Why is "Saying iodine 'turns brown' for starch" a common mistake in Investigating Photosynthesis?

Why it happens: Students remember a colour change but forget which way round it is — orange-brown is iodine's own colour. How to avoid it: Starch present → **blue-black**; starch absent → iodine stays **orange-brown** (its normal colour). The positive result is blue-black. Source: Edexcel Biology examiner reports

Why is "Forgetting to destarch the plant first" a common mistake in Investigating Photosynthesis?

Why it happens: It happens before the visible experiment, so students skip it. How to avoid it: Always state: keep the plant in the dark to destarch, so any starch found afterwards proves photosynthesis occurred during the test. Source: Edexcel Biology examiner reports 2023–2024

Why is "Describing only the test, not a control/comparison" a common mistake in Investigating Photosynthesis?

Why it happens: Students focus on the leaf that 'goes wrong' and ignore the comparison. How to avoid it: Name the control explicitly: variegated leaf (green vs white), covered vs uncovered, soda lime vs sodium hydrogencarbonate. No control = lost marks. Source: Edexcel Biology examiner reports

Why is "Giving the wrong reason for boiling in ethanol" a common mistake in Investigating Photosynthesis?

Why it happens: Students confuse the water step (kill cells) with the ethanol step. How to avoid it: Ethanol **removes the chlorophyll** so the iodine colour is easy to see; boiling in **water** kills the cells / lets chemicals in. Source: Edexcel Biology examiner reports

Why is "Not naming the gas, or calling the bubbles 'air'" a common mistake in Investigating Photosynthesis?

Why it happens: Students describe what they see (bubbles) rather than the biology. How to avoid it: The gas is **oxygen**, a product of photosynthesis. Link it: more light → faster photosynthesis → more oxygen bubbles. Source: Edexcel Biology examiner reports

Why is "Confusing 'reliable' with 'accurate' when improving the method" a common mistake in Investigating Photosynthesis?

Why it happens: The terms are used loosely in everyday speech. How to avoid it: **Reliable** = repeat and take a mean. **Accurate** = better measurement (measure oxygen volume with a syringe instead of counting variable-sized bubbles). Source: Edexcel Biology examiner reports 2022–2024

Structure and Functions in Living OrganismsEdexcel IGCSE Science(Double Award)-Biology (4WSD0-1B)

Investigating Photosynthesis

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Investigating Photosynthesis — Practical (Spec 2.23) — Edexcel International GCSE Science (Double Award) Biology (4WSD0-1B) Study Notes

The required practical work on photosynthesis: showing a pondweed gives off oxygen as light intensity changes, testing a leaf for starch with iodine, and proving that light, carbon dioxide and chlorophyll are all needed. Method, controls and results — Double Award Biology spec 2.23, assessed on Biology Paper 1 (4WSD0/1B).

At a glance

Spec 2.23 is a PRACTICAL — examiners test method, controls and how you read results.
Oxygen test: count bubbles from Elodea (pondweed) as you change light intensity (move the lamp nearer/further).
Light intensity ∝ 1 / distance² — moving the lamp closer increases intensity, so more bubbles per minute.
Starch test on a leaf: destarch the plant in the dark first, then boil → ethanol → iodine to test for starch.
Iodine turns from orange-brown to blue-black where starch is present.
Light needed: a leaf kept in the dark makes no starch (stays orange-brown).
Chlorophyll needed: a variegated leaf only goes blue-black in the green parts, not the white parts.
Carbon dioxide needed: a leaf in a flask with soda lime (removes CO₂) makes no starch.
Always use a water bath/heat off when adding ethanol — it is flammable.

What you’ll learn

Mapped to the Edexcel IGCSE 4SD0 syllabus (2026 onwards).

2.23 (practical) — Investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch, and the requirements for light, carbon dioxide and chlorophyll.

Showing oxygen is given off (Elodea / pondweed)

Count bubbles from pondweed to measure the rate of photosynthesis.

Photosynthesis releases oxygen. You can see this by collecting the gas bubbling out of an underwater plant such as Elodea (Canadian pondweed).

