First Living Things on Earth

ВВЕДЕНИЕ

Данные методические указания предназначены для формирования и развития навыков самостоятельной работы и чтения научно-популярных текстов студентов младших курсов биологических специальностей.

Основываясь на дидактических особенностях модульно-рейтинговой технологии, при разработке методических указаний учитывались основные принципы и этапы работы над модулем, разработана рейтинговая шкала для оценивания самостоятельной работы студентов, т. е. реализован модульный подход к развитию навыков самостоятельной работы.

Методические указания состоят из 8 модулей, каждый из которых содержит аутентичные тексты различных уровней сложности. Исходными моментами при отборе текстов были актуальность темы, а также увлекательность и доступность изложения материала. Тематика каждого модуля связана с исследованием наиболее интересных фактов о живой и неживой природе.

Каждый раздел (модуль) содержит комплекс упражнений на активизацию и закрепление лексического материала, а также упражнения, направленные на проверку понимания прочитанного, развития навыков конспектирования.

Методические указания могут быть использованы студентами для подготовки индивидуального домашнего чтения, а также как дополнительный материал для развития навыков просмотрового и поискового чтения.

Внедрение данных методических указаний в учебный процесс будет способствовать формированию и развитию навыков самостоятельной работы, расширению лексического запаса, а также повышению интереса студентов к изучению специализированного английского языка.

 

Рейтинговая шкала для оценивания самостоятельной работы студента над модулем

	Макс. балл	Module	Module	Module	Module	Module	Module	Module	Module
Знание лексических единиц
Устный перевод текстов модуля
Пересказ содержания прочитанных текстов
Собеседование по содержанию прочитанного
Письменное выполнение задания к модулю
Подбор дополнительного материала
Конспектирование изучаемого материала
Выполнение контрольного тестирования
Итого

Module 1: EVOLUTION

Stages of Life on Earth

Life has been evolving on Earth for close to four billion years. The earliest life on Earth appeared during the Precambrian super eon, which began when the Earth was formed and ended about 550 million years ago. Fossils show that simple cells, known as prokaryotes, appeared on Earth between about three and a half and four billion years ago. The stage of the Earth's history after the Precambrian, in which we are living today, is known as the Phanerozoic eon. Phanerozoic means "visible life". Multicellular plants and animals became abundant during the Phanerozoic eon. The Phanerozoic eon is divided into three eras: the Paleozoic, the Mesozoic and the Cenozoic eras. Paleozoic means "ancient life". The Paleozoic era lasted from about 550 million to about 250 million years ago. Life forms that arose during the Paleozoic include animals such as insects, crustaceans, fish, amphibians and reptiles, and plants such as ferns, conifers and cycads. Many of the animals that flourished during the Paleozoic are now extinct. The Mesozoic ("middle life") era lasted from about 250 million to about 65 million years ago. Dinosaurs lived during the Mesozoic era. Mammals, birds and flowering plants appeared on Earth for the first time. The latest stage of life on Earth has been the Cenozoic era. Cenozoic means "modern life". The Cenozoic era, which began about 65 million years ago, is the geologic era that we are living in now. During the Cenozoic era, insects, mammals, birds, and flowering plants became widespread. Humans appeared on Earth in the last few million years of the Cenozoic.

X-Woman

In March 2010, a pinky bone from a young hominin – a member of the group of animals that includes humans, chimpanzees and bonobos (sometimes known as pygmy chimpanzees) – was discovered in Denisova Cave in Altay Krai in southern Siberia. Carbon dating of artifacts found in the cave, including a bracelet, shows that the owner of the pinky bone, who became known as X-woman, probably lived between about 30,000 and 50,000 years ago. X-woman was probably between 5 and 7 years old when it died. DNA from the X-woman's mitochondria (structures in cells that are involved with energy, respiration and growth) was compared to the mitochondrial DNA (mtDNA) of 54 modern humans from Russia, the mtDNA of six Neanderthals, and the mtDNA of one chimpanzee and one bonobo. (The name X-woman comes from the fact that mitochondrial DNA passes only through the female line; we don't know the actual sex of X-woman.) X-woman's mtDNA was found to be twice as different from that of modern humans as the mtDNA of Neanderthals is from us. Modern humans, Neanderthals and the species to which X-woman belonged had a common ancestor that lived about one million years ago. Scientists think that modern humans migrated from Africa about one million years ago – after Homo erectus, an ancestor of modern humans, left Africa and before the ancestors of Neanderthals migrated from Africa. X-woman's species, Neanderthals and modern humans may have interacted with one another and may even have interbred. In fact, X-woman may have been a hybrid – with a father who was a modern human or a Neanderthal. X-woman's mitochondrial DNA tells us nothing about its male ancestry. Researchers believe that X-woman's species probably hunted wooly rhinos and wooly mammoths and wore heavy clothing to keep warm. The discovery of X-woman shows that at least four different species of human beings – Homo sapiens (modern humans), Homo neanderthalensis (Neanderthals), Homo floresiensis, and X-woman – have lived in Europe and Asia. This is the first time that DNA sequencing – rather than the structure of fossils – has been used to describe a hominid. (Hominids include humans, chimpanzees, bonobos, orangutans and gorillas.)

