P Flashcards
c) The ecologists also investigated photosynthesis in two species of plant found in the
woodland. One of the species was adapted to growing in bright sunlight (sun plant)
and the other was adapted to growing in the shade (shade plant). The ecologists’
results are shown in the figure below.
(i) Give two factors which could be limiting the rate of photosynthesis in the sun
plant between points A and B on the figure
i) Temperature and carbon dioxide;
Neutral: water, chlorophyll.
(iii) Use the information in the figure to explain how the shade plant is better
adapted than the sun plant to growing at low light intensities.
- (Shade plant) has lower (rate of) respiration / respiratory losses /
less CO2 released at 0 light intensity / in dark;
Accept use of figures.
Accept: lower compensation point. - Greater (net) productivity / less sugars / glucose used / more
sugars / glucose available;
Neutral: any references to rate of photosynthesis.
0 1 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.
0 1 . 1 Heat stress decreases the light-dependent reaction of photosynthesis.
Explain why this leads to a decrease in the light-independent reaction.
[2 marks]
- (Less/no) ATP;
- (Less/no) reduced NADP;
Approximately 41% of students obtained both marks by referring to the reduction in ATP
and reduced NADP. Students who gained a single mark usually did so by stating that there
was less ATP rather than less reduced NADP. Many of these students incorrectly stated
that less NADP or less reduced NAD was produced. Some students suggested that heat
stress caused stomata to close and this limited carbon dioxide uptake and photosynthesis.
Many students used additional pages for this question due to initially describing
denaturation of rubisco or ATP synthase before realising that they needed to name specific
products of the light-dependent reaction. It was disappointing to note that nearly a third of
students scored zero.
0 1 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.
1 . 2 Another effect of heat stress is a decrease in the activity of the enzyme rubisco. A
decrease in the activity of an enzyme means that the rate of the reaction it catalyses
becomes slower.
A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis.
Explain why [2 marks]
- (Less/no) carbon dioxide (reacts) with RuBP;
- (Less/no) GP;
failed to mention both RuBP and
carbon dioxide, or simply referred to a six-carbon product with no mention of GP being
formed
A minority of students incorrectly referred to GP as glucose phosphate.
1 . 3 Where precisely is rubisco found in a cell?
[1 mark]
- Stroma (of/in chloroplast);
01.3 Almost two-thirds of students correctly named the stroma as the location of rubisco. The
thylakoids, crista and matrix were common incorrect responses
Scientists investigated the effect of temperature on the activity of two enzymes box
isolated from the leaf cells of cotton plants.
• Rubisco
• Rubisco activase – an enzyme that activates rubisco
Figure 1 and Figure 2 show their results.
Figure 1 Figure 2
0 1 . 4 The scientists concluded that heat stress reduces the activity of rubisco in plant
leaves by affecting rubisco activase.
Use all the information to evaluate their conclusion.
[4 marks]
- Rubisco activity increases with temperature
OR
Rubisco optimum temperature is above (rubisco activase); 2. (Rubisco) activase activity decreases at high temperatures (allow any temperature above 25 ºC.) OR (Rubisco) activase optimum (allow in range) 25 to 30 ºC.; 3. (Results/graphs suggest) activase cannot/does not affect activity of rubisco; 4. (Results are) only for cotton; 5. (Results are) for isolated enzymes; 6. No stats test
01.4 The majority of students (56%) obtained two marks, invariably for describing the effects of
an increase in temperature on the activity of rubisco and rubisco activas
e.
use all the information successfully to evaluate the scientists’
conclusion.
Only 25% of students obtained more than two of the four marks available.
Relatively few students clearly stated that the results indicate that rubisco activase does not
activate rubisco. Similarly, very few students stated that these results were only for cotton
plants and were for isolated enzymes. Equally surprising was the scarcity of responses
which referred to the lack of a statistical test. Consequently, this question did not
discriminate as effectively as had been expected.
7 box . 1 In photosynthesis, which chemicals are needed for the light-dependent reaction?
Tick () one box.
