Photosynthesis is how plants make their own food. You eat food to get energy. A plant cannot eat, so it makes its food using light.
It matters because almost all life depends on it. Plants make the food and the oxygen that animals need.
By the end you will be able to write the equation and say what the plant does with its glucose. You will explain what limits the speed of photosynthesis. You will also describe the pondweed practical.
What it is, and the equation
A plant takes in carbon dioxide and water. It uses energy from light to turn them into glucose, a sugar. Oxygen is released. Think of a leaf as a tiny kitchen, with light as the cooker.
Word equation: carbon dioxide + water → glucose + oxygen. The reaction takes in energy from light, so it is endothermic. Endothermic means it takes in energy.
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. There are 6 carbon, 12 hydrogen and 18 oxygen atoms on each side. So it is balanced.
Where it happens
Photosynthesis happens in chloroplasts. These are tiny green parts inside plant cells. Leaves have lots of them, especially in the palisade cells near the top.
Chloroplasts contain chlorophyll. This is a green pigment, which means a coloured substance. It absorbs light energy and drives the reaction.
Carbon dioxide enters the leaf through tiny holes called stomata. Water is taken up by the roots and carried up the stem in the xylem. The oxygen made leaves through the stomata.
What the plant does with glucose
Plants do not waste their glucose. Some is used in respiration, which releases energy for the cell to use.
Some is turned into starch for storage. Starch is insoluble, which means it does not dissolve, so it does not draw water in by osmosis. Some becomes cellulose, which builds strong cell walls. Some is made into fats and oils for seeds.
Glucose can also join with nitrate ions from the soil to make amino acids. Amino acids are built into proteins.
Limiting factors
A limiting factor is the thing in shortest supply. It stops the rate of photosynthesis going up. Think of making cheese sandwiches. If you run out of bread, extra cheese does not help.
The main limiting factors are light intensity, carbon dioxide concentration and temperature. The amount of chlorophyll can limit it too.
On a graph, the rising part shows the factor on the x-axis is limiting. The flat part means a different factor is now limiting. At about 45 °C the enzymes are denatured, so the rate falls.
A graph of rate of photosynthesis against light intensity for high and low carbon dioxide. Both rise at first. The low carbon dioxide line levels off at a rate of 5, and the high carbon dioxide line levels off at a rate of 10.
Light, distance and the pondweed practical
Put pondweed in water with a lamp at a measured distance. The oxygen bubbles show the rate. Count them for one minute. Move the lamp and repeat. Keep the temperature the same. Add sodium hydrogencarbonate to keep the carbon dioxide the same.
Higher tier only: light gets weaker with distance. Intensity is proportional to 1 ÷ d². Doubling the distance gives a quarter of the intensity. While light is limiting, the rate falls to a quarter too.
Limiting factors in farming
Growers of crops in greenhouses can control the limiting factors. They add artificial light, heat the greenhouse and add carbon dioxide. The plants grow faster and give a higher yield.
The grower must weigh the extra cost of heating, lighting and carbon dioxide against the extra profit.
First work out which factor is limiting. Then change that one. There is no point adding more of a factor if another factor is now limiting. The extra money would be wasted.
Recap
Photosynthesis: carbon dioxide + water → glucose + oxygen, using light. It is endothermic.
Glucose is used for respiration, starch, cellulose, fats and proteins.
Limiting factors are light, carbon dioxide, temperature and chlorophyll.
Light intensity ∝ 1 ÷ distance² (higher tier).
Key points
carbon dioxide + water → glucose + oxygen, using light. It is endothermic.
Glucose is used for respiration, starch, cellulose, fats and proteins.
Limiting factors: light, carbon dioxide, temperature and chlorophyll.
Light intensity ∝ 1 ÷ distance².
Worked examples
Example 1
Write the word equation for photosynthesis.
The reactants are carbon dioxide and water.
The products are glucose and oxygen.
carbon dioxide + water → glucose + oxygen.
Answer: carbon dioxide + water → glucose + oxygen
Common slips
Slip: glucose + oxygen → carbon dioxide + water This is respiration, not photosynthesis.
Slip: water + oxygen → glucose Carbon dioxide is a reactant, and oxygen is a product.
Slip: carbon dioxide + glucose → water + oxygen Glucose is a product of photosynthesis.
Example 2
In an investigation the lamp is 10 cm from the pondweed and 30 bubbles are given off per minute. Another student suggests that moving the lamp to 20 cm will halve the bubbles. Explain why this is wrong, and say what you would expect.