Method

Cut a piece of pondweed and place it (cut end up) in a beaker of water; add a little sodium hydrogencarbonate to supply plenty of carbon dioxide.
Place a lamp a measured distance away and switch it on.
Leave 5 minutes to settle, then count the bubbles of gas given off in one minute (or use a gas syringe / capillary tube to measure the volume of oxygen).
Move the lamp to a series of distances (e.g. 10, 20, 30, 40 cm) and repeat the count at each.

Light intensity and distance

The lamp is the source of light intensity — the factor you are changing (the independent variable).
Light intensity is proportional to 1 / distance², so moving the lamp closer gives a higher light intensity and more bubbles per minute.

Controlling variables (so it's a fair test)

Keep the temperature constant — put a beaker of water (heat shield) between the lamp and the plant to absorb heat, so only light changes.
Keep the same piece of pondweed, same water and same CO₂ supply throughout.

Results

More light → faster photosynthesis → more bubbles per minute, until another factor (CO₂ or temperature) becomes limiting and the rate plateaus.

Move the lamp to different distances and count oxygen bubbles per minute. The water heat shield keeps temperature constant.

Use Elodea (pondweed) + sodium hydrogencarbonate for CO₂.
Change light intensity by moving the lamp; count bubbles per minute.
Heat shield (beaker of water) keeps temperature constant.

See the full worked example for investigating photosynthesis →

Testing a leaf for starch (the iodine test)

Glucose made in photosynthesis is stored as starch, which iodine detects.

A photosynthesising leaf turns its glucose into starch for storage, so the presence of starch is the evidence that photosynthesis has happened. You detect starch with iodine solution.

Why destarch first?

Before the test, leave the plant in the dark for 24–48 hours to destarch it (it uses up the starch already in its leaves).
This guarantees that any starch found afterwards must have been made during the experiment, not before it.

Method — testing a leaf

Boil the leaf in water (~1 minute) — this kills the cells / breaks the membranes so chemicals can get in.
Boil the leaf in ethanol in a water bath (turn the Bunsen off first — ethanol is flammable) — this removes the green chlorophyll so the colour change is easy to see.
Dip in hot water to soften the brittle leaf.
Spread the leaf flat and add iodine solution.

Result

Starch present → iodine turns blue-black.
No starch → iodine stays orange-brown.

Safety: ethanol is flammable, so heat it in a water bath with the flame removed, never directly over a Bunsen.

Destarch in the dark first, so new starch proves photosynthesis.
Boil in water (kill cells) → ethanol (remove chlorophyll) → iodine test.
Iodine: blue-black = starch present; orange-brown = none.

See the full worked example for investigating photosynthesis →

Proving light, CO₂ and chlorophyll are all needed

Three classic experiments, each removing one requirement.

Each test removes one requirement and uses the starch test to show photosynthesis has stopped. Always destarch the plant first.

1. Is LIGHT needed?

Cover part of a leaf with foil/black card (or keep a whole leaf in the dark), leave in light, then test for starch.
Covered part → orange-brown (no starch); uncovered part → blue-black. Light is needed.

2. Is CHLOROPHYLL needed?

Use a variegated leaf (green-and-white, e.g. variegated geranium). Only the green parts contain chlorophyll.
After the starch test, only the green parts go blue-black; the white parts stay orange-brown. Chlorophyll is needed.

3. Is CARBON DIOXIDE needed?

Seal a destarched plant (or one leaf) in a flask with soda lime, which absorbs the carbon dioxide. Keep an identical plant with a dish of sodium hydrogencarbonate (gives off CO₂) as the control.
The leaf without CO₂ → orange-brown; the control with CO₂ → blue-black. Carbon dioxide is needed.

The control idea: in each experiment everything is kept the same except the one factor being tested, so any difference in starch must be due to that factor.

Each test removes one requirement; only where photosynthesis happened does iodine give blue-black.

Light: covered/dark leaf stays orange-brown.
Chlorophyll: variegated leaf — only green parts go blue-black.
CO₂: soda lime removes CO₂, so leaf stays orange-brown.

