Click4Biology: 8.2 Photosynthesis

8.2.1 Chloroplast Structure.

(a) Cell wall

(b) Double membrane

(c) Starch grain

(d) Grana

(e) Thylakoid

(f) Stroma

 

 

 

 

• Internal membranes called thylakoids which is the location of the light dependent reaction

• Stroma surrounding the thylakoids and inside the double membrane. This is the location of the light independent reaction that includes the Calvin cycle.

• The stroma often contains starch grains and oil droplets both products of photosynthesis

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8.2.2 Stages of photosynthesis.

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

• Light Dependent Reaction in which

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.

 

• The Light Independent Reaction

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.

• Light energy is converted into chemical energy.

• Chlorophyll molecules are attached to the thylakoid membranes.

• They are often associated with accessory pigments and other proteins to form Photosystem.

• At the centre of all photosystem are forms of chlorophyll a each of which is specialised to absorb a particular wavelength of light.

• Electrons within the chlorophyll absorb the energy from photons and this raises them to higher 'excited' states.

• Excited electrons are more easily lost from the chlorophyll which is a form of oxidation

 

 

 

 

 

 

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:

 

• Light is absorbed by chlorophyll a in Photosystem II (number of different chlorophyll's working together).

• PSII absorbs light best at 680nm and is designated as P₆₈₀

• The chlorophyll absorbs the light energy and converts this to chemical energy in the form of electrons.

• Photosystem II is oxidised, releasing electrons.

 

 

 

 

• The electrons from PS II pass along membrane proteins in a series of redox reactions(in thylakoids)

• The reduced membrane protein pumps H⁺from the stroma into the space inside the thylakoids.

 

 

 

• At the same time as a), Photosystem I (different chlorophyll combination) absorbs light with a peak absorption of 700 nm.

• The chlorophyll molecule releases electrons in the oxidation of PS I

• PS I is now oxidised.

 

 

• The electrons pass from PSI to other membrane proteins named here as ferrodoxins

• These proteins bring about the reduction of NADP⁺ to NADPH + H⁺

• NADPH is found in the stroma and is used in the light independent reaction

 

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• PS I has been oxidised and lost its electron.

• To continue absorbing light PSI must be reduced back to its 'ground state'

• The source of electrons for this reduction are those passed to cytochrome from PSII

 

 

 

• PS II must also be reduced and returned to its 'ground state' to maintain light absorption

• The source of electrons is water.

• Electrons are removed form water for PS II which leaves a source of H⁺and Oxygen

• Oxygen is a waste product of photosynthesis but very important to aerobic organisms

 

 

 

• There is a high concentration of H⁺ in the thylakoid lumen

• The H⁺diffuse back to the stroma through the channel pore of the ATP Synthetase

• This drives the phosphorylation of ADP to ATP

 

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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:

• When the ratio of NADPH + H⁺: NADP⁺ is high then only ATP is produced in a cyclic process.

• PS I does not generate NADPH + H⁺ but sends its 'excited' electron to a proton pump.

 

 

 

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:

• Accumulation of a high concentration of H⁺ which is due to proton pumping.

• There being a concentration difference between two places which is the high concentration of H⁺in the thylakoid space and a lower concentration in the stroma.

• The protons diffuse through the core of the ATP synthetase.

• This drives the motor mechanism of the structure resulting the in the reduction of ADP to ATP.

• Note how the same mechanism was seen on the cristae membranes in respiration.

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.

• The energy trapped from sunlight in the light dependent reaction (ATP and NADPH) is used to fix carbon from carbon dioxide into organic molecules.

• The reaction called the Calvin Cycle takes place in the stroma and is controlled by enzymes.

• Ribulose Bisphosphate Carboxylase (Rubisco) allows carbon (carbon dioxide) to be fixed into an initial organic molecule

• RBCase therefore can be seen as a link between inorganic(non-living) and the organic (living) e.g. Primary productivity

 

 

 

 

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

Carbon Fixation:

 

• The single carbon in carbon dioxide is first trapped by Ribulose bisphosphate (5C) to form a 2 molecules of Glycerate-3-phosphate (GP).

 

Reduction:

 

• GP is reduced to a Triose-phosphate(TP) in the process NADPH and ATP are oxidised to provide the energy

 

 

 

• TP is used to manufacture a variety of organic molecules including Glucose-phosphate

 

Regeneration:

 

• Some of the TP is used to regenerate Ribulose Bisphosphate. This will allow more carbon dioxide to be fixed from the atmosphere.

 

 

8.2.6 Structure and function of the chloroplast.

 

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8.2.7 Action spectrum and absorption spectrum.

• White light from the sun is a short section of the much larger electromagnetic spectrum.

• White light is made up of a range of wavelengths that correspond to the colours we can see.

• Longer wavelength have less energy (red light) whilst shorter wavelengths have more energy include blue light.

 

 

 

 

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:

• absorption of blue light

• absorption of red light

• green light is reflected.

 

 

 

• Notice the Y-axis is rate of photosynthesis

• The rate of photosynthesis is measured at different wavelengths.

• The maximum rate are at the blue end and red end of the visible spectrum.

• The lowest rates are in the yellow greens.

• Chlorophylls are absorbing blue and red light well but not green.

 

 

 

 

Relationship between the two graphs:

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

• Blue light and Red light are the main peaks of light absorption and responsible for the peaks in the rate of photosynthesis

• The low absorption of green light corresponds to its reflection from the chlorophyll and the apparent green colour of chlorophyll and plants

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8.2.8 Limiting factors on the rate of photosynthesis.

• Light Intensity

• Carbon dioxide concentration

• Temperature

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

• The rate of complex biochemical pathways like respiration and photosynthesis are affected by a number of factors.

• The rate of photosynthesis can be affected by light intensity, carbon dioxide concentration and temperature.

• Under a given set of conditions only one factor will affect the rate of photosynthesis this factor is at its minimum and is called the limiting factor.

• As has been shown photosynthesis is a process with many individual steps or stages.

• The overall rate of photosynthesis is determined by the step that is proceeding most slowly (rate-limiting step).

• Therefore each factor e.g.. light, temperature etc can become the limiting factor in any on the rate-limiting steps.

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