Discovering Fossils

In 1887 Eugene Dubois, a young Dutch physician, uncovered on the island of Java in what was then the Dutch East Indies the fossil bones of a specimen he called Pithecanthropus erectus (the upright ape-man). Java man, as he has since become known, possessed a flattish skull somewhat apelike in appearance. The shape of his thigh bone, though, indicated that he had walked upright, like a human. Public outrage ran high at Dubois' claim to have found an early ancestor of man. Many people could not accept the possibility that modern man had descended from such primitive stock. Shortly after the discovery of Java man, however, a similar fossil skull was found in a cave at Zhoukoudian, near Beijing. It was called the Peking (former name of Beijing) man. The two pieces of evidence at such widely separated points in Asia suggested that a species of man more primitive than Homo sapiens had once ranged over much of the Orient.

In 1924 Dr. Raymond Dart, a professor of anatomy at South Africa's University of Witwatersrand, identified a fossil from a limestone quarry at Taung as the skull of a six-year-old child with a brain case no larger than that of a young ape, but with other clearly human traits. Not until 20 years later were enough adult skulls of this same type recovered by another South African paleontologist, Dr. Robert Broom, to confirm that these early creatures were not slightly unusual chimpanzees but higher forms of primates more closely allied to humans, despite their small brain case and small stature. These creatures are known as Australopithecines (southern apes). The potassium-argon method of radio­active dating has found some of the Australopithecine remains to be as old as 2.5 million years. It is in this same region of Africa where the Australopithecines once flourished that more advanced and recognizably manlike beings eventually are believed by anthropologists to have arisen, probably as descendants of the Australopithecines. If the Australopithecines can not quite qualify as the first men on the Earth, their descendants do and it is widely accepted that all humans originally descended from ancestors in what is now Africa.

Evolution of Land Plants

Life on Earth originated in the oceans. Plants, like animals, evolved through a number of stages which enabled them to live and reproduce successfully on land.

Algae

Around 420 million years ago, some marine algae developed the ability to live outside the water by growing waxy cuticles that kept them from drying out. However, the algae that had these cuticles still needed to enter the ocean in order to reproduce sexually. Algae reproduce both asexually, by division, as well as asexually. Sexual reproduction – and the genetic variation that it brings – is necessary for evolution to occur.

Mosses and Liverworts

Mosses and liverworts were the first plants to use the wind to assist in reproduction. These plants alternate between sexual reproduction and asexual reproduction with each generation. During sexual reproduction, the egg is attached to a stem. Sperm cells are released in the water and swim to the egg in order to fertilize it. The egg that is fertilized remains attached to the parent plant.

The next generation reproduces asexually. The plants of this generation consist of a thin stem with a hollow capsule in which many small spores are produced. When the atmosphere dries up, the wall of the capsule breaks and the spores are distributed by the wind. Those that land on a suitable site grow to become new plants. These plants cannot grow very high. They have soft cells that aren't strong enough to allow their stems to stand up straight.

Plants with Upright Stems

Over 400 million years ago, the first plants that could stand up appeared on Earth. These plants, which could grow to heights up to a few centimeters, had long cells with thick walls that allowed water to travel up a stem. They had a horizontal stem that lay along or just below the ground. The upright stem was an extension of the horizontal stem. This meant that these plants could only survive in places where there was a constant supply of aboveground water.

The First Trees – with Roots and Leaves

Three groups of plants – ferns, clubmosses and horsetails – went on to develop roots. This allowed them to absorb water from underground sources. These plants had strong, woody vessels in their stems. This allowed the water that the roots absorbed to be transported up the stems. The evolution of rigid stems allowed plants to grow very tall. Because green plants depend on sunlight for photosynthesis, there was intense competition to grow taller. If a plant isn't as tall as its neighbors, it will be blocked from receiving sunlight and may die. As a result of this competition for height, these plants grew very tall and became the first trees.

