photosynthesis

Question 1

Explain the photoactivation of chlorophyll [2m]

Answer 1

1. when a photon of light** strikes a chlorophyll molecule, one of its electron is excited to a higher energy state;
2. energy is relayed from pigment to pigment via resonance transfer of energy, until it reaches one of the 2 specialised chlorophyll a, P680, in the reaction centre of photosysytem** II
3. excited electron emitted from a chlorophyll a is captured by the primary electron acceptor* in the reaction centre

Question 2

Function of the thylakoid membrane in photophosphorylation [3m]

Answer 2

1. Provides a large surface area to embed many photosynthetic pigments/chlorophyll molecules for light absorption***
2. maintains the sequential arrangement of photosystems** and electron carriers of electron transport chain***** for the flow of electrons
3. maintains a proton gradient for ATP synthesis since the hydrophobic core*** of the membrane is impermeable to protons and this is essential for chemiosmosis
4. allows many ATP synthase** to be embedded so that ATP can be produced as the protons flow down their gradient via chemiosmosis** from thylakoid space to stroma

Question 3

State 2 ways in which oxidative phosphorylation in mitochondria resembles photophosphorylation in chloroplasts [2m]

Answer 3

1. energy lost from the flow of electrons along an electron transport chain** is used to actively pump protons across membrane to generate proton gradient
2. ADP is phosphorylated to form ATP via ATP synthase** using energy directly from the flow of protons down its gradient
3. both processes take place on membranes

Question 4

GP and RuBP increases in the presence of light but RuBP decreases once it is in dark conditions while GP continues to increase.

Why is this so?

Answer 4

1.In the dark, light-dependent reactiondoes not occur and therefore ATP and NADPH*is not produced;

2. The amount of RuBP decreases as it is carboxylated and converted to GP;
3. since carbon fixation continues as it doesn’t require ATP or NADPH;
4. RuBP regeneration* is slowed due to the lack of ATP and/or NADPH to reduce GP to G3P , leading to a lack of G3P for regeneration.
5. GP accumulates in the dark because GP is not reduced into G3P due to lack of ATP and NADPH. Thus, the amount of GP increases;
6. (data) The decrease in RuBP corresponds to an approximate two-fold increase of GP. This is because 2 GP is formed from every 1 RuBP;

Question 5

Describe how carbon dioxide is fixed in the Calvin cycle. [2]

Answer 5

1. Carbon dioxide combines with RuBP*(5C compound);
2. to form an unstable 6C compound* that will immediately split* to form 2 molecules of (3C) glycerate phosphate* (GP);
3. Carbon fixation is catalysed by enzyme, RuBP carboxylase*(Rubisco);

Question 6

Explain what initially happens to the concentration of RuBP and GP if the supply of carbon dioxide is reduced. [3]

Answer 6

1. RuBP being the carbon dioxide acceptor, would increase, as less is used in carbon fixation at low CO2 concentration;
2. RuBP regeneration continues as GP continues to be converted to RuBP, giving rise to more RuBP and less GP;
3. GP (in the presence of ATP and NADPH) reduced to triose phosphate/G3P*and eventually into, RuBP, therefore quantity of GP drops;
4. Little RuBP is fixed, due to low CO2 concentration, therefore there is very little GP replacement;

Question 7

Explain why the CO2assimilation rate levels off at higher light intensity. [3]

Answer 7

1. At higher light intensity when CO2 assimilation rate levels off, the chloroplasts are saturated with light;
2. and that photosynthesis is occurring at maximum rate;
3. Light is no longer a limiting factor and other factors such as temperature is the limiting factor;

Question 8

Explain why the CO2 assimilation rate is negative at the lowest light intensity. [3]

Answer 8

TAKE NOTE OF POINT 3 &4

1. At the lowest light intensity there is low/no photosynthesis occurring;
2. because there is less photons of light for photoactivation***of chlorophyll molecules in the light dependent stage of photosynthesis thus less ATP and NADPH produced needed for light independent reaction;
3. Respiration occurs at a higher rate than photosynthesis (so more CO2 is produced in respiration than used in photosynthesis);
4. This is occurring below the light compensation point*;

Question 9

Describe the significance of the light compensation point to the growth of the plant [3m]

Answer 9

1. The light compensation point*where at that value of light intensity, the rate of respiration equals the rate of photosynthesis;
2. amount of CO2 given out during respiration is equivalent to the amount of CO2 fixed during the light independent stage of photosynthesis;
3. ** Thus, there is no net gain in dry mass and no growth as the products of photosynthesis (e.g. glucose) are used up in respiration;

