The light-dependent reaction

Respiration allows cells to carry out processes such as DNA replication.

Outline why cellular respiration is necessary in cells that are carrying out DNA replication.

Chemiosmosis is a mechanism that is thought to occur in both respiration and photosynthesis.

Evidence for chemiosmosis during photosynthesis can be demonstrated by:

• isolating chloroplasts from plant cells

• placing the chloroplasts in an acidic solution, in the dark

• shining light onto the chloroplasts in the solution

• measuring the final pH of the solution.

Explain what is likely to happen to the pH of the solution when light is shone on the chloroplasts.

André Jagendorf provided evidence for chemiosmosis in an experiment carried out in the 1960s.

In his experiment, Jagendorf:

• broke open chloroplasts to expose thylakoids, in the dark

• placed the thylakoids into a solution buffered at pH 4 (solution 1)

• after a short time period, transferred some of these thylakoids to a new solution buffered at pH 4 that contained ADP and P_i (solution 2)

• transferred other thylakoids from solution 1 to a new solution buffered at pH 8 that contained

ATP production was measured in solution 2 and solution 3.

Explain what you would expect to observe in solution 2 and solution 3 after the transfer of the thylakoids.

The diagram shows the absorption spectra of different photosynthetic pigments.

Which of the statements about the absorption spectra shown in the diagram is not correct?

A Absorption of light energy by accessory pigments such as chlorophyll b and carotenoids increases the efficiency of light harvesting.

B Chlorophyll a absorbs red light but not blue light and chlorophyll b absorbs blue light but not red light.

C Chlorophyll b and carotenoids are described as accessory pigments because they are not involved directly in the light dependent reaction of photosynthesis.

D The absorption spectra explain why plants appear green because they absorb red and blue wavelengths of light.

Your answer

The Hill reaction is a model system used to study the light-dependent stage of photosynthesis. It uses a blue dye, DCPIP, which is colourless when reduced.

A student was provided with the following method:

• Cut three small spinach leaves into small pieces and place in a mortar containing 20 cm³ of 0.5 mol dm^–3 sucrose solution.

• Homogenise by grinding vigorously.

• Filter the mixture and pour the filtrate into a centrifuge tube.

• Centrifuge at high speed for 5 minutes.

• Gently pour the supernatant (liquid part) into a clean tube labelled A.

• Resuspend the pellet (sediment) in 20 cm³ of pH 7.0 buffer.

• Add 5 cm³ of resuspended pellet to each of tubes labelled B – D.

• Boil tube B for 5 minutes and then cool.

• Add 10 cm³ DCPIP solution to each of tubes A – E.

• Transfer tube C to a dark cupboard.

• Observe colour of the tubes after 5 minutes.

The table shows the results of the experiment.

State the name of the final electron acceptor in the light-dependent stage of photosynthesis.

DCPIP is reduced in the Hill reaction.

Suggest and explain the function of DCPIP in the Hill reaction.

Using the results shown in the table, explain what can be concluded from each tube, or pair of tubes, about the light-dependent stage of photosynthesis.

Tubes C & D........

The student knew that it was important to use sucrose solution when homogenising the leaves.

Explain why it was important that the pellet was suspended in buffer solution and why it did not contain sucrose.

Suggest and explain two improvements that would increase the validity of the method.

Terrariums are popular for growing houseplants.

A terrarium is a glass container containing soil and small plants.

Once established, a terrarium can be sealed and the plants will be able to grow for months or even years despite not being in contact with the outside atmosphere.

The terrarium maintains moist conditions for the plants.

Suggest one other reason why the plants in a sealed terrarium continue to grow.

Cacti are popular house plants.

Suggest why cacti do not grow well in a terrarium.

Cyanobacteria are photosynthetic prokaryotes.

A scientist exposed cyanobacteria to light of different colours and intensities and made the following observations:

• Most cyanobacteria are blue in colour.

• At low light intensities, glucose production in cyanobacteria is low.

• When light intensity reaches a certain level the rate of glucose production in cyanobacteria stops increasing.

Which of the following statements, A to D, correctly explains these observations?

A The pigments in cyanobacteria absorb blue light and light intensity is a limiting factor for the rate of photosynthesis.

B The pigments in cyanobacteria absorb red light and light intensity is not a limiting factor for the rate of photosynthesis.

C The pigments in cyanobacteria absorb blue light and light intensity is not a limiting factor for the rate of photosynthesis.

D The pigments in cyanobacteria absorb red light and light intensity is a limiting factor for the rate of photosynthesis.

Your answer

Which of the following, A to D, is a similarity in the way ATP is made in respiration and photosynthesis?

A both involve NAD

B both involve substrate level phosphorylation

C both involve photons

D both involve proton gradients

Your answer

Chromista are photosynthetic protoctists that live in water.

Chromista are different from other photosynthetic organisms because they contain the pigment chlorophyll c.

Chlorophyll c is not found in plants.

Outline the importance of photosynthetic pigments in photosynthesis.

The wavelengths of light absorbed by chlorophyll c are different from those wavelengths absorbed by chlorophyll a and chlorophyll b.

Suggest why Chromista need pigments that are different from those of other photosynthetic organisms.

Fig. 17.1 is a diagram of the chloroplast found in a Chromista cell.

Outline the structural differences between the Chromista chloroplast in Fig. 17.1 and the chloroplasts found in flowering plants.

Fig. 17.2 is a diagram of part of the plasma membrane of a Chromista cell.

State and explain how one property of region A in Fig. 17.2 contributes to the stability of the plasma membrane.

There are differences between the plasma membrane and membranes within cells.

Outline the role of membranes within cells.