Photosynthesis: Biology 9700 (Cambridge International AS & A Level)

A biology student is comparing the light-dependent stage and the light-independent stage (the Calvin cycle) of photosynthesis inside a chloroplast.

Which row correctly identifies the membrane system or region in which each stage takes place, together with one substance produced there?

Chlorophyll a, chlorophyll b and carotenoids (carotene and xanthophyll) are photosynthetic pigments found within the thylakoid membranes of a chloroplast, arranged together with proteins into photosystems.

(a) Describe how having several different pigments arranged together within a photosystem allows a wider range of wavelengths of light to be absorbed and used in photosynthesis. [3]

(b) A student produces an absorption spectrum for extracted chloroplast pigments and, separately, an action spectrum showing the rate of photosynthesis at each wavelength of visible light, using the same plant. Explain why the general shape of the action spectrum closely matches the shape of the absorption spectrum. [3]

(c) The student notes that carotenoids absorb almost no light in the green part of the spectrum, but absorb strongly in the blue region, similar to chlorophyll a and chlorophyll b. Suggest why plant leaves nonetheless typically appear green to the human eye. [2]

Within the thylakoid membranes of a chloroplast, light energy absorbed by chlorophyll drives two related pathways: non-cyclic photophosphorylation and cyclic photophosphorylation.

(a) Describe how non-cyclic photophosphorylation results in the production of ATP, reduced NADP and oxygen, including the roles of photosystem II, photosystem I, the electron transport chain and photolysis. [6]

(b) Describe how cyclic photophosphorylation differs from non-cyclic photophosphorylation, in terms of which photosystem(s) are involved and which products are formed. [3]

(c) Suggest why a chloroplast benefits from being able to carry out cyclic photophosphorylation as well as non-cyclic photophosphorylation. [2]

The light-independent reactions of photosynthesis (the Calvin cycle) take place in the stroma of the chloroplast and depend on the products of the light-dependent stage.

(a) Describe how carbon dioxide is fixed in the Calvin cycle and how the resulting product is converted into triose phosphate (TP), naming the enzyme and molecules involved. [4]

(b) Describe the two possible fates of the triose phosphate (TP) molecules produced in the Calvin cycle. [2]

(c) Using the term "limiting factor", explain why the rate of photosynthesis in a plant does not keep on increasing indefinitely as light intensity is increased, even though carbon dioxide concentration and temperature are kept constant throughout. [3]

A plant is photosynthesising steadily in bright light with a plentiful supply of carbon dioxide. The concentration of carbon dioxide around the plant is then suddenly and substantially reduced, while light intensity and temperature are kept constant.

What is the immediate effect on the concentrations of glycerate 3-phosphate (GP) and ribulose bisphosphate (RuBP) in the Calvin cycle?

A student examines an electron micrograph of a chloroplast from a palisade mesophyll cell and identifies three structures, described in the table below.

(a) Give the name for a stack of sacs such as structure P, and explain how this stacked arrangement makes the light-dependent stage more efficient. [3]

(b) Suggest why it is significant for the chloroplast's role in photosynthesis that it contains its own DNA (structure Q) and its own ribosomes, rather than relying only on proteins made using the cell's nuclear DNA. [3]

(c) Structure R is present in much greater amounts in a chloroplast that has been photosynthesising in bright light for several hours than in a chloroplast that has been kept in darkness for the same length of time. Explain this difference. [3]

A student extracts photosynthetic pigments from a spinach leaf and separates them by paper chromatography. A concentrated spot of the pigment extract is placed on an origin line drawn near the bottom of a strip of chromatography paper, which is then stood in a shallow layer of a non-polar solvent. After the solvent front has moved 9.6 cm from the origin, four separate coloured spots are visible. The table shows the distance each pigment has travelled from the origin.

(a) Calculate the $R_f$ value of chlorophyll b. Show your working and give your answer to 2 decimal places. [2]

(b) Carotene has the highest $R_f$ value of the four pigments, and chlorophyll b has the lowest. Explain this difference in terms of the solubility of the pigments in the non-polar solvent used. [2]

(c) The student wants to confirm that the spot she has identified as chlorophyll a is definitely chlorophyll a, and not a different pigment that happens to have a similar $R_f$ value in this solvent. Explain how repeating the chromatography using a second, different solvent could help her confirm the identity of the spot. [3]

A plant is photosynthesising with light intensity and carbon dioxide concentration both kept high enough that neither is limiting the rate. As the surrounding temperature is raised from 10°C, the rate of photosynthesis increases, reaches a maximum at around 25–30°C, and then falls sharply as the temperature is raised further towards 45°C.

Which of the following best explains this pattern?

A student investigates the effect of light intensity on the rate of photosynthesis in a sprig of the aquatic plant Cabomba. The cut end of the shoot is held underwater in a beaker of sodium hydrogencarbonate solution (as a source of dissolved carbon dioxide), and a lamp is placed at various distances from the plant. At each distance, once the rate of bubbling has become steady, the student counts the number of oxygen bubbles released from the cut stem in one minute and uses this as a measure of the rate of photosynthesis. Light intensity reaching the plant is inversely proportional to the square of the distance between the lamp and the plant. The student intends to keep temperature constant throughout.

(a) The lamp is placed 10 cm from the plant, and later moved to 40 cm from the plant. Calculate the light intensity reaching the plant at 40 cm as a fraction of the light intensity reaching the plant at 10 cm. Show your working. [3]

(b) The student plans to plot a graph of the number of bubbles produced per minute against distance from the lamp. Explain why plotting the number of bubbles produced per minute against $\dfrac{1}{(\text{distance})^2}$ instead would be expected to give a more linear relationship, assuming light intensity is the limiting factor throughout. [2]

(c) The student increases light intensity simply by moving the lamp closer to the plant, without taking any other precautions. Identify one variable, other than light intensity, that is not adequately controlled by this method, and explain how it could make the results of the investigation misleading. [3]

A student investigates the rate of photosynthesis of an aquatic plant at a constant temperature of 25°C, using two different concentrations of dissolved carbon dioxide: 0.04% (found in normal, air-saturated water) and 0.4% (a carbon-dioxide-enriched solution). At each of several light intensities, the student measures the rate of photosynthesis as the volume of oxygen produced per minute. The results are shown in the table.

(a) Explain why the rate of photosynthesis is the same at both carbon dioxide concentrations when light intensity is 10 units. [2]

(b) Explain why the rate of photosynthesis at 0.04% CO₂ stops increasing once light intensity reaches 20 units, whereas the rate at 0.4% CO₂ continues to increase up to a light intensity of 40 units. [3]

(c) At a light intensity of 50 units and 0.4% CO₂, the rate of photosynthesis has levelled off at 26 mm³ O₂ min⁻¹. Predict, with a reason, the effect on this rate of raising the temperature from 25°C to 45°C, keeping light intensity and carbon dioxide concentration the same. [2]