PHOTOSYNTHESIS

AQA GCSE Biology · Lessons 1–2
⬇ Student Booklet ⬇ Teacher Booklet

Key Terms — Photosynthesis

All Terms

01
Limiting factor
DefinitionSomething that slows down the rate of photosynthesis if it is in short supply. The three main limiting factors are light intensity, carbon dioxide concentration and temperature.
02
Light intensity
DefinitionLight provides the energy for photosynthesis. As light intensity increases the rate increases, until light is no longer limiting.
03
Carbon dioxide concentration
DefinitionCarbon dioxide is a reactant in photosynthesis. As its concentration increases the rate increases; on a sunny day it is often the main limiting factor.
04
Temperature
DefinitionPhotosynthesis is controlled by enzyme-catalysed reactions, so the rate rises with temperature up to an optimum and then falls.
05
Optimum temperature
DefinitionThe temperature at which the rate of photosynthesis reaches its maximum.
06
Denature
DefinitionWhat happens to enzymes at high temperatures (40–50 °C): they can no longer catalyse the reactions, so the rate falls.
07
Plateau
DefinitionThe flat part of a rate graph, where the factor plotted is no longer limiting and another factor has become the limiting factor.
08
Greenhouse
DefinitionA structure in which growers control light intensity, temperature and carbon dioxide concentration to keep photosynthesis at its optimum and maximise crop yield.
09
CO₂ generator
DefinitionEquipment used in a commercial greenhouse to increase the carbon dioxide concentration.
10
Sodium hydrogencarbonate
DefinitionThe solution added to the boiling tube in the required practical. It supplies carbon dioxide, so CO₂ does not become a limiting factor.
11
Pondweed
DefinitionThe aquatic organism used in the required practical. The oxygen bubbles it releases are counted to measure the rate of photosynthesis.
12
Dependent variable
DefinitionThe variable you measure. In the required practical it is the number of bubbles produced (the rate of photosynthesis).
13
Control variable
DefinitionA variable kept the same so the test is fair — here temperature, carbon dioxide concentration and the type or size of pondweed.
14
LED lamp
DefinitionA lamp used in place of a normal bulb because it gives off less heat, so temperature does not affect the results.
15
Inverse square law
DefinitionLight intensity is inversely proportional to the square of the distance from the source (intensity ∝ 1/d²), because the light energy spreads out over a larger area.
16
Anomalous reading
DefinitionA result that does not fit the pattern of the others. It is ignored when calculating a mean.
17
Mean
DefinitionThe average of the repeated readings. Repeating and calculating a mean increases reliability.
18
Coloured filter
DefinitionA filter placed in front of the lamp to change the colour (wavelength) of the light, so its effect on the rate can be investigated.
19
Starch
DefinitionThe insoluble store that some of the glucose made in photosynthesis is changed into. It is stored for later use.
20
Osmosis
DefinitionThe movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
21
Active transport
DefinitionThe movement of substances against a concentration gradient, using energy from respiration.
22
Photosynthesis equation
Definitioncarbon dioxide + water → glucose + oxygen  ·  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Lesson 1 · Effect of Limiting Factors on Photosynthesis

Do Now

Q1
Write the balanced symbol equation for photosynthesis.
Model Answer6CO2 + 6H2O -> C6H12O6 + 6O2.
Q2
Define active transport.
Model AnswerThe movement of substances against a concentration gradient, using energy from respiration.
Q3
Why does transpiration decrease when humidity increases?
Model AnswerHigh humidity reduces the concentration gradient, so less water evaporates.

Part 1 · What is a limiting factor?

Read the passage.

The rate of photosynthesis is affected by limiting factors. A limiting factor is something that slows down the rate of photosynthesis if it is in short supply. The three main limiting factors are:

  • Light intensity
  • Carbon dioxide concentration
  • Temperature

If any of these factors is below its optimum level, it limits the rate of photosynthesis. For example, plants grow faster in summer because there is:

  • More light
  • Higher temperatures

This increases the rate of photosynthesis.

