Click4Biology: 8.2 Photosynthesis

Photosynthesis:

8.2.1 Chloroplast Structure.

8.2.2 Stages of photosynthesis.

8.2.3 Light-dependent reaction.

8.2.4 Photophosphorylation.

8.2.5 Light-Independent reaction.

8.2.6 Structure and function of the chloroplast.

8.2.7 Action spectrum and absorption spectrum.

8.2.8 Limiting factors on the rate of photosynthesis.

8.2.1 Chloroplast Structure.

(a) Cell wall

(b) Double membrane

(c) Starch grain

(d) Grana

(e) Thylakoid

(f) Stroma

 

 

 

 

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8.2.2 Stages of photosynthesis.

Photosynthesis occurs in two main phases(see menu diagram):

  1. Energy of sun is trapped by chlorophyll molecules (oxidation)

  2. Photon energy is used to raise the energy of electrons which escape the chlorophyll (oxidation)

  3. This energy is coupled to the reduction of ADP to ATP and the coenzyme NADP+ is reduced to NADPH + H+ .

  4. The reaction must have light to take place.

  5. This reaction takes place on the thylakoid membranes.

 

  1. which uses the chemical energy from the LDR to fix atmospheric carbon into organic molecules such as glucose.

  2. The process does not require light and can occur in both the light and dark periods.

  3. This reaction takes place in the stroma

8.2.3 Light-dependent reaction.

 

 

 

 

 

 

Photons raise the energy level of the electron from the 'ground' state to the higher 'excited' state causing oxidation.

 

 

 

 

 

 

Summary of Non-Cyclic Photophosphorylation:

 

 

 

 

 

Non-cyclic photophosphorylation has the following feature:

1. Light energy is trapped in two Photosystem.

2. ATP is produced.

3. The co-enzyme NADP+is reduced

 

 

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Breakdown of Non-cyclic photophosphorylation:

 

 

 

 

 

 

 

 

 

 

 

S

 

 

 

 

 

 

 

 

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HIll Z Scheme: energy considerations

This model considers the changing energy levels of molecules and electrons during the redox reactions of the light dependent reaction.

This is another way to visualize the steps already covered above.

1) PS II absorbs light at a peak of 680 nm

2) Electrons in chlorophyll are 'excited' and raised to the next energy level.

3) Electrons are lost (oxidation) from PS II and picked up by acceptors like cytochrome

4) The electrons transfer to other membrane proteins releases free energy in pumping H+into the thylakoid space which in turn birng about the synthesis of ATP

5) The electron is taken up by PS I reducing this back to ground state.

6) PS I has already absorbed lower energy electrons 700nm and released electrons at the more 'excited' higher energy level.

7) The electron is passed in a series of redox reactions along membrane proteins.

8) Free energy released is coupled to the reduction of co-enzyme NADP+

 

 

 

 

 

The model above is known as the Hill -Z scheme after Robin Hill its author. There are many versions of this diagram but for my money I prefer the one found in this essay by David Walker. David walker made extraordinary contributions to the understanding of photosynthesis and can still be found on his website in UK. For those of you who study Biology because you love it rather than for the sake of an an examination, read on. The Z-scheme - Down Hill all the Way. Do not confuse David walker with John Walker (below) the latter could only manage a Nobel Prize!

 

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Cyclic Photophosphorylation:

 

 

 

1. PS I is oxidised releasing an 'excited' electron.

2. The electron reduces the membrane proton pump. Protons are pumped into the thylakoid space. This generates ATP.

3. The electrons are cycled back to PS I for its own reduction.

 

 

 

 

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8.2.4 Photophosphorylation.

Photophosphorylation in terms of chemiosmosis.

Chemiosmosis theory is based on:

The Nobel Prize for Chemistry was awarded to Paul Boyer, John Walker and Jens Skou in 1997 for their work on the mechanism of ATP Synthetase enzymes.

 

8.2.5 Light-Independent reaction.

 

 

 

 

Step by Step: The above diagram has the light independent reaction split into three sections.

Carbon Fixation:

 

 

Reduction:

 

 

 

 

 

Regeneration:

 

 

 

8.2.6 Structure and function of the chloroplast.

 

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8.2.7 Action spectrum and absorption spectrum.

 

 

 

 

Absorption spectra are obtain from samples of pigment.

Using a colorimeter different wavelengths of light are passed through and the absorption is measured .

This absorption spectra for chlorophyll shows:

 

 

 

 

 

 

 

Relationship between the two graphs:

Through comparison of Action spectra and absorption spectrum the following correlations are seen:

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8.2.8 Limiting factors on the rate of photosynthesis.

All these factors have been covered in sufficient detail in section 3.8.8 . However here we consider the concept of the limiting factor.

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