Ch 1 · Photosynthesis · O Level Biology 5090
Topic 1 of 3 in Plant Nutrition — O Level Biology 5090 — 54 lessons.
Introduction
Photosynthesis is the process on which almost every food chain depends. Plants use energy from light to build carbohydrates from carbon dioxide and water, releasing oxygen. This lesson covers the equations, the uses of carbohydrate, the starch-test experiments and, for candidates, limiting factors and glasshouses.
Key ideas — Definition and equations
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide (entering through the stomata) and water (absorbed by the roots). Light is absorbed by chlorophyll, the green pigment in chloroplasts. Chlorophyll transfers energy from light into energy in chemicals, which is used for the synthesis of carbohydrates.
🧑🏫 Teacher's explanation
Think of a plant like a solar-powered bakery. The raw ingredients are carbon dioxide (from the air) and water (from the ground). The oven is the chlorophyll, which captures sunlight to provide the energy needed to bake these ingredients into 'bread' (glucose). Without the sun's energy, the ingredients just sit there; the chlorophyll's job is to transform that light energy into chemical energy stored in the bonds of the sugar molecules.
📖 Definitions
🔍 Worked example — Easy
Write out the balanced chemical equation for photosynthesis and identify the gas released as a byproduct.
🔍 Worked example — Standard
A plant uses 12g of glucose to build cell walls. If the plant produces glucose via photosynthesis, state the primary use of the glucose produced for building cell walls and the name of the polymer formed.
🔍 Worked example — Exam-style
In an experiment to investigate the effect of light intensity on the rate of photosynthesis, a student places a water plant in a beaker and covers it with a glass tube. The student moves a lamp closer to the plant. Predict the effect on the rate of oxygen bubble production and state the independent variable.
⚠️ Common errors
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🎯 Exam question
Key ideas — Uses of the carbohydrates produced
Glucose made in photosynthesis is quickly converted into other substances.
🧑🏫 Teacher's explanation
Think of a plant like a high-end bakery. Photosynthesis is the process of taking raw ingredients (water and carbon dioxide) and using solar energy to bake bread (glucose). However, a bakery doesn't just leave loaves of bread sitting on the counter; they package them for storage, use some to feed the staff, or turn them into different products like cakes or pastries to sell.
📖 Definitions
🔍 Worked example — Easy
A plant leaf contains 15\% starch by mass. After a period of intense photosynthesis, the starch content increases to 25\% by mass. Calculate the percentage increase in starch content.
🔍 Worked example — Standard
Explain why plants convert glucose into starch for storage rather than storing it as glucose.
🔍 Worked example — Exam-style
A student investigates the effect of light intensity on the rate of photosynthesis. The student finds that at low light intensities, increasing light intensity increases the rate, but at high light intensities, the rate remains constant. Identify the limiting factor in the first scenario and explain the second scenario.
⚠️ Common errors
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🎯 Exam question
Key ideas — Investigating what photosynthesis needs
All three classic experiments use the starch test: the leaf is placed in boiling water to kill it, then in hot ethanol (in a water bath, not over a flame) to remove chlorophyll, rinsed in water to soften it, and covered with iodine solution. Blue-black shows starch, so photosynthesis has occurred.
🧑🏫 Teacher's explanation
Think of photosynthesis like baking a cake. To make a cake, you need specific ingredients (raw materials like carbon dioxide and water), a way to mix them (chlorophyll), and an oven to provide the energy (light). If you run out of flour, the baking stops; if you turn off the oven, the baking stops. In biology, we call these 'limiting factors'.
📖 Definitions
🔍 Worked example — Easy
A student performs a starch test on a leaf. The leaf was placed in boiling water, then ethanol, then rinsed, and finally covered with iodine. Describe the expected color change if the leaf has been actively photosynthesizing.
🔍 Worked example — Standard
A student is investigating the effect of light intensity on the rate of photosynthesis using pondweed. They observe that as the lamp is moved closer to the plant, the number of oxygen bubbles released per minute increases. Calculate the percentage increase in bubble production if the rate increases from 10 bubbles/min to 25 bubbles/min.
