected to identify in a dicotyledonous plant

  • Chloroplasts
  • Cuticle
  • Guard cells
  • Stomata
  • Upper and lower epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Air spaces
  • Vascular bundles (xylem and phloem)
• Pathway of carbon dioxide from the atmosphere to chloroplasts by diffusion: atmosphere → air spaces around spongy mesophyll tissue → leaf mesophyll cells → chloroplast