Answer:

1. Ozone layer protects the earth and the organisms from the harmful Ultraviolet rays of the Sun. ... The depletion of the ozone layer causes more UV radiation to reach the Earth and thus it is a cause of concern.

2. (i) Arteries have thick, elastic, muscular walls whereas veins have thin walls with few elastic fibres. Arteries need these properties to give them the strength and elasticity needed to cope with the high pressure surges of oxygenated blood coming from the heart. The elasticity allows the walls to recoil back to their original shape after they have increased in diameter to allow the surge of blood through.

(ii) Veins have one-way valves whereas arteries do not have any valves.Veins need valves as the blood flowing through them is at such a low pressure that they need something to prevent it going backwards! They are needed to prevent blood pooling. Arteries do not need valves as the blood is flowing at such a high pressure that it cannot GO backwards.

3. Carbon dioxide is essential for the plants to sustain, as it is the carbon fixed from the carbon dioxide during photosynthesis is used for synthesizing glucose. This glucose is then LATER used during cellular respiration to make ATP, the energy molecule. Also, glucose is also used by plants as a vital source of carbon to produce a major part of the plant mass.

The importance of carbon dioxide for plants can be proved by a small experiment;

Take two healthy potted plants.

Keep them in dark for 2-3 days, so that all the carbon dioxide fixed till that gt used up by the plant.

Take the plants and keep both in two separate closed bell jar containers that are air-tight. In one of the jar, keep an open beaker containing potassium hydroxide (KOH) solution along with the plant. KOH can absorb carbon dioxide, so the jar containing KOH is devoid of carbon dioxide.

Keep both the jars in sunlight for 4 hours.

Pluck the leaves from plants in both jars and remove chlorophyll by boiling in ethanol.

Apply IODINE solution over the leaves. The leaves of the plant present in the jar without KOH solution turns blue-black color, as the starch in the leaves reacts with iodine. But, the leaves of the plant in the jar with KOH does not change color as in the absence of carbon dioxide the plant cannot produce starch.