Flowers

So far, plants had relied on water and wind for reproduction. Water is not always available. Depending on wind can be very wasteful. When spores, which are used in asexual reproduction, are distributed by the wind, they will only develop into plants if they land in a spot with precisely the right conditions. A pollen grain, used in sexual reproduction, which is blown by the wind, will only develop into a plant if it lands on a female cone. This means that plants such as pines must produce enormous quantities of pollen, most of which never develops into a plant. Insects and plants evolved together. Insects are much more efficient than wind for transporting pollen. An insect can take a small amount of pollen and place it precisely where it is needed in order for the egg to be fertilized. Flowers developed as a way of placing the male and female parts of a plant next to each other, making it easier for insects to move pollen from the male part to the female part. The first flowers, which appeared about a hundred million years ago, grew on magnolias. In magnolias, the eggs are clustered in the center of the flower. The eggs are protected by a green coat with a spike on the end. This is known as a stigma. Many stamens, organs which produce pollen, are arranged around the eggs. The pollen has to be placed on the stigma in order for the egg to be fertilized. Around this entire structure, there are leaves that have been modified to be brightly colored in order to attract insects. These modified leaves are known as petals. In magnolias, as in many other flowering plants, the eggs and pollen develop at different times. This prevents cross-fertilization – pollen fertilizing an egg from the same plant. With cross-fertilization, there is no genetic variation. The stigma of a magnolia flower will accept pollen as soon as the flower opens. However, the stamens won't produce pollen until after the eggs have already been fertilized, with the help of insects, by pollen from other plants. Some flowers developed attractive scents to attract insects. Others developed nectar as an insect lure. Later on, some plants evolved flowers that encourage pollination by birds and bats. Flowers that are pollinated by birds do not have a scent, but are brightly colored. Flowers that are pollinated by bats have a musty smell.

Exercises

Find the missing word.

1. Archaea may been the earliest form of life on Earth.	______
2. Cyanobacteria use the process of photosynthesis in to obtain energy.	______
3. Prokaryotes not have a cell nucleus.	______
4. Roots allow plants to absorb water underground sources.	______
5. Prokaryotes appeared on Earth between about three and a half and four billion years.	______
6. When the Earth first formed, it was extremely hot.	______
7. In 1887 Eugene Dubois uncoveredthe fossil bones belonged to a specimen Pithecanthropus erectus.	______
8. Fossils are likely to appear volcanoes erupt.	______
9. Phanerozoic can be explained "visible life".	______
10. Sexual reproduction is necessary evolution to occur.	______
11. The first flowers appeared Earth grew on magnolias.	______
12. Cyanobacteria also known as blue-green algae.	______
13. Dinosaurs lived the Mesozoic era.	______
14. Mosses and liverworts alternate sexual and asexual reproduction with each generation.	______
15. Insects and plants evolved.	______

Module 2: NATURE

Life in Streams and Rivers

Streams are usually well-oxygenated at their headwaters. There are usually no plankton – drifting organisms that are often microscopic – in the turbulent headwaters of a stream. Green algae, diatoms and water mosses may be attached to stones or other objects in the water. These organisms may completely cover the bottom of the stream. These organisms are eaten by insect larvae, which are themselves eaten by small fish. Insects continually fall into the stream, and rain washes detritus (dead organic matter) into it as well. Whatever is not eaten immediately washes further downward, so there is very little food for detritivores (also known as saprovores or saprophages), organisms that eat detritus, in the headwaters.

As the water moves on, it begins to move more slowly. The bed of the stream becomes larger, and the total volume of water increases. Some of the sediment from upstream is deposited. Dead organic matter accumulates, so there is now food for detritivores to consume. As the stream grows wider and starts to become a river, less water is shaded by the trees belong the bank. This means that direct sunlight can reach most of the surface of the water. As the level of light increases, the rate of photosynthesis increases. There are some plankton, but many plankton organisms are swept downstream. Plants with roots grow in the sediments at the bottom. During floods, these plants may be washed away. Snails, mussels, crayfish and insect larvae live at the bottom. These may be eaten by perch and trout. Leeches may feed on these fish.

