Investigating Respiration

Aerobic respiration in living cells can be summarised as:

glucose + oxygen → carbon dioxide + water (+ energy released)

This practical gives direct evidence that living organisms respire, by detecting two products of respiration:

• Carbon dioxide (CO₂) — shown with limewater or hydrogencarbonate indicator.
• Heat (thermal energy) — shown with a thermometer in an insulated vacuum flask.

Why germinating (sprouting) seeds?

• Germinating seeds are alive and growing, so they respire very rapidly to release the energy needed for growth.
• A fast respiration rate means CO₂ and heat build up quickly, so results appear within a lesson — easy to measure.
• Other living organisms (e.g. small invertebrates such as woodlice or maggots, or yeast) work too, but germinating seeds (peas/beans) are the standard, easy choice.

Exam tip. If asked "why germinating seeds and not dry seeds?", the answer is: dry/dormant seeds barely respire, so they release almost no CO₂ or heat. Germinating seeds respire rapidly.

There are two common CO₂ tests you must know.

1. Limewater

• Limewater is a clear, colourless solution of calcium hydroxide.
• When CO₂ bubbles through it, it turns cloudy / milky white.
• The more CO₂, the faster and cloudier the change.

2. Hydrogencarbonate indicator (the more sensitive test)

• This indicator changes colour with the amount of dissolved CO₂:

CO₂ level	Colour of hydrogencarbonate indicator
Low CO₂ (CO₂ removed)	Purple / magenta
Normal (atmospheric)	Red / orange
High CO₂	Yellow

Setting it up with seeds

• Seal living germinating seeds in a sealed tube/container with a small beaker of indicator (or limewater) — but make sure the seeds do not touch the liquid.
• As the seeds respire, they release CO₂ into the air around them; the gas dissolves into the indicator and changes its colour.

Exam tip. Limewater goes cloudy (not "clear" — clear means see-through!). Use the word cloudy or milky to score the mark.

Respiration also releases heat (thermal energy). To detect it:

Method

1. Soak peas/beans so they germinate, then place them in a vacuum flask (Thermos).
2. Insert a thermometer through the cotton-wool/foam bung and record the starting temperature.
3. Invert/seal the flask and leave it; record the temperature again after 24–48 hours (or read at intervals).

What you see

• The temperature in the living-seed flask rises (often by several °C).
• The heat comes from respiration in the living seeds.

Why a vacuum flask?

• A vacuum flask is insulated — it greatly reduces heat loss to the surroundings.
• The amount of heat from a small number of seeds is small, so without insulation it would escape and the rise would be too small to detect.

Why is the flask usually turned upside down?

• So that CO₂ (denser than air) and warm air don't simply flow out of the open neck — and the thermometer bulb stays among the seeds where the heat is produced.

Exam tip. Don't say the seeds "make energy". Energy is released (transferred), not created. The temperature rise is the evidence that respiration is happening.

Every version of this practical needs a control to prove the CO₂/heat comes from respiration, not from something else.

The control = boiled (dead) seeds

• Identical seeds are boiled (killed) so they cannot respire.
• They are usually then soaked in disinfectant and rinsed, to kill any microbes (bacteria/fungi) on their surface.

Why disinfect the dead seeds?

• Microbes (bacteria and fungi) also respire and release CO₂ and heat.
• If the dead seeds were covered in microbes, the control would warm up / release CO₂ too — and you couldn't be sure the heat came from seed respiration.
• Disinfecting removes this confounding variable, making the control valid.

What the control should show

• No colour change (limewater stays clear, indicator stays orange).
• No temperature rise (stays at room temperature).

The logic

• Living seeds: CO₂ released + temperature rises.
• Dead seeds (control): no CO₂ + no rise.
• ∴ The CO₂ and heat must come from respiration in living cells. This is what makes the experiment a fair, valid test.

Exam tip. A favourite mark: "Explain the purpose of the boiled seeds." → They are a control; they are dead so cannot respire, so they show the CO₂/heat in the other flask is due to respiration (and not the apparatus or microbes).

Making it a fair test (controlling variables)

Keep these the same for the living set and the dead (control) set:

• Mass / number of seeds (e.g. 50 g in each flask).
• Type of seed (e.g. all peas).
• Volume of the container / flask.
• Starting temperature and the time left.
• Same room / position (avoid one flask in sunlight).

The only thing you change (the independent variable) is whether the seeds are alive or dead (boiled). The thing you measure (the dependent variable) is the temperature (heat experiment) or the colour change (CO₂ experiment).

Improving accuracy / reliability

• Use a data logger / temperature probe to record continuously, rather than reading a thermometer once.
• Repeat the experiment and look for consistent results.
• Leave for long enough (e.g. 24–48 h) for a clear change.

Evaluation — limitations to mention

• The temperature rise is small, so even a vacuum flask loses some heat — the rise underestimates the true heat released.
• A thermometer read by eye can give reading errors (parallax) — a probe is better.
• Results show CO₂ and heat are released, but this practical alone doesn't measure the rate precisely.

Safety

• Disinfectant is an irritant — wash hands; avoid skin/eye contact.
• Take care with hot water / boiling when killing the control seeds.

Exam tip. "Suggest one improvement" almost always scores for using a data logger/probe or repeating and averaging. "Suggest a reason the rise was smaller than expected" scores for heat lost to the surroundings.