🔍 Worked example — Exam-style
In an experiment to test the necessity of carbon dioxide, a leaf is placed in a sealed container with soda lime. Another leaf is placed in a sealed container with sodium hydrogencarbonate solution. Predict the result of the iodine test for both leaves and explain the difference.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the student uses a water bath instead of just leaving the plant in the room. (b) Predict the effect on the rate of photosynthesis if the temperature is increased to 45circC. Explain your answer. [3 marks]
Key ideas — Limiting factors and glasshouses
limiting factors. A limiting factor is the factor in shortest supply that restricts the rate of a process. On a graph of rate against light intensity, the line first rises: light is limiting, so more light gives a faster rate. It then levels off: light is no longer limiting and another factor, carbon dioxide concentration or temperature, now sets the rate. If repeating the experiment with more carbon dioxide gives a higher plateau, carbon dioxide was the limiting factor. Temperature limits the rate because photosynthesis is controlled by enzymes; too high a temperature denatures them and the rate falls.
🧑🏫 Teacher's explanation
Think of photosynthesis like a factory assembly line. To make a product, you need workers (enzymes), raw materials (carbon dioxide and water), and electricity to run the machines (light). If you have plenty of materials and electricity, but only two workers, the factory can only produce at the speed of those two workers. In this case, the number of workers is the 'limiting factor'.
📖 Definitions
🔍 Worked example — Easy
A student measures the rate of photosynthesis at different light intensities. At 100 lux, the rate is 5 units. At 200 lux, the rate is 10 units. At 300 lux, the rate is 10 units. Identify the limiting factor at 100 lux and 300 lux.
🔍 Worked example — Standard
Calculate the percentage increase in the rate of photosynthesis when light intensity is increased from 50 units to 150 units, assuming light is the limiting factor throughout.
🔍 Worked example — Exam-style
An experiment shows that as temperature increases from 20^° C to 40^° C, the rate of photosynthesis increases. However, at 50^° C, the rate drops to zero. Explain why the rate falls at 50^° C.
⚠️ Common errors
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🎯 Exam question
(a) State two factors the grower can control in the glasshouse to achieve this. [2] (b) Explain why increasing the temperature beyond the optimum temperature will decrease the rate of photosynthesis. [2] [4 marks]
Key ideas — Aquatic plants and hydrogencarbonate indicator
Submerged aquatic plants such as pondweed are used because the oxygen they release is visible as bubbles, and because changes in dissolved carbon dioxide can be shown with hydrogencarbonate indicator solution.
🧑🏫 Teacher's explanation
Think of photosynthesis like baking a cake. To make a cake, you need ingredients (carbon dioxide and water) and an oven (light energy). If you take away the flour, you can't make a cake, no matter how hot the oven is. In plants, the 'flour' is carbon dioxide.
📖 Definitions
🔍 Worked example — Easy
A plant is placed in a dark cupboard for 48 hours. What is the purpose of this 'destarching' process?
🔍 Worked example — Standard
Calculate the percentage increase in the rate of photosynthesis when light intensity is increased from 20 mol/m^2s to 50 mol/m^2s, assuming the rate increases from 5 mg/h to 15 mg/h.
🔍 Worked example — Exam-style
A student investigates the effect of light intensity on photosynthesis. They observe that as light intensity increases, the rate of photosynthesis increases, but then levels off. Explain why the rate levels off.
⚠️ Common errors
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🎯 Exam question
(a) Why was the plant kept in the dark? [1] (b) Describe the color change observed in the leaf if photosynthesis occurred successfully. [1] (c) State the balanced chemical equation for photosynthesis. [2] [4 marks]
Worked example
Question. A destarched potted plant was set up with one leaf enclosed in a flask containing soda lime and another leaf in a flask containing sodium hydrogencarbonate solution. After six hours in bright light both leaves were tested for starch. Explain the purpose of each flask and predict the results. [4]
Common mistakes
Practice
Answers Carbon dioxide + water → glucose + oxygen [B1]; light energy and chlorophyll shown as conditions [B1]. Chlorophyll transfers energy from light into energy in chemicals [B1]. Destarch the plant in the dark for 48 hours [M1]; cover part of a leaf with foil and leave the plant in light for several hours [M1]; kill in boiling water, decolourise in hot ethanol, rinse, add iodine solution [M1]; the uncovered region turns blue-black and the covered region stays orange-brown [A1]. Any three of: starch as an energy store; cellulose for cell walls; glucose for respiration; sucrose for transport or nectar [B1 each]. In the rising part, light intensity is the limiting factor [B1]; on the plateau, carbon dioxide concentration or temperature limits the rate [B1]; the grower should increase carbon dioxide or temperature, not light [B1]; this raises the rate and the crop yield [B1].