As a river comes close to the sea, it usually slows down even further. It drops large amounts of sediment. Banks along the lower reaches of the river may grow higher than the land behind it, forming natural levees. Waters in this part of a river are usually muddy. Because of this, the amount of sunlight that can penetrate the water is reduced, and organisms that depend on photosynthesis cannot live at the bottom. However, many plankton – some of which perform photosynthesis – live in the waters near the surface. Floating plants and emergent plants – plants that extend from below the water into the air – grow in the swampy lands along a river. During floods, the fruits and seeds of these plants are swept into the river. Large predators at the lower end of a river eat zooplankton – animal forms of plankton. Large fish, large crustaceans and large mollusks live at this end of the river. Many birds and mammals come here to obtain food. Crocodiles can be found at the lower ends of tropical rivers.

Nitrogen Cycle

The nitrogen cycle is the movement of nitrogen from the Earth's soil to the atmosphere and back again. Most of the Earth's atmosphere – about 78 % – is made up of nitrogen (N₂). Most living things, including human beings, cannot use it in this form. When we breathe, we inhale nitrogen and then exhale it. We don't use the nitrogen that we take in from the air. However, nitrogen is necessary for life. All organisms contain nitrogen compounds. Nitrogen is an essential component of proteins and nucleic acids (DNA and RNA). Many microorganisms that live in soil decompose proteins, breaking it down into simpler materials. During this process, ammonia (NH₃) is produced as a waste product. When ammonia, which is a gas, is dissolved in soil water, it reacts with hydrogen ions to form ammonium ions (NH₄+). Sometimes, the roots of plants will absorb these ammonium ions and convert them back into proteins. However, plants often do not absorb ammonium ions directly. Instead, bacteria in soil convert ammonium ions to nitrates, which are the main source of nitrogen for most plants. Soil contains two groups of bacteria known as nitrifying bacteria. One group converts ammonium ions to nitrite ions (NO₂-). The second group then changes these nitrite ions into nitrate ions (NO₃-). Nitrifying bacteria only break down ammonium ions when there is oxygen in the soil water.

Oxygen dissolves into soil water from the air spaces that normally occur in soil. However, if all the spaces become filled with water, leaving no room for air, then the soil water has no source of oxygen, and nitrifying bacteria cannot do this. In anaerobic conditions (when there is no oxygen available) a group of bacteria known as denitrifying bacteria change remaining nitrates to nitrogen gas, which gradually escapes into the atmosphere. Lightning changes smalls amount of gaseous nitrogen to nitrogen compounds. When these are dissolved in rainwater, they form weak nitrous and nitric acids. In soil, these combine with other elements to form nitrates and nitrites. Some bacteria, known as nitrogen-fixing bacteria, change nitrogen from the atmosphere into nitrogen compounds. They play an important role in returning gaseous nitrogen to the soil.

Pond Life

A pond is a relatively small body of water. Ponds are usually smaller than lakes.

Ponds contain standing water. However, most ponds are fed by springs or streams and most have an outlet, so a small current passes through the water and the water gradually changes. Huge numbers of plankton – drifting organisms that are mostly microscopic in size – float through ponds. Some of these organisms are plants, some are animals, some are bacteria and some are archaea. Bacteria and archaea are the two types of prokaryotes – one-celled organisms that do not have nuclei. Prokaryotes were the first living things to have existed on Earth. Most of the photosynthesis in ponds is performed by plankton known as phytoplankton. Diatoms are usually the most common form of plankton in ponds. Dinoflagellates, a type of plankton that has a flagellum – a tail-like structure that assists with locomotion – are also common in some ponds. Zooplankton, animal forms of plankton, can include rotifers and tiny crustaceans. Enough light penetrates through to the bottom of most ponds so that photosynthesis can take place at the bottom. At the bottom of a pond, Chlorophyta, a type of green algae, may wave filaments back and forth in the current while diatoms turn the mud a golden-green color. Sometimes green algae can become so abundant in a pond that the whole surface of the pond turns green. Large emergent plants – plants that pierce the surface of the water and grow partially in the air – such as irises, may grow around the edge of a pond, but there usually aren't any rooted plants growing in the center of a pond. Plants may grow within the water, some floating on the surface and others remaining submerged.

Mussels may live at the bottom of a pond.Fish can be found mostly around the edge of a pond, where they can hide among plants. Large predatory fish, such as pike, may swim through a pond in search of prey. Fish may eat insects and other invertebrates, or other fish. Numerous insects can often be found above and around ponds. Frogs eat these insects. Herons, kingfishers and otters come to ponds to eat pond fish. As dead creatures sink to the bottom of a pond, large amounts of organic matter accumulate at the bottom.