See the full worked example for investigating photosynthesis →

Controls, variables and reliability

What makes each photosynthesis practical a fair, reliable test.

Examiners give a lot of marks for good experimental design, not just the result.

Independent, dependent and control variables

Independent variable = the thing you change (e.g. distance of lamp / which requirement you remove).
Dependent variable = the thing you measure (e.g. bubbles per minute, or starch present/absent).
Control variables = everything kept the same (temperature, CO₂ supply, type/age of leaf, time).

Why a control experiment?

A control is an identical set-up with the test factor present, so you can compare. Without it you cannot say the missing factor caused the result.

Improving reliability and accuracy

Repeat counts/tests and take a mean to reduce the effect of random error.
For the bubble test, counting gas volume with a syringe is more accurate than counting bubbles (bubbles vary in size).
Allow the plant to acclimatise for a few minutes before counting.

Common safety points

Ethanol is flammable — heat in a water bath with the flame off.
Boiling water / hot apparatus — handle with care, use tongs/eye protection.

Identify independent, dependent and control variables clearly.
A control = same set-up but with the tested factor present.
Repeat and average; measuring gas volume beats counting bubbles.

See the full worked example for investigating photosynthesis →

How it’s examined

Assessed on Biology Paper 1 (4WSD0/1B), 2 hours, 110 marks, untiered — Double Award has no Biology Paper 2. Practical 2.23 appears as: describe/order the steps of the starch test (3–4 marks), explain a step (e.g. 'why boil in ethanol?'), design or improve a fair test for light/CO₂/chlorophyll, and read a bubble-count or distance table/graph. Method-order, the iodine colour change and naming the right control reagent (soda lime, variegated leaf) are reliable marks.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel International GCSE Biology 4BI1 specification — Issue 4 (shared Biology Paper 1 practical content); Pearson Edexcel 4SD0 / 4BI1 examiner reports 2022–2024. Last reviewed 2026-06-04.

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Step-by-step worked examples — Investigating Photosynthesis

Step-by-step solutions to past-paper-style questions on investigating photosynthesis, written exactly the way a tutor would explain them at the board.

