ENERGY TRANSFERS IN & BETWEEN ORGANISMS

• PHOTOSYNTHESIS

  In photosynthesis, which chemicals are needed for the light-dependent

  reaction? (1)

  NADP, ADP, Pi and water;
• Describe what happens during photoionisation in the light-dependent

  reaction. (2)

  1. Chlorophyll absorbs light.

  2. Electron/s are lost
• Explain why the student marked the origin using a pencil rather than using

  ink. (1)

  Ink and (leaf) pigments would mix
• Describe the method the student used to separate the pigments after the

  solution of pigments had been applied to the origin. (2)

  1. Level of solvent below origin/line;.

  2. Remove/stop before (solvent) reaches top/end;
• The pigments in leaves are different colours. Suggest and explain the

  advantage of having different coloured pigments in leaves. (1)

  (Absorb) different/more wavelengths (of light) for photosynthesis;
• Heat stress decreases the light-dependent reaction of photosynthesis.

  Explain why this leads to a decrease in the light-independent reaction. (2)

  1. (Less/no) ATP;

  2. (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)

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

  2. (Less/no) GP;
• Where precisely is rubisco found in a cell? (1)

  1. Stroma (of/in chloroplast);
• The solution that the student used to produce the chloroplast suspension

  had the same water potential as the chloroplasts.

  Explain why it was important that these water potentials were the same.(2)

  1. Osmosis does not occur;

  2. Chloroplast / organelle does not burst / lyse / shrivel / shrink;
• 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
• Explain how chemicals which inhibit the decolourisation of DCPIP could

  slow the growth of weeds (1)
• Other than the thermometer, explain how two features of the calorimeter

  shown in the diagram above would enable a valid measurement of the total

  heat energy released.(2)

  1. Stirrer distributes heat (energy);

  Accept stirrer ensures equal/even temperature or

  prevents build up of 'hot spots'.

  2. Insulation/space/air reduces loss/gain of heat
• In natural ecosystems, most of the light falling on producers is not used in

  photosynthesis.

  Suggest two reasons why. (2)

  1. (Light is) reflected;

  Light is not absorbed on its own is not enough.

  2. (Light is) wrong wavelength;
• The light absorbed by chlorophyll is used in the light-dependent reaction.

  Name the two products of the light-dependent reaction that are required for

  the light-independent reaction.(2)

  1. ATP;

  2. Reduced NADP;
• 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)

  1. No error bars / SD;

  2. To show if overlap occurs so difference (in means) is not significant /

  due to chance
• Atrazine binds to proteins in the electron transfer chain in chloroplasts of

  weeds, reducing the transfer of electrons down the chain.

  Explain how this reduces the rate of photosynthesis in weeds. (4)

  1. Reduced transfer of protons across thylakoid membrane

  OR

  Reduced chemiosomotic gradient / proton gradient across thylakoid

  membrane;

  2. (So) less ATP produced;

  3. (So) less reduced NADP produced;

  Accept NADPH / NADPH2 / NADPH+

  Reject reduced NAD

  4. (So) light-independent reaction slows / stops;

  OR

  Less reduction of GP to triose phosphate.
• 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)
• RESPIRATION

  Describe the process of glycolysis. (4)

  1. Phosphorylation of glucose using ATP;

  2. Oxidation of triose phosphate to pyruvate;

  3. Net gain of ATP;.

  4. NAD reduced;
• Malonate inhibits a reaction in the Krebs cycle.

  Explain why malonate would decrease the uptake of oxygen in a respiring

  cell. (2)

  1. Less/no reduced NAD/coenzymes

  2. 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)

  1. Regenerates/produces NAD

  2. (So) glycolysis continues;'.
• Suggest one reason why it was important that the student left the

  apparatus for one hour after the yeast culture reached a constant

  temperature. (1)

  (So the) oxygen is used/absorbed/respired;
• During her investigation, the coloured liquid moved to the right.

  Explain why it moved to the right. (2)

  1. Anaerobic respiration produces carbon dioxide;

  2. Increase in pressure/volume (of gas)
• Explain why a log scale is used to record the number of cells. (1)

  Large range/difference/increase in numbers;
• Many yeast cells die during the death phase.

  Suggest one reason why (1)

  Decrease/no glucose/substrate

  OR

  Increase in ethanol/carbon dioxide/acidity;
• For the first 10 minutes, the tap attached to tube A was left open and the

  syringe from tube B was removed.

  Suggest three reasons why the apparatus was left for 10 minutes. (3)

  1. Equilibrium reached.

  2. Allow for expansion / pressure change in apparatus;

  3. Allow respiration rate of seeds to stabilise.
• EPO is another performance-enhancing drug. It can increase the haematocrit

  (the percentage of red blood cells in blood).

  (f) A heart attack is caused by a lack of glucose and oxygen being delivered to

  cardiac muscle via the coronary arteries. The overuse of EPO can increase

  the risk of a heart attack.

