Photosynthesis

• Plants, unlike animals, can make their own food.
• Photosynthesis equations

  Carbon Dioxide + Water =(light energy) Glucose + Oxygen

  A) 6

  B) 6

  C) 6

  
• Photosynthesis requires energy in the form of light to drive the chemical reaction.
• Photosynthesis is an endothermic reaction.
• The light energy required is absorbed by a green pigment called chlorophyll in leaves.

  Chlorophyll is located in chloroplasts in plant cells, particularly the palisade and spongy mesophyll cells.
• Plant leaves are the main photosynthetic organ, but any part of the plant exposed to the light will develop chlorophyll and photosynthesise.
• The carbon dioxide required for photosynthesis comes from air.

  It enters leaves through the stomata.
• Water enters the plant through the roots, and is transported to the leaves in the xylem.
• Oxygen is formed as a waste product.

  Some is used for the plant's respiration, and also released which makes it available for respiration to animals and many microorganisms.

  During the day, provided the rate of photosynthesis is sufficiently high, plant leaves, and water plants, give out oxygen.
• Biosynthesis

  • Some of the glucose produced by photosynthesis is used for respiration.
  • Glucose is the starting point for the biosynthesis of materials that plants need to live.
• Glucose from photosynthesis makes 4 substances:

  • Starch
  • Cellulose
  • Lipids
  • Amino acids
• Glucose from Photosynthesis - Starch

  • Many glucose molecules joined together forms starch.
  • Starch is insoluble.
  • Starch can be storage for carbohydrates.
• Glucose from Photosynthesis - Cellulose

  • Cellulose is used to build cell walls.
  • Cellulose adds strength to cell walls.
• Glucose from Photosynthesis - Lipids

  • Lipids are used for storage in seeds.
• Glucose from Photosynthesis - Amino Acids

  • Glucose + Nitrate ions from soil = amino acids.
  • Many amino acids joined together makes proteins.
• The several ways of measuring the rate of photosynthesis include:

  • The rate of oxygen output.
  • The rate of carbon dioxide uptake.
  • The rate of carbohydrate production.
• Several factors can affect the rate of photosynthesis:

  • Light intensity.
  • Carbon dioxide concentration.
  • Temperature.
  • Amount of chlorophyll.
• Amount of chlorophyll affecting the rate of photosynthesis:

  • Plants in lighting conditions unfavourable for photosynthesis synthesise more chlorophyll, to absorb the light required.
  • The effects of some plant diseases affect the amount of chlorophyll, and therefore the ability of a plant to photosynthesise.
• Light intensity affecting Photosynthesis

  • Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature.
  • Increasing light intensity increases the rate of photosynthesis, until some other factor – a limiting factor – becomes in short supply.
• At very high light intensities, photosynthesis is slowed and then inhibited, but these light intensities do not occur in nature.
• Temperature affecting Photosynthesis

  • The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by enzymes.
  • As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.
  • At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates.
  • At high temperatures, enzymes are denatured.

  
• Carbon Dioxide Concentration affecting Photosynthesis

  • Carbon dioxide – with water – is one of the reactants in photosynthesis.
  • If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.
  • At some point, another factor may become limiting.

  
• Water

  In the UK, water is often the main limiting factor for photosynthesis.

  Water is important in many other areas of a plant's life, and not just photosynthesis. Most important is its role as a solvent for all the chemical reactions in cells.
• You won't see graphs for water's effects on photosynthesis.
• Light

  Light intensity affects the rate of photosynthesis.

  The light intensity fluctuates during the day, and will also be affected by the weather.

  The rate of photosynthesis will change with the time of day.
• Carbon dioxide

  Levels of carbon dioxide in the atmosphere are rising because of greenhouse gas emissions.

  This concentration is still very low in terms of being the optimum for photosynthesis.
• Temperature

  Plants can photosynthesise over a wide range of temperatures from 0°C to around 50°C.

  The optimum temperature for most plants is 15°C to around 40°C.

  Temperature affects the rate of photosynthesis in crop plants and affects where certain crops can be grown.
• Chlorophyll

  The position of the compensation point is different from plants grown in brighter conditions.

  For shade-adapted plants, the compensation point is lower – their rate of photosynthesis will exceed the rate of respiration at lower light intensities than the plants adapted to sun.
• With the law of limiting factors, factors do not work in isolation. Several factors may interact, and it may be any one of them that is limiting photosynthesis.
• One example of how factors might interact:

  Carbon dioxide concentration and temperature interact with the effect of light intensity on photosynthesis:

  • The rate of photosynthesis increases until factors becoming limiting.
  • If carbon dioxide concentration is increased, the rate increases further, and then another factor becomes limiting.
  • The rate can be increased further if the temperature is increased.
  • The rate increases again until another factor becomes limiting.
• The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops.

  The plants will release bubbles of oxygen – a product of photosynthesis – which can be counted.

  The bubbles produced over one minute periods are recorded.
• A lamp with an LED bulb is set up at different distances from the plant in a beaker of water:

  • An LED bulb is best as this will not raise the temperature of the water.
  • Sodium hydrogencarbonate – formula NaHCO3 – is added to the water to supply carbon dioxide – a reactant in photosynthesis – to the plant.
  • Light intensity is proportional to distance – it will decrease as the distance away from the bulb increases – so light intensity for the investigation can be varied by changing the distance from the lamp to the plant.
• The aim of the photosynthesis practical is to investigate the effect of light intensity on the rate of photosynthesis.
• 1. Set up a boiling tube containing 45 cm3 of sodium hydrogencarbonate solution (1%). Allow the tube to stand for a few minutes and shake to disperse any air bubbles that might form.
  2. Cut a piece of the pondweed, Cabomba. The pondweed should be 8 cm long.
  3. Use forcepts to place the pondweed in the boiling tube carefully. Make sure that you don't damage the pondweed, or cause the liquid to overflow.
• 4. Position the boiling tube so that the pondweed is 10 cm away from the light source. Allow the boiling tube to stand for five minutes. Count the number of bubbles emerging from the cut end of the stems in one minute. Repeat the count five times and record your results.

  5. Calculate the average number of bubbles produced per minute. Repeat the experiment at different distances away from the light source.
• The independent variable is the distance from the light source/light intensity.
• The dependent variable is the number of bubbles produced per minute.
• Control variables is the concentration of sodium hydrogencarbonate solution, temperature, using the same piece of Cabomba pondweed each time.
• Risks

  • Care must be taken when using water near electrical equipment.
  • Ensure that your hands are dry when handling the lamp.
• The volume of oxygen produced could be measured by collecting the gas produced in a gas syringe.