Questions — What is a limiting factor?

Q1
What is a limiting factor? (2 marks)
Model AnswerSomething that slows down the rate of photosynthesis when it is in short supply.
Q2
State the three factors that can affect the rate of photosynthesis. (3 marks)
Model AnswerLight intensity, carbon dioxide concentration and temperature.
Q3
Why do plants grow faster in summer? (2 marks)
Model AnswerThere is more light and higher temperatures, which increase the rate of photosynthesis.
Q4
What does it mean if a factor is below its optimum level? (1 mark)
Model AnswerIt limits (slows) the rate of photosynthesis.
Q5
Which limiting factor is also a reactant in photosynthesis? (1 mark)
Model AnswerCarbon dioxide concentration.
Q6
Is light intensity a limiting factor? (1 mark)
Model AnswerYes.

Part 2 · How each factor affects the rate

Read about each factor and look at the graphs.

Light intensity

Light provides the energy for photosynthesis.

  • As light intensity increases, the rate of photosynthesis increases.
  • At low light intensity, photosynthesis is slow or may stop.
  • A maximum rate is reached where light is no longer limiting.
  • At this point another factor (carbon dioxide or temperature) becomes the limiting factor.
Fig 1.1 — Light is the limiting factor on the steep part; another factor limits the rate on the plateau.
Fig 1.1 — Light is the limiting factor on the steep part; another factor limits the rate on the plateau.

Carbon dioxide concentration

Carbon dioxide is a reactant in photosynthesis.

  • As carbon dioxide concentration increases, the rate of photosynthesis increases.
  • At low concentrations, photosynthesis is limited.
  • A maximum rate is reached where carbon dioxide is no longer limiting; another factor then becomes limiting.
  • On a sunny day, carbon dioxide is often the main limiting factor.
Fig 1.2 — Carbon dioxide is the limiting factor on the steep part; another factor limits the rate on the plateau.
Fig 1.2 — Carbon dioxide is the limiting factor on the steep part; another factor limits the rate on the plateau.

Temperature

Photosynthesis is controlled by enzyme-catalysed reactions.

  • As temperature increases, the rate increases because enzyme activity rises (molecules collide more often).
  • The rate reaches a maximum at the optimum temperature.
  • At high temperatures (40-50C) the enzymes denature, so the rate falls.
  • At low temperatures, enzyme activity is low, so the rate is slow.
Fig 1.3 — The rate rises to an optimum temperature, then falls as the enzymes denature.
Fig 1.3 — The rate rises to an optimum temperature, then falls as the enzymes denature.

Questions — How each factor affects the rate

Q7
What happens to the rate of photosynthesis when light intensity increases? (1 mark)
Model AnswerThe rate increases (until another factor becomes limiting).
Q8
What happens to the rate when the carbon dioxide concentration increases? (1 mark)
Model AnswerThe rate increases (until another factor becomes limiting).
Q9
What happens to the rate between 0C and 40C? (1 mark)
Model AnswerIt increases as temperature rises (faster enzyme activity).
Q10
Why does the rate decrease after about 40C? (2 marks)
Model AnswerThe enzymes start to denature, so they can no longer catalyse the reactions.
Q11
What provides the energy for photosynthesis? (1 mark)
Model AnswerLight.
Q12
What happens at maximum light intensity? (2 marks)
Model AnswerLight is no longer limiting, so another factor (CO2 or temperature) becomes the limiting factor.
Q13
Why is carbon dioxide often the limiting factor on a sunny day? (2 marks)
Model AnswerOn a bright, warm day light and temperature are high, so CO2 is the factor in shortest supply.

Part 3 · Greenhouses

Read about greenhouses and look at the diagram.

Commercial greenhouses use knowledge of limiting factors to increase the rate of photosynthesis and maximise crop yield. Conditions are controlled to keep factors at their optimum levels:

  • Light intensity is increased using artificial lighting.
  • Temperature is controlled using heaters.
  • Carbon dioxide concentration is increased using CO2 generators.
Fig 1.4 — A greenhouse lets growers control light, temperature and carbon dioxide.
Fig 1.4 — A greenhouse lets growers control light, temperature and carbon dioxide.

