Ch 1 · Photosynthesis · IGCSE Biology 0610
Topic 1 of 3 in Plant Nutrition — IGCSE Biology 0610 — 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 Extended 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 kitchen. To bake a loaf of bread (glucose), you need raw ingredients (carbon dioxide and water) and a power source (light). The chlorophyll acts like the solar panels on the roof of the kitchen, capturing the sun's energy and converting it into the 'electricity' needed to run the oven.
📖 Definitions
🔍 Worked example — Easy
A student is given the following word equation for photosynthesis: \(\text{Carbon dioxide} + \text{Water} \rightarrow \text{Glucose} + \text{Oxygen}\). Identify the two substances that act as the 'raw materials' for this process.
🔍 Worked example — Standard
Complete the balanced chemical equation for photosynthesis: \(6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{?}\)
🔍 Worked example — Exam-style
A plant uses glucose produced during photosynthesis for different purposes. Match the carbohydrate to its specific function: (i) Starch, (ii) Cellulose, (iii) Sucrose.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 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 professional bakery. The plant uses light energy to bake 'bread' (glucose). However, a baker doesn't just leave loaves of bread sitting on the counter; they might turn them into flour for later (starch), use them to build the bakery walls (cellulose), eat them for energy to keep working (respiration), or package them into sweet treats to attract customers (nectar).
📖 Definitions
🔍 Worked example — Easy
A plant cell contains a certain concentration of glucose. If the plant converts this glucose into starch, explain the effect on the water potential of the cell.
🔍 Worked example — Standard
A student observes a plant leaf under a microscope. They identify a large area of starch granules. State the purpose of this starch in the context of the plant's life cycle.
🔍 Worked example — Exam-style
Calculate the percentage increase in glucose concentration if it rises from 0.5 mmol/L to 0.8 mmol/L during a period of high light intensity.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the form in which this carbohydrate is transported from the leaves to the roots. [1] (b) Explain why glucose is converted into starch for storage in tubers like potatoes. [2] [3 marks]
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 a professional kitchen. To bake a cake (carbohydrates), you need ingredients (carbon dioxide and water), a chef (chlorophyll), and an oven (light energy). If you take away the chef, the oven, or the flour, you won't get a cake. In biology, we use the 'starch test' to see if the plant has been 'baking' during our experiment. Since plants store excess glucose as starch, finding starch in a leaf proves photosynthesis has been happening.
📖 Definitions
🔍 Worked example — Easy
A student is testing the effect of light intensity on the rate of photosynthesis using pondweed. The student counts 20 bubbles of oxygen per minute when the lamp is 10 cm away. When the lamp is moved to 20 cm away, the count drops to 10 bubbles per minute. Calculate the percentage decrease in the rate of photosynthesis.
🔍 Worked example — Standard
Describe the procedure to test if carbon dioxide is required for photosynthesis using a variegated leaf and two identical plants.
🔍 Worked example — Exam-style
A graph shows the rate of photosynthesis increasing with temperature up to 35circC, then dropping sharply to zero at 45circC. Explain why the rate drops to zero at 45circC.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State why a water bath was used. [1] (b) The student found that as the lamp was moved closer to the plant, the number of bubbles per minute increased, until the lamp reached a certain distance, after which the rate remained constant. Explain why the rate remained constant. [2] [3 marks]
Key ideas — Limiting factors and glasshouses
🧑🏫 Teacher's explanation
Imagine you are running a busy pizza shop. The speed at which you can make pizzas (the rate) depends on how many chefs you have, how many ovens you have, and how many customers are ordering. If you have 10 chefs but only 1 oven, the oven is your 'limiting factor'. No matter how many chefs you hire, you can't make pizzas any faster because the oven is the bottleneck.
📖 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 500 lux, the rate remains 10 units. Identify the limiting factor at 500 lux.
🔍 Worked example — Standard
Calculate the percentage increase in the rate of photosynthesis when light intensity increases from 50 mol/m^2s to 150 mol/m^2s, assuming the rate increases from 2.0 mg/cm^2s to 5.0 mg/cm^2s.
🔍 Worked example — Exam-style
Explain why a graph showing the rate of photosynthesis against temperature shows a peak followed by a rapid decline.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Gas exchange in an aquatic plant: light and dark
Hydrogencarbonate indicator solution shows changes in dissolved carbon dioxide:
🧑🏫 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
🧠 Quick check 1
🧠 Quick check 2
🎯 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].