1Reading the iodine result (Getting started)
Getting started• 4WSD0/1B style — short answer• starch, iodine, spec-2.23
Question
A leaf is tested with iodine solution. Part of the leaf turns blue-black and part stays orange-brown. What does each colour tell you? (2 marks)
Step-by-step solution
1. Step 1
  Blue-black (1). Starch is present here — photosynthesis has taken place.
2. Step 2
  Orange-brown (1). Starch is absent here — no photosynthesis (iodine's normal colour, unchanged).
Answer
Blue-black = starch present (photosynthesis happened); orange-brown = no starch (no photosynthesis).
Examiner tip
Don't write 'iodine turns brown' — its normal colour IS orange-brown; the positive result is blue-black.
2Bubbles and lamp distance (Getting started)
Getting started• 4WSD0/1B style — short answer• pondweed, oxygen, light-intensity, spec-2.23
Question
A student moves a lamp from 30 cm to 10 cm from some pondweed. The bubble count rises from 12 to 30 bubbles per minute. Explain this result. (2 marks)
Step-by-step solution
1. Step 1
  Closer lamp → higher light intensity (1). Light intensity is proportional to 1 / distance², so moving from 30 cm to 10 cm raises the light intensity.
2. Step 2
  More light → faster photosynthesis (1), so more oxygen is given off — the bubble count rises from 12 to 30 per minute.
Answer
Moving the lamp closer increases light intensity, so the rate of photosynthesis increases and more oxygen bubbles are released per minute.
Examiner tip
Name the gas as OXYGEN, and link 'more light → faster photosynthesis → more O₂'. The bubbles are evidence, not the cause.
3Order the starch-test steps (Building confidence)
Building confidence• 4WSD0/1B style — structured• starch, method, spec-2.23
Question
Put these starch-test steps in the correct order and give a reason for the ethanol step: add iodine; boil leaf in water; boil leaf in ethanol; dip in hot water. (4 marks)
Step-by-step solution
1. Step 1
  Boil leaf in water (1). Kills the cells / breaks membranes so chemicals can enter.
2. Step 2
  Boil leaf in ethanol (1). Removes the green chlorophyll so the iodine colour change can be seen clearly.
3. Step 3
  Dip in hot water (1). Softens the now-brittle leaf so it can be spread flat.
4. Step 4
  Add iodine (1). Tests for starch — blue-black if present.
Answer
Order: boil in water → boil in ethanol → dip in hot water → add iodine. Ethanol removes the chlorophyll so the iodine colour change is visible.
Examiner tip
The 'ethanol removes chlorophyll' reason is asked almost every series. Heat ethanol in a water bath — it is flammable.
4Variegated-leaf result (Building confidence)
Building confidence• 4WSD0/1B style — structured• chlorophyll, variegated, spec-2.23
Question
A green-and-white (variegated) leaf is destarched, left in light, then tested with iodine. Only the green parts turn blue-black. What does this prove, and why are the white parts orange-brown? (4 marks)
Step-by-step solution
1. Step 1
  Conclusion (1). It proves chlorophyll is needed for photosynthesis.
2. Step 2
  Green parts (1). Contain chlorophyll, so they photosynthesise and make starch → blue-black.
3. Step 3
  White parts (1). Have no chlorophyll, so they cannot photosynthesise → no starch made.
4. Step 4
  Iodine on white parts (1). No starch to react with, so iodine stays its normal orange-brown.
Answer
It proves chlorophyll is needed. Green parts have chlorophyll, photosynthesise and store starch (blue-black). White parts have no chlorophyll, make no starch, so iodine stays orange-brown.
Examiner tip
The variegated leaf is a self-controlled experiment — green vs white on the SAME leaf keeps all other variables equal.
5Design a fair test for CO₂ (Stretch)
Stretch• 4WSD0/1B style — extended• carbon-dioxide, control, design, spec-2.23
Question
Describe how you would show that carbon dioxide is needed for photosynthesis, including the control and how you reach a conclusion. (5 marks)
Step-by-step solution
1. Step 1
  Destarch (1). Keep the plant in the dark 24–48 h so existing starch is removed.
2. Step 2
  Remove CO₂ (1). Enclose one leaf/plant in a flask with soda lime to absorb the carbon dioxide; seal it.
3. Step 3
  Control (1). Set up an identical plant with a dish of sodium hydrogencarbonate (gives off CO₂) — everything else the same.
4. Step 4
  Light + test (1). Leave both in light for several hours, then test each leaf for starch with iodine.
5. Step 5
  Conclusion (1). No-CO₂ leaf stays orange-brown (no starch); control goes blue-black — so CO₂ is needed for photosynthesis.
Answer
Destarch first; enclose one leaf with soda lime to remove CO₂ and a control with sodium hydrogencarbonate to supply CO₂; leave in light; starch-test both. The leaf without CO₂ stays orange-brown, the control goes blue-black — so CO₂ is needed.
Examiner tip
Marks are lost without the CONTROL and without DESTARCHING. Name soda lime (removes CO₂) precisely.
6Analyse a bubble-count table (Stretch)
Stretch• 4WSD0/1B style — extended• pondweed, data, reliability, spec-2.23
Question
Pondweed gives bubbles per minute at four distances: 10 cm → 40; 20 cm → 22; 30 cm → 11; 40 cm → 9. Describe the pattern, suggest why the drop from 30 to 40 cm is small, and state one improvement to the method. (6 marks)
Step-by-step solution
1. Step 1
  Pattern (1 + 1). As distance increases, bubbles per minute decrease (40 → 22 → 11 → 9): closer lamp = higher light intensity = faster photosynthesis (light intensity ∝ 1 / distance²).
2. Step 2
  Why the small drop 30→40 cm (1 + 1). At low light intensity another factor (e.g. CO₂ or temperature) is now limiting, so reducing the light further has little effect — the rate is levelling off.
3. Step 3
  Improvement (1 + 1). Repeat each count and take a mean (reduces random error), OR collect the gas in a syringe and measure its volume (bubbles vary in size, so volume is more accurate); also use a heat shield to keep temperature constant.
Answer
Bubbles fall as distance increases (light intensity ∝ 1/distance²). The 30→40 cm change is small because a different factor (CO₂/temperature) is now limiting. Improve by repeating and averaging, or measuring oxygen volume with a syringe instead of counting bubbles.
Examiner tip
Higher-mark data questions want a manipulation/quotation of the FIGURES, a 'limiting factor' explanation, and a specific, justified improvement.

