Chapter 11: Photosynthesis

Chapter 11: Photosynthesis

  • Photosynthesis is the process of using sunlight energy and chlorophyll to produce glucose from carbon dioxide and water.
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  • Photosynthesis occurs in the chloroplast.

Structure of Chloroplast

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General Process of Photosynthesis

  • Sunlight is absorbed by chlorophyll (found in thylakoid membranes of the chloroplast).
  • This absorbed energy is used to split water into three different components:
    1. Hydrogen ions (also called protons) are stored in the chloroplast – an area called the proton pool.
    2. Oxygen passes out of plant cell and leaf into the atmosphere.
    3. Electrons are given to chlorophyll.
  • Chlorophyll that has energy because of sunlight transfers that energy to its electrons.
  • These electrons are combined with protons and carbon dioxide to make glucose.
Detailed Process of Photosynthesis
Photosynthesis occurs in the chloroplast in two stages:

  • Light stage – requires the direct input of light and occurs in the thylakoid membranes..
  • Dark stage (light-independent stage OR Calvin Cycle) – does not require the direct input of light and occurs in the stroma.

Light Stage

  • Sunlight photon strikes a cluster of chlorophyll molecules called a photosystem.
  • The chlorophyll molecules transfer the energy to a reaction center chlorophyll (RCC).
  • The energy is absorbed by an electron which travels to a higher orbit as a result.
  • The energised electron is released from the RCC and can take one of two paths:

1. Cyclic pathway (pathway 1)
2. Non-cyclic pathway (pathway 2)

Cyclic pathway

  • The energised electron is picked up by an electron acceptor.
  • It is passed from electron acceptor to electron acceptor losing energy along the way.
  • This energy is used to power the production of ATP from ADP and a phosphate.
  • Once the electron has been passed through it is taken back up by the RCC.
  • The ATP is passed onto the next stage of photosynthesis – the dark stage.
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Non-cyclic pathway

  • The energised electron is picked up by an electron acceptor.
  • It is passed from electron acceptor to electron acceptor losing energy along the way.
  • This energy is used to power the production of ATP from ADP and a phosphate.
  • The photosystem is deficient in electrons and splits water into electrons, protons and oxygen gas.
  • The electrons are taken up by the photosystem, the protons are stored in a proton pool within the chloroplast and the oxygen gas is either released into the atmosphere or used in respiration.
  • The electrons that passed through the electron acceptors is now passed onto another photosystem.
  • Light strikes this second photosystem and the electrons are re-energised.
  • The electrons are released and captured by NADP+ to become NADP-.
  • Protons are attracted towards and taken up by NADP- to become NADPH.
  • NADPH and ATP are passed onto the next stage of photosynthesis – the dark stage.
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Dark Stage

  • NADPH and ATP from the light stage are used to reduce (addition of protons and electrons) carbon dioxide in the stroma of the chloroplast.
  • As a result, NADPH is converted back to NADP+ and ATP is converted back to ADP and a phosphate.
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Mandatory Experiment: to investigate the effect of light intensity OR carbon dioxide concentration on the rate of photosynthesis

Equipment:

  • Pondweed (Elodea)
  • Metre stick
  • Pond water
  • Thermometer
  • Paperclip
  • Sodium hydrogen carbonate
  • Stopwatch
  • Backed blade
  • Strong fluorescent light source

Method:

  • Obtain fresh pondweed.
  • Cut a small section using the backed blade and crush the cut end slightly between your fingers.
  • Place pondweed in a test tube with pond water.
  • Ensure the cut end is facing upwards and weigh the pondweed down by attaching a paperclip.
  • Shine the strong fluorescent light source on the pondweed for 5 minutes to allow the pondweed to adjust.
  • Move the light source various distances from the pondweed.
  • Allow the pondweed to adjust to each new light intensity for 5 minutes before counting the number of bubbles per minute.
  • Count the number of bubbles per minute at each distance.
  • Calculate the light intensity at each distance by using the following formula:
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Result:

  • As the light source is moved further away from the pondweed, the rate of bubbles per minute decreases.

Conclusion:

  • The rate of photosynthesis increases with light intensity
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