What you need to know
- 5.8Core Practical 10: effects of light intensityThe amount of light energy reaching a given area in a given time., light wavelength, temperature and carbon dioxide availability on photosynthetic rate using a suitable aquatic plant.
This is preparation for supervised school work. Teachers must approve species, lamp type, buffers and how gas is collected. Fictional rates below are teaching data, not results you should fake in a book.
By the end, you should be able to…
- Name independent, dependent and control variableA variable kept the same throughout an investigation so that it does not affect the result. for each factor.
- Describe a workable method with an aquatic plant.
- Evaluate bubble counts, inverse-square lamps and coloured filters.
Build your understanding
What you are measuring
The dependent variable is rate of photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen., often as oxygen produced per unit time. An aquatic plant such as Cabomba or Elodea can release bubbles from a cut stem. Counting bubbles is simple but treats every bubble as equal. A photosynthometer collects gas so you can read a volume; that is still not a pure oxygen mole count, because the gas is a mixture.
Investigate one independent variable at a time: light intensityThe amount of light energy reaching a given area in a given time., wavelength, temperature, or carbon dioxide availability. Hold the others as constant as the apparatus allows. Use the same length of healthy shoot, the same side of the lamp, and time to equilibrate after each change.
A workable school method
Place the shoot in a dilute sodium hydrogencarbonate solution so carbon dioxide is not immediately limiting unless CO₂ is the independent variable. For intensity, change lamp distance and, if asked, treat intensity as proportional to 1/d², stating that this assumes a point source and no other light. For wavelength, use filters of known colour and keep intensity as similar as you can, because a dark filter also cuts total light. For temperature, use a water bath and a thermometer in the tube. For carbon dioxide, change hydrogencarbonate concentration, or compare with boiled, cooled water as a low-CO₂ control if that is the approved protocol.
A workable school method (continued)
Allow several minutes at each setting so the plant and dissolved gases settle. Record repeats with a fresh shoot or a rest in dim light if the teacher’s method says so. A heat shield of water between lamp and tube reduces warming when intensity is the independent variable. Follow local rules for lamps, water and electrical leads.
Follow the process
A factor changes the rate only while it is limiting. More useful light, while light is scarce → more chlorophyllThe green pigment in chloroplasts. It absorbs mainly red and blue light. electrons are excited → more ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. and reduced NADPNADP that has accepted hydrogen in the light-dependent reactions. It supplies the hydrogen to reduce GP to GALP. → the Calvin cycleThe cycle of light-independent reactions: CO₂ is fixed by RuBP to make GP, GP is reduced to GALP, and RuBP is regenerated. can run faster, until another factor becomes scarce. Less carbon dioxide → RUBISCOThe enzyme that catalyses carbon fixation, joining CO₂ to RuBP. has less substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. → less GP is made → the rate stays low even in bright light. A moderate rise in temperature → Calvin-cycle enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. work faster; too high a temperature → those enzymes denature and the rate falls. Adding more of a factor that is already plentiful does not raise the rate.
Risk, variables and evaluation
Risks include hot lamps, water near electrics, broken glass and, if using wild plants, collecting without permission. Use the provided cultured shoots. Cabomba and Elodea are invasive in many regions: never release shoots, fragments or their water into ponds, drains or rivers, and dispose of them as your teacher instructs. Do not stare into the lamp. Report damaged leads.
Bubble rate overestimates precision if bubbles differ in size or include trapped nitrogen. Filters never isolate a single wavelength. Inverse-square calculations fail if the lamp is large or the room is bright. Temperature changes enzyme activity in the Calvin cycle and also gas solubility, so a peak then a fall can be denaturationA permanent change to a protein’s tertiary structure, for example by high temperature or extreme pH, so an enzyme’s active site no longer fits its substrate. plus physical effects. A fictional class set might show 12, 19, 24, 18 bubbles min⁻¹ as distance falls then the tube overheats — those numbers are for discussing evaluation, not for copying as your own.
| Independent variable | How you change it | What you must still control |
|---|---|---|
| Light intensity | Lamp distance (and 1/d² if used) | Temperature, CO₂, wavelength, shoot size |
| Wavelength | Coloured filters | Intensity as far as possible, temperature, CO₂ |
| Temperature | Water bath | Light, CO₂, shoot size |
| CO₂ availability | Hydrogencarbonate concentration | Light, temperature, shoot size |
The pondweed photosynthesis lab
Change one factor at a time, wait for the shoot to settle, then count oxygen bubbles for a minute. The shoot, rates and light units are fictional.
Simulation only. Keep water away from mains leads, let hot lamps cool before moving them, and never release Cabomba, Elodea or their water into ponds, drains or rivers.
