Plant nutrition

• How to use this set. Six questions, and only two of the fourteen statements in Topic 6. That ratio is the point: the Supplement delta in this topic is small, so there is no excuse for dropping it, and most of your revision time still belongs to the Core. Target: all six from cold, twice, on different days.

• Core 6.1.1 Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light, and state where in the cell it happens.
• Core 6.1.2 State the word equation for photosynthesis, with light and chlorophyll placed correctly around the arrow, and explain why neither belongs on the reactant side.
• Core 6.1.3 State that chlorophyll is a green pigment found in chloroplasts, and keep the pigment, the organelle and the cell apart in your wording.
• Core 6.1.4 State that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.
• Core 6.1.5 Outline all five uses and stores of the carbohydrate made, naming the substance formed in each: starch, cellulose, glucose for respiration, sucrose and nectar.
• Core 6.1.6 Explain the importance of nitrate ions for making amino acids and of magnesium ions for making chlorophyll, and describe what a shortage of each one looks like.
• Core 6.1.7 Investigate the need for chlorophyll, light and carbon dioxide, naming the control and the controlled variables in each design.
• Core 6.1.7 Describe the starch test in the correct order with a reason for every step, including the safety reason for the water bath, and explain destarching.
• Core 6.1.8 Investigate and describe the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis, using rises, levels off and falls.
• Core 6.1.9 Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution, and read every colour as a net result.
• Core 6.2.1 Explain how a large surface area and a thin lamina adapt most leaves for photosynthesis.
• Core 6.2.2 Identify all eleven named structures of a dicotyledonous leaf in a diagram or image, including xylem above phloem, and keep the stoma distinct from its guard cells.
• Core 6.2.3 Explain how each of those structures adapts the leaf for photosynthesis, using the feature → location → effect chain.
• Supplement 6.1.10 State the balanced chemical equation for photosynthesis.
• Supplement 6.1.11 Define a limiting factor and identify which factor is limiting at a named point on a rate graph.
• Supplement 6.1.11 Explain the limiting factors of photosynthesis in different environmental conditions — including a graph carrying several curves, and unfamiliar contexts such as a glasshouse or a shaded crop.
• Practical skill — Papers 5 and 6 Plan an investigation through all eleven headings: variables, range and intervals, repeats, apparatus and precision, hazards with matched precautions, table, graph choice, conclusion logic, limitations with matched improvements.
• Practical skill — Papers 5 and 6 Calculate a rate from a volume and a time, a mean with an anomaly identified and excluded, and a percentage change — giving the unit every time.
• Practical skill — Papers 5 and 6 Use the inverse square model to compare relative light intensities at different lamp distances, and state what the model does not tell you.
• Practical skill — Papers 5 and 6 Evaluate bubble counting against gas collection, and choose a line graph or a bar chart according to whether the independent variable is continuous or categorical.
• Practical skill — Papers 5 and 6 Draw and label a transverse section of a dicotyledonous leaf to biological-drawing standards.

Supporting context: the compromise a leaf cannot escape. This paragraph is not a Topic 6 statement — water loss from leaves is transpiration, Topic 8 — but it answers the obvious question about why the pores are where they are. An open stoma lets carbon dioxide in — and it lets water vapour out. A plant cannot have one without the other, because both gases move through the same pore. That is why guard cells exist: they let the plant adjust the pore, opening it when photosynthesis can use the carbon dioxide and closing it when losing water matters more. Most stomata being on the shaded, cooler lower surface is part of the same compromise.

