Plants prepare glucose in the process of:
- (a)respiration
- (b)photosynthesis
- (c)degradation
- (d)mineral absorption
Correct — B, photosynthesis. Photosynthesis is the process by which green plants build glucose out of carbon dioxide and water, using light energy captured by chlorophyll: 6CO₂ + 6H₂O, in the presence of light and chlorophyll, gives C₆H₁₂O₆ + 6O₂. Two things are worth being exact about. The carbon in the sugar comes from carbon dioxide taken in through the stomata, not from the soil. And the oxygen released comes from the splitting of water, not from the carbon dioxide — a point established by tracing water labelled with a heavy isotope of oxygen. The glucose made this way is either used at once in respiration, stored as starch, or built into cellulose for the cell wall.
- (a)respiration — The opposite process. Respiration breaks glucose down to release the energy stored in it, consuming oxygen and giving out carbon dioxide; it uses sugar rather than making it, and plants do it as continuously as animals.
- (c)degradation — A general word for breaking a large molecule into smaller ones. Making glucose is a building process, so the label is the wrong way round in every sense.
- (d)mineral absorption — What the roots do — taking up nitrogen, phosphorus, potassium and other ions dissolved in soil water. Those minerals are needed to build proteins and chlorophyll, but they are not the source of the carbon in glucose.
Photosynthesis has a light-dependent stage and a light-independent stage. In the first, chlorophyll in the thylakoid membranes of the chloroplast absorbs light, and the energy is used to split water into hydrogen and oxygen; the oxygen is released and the hydrogen is held on carrier molecules along with the energy captured. In the second, that hydrogen and energy are used to reduce carbon dioxide into carbohydrate. The raw materials are carbon dioxide, water, chlorophyll and light; the products are glucose and oxygen. Anything the plant needs beyond carbon, hydrogen and oxygen comes in as mineral ions from the soil.
This is one of the easiest marks on the paper, and the value in it is the chance to fix the two facts that harder versions of the question turn on. The first is the origin of the released oxygen, which is water and not carbon dioxide. The second is that respiration and photosynthesis run at the same time in a green plant rather than by day and by night respectively — what changes with the light is which of the two is faster, so a plant is a net producer of oxygen in daylight and a net consumer at night. Questions that ask which process 'adds carbon dioxide to the carbon cycle' are testing exactly that pair.
- The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and chlorophyll.
- The oxygen released comes from the splitting of water, not from carbon dioxide.
- Chlorophyll sits in the chloroplast, the organelle in which the whole process takes place.
- Glucose made in photosynthesis is respired, stored as starch, or built into cellulose.
- Respiration continues in a plant day and night; light changes which of the two processes runs faster, not whether respiration happens.
- Believing plants respire only at night.
- Assuming the oxygen given off comes from carbon dioxide.
- Treating mineral uptake from the soil as the source of the plant's food rather than of its nutrients.
As a one-line identification like this one, or in a harder form asking what the equation's oxygen comes from, or which processes add carbon dioxide to the carbon cycle.
Which one of the following is the process involved in photosynthesis?
- (a) Potential energy is released to form free energy
- (b) Free energy is converted into potential energy and stored
- (c) Food is oxidized to release carbon dioxide and water
- (d) Oxygen is taken in, and carbon dioxide and water vapour are given out
Answer(b) Free energy is converted into potential energy and stored
The same process described in the language of energy rather than of products. Light energy is trapped and locked into the chemical bonds of a sugar, and the two options that describe oxidation and the taking in of oxygen are respiration wearing the wrong label.
Which one of the following statements about the process of photosynthesis is correct?
- (a) Light energy is directly converted to kinetic energy which oxidizes carbon dioxide.
- (b) Light energy is directly converted to chemical energy which causes reduction of water.
- (c) Chlorophyll absorbs the chemical energy of light which causes oxidation of carbon dioxide.
- (d) Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
Answer(d) Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
The harder version of this question, set in the second session of the same year. It turns on the two details worth carrying away from an easy item — that water is split and that carbon dioxide is reduced, not the other way round.
- practice — not a real PYQ
The oxygen released during photosynthesis comes from
- (a)carbon dioxide
- (b)water
- (c)glucose
- (d)chlorophyll
Answer(b) water — the light-dependent stage splits water molecules, releasing the oxygen and passing the hydrogen on to reduce carbon dioxide.
- practice — not a real PYQ
Which one of the following is NOT a raw material for photosynthesis?
- (a)Carbon dioxide
- (b)Water
- (c)Oxygen
- (d)Light
Answer(c) Oxygen — it is a product of photosynthesis, not a requirement for it; the plant releases it rather than taking it in for this process.