My Science School

            There is a close link between the sense of taste and the sense of smell. Much of what we think of as taste has a big smell component. The taste buds in the tongue monitor relatively crude sensation of the four basic tastes – sweet, salty, sour and bitter. Faint vapours of whatever we eat drift into the nasal cavity where the smell receptors add more detail to the information given by the taste buds.

When we have a cold our nose becomes blocked and the sensation of smell ceases. The gas and vapours from the food cannot be sensed by the smell receptors; consequently we cannot perceive the smell or aroma of the food, which in turn makes the food appear tasteless.

            Tears produced by the tear glands keep the eyes moist and are normally drained off through the nose. Onions contain a number of water soluble volatile substances containing sulfur. On cutting the onion these substances quickly spread into the air and cause irritation in eyes. The tear glands start secreting more and more tears to wash out the irritants. The tears are produced so rapidly that they cannot be drained out through nose and so run down the eyes.

Potato is a living plant tuber (a modified stem specialized for storage of food). There is a lot of difference in the cellular structure and chemical nature of the potato peel and that of the flesh. The flesh is made up of parenchyma cells, specialized to store starch. The walls of these cells are made up of cellulose and they are joined together by a sort of glue called pectin. The peel or the skin of potato is made up of cork cells, the walls of which have a lot of lignin and suberin deposits. These substances give a greater mechanical and structural strength to the corky cells and hence are easy to remove as a continuous peel. In un-boiled potato the corky skin and the flesh of potato are sealed together by pectin. Hence peeling is difficult. During boiling, little change occurs in the corky peel, but the fleshy parenchyma becomes soft. Also the glue-like substances present between the cells, swell up and break down. As a result, the adhesion between the peel and the flesh is reduced and the effort required to separate the peel from the flesh is highly reduced. Hence it is easier to peel a boiled potato.

            Spoilage of foodstuffs is caused by microorganisms which decompose them to produce acid, alcohol, etc. Bacteria are always present in the atmosphere. When freshly prepared food is kept in the open for long periods it gets contaminated. The bacteria then act upon the foodcomponents such as proteins, carbohydrates and fats and break them down to products which due to their off-odours or bad-taste are not suitable for consumption. Factors such as high temperature and humidity help the bacteria to grow.

    Spoilage of food can be prevented by treatments like heat sterilization, refrigeration, deep-freezing and desiccation.

                                    

  Flowering is a natural mechanism by which the majority of plants perpetuate their species through the production of seeds. All the parts of a flower take part in one way or another in the reproductive process. The stamens consist of thread-like stalks supporting sac-like anthers which produce the pollen. The central pistil contains the ovary and a sticky stigma. The colourful petals serve as signals to attract insects and other pollinators. When pollinating insects or birds visit the flowers for nectar, they help transfer pollen to the stigma which leads to fertilization of the egg cells in the ovary. The ovary then grows into a fruit containing seeds that would, if planted, grow into new plants thus perpetuating the species.

            Like any living organism, plants need, besides air and sunlight, water and nutrients for growth and survival which they take up through their roots. For land plants, the soil provides the moisture and nutrients. The soil also protects the roots from rapid fluctuations in temperature, as just below the surface the temperature scarcely changes from day to night. But more importantly, soil provides the firm support which holds the plants on the ground. Some plants, like the water hyacinth which grow in water, and orchids which grow on trees do not need soil to grow on as they can get their nutrients from water or the host                                                                                              plant.

            All plants respond to touch, but Mimosa pudica, or the “touch-me-not” plant is more sensitive than the others. Whenever touched, the leaves of the mimosa plant close and drop immediately, appearing like a dead plant. Possibly it is a defense mechanism by which the plant protects itself against its enemies.

   The opening and closing of the leaflets and the entire leaf are controlled by fluid-filled sac-like structures found at the base of not only the compound leaf but of each leaflet as well. When the leaf is touched, a chemical signal immediately moves to the base of each leaflet making the fluid drain away from the sac which makes the leaflet fold. When the signal reaches the base of the whole leaf, it makes the entire leaf drop. If left untouched for some time, the sacs fill up and the leaflets open again.