• “Plants get their food from the soil.” Why it is wrong: plants absorb water and mineral ions from the soil, and neither is food. Food means an energy-containing organic substance, and the plant makes its own. Say instead Plants make their own carbohydrate by photosynthesis. From the soil they take up water and mineral ions such as nitrate and magnesium, which are raw materials and building materials rather than food.
• “Light is a reactant in photosynthesis.” Why it is wrong: a reactant is a substance that is chemically changed. Light has no atoms to appear in glucose or oxygen; it is a source of energy. Say instead Light provides the energy for the reaction and is written above the arrow, not on the reactant side.
• “Chlorophyll is used up during photosynthesis.” Why it is wrong: a leaf that has photosynthesised all day is still green. Chlorophyll absorbs light, passes the energy on, and is ready to do it again. Say instead Chlorophyll is not consumed. It is a condition of the reaction and is written below the arrow.
• “Plants respire only at night.” Why it is wrong: every living cell needs energy released continuously. Respiration never stops in a living plant — what changes with light is whether photosynthesis is also happening, and how fast. Say instead Plants respire continuously, day and night. In the light, photosynthesis happens as well, and the gas change you observe is the net result of both.
• “The starch test measures the rate of photosynthesis.” Why it is wrong: iodine solution detects starch that has accumulated over several hours. It gives a yes-or-no answer about a store, not a measurement per unit time. Say instead The starch test shows whether starch was made. To measure a rate, collect the oxygen released and measure its volume per unit time.
• “Heat the leaf in ethanol directly over a Bunsen flame.” Why it is wrong: ethanol is highly flammable and boils well below the temperature of a flame. Its vapour can ignite. Say instead Heat water first, remove or turn off the flame, then stand the tube of ethanol in the hot water. Describe it as a hot water bath, away from a naked flame.
• “Twice as many bubbles means twice as much oxygen.” Why it is wrong: bubbles are not a unit of volume. They vary in size, they can merge, and they can stick to the glass, so the count is not proportional to the gas released. Say instead Bubble counting gives an approximate measure of the rate. Collecting the gas in a graduated tube or gas syringe and measuring its volume is the stronger method.
• “The graph levels off, so photosynthesis has stopped.” Why it is wrong: a level region is a constant rate, usually a high one. Stopping would be a line falling to zero, which is a different shape entirely. Say instead Core: the rate has become constant — it is not rising any further, and it has not stopped. Supplement 6.1.11: it is constant because the factor on the x-axis is no longer limiting; another factor has become the one in shortest supply.
• “Increasing any factor always increases the rate.” Why it is wrong: increasing a factor that is not in shortest supply changes nothing, and increasing temperature past the optimum makes things worse rather than better. Say instead Core: beyond the optimum temperature, further heating reduces the rate because the enzymes are denatured; and on a light or carbon dioxide graph the rate stops rising once the curve has levelled off. Supplement 6.1.11: increasing a factor raises the rate only while that factor is the limiting one.
• “Above the optimum temperature the enzymes are killed.” Why it is wrong: enzymes are proteins. They were never alive, so they cannot die. Say instead Core: the enzymes are denatured — the shape of the active site changes, the substrate no longer fits, and the rate falls. Supplement: add that fewer enzyme–substrate complexes form, which is Topic 5 Supplement vocabulary.
• “Nitrate ions supply the plant with energy.” Why it is wrong: energy in a plant is released by respiring carbohydrate. Nitrate contributes atoms, not energy. Say instead Nitrate ions supply nitrogen for making amino acids — the words statement 6.1.6(a) actually asks for — which are then joined into proteins. A shortage causes poor or stunted growth.
• “Magnesium ions are used to make protein.” Why it is wrong: that is nitrate’s job. The two ions are constantly swapped in answers. Say instead Magnesium ions are needed to make chlorophyll. A shortage causes chlorosis — yellowing — and so a lower rate of photosynthesis.
• “Stomata absorb light for the plant.” Why it is wrong: a stoma is a hole. Light is absorbed by chlorophyll inside chloroplasts, and a pore contains neither. Say instead Stomata are pores that allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out. Light absorption is done by the chloroplasts in the mesophyll.
• “Xylem transports sucrose.” Why it is wrong: xylem carries water and mineral ions into the leaf. Sucrose leaves the leaf, and it leaves in the phloem. Say instead Xylem brings water and mineral ions in and lies above the phloem in the bundle. Phloem carries sucrose and amino acids away.
• “Every cell in the leaf has the same number of chloroplasts.” Why it is wrong: the distribution is uneven, and the unevenness is the point — it is one of the leaf’s adaptations. Say instead Palisade cells contain the most chloroplasts because they receive the most light; spongy mesophyll cells contain fewer; epidermal cells contain none, except the guard cells.