Model Answers — Investigating Photosynthesis

High-scoring sample answers for investigating photosynthesis on the Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1B style2 marks
State the colour iodine solution turns when starch is present, and the colour when starch is absent. (2 marks)
Model answer
Starch present → iodine turns blue-black. (1)

Starch absent → iodine stays its normal orange-brown colour. (1)
Why this scores
1 mark each. 'Brown' alone is not enough — orange-brown is iodine's UNCHANGED colour; the positive result is blue-black.
Question 2
4WSD0/1B style3 marks
Before testing a plant for the effect of light, the plant is left in the dark for two days. Explain why. (3 marks)
Model answer
Leaving the plant in the dark makes it destarch. (1) In the dark it cannot photosynthesise, so it uses up the starch already stored in its leaves. (1) This means any starch found after the experiment must have been made during the experiment, so the result is a fair test of the factor being investigated. (1)
Why this scores
The key idea is 'uses up existing starch so new starch proves photosynthesis'. Many lose the third mark by not linking it to a fair test.
Question 3
4WSD0/1B style5 marks
Describe how you would test a leaf to show that it contains starch. Include a reason for each heating step. (5 marks)
Model answer
Boil the leaf in water for about a minute. (1) This kills the cells / breaks the cell membranes so that chemicals can pass in and out. (1)

Boil the leaf in ethanol in a water bath (Bunsen turned off, as ethanol is flammable). (1) This removes the green chlorophyll, so the iodine colour change can be seen clearly. (1)

Dip the leaf in hot water to soften it, spread it flat, then add iodine solution: it turns blue-black where starch is present. (1)
Why this scores
Order matters. Reason marks: 'kills cells/lets chemicals in' (water) and 'removes chlorophyll' (ethanol) are the high-value points.
Question 4
4WSD0/1B style6 marks
Describe how you would investigate the effect of light intensity on the rate of photosynthesis using pondweed. (6 marks)
Model answer
Place a piece of pondweed (Elodea) in a beaker of water and add sodium hydrogencarbonate to supply carbon dioxide. (1)

Position a lamp a measured distance from the pondweed; this controls the light intensity (the independent variable). (1)

After a few minutes to settle, count the number of oxygen bubbles given off per minute (the dependent variable). (1)

Repeat the count at several distances (e.g. 10, 20, 30, 40 cm) — or better, collect the gas and measure its volume with a syringe. (1)

Control variables: keep temperature constant using a beaker of water as a heat shield, and keep the same pondweed and CO₂ supply. (1)

Repeat each reading and take a mean to make the results more reliable. (1)
Why this scores
Examiners reward a named independent variable (light intensity via distance), a measurable dependent variable (bubbles/volume per minute), and at least one sensible control (temperature).
Question 5
4WSD0/1B style — extended open response8 marks
Describe three experiments, one each, to show that light, carbon dioxide and chlorophyll are needed for photosynthesis. For each, state the control or comparison used and the expected starch-test result. (8 marks)
Model answer
In every case destarch the plant in the dark first, then leave it in light and test for starch with iodine.

Light — cover part of a leaf with foil/black card and leave the rest exposed. (1) Comparison: the uncovered part of the same leaf is the control. (1) Result: covered part orange-brown (no starch), uncovered part blue-black → light is needed.