            Lightning occurs when a massive electrical discharge takes place between two oppositely charged clouds or between a charged cloud and the ground. The charges develop in thunder clouds due to the friction of water droplets with air as the droplets move up and down with the rising and descending air currents within the cloud. During a bolt of lightning, thousands of amperes of electricity flows through the air in a fraction of a second. This rapidly heats up the air along its path which expands very fast producing shock waves which we hear as thunder. Although the lightning and thunder are produced at the same instant, we hear the thunder later because light travels faster than sound.

    The reason lies in the water cycle. The total quantity of water in the lakes, rivers and oceans of the world remains almost the same always; only the distribution changes periodically. The water cycle begins as the Sun evaporates water from oceans, lakes, rivers and soil. The vapour forms clouds which precipitate as rain or snow, 75 per cent of which falls on the oceans. The remainder falls on land. Some of this water seeps into the earth as groundwater while some is returned to the oceans by the rivers. Thus there is constant balance between water flowing into the oceans and water evaporating from them. That is why the oceans do not overflow.

            The Earth’s axis – the imaginary line around which the Earth turns daily is titled at an angle of  to the plane of its orbit. The seasons are caused by this tilt and by the fact that the Earth’s axis always holds the same orientation in space. As the Earth travels round the Sun, the North Pole is sometimes directed towards the Sun and sometimes directed away from it. When the North Pole is directed towards the Sun, sunray’s strike the northern hemisphere almost directly. This causes summer in the northern hemisphere and the southern hemisphere has winter. Three months later, the North Pole begins to slant away from the Sun. This causes the northern autumn and southern spring. After another three months the North Pole it tilted away from the Sun, this causes winter in the north and summer in the south. Finally the North Pole tilts towards the Sun again, bringing spring to north and autumn to south.

The main reason why some trees shed leaves is to conserve water during the winter months. When root activity and water absorption decrease, deciduous trees drop their leaves. This helps them keep water loss at the minimum. Unlike deciduous trees, which lose their leaves all at the same time, evergreen trees, mostly conifers, shed and replace leaves continuously. Before the leaves fall, triggered by changes in temperature and daylight, groups of the soft cells from around the base of the leaf stalk. These cells produce an enzyme which dissolves the stalk cell walls and causes leaf to detach. Discarding their leaves also helps trees dispose of toxic and waste products which accumulate in the leaves before they fall.

            Sea water is salty because it contains many dissolved salts, mostly sodium chloride with small proportions of potassium, magnesium, calcium and carbonates. The salt comes mainly with river water that flows through rocks eroded by frost and rain. The gradual wearing a way of mountains releases chemicals which are carried down by rivers to the ocean as dissolved salts. Some salt also enters sea water from the rocks beneath the sea bed. The river water carrying the salts to the sea does not taste salty because the salts are present in extremely small concentration. Whereas in the case of the oceans, water is continuously evaporating even as more and more salt continues to be added with river water. This, over millions of years has led to the high concentration of dissolved salts in sea water.

            During a total solar eclipse the Moon moves across the bright disc of the Sun from west to east. So, as the eclipse progresses, the Moon’s shadow too moves in the same direction. The Earth too spins from west to east. But the speed of movement of the Moon’s shadow is much greater than the speed at which the surface of the Earth moves from west to east. As a result, the Moon’s shadow still moves from west to east along the path of totality.

                                    

  All the planets of our solar system were formed out of the same cloud of gas and dust that gave rise to the Sun, some five billion years ago. As random collisions and gravitational forces compacted the gas and dust particles into the various planets, the heat generated brought the masses into a molten state. Since surface tension of a liquid tends to make the surface area minimum and the sphere has the minimum surface area for a given volume, all the planets in the molten state became spherical and have retained that shape as they cooled.

 The phase of the Moon as seen from the Earth arises because of the constantly changing position of the Moon relative to the Sun and the Earth. As the Moon goes round the Earth, one half of its surface that faces the Sun is always illuminated. But from Earth, the entire illuminated side is visible only when it faces us, that is, only at full Moon. At other times, depending on the position of the Moon in the sky relative to the Sun, only a part of the illuminated face becomes visible, which grows and shrinks in size, giving rise to the familiar phases of the Moon.