[1 mark]
Reduced NADP, ADP, Pi, water and oxygen.
NADP, ATP and water.
Reduced NADP, ATP, water and carbon dioxide.
NADP, ADP, Pi and water.
NADP, ADP, Pi and water;
07.1 Almost 70% of students correctly identified the chemicals needed for the light-dependent
reaction.
0 7 . 2 Describe what happens during photoionisation in the light-dependent reaction.
[2 marks]
- Chlorophyll absorbs light
OR
Light excites/moves electrons in chlorophyll;
2. Electron/s are lost
OR
(Chlorophyll) becomes positively charged;
07.2 Photoionisation in the light-dependent reaction was clearly described by many students,
with 44% obtaining both marks and approximately 77% obtaining at least one mark. Some
students limited their description to photolysis and gained no marks. Many students
referred to chlorophyll absorbing light and/or light exciting the electrons in chlorophyll.
Some answers referring only to chloroplasts or photosystems did not obtain this mark point.
Many students referred to electrons being lost or being passed to the electron transport
chain. Few students referred to chlorophyll molecules becoming positively charged.
A student obtained a solution of pigments from the leaves of a plant. Then the
student used paper chromatography to separate the pigments.
Figure 5 shows the chromatogram produced.
Figure 5
0 7 . 3 Explain why the student marked the origin using a pencil rather than using ink.
[1 mark]
nk and (leaf) pigments would mix OR
(With ink) origin/line in different position
OR
(With pencil) origin/line in same position
OR
(With pencil) origin/line still visible;
07.3 This question was not well answered and was a poor discriminator. Slightly more than 20%
of students obtained the mark. Only the minority of students could clearly explain that
pencil was used to mark the origin so that the origin/line was still visible after running the
chromatogram. The problem of ink dissolving in the solvent was appreciated by many but
the consequences of this were not explained; the mixing of the ink and leaf pigments was
not appreciated by most students. Answers often only referred to the ink
running/smudging/dissolving in the solvent and affecting results. It was not always clear
whether students were writing about pencil or ink
0 7 . 4 Describe the method the student used to separate the pigments after the
solution of pigments had been applied to the origin.
[2 marks]
- Level of solvent below origin/line;
- Remove/stop before (solvent) reaches top/end;
only mentioned placing the
chromatogram in a solvent, adding a lid and running the chromatogram for a set time or
until the pigments had separated. More students obtained a mark for the level of the
solvent being below the origin than for marking the solvent front or for removing the
chromatogram before it reached the top/end. Some students explained the principle of
chromatography rather than providing details on the method. A range of inappropriate
solvents were referred to including water, glucose and hydrochloric acid.
7 . 6 The pigments in leaves are different colours. Suggest and explain the advantage of
having different coloured pigments in leaves.
[1 mark]
(Absorb) different/more wavelengths (of light) for
photosynthesis;
07.6 Approximately 46% of students obtained this mark, explaining that different
wavelengths/frequencies of light could be absorbed for photosynthesis. Students failing to
gain credit often omitted to mention photosynthesis or did not refer to wavelength or
frequency of light. A few students suggested the different colours of pigments enabled
them to be identified during chromatography.
1- TOPIC test
Using figure 1 explain the results obtained in figure 2 (2)
- More o2 there is the less likely RUBP will react with cO2
2. Competitive inhibition/ competition between o2 and co2 for rubisco
c) Use the infomation provided and your knowledge of the LIR to explain why the yield from soya bean plants is decreased at higher concentrations of o2. Phosphoglycerate is not used in the light independent reaction (3)
- Less GP produced
- Less triose phosphate to form sugars
- less RUBP regenerated
2c- Explain why the increase in dry mass of a plant over 12 months is less than the mass of hexose produced over the same period (3)
- some hexose used in respiration
- co2 produced
- some parts of the plant are eaten
Describe 1 function of each of these substances in the light-independent reactions (2)
- ATP provides energy for reduction of GP to TP / provides P for RUP
- Reduced NADP provides electrons for GP to TP