This increases the rate of photosynthesis, leading to faster plant growth, increased crop yield and higher profits.

AdvantagesDisadvantages
Increased yield and faster growthHigh cost of energy (lighting and heating)
Ability to grow crops all year roundCost of equipment and monitoring systems

Growers must balance the increased profit against the costs of maintaining optimal conditions.

Questions — Greenhouses

Q14
What conditions can be controlled in a greenhouse? (3 marks)
Model AnswerLight intensity, temperature and carbon dioxide concentration.
Q15
What are the advantages of using commercial greenhouses? (2 marks)
Model AnswerIncreased yield and faster growth, and the ability to grow crops all year round.
Q16
What are the disadvantages of commercial greenhouses? (2 marks)
Model AnswerThe high cost of energy (lighting and heating) and of equipment and monitoring.
Q17
How is light increased in a greenhouse? (1 mark)
Model AnswerWith artificial lighting.
Q18
How is carbon dioxide increased in a greenhouse? (1 mark)
Model AnswerWith CO2 generators.
Q19
Why must growers balance costs against profit? (1 mark)
Model AnswerThe higher yield earns more money, but only if it outweighs the cost of maintaining optimum conditions.

Exam-style questions

Q1
The graph shows how the rate of photosynthesis is affected by different conditions. (7 marks)
Q1 — The graph shows how the rate of photosynthesis is affected by different conditions.
(a) What patterns can you find from this graph?(b) How useful could this information be to a grower using glasshouses? Give reasons for your answer.
Mark Scheme(a) As light intensity increases the rate increases then levels off (plateaus); a higher carbon dioxide concentration (4% vs 0.03%) gives a much higher rate; a higher temperature (25C vs 15C) gives a higher rate; at 0.03% CO2 the rate is low and plateaus quickly, so CO2 is limiting.(b) Very useful: it shows that increasing CO2, temperature and light all raise the rate of photosynthesis, so a grower could increase yield by controlling these in a glasshouse - but must weigh the higher yield against the cost.
Q2
(6 marks)(a) The equation describes the process of photosynthesis: carbon dioxide + ______ + light energy -> glucose + ______.(i) Write in the names of the two missing substances.(ii) Name the green substance which absorbs the light energy.(b) (i) In bright sunlight, the concentration of carbon dioxide in the air can limit the rate of photosynthesis. Explain what this means.(ii) Give one environmental factor, other than light intensity and carbon dioxide concentration, which can limit the rate of photosynthesis.
Mark Scheme(a) (i) water; oxygen.(ii) Chlorophyll.(b) (i) It means carbon dioxide is in short supply, so even though there is plenty of light the lack of CO2 holds back (limits) the rate of photosynthesis.(ii) Temperature.
Q3
(5 marks)
Q3 —
(a) Complete the equation for photosynthesis: carbon dioxide + ______ ->(light energy) ______ + oxygen.(b) A farmer grew tomato plants in a greenhouse. The graph shows the effect of light intensity on the rate of photosynthesis in the tomato plants.(i) At which light intensity was light a limiting factor for photosynthesis (1 / 4 / 10 arbitrary units)?(ii) What was the highest rate of photosynthesis?(iii) Apart from light intensity, name one factor the farmer could change to increase the rate of photosynthesis.
Mark Scheme(a) water; glucose.(b) (i) 1 arbitrary unit (the rate is still rising, so light is limiting).(ii) About 210 arbitrary units (the plateau).(iii) Carbon dioxide concentration (or temperature).
Q4
Plants are grown in glasshouses to protect them from the weather or extend the growing season. In winter, when days are shorter, glasshouses are heated to keep the enzyme reactions at optimum rates. What else should a grower do to make sure the plants are photosynthesising at the optimum rate? Give a reason for your answer. (3 marks)
