Photosynthesis (3.5.1) A2 Biology

What are the 2 steps in photosynthesis?
[1 mark]

Light dependent reaction

Calvin cycle (Light independent reaction)

Describe the whole process of the light dependent reaction
[6 marks]

1) Photo-ionisation occurs - where the photosynthetic pigments in the chlorophyll absorb light. This gained energy causes electron excitation, where the electrons move to a higher energy level

2) If they gain enough energy, the electrons can leave the chlorophyll and the chlorophyll becomes oxidised

3) As a result photolysis of water occurs, where water is split by light into hydrogen ions, electrons and oxygen to replace the lost electrons by photo-ionisation

4) The lost electrons from the chlorophyll enter the electron transport chain in the thylakoid membrane and are passed between electron carrier proteins in redox reactions

5) This releases energy, which is used to provide the energy to actively transport hydrogen ions out of the stroma and into the thylakoid lumen.

6) The thylakoid membrane is impermeable to H+ ions so they accumulate in the thylakoid lumen, creating an electrochemical proton gradient

7) As a result the hydrogen ions move back into the stroma down the protein channel ATP synthase by facilitated diffusion as it is the only channel permeable to them

8) The movement of H+ ions releases energy which is used to phosphorylate ADP into ATP

9) The co-enzyme NADP accepts H+ and electrons to form NADPH (reduced NADP) at the end of the electron transport chain

What is the equation for the photolysis of water?
[1 mark]

2H₂O → 4H⁺ + 4e^-- + O₂

What are the overall products of the light dependent reaction?
[1 mark]

ATP

NADPH (reduced NADP)

How are the chloroplasts structurally adapted for the light dependent reaction?
[4 marks]

1) Chloroplasts contain many thylakoids stacked into grana which massively increase the surface area for more photosynthetic pigments to be arranged for the maximum absorbtion of light

2) Large surface area created by many thylakoids stacked into grana also allows for more electron transport chains to be embedded in the thylakoid membranes, producing more ATP and NADPH

3) A large surface area created by many thylakoids stacked into grana, which allows more ATP synthase enzymes to be embedded in thylakoid membrane, increasing the rate of ATP production by chemiosmosis

4) Contain chloroplast DNA which can manufacture the enzymes and proteins needed for the light dependent and light independent reaction

5) ATP synthase present which is selectively permeable to H+ ions, whilst the thylakoid membrane is selectively impermeable to H+ ions allowing for a proton gradient to be established and ATP to be synthesised due to the movement of H+ ions down ATP synthase

Describe the whole process of the calvin cycle
[6 marks]

1) The enzyme Rubisco catalyses carbon fixation by attaching carbon dioxide to ribulose bisphosphate (RuBP)

2) This forms an unstable 6 carbon intermediate which immediately breaks down to form two molecules of glycerate-3-phosphate (GP)

3) Glycerate-3-phosphate is reduced into triose phosphate. Reduced NADP (NADPH) provides the hydrogen for reduction (oxidising NADPH back into NADP). Energy is released from the hydrolysis of ATP, which was produced in the light dependent reaction

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

5) 5/6 of triose phosphate molecules are used to regenerate ribulose bisphosphate RuBP so the calvin cycle can continue

6) 1/6 of triose phosphate is used to produce useful organic products like glucose, amino acids and lipids

How are the chloroplasts structurally adapted for the light independent reaction?
[4 marks]

1) Fluid of the stroma contains enzymes that are needed to carry out the light independent reaction

2) The stroma fluid directly surrounds the thylakoids, allowing ATP and reduced NADP produced during the light-dependent reaction to rapidly diffuse into the stroma

3) The stroma contains circular DNA and 70S ribosomes, allowing rapid synthesis of enzymes needed specifically for the light-independent reaction (such as Rubisco)

What are the environmental factors that limit the rate of photosynthesis?
[1 mark]

1) Carbon dioxidie concentration

2) Light intensity

3) Temperature

How does carbon dioxide concentration affect the rate of photosynthesis?

[4 marks]

1) Carbon dioxide is a limiting factor, so as the concentration of carbon dioxide increases the rate of photosynthesis increases

2) This is because more carbon dioxide is available to react with ribulose bisphosphate (RuBP) catalysed by Rubisco for the light independent reaction to occur

3) So the rate of calvin cycle is faster, therefore rate of photosynthesis is higher

4) After a certain point, carbon dioxide is no longer the limiting factor and other external factors such as light intensity and the temperature are limiting factors

5) So the rate of photosynthesis plateaus

How does light intensity affect the rate of photosynthesis?

[4 marks]

1) Light intensity is a limiting factor, therefore as the light intensity increases the rate of photosynthesis increases

2) Higher light intensity means more photoionisation, therefore more photolysis.
This increases the rate of the light dependent reaction, so more ATP and NADPH is produced

3) As a result, more glycerate-3-phosphate is reduced into triose phosphate so the rate of the calvin cycle increases, which means a faster rate of photosynthesis

4) After a certain point, light intensity is no longer the limiting factor and other external factors such as carbon dioxide and the temperature are limiting factors

5) So the rate of reaction plateaus

How does temperature affect the rate of photosynthesis up to the optimum temperature?
[4 marks]

1) As you increase the temperature, the rate of photosynthesis increases. This is because the enzyme (Rubisco) and the substrates (Ribulose bisphosphate and carbon dioxide) gain kinetic energy and move faster

2) More particles therefore have energy equal to or greater than the activation energy, so when they collide they form more enzyme substrate complexes

3) This increases the frequency of successful collisions and therefore increases the rate of the light independent reaction (Calvin cycle)

4) So the rate of photosynthesis increases until the optimum temperature, where the rate is at its highest

How does temperature affect the rate of photosynthesis beyond the optimum temperature?
[4 marks]

1) Beyond the optimum temperature, the high temperature causes the molecules in Rubisco to gain kinetic energy and vibrate more — damaging the hydrogen and ionic bonds in the tertiary structure.

2) This changes the specific 3D tertiary structure of Rubisco, and in turn changes the shape of the specific active site

3) So carbon dioxide and ribulose bisphosphate are no longer complementary to the active site so cannot form enzyme substrate complexes with Ribulose

4) As a result the rate of the light independent reaction drops to zero. The enzyme is said to be denatured at this point.