Mark Scheme Answers Flashcards
During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how (6)
- Carbon dioxide reacts with ribulose biphosphate/ RuBP using rubisco
- Produces two glycerate 3-phosphate/ GP
- GP reduced to two triose phosphate
- Using reduced NaDP
- Using energy from hydrolysis of ATP
- Triose phosphate converted to useful organic substances e.g glucose/ hexose or regenerated RuBP
Describe how plants use light energy during the light-dependent reaction (6)
- Excites electrons/ electrons removed from chlorophyll
- Electrons move along carriers/ electron transfer chain in series of redox reactions releasing energy
- Energy is used to pump H+ across thylakoid membrane
- H+ move back across thylakoid membrane through ATP Synthase releasing energy used to join ADP and Pi to form ATP
- Photolysis of water produces protons, electrons and oxygen
- Electron from end of chain combines with proton from photolysis to form hydrogen
- NADP reduced by hydrogen to produce reduced NADP
How is a leaf adapted to carry out photosynthesis (6)
- Large surface area for absorption of light
-Transparent cuticle to let light pass through the mesophyll cells for photosynthesis - Long narrow tightly packed mesophyll cells packed with chloroplasts for light absorption
- Many stomata for gas exchange, which can open and close in different light intensities
- Many air spaces allow short diffusion pathway for CO2 and O2
- Network of xylem which brings water to leaf for photosynthesis and phloem which transport sugars made in photosynthesis away
Describe what happens during photoionisation in the light-dependent
reaction.
- Chlorophyll absorbs light/ electrons are excited
- Electrons are lost, chlorophyll becomes positively charged
Explain why the student marked the origin using a pencil rather than using
ink.
Ink and leaf pigments wExplain why the student marked the origin using a pencil rather than using
ink.ill mix
Describe the method the student used to separate the pigments after the
solution of pigments had been applied to the origin
- Level of solvent below origin/line
- Remove/stop before (solvent) reaches top/end
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.
(a) Heat stress decreases the light-dependent reaction of photosynthesis.
Explain why this leads to a decrease in the light-independent reaction
- (Less/no) ATP;
- (Less/no) reduced NADP;
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
- (Less/no) carbon dioxide (reacts) with RuBP;
- (Less/no) GP;
Why does a potometer apparatus need to be air tight
- Air bubbles might enter/ gases may escape so the rate of photosynthesis would be affected
How does temperature affect the rate of photosynthesis
Temperature would affect the enzymes which would affect the rate of photosynthesis
Suggest why the investigation of photosynthesis takes place in the dark with one light source and why the plant tissue is kept in the dark before the experiment
- Light intensity affects photosynthesis so only one light source
- Plant is kept in the dark so no residual product is formed
How is starch stored in plants
- Amylose and amylopectin which is synthesized from glucose
Where in the thylakoid is chlorophyll stored?
In the outer membrane
Describe the process of glycolysis (4)
- Phosphorylation of glucose using ATP;
- Oxidation of triose phosphate to pyruvate;
Accept removal of hydrogen from triose phosphate
for oxidation. - Net gain of ATP;
Accept any description that indicates a net gain
e.g., 4 produced, 2 used. - NAD reduced;
Accept NADH/NADH2/NADH + H+ produced.
Malonate inhibits a reaction in the Krebs cycle.
Explain why malonate would decrease the uptake of oxygen in a respiring
cell (2)
- Less/no reduced NAD/coenzymes
OR
Fewer/no hydrogens/electrons removed (and passed to
electron transfer chain);
Accept less/no FAD reduced. - Oxygen is the final/terminal (electron) acceptor
In muscles, pyruvate is converted to lactate during prolonged exercise.
Explain why converting pyruvate to lactate allows the continued production
of ATP by anaerobic respiration (2)
- Regenerates/produces NAD
OR
oxidises reduced NAD;
Reject NADP and any reference to FAD.
Accept descriptions of oxidation e.g. loss of hydrogen. - (So) glycolysis continues;
Accept description of glycolysis e.g. glucose to pyruvate.
Accept ‘for oxidising/converting triose phosphate to
pyruvate’.
Many yeast cells die during the death phase.
Suggest one reason why
- Decrease/no glucose/substrate
OR
Increase in ethanol/carbon dioxide/acidity; - Accept decrease/no oxygen as Figure 2 is not
linked to Figure 1. - Accept competition for glucose/oxygen.
- Accept any named sugar
- Accept decrease in pH
- Accept increase in toxin
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.
(a) Heat stress decreases the light-dependent reaction of photosynthesis.
Explain why this leads to a decrease in the light-independent reaction (2)
- (Less/no) ATP;
- (Less/no) reduced NADP
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)
- (Less/no) carbon dioxide (reacts) with RuBP;
- (Less/no) GP;
Explain why the student set up a control when testing for respiration
- To show light does not affect DCPIP;
- To show chloroplasts are required;
The student evaluated the effectiveness of different chemicals as
weed-killers by assessing their ability to prevent the decolourisation of
DCPIP in chloroplast suspensions.
He added different concentrations of each chemical to illuminated
chloroplast suspensions containing DCPIP. He then determined the IC50 for
each chemical. The IC50 is the concentration of chemical which inhibits the
decolourisation of DCPIP by 50%.
Explain the advantage of the student using the IC50 in this investigation. (1)
- Provides a standard / reference point
OR - Can compare different chemicals/weed-killers
OR - Can compare different concentrations of chemicals / weed-killers;
A teacher studying these data with her students told her class that no
definite conclusions could be drawn when comparing the mean values in
the graph.
Suggest why the teacher said this (2)
- No error bars / SD;
- To show if overlap occurs so difference (in means) is not significant /
due to chance
OR
To show if no overlap occurs so difference (in means) is significant /
is not due to chance
When treated with Atrazine, weeds have been shown to give off small
amounts of heat.
Suggest an explanation for this observation (1)
Idea that energy is released from high energy / excited electron/s (that
were lost from chlorophyll)
Give one piece of evidence from the diagram which suggests that the conversion of pyruvate to ethanol involves production (2)
- Oxygen removed from pyruvate
- Reduced NAD is oxidised/ donates electrons/donates hydrogens
