Photosynthesis and Plant Responses Section 1

Biology iGCSE Topic 6 Plants as Organisms Photosynthesis and Plant Responses. Section 1 which includes the basics of photosynthesis, how leaves are adapted for photosynthesis, mineral nutrition in plants and testing leaves for starch.

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  • Created by: emma
  • Created on: 24-11-12 14:56

TARGETS - Section 1

Targets for revision section 1:

  • Write out the word equation for photosynthesis
  • Write out the chemical equation for photosynthesis
  • Explain that light energy is converted to chemical energy during photosynthesis
  • Explain where chloroplasts are found
  • Explain how chlorophyll is used to absorb light energy to produce food in plantgs and algae
  • Carry out an experiment to test leaves for the presence of starch 
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Photosynthesis - Basics

What is the difference between animal and plant nutrition?

Animals eat and digest large, complex molecules, such as starch and proteins. They break them down into small, soluble molecules.

They rebuild the small molecules into large molecules that the animal needs. Any left over molecules are broken down and toxic bits are exreted.

Plants only take in very small, simple molecules. They build them unto the complex molecues they need. This is very efficient so there is hardly any left-over material left to excrete. 

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The Process of Photosynthesis

Photosynthesis is the process which plants use to make food from carbon dioxide and water using energy in the worm of light which is trapped by a green pigment called chlorophyll found in cells structures called chloroplasts which are found in palisade and spongy cells.

Light energy is converted into chemical energy.

The word equation for this process is:

carbon dioxide + water --------light energy-----> oxygen + glucose

The chemical equation for this process is:

6CO2  +   6H2O   -------Light energy-------> 6O2 + C6H12O6

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Cross section of a leaf


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How are leaves adapted for photosynthesis?

The diagram on the previous page shows a cross section of a leaf. This shows the adaption the leaf has to be able to photosynthesise.

  • The waxy cuticle let's light through because it is transparent and it prevents water loss
  • Palisade cells have a large surface, lot's of chloroplasts and are tightly bunched together. It's close to the top so it can be closer to the sun
  • The spongy layer contains air spaces between the celss to allow CO2 in and O2 out of the leaf.
  • Stomata allow gases in and out of the leaf. The opening is controlled by guard cells. Guard cells prevent water loss. 
  • Xylem carry water from the roots to the leaves. 
  • Leaves are flat and thin to give them a large surface area. 
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The Products of Photosynthesis

The intial product of photosynthesis is glucose some of which is used straight away for the process of respiration. This provides energy to build up larger molecules from smaller ones.

A faster rate of photosynthesis means a faster rate of growth for the plant.

Glucose that is not used straight away is converted to starch for storage.

Why is starch a better storage molecule than glucose?

 It's more compact and is a large insoluble molecule so that it can stay in one place.

Energy is also used to convert sugars into fatty acids and glycerol, which are then built up into oils for storage in seeds.

Energy is used to convert glucose into cellulose to strengthen cell walls.

Energy is used to convert sugar, nitrates and other nutrients into amino acids which are then built up into protein.

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Mineral nutrition in plants

Plants need more than carbon dioxide and water for healthy growth. They also need a range of mineral ions which are absorbed by the roots by the process of active transport. Basic fertiliser is often called N P K fertiliser because it contains the three main mineral salts:

N - nitrogen       P - phosphorus      K - potassium

Nirates are very important because the plant needs them to make amino acids that are joined up to make many different proteins.

Proteins in plants include:

- Enzymes

- Chlorophyll

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Testing Leaves for Starch

This procedure kills a leaf, disrupts the cell membrane and softens the cuticle and cell walls, This makes it possible to extract the chlorophtyll with hot ethanol and also allows the iodine solution to penetrate the cells and react with any starch present.

You will need:

Beaker with 250 ml of water                                   Ethanol in a boiling tube
Bunsen burner, tripod and gauze                            Iodine
Glass rod                                                             Geranium leaf
Test tube rack                                                      Goggles
White tile                                                      


  • Collect one geranium leaf
  • Boil the water in the beaker and add the leaf. Remove after one minute with the glass rod.  
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Testing Leaves for Starch Continued

  • Switch off the bunsen burner.
  • Drop the lead into a boiling tube with ethanol and push it to the bottom with a glass rod. Stand the boiling tube in your beaker of hot water. 
  • Watch as the ethanol boils and the green colouring (chlorophyll) is removed from the leaf this will take a few minutes. 
  • Using the glass rod, remove the leaf from the boiling tube and rinse the lead in the beaker of water 
  • Spread out the leaft flat on a white tile
  • Add iodine solution to the leaf from the dropper bottle. Make sure the leaf is completely covered. 
  • Watch for a few minutes to see if a blue black colour develops in any part of the leaf. A blue black colour with iodine solution indicates that starch is present. 

Why do we test the leaves for starch and not glucose?

Leaves store glucose as starch, so this is how we know plants photosynthesise.

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