Inglês 2017

Water and Plants

With a few exceptions, plants make their own food from water and air. In order to survive, they act like pipelines, taking water out of the soil, delivering it to cells for use, and allowing whatever is left over to disperse into the air. The water, which is absorbed through fine root hairs underground, travels upwards through labyrinths of long microscopic tubes penetrating the stem and branches, and passes back to the atmosphere through tiny leaf pores called stomata. This latter process is called transpiration. A leaf with an area of 2.5 square centimetres may contain as many as 30,000 stomata, most of which are on the underside, and they release an astonishing amount of water. Although transpiration varies with conditions of temperature, humidity, light, wind and the moisture of the soil, it usually totals several hundred times the dry weight of the plant itself during a single growing season. During its lifetime, a crop of maize, for example, may release water sufficient to cover the entire field in which it has grown, to a depth of 28 cm. In one warm day, a single birch tree can dispose of 225 – 315 litres of water.

The mechanics of this remarkable capacious water-handling system are still not completely understood and the movement of water in certain plants – very tall trees, for example – poses one of the most intriguing puzzles of plant biology. What is known, however, testifies again to the distinctive characteristics of water. One of these characteristics is called osmosis.

Ground water enters the root hairs of a plant by a special kind of diffusion (osmosis) which is a fundamental process that goes on in nearly all living tissues. Through this process, water molecules are able to cross living membranes even though these membranes do not seem to admit water in the form of drops of liquid. This apparent paradox can be demonstrated with a piece of cellophane which is a synthetic membrane quite similar to natural ones. Cellophane is watertight in the sense that a drop of water placed on its surface will not drip through; a microscope reveals no pores. Yet, somehow, water crosses to enter the plant. This puzzling circumstance is resolved by a closer examination of the membrane. Pores do exist, but they are too small to be seen with an ordinary microscope. Like all substances, a membrane is composed of molecules, and the molecules, no matter how tightly they are packed together, have spaces between them. The spaces are large enough to accommodate water molecules but far too small to allow the penetration of water drops. Thus a drop may pass through the barrier, but only a few molecules at a time.

Diffusion takes place because of the random movement of molecules. They bounce against one another and fly apart, tending always to spread from a region where they are closely packed together to regions of lesser concentration. This action is the same one that diffuses dissolved molecules through a liquid – and is the reason why a lump of sugar eventually sweetens a cup of tea or coffee whether the liquid is stirred or not.

The rate at which molecules slip through a plant’s intermolecular structure depends on the size of both the molecules and the pores. Small molecules like those of water travel through the pores of living membranes at a fairly rapid rate. The larger molecules of soluble substances, like minerals, travel through much more slowly. This difference in the rate of progress across the membrane makes the membrane a kind of sieve, and this sieving action can build up a substantial pressure. The reason is that there are relatively more water molecules outside the plant than inside, where minerals are present in the liquid. As indicated above water molecules move to regions where their concentration is less and hence ‘osmotic’ pressure builds up.

Tomatoes – the perfect fruit

It is difficult to imagine a world without tomatoes. High in the Andes mountains of modern-day Peru, the local inhabitants have been cultivating and eating tomatoes since prehistoric (11) _____, but the food has only become (12) _____ in the rest of the world (13) _____ recently. These days, the bright red fruit (14) _____ an important role in the cooking of many cultures and is a key ingredient in many types of fast food, (15) _____ both taste and colour to dishes that (16) _____ would be rather ordinary.

The tomato (17) _____ to the nightshade family of plants, many members of which are poisonous. When they were first (18) _____ into North America, therefore, tomatoes were viewed with (19) _____ and people tended to use them as table decorations (20) _____ than as food. In Europe, the tomato was first grown in Italy in 1555, although it wasn’t (21) _____ with pasta until much later. The first recipe for tomato ketchup dates from 1727 and in the 1800s, tomatoes began to be used more (22) _____ in sauces and soups. These days, as well as tasting good, tomatoes are (23) _____ to contain substances which are good for our health. Nutritionists (24) _____ out, however, that many processed tomato products also contain additives such as salt and sugar which can (25) _____ the beneficial effects of the fruit.

Promoting hygiene in our communities

Hygiene plays a very important role in the promotion of health, well-being and comfort. When it comes to health, staying hygienic is vital especially because most new diseases in our world nowadays are associated with lack of hygiene. Some of those diseases include bird flu and the swine flu. In Mozambique, a killer disease that is associated with hygiene is cholera. Therefore, it is important to inculcate in our population, especially in the children the habit of hygiene. Hygiene is not only about our bodies. It is also about the surroundings in our communities.

Here are some tips on hygiene:

1. Bodily hygiene

It is recommended that you always brush the teeth twice a day. While brushing your teeth will avoid that they get damaged, it is said that dental problems can cause serious conditions such as high blood pressure or heart attack. We must always take a bath once or more every day. Taking bath often is very important, especially in the city, where humidity and pollution will attract bacteria to our body. Another hygienic move has to do with always covering the mouth when coughing. You don’t want to pass germs on to others. Changing clothes as often as possible may prevent very serious skin disorders. A final hygienic tip relating to our body concerns our genital areas. We have to keep these areas extremely clean. If we don’t do that, we are prone to infections and bacterial attacks. And then, there are our hands. Let’s wash our hands as often as possible. Many places that we touch are not clean.

2. Hygiene in the environment

The environment where we live in can be a source of diseases caused by bacteria and viruses. So, here are some tips to keep our environment safe. One of the first things that we must all do is dispose of waste properly. If waste material is inappropriately disposed of, it can cause an outbreak of deadly diseases. Cholera is one of those diseases, which can be caused by improper disposal of garbage. Do not keep swamps near residence areas. They may be mosquito breeding locations. One type of mosquitoes, the anopheles, causes malaria. So, this is what we should always do: keep the home clean; do not throw garbage around residential areas; use garbage cans (in cities) or bury garbage (rural areas. Do not incinerate it in the open. You will pollute the environment); never spit or urinate in public places. Such actions cause the entire surrounding area to stink and breed bacteria and viruses that are dangerous to our health.