Topic 5A: Photosynthesis

Question 1

Why can’t the detail seen using an electron microscope be seen using an optical microscope? [2]

Answer 1

1. Light has long(er) wavelength;
2. (So) low(er) resolution

Question 2

Name an organelle found in both a chloroplast and a prokaryotic cell. [1]

Answer 2

1. (70S) Ribosome

Question 3

State one difference between the DNA in a chloroplast and on the nucleus of a plant cell. [1]

Answer 3

Chloroplast DNA:
Is not associated with histones but nuclear DNA is
OR
Is circular but nuclear DNA is linear
OR
Is shorter than nuclear DNA

Question 4

State one difference between the ribosomes in a chloroplast and those in the cytoplasm of a plant cell. [1]

Answer 4

1. Chloroplast ribosomes are smaller (than those in the cytoplasm)

Question 5

Describe what happens during photoionisation in the light-dependent reaction. [2]

Answer 5

1. Chlorophyll absorbs light
   OR
   Light excites electrons in chlorophyll;
2. Electron(s) are lost

Question 6

Which chemicals are needed for the light-dependent reaction? [1]

Answer 6

1. NADP, ADP, Pi and water

Question 7

Describe how crop plants use light energy during the light-dependent reaction. [5] (Old specification question)

Answer 7

1. Excites electrons / electrons removed (from chlorophyll);
2. Electrons move along electron transfer chain releasing energy;
3. Energy used to join ADP and Pi to form ATP;
4. Photolysis of water produces protons, electrons and oxygen;
5. NADP reduced by electrons and protons

Question 8

Describe the Light Dependent Reaction. [6]

Answer 8

1. Chlorophyll absorbs light energy;
2. Electrons in chlorophyll are excited and move from chlorophyll to electron transfer chain;
3. Electrons release energy as they move through the electron transfer chain;
4. The release of energy is used to actively pump protons into the thylakoids space create a electrochemical gradient across the thylakoids membrane;
5. The energy stored in the electrochemical gradient is used to join ADP to Pi to form ATP.
6. The electrons are transferred from the electron transport chain to NADP forming reduced NADP;
7. Photolysis of water occurs, releasing electrons, protons and oxygen. The electrons replace those lost by the excitation of chlorophyll

Question 9

During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances.

Describe how. [6]

Answer 9

1. Carbon dioxide reacts with ribulose bisphosphate/RuBP;
2. Produces two glycerate 3-phosphate/GP using (enzyme) Rubisco;
3. GP reduced to triode phosphate;
4. Using reduced NADP;
5. Using energy from ATP;
6. Triode phosphate converted into glucose

Question 10

Explain why the amount of GP in a cell increases and the amount of RuBP decreases when the light source is removed. [3]

Answer 10

1. (LDR stops so) ATP and reduced NADP not produced;
2. (therefore) GP builds up because it cannot be reduced to triose phosphate;
3. RuBP falls because it reacts with carbon dioxide to form GP / cannot be reformed from triose phosphate

Question 11

Explain why the light-independent reaction slows down at low temperatures. [2]

Answer 11

1. Less kinetic energy;
2. Less enzyme-substrate complexes formed

Question 12

Atrazine is a chemical that inhibits the transfer of electrons along the electron transfer chain of chloroplasts.

Explain how atrazine could be used to kill weeks. [5]

Answer 12

1. No ATP produced (in LDR);
2. No reduced NADP produced (in LDR);
3. (so) no GP reduced to triose phosphate;
4. (so) no glucose formed;
5. (so) no (glucose to be used in) respiration

Question 13

Atrazine is a chemical that inhibits the transfer of electrons along the electron transfer chain of chloroplasts. Weeds treated with atrazine have been shown to give off small amounts of heat.

Suggest an explanation for this observation. [1]

Answer 13

1. Energy is released from excited electrons as heat

Question 14

Heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time. Heat stress is a major factor in limiting the rate of photosynthesis. Heat stress decreases the LDR of photosynthesis.

Explain why this leads to a decrease in the light-independent reaction. [2]

Answer 14

1. (Less/no) ATP;
2. (Less/no) reduced NADP

Question 15

Another effect of heat stress is a decrease in the activity of the enzyme rubisco which results in the rate of the reaction it catalyses becoming slower. A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis.

Explain why. [2]

Answer 15

1. (Less/no) carbon dioxide reacts with RuBP;
2. (Less/no) GP

Question 16

Where precisely is rubisco found in a cell? [1]

Answer 16

1. Stroma (in chloroplast)

Question 17

Describe how a pure sample of chloroplasts could be isolated from leaves. [4]

Answer 17

1. Break open cells and filter;
2. In ice-cold, isotonic, buffered solution;
3. Centrifuge and remove nuclei;
4. (Centrifuge) at higher speed, chloroplasts settle out

Question 18

What is the purpose of filtration during chloroplast extraction? [1]

Answer 18

1. To remove (intact) tissue/cells/debris

Question 19

Explain why the solution used to extract the chloroplasts should be cold, buffered and of an appropriate water potential. [3]

Answer 19

1. Cold to prevent enzyme activity;
2. Buffered to stop protein denaturing;
3. Same water potential to prevent osmosis / organelles bursting/shrinking

Question 20

In the practical investigating the effect of light on the rate of dehydrogenase activity in extracts of chloroplasts, explain why it is important that the water potentials of the solution used to produce the chloroplast suspension and of the chloroplasts is the same. [2]

Answer 20

1. Osmosis doesn’t occur;
2. Chloroplast / organelles don’t burst/lyse/shrivel/shrink

Question 21

In the practical investigating the effect of light on the rate of dehydrogenase activity in extracts of chloroplasts, explain why a test up is set up with 1cm3 of solution without chloroplasts and 9cm3 of DCPIP solution in the light. [2]

Answer 21

1. To show light does not affect DCPIC (on its own);
2. To show chloroplasts are required

Question 22

In the practical investigating the effect of light on the rate of dehydrogenase activity in extracts of chloroplasts, explain why the DCPIP in the test tube with the chloroplast solution in the light turns from blue to colourless. [2]

Answer 22

1. Reduction of DCPIP by electrons;
2. (From) chlorophyll / light dependent reaction

Question 23

Explain how chemicals which inhibit the decolorisation of DCPIP could slow the growth of weeds. [2]

Answer 23

1. Less / no ATP produced;
2. Less / no NADP produced;
3. Less / no GP reduced to triose phosphate

Question 24

State one measurement that could be taken to determine the rate of photosynthesis in an experiment. [1]

Answer 24

1. Volume of oxygen produced per unit time / CO2 absorbed per unit time

Question 25

In the practical investigating the effect of light on the rate of dehydrogenase activity in extracts of chloroplasts, all the tubes are placed at the same distance from the lamp.

Explain why. [1]

Answer 25

1. So that the tubes receive the same amount of heat

Question 26

When CO2 concentration was increased from 1% to 2%, the relative amount of GP and RuBP remained the same.

Suggest two reasons why. [2]

Answer 26

1. Temperature is a limiting factor;
2. Light is a limiting factor;
3. Limited by RuBP (available/produced);
4. Limited by enzyme

Question 27

Some bacteria use H2S to produce organic compounds. The H2S has a similar role to that if water in photosynthesis.

A simple equation for this process in bacteria is shown below:

Hydrogen sulphide + carbon dioxide —) glucose + sulphur + water

Suggest what the hydrogen sulphide is used for in these bacteria. [2]

Answer 27

1. Provides H+ and electrons;
2. For reduction;
3. Source of electrons for chlorophyll