Photosynthesis

• Plants use light for their source of energy. The reaction that plants use to trap this light energy is called photosynthesis.
• Because photosynthesis takes in energy, it is an example of an endothermic reaction.
• Photosynthesis takes place in the leaves of a plant. Leaves contain the green chemical chlorophyll. Chlorophyll can absorb light energy.
• What is the word equation for photosynthesis?

  Here it is...

  
• What is the symbol equation for photosynthesis?

  Here it is...

  
• In order for photosynthesis to take place, we need carbon dioxide and light. But what happens if there's not enough of these...
• When the light intensity is zero, the rate of photosynthesis is zero. Plants need light to carry out photosynthesis. Like this...

  
• As we increase the light intensity, the rate of photosynthesis increases. Because the plant now, has more light energy to carry out the photosynthesis reaction, so the reaction gets faster. Like this...

  
• However, if we increase the light intensity and the rate of photosynthesis also increases, that tells us that the light intensity was limiting. Basically, photosynthesis was not as fast as it could have been, because there was not enough light. So at this point, light intensity is a limiting factor. Like this...

  
• If we keep increasing the light intensity, there comes a point where the rate of photosynthesis no longer increases. Like this...

  At this point, light intensity is no longer the limiting factor. Then this means that something else is now in short supply. This is the similar explanation towards carbon dioxide.

  
• Also the amount of chlorophyll in the leaf, can affect the rate of photosynthesis. Because these leaves can trap less light energy than normal leaves, they will have a lower rate of photosynthesis.

  
• Temperature can also affect the rate of photosynthesis, as we increase the temperature, the enzymes involved in photosynthesis work faster so the rate increases. If we keep increasing the temperature, the enzymes will denature and the rate of photosynthesis falls. Like this...

  
• Photosynthesis is food making: (part1)

  Plants and algae are autotrophs – this means that they can make their own food.

  • Autotrophic organisms make complex molecules (that can be used as an energy store) from simple molecules (carbon dioxide and water).
• Photosynthesis is food making: (part 2)

  Plants and algae make glucose using light energy, water and carbon dioxide in the process of photosynthesis.

  • The glucose can be used as a source of energy in cellular respiration or used to make more complex molecules that are used to help the plant or algae grow.
  • This makes-up the organism's biomass (mass of living material).
• Where do the reactants come from and where do the products go?

  
• The leaves of most plants contain specialised mesophyll cells which are packed with chloroplasts.

  • This helps maximise the amount of photosynthesis.
  • Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light.
• Photosynthesis is a two-stage process:

  • Photosynthesis occurs in two main stages.
  • Energy transferred from light is used to split water into oxygen gas and hydrogen ions.
  • Carbon dioxide gas taken from the atmosphere then combines with the hydrogen ions to make glucose.
• Required Practical: (part 1)

  1. Start by taking a boiling tube and placing it 10cm away from an LED light source. An LED light is used as these do not release very much heat. We use this because too much heat would change the temperature of the experiment.
  2. Fill the boiling tube with sodium hydrogen carbonate solution. Sodium hydrogen carbonate solution releases cardon dioxide, which is needed for photosynthesis.
  3. Put a piece of pondweed into the boiling tube with the cut end at the top.
  4. Leave this for five minutes to acclimatise to the conditions in the boiling tube. Like this so far...

  
• Required Practical: (part 2)

  We should see bubbles of gas being produced from the cut of the pondweed. This gas is oxygen and that is produced by photosynthesis.

  5. Start a stop-watch and count the number of bubbles produced in one minute.

  6. Repeat this two more times and calculate the mean number of bubbles produced in one minute.

  7. Do the whole experiment again from the start at 20cm, then 30cm, then 40cm. Like this so far...

  
• Required Practical:

  There are two main problems in this practical.

  1. The number of bubbles can be too fast to count accurately.
  2. The bubbles are not always the same size. A large bubble would count the same as a small bubble.
• But there is a solution to this practical:

  By measuring the volume of oxygen produced instead of counting bubbles. We would use this equipment...

  
• We place the pond-weed under a funnel and catch the bubbles in a measuring cylinder, filled with water. We then use the measuring cylinder to measure the volume, of the oxygen gas produced.