Respiration in Plants — ICSE Class 9 Biology Important Questions

Answer: (c) Mitochondrion.

The mitochondrion is called the powerhouse of the cell because it is the site of aerobic respiration, where glucose is oxidised to release ATP energy.

Answer: (b) Ethyl alcohol and carbon dioxide.

Yeast carries out alcoholic fermentation: $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{energy}$C_6H_12O_6 → 2C_2H_5OH + 2CO_2 + energy.

Answer: (b) Lenticels.

Lenticels are small pores in the bark of woody stems that allow diffusion of oxygen and carbon dioxide between the inner tissues and the atmosphere.

Answer: (b) The seeds absorb oxygen and the released CO2 is absorbed by KOH, lowering the pressure.

The peas use up oxygen in respiration while the KOH absorbs the carbon dioxide they release. The gas volume in the flask falls, pressure drops and the coloured drop moves inward — proving oxygen is consumed during respiration.

Answer: (a) Anaerobic respiration breaks glucose down only partly to alcohol (or lactic acid), so most of the chemical energy stays trapped in these products; this is exactly why far less energy is released. R correctly explains A.

Respiration is the biochemical process by which living cells oxidise food (mainly glucose) to release energy for life processes, along with carbon dioxide and water.

The overall equation for aerobic respiration is:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (38 ATP)}$C_6H_12O_6 + 6O_2 → 6CO_2 + 6H_2O + energy (38 ATP)

1. Oxygen: Aerobic respiration requires oxygen, whereas anaerobic respiration takes place in the absence of oxygen.

2. Energy and end products: Aerobic respiration completely oxidises glucose to carbon dioxide and water, releasing a large amount of energy (38 ATP); anaerobic respiration only partly breaks glucose into ethyl alcohol and carbon dioxide (in plants/yeast), releasing very little energy (2 ATP).

Aim: To show that respiring seeds give out carbon dioxide.

Apparatus: A conical flask with living germinating seeds on moist cotton, closed with a two-holed cork; a delivery tube leading into a test tube of clear lime water.

Procedure and observation: The set-up is left for a few hours. The seeds respire and release carbon dioxide, which passes through the delivery tube into the lime water. The lime water turns milky (turns chalky).

Control: An identical flask with boiled (dead) seeds does not turn the lime water milky.

Conclusion: Since only the living seeds turn lime water milky, carbon dioxide is produced during respiration, because $CO_2$CO_2 reacts with lime water: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$Ca(OH)_2 + CO_2 → CaCO_3 + H_2O.

Respiration causes a loss of dry weight, while photosynthesis leads to a gain in dry weight of the plant.

A green leaf carries out both processes, but at different rates depending on light:

• Respiration goes on continuously, day and night, using oxygen and releasing carbon dioxide.
• Photosynthesis occurs only in light and is usually much faster than respiration during bright daylight.

In the day, photosynthesis uses up all the carbon dioxide produced by respiration and needs still more, so carbon dioxide is drawn in from the air; likewise all the oxygen needed for respiration is supplied internally and the surplus oxygen is released. Thus the leaf appears only to take in $CO_2$CO_2 and give out $O_2$O_2, even though respiration continues. At the compensation point (dawn/dusk) the two rates are equal and there is no net exchange.

Plants exchange respiratory gases (oxygen and carbon dioxide) with the surroundings simply by diffusion through several surfaces:

1. Leaves — through stomata: Tiny pores called stomata, mostly on the lower epidermis, open into intercellular air spaces. Oxygen diffuses in and carbon dioxide diffuses out; the moist mesophyll cell walls allow gases to dissolve and diffuse.

2. Stems:

• In young green stems, gaseous exchange occurs through stomata in the epidermis.
• In older woody stems the cork prevents diffusion, so exchange occurs through lenticels — loose masses of cells forming pores in the bark.

3. Roots — through the root surface: In young roots, oxygen dissolved in the soil water diffuses in through the thin walls of the root hairs and general root surface, and carbon dioxide diffuses out.

Why no specialised system is needed:

• Each plant cell exchanges gases directly with its immediate environment, so gases need not be transported far.
• Plants have a large surface area exposed to air and a low rate of respiration compared with active animals.
• Much of the plant body is non-living (wood) or has intercellular air spaces, so the demand for oxygen is low and simple diffusion is sufficient.

Aim: To show that heat is released during respiration.

Apparatus: Two vacuum (thermos) flasks each fitted with a thermometer through a cotton-plugged cork.

Procedure:

1. In flask A place soaked, living germinating seeds (e.g. gram or wheat).
2. In flask B (the control) place an equal quantity of seeds that have been boiled and then disinfected with a little formalin/dilute antiseptic to prevent decay.
3. Note the temperature in each flask at the start and again after 24 to 48 hours.

Observation: The thermometer in flask A (living seeds) shows a marked rise in temperature, while the thermometer in flask B (dead seeds) shows little or no rise.

Conclusion: The rise in temperature in flask A is due to heat released by the respiring living seeds, proving that respiration is exothermic.

Precautions / why the control is needed:

• A vacuum flask is used so that the small amount of heat produced is not lost to the surroundings.
• The dead seeds in the control must be disinfected; otherwise micro-organisms (bacteria and fungi) would themselves respire and raise the temperature, giving a false result.
• The flasks should be inverted or the thermometer bulb kept among the seeds, and both flasks kept under identical conditions so that the only difference is whether the seeds are living or dead.

(i) The yeast carries out anaerobic respiration (alcoholic fermentation).

(ii) Word equation:

$\text{Glucose} \rightarrow \text{Ethyl alcohol} + \text{Carbon dioxide} + \text{energy}$Glucose → Ethyl alcohol + Carbon dioxide + energy

(iii) The dough rises and develops holes because the carbon dioxide gas produced by fermentation is trapped as bubbles within the sticky, elastic dough, making it swell and giving it a porous, spongy texture.

(iv) The dough is kept in a warm place because the enzymes of the yeast that carry out fermentation work best at a warm optimum temperature; in the cold the enzymes act very slowly and the dough would hardly rise.