Soil

Soil is vital to the many organisms that live in it and to all animals that eat plant food. A soil starts to form when bacteria and small plants, such as mosses, begin to grow in decomposed, weathered rock. Humus, a dark organic material, is added when plants and animals die and rot. Then plant roots, as well as burrowing animals, mix the contents of the new soil, keeping it porous and spongelike. This allows water, air and minerals to circulate. Plants stabilize the soil by their root systems. It takes about 50 years for one centimeter of soil to form. Most soils contain three distinct layers, or horizons. The A horizon (topsoil) consists of decomposed rock and humus. Horizon B (subsoil) is red or brown in color and is made up of clay and iron with little organic material. The C horizon consists of partly decomposed rock. It grades down to solid, unaltered rock. Soil types and colors depend partly on the type of rock from which they develop and partly on the climate of the area in which they occur. Different types of soils include chernozems (black earths), chestnut-brown soils, podsoils, prairie soils and red, tropical laterites.

Soil Decomposition

Most microorganisms that live in soil are detritivores, also known as saprovores or saprophages. Detritivores are organisms that eat detritus – decomposing organic matter. Many detritivores only consume certain substances in a dead organism, rather than the whole body. As they decay, organic substances in soil pass through a food chain, in which the most complex pieces of detritus are reduced to simpler and simpler substances. For example, some detritivores consume cellulose, pectin and lignin – substances found in plant cell walls – from dead leaves that fall on top of soil. They then leave simpler organic substances as waste, and other detritivores will consume this waste. These detritivores will leave even simpler waste products, such as sugars, which then will be used by another group of detritivores. Food continues to pass downward through this food chain until eventually only inorganic materials – minerals, water and carbon dioxide – are left. The many intermediate organic substances that are formed during this process affect the environment of the soil. Many soil organisms create acids as waste products. These tend to accumulate in woodland soils, making woodland soil intolerable for many bacteria. However, fungi can tolerate acidic soil and therefore thrive in this type of soil. Some microorganisms in soil produce antibiotics, which prevent competing organisms from growing. Some of these, such as Tetracycline and Streptomycin, have been used to fight bacterial infections in human beings.

Soil Life

Bacteria

Actinobacteria, or actinomycetes, are a group of bacteria that are very common in soil. Actinobacteria are the chief agents of decomposition in the relatively dry and alkaline soil of grasslands. In 1900s Martinus Beijerinck, a Dutch botanist and microbiologist, was the first to point out how essential actinomycetes are to soil health. In the 1920s, soil microbiologists discovered that when soil samples are grown in a laboratory, between 30 and 40 percent of the colonies that develop are actinomycetes. Some actinomycetes decompose cellulose, which is one of the most abundant materials in the plant remains, while others act on the substances that result from this decomposition. In 1940, Selman Waksman, an American microbiologist and biochemist discovered that some actinobacteria produce actinomycin, which is an antibiotic. His work led to the discovery of other natural antibiotics, such as streptomycin. He received the 1952 Nobel Prize in Medicine for his work.

Fungi

Fungi live in many different soils, but they are especially important as agents of decay in woodland soils. They seem to tolerate the acid conditions of woodland soils better than bacteria. If you turn over a few inches of woodland soil, you will probably expose an irregular network of thin grey or white filaments. These filaments are branching fungal cells known as hyphae. Soil fungi decompose cellulose, as well as pectins – substances found in plant cell walls – and chitins, which are found in insect exoskeletons.

Algae

Although we often think of algae as water organisms, many species of algae live in soil. Algae may form surface crusts in desert soils and so help reduce soil erosion. In rice paddy soils, algae increase crop yields by adding nitrogen and oxygen to the soil.

Invertebrates

Common soil animals include earthworms, nematodes (roundworms), small insects, millipedes, centipedes and mites.

Tree Bark

Tree bark protects the tissues of a tree from weather extremes, from disease and from attacks by animals. Bark consists of two layers. The inner layer consists of living tissue. It is known as the phloem. The outer layer is made up of dead tissue. The phloem transforms sugars from the leaves to other parts of the tree. The outer layer is waterproof, which prevents the living tissues from drying out. In parts of the world where there are seasonal forest fires, some trees have very thick bark, which acts as insulation against heat. The cork oak of the Mediterranean region is an example of one of these trees. Trees with thin barks often have much thicker barks near the base of the trunk. This helps protect the tree against large herbivores.