Carbon dioxide — enclose one plant with soda lime to remove CO₂. (1) Control: an identical plant with sodium hydrogencarbonate (supplies CO₂). (1) Result: no-CO₂ leaf orange-brown, control blue-black → CO₂ is needed. (1)

Chlorophyll — use a variegated leaf (green-and-white). (1) The green and white areas on the same leaf act as the comparison. (1) Result: only the green parts go blue-black; white parts stay orange-brown → chlorophyll is needed. (1)
Why this scores
Up to ~3 marks per experiment (method + control/comparison + result). Examiner reports penalise answers that forget the CONTROL or the destarch step.
Question 6
4WSD0/1B style — extended open response7 marks
A student counts oxygen bubbles from pondweed to study light intensity but gets results that are hard to repeat. Explain the weaknesses in counting bubbles and describe how the investigation could be improved to give more reliable and accurate results. (7 marks)
Model answer
Weaknesses of counting bubbles:

Bubbles are different sizes, so equal numbers do not mean equal volumes of oxygen — the measure is inaccurate. (1)

At high rates the bubbles come too fast to count accurately. (1)

Bubble production can be irregular, and a single count includes random error. (1)

Improvements:

Collect the gas in a gas syringe / capillary tube and measure the volume of oxygen per minute instead of counting bubbles. (1)

Repeat each measurement and take a mean to reduce the effect of random error. (1)

Control the temperature with a heat shield (beaker of water) so only light intensity changes. (1)

Allow the pondweed to acclimatise for a few minutes before each reading, and use the same piece of pondweed throughout. (1)
Why this scores
Top answers distinguish RELIABILITY (repeat + mean) from ACCURACY (measure volume not count bubbles) and tie each improvement to a named weakness.

Key Definitions and Keywords — Investigating Photosynthesis

Definitions to memorise and the exact keywords mark schemes credit for investigating photosynthesis answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) sitting.

Destarch
Examiner keyword
Keeping a plant in the dark (usually 24–48 hours) so it uses up the starch stored in its leaves, before a photosynthesis experiment. Any starch found afterwards must have been made during the experiment.
Iodine (starch) test
Examiner keyword
Adding iodine solution to a treated leaf: it turns blue-black if starch is present and stays orange-brown if starch is absent.
Variegated leaf
Examiner keyword
A leaf with green parts (containing chlorophyll) and white parts (no chlorophyll). Used to show chlorophyll is needed — only the green parts make starch.
Soda lime
Examiner keyword
A chemical that absorbs carbon dioxide. Sealing a plant with soda lime removes CO₂, so it cannot photosynthesise — used in the 'is CO₂ needed?' experiment.
Elodea (pondweed)
Examiner keyword
A water plant used to demonstrate oxygen evolution — the oxygen made in photosynthesis is seen as bubbles rising through the water.
Light intensity
Examiner keyword
How bright the light is. It is proportional to 1 / distance², so moving a lamp closer increases light intensity and speeds up photosynthesis.
Control experiment
Examiner keyword
An identical set-up in which the factor being tested is present (e.g. CO₂ supplied), used for comparison so any difference can be attributed to that factor.
Limiting factor
The factor in shortest supply that holds back the rate of photosynthesis. When light is low it limits the rate; raising another factor then has little effect.

Common Mistakes and Misconceptions — Investigating Photosynthesis

The traps other students keep falling into on investigating photosynthesis questions — taken from recent Pearson Edexcel IGCSE Biology Paper 1 (4WSD0/1B) examiner reports and mark schemes — and how to avoid them.

