ما هي معادلة البناء الضوئي الكيميائية الموزونة؟ What is the balanced equation for photosynthesis?

المعادلة الموزونة هي: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂ (ستة جزيئات ثاني أكسيد الكربون + ستة جزيئات ماء في وجود طاقة الضوء وصبغة الكلوروفيل تعطي جزيء جلوكوز وستة جزيئات أكسجين). وتتطلب العملية تفاعلات ضوئية لإنتاج ATP و NADPH يعقبها تفاعلات لاضوئية لتثبيت الكربون. [الصيغة الرمزية الموزونة: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂]

2.6.3 Practical: Investigating Respiration · 2. Structure and Function in Living Organisms | Edexcel International GCSE Science Double Award 4SD0: Biology

Section 1 of 9What can be measured?

Hydrogencarbonate indicator colour changes

The rate of respiration can be investigated by measuring the product made or the reactant used. Common methods are:

What can be measured?

Heat released

Heat released by respiring organisms (germinating seeds in a vacuum flask with a thermometer).

Oxygen uptake

Oxygen uptake using a respirometer.

Carbon dioxide production

, such as using hydrogencarbonate indicator, limewater, or counting bubbles of CO₂ from yeast.

These methods allow you to investigate how a factor such as temperature, type of sugar or the species affects the rate.

Section 2 of 9Germinating seeds in a vacuum flask

Respiring seeds release heat energy. Equipment: two vacuum flasks, a thermometer in each, germinating peas (live) in one flask and boiled or dead peas (control) in the other. To prevent microbial growth the seeds are first washed in a disinfectant solution. The cotton wool plug is used to allow gas exchange but reduce heat loss.

1

Soak a set of peas, let some germinate. Boil another equal set to kill them, then cool.

2

Rinse both in disinfectant, then place into the flasks of equal mass.

3

Place a thermometer in each flask and plug with cotton wool.

4

Record the temperature at regular intervals (for example every 10 minutes for several hours, or daily).

The temperature in the flask with the living peas rises because respiration releases heat, while the boiled peas show no rise. This is evidence that respiration releases heat energy.

Section 3 of 9Yeast and carbon dioxide production

Measuring respiration in yeast with methylene blue

Yeast is a single-celled fungus that respires anaerobically if oxygen is absent.

1

Suspend yeast in a sugar solution in a test tube, keep it in a water bath at a set temperature,

and measure the volume of gas collected in an upturned measuring cylinder,
or count bubbles through a delivery tube into water.

2

Repeat at different temperatures (10, 20, 30, 40 and 50 °C) to find the effect of temperature.

3

Rate = volume of CO₂ divided by time.

4

The rate rises as temperature increases up to the optimum (around 35-40 °C), then falls as the enzymes in the yeast are denatured.

Section 4 of 9Hydrogencarbonate indicator and limewater

Yeast fermentation experiment apparatus (limewater, oil)

Yeast fermentation apparatus (limewater, vegetable oil)

Hydrogencarbonate indicator tubes A-D: colour changes

Hydrogencarbonate indicator is red at atmospheric CO₂ concentration, turns yellow when more CO₂ is present (acidic) and purple when less CO₂ is present.

Hydrogencarbonate indicator and limewater

Place small organisms or seeds in tubes separated from the indicator by a gauze platform; the time taken to change colour gives a measure of respiration rate.

Limewater turns cloudy (milky) in the presence of carbon dioxide, a test for the gas, but is mostly qualitative.

Variable	How to control
Independent	Temperature (water bath), type of sugar
Dependent	Volume of CO₂ or temperature rise, or time for colour change
Control	Mass of seeds or yeast, volume and concentration of solution, time

Section 5 of 9Respirometer method and safety and ethics

Respirometer apparatus with germinating seeds and control tube

A simple respirometer consists of a boiling tube containing small organisms such as woodlice or germinating seeds, with a small beaker of soda lime or potassium hydroxide solution to absorb the carbon dioxide produced.

1

As the organisms use oxygen, the gas volume inside decreases, and coloured liquid in a connected capillary tube moves towards the tube.

2

The distance moved per minute measures the rate of oxygen uptake.

3

A control tube with glass beads of the same volume checks for changes in temperature or pressure.

4

Keep the apparatus in a water bath, as heat from your hands would make the gas expand and distort the readings.

Wash hands after handling germinating seeds, and use disinfectant carefully, as it can irritate skin.

1

Potassium hydroxide and soda lime are corrosive, so wear eye protection.

2

When using animals such as woodlice, handle them gently, keep them for a short time only and return them to where they were found.

3

Using plants and yeast avoids animal welfare concerns.

4

Hot water baths should be set up away from the edge of the bench, and electrical items kept away from water.

Section 6 of 9Improving reliability and drawing conclusions

Gas molecules in a beaker — interactive drawing

To obtain valid and reliable data, repeat each condition at least three times and calculate a mean, ignoring anomalies.

1

Use a thermometer or digital probe with a suitable resolution, and keep experiments in a draught-free place.

2

The control (dead seeds) shows that any change is caused by living organisms, not by the surroundings.

3

Possible sources of error include heat loss to the surroundings, bubbles escaping from the apparatus, and differences in yeast quantity.

4

Conclusions must be limited to the data: for example, "as temperature increased from 10 to 40 °C the volume of gas rose from 2 to 18 cm³ in 10 min",

then explain with kinetic energy of molecules and enzyme activity.

5

Calculate the rate as 18 / 10 = 1.8 cm³ per minute.

Section 7 of 9Exam-style questions

A student put germinating peas in one flask and boiled peas in another. Explain why the boiled peas are used. [2]2 marks
Describe a method to investigate how temperature affects the rate of respiration of yeast. [4]4 marks
A tube of yeast at 30 °C produced 24 cm³ of gas in 8 minutes. Calculate the rate in cm³ per minute. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Control	A sample treated identically but without the factor being tested.
Respirometer	Apparatus used to measure oxygen uptake.
Hydrogencarbonate indicator	Indicator that changes colour with carbon dioxide concentration.
Limewater	Solution that turns cloudy in the presence of carbon dioxide.
Water bath	Container of water used to maintain a constant temperature.
Reliability	The extent to which results can be repeated.

Section 9 of 9Exam technique

1

Not including a control (dead or boiled organisms).

2

Describing the independent and dependent variable the wrong way round.

3

Saying yeast produces oxygen; it produces carbon dioxide.

4

Not repeating the experiment to calculate a mean.

5

Forgetting to keep the volume or mass constant between conditions.