Light intensity is proportional to 1 ÷ distance squared.
Doubling the distance reduces the intensity to one quarter.
So the rate would fall to about a quarter, which is about 7 or 8 bubbles per minute.
Answer: The intensity falls to a quarter, so about 7 to 8 bubbles per minute
Common slips
Slip: The rate does not change. Light is limiting, so the rate does fall.
Slip: The rate falls to 15. The intensity falls to a quarter, not a half.
Slip: The rate doubles. The lamp is further away, so the rate falls.
Example 3
Explain why starch is a good way for a plant to store glucose.
Glucose is soluble and would change the concentration of cell sap.
That would draw water into cells by osmosis.
Starch is insoluble, so it does not affect osmosis and can be stored in large amounts.
Answer: Starch is insoluble so it does not affect osmosis
Common slips
Slip: Starch dissolves so it is easy to move. Starch is insoluble. That is the advantage for storage.
Slip: Starch gives energy faster than glucose. Glucose is used for fast energy.
Slip: Starch contains more carbon dioxide. Starch is made from glucose, and does not contain carbon dioxide.
Questions and answers
Photosynthesis and limiting factors practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the green pigment that absorbs light for photosynthesis.
Show answer
Answer: chlorophyll
Plants are green because of it.
It is chlorophyll.
Question 2 Easy
Name the gas released by photosynthesis.
Show answer
Answer: oxygen
Photosynthesis makes glucose and a gas.
The gas is oxygen.
Question 3 Easy
Name the cell organelle where photosynthesis takes place.
Show answer
Answer: chloroplast
It contains chlorophyll.
It is the chloroplast.
Question 4 Easy
Name the insoluble carbohydrate that plants use to store glucose.
Show answer
Answer: starch
It is made of many glucose units.
It is starch.
Question 5 Medium
Name one factor that can limit the rate of photosynthesis.
Show answer
Answer: light intensity
The factor in shortest supply limits the rate.
Light intensity is one.
Question 6 Medium
Is photosynthesis exothermic or endothermic?
Show answer
Answer: endothermic
It takes in energy from light.
So it is endothermic.
Question 7 Medium
What is the formula of glucose?
Show answer
Answer: C6H12O6
It has 6 carbon atoms.
The formula is C₆H₁₂O₆.
Question 8 Medium
A lamp is moved from 10 cm to 20 cm from a plant. By what factor does the light intensity fall? Give the number.
Show answer
Answer: 4
Intensity is 1 ÷ distance².
2² = 4, so it is divided by 4.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
(a) Write the balanced symbol equation for photosynthesis. [2 marks] (b) Explain why the rate of photosynthesis may not increase when light intensity is increased. [2 marks]
Show answer and marking points
Answer: (a) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (b) another factor, such as carbon dioxide concentration or temperature, is limiting
(a) correct reactants 6CO₂ and 6H₂O (1)
(a) correct products C₆H₁₂O₆ and 6O₂ (1)
(b) another factor is limiting (1)
(b) for example carbon dioxide concentration or temperature (1)
Exam-style question 24 marks
A student moves a lamp from 10 cm to 20 cm from a plant. (a) State how the light intensity changes. [1 mark] (b) The plant made 48 bubbles of oxygen per minute at 10 cm. Predict the rate at 20 cm and explain your answer. [3 marks]
Show answer and marking points
Answer: (a) it falls to a quarter (b) 12 bubbles per minute, because intensity is proportional to 1 ÷ distance², and the rate is proportional to the intensity
(a) the intensity falls to a quarter (1)
(b) 48 ÷ 4 (1)
(b) 12 bubbles per minute (1)
(b) intensity is proportional to 1 ÷ distance squared (1)
Common mistakes
Writing the equation for respiration instead of photosynthesis
Photosynthesis takes in carbon dioxide and water, and makes glucose and oxygen.
Saying a plant only respires at night
Plants respire all the time. They photosynthesise only in the light.
Saying halving the distance doubles the light intensity
The intensity depends on distance squared, so halving the distance gives four times the intensity.
Saying the factor on the x-axis limits the flat part of the graph
On the flat part, a different factor is limiting.
Questions students ask
Is Photosynthesis and limiting factors on the Foundation or Higher tier?
Photosynthesis and limiting factors is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
What is a common mistake on Photosynthesis and limiting factors?
Writing the equation for respiration instead of photosynthesis. Photosynthesis takes in carbon dioxide and water, and makes glucose and oxygen.
Where are the answers to the Photosynthesis and limiting factors questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.