Question 10

Describe the role of NADP in photosynthesis

Answer 10

1. NADP is a coenzyme** which carries both protons and high energy electrons
2. NADP is the final electron acceptor* in the non-cyclic light dependent reaction in the thylakoid membrane
3. Electrons carried in NADPH are used in calvin cycle in the stroma of the chloroplast to reduce glycerate phosphate(GP) to glyceraldehyde-3-phosphate(G3P)
4. when GP is reduced to G3P, NADP is regenerated to carry out its role as an electron carrier from the light dependent reactions

Question 11

Describe what happens during the reduction stage of the calvin cycle

Answer 11

1. NADPHis the reducing power used to reduce*GP to glyceraldehyde-3-phosphate (G3P)
2. ATP is the source of energy required;
3. Triose phosphate/G3P is the first sugar formed in photosynthesis and the end product of Calvin cycle;

Question 12

Describe what happens during the RuBP regeneration step

Answer 12

1.5 molecules of G3P are used to regenerate* 3 RuBPs o that the cycle of carbon dioxide fixation can continue. 2.This requires 3 ATP;

Question 13

State two environmental factors that can directly limit the rate of the Calvin cycle and explain how they act. [2]

(concept: ENZYMES!!!!)

Answer 13

Factor 1: Low carbon dioxide concentration(0.04%) in atmosphere

Explanation: Low carbon dioxide concentration decreases frequency of effective collisions* between CO2, ribulose bisphosphate (RuBP) and rubisco thus, decrease enzyme-substrate complex* formed per unit time, lowering rate of carbon fixation, which limits rate of Calvin cycle.

Factor 2: Low temperatures (winter in temperate countries)
Explanation: Low temperature, leads to lower kinetic energy of CO2, ribulose bisphosphate (RuBP) and rubisco, decreases frequency of effective collisions* between them thus, decrease enzyme-substrate complex* formed per unit time, lowering rate of carbon fixation, which limits rate of Calvin cycle.

Question 14

Difference between calvin cycle and krebs cycle

Answer 14

Points of comparison

1. location
2. nature of process - (calvin) anabolic pathway involving the formation of G3P or sugars vs (krebs) catabolic pathway involving the breakdown of acetyl coA and intermediates
3. fate of c02
4. role of coenzymes involved - NADPH as electron donor while NAD+/FAD+ accepts electron
5. role of ATP - (calvin) Expenditure of ATP - in the reduction of GP to G3P and in the regeneration of RuBP vs ( krebs) Synthesis of ATP by substrate level phosphorylation
6. substance generated - ribulose bisphosphate vs oxaloacetate

Question 15

Explain the effect of carbon dioxide concentration on the rate of photosynthesis [4m]

Answer 15

CO2 fixation** in the Calvin cycle; carboxylation of RuBP is catalysed by RuBisCo in the stroma**
•As CO2 concentration increases, rate of effective collisions between enzyme RuBisCo and substrate increases, rate of formation of ES complexes increases, resulting in an increased rate of carbon fixation and the Calvin cycle.
•This increases the rate of use of ATP and NADPH, and the rate of NADP+ regeneration, thus increasing the rate of photolysis of water and oxygen production.

Question 16

Explain the effect of light intensity on the rate of photosynthesis ( measured from amount of oxygen production)

Answer 16

2. Increasing light intensity increases the amount of light energy absorbed by pigment molecules in the light harvesting complexes of photosystems I and II*
3. This increases the rate of photoactivation where electrons from the specialized chlorophyll a(P680/700)in the reaction center of photosystem I and II are excited and lost.
4. The rate of photolysisincreases to replace the electrons lost from the P680 electron in PSII,increasing the rate of oxygen production.
5. Oxygen production increases the buoyancy/decrease density of the leaf discs and will allow the leaf discs to rise to the surface.
6. Thesmaller the distance between the leaf disc and the bench lamp, the greater the light intensity and the shorter the time taken for the disc to rise to the surface, thus indicating a higher rate of photosynthesis.

Question 17

Explain the shape of the action spectrum [2m]

Answer 17

1. Peaks of action spectrum correspond to the absorption peaks
2. Light absorbed by different photosynthetic pigments is used for photo activation thus the higher percentage of light absorption leads to higher rate of photosynthesis
3. The region between (quote value: lowest point of the absorption spectrum of chl a and b) is not an exact match between absorption and action spectra
   + carotenoids absorbed at blue and red light broadens the wavelength of light that can be used for photosynthesis
4. Due to the role of carotenoids and other accessory pigments in light absorption that pass the energy via resonance to the reaction centre