Mark SchemeProvide extra artificial lighting (and/or add carbon dioxide). In winter the days are short and light intensity is low, so light would be the limiting factor; adding light (and CO2) keeps photosynthesis at its maximum rate, increasing yield.
Q5
The graph shows the concentration of carbon dioxide in the air in a greenhouse full of tomato plants, measured over a period of 24 hours. (4 marks)
Q5 — The graph shows the concentration of carbon dioxide in the air in a greenhouse full of tomato plants, measured over a period of 24 hours.
(a) Explain why the concentration of carbon dioxide in the greenhouse increased between X and Y.(b) Explain why the concentration of carbon dioxide in the greenhouse decreased between Y and Z.
Mark Scheme(a) Between X and Y it is night, so the plants respire but do not photosynthesise; they release carbon dioxide, so its concentration rises.(b) Between Y and Z it is daytime, so the plants photosynthesise faster than they respire, using up carbon dioxide faster than it is produced, so its concentration falls.
Q6
The diagram shows some plants growing in a greenhouse on a hot summer's day. Which one of the following factors is most likely to limit the rate of photosynthesis at this time (carbon dioxide concentration / light intensity / temperature)? Explain the reason for your answer. (4 marks)
Q6 — The diagram shows some plants growing in a greenhouse on a hot summer's day. Which one of the following factors is most likely to limit the rate of photosynthesis at this time (carbon dioxide concentration / light intensity / temperature)? Explain the reason for your answer.
Mark SchemeCarbon dioxide concentration. On a hot, sunny day the light intensity and temperature are both high (so they are not limiting), so carbon dioxide is the factor in shortest supply and limits the rate of photosynthesis.
Q7
Photosynthesis uses carbon dioxide to make glucose. (5 marks)
Q7 — Photosynthesis uses carbon dioxide to make glucose.
(b) The graph shows the effect of the concentration of carbon dioxide on the rate of photosynthesis in tomato plants at 20C.(i) What is the maximum rate of photosynthesis of the tomato plants shown in the graph?(ii) At point X, carbon dioxide is not a limiting factor. Suggest one factor that is limiting the rate of photosynthesis at point X.(c) A farmer plans to grow tomatoes in a large greenhouse. The atmosphere is 0.04% carbon dioxide; the farmer adds carbon dioxide so its concentration is 0.08%.(i) Why does the farmer use 0.08% carbon dioxide (to increase the rate of growth / to increase the rate of respiration / to increase water uptake)?(ii) Why does the farmer not use a concentration higher than 0.08% (tick two: it would cost more money / it would decrease the temperature / it would not increase the rate any further / it would increase water loss)?
Mark Scheme(b) (i) About 20 arbitrary units (the plateau).(ii) Light intensity (or temperature).(c) (i) To increase the rate of growth of the tomato plants.(ii) Because it would cost more money, and because it would not increase the rate of photosynthesis any further (another factor has become limiting).

Lesson 2 · Required Practical: Light Intensity & Photosynthesis

Do Now

Q1
Define osmosis.
Model AnswerThe movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
Q2
Give two differences between a eukaryotic and a prokaryotic cell.
Model AnswerProkaryotes have no nucleus (DNA is circular in the cytoplasm) and no membrane-bound organelles.
Q3
Explain how light intensity affects the rate of transpiration.
Model AnswerHigher light intensity opens the stomata for photosynthesis, increasing transpiration.

Part 1 · The method

Read the method and look at the diagram.

Risk assessment

  • Take care when handling glassware.
  • Take care with lamps, which may get hot.
  • Take care with water near the electrical power supply for the lamp.

