Photosynthesis (3.5.1)

To produce organic molecules from inorganic molecules using light energy from carbon dioxide and water

There are 2 stages:

1) Light dependent reaction

2) Calvin cycle (light independent)

On the thylakoid membrane present on the grana

Adaptations of the chloroplasts for LDR:

1) Grana (stacks of thylakoids) provide a large surface area for photosynthetic pigments, electron carriers and ATP synthase

2) Thylakoid membranes contain photosystems, electron carriers and ATP synthase needed for electron transport and ATP production

3) Thylakoid membrane is relatively impermeable to H⁺ which maintains the proton gradient so H⁺ flows through ATP synthase, driving ATP synthesis.

4) Photosynthetic pigments, e.g. chlorophyll which absorb light energy to excite electrons during photoionisation.

In the stroma

Adaptations of the chloroplasts for Calvin Cycle:

1) Stroma contains enzymes, including Rubisco which catalyses the fixation of CO₂ by RuBP in the Calvin cycle

2) Chloroplast DNA contains genes coding for some proteins needed for photosynthesis

5 processes in the light dependent reaction:

1) Photoionisation

2) Photolysis

3) Electron transport chain

4) Chemiosmosis

5) Formation of NADH

In photoionisation:

1) The photosynthetic pigments absorb light energy in the choloroplasts, causing electrons in the pigment to become excited

2) They are lost from the chlorophyll, which becomes oxidised

In photolysis:

1) To replace the electrons lost from the photosynthetic pigments, light energy is used to split water molecules

2) H₂O → 2H⁺ + 2e⁻ + O₂

3) This releases electrons which replaces those lost in photoionisation

In the electron transport chain (LDR):

1) The lost electrons from the photosynthetic pigments in the chlorophyll enter the electron transport chain

2) They are passed between electron carrier proteins, through a series of redox reactions

3) This releases energy

In chemiosmosis:

1) The energy released from the redox reactions as electrons pass from each electron carrier protein is used to actively pump H⁺ ions into the intermembrane space (thylakoid membrane)

2) This creates an electrochemical H⁺ concentration gradient

3) As a result, H⁺ ions move back into the stroma by facilitated diffusion down the ATP synthase channel protein

4) The movement of H⁺ ions releases energy which is used to phosphorylate ADP (addition Pᵢ) into ATP

In the formation of NADPH:

1) Co-enzyme NADP accepts H (made from H⁺ and e⁻) and becomes reduced

2) This forms NADPcan H at the end of the electron transport chain

The products of the light dependent reaction

1) NADPH

2) ATP

3 processes in the light independent reaction:

1) Carbon fixation

2) Reduction of GP into TP

3) Production of organic compounds

In carbon fixation:

1) CO₂ reacts with Ribulose bishphosphate to form an unstable 6 carbon intermediate

2) This immediately splits to form 2 molecules of glyercate-3-phosphate

3) Carbon fixation is catalysed by Rubisco

In the reduction of GP into TP:

1) The 2 molecules of glycerate-3-phosphate are reduced by NADPH produced in the light dependent reaction

2) ATP is phosphorylated to provide the energy required to reduce glycerate-3-phosphate into triose phosphate

4) NADP and ADP and Pᵢ return to the thylakoid membrane to be used in the light dependent reaction

In the production of organic compounds:

1) 5/6 of the molecules of triose phosphate are converted back into Ribulose bisphosphate

2) 1/6 of the molecules of triose phosphate are converted into useful organic products (glucose, amino acids, lipids)

Factors that limit the rate of photosynthesis:

1) CO₂

2) Light intensity

3) Temperature

CO₂ affects the rate of photosynthesis:

CO₂ is involved in the Calvin cycle as a reactant for photosynthesis, which reacts with RUBP to form a 6C compound

This breaks down to form glycerate-3-phosphate and is reduced into triose phosphate

So increased CO₂ means greater carbon fixation, so a faster rate of the Calvin cycle, so rate increases

Light intensity affects the rate of photosynthesis:

Light energy is used to excite the electrons in the chlorophyll causing the chlorophyll to be oxidised

This causes the photolysis of water to replace the lost electrons, and ensures that NADPH and ATP can be formed through oxidative phosphorylation

This means more products of the light dependent reaction can be used to reduce glycerate-3-phosphate into triose phosphate

Therefore the rate of the Calvin cycle increases and the rate of photosynthesis increases

Temperature affects the rate of photosynthesis:

Temperature can affect the action of the enzyme Rubsico and increase the kinetic energy of reactant particles

Increasing temperature up until the optimum temperature causes the rate of photosynthesis to increase

Beyond the optimum temperature, the rate of reaction falls as Rubisco becomes denatured