Cambridge O Level Biology · Syllabus 5090 · Plant Nutrition
Photosynthesis
What is 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.
This definition is part of the Plant Nutrition chapter in Cambridge O Level Biology.
Photosynthesis in context
Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light. It happens in chloroplasts, and the green pigment inside them, chlorophyll, absorbs light and transfers that light energy into chemical energy stored in carbohydrate. Two raw materials go in — carbon dioxide and water. Two products come out — glucose and oxygen. Light and chlorophyll are conditions, not reactants: light supplies the energy and chlorophyll transfers it, and neither appears on the reactant side of the equation. The glucose then goes four ways: it is respired, stored as starch, built into cellulose for cell walls, or converted to sucrose for transport in the phloem. How fast the whole thing runs is set by whichever necessary factor is in shortest supply — the limiting factor — and the leaf is built to keep light, carbon dioxide and water in good supply while letting oxygen out.
The rate of photosynthesis is measured by collecting the oxygen an aquatic plant releases. The plant is placed in water containing sodium hydrogencarbonate solution to supply carbon dioxide, under a funnel that guides the gas into an inverted graduated tube. Counting bubbles per minute is quick but only approximate, because bubbles vary in size. Measuring the volume of oxygen collected per unit time using a graduated tube or a gas syringe is the stronger quantitative method, because volume is a genuine measurement rather than a count of unequal units.
Common mistakes with Photosynthesis
- “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.
- “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.
- “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.
Examiner tips on Photosynthesis
- 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.
- 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.
Questions students ask about Photosynthesis
Is light a reactant in photosynthesis?
No. Light is a source of energy, not a substance that is chemically changed, so it has no atoms to appear in glucose or oxygen. It is written above the reaction arrow, in the place a chemist writes any other condition. The reactants are carbon dioxide and water only.
Why is chlorophyll not written in the photosynthesis equation?
Because it is not used up. Chlorophyll absorbs light, transfers the energy into the reaction, and is unchanged afterwards — which is why a leaf that has photosynthesised all day is still green. Substances that are not consumed are written below the arrow as conditions, never as reactants or products.
Do plants respire at night only?
No. Plants respire continuously, in every living cell, day and night. What changes with light is whether photosynthesis is also happening and how fast. The gas exchange you observe is therefore the net result of both processes together, which is why a plant in bright light appears to take in carbon dioxide and give out oxygen.
Why must a plant be destarched before testing for photosynthesis?
So that any starch found afterwards must have been made during the investigation. Keeping the plant in darkness stops photosynthesis while respiration continues, so the stored starch is used up. Without destarching, a positive starch test could simply be starch that was already there, and the result would support no conclusion at all.
What does a plateau on a photosynthesis graph mean?
It means the factor on the horizontal axis is no longer the limiting factor. Photosynthesis is still occurring, at a constant and usually high rate; it simply cannot go faster because a different factor — carbon dioxide concentration or temperature — is now the one in shortest supply. A plateau never means photosynthesis has stopped.
Why does the rate of photosynthesis fall at high temperatures?
Because photosynthesis is controlled by enzymes. Above the optimum temperature the enzymes are denatured: the shape of the active site changes, so the substrate no longer fits, fewer enzyme–substrate complexes form and the reaction slows. Enzymes are proteins, so they are denatured rather than “killed”.
Why is counting bubbles not a good measure of the rate of photosynthesis?
Because bubbles are not all the same size, so twice as many bubbles does not mean twice as much oxygen. Bubbles can also merge or stick to the apparatus. Counting is quick and useful as an approximate measure, but collecting the gas and measuring its volume per unit time, in a graduated tube or gas syringe, is the stronger quantitative method.
Do nitrate ions give a plant energy?
No. Nitrate ions supply nitrogen, which the plant uses to make amino acids and therefore proteins. They are a building material. A plant’s energy is released by respiring the carbohydrate made in photosynthesis, and a nitrate shortage shows as poor or stunted growth rather than as a lack of energy.
What causes yellow leaves in a plant grown without magnesium?
Magnesium ions are needed to make chlorophyll. Without enough of them less chlorophyll is produced, so the leaves turn yellow — a condition called chlorosis. Because there is less chlorophyll, less light can be absorbed, so the rate of photosynthesis also falls.