Photosynthesis - revision notes

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Biology revision notes.

Photosynthesis = The process that makes food (glucose) in plants.

  • It takes place in the green leaves of plants.
  • Sun light is very essential and is taken in by the green pigment in plants: Chlorophyll.
  •  Chlorophyll is in the chloroplasts. Chlorophyll changes the light energy from the sun to chemical energy.
  • The raw materials = carbon dioxide and water.
  • The products = glucose (which is later turned into other substances like starch) and oxygen.

  • The  word equation =

 

carbon dioxide + water    sun light energy absorbed by chlorophyll    glucose + oxygen

or the chemical equation:

                 6CO2 + 6H2O   sun light energy absorbed by chlorophyll    C6H12O6 + 6O2

Experiments for photosythesis:

  • Iodine shows if starch is present.
  • The factor/thing we want to investigate is taken away from the plant.
  • We use a control as a comparison. This is a plant that is given normal conditions INCLUDING the factor we are investigating.

                

Detailed notes:

Plants make their own organic food such as starch, but how do they do it?????


What does a plant need for photosynthesis??

Experiments to show factors needed for photosynthesis:

  1. Remove all starch from leaves of plant (de-starch). (Put it in the dark for a few days)
  2. Give the plant everything it needs apart from the factor we’re investigating.
  3. Do starch test on the leaves to see if it has been making starch.
  • If plant did not produce any starch it means there was no photosynthesis meaning that the factor we were investigating is needed for production of starch/photosynthesis to take place.
  • No starch = factor is needed for photosynthesis.

The individual experiments:

To see if photosynthesis has taken place you use the starch test:

The starch test:

(1) Boil leaf in water to kill it.

(2) Boil leaf in ethanol to remove chlorophyll.

(3) Wash leaf in water to soften the leaf.

(4) Cover with iodine solution to stain starch.

(1) Do plants need carbon dioxide to make starch??

We remove carbon dioxide from the air surrounding a plant using soda lime while another (the control) has carbon dioxide. We then test the leaves of both the plants with iodine to see if any starch was made.

(2) Do plants need chlorophyll to make starch??

It’s impossible to remove chlorophyll from a leaf without killing it!!!!

So we have to use a plant that has variegated leaves.

  • These are plants which have parts that are green (contain chlorophyll) and parts that aren’t (don’t contain chlorophyll).

Examples of plants with variegated leaves: Hibiscus and certain types of ivy.

So we carry out a starch test on a variegated leaf.

     

(3) Do plants need light for making starch??

Join now!

1 plant in the dark, control in the light for a few days.

Do starch test.

OR

Cover part of a leaf with black paper and leave rest of plant in light.

Do starch test on covered leaf.

(4) Do plants need water to make starch??

No simple experiment. Taking away water kills plant.

What does photosynthesis produce??

    (Apart from starch what else is produced in the process of      photosynthesis??)

     

     

Is oxygen produced during photosynthesis??

Joseph Priestly’s experiment (in 1771).

Joseph Priestly did an experiment ...

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