8.2a Photosynthesis Light Reactions

09/01/2013 § Leave a comment

Today I flipped to the pages of this reading assignment and was met with the uncannily familiar sight of the HL ribbon streaking down the lefthand side of page 109. Phase two has begun.

Light Absorption and Photosystems

The substances chlorophyll and accessory pigments are parts of groups of hundreds of molecules called photosystems, which are located in the thylakoids of chloroplasts. Photosystems are what harvest and absorb light for photosynthesis and there are two types of these “light harvesting arrays”: photosystems I and II. Both photosystems contain hundreds of stacks of chlorophyll (called grana), which absorbs energy from sunlight and raises its electron to a higher energy level. This makes the electron an excited electron, and makes that chlorophyll photoactivated. When electrons are excited anywhere in the photosystem, they are passed among the molecules until a special chlorophyll gets its hands on it at the reaction centre. This particular chlorophyll donates the electron to an electron acceptor, or a chain of electron carriers.

Photolysis and Byproducts of Photosynthesis

We know that oxygen is one of the byproducts of photosynthesis. It’s how we (humans) breathe. The production of oxygen occurs during photolysis, which is the splitting of water molecules in the light. In the thylakoid space, an enzyme at the reaction center splits water molecules and the electrons are given to chlorophyll. The oxygen and H+ ions (protons!) are the result of this process. The oxygen acts as waste and the protons add to the chemiosmotic (remember that from last unit?) concentration gradient in the membrane.

Photophosphorylation and ATP Production

Photophosphorylation is the non-cyclic process that produces ATP using the energy from excited electrons retrieved from Photosystem II, where light-dependent reactions begin. Thylakoids are in charge of this process with the following structures:

  • photosystem II
  • a chain of electron carriers (electron acceptors!)
  • ATP synthase
  • photosystem I

Electrons are carried throughout the chain of electron carriers and the energy released pumps protons across the thylakoid membrane. The diffusion of protons down their concentration gradient releases energy that ATP synthase uses to make ATP from ADP and inorganic phosphate – just like in respiration.

Completing Light-Dependent Reactions

Photosystem I is in charge of these last reactions in which NADPH is produced. The chlorophyll in photosystem I absorb light energy that raises electrons to a higher energy level, which is also photoactivation. A protein called ferredoxin are reduced with these electrons, and two molecules of reduced ferredoxin are used to reduce NADP+ (the oxidized form) to NADPH + H+. Photosystems I and II are linked and electrons are passed from photosystem II to photosystem I. The electrons are used to reduce NADP+ , and later, when NADP+ runs out (this will be explained later, I promise) electrons return to Photosystem II along the chain of electron carriers. The proton pumping the results allows for ATP production. This is called cyclic photophosphorylation.

Essay Questions

  1. Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. 6 marks
  2. Explain the effect of light intensity and temperature on the rate of photosynthesis. 8 marks
  3. Explain how the rate of photosynthesis can be measured. 7 marks
  4. Explain the role of water in photosynthesis. 4 marks
  5. Outline the light-dependent reactions of photosynthesis. 6 marks
  6. Explain photophosphorylation in terms of chemiosmosis. 8 marks
  7. Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast. 8 marks
  8. Outline the light-independent reactions of photosynthesis. 8 marks
  9. Explain why the light-independent reactions of photosynthesis can only continue for a short time in darkness. 6 marks
  10. Explain how the light-independent reactions of photosynthesis rely on light-dependent reactions. 8 marks
  11. Outline the formation of carbohydrate molecules in photosynthesis starting from the absorption of light energy. 6 marks
  12. Compare the structure of a chloroplast and a mitochondrion in relation to function. 8 marks
  13. Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope. 4 marks
  14. Photosynthesis and transpiration occur in leaves. Explain how temperature affects these processes. 8 marks



Page 110, evidence for chemiosmosis


a) Describe the relationship between pH of ADP solution and ATP yield, when acid incubation was at pH 3.8.

As the pH of the ADP solution increased, so did the production of ATP.

b) Explain why the pH of the ADP solution affects the ATP yield.

The pH is a measure of how many protons are present in the solution. The presence of protons can help us to determine how much ATP is being produced, since the diffusion of protons across the chemiosmotic gradient provides the thylakoids with the energy to convert ADP into ATP. If there is a lower pH, it means there are many protons and as the pH increases, it means that protons are moving across some gradient and producing ATP as a result.

2. Explain the effect of changing the pH of acid incubation on the yield of ATP.

When the pH is higher, it means that the substance is more basic, or alkaline, therefore there are less protons. Because there are less protons, there will generally be less yield of ATP since not as many protons are moving across the concentration gradient as there would be in a lower pH substance.

3. Explain why there was only a short burst of ATP production.

There was only a short burst of ATP production because the thylakoids produced as much ATP as they could at that time with the ADP available to them in the substance. The protons have to diffuse down their concentration gradient first and once the gradient is equal again, ATP production stops because no more protons are moving.

4. Explain the reason for performing the experiment in darkness.

The presence of light will alter the pH of the ADP solution because light energy contributes to photolysis, which could create additional protons that will change the alkalinity or acidity of the thylakoids and render the data insufficient.

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