Method 7 steps
- Cut a fresh, healthy shoot of cultured Cabomba or Elodea (same length each time) under water, and place it cut end up in a boiling tube of dilute sodium hydrogencarbonate solution.
- Stand the tube in a beaker of water at a set temperature, with a thermometer in the tube. Put a water heat shield between the lamp and the tube, or use an LED lamp.
- Darken the room. Place the lamp at the first distance, measured from the lamp to the shoot with a metre rule.
- Leave the shoot for at least 5 minutes so the rate settles.
- Count the bubbles leaving the cut stem in 1 minute (or collect the gas in a photosynthometer and measure its length). Repeat to get three counts.
- Change the distance and repeat steps 4–5 for at least five distances. Keep temperature, NaHCO₃ concentration, wavelength and the shoot the same.
- Calculate the mean rate at each distance, plot it against distance (and against 1/d² for intensity), and describe the shape.
Aligned with the order of the specification practical. Simulation only: a teacher sets the real volumes, concentrations and risk controls; this is not a laboratory protocol.
Variables
- Independent
- Light intensity, changed by lamp distance (or wavelength, temperature or CO₂ availability, one at a time).
- Dependent
- Rate of photosynthesis: bubbles per minute, or gas volume per minute from a photosynthometer.
- Controlled
- Temperature in the tube (water bath, heat shield or LED)
- NaHCO₃ concentration (CO₂ supply)
- Wavelength: the same lamp and no filter
- Background light: room lights off
- The same shoot, or shoots of the same length and health
- Time to settle after each change
Safety
- Keep water away from mains leads and sockets; dry hands before touching the lamp. Report damaged leads.
- Filament and halogen lamps get hot: do not touch the bulb or shade, and let it cool before moving it.
- Do not stare into the lamp. Handle boiling tubes and beakers carefully and clear up broken glass with a brush.
- Use cultured shoots from a supplier. Cabomba and Elodea are invasive in many places: never release them or the water into ponds, drains or rivers, and dispose of them as your teacher instructs.
Follow your school’s risk assessment; these notes are not a substitute for it.
Interpreting
- Intensity ∝ 1/d² for a point source with no other light: at double the distance, intensity is a quarter.
- Rate rises in proportion to light only while light is limiting. Where the graph levels off, another factor (CO₂ or temperature) is limiting.
- Bubble counts are a comparative rate: bubbles differ in size and the gas is a mixture, not pure oxygen. A photosynthometer gives a volume but still not moles of O₂.
- Coloured filters change intensity as well as wavelength, and green light still drives some photosynthesis. NaHCO₃ also raises the pH, which changes how much is present as dissolved CO₂.
Guided method · step 1 of 10
- In 1 · Plan, just below these steps, keep Independent variable on Light intensity (or choose Wavelength, Temperature or CO₂ availability).
- Press Fix the other three at the end of the same row.
Light intensity, wavelength, temperature and CO₂ all change the rate. Change one at a time and hold the other three constant, or you cannot say which one caused a change.
1 · Plan
Choose one independent variable, then set up a fresh shoot.
2 · Shoot
Keep the same shoot for a whole series. A fresh shoot differs a little in size and health.
3 · Controls
Conditions
4 · Wait and count
Changed 0.0 min ago: wait until 5 min before counting.
5 · Analyse
Evaluate the method
Rates are net: the shoot also respires, so a little light gives no bubbles at all. NaHCO₃ raises the pH as well as the CO₂ supply.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: extra light stops increasing the bubble rate. Y: light is no longer limiting, so carbon dioxide or temperature now caps the Calvin cycleThe cycle of light-independent reactions: CO₂ is fixed by RuBP to make GP, GP is reduced to GALP, and RuBP is regenerated.. Z: you have to change that second factor, not move the lamp again. Incomplete: “The plant is full of light.” Improved: “At saturating light, more photons do not make more GP if RUBISCOThe enzyme that catalyses carbon fixation, joining CO₂ to RuBP. is short of carbon dioxide, so the rate levels off.”
Name the limiter
A factor only changes rate while it is limiting. At saturating light, adding more light does little if carbon dioxide is scarce.
Do not treat bubbles as moles
State that volume, or count, is a proxy for rate.
Filters cut intensity too
A red filter that also darkens the light is not a fair wavelength comparison (the 5.6 action spectrum idea) unless intensity is matched, for example with a light meter.
Avoid this misconception
The claim: The number of bubbles is the volume of pure oxygen, so the rate is exact.
The correction: Bubbles vary and the gas is mixed. Treat the count as a comparative rate, or collect a volume and still discuss composition.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
State suitable independent and dependent variables for investigating light intensity with Cabomba.
