AIDS is caused by a virus called Human Immunodeficiency Virus or HIV. The presence of this virus is detected by screening the blood of a person for special kinds of proteins called antibodies which are produced in the body in response to viral infection.
A characteristic feature of antibody molecules is that they are highly specific. They bind only to certain regions of the viral - proteins which act as the antigen. This is similar to a key fitting only a specific lock. AIDS detection tests are actually based on detecting the binding of the antibodies present in the blood with an artificially synthesized antigen attached to a solid surface.
If the HIV antibody is present in the blood, it binds with the synthetic antigen. This reaction can be confirmed by tests which give a coloured product.
Besides chemicals called pheromones, bees use a kind of dance language to tell other bees in a hive where the food is. When a worker bee locates a source of food such as a tree in bloom, it returns to the hive and performs a ‘dance’. If it is a round dance, that is, if the bee just goes round and round, it only says that food is nearby. A more complicated waggle dance, in which the bee executes a kind of ‘figure of eight’ movement, gives information not only about the distance and direction of the source of food but also the quantity of food expected. It is now known that the bees use gravity and not the sun as the reference point while dancing. That the dance language really works has now been demonstrated beyond doubt by using robot bees.
Honeybees suck nectar from a wide variety of flowers. But each bee usually keeps to a particular kind of flower until its supply diminishes. The nectar is collected by the forager bee in an organ called the honey sac. Here the nectar is broken down to two sugars, fructose and dextrose, by the action of a special enzyme. On returning to the hive the bee empties the nectar load by disgorging it onto the mouth parts of the younger bees. They evaporate some of the water and concentrate the nectar. Glandular secretions further transform it. After this, the nectar is deposited in an empty storage cell in the honeycomb, where the nectar gradually thickens into honey by evaporation. To speed up evaporation bees in the hive fan their wings (26,400 times a minute) to set up additional air currents. After the cells are filled with honey these are sealed with wax by the bees. 
Camels can survive for days in deserts with little food and no water due to their unique physiology. Contrary to the common belief, a camel does not store water in its stomach or its hump. Instead the hump stores fat which serves as a reserve for energy and a source of water when in need. When the fat is utilized by the camel’s body, hydrogen is released which combines with oxygen to form water. Camels also use water from other body tissues in a similar way. As body tissues dehydrate, it can lose as much as 25 per cent of its body weight without suffering any permanent damage. But when it does find water, a thirsty camel may drink up to 150 litres in 15 minutes. The water passes rapidly into its body tissues, making the animal fit for another desert crossing.
Electricity in fish is generated by the normal functions of nerves and muscles together with a chemical reaction. The electricity is produced in modified muscle tissue called electric organs. The nerves from these organs are joined to the brain. The electricity is produced by the passage of a stimulus along a nerve and a contraction in a muscle. The discharge goes from the belly to the back. It is a defensive mechanism in most cases, but is also used to kill the prey. Fishes are immune to their own shocks because of the insulating layers of fat.
It is not quite correct to say that a chameleon can change its colour to match coloured surroundings. A chameleon merely lightens or darkens its body colour in response to changes in the surrounding temperature and brightness. Its colour darkens in dim, cool surroundings and lightens when conditions are reversed. Chameleons also show lightened body colour if suddenly frightened or excited in which case the body colour may not blend with the environment.
All dogs have keenly developed senses of smell and hearing. Dogs naturally depend on their sense of smell to identify friends and foes. They also mark off their territory with their own urine which is individually recognizable to themselves and other dogs. Human beings also have individual ‘smells’ although they are usually unable to identify it. Each individual has a unique odour -the product of diet and subtle differences in metabolism.
Artificial insemination is a method used in animal husbandry to genetically upgrade stock by producing good quality off springs. In humans, the process is used to induce pregnancy in case of reproductive blocks.
IQ, or intelligence quotient, is commonly used as a measure of an individual’s level of intelligence. Right from early childhood an individual’s level of intelligence grows with age. A normal child of a certain age is taken to have a certain level of intelligence which can be measured by a set of carefully designed tests, such as identifying objects, answering simple objective tests and solving simple arithmetical or logic problems. If the performance of the child matches with that expected from an average child of the same age; that is, if his age in years is the same as his mental age, the , child has an IQ of 100. If the performance is below average, it is less than 100, and if above average it is more than 100. The IQ is calculated by dividing the mental age by the actual age in years and multiplying the ratio by 100.
Vitamin D is a group of about 10 fat soluble compounds. The form of vitamin D produced in humans is called vitamin and that in plants is called vitamin . Both vitamin and can be formed from their respective precursors in the presence of sunlight. In humans, exposure to sunlight leads to secretion of a compound called 7-dehydrocholesterol at the surface of the skin. The ultraviolet rays in sunlight convert 7-dehydrocholesterol into vitamin D by means of a series of chemical reactions. The vitamin D formed at the surface of the skin is reabsorbed into the blood stream.
Sugars in our diet are converted into the glucose due to metabolism and which then goes into the blood. It is generally measured by a method using chemicals called enzymes. The enzymes, glucose oxidase and glucose peroxidase convert glucose into gluconic acid and hydrogen peroxide which produces a red coloured substance.
Pain is felt when the message reaches the brain via the nerves and the spinal cord. The spinal cord is a sort of junction for all nervous signals to the brain. Pain-balms contain volatile oils that irritate the nerves on the skin surface. Since the messages from the skin counteract the pain message being communicated to the brain, the brain senses lesser pain than before. Muscle relaxants added to pain-balms also ease tense muscles. Massages increase blood flow to the affected areas. All these add up to a sensation of relief.
