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Respiration and Photosynthesis

April 13, 2014

Respiration and Photosynthesis

With air quality going downhill, there may be chances that we need to carry plants around as our oxygen tank.  The working mechanism is based on 2 simple concepts of Primary 5 Science, Respiration and Photosynthesis.

As seen from the picture, the man is wearing a mask connected to the plant. As the man respires, it gives out carbon dioxide.

The carbon dioxide, together with water and light, the green plant with cholorphyll photosynthesises and produces oxygen for the man to respire and survive.

A summary of the both respiration and photosynthesis processes are below:


Respiration is the breaking down of digested food with oxygen to produce energy.

The equation of Respiration

Sugar (from digested food) +  Oxygen (from inhaled air)  è

ENERGY (for body to use) +  Carbon Dioxide (to exhaled air) + Water Vapour (to exhaled air)

*NOTE* All living things respire including plants. Respiration takes place ALL the time.


Photosynthesis is the process of plant making food in the presence of  chlorophyll trapping sunlight, carbon dioxide and water.

The equation of Photosynthesis

Light energy  + Carbon Dioxide (enter through stomata from air) + Water (absorbs by roots) è

Sugar (excess sugar will become starch) + Oxygen (release through stomata through air)

Hence if we put it side by side, we will get this:

Respiration and Photosynthesis Equations

Primary 5 Science: Photosynthesis and Respiration Application Questions

Question 1

Elaine set up an experiment as shown below. The setups were left in a room for a day. The colour of the limewater in all setups before and after the experiment was observed.

Limewater experiment with plants in different setups

Which of the following correctly shows the colour of the limewater setups before and after the experiment?

Set Up Before After
1 R Clear Chalky
2 S Clear Clear
3 T Chalky Clear
4 U Chalky Clear

Answer: (2)

Explanation: In the beginning, limewater is always clear. Hence we can eliminate option (3) and (4).

R has no livings things to respire. Thus limewater will not turn chalky as amount of carbon dioxide did not turn clear limewater chalky. Hence it should not have been chalky. Instead, it should be clear.

S has a pot a  plant. The carbon dioxide from respiration of plant is used by the plant for photosynthesis. Thus, amount of carbon dioxide in the set up did not increase to turn limewater chalky. Hence limewater remains clear. 

Question 2

The graph below shows the amount of oxygen in a glass box with a green plant over a 24-hour period.

Oxygen Level Graph of Plant


Why did oxygen level increase from point A to B.

It is because from 10am tp 6pm it is in the daytime. In the presence of light, water and carbon dioxide, the plant photosynthesises to release oxygen. Although the plant also respire, using oxygen, the rate of photosynthesis is much more than respiration. Hence amount of oxygen level increase from point A to point B.

Why did oxygen level decrease from point B to C.

It is because from 6pm to 6am, it is night timeWithout the presence of light, the plant can only respireusing oxygen to break down glucose for energy and its survival. Therefore volume of oxygen decreases from point B to C.

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