Method

  1. Set up a boiling tube 40 cm from the light source. (Use an LED, which gives off less heat; if a normal bulb is used, place a beaker of water between the bulb and the tube to absorb heat.)
  2. Fill the boiling tube with sodium hydrogencarbonate solution. (This supplies carbon dioxide, so it is not a limiting factor.)
  3. Put a piece of pondweed into the boiling tube.
  4. Leave it for a couple of minutes. (This lets the pondweed adjust to the new light intensity.)
  5. Start the stopwatch and count the number of bubbles produced in 1 minute.
  6. Repeat twice more and calculate a mean. (This increases reliability; ignore any anomalous readings.)
  7. Move the lamp to 30 cm, 20 cm and 10 cm and repeat the measurements.
  8. Record your results in a table and plot a line graph.
Fig 2.1 — Counting oxygen bubbles from pondweed at different lamp distances.
Fig 2.1 — Counting oxygen bubbles from pondweed at different lamp distances.
Distance from light (cm)Test 1Test 2Test 3Mean
40
30
20
10

Questions — The method

Q1
Why do you need to add sodium hydrogencarbonate to the water? (1 mark)
Model AnswerIt supplies carbon dioxide, so CO2 does not become a limiting factor.
Q2
What is the dependent variable in this experiment? (1 mark)
Model AnswerThe number of bubbles produced (the rate of photosynthesis).
Q3
What variables should be controlled? (2 marks)
Model AnswerTemperature, carbon dioxide concentration, and the same piece of pondweed (type/size).
Q4
What do you need to measure to calculate the rate of photosynthesis? (1 mark)
Model AnswerThe number of bubbles produced in a set time.
Q5
Why should LED bulbs be used? (1 mark)
Model AnswerThey give off less heat, so temperature does not affect the results.
Q6
Which aquatic organism is used in this practical? (1 mark)
Model AnswerPondweed.
Q7
Describe the method briefly. (3 marks)
Model AnswerPlace pondweed in sodium hydrogencarbonate solution, put the lamp at a set distance, count the bubbles per minute, repeat for a mean, then change the distance and repeat.

Part 2 · The inverse square law (HT only)

Read about light intensity and distance.

There is an inverse relationship between distance and light intensity:

  • As the distance from the light source increases, the light intensity decreases.
  • This is because the light energy spreads out over a larger area.

The light intensity is inversely proportional to the square of the distance - this is the inverse square law. For instance, for the lamp 10 cm away from the plant:

relative light intensity = 110 x 10 = 1100 = 0.01

Questions — The inverse square law (HT only)

Q8
What is the inverse square law? (2 marks)
Model AnswerLight intensity is inversely proportional to the square of the distance from the source (intensity proportional to 1/d²).
Q9
How does distance affect light intensity? (2 marks)
Model AnswerAs distance increases, light intensity decreases.
Q10
Why does light intensity decrease with distance? (2 marks)
Model AnswerThe light energy spreads out over a larger area as it travels further.
Q11
Calculate the relative light intensity at a distance of 10 cm using 1/d². (2 marks)
Model Answer
Worked Answer (VESSU)
V
distance d = 10 cm
light intensity is proportional to 1/d²
relative intensity = ?
E
intensity proportional to 1 / (d x d)
S
= 1 / (10 x 10)
S
= 1100 = 0.01
U
arbitrary units
Answer: 0.01 (1/100)
Q12
Is light intensity proportional or inversely proportional to the square of the distance? (1 mark)
Model AnswerInversely proportional.
Q13
What happens to the light intensity if the distance is doubled? (1 mark)
Model AnswerIt falls to a quarter (1/4) of its value.

Part 3 · Adapting for other factors

Read about how the practical can be adapted.

This practical can be adapted in the following ways to test different factors:

FactorAdaptationControl variables
Carbon dioxide concentrationChange the concentration of sodium hydrogencarbonate by adding a different mass to the beaker.Keep light intensity the same; keep temperature constant.
Colour / wavelength of lightUse coloured filters in front of the lamp to change the colour of the light.Keep light intensity the same; keep temperature constant.
TemperaturePlace the beaker in a water bath at different temperatures.Keep light intensity the same; keep carbon dioxide concentration constant.

