It is the physical, chemical and biological conditions of a particular area or the whole world, as the case may be.

(i) Small intestine.
(ii) Large intestine

An autograph is an organism which prepares its own food, while a heterotroph takes food from another organism. Autotrophs do not show locomotion because they don't need to move in search of food. On the other hand, most of the heterotrophs show locomotion because they need to move in search of food.

Most of the carbon dioxide released during respiration exits from the plant cells through various openings. The plant cells through various openings. The carbon dioxide, required for photosynthesis is taken through stomata. So, even if a plant keeps on taking in oxygen and releasing carbon dioxide during the day, it does not mean that no photosynthesis is happening in the plant.

Ozone is present in the stratosphere. It protects the earth from harmful ultraviolet radiations. UV ray causes various diseases to organisms e.g. skin cancer, cataract in human beings.
Ozone layer is depleting because of chlorides and fluorides. They act on ozone molecules and deplete it. Chlorides, fluorides are present in CFC's which are used in refrigerants and fire extinguishers.
CFC \(\longrightarrow \) Chlorofluorocarbon

(i) Digestive enzymes - Foods need to be broken into their small or simpler molecules so that they can be absorbed into the bloodstream. However, the physical breakdown of food is not enough. Enzymes are hence needed for the chemical breakdown of food and speeding up the digestive process. The products of digestion should be smaller enough to be absorbed.
(ii) Hydrochloric acid helps to kill the germs which might have entered into the system through food. It creates acidic medium for the pepsin to act on food to break down proteins.
 (iii) Villi are finger-like projections in the small intestine. They help to increase the surface area for absorption of the digested food. Villi are richly supplied with blood vessel which help to absorb digested food from the bloodstream.

(i)The glottis is gaurded by epiglottis because it prevents the entry of food in wind pipe. If the food enters wind pipe, then it chokes it, leading to the death of the person.
(ii)The lung alveoli are covered with blood vessels because the oxygen that we inhale dissolves in the blood, present in these blood capillaries only, as it reaches the lungs.
(iii)The wall of trachea is supported by cartilage rings because these rings prevent the air passage from collapsing.

(a) Functions of blood:
(i) Plasma transports food, carbon dioxide and nitrogenous waste in dissolved form.
(ii) Red blood cells carry oxygen to each and every cells.
(iii) White blood cells act as soldiers of our body and kill germs.
(iv) Platelets help in blood clotting, plugs the leakage. (v) It maintains our body temperature.
(b) Fishes have only two chambers in their hearts, the blood is pumped to the gills for getting oxygenated and from there it passes directly to rest of the body. Thus, the blood goes only once through the heart during one cycle of passage through the body.

(b) Lungs have bronchioles (small tubes) coming from the bronchi. The bronchioles at the terminals form a balloon like structure called alveoli which increases the  surface area for exchange of gases and is richly supplied with blood capillaries.

(b) Both A and R are true, but R is not the correct explanation of A.
The waste we generate daily may be biodegradable like fruit peel, vegetable waste, etc. and can be decomposed by decomposers or it can be non-biodegradable like plastics, glass, etc. Non-biodegradable wastes cannot be broken down by decomposers and tend to pile up and cause pollution.

(c) : Landfills are low lying areas of ground where waste is dumped and covered with soil to the level of ground.

(i) (b):Nitrogenous waste substances such as ammonia, urea or uric acid are produced during protein metabolism. Ammonia is the most toxic, followed by urea and uric acid. Uric acid is least toxic.
(ii) (c)
(iii) (b): Urea is a nitrogenous waste formed as a result of metabolism of various food/beverages in the body and is excreted in the urine. So,if a person is not taking food/beverages he will have less urea in his urine.
(iv) (c)
(v) (a): Malpighian corpuscle (A) is the site of ultrafiltration. A considerable amount of water is reabsorbed in the collecting duct (E) under the influence of ADH. Proximal convoluted tubule (C) is the main site for the reabsorption of glucose and amino acids. In distal convoluted tubule (D), bothhydrogen ions and ammonium ions are secreted, thus it maintains blood pH.