Photosynthesis

  1. Chlorophyll is used in photosynthesis;
  2. Photosynthesis uses light energy to make glucose;
  3. Photosynthesis takes place in the mesophyll of a leaf;
  4. Photosynthesis also requires Carbon Dioxide and water;
  5. Glucose can be used in respiration or stored as starch;
  6. Iodine and starch turn black;
  7. Green light is not used in photosynthesis;
  8. Increasing the light intensity will make photosynthesis faster.

Please make sure that you have memorized the word equation for photosynthesis, and the balanced chemical equation if you are doing the higher papers. You can find these equations and an explanation of how to balance chemical equations here.

Word Equation:

Carbon Dioxide + Water + Light = Glucose + Oxygen  (this needs chlorophyll).

Balanced Chemical Equation:

6CO2        +    6H2O     +    Light   =      C6H12O    +    6O2

The Stages of Photosynthesis:

Photosynthesis occurs in two separate stages: these are called the “Light Reaction” and the “Dark Reaction”.

In the light reaction, the energy of light is captured by chlorophyll. Electrons are “knocked off” a chlorophyll molecule when a photon (light particle) bumps into a molecule of chlorophyll. The energy in this excited molecule of chlorophyll is used to split water into Oxygen and hydrogen atoms that are used to convert NADP into NADPH.

NADP (Nicotimamide Adenine Dinucleotide Phosphate) is a Hydogen acceptor. When NADP is converted into NADPH the energy from light is stored in this molecule and the electron is given back to the chlorophyll molecule. The chlorophyll molecule is ready to capture another photon.

In the Dark Reaction(this does NOT need light) Carbon Dioxide is converted into a sugar. This needs the energy already captured in NADPH. The extra Hydrogen atom in the NADPH is bonded with the Carbon of  CO2.

Investigating Photosythesis:

If you are investigating Photosythesis, you should use the knowledge that you have already acquired here. Since starch is turned blue or black with Iodine, you can tell whether a leaf has been photosynthesising by adding Iodine. If it goes black it has, but if it stays yellow it has not.

If you want to know how fast photosynthesis is going, you need to measure how fast it is making Oxygen.

You could use different colours of light to see which ones are needed for photosynthesis. Green light is not. Leaves appear green because they use red and blue for photosynthesis and reflect the rest.

Chlorophyll is green and so are most leaves. There is no co-incidence here, but leaves are not all exactly the same shade of green. Why not? You could extract the pigments from a leaf and separate them using chromatography. There are several different pigments, e.g. Chlorophyll a, chlorophyll b, chlorophyll c, xanthophyll, carotene etc..

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