Questions — Adapting for other factors

Q14
How could you measure the effect of light wavelength on the rate of photosynthesis? (2 marks)
Model AnswerUse coloured filters in front of the lamp, keeping light intensity and temperature constant, and count the bubbles.
Q15
How could you measure the effect of carbon dioxide concentration? (2 marks)
Model AnswerAdd different masses of sodium hydrogencarbonate to change the CO2 concentration, keeping light and temperature constant.
Q16
How could you test the effect of temperature? (2 marks)
Model AnswerPlace the beaker in a water bath at different temperatures, keeping light intensity and CO2 constant.
Q17
What must you control when testing CO2 concentration? (1 mark)
Model AnswerLight intensity and temperature.
Q18
Bubble counts in 2 minutes were 22, 34, 31 and 31. Calculate the mean rate of bubbles per minute. (3 marks)
Model Answer
Worked Answer (VESSU)
V
counts in 2 min: 22, 34, 31, 31
mean = (22+34+31+31) / 4 = 29.5
rate per minute = ?
E
rate = mean count / time
S
= 29.5 / 2
S
= 29.52 = 14.75
U
bubbles per minute
Answer: 14.75 bubbles per minute

Exam-style questions

Q1
Students investigated the effect of light intensity on the rate of photosynthesis in pondweed. The diagram shows the equipment. Method: place the lamp 50 cm from the pondweed; count the number of bubbles of gas released in two minutes; repeat with the lamp at different distances. (8 marks)
Q1 — Students investigated the effect of light intensity on the rate of photosynthesis in pondweed. The diagram shows the equipment. Method: place the lamp 50 cm from the pondweed; count the number of bubbles of gas released in two minutes; repeat with the lamp at different distances.
Distance of lamp (cm)1234Mean
5058656
40108949
301212151714
202517232424
1022343131X
(a) The students could not make a firm conclusion because their method did not control enough variables. Give two variables the students have not controlled that would affect the rate of photosynthesis.(b) Calculate the mean rate of bubbles produced per minute when the lamp was 10 cm from the pondweed.(c) The mean number of bubbles at 30 cm was greater than at 50 cm. How many times greater?(d) Describe how the method could be adapted to find the effect of different wavelengths of light.
Mark Scheme(a) Any two of: temperature, carbon dioxide concentration, the type/size of pondweed, the time counted.(b) Mean count = (22+34+31+31)/4 = 29.5 in 2 minutes; per minute = 29.5 / 2 = 14.75 bubbles per minute.(c) 14 / 6 = 2.3 times greater.(d) Place coloured filters in front of the lamp to change the colour (wavelength) of the light, keeping the light intensity, distance and temperature the same, and count the bubbles.
Mark Scheme (VESSU)
V
counts in 2 min: 22, 34, 31, 31
mean = (22+34+31+31) / 4 = 29.5
rate per minute = ?
E
rate = mean count / time
S
= 29.5 / 2
S
= 29.52 = 14.75
U
bubbles per minute
Answer: 14.75 bubbles per minute
Q2
This question is about photosynthesis. (7 marks)
Q2 — This question is about photosynthesis.
Q2 — This question is about photosynthesis.
(a) Plants make glucose during photosynthesis. Some of the glucose is changed into insoluble starch. What happens to this starch (it is converted into oxygen / it is stored for later use / it is used to make the leaf green)?(b) A student investigated the effect of temperature on the rate of photosynthesis in pondweed (see diagram).(i) The student needed to control some variables to make the investigation fair. State two of these variables.(ii) The bubbles of gas are produced only while photosynthesis is taking place. What two measurements would the student make to calculate the rate of photosynthesis?(c) The graph shows the effect of temperature on the rate of photosynthesis.(i) Name the factor that limits the rate between the points labelled A and B.(ii) Suggest which factor - carbon dioxide, oxygen or water - might limit the rate between the points labelled C and D.
Mark Scheme(a) It is stored for later use.(b) (i) Any two of: light intensity, carbon dioxide concentration, distance of lamp, type/size of pondweed.(ii) The number of bubbles produced and the time taken (to give bubbles per minute).(c) (i) Temperature.(ii) Carbon dioxide.
Q3
Some students investigated the effect of light intensity on the rate of photosynthesis using the apparatus shown in Diagram 1. They placed the lamp 10 cm from the pondweed, counted the number of bubbles released in 1 minute, and repeated this for different distances. (3 marks)
