Biology notes Nutrition and Gas exchange in plants Page 1

The importance of minerals for proper growth in plants 1. Green plants can produce their own organic foods by photosynthesis. However, they need to absorb water and

some minerals from the soil for proper growth. 2. In general, the minerals are absorbed from the soil in form of ions, e.g., nitrates ( NO3

- ) and magnesium ion ( Mg2+ ).

The application of fertilizers in agriculture 1. In crop fields, the minerals in the soil are constantly absorbed by the crops to support their growth. When the

crops are harvested, these minerals are removed from the soil. 2. In order to replace the lost minerals and to maintain soil fertility, fertilizers are added to the soil. 3. There are two types of fertilizers : Organic and inorganic.

Biology Notes Section 3a Topic : Nutrition and gas exchange in plants

Minerals Function(s) Deficiency symptoms

1. Nitrates for making proteins and DNA 1. poor growth 2. yellowing of leaves

2. Magnesium ions for making chlorophyll 1. yellowing of leaves

Type of fertilizers

Sources Advantages Disadvantages

1. Organic

From dead bodies, crop residue, animal manure The organic matter will be decomposed and minerals are released into the soil gradually.

1. Forming humus (腐殖質) which improves soil texture. 2. Nutrients are released gradually, long-lasting effects. 3. Cheaper

1. Actions are slower. 2. Not hygienic 3. Lower nutrient content

2. Inorganic Artificially made, highly concentrated salts which can be added directly to the soil.

1. Faster actions 2. High nutrient content, readily available. 3. Concentration can be carefully adjusted.

1. Easily leach away by rainwater. 2. Decrease water potential of soil solution, may kill root cells. 3. It cannot improve soil texture

Biology notes Nutrition and Gas exchange in plants Page 2

The gas exchange in plants 1. Plants do not have specialized organs for gaseous exchange. As the stems and leaves are covered by cuticle

which is impermeable to water and gases, gaseous exchange can only take place at a few sites :

(a) Through the stomata (氣 孔 ) of leaves

(b) Through the stomata of green stems or the lenticels ( 皮 孔 ) in the bark ( 樹 皮 ) of woody stems. (c) At the roots ( not covered by cuticle and have root hairs ) Effects of light intensity on gaseous exchange in plants 1. In plants, gaseous exchange in green plants is controlled by two processes : respiration and photosynthesis. 2. Respiration takes place at all times while photosynthesis occurs only in the presence of light. The net flow of

gases into and out of a plant depends largely on the balance of the two processes. 3. In general, under strong light conditions, the rate of photosynthesis is faster. As a result, CO2 moves into the

leaves through the stomata while oxygen is released. Water vapour also diffuses out through the stomata due to transpiration.

4. Absorption and release of CO2 in plants under different light intensity : i) At point X, only respiration occurs. CO2 is given out. ii) Between points X and Y, the rate of respiration is faster than that of photosynthesis. More CO2 is given out than

taken in. iii) At point Y, rate of respiration and rate of photosynthesis are the same. CO2 released from respiration is equal to

the CO2 absorbed for photosynthesis. This is known as the compensation point. At this point, the net CO2 uptake ( or release ) is 0.

iv) Between points Y and Z, rate of photosynthesis is faster than rate of respiration. More CO2 is taken in than given

out. Note: The net CO2 uptake for photosynthesis at any time can be read from the graph. However, the total CO2

uptake is the sum of the net CO2 uptake and the amount of CO2 released from respiration. The total CO2 uptake can only be found if we assume that the CO2 released from respiration is constant throughout the day.

Biology notes Nutrition and Gas exchange in plants Page 3

An experiment to study the effect of light intensity on CO2 exchange in plants

Work examples :

At 0600 and 1800 hr, rate of PHS = respiration ( net uptake of O2 = net release )

Fastest rate of PHS at 0930 hr At 1200 hr, PHS rate drops because the light

intensity is too strong, some stomata may close After 1830 hr, no PHS Assume that the rate of respiration is constant,

Total amount of O2 released at 1200 hr is 12 + 44= 56 mg

Dark period = 11 hr , rate of CO2 released = 12 mg/hr Amount of CO2 released in the dark = 12 x 11 =132 mg Assume that the rate of respiration is constant, the rate of

PHS at 0930 hr ( in terms of CO2 uptake ) is 76 + 12 = 88 mg/ hr ( fastest )

area A area B PHS rate rate of respiration food production is faster than food oxidation positive growth due to accumulation of food

Set Condition Colour of indicator soltuion A dark yellow ( CO2 0.03% ) B dim light red ( CO2 0.03% ) C strong light purple ( CO2 0.03% ) D as a control red