✕Saying iodine 'turns brown' for starch
Edexcel Biology examiner reports
Why it happens
Students remember a colour change but forget which way round it is — orange-brown is iodine's own colour.
How to avoid it
Starch present → blue-black; starch absent → iodine stays orange-brown (its normal colour). The positive result is blue-black.
✕Forgetting to destarch the plant first
Edexcel Biology examiner reports 2023–2024
Why it happens
It happens before the visible experiment, so students skip it.
How to avoid it
Always state: keep the plant in the dark to destarch, so any starch found afterwards proves photosynthesis occurred during the test.
✕Describing only the test, not a control/comparison
Edexcel Biology examiner reports
Why it happens
Students focus on the leaf that 'goes wrong' and ignore the comparison.
How to avoid it
Name the control explicitly: variegated leaf (green vs white), covered vs uncovered, soda lime vs sodium hydrogencarbonate. No control = lost marks.
✕Giving the wrong reason for boiling in ethanol
Edexcel Biology examiner reports
Why it happens
Students confuse the water step (kill cells) with the ethanol step.
How to avoid it
Ethanol removes the chlorophyll so the iodine colour is easy to see; boiling in water kills the cells / lets chemicals in.
✕Not naming the gas, or calling the bubbles 'air'
Edexcel Biology examiner reports
Why it happens
Students describe what they see (bubbles) rather than the biology.
How to avoid it
The gas is oxygen, a product of photosynthesis. Link it: more light → faster photosynthesis → more oxygen bubbles.
✕Confusing 'reliable' with 'accurate' when improving the method
Edexcel Biology examiner reports 2022–2024
Why it happens
The terms are used loosely in everyday speech.
How to avoid it
Reliable = repeat and take a mean. Accurate = better measurement (measure oxygen volume with a syringe instead of counting variable-sized bubbles).

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Investigating Photosynthesis — Practical (Spec 2.23) — Edexcel International GCSE Science (Double Award) Biology (4WSD0-1B) Study Notes

Photosynthesis releases oxygen. You can see this by collecting the gas bubbling out of an underwater plant such as Elodea (Canadian pondweed).

Method

Cut a piece of pondweed and place it (cut end up) in a beaker of water; add a little sodium hydrogencarbonate to supply plenty of carbon dioxide.
Place a lamp a measured distance away and switch it on.
Leave 5 minutes to settle, then count the bubbles of gas given off in one minute (or use a gas syringe / capillary tube to measure the volume of oxygen).
Move the lamp to a series of distances (e.g. 10, 20, 30, 40 cm) and repeat the count at each.

Light intensity and distance

The lamp is the source of light intensity — the factor you are changing (the independent variable).
Light intensity is proportional to 1 / distance², so moving the lamp closer gives a higher light intensity and more bubbles per minute.

Controlling variables (so it's a fair test)

Keep the temperature constant — put a beaker of water (heat shield) between the lamp and the plant to absorb heat, so only light changes.
Keep the same piece of pondweed, same water and same CO₂ supply throughout.

Results

More light → faster photosynthesis → more bubbles per minute, until another factor (CO₂ or temperature) becomes limiting and the rate plateaus.

Move the lamp to different distances and count oxygen bubbles per minute. The water heat shield keeps temperature constant.

Investigating Photosynthesis

Detailed Notes

What you’ll learn

How it’s examined

1Reading the iodine result (Getting started)

2Bubbles and lamp distance (Getting started)

3Order the starch-test steps (Building confidence)

4Variegated-leaf result (Building confidence)

5Design a fair test for CO₂ (Stretch)

6Analyse a bubble-count table (Stretch)

Destarch

Iodine (starch) test

Variegated leaf

Soda lime

Elodea (pondweed)

Light intensity

Control experiment

Limiting factor

✕Saying iodine 'turns brown' for starch

✕Forgetting to destarch the plant first

✕Describing only the test, not a control/comparison

✕Giving the wrong reason for boiling in ethanol

✕Not naming the gas, or calling the bubbles 'air'

✕Confusing 'reliable' with 'accurate' when improving the method

Past paper style quiz

Investigating Photosynthesis

Detailed Notes

What you’ll learn

How it’s examined

1Reading the iodine result (Getting started)

2Bubbles and lamp distance (Getting started)

3Order the starch-test steps (Building confidence)

4Variegated-leaf result (Building confidence)

5Design a fair test for CO₂ (Stretch)

6Analyse a bubble-count table (Stretch)

Destarch

Iodine (starch) test

Variegated leaf

Soda lime

Elodea (pondweed)

Light intensity

Control experiment

Limiting factor

✕Saying iodine 'turns brown' for starch

✕Forgetting to destarch the plant first

✕Describing only the test, not a control/comparison

✕Giving the wrong reason for boiling in ethanol

✕Not naming the gas, or calling the bubbles 'air'

✕Confusing 'reliable' with 'accurate' when improving the method

Past paper style quiz