Plant Nutrition

• How to use this set properly. Do all twenty with the answers hidden, then reveal and mark yourself honestly. Note only the ones you got wrong, wait a day, and redo those. Repeating questions you already answer correctly feels productive and teaches you almost nothing new. Target: 18 out of 20 from cold, twice, on different days.
• If you have twenty minutes and nothing else. Write the two equations. Draw the three graphs. Draw the leaf section. Write the two ions and what each builds. Those four pieces of paper carry the majority of the marks in this topic. Four things, twenty minutes, no notes.

• Define photosynthesis and state where in the cell it happens.
• State the role of chlorophyll: it absorbs light and transfers light energy into chemical energy used to make carbohydrate.
• Write both equations — the word equation and the balanced symbol equation — with light and chlorophyll placed correctly around the arrow.
• Explain why light is not a reactant and why chlorophyll is not used up.
• State four uses of the carbohydrate made in photosynthesis, and name the substance formed in each.
• Describe the starch test in the correct order and give a reason for every step, including the safety reason for the water bath.
• Explain destarching and why a requirement investigation is worthless without it.
• Design and interpret controlled investigations into the need for light, chlorophyll and carbon dioxide, naming the control and the controlled variables.
• Define a limiting factor and identify which factor is limiting at a named point on a graph.
• Describe and explain the effect of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis.
• Use the inverse square model to compare relative light intensities at different lamp distances, and state its limits.
• Calculate a rate of photosynthesis from a volume of oxygen and a time, and evaluate bubble counting against gas collection.
• Interpret hydrogencarbonate indicator colours through the net balance of photosynthesis and respiration.
• Label a transverse section of a dicotyledonous leaf and relate each structure to photosynthesis or gas exchange.
• Explain why plants need nitrate ions and magnesium ions, and describe the effects of deficiency.

Why nitrogen has to come from the soil at all. Photosynthesis makes carbohydrate, which contains only carbon, hydrogen and oxygen — the atoms available in carbon dioxide and water. Proteins also contain nitrogen, and there is no nitrogen in either raw material. So a plant that has everything it needs for photosynthesis still cannot make a single protein without a nitrogen source, and that source is nitrate ions from the soil.

Uses of Photosynthetic Carbohydrate

The glucose made in photosynthesis has four required fates in a plant: it is used in respiration to release energy, converted to starch for storage, converted to cellulose to build cell walls, and converted to sucrose for transport in the phloem. Starch is the storage form because it is insoluble and so does not affect the water potential of the cell.

Balanced Equation for Photosynthesis

The balanced symbol equation for photosynthesis is 6CO2 + 6H2O to C6H12O6 + 6O2, with light energy and chlorophyll written around the reaction arrow rather than as reactants. Six molecules of carbon dioxide and six of water yield one molecule of glucose and six of oxygen, so carbon, hydrogen and oxygen atoms balance on both sides.

Chlorophyll

The green pigment found inside chloroplasts. Chlorophyll absorbs light and transfers the light energy into chemical energy that is used to make carbohydrate from carbon dioxide and water. It is a condition for photosynthesis rather than a reactant, because it is not consumed by the reaction. A shortage of chlorophyll, from magnesium deficiency or in the non-green regions of a variegated leaf, reduces or prevents photosynthesis there.

Carbon Dioxide Concentration as a Limiting Factor

Carbon dioxide concentration limits photosynthesis when it is the factor in shortest supply. Increasing it increases the rate, because more carbon dioxide is available to be combined with water to form carbohydrate, until light intensity or temperature becomes limiting and the curve plateaus. In the laboratory sodium hydrogencarbonate solution is used to supply carbon dioxide to an aquatic plant.

Light Intensity as a Limiting Factor

Light intensity limits photosynthesis when it is the factor in shortest supply. As light intensity increases from low values, more light energy is absorbed by chlorophyll and transferred into chemical energy, so the rate of photosynthesis increases. The curve then reaches a plateau, at which point a different factor such as carbon dioxide concentration or temperature has become limiting.

Magnesium Ions

Mineral ions absorbed from the soil by plant roots and required to make chlorophyll. A shortage of magnesium ions means less chlorophyll can be made, so the leaves become yellow, a condition called chlorosis, and less light can be absorbed, which reduces the rate of photosynthesis.

Destarching

Keeping a plant in darkness for a period long enough for the starch already stored in its leaves to be used up in respiration, before an investigation into the requirements for photosynthesis. Destarching means any starch detected afterwards must have been made during the investigation itself, so the result can be attributed to the treatment rather than to starch stored beforehand.

Photosynthesis

The process by which plants manufacture carbohydrates from raw materials using energy from light. Carbon dioxide and water are the raw materials, light provides the energy and chlorophyll transfers it, and glucose and oxygen are the products. It takes place in chloroplasts and transfers light energy into chemical energy stored in carbohydrate.

