Biology 3.2 Photosynthesis

Question 1

Define photosynthesis.

Answer 1

Photosynthesis is the process by which green plants capture light energy and transduce it into chemical energy stored in molecules of carbohydrate.

Question 1

What is the equation for photosynthesis?

Answer 1

6CO2 + 6H2O > C6H12O6 + 6O2

Question 2

What does photosynthesis require?

Answer 2

1: Energy in the form of light
2: Chlorophyll and other pigments to absorb light energy

Question 3

What is the law of limiting factors?

Answer 3

The rate of a physiological process will be limited by the factor which is in shortest supply.

Question 4

What is a photosynthetic pigment?

Answer 4

A pigment is a molecule that absorbs specific wavelengths of light. In the chloroplasts, light energy is trapped by photosynthetic pigments, with different pigments trapping different wavelengths.

Question 5

Why is it good that different pigments absorb different wavelengths of light?

Answer 5

This allows a large range of wavelengths to be absorbed and is consequently more useful than if there were just one pigment, absorbing a small range of wavelengths.

Question 6

What are the two pigment classes?

Answer 6

Chlorophylls: Chlorophyll a and b
Carotenoids: B-carotene and xanthophylls.

Question 7

What is the Rf value?

Answer 7

Distance moved by solute /
Distance moved by solvent front

Question 8

What is the absorption spectrum?

Answer 8

The amount of light absorption by a pigment at different wavelengths of light.

Question 9

What is the action spectrum?

Answer 9

The rate of photosynthesis at different wavelengths of light.

Question 10

What is the structure of photosystems?

Answer 10

They have an antenna complex which contains most of the pigment molecules that capture light energy.
They also have a reaction centre that contains special chlorophyll molecules that are sensitive to certain wavelengths of light - these release high energy electrons to provide energy for subsequent reactions.

Question 11

What is the absorption maximum of photosystem 1?

Answer 11

680nm so it’s called P680.

Question 12

What is the absorption maximum of photosystem 2?

Answer 12

700nm so it’s called P700.

Question 13

What does the light dependent stage produce?

Answer 13

1: ATP, which provides the chemical energy transduced from light energy, to synthesise energy-rich molecules such as glucose.
2: NADPH/H+, which provides the reducing power to synthesise molecules such as glucose from CO2.
3: O2, a by-product, derived from water. O2 diffuses out of the chloroplast, out of the photosynthetic cells and out of the leaf through the stomata.

Question 14

What is photophosphorylation?

Answer 14

The addition of a phosphate ion to ADP. The energy from this reaction comes from light.

Question 15

Explain the photolysis of water.

Answer 15

In the thylakoid spaces, water molecules absorb light, which indirectly causes them to dissociate into hydrogen, oxygen and electrons: H2O > 2H+ + 2e- + 1/2O2. The splitting of water by light is called photolysis.

Question 16

What happens with the electrons produced in photolysis?

Answer 16

They replace those lost from PSII.

Question 17

What is reduced as a result of photolysis?

Answer 17

Protons from water and electrons from PSI reduce NADP.

Question 18

What happens to oxygen as a result of photolysis?

Answer 18

It diffuses out of the chloroplast and cell into intercellular air spaces, and then out through the stomata as a waste product.

Question 19

What happens as electrons pass through a proton pump in the thylakoid membrane? (1)

Answer 19

As electrons pass through a proton pump in the thylakoid membrane, they provide energy to pump protons from the stroma into the thylakoid space. The protons join the H+ ions from the photolysis of water and accumulate. They generate an electrochemical gradient, since there is more inside the thylakoid space than there are outside, in the stroma. This gradient is a source of potential energy.

Question 20

What happens as electrons pass through a proton pump in the thylakoid membrane? (2)

Answer 20

Chemiosmosis occurs. The H+ ions diffuse down their electrochemical gradient through ATP synthetase in the thylakoid membrane, into the stroma. This diffusion makes available the energy derived from light and carried by the electrons. As they pass through ATP synthetase, ADP is phosphorylated to ATP.

Question 21

What happens as electrons pass through a proton pump in the thylakoid membrane? (3)

Answer 21

Once in the stroma, H+ ions are passed to oxidised NADP, reducing it: NADP + 2H+ + 2e- > NADPH/H+. This removal of H+ ions from the stroma, in conjunction with the proton pump, contributes to maintaining the proton gradient across the thylakoid membranes.

Question 22

What 3 factors maintain the proton gradient between the thylakoid space and the stroma?

Answer 22

1: The proton pump associated with the ETC pushing protons into the thylakoid space.
2: The photolysis of water in the thylakoid space.
3: The removal of protons from the stroma, reducing NADP.

Question 23

Where does the light independent stage happen?

Answer 23

In solution in the stroma and it involves many reactions, each catalysed by a different enzyme.

Question 24

How do the reactions of the light independent stage use the products of the light dependent stage?

Answer 24

1: ATP is a source of energy.
2: NADPH/H+ is the source of the reducing power, reducing CO2.

Question 25

How are carbohydrates made by plants?

Answer 25

First hexose made is fructose bisphosphate. This can be converted to glucose and combined with the fructose to make sucrose, for transport around the plant. The a-glucose molecules may be converted to starch, for storage, or to B-glucose, which is polymerised into cellulose for cell walls.

Question 26

How are fats made by plants?

Answer 26

Acetyl coenzyme A (AcCoA) can be synthesised from GP, made in the Calvin Cycle, and converted to fatty acids. TP can be converted directly to glycerol, another 3C compound. Fatty acids and glycerol undergo condensation reactions to form triglycerides.

Question 27

How are proteins made by plants?

Answer 27

GP can be converted into amino acids for protein synthesis. The amino group is derived from NH4+ ions, made from nitrate ions taken in at the roots and transported throughout the plant.

Question 28

What do plants need in order to be efficient at photosynthesis?

Answer 28

1: The reactants, CO2 and water.
2: Light at a high enough intensity and of suitable wavelengths.
3: A suitable temperature.

Question 29

What is the light compensation point?

Answer 29

At a particular light intensity, so little CO2 is needed that respiration provides all that is required and none is absorbed. Similarly, all the oxygen needed for respiration is provided by photosynthesis. There is therefore no net gas exchange. The light intensity at which this happens is called the light compensation point.

Question 30

What happens to a plant if nitrogen is in short supply?

Answer 30

Because of its role in protein and nucleic acid synthesis, symptoms of nitrogen deficiency include reduced growth in the whole plant. Nitrogen is a component of chlorophyll and so its deficiency also causes chlorosis.

Question 31

What are the roles of magnesium in plants?

Answer 31

Forms part of the chlorophyll molecule and so the main symptom of magnesium deficiency is chlorosis. Magnesium ions are also important enzyme activators such as ATPase.

Question 32

What is the source of hydrogen ions in the light dependent stage of photosynthesis?

Answer 32

Water.

Question 33

What acts as the final electron acceptor in the light dependent stage of photosynthesis?

Answer 33

NADP - electrons (from ETC) and protons ( H+ - from the stroma) are used to reduce the NADP to NADPH/H+