Photosynthesis

• Photosynthesis


  Process that takes place in the leaves and more specifically in the subcellular structures called chloroplasts
• Photosynthesis

  Plants make their food from sunlight
• Photosynthetic organisms


  • Green plants and algae
  • Use energy from the sun to make glucose
• Some of the glucose is used during cellular respiration
• Some prokaryotes can also photosynthesise, although they don't have chloroplasts
• Some glucose is converted to starch and stored
• Some glucose is also used to make larger, complex molecules that the plants or algae need to grow, e.g. lipids, proteins, and carbohydrates besides starch
• These make up the organism's biomass
• Chloroplasts


  Subcellular structures where photosynthesis happens
• Chlorophyll

  •  is a pigment that can absorb light, which it uses to convert carbon dioxide and water into glucose and oxygen.
• Chloroplasts


  • Contain the enzymes that catalyse reactions in photosynthesis
  • Contain chlorophyll, which absorbs light
• Energy transfer in photosynthesis

  Energy is transferred to the chlorophyll from the environment by light
• 4 things needed for Photosynthesis:

  • Sunlight
  • Water
  • Chlorophyll
  • Carbon Dioxide
• equation for photosynthesis:

  6CO2 + 6H2O—(chlorophyll) — C6H12O6 + 6O2

  carbon dioxide + water—(light) — glucose + oxygen

  
• Photosynthesis is endothermic — energy is transferred from the environment during photosynthesis.
• Lots of chemical reactions happen during photosynthesis, but it takes place in two main stages. 

  1. First, the energy is transferred by light to the chlorophyll and is used to split water into oxygen gas and hydrogen ions — the oxygen is released as a waste product.
  2. Carbon dioxide gas then combines with the hydrogen ions to make glucose.
• The starch test shows whether photosynthesis is taking place.

  • Remember, glucose is stored by plants as starch. If a plant can't photosynthesise, it can't make starch, you can use this to show that light and CO2  are needed for photosynthesis.
• Testing a leaf for starch

  1. Dunk leaf in boiling water

  2. Put leaf in boiling tube with ethanol, heat gently

  3. Rinse leaf in cold water

  4. Add iodine solution
• If starch is present, the leaf will turn blue-black
• Destarching plants


  Leave plants in the dark for 48 hours
• Showing light is required for photosynthesis


  1. Keep one plant in the dark

  2. Move other plant into the light

  3. Perform starch test on leaf from each plant
• Leaf from plant moved to the light should turn blue-black, but leaf from plant kept in the dark won't
• This shows that light is needed for photosynthesis, as no starch has been made in the leaf grown without light
• Showing CO2 is needed for photosynthesis


  1. Use apparatus shown on left

  2. Soda lime absorbs CO2 out of the air

  3. Leave plant in jar for a while

  4. Test leaf for starch
• Leaf won't turn blue-black, showing no starch has been made, which means CO2 is needed for photosynthesis to happen
• Oxygen production


  Shows the rate of photosynthesis
• Factors affecting the rate of photosynthesis

  • Light intensity
  • Concentration of CO2
  • Temperature
• Limiting factors


  Factors that can stop photosynthesis from happening any faster
• Investigating how factors affect the rate of photosynthesis


  1. Use pondweed

  2. Measure oxygen production over time
• Rate of oxygen production by pondweed

  Corresponds to the rate of photosynthesis
• The faster the rate of oxygen production, the faster the rate of photosynthesis
• Not enough light


  Slows down the rate of photosynthesis
• Light


  Transfers the energy needed for photosynthesis
• As the light level is raised

  The rate of photosynthesis increases steadily
• Beyond a certain point, increasing light level will not make a difference - it'll be either the temperature or the CO2 level which is the limiting factor
• Investigating light intensity


  Moving a lamp closer to or further away from the plant
• If you just plot the rate of photosynthesis against "distance of lamp from the plant", you get a weird-shaped graph
• To get a graph showing the relationship between light and photosynthesis

  1. Measure the light intensity at the plant using a light meter

  2. Do a bit of nifty maths with your results
• Inverse square law


  Light intensity decreases in proportion to the square of the distance
• CARBON DIOXIDE

  Too little carbon dioxide also slows it down.

  • CO2 is one of the substrates needed for photosynthesis.
  • This means that increasing the concentration of CO2 will increase the rate of photosynthesis — but only up to a point.
  • If CO2 is in plentiful supply, then the factor limiting photosynthesis must be light or temperature.
  • The same amounts of sodium hydrogencarbonate in the same volume of water will vary the CO2 concentration.