Show suggested answer
Independent: light intensity, for example by changing lamp distance. Dependent: rate of photosynthesis, for example bubbles or oxygen volume per minute.
Practice question
Retrieval and explanation · suggested answer
Why place a water-filled flask between a lamp and the tube when intensity is the independent variable?
Show suggested answer
To absorb heat so temperature, which also affects rate, is less likely to change with the lamp.
Practice question
Retrieval and explanation · suggested answer
A student uses a blue filter and a red filter but the blue filter transmits less total light. Why is the comparison unfair?
Show suggested answer
Wavelength and intensity have both changed, so a lower rate might be less light rather than a less useful wavelength.
Practice question
Retrieval and explanation · suggested answer
Lamp distances are 10 cm and 20 cm. What happens to relative intensity if 1/d² applies?
Show suggested answer
At double the distance, relative intensity falls to one quarter, assuming a point source and no other light.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Rates in arbitrary units at 15, 25 and 35 °C are 8, 18 and 9 with the same light and hydrogencarbonate. Interpret the pattern and give two reasons 35 °C is not “twice as good as 15 °C”.
Show a hint
Enzymes and gases both feel temperature.
Explore the reasoning
Rate rises from 15 to 25 °C as enzyme activity increases, then falls at 35 °C as enzymes such as RUBISCO denature or as other damage occurs. You cannot say 35 °C is twice 15 °C because 9 is not 16, and because oxygen solubility and the plant’s condition also change. The numbers are a pattern to explain, not a Q10 you must calculate unless asked.
Recall this lesson
Lesson recall questions
Question 1 of 5: Outline, in order, how to investigate the effect of light intensity using a lamp and an aquatic plant.
Core practical 10: rate of photosynthesis
Practice question
Retrieval and explanation · suggested answer
Outline, in order, how to investigate the effect of light intensity using a lamp and an aquatic plant.
Lesson: CP10: Factors Affecting the Rate of Photosynthesis
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The same four factors, at field scale
CP10 · Factors Affecting the Rate of PhotosynthesisNASA maps chlorophyll in ocean water from the colour of reflected light. The observation connects the pigment biology of photosynthesis to a global research question, but it is…Specification 5.8Read & use the scienceClose article
Why this matters
NASA maps chlorophyll in ocean water from the colour of reflected light. The observation connects the pigment biology of photosynthesis to a global research question, but it is not a direct rate measurement. In Core Practical 10, an aquatic plant gives an observable gas or bubble-rate proxy while you vary light, carbon dioxide or temperature. The interesting difference is what each instrument actually measures. The school plant gives a gas-volume or bubble-rate proxy over minutes; an orbiting sensor estimates pigment from reflected light over kilometres. Neither directly counts every molecule of glucose made. Ocean scientists must separate biological variation from clouds, suspended sediment and changing water depth before calling a colour pattern a change in production.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32 · assessed through Unit 6. The application extends beyond required recall.
- 5.8 Core Practical 10: investigate how light intensity, wavelength, temperature and carbon dioxide availability affect photosynthesis in an aquatic plant.
Use the science
Why might a bright, cold day still give a low aquatic-plant rate in the lab even though a satellite map looks green?
Explore a possible response
The map shows pigment, not instantaneous rate. In the tube, low temperature can limit Calvin-cycle enzymes even when chlorophyll and light are present.
Explore the source
An ocean-colour map needs a water sample
CP10 · Factors Affecting the Rate of PhotosynthesisNASA's PACE mission measures ocean colour to estimate chlorophyll in surface waters. Phytoplankton pigments absorb and scatter different wavelengths, so their abundance changes the light a satellite receives.…Specification 5.8Read & use the scienceClose article
Why this matters
NASA's PACE mission measures ocean colour to estimate chlorophyll in surface waters. Phytoplankton pigments absorb and scatter different wavelengths, so their abundance changes the light a satellite receives. This extends Core Practical 10's question about light wavelength and photosynthesis, but the instrument is measuring an optical proxy, not oxygen bubbles or instantaneous carbon fixation. Scientists compare satellite results with water samples because sediment, dissolved matter and mixed communities can affect colour. A strong investigation states what the sensor measures before using it to infer a biological rate.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32 · assessed through Unit 6. The application extends beyond required recall.
- 5.8 Core Practical 10: investigate how light intensity, wavelength, temperature and carbon dioxide availability affect photosynthesis in an aquatic plant.
Use the science
If two sea areas have equal estimated chlorophyll but different oxygen production, why is the satellite map not necessarily wrong?
Explore a possible response
Chlorophyll estimates pigment or biomass, whereas oxygen production is a rate. Light, nutrients, depth or temperature can change photosynthetic rate without an equal change in pigment.