Optimum Temperature for Photosynthesis

The temperature at which the rate of photosynthesis is greatest. Below it, raising the temperature increases the kinetic energy of the molecules, so there are more successful collisions between the enzymes controlling photosynthesis and their substrates and the rate rises. Above it, the enzymes denature: the shape of the active site changes, substrates no longer fit, and the rate falls.

Palisade Mesophyll

The layer of tall, closely packed, column-shaped cells lying immediately beneath the upper epidermis of a leaf. Palisade cells contain more chloroplasts than any other cell type in the leaf, and their position just below the transparent upper tissues means they receive the most light, so most of the leaf's photosynthesis takes place there.

Hydrogencarbonate Indicator

A solution whose colour depends on the concentration of dissolved carbon dioxide, used to provide evidence about gas exchange. It is yellow when the carbon dioxide concentration is above atmospheric equilibrium, red or orange at atmospheric equilibrium, and purple when the carbon dioxide concentration is below it. The colour observed with a plant reflects the net balance between photosynthesis, which removes carbon dioxide, and respiration, which adds it.

Control Experiment

A second set-up that is identical to the treatment in every respect except the one variable being investigated. Its purpose is to rule out alternative explanations, so that any difference in the result can be attributed to that single variable. In the carbon dioxide investigation the control is a sealed plant with carbon dioxide still available, matched to the treatment for plant, light, temperature, water and duration.

Stoma

A pore in the epidermis of a leaf, found mainly in the lower epidermis and bounded by two guard cells. Carbon dioxide diffuses into the leaf through the stoma and oxygen and water vapour diffuse out. The guard cells alter the size of the pore, and so control the rate of gas exchange and of water loss.

Dicotyledonous Leaf

The leaf of a dicotyledonous plant, seen in transverse section as a series of layers: a waxy cuticle over an upper epidermis, a palisade mesophyll of tall closely packed cells containing many chloroplasts, a spongy mesophyll of rounded cells separated by air spaces, vascular bundles containing xylem above phloem, and a lower epidermis containing stomata, each bounded by two guard cells, covered by a lower cuticle.

Nitrate Ions

Mineral ions absorbed from the soil by plant roots and used as a source of nitrogen. The nitrogen is needed to make amino acids, which are used to build proteins. A shortage of nitrate ions causes poor or stunted growth, and older leaves may turn yellow. Nitrate ions are a building material, not a source of energy.

Variegated Leaf

A leaf with both green and non-green regions, the green regions containing chlorophyll and the non-green regions containing little or none. Because both kinds of region belong to the same leaf and share its light, air, water and temperature, a variegated leaf provides a built-in comparison for investigating whether chlorophyll is needed for photosynthesis.

Plateau on a Photosynthesis Rate Graph

The flat region of a rate graph, in which further increases in the factor on the horizontal axis produce no further increase in the rate of photosynthesis. It shows that this factor is no longer in shortest supply and that a different factor has become limiting. A plateau is a constant high rate, not a stopped process.

Limiting Factor

The factor in shortest supply that restricts the rate of a process. In photosynthesis the candidates are light intensity, carbon dioxide concentration and temperature: whichever is least available at that moment sets the rate, and increasing any of the others has little or no effect until the limiting one is increased.

• “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 plateaus, so photosynthesis has stopped.” Why it is wrong: a plateau is a constant rate, usually a high one. Stopping would be a line falling to zero, which is a different shape entirely. Say instead The rate has become 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 Increasing a factor raises the rate only while that factor is the limiting one. Beyond the optimum temperature, further heating reduces the rate because enzymes denature.
• “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 The enzymes are denatured: the shape of the active site changes, the substrate no longer fits, fewer enzyme–substrate complexes form and the rate falls.
• “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, and therefore 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.

• Two-mark structure for “why starch?” One mark for starch is insoluble; one mark for so it does not affect the water potential of the cell / does not cause water to enter by osmosis. Answers that only say “starch stores more energy” miss both.
• 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.
• 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.
• 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.

• Plant nutrition is unusually rich in practical and data marks. 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.
• Definitions, both equations, the four uses of glucose, the two ions. These are all-or-nothing marks: a definition that omits “using energy from light” scores nothing, 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, quote values from the axes, identify the limiting region, then explain the biology. Description and explanation earn separate marks, so write both.
• Leaf questions want a chain: feature → where it is → what that achieves. “Palisade cells contain many chloroplasts” is half a mark; adding “so more light is absorbed near the upper surface” completes it.
• Rate as volume divided by time, percentage change, and relative light intensity from lamp distance. Show the substitution and give the unit — both are marked.