Photosynthesis: An Introduction - (GCSE Biology)

Photosynthesis: An Introduction

Photosynthesis

Word Equation

• Photosynthesis makes glucose using light energy. Light energy is used by cells in plants in order to convert carbon dioxide and water into glucose and oxygen.

• Photosynthesis occurs in 2 main stages:
  1. Water is split into oxygen gas and hydrogen ions with the help of the energy transferred by light.
  2. The hydrogen ions combine with carbon dioxide to form glucose.

Chemical Equation

Energy Transfer in Photosynthesis

• Photosynthesis takes place in chloroplasts. Chloroplasts are small organelles in plant cells. Chloroplasts have pigments that can be used to trap light. The most common pigment is the green-coloured chlorophyll. Chlorophyll transfers light energy into chemical energy in molecules to make carbohydrates including glucose.
• Photosynthesis is an endothermic reaction. Energy is taken in by living cells as heat energy in photosynthesis. A reaction that takes in heat energy is an endothermic reaction.
• Photosynthetic organisms are the main producers of food on earth. They use light energy to build carbohydrates that can be passed down to other organisms down the food chain. Thus, photosynthetic organisms contribute the most to biomass on earth.

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Respiration and Photosynthesis

You might have noticed that the reactants and products of aerobic respiration and photosynthesis are the opposite – the reactants of respiration are the products of photosynthesis and the reactants of photosynthesis are the products of respiration. So the net exchange of carbon dioxide and oxygen in plants depends on the intensity of light:

• Plants respire all the time during the day and the night. They respire constantly so that they can continuously get the energy they need for internal processes and survive.
• Photosynthesis only occurs during the day. Photosynthesis is driven by light energy so can only occur during the day when light intensity is high.
• Plants produce more oxygen and glucose during the day due to photosynthesis than they use in respiration. So in the day, they use more carbon dioxide than they produce.
• During the night, no photosynthesis takes place. This is because there is no sunlight. Only respiration occurs so at night, they take up oxygen and release carbon dioxide.

Investigating Net Exchange in Plants

You can investigate the changes in carbon dioxide by using hydrogencarbonate which is initially orange. When carbon dioxide concentration increases, it turns yellow. When carbon dioxide concentration decreases, it turns purple.

1. Set up 4 tubes with hydrogen-carbonate.
2. Place similar sized leaves in 3 tubes, trapping them with a bung.
   Leave the 4th tube empty – this will be your control.
3. Wrap one of the tubes with a leaf in foil, another in gauze and leave the third unwrapped.
4. Leave the tubes for an hour or two and observe the colour changes:

• Control tube – no colour change
• Tube with leaf and with foil – changes to yellow as only respiration takes place and no photosynthesis, so carbon dioxide concentration increases
• Tube with leaf and gauze – no colour change as respiration is taking place but also little photosynthesis so any carbon dioxide produced will be taken up
• Uncovered tube with leaf – changes to purple as both respiration and photosynthesis are taking place but due to the high light intensity, more carbon dioxide is taken up in photosynthesis so the concentration of carbon dioxide decreases.