  Suggest how. (2)

  1. (EPO) causes blood to thicken;

  2. (The thickened blood) could block the coronary arteries
• The normal haematocrit for human males is 47(±5)%. For professional

  male cyclists, the maximum haematocrit allowed is 50%.

  A student suggested that professional male cyclists should be allowed to

  use EPO until their haematocrit is 50%.

  Give two reasons why this suggestion is not valid. (2)

  1. Some cyclists will gain a bigger advantage/increase

  2. There are health risks (associated with) taking EPO;
• Explain how you could determine the total amount of carbon dioxide

  secreted at 30 °C during the period of recording. (1)

  1. Determine the area under the curve;
• Suggest an explanation for the effect of temperature on the rate of carbon

  dioxide release (3)

  1. Enzymes / metabolism faster;

  2. Higher rate of respiration and carbon dioxide production / release;

  3. Spiracles open more often / remain open to excrete / get rid of carbon

  dioxide / get more oxygen;
• ENERGY AND ECOSYSTEMS

  Freshwater marshes have one of the highest rates of gross primary production

  (GPP) and net primary production (NPP) of all ecosystems.

  Carbon use efficiency (CUE) is the ratio of NPP:GPP. Freshwater marshes have

  a high CUE.

  (a) Use your knowledge of NPP to explain why freshwater marshes have a

  high CUE and the advantage of this.

  Do not refer to abiotic factors in your answer. (2)

  1. Low respiration;

  2. More growth/biomass/colonisation;
• Freshwater marsh soils are normally waterlogged. This creates anaerobic

  conditions.

  Use your knowledge of the nitrogen cycle to suggest why these soils

  contain relatively high concentrations of ammonium compounds and low

  concentrations of nitrite ions and nitrate ions. (2)

  1. Less nitrification

  2. More denitrification
• The student used the equation above; however, she substituted height for

  biomass. This was because she did not want to destroy the plants to measure

  their biomass.

  (c) State the assumption the student has made and suggest why this

  assumption might not be valid. (2)

  1. Assumed that height is (directly) proportional to biomass;

  2. Height does not include the roots
• The scientist determined the dry mass of the spinach plants. First, he

  heated each sample at 80 °C for 2 hours.

  Suggest what the scientist should do to ensure that he has removed all the

  water from the sample. (2)

  1. Weigh and heat;.

  2. (Until) mass is constant;
• Arbuscular mycorrhiza fungi (AMF) are fungi which grow on, and into, the roots

  of plants. AMF can increase the uptake of inorganic ions such as phosphate.

  (a) Suggest one way in which an increase in the uptake of phosphate could

  increase plant growth. (1)

  Used to produce named phosphate compound in cells;

  e.g. ATP / ADP / phospholipids / DNA / RNA / RuBP / TP /GP etc.
• Suggest one way in which AMF may benefit from their association with

  plants. (1)

  Example of a carbon-containing biological compound e.g.

  carbohydrate / amino acid / vitamin;

  Accept: sugars / organic (compounds).
• Explain why an increase in shoot biomass can be taken as a measurement

  of net primary productivity (2)

  1. Positive correlation (between diameter and biomass);

  2. Not linear / geometric / exponential / gradient gets steeper;
• The ecologists dried the samples in an oven at 103 °C for 24 hours.

  Describe how the ecologists could have determined whether or not this

  drying removed all the water from a sample of wood.(2)

  1. Record mass and reheat;

  Accept return to oven = reheat

  2. Until constant mass recorded;
• Plantations of trees are often created to remove carbon dioxide from the

  atmosphere, to help to balance the carbon dioxide released by burning

  fossil fuels.

  For different species of tree, information is available for:

  • the relationship between diameter of trunk and freshly cut biomass

  • the percentage of water in fresh-cut wood

  • the mean dried density of wood.

  Using only the information provided in part (c), suggest how the mass of

  carbon in the wood of a plantation of trees of a particular species could be

  estimated.

  Start with measuring the diameter of a large number of trees.

  Assume that the dry biomass of a tree consists of biological molecules that

  contain carbon. (4)

  1. Calculate a mean diameter;

  2. (Use this to) estimate / determine the mean fresh biomass of trees;

  3. Use the percentage water content to find the dried biomass;

  4. Use the dried density to calculate the mass of tree;

  5. Count / estimate the number of trees in plantation and multiply by

  (mean) carbon content (to find total carbon);
• NUTRIENT CYCLES

  Freshwater marsh soils are normally waterlogged. This creates anaerobic

  conditions.

  Use your knowledge of the nitrogen cycle to suggest why these soils

  contain relatively high concentrations of ammonium compounds and low

  concentrations of nitrite ions and nitrate ions. (2)

  1. Low respiration;

  2. More growth/biomass/colonisation;
• Describe the role of saprobionts in the nitrogen cycle. (2)

  1. (They use enzymes to) decompose proteins/DNA/RNA/urea;

  2. Producing/releasing ammonia/ammonium

  compounds/ammonium ions;