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Summary Topic 6: Plant nutrition
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Topic 6: Plant nutrition: IGCSE Biology course notes which will help you study for your papers. By studying these notes I achieved a 9 - A* on my Igcse
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TOPIC 6: PLANT NUTRITION6.1 PHOTOSYNTHESIS
→Photosynthesis: the process by which plants manufacture carbohydrates from raw materials using energy from light.
(occurs in chloroplasts)
WORD EQUATION: CHEMICAL EQUATION:
→Green plants make the carbohydrate glucose from the raw
materials carbon dioxide and water
→At the same time oxygen is made and released as a waste
product
→The reaction requires energy which is obtained by the
pigment chlorophyll trapping light from the Sun
→The light energy is converted into chemical energy in the bonds holding the atoms in the glucose molecules together.
Chlorophyll
-In photosynthesis the sunlight energy has to be trapped and then used in the reaction and that is why chlorophyll is needed.
-Chlorophyll is the pigment that makes plants look green and it absorbs sunlight which is then released as energy.
6.2 INVESTIGATING PHOTOSYNTHESIS
Starch Production & Need for Chlorophyll
→Although plants make glucose in photosynthesis, leaves
cannot be tested for its presence as the glucose is quickly used,
converted into other substances and transported or stored as
starch.
→Starch is stored in chloroplasts where photosynthesis occurs
so testing a leaf for starch is a reliable indicator of which parts of
the leaf are photosynthesising.
-Leaves can be tested for starch using the following procedure:
→A leaf is dropped in boiling water to kill and break down the
cell walls
→The leaf is left for 5-10 minutes in hot ethanol in a boiling tube
to remove the chlorophyll so colour changes from iodine can be
seen more clearly (if not green colour of the leaf and the brown colour of iodine mix and black is created).
→The leaf is dipped in boiling water to soften it
→The leaf is spread out on a white tile and covered with IODINE SOLUTION
→In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf
→This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is
partially green and partially white)
→The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain
blue-black
→The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored.
, →Care must be taken when carrying out this practical as ethanol is extremely flammable, so at
that stage of the experiment the Bunsen burner should be turned off.
→The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a
Bunsen burner with an open flame
The Need for Light in Photosynthesis
→The same procedure as above can be used to investigate if light is needed for photosynthesis
→Before starting the experiment, the plant needs to be destarched by placing in a dark cupboard
for 24 hours/covered with black paper.
→This ensures that any starch already present in the leaves will be used up and will not affect the
results of the experiment
→Following destarching, a leaf of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a
day (new starch created)
→The leaf can then be removed and tested for starch using iodine
→The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and
could not photosynthesise, while the area exposed to sunlight will turn blue-black
→This proves that light is necessary for photosynthesis and the production of starch.
The Need for Carbon Dioxide in Photosynthesis
→Destarch a plant
→Tie a clear bag containing sodium hydroxide, which will absorb carbon dioxide from the surrounding air, around one leaf.
→Tie a clear bag containing water (control experiment), which will not absorb carbon dioxide from the surrounding air, around
another leaf
→Place the plant in bright light for several hours.
→Test both leaves for starch using iodine
→The leaf from the bag containing sodium hydroxide will remain orange-brown as it could not photosynthesise due to lack of
carbon dioxide
→The leaf from the control bag containing water should turn blue-black as it had all necessary requirements for photosynthesis
Investigating the Rate of Photosynthesis (through collecting oxygen released):
→As photosynthesis occurs, oxygen gas produced is released
→As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pondweed
→The number of bubbles produced over a minute can be counted to record the rate
→The more bubbles produced per minute, the faster the rate of photosynthesis
→A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the
pondweed over a longer period of time and then measure the volume of oxygen collected
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