• Where the tiering bites in Topic 6. The topic has twelve Core statements and two Supplement statements, and both Supplement statements sit inside subtopic 6.1. They are 6.1.10, the balanced chemical equation, and 6.1.11, identifying and explaining the limiting factors of photosynthesis. Subtopic 6.2, leaf structure, is entirely Core — there is no Supplement leaf content at all.
• A complete “why starch?” answer has two ideas. First, starch is insoluble. Second, so it does not affect the water potential of the cell / does not cause water to enter by osmosis. An answer that only says “starch stores more energy” gives neither.
• Diagnosing from a description. If the plant is described as small, stunted or short, think nitrate. If it is described as yellow, pale or chlorotic at close to normal size, think magnesium. If both are described, say so and explain both — a plant can be short of more than one ion, and the question may be testing whether you notice.
• What this test can and cannot tell you. A blue-black leaf tells you starch had accumulated by the time the leaf was taken. It does not tell you how fast photosynthesis was going, and it does not tell you when the starch was made. That is why destarching — the next section — is not optional housekeeping but the thing that makes the whole experiment mean anything.
• An explanation, not a restatement. “More carbon dioxide means more photosynthesis” simply says the graph again. The explanation is that carbon dioxide is a raw material: more of it means more is available to be combined with water, so more carbohydrate can be made per unit time.
• Answering “evaluate the use of bubble counting”. Give the strength and the weakness, then the improvement. “Counting bubbles is quick and simple, but bubbles vary in size so the count is only an approximate measure of the volume of oxygen released. Collecting the gas in a graduated tube or gas syringe and measuring its volume per unit time would give a more reliable measure of the rate.” That is a complete evaluation in three clauses.
• Observation and inference are different marks. “The indicator turned purple” is the observation. “The carbon dioxide concentration fell” is the inference from the colour key. “Because photosynthesis was faster than respiration” is the explanation. A question worth three marks usually wants all three, in that order.
• Two marks, two features, and both need the ending. “Leaves are broad and thin” names the features and stops short of the explanation the statement asks for. Each one needs its consequence, and the consequence has to be about light absorption or carbon dioxide supply — that is, about photosynthesis.
• Three habits that protect calculation marks. Put the unit in the column heading, not beside every number, so each cell holds a bare value. Give every answer a unit unless it is genuinely dimensionless — relative light intensity has none, a rate always does. And match the significant figures of your answer to the data you were given, rather than copying every digit from the calculator.

• Which components you sit depends on your route. A Core candidate takes Paper 1 and Paper 3. An Extended candidate takes Paper 2 and Paper 4. Every candidate, on either route, also takes Paper 5 or Paper 6. Topic 6 can appear in all of them, and it looks different in each.
• Do not assume the papers are interchangeable. Core and Extended do not share a theory paper. If you are entered for the Extended route you sit Papers 2 and 4, and both Supplement statements in this topic are fair game in both.
• Plant nutrition is unusually rich in practical and data marks — three of its twelve Core statements begin with the word investigate. Knowing the biology is only half of it; the other half is knowing how to write about an experiment and how to read a curve. The practical components are the same two papers whichever route you are on, so every piece of method, control and evaluation work in this chapter is Core content for everybody.
• The definition, the word equation, the five uses of the carbohydrate, the two ions — and for Extended candidates the balanced chemical equation as well. These have to be stated precisely: a definition that omits “using energy from light” has not given the statement, however long it is.
• Describe a method, name a control, list controlled variables, state a hazard. Use the sequence Change → Observe or measure → Repeat → Record → Evaluate → Control → Time or safety.
• Describe the trend and quote values from the axes — that is the Core requirement, 6.1.8. Extended candidates then identify the limiting factor in each region and explain it, which is 6.1.11. Description and explanation earn separate marks.