Photosynthesis and Respiration
ICSE Class 7 · Biology · Chapter 4
What this chapter covers
- Photosynthesis
- The leaf: stomata and chloroplasts
- Factors affecting photosynthesis
- Experiments to demonstrate photosynthesis
- Significance of photosynthesis
- Respiration
- Aerobic and anaerobic respiration
- Photosynthesis versus respiration
Key points
- Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using sunlight trapped by chlorophyll, and give out oxygen.
- Word equation: carbon dioxide + water → (sunlight, chlorophyll) → glucose + oxygen.
- Extra glucose is stored as starch, which turns blue-black with iodine solution.
- Stomata are leaf pores for gas exchange, opened and closed by guard cells; chloroplasts are green bodies containing chlorophyll, where photosynthesis takes place.
- Light, carbon dioxide, water and chlorophyll all affect photosynthesis; a shortage of any one slows it down, and the absence of any one stops it.
- Experiments with a black paper strip, a variegated leaf and potassium hydroxide show that light, chlorophyll and carbon dioxide are necessary; the Hydrilla experiment shows that oxygen is given off.
- Photosynthesis supplies food and oxygen to living things and helps balance carbon dioxide and oxygen in the air, which is why more trees need to be planted.
- Respiration is the breakdown of food in living cells to release energy, used for movement, growth, repair, warmth and all life activities.
- Aerobic respiration: glucose + oxygen → carbon dioxide + water + a large amount of energy.
- Anaerobic respiration happens without oxygen and releases a small amount of energy: yeast makes alcohol and carbon dioxide, and tired muscles make lactic acid.
- Photosynthesis stores energy and happens only in green cells in light; respiration releases energy and happens in all living cells at all times.
Questions answered
Define photosynthesis and write its word equation.
Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using sunlight trapped by chlorophyll, and release oxygen. Word equation: carbon dioxide + water → (sunlight, chlorophyll) → glucose + oxygen.
Name the two raw materials and the two conditions needed for photosynthesis.
The raw materials are carbon dioxide and water. The conditions are sunlight and chlorophyll.
What are stomata, and what controls their opening and closing?
Stomata are tiny pores, mostly on the lower surface of a leaf, through which gases pass in and out. Each is opened and closed by a pair of guard cells, which open the pore when they swell with water and close it when they lose water.
Why is a leaf destarched before an experiment on photosynthesis, and how is it done?
A leaf is destarched so that any starch found at the end must have been made during the experiment. The plant is kept in complete darkness for about two days, so it uses up the starch already in its leaves.
In the starch test, why is the leaf boiled in alcohol, and why is a water bath used?
Alcohol removes the green chlorophyll so that the blue-black colour of iodine with starch can be seen clearly. A water bath is used because alcohol catches fire easily and must never be heated over a direct flame.
A croton leaf with green and yellow patches is tested for starch after a sunny day. Which parts turn blue-black, and what does this show?
Only the green parts turn blue-black. This shows that chlorophyll is necessary for photosynthesis.
How can you show that the gas given off by Hydrilla in sunlight is oxygen?
Collect the gas in an upside-down test tube over the plant, then bring a glowing splinter to its mouth. The splinter relights, which happens only with oxygen.
Give three reasons why photosynthesis is important to animals and people.
It provides the food on which all animals depend, it releases the oxygen that living things breathe, and it removes carbon dioxide from the air, helping to keep the amounts of the two gases balanced.