Q3 — Some students investigated the effect of light intensity on the rate of photosynthesis using the apparatus shown in Diagram 1. They placed the lamp 10 cm from the pondweed, counted the number of bubbles released in 1 minute, and repeated this for different distances.
Distance (cm)Number of bubbles per minute
1084
1584
2076
4052
5026
(a) The lamp gives out heat as well as light. What could the students do to make sure that heat from the lamp did not affect the rate of photosynthesis?(b) The table shows the students' results.(i) At distances between 15 cm and 50 cm, light was a limiting factor for photosynthesis. What evidence is there for this in the table?(ii) Give one factor that could have limited the rate of photosynthesis when the distance was between 10 cm and 15 cm.
Mark Scheme(a) Place a beaker (or tank) of water between the lamp and the pondweed to absorb the heat (or use an LED lamp / keep the temperature constant with a water bath).(b) (i) As the distance decreases from 50 cm to 15 cm the number of bubbles increases (26 -> 84), so changing the light intensity changed the rate - light was limiting.(ii) Carbon dioxide concentration (or temperature) - the rate did not increase between 15 cm and 10 cm, so another factor was limiting.
Q4
(13 marks)
Figure note: Part (c)(ii) requires graph paper - provide a grid for the student to plot the means and draw a line of best fit.
Q4 —
Distance d (cm)1234Mean
105252545453
204951485250
303230273130
403010911
(a) Complete the equation for photosynthesis (draw a ring around each correct answer): carbon dioxide + (hydrogen / nitrogen / water) ->(light energy) (alcohol / glucose / methane) + oxygen. Some students investigated the effect of light intensity on the rate of photosynthesis in pondweed (see diagram); the closer the lamp, the more light the pondweed receives. They counted the number of bubbles released in 1 minute for each distance d.(b) A thermometer was placed in the glass beaker. Why was it important to use a thermometer in this investigation?(c) The students counted the bubbles four times at each distance and calculated the mean (table).(i) Calculate the mean number of bubbles released per minute when the lamp was 40 cm from the pondweed.(ii) On graph paper, draw a graph to show the students' results (plot the means and draw a line of best fit).(iii) One student concluded that the rate of photosynthesis was inversely proportional to the distance of the lamp from the plant. Does the data support this conclusion? Explain your answer.
Mark Scheme(a) carbon dioxide + water ->(light energy) glucose + oxygen.(b) The lamp gives out heat, which would raise the temperature and could change the rate of photosynthesis; the thermometer checks the temperature stays constant (a control variable), so only light intensity affects the results.(c) (i) Ignoring the anomalous reading (30): (10 + 9 + 11) / 3 = 10 bubbles per minute.(ii) Plot distance d on the x-axis and mean bubbles per minute on the y-axis - points (10, 53), (20, 50), (30, 30), (40, 10) - with a smooth line of best fit.(iii) No. Inversely proportional means doubling the distance would halve the rate (and rate is proportional to 1/d²); the data do not show this exact relationship, so the rate is not inversely proportional to the distance.
Challenge
Figure 1 shows apparatus that can be used to measure the rate of photosynthesis. The rate of photosynthesis in the pondweed is affected by different colours of light. Describe a method you could use to investigate this. You should include what you would measure and the variables you would control. (6 marks)
Challenge — Figure 1 shows apparatus that can be used to measure the rate of photosynthesis. The rate of photosynthesis in the pondweed is affected by different colours of light. Describe a method you could use to investigate this. You should include what you would measure and the variables you would control.
Mark SchemePlace pondweed in sodium hydrogencarbonate solution a fixed distance from a lamp. Put a coloured filter in front of the lamp and count the number of bubbles released per minute; repeat and find a mean. Change the colour of the filter and repeat for each colour. Measure: the number of bubbles per minute (the rate). Control variables: light intensity, distance of the lamp, temperature, carbon dioxide concentration and the same piece of pondweed.
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