How Tree Bark is Formed

Underneath tree bark, there is a layer of wet, green tissue known as bark cambium or cork cambium. The bark cambium creates corky cells. The bark cambium, together with these cells, is known as the periderm. When a tree is young, the periderm first appears in the outer tissues of a shoot. Waxes and other materials in the periderm cause the color of the shoot to change, usually from green to grey.

In most trees, a succession of periderms arise one after another, each coming from deeper and deeper layers of the stem. Periderms contain lenticels, small pores that allow gases to be exchanged with the outer atmosphere. As each periderm forms, the tissue layers outside it die because they can no longer receive water or nutrients. New layers of bark are always being created in the stem. This causes the bark to increase in size. In some trees, old layers of bark easily peel or break off. With other trees, the older layers remain attached so that the bark becomes very thick. These trees often have cracks in their bark because the trunk grows faster than the outside layers of bark can expand.

Tree Decay

Many nutrients are stored in a tree's wood and bark. When a tree dies and then decays, nutrients are returned to the soil, to be recycled and then used by the next generation of trees. It takes about 20 years for a large log to decay completely. Insect activity, as well as the growth of fungi, can speed up the decay of dead wood. As a tree decays, various organisms began to colonize it. The first to arrive are those that invade the tree as it is dying. Next come organisms that live on wood that has recently died. Other creatures, which specialize in different stages of decay, come later, with the last group specializing in wood that has become crumbly. The species that colonize wood in its earlier stages of decay tend to be more specialized than those that colonize it in the later stages.

Exercises

Find the missing word.

1. The outer layer is waterproof, which prevents the living tissues from drying.	______
2. Many detritivores only consume certain substances in a dead organism than the whole body.	______
3. Old layers of bark easily peel or break.	______
4. Lenticels allow gases exchanged with the outer atmosphere.	______
5. New layers of bark are always created in the stem.	______
6. Organic substances in soil pass a food chain.	______
7. If you turn a few inches of woodland soil, you will probably expose an irregular network of thin grey or white filaments.	______
8. Crocodiles can found at the lower ends of tropical rivers.	______
9. In anaerobic conditions a group of bacteria known as denitrifying bacteria change remaining nitrates nitrogen gas.	______
10. If all the spaces become filled water, leaving no room for air, the soil water has no source of oxygen.	______
11. Sometimes green algae can become so abundant in a pond the whole surface of the pond turns green.	______
12. Plant roots, as burrowing animals, mix the contents of the new soil, keeping it porous and spongelike.	______
13. Most soils contain three distinct layers horizons.	______
14. However, plants often not absorb ammonium ions directly.	______
15. The first organisms to arrive are that invade the tree as it is dying.	______

Module 3: ATMOSPHERE

Atmospheric Weight and Heat

The atmosphere weighs an estimated 5,000 million tons, and about half of this total mass is in the lower layers, within 5km of the Earth's surface. At sea level the average atmospheric pressure is 1.05kg/cm² (or 1,013 millibars) – that is, the weight of air above each square centimeter is 1.05kg. The pressure (and density) of the atmosphere decreases with increasing altitude; at a height of 5km the average pressure is 500 millibars – about half that at sea level – and at 16km above the ground it is only 100 millibars.

Variations in pressure are also caused by temperature changes. The chief source of heat is solar radiation, although little heat comes directly from the Sun's short wavelength radiation. Of the radiation that reaches the outer atmosphere, only about 46 per cent reaches the Earth's surface, most of the rest having been scattered or reflected back into space. At the surface, however, solar radiation is absorbed (thereby heating the surface) then reradiated in the form of longer wavelength radiation. It is this long-wavelength radiation that is absorbed by the carbon dioxide, water vapor and clouds in the lower atmosphere, creating the greenhouse effect. Hence the atmosphere is heated principally from below and, as a result, temperatures decrease with increasing altitude in the lower part of the atmosphere.

Heating by long-wavelength radiation near ground level makes the air expand so that it becomes less dense than the overlying cold air. As a result, the warm air tends to rise, leaving behind an area of comparatively low pressure. This contrasts with cold, dense air, which tends to sink, creating relatively high air pressure.

Carbon Dioxide

In one respect, the composition of the atmosphere has been changing in the last 200 years. Scientists have estimated that the concentration of carbon dioxide in the atmosphere before the Industrial Revolution was between 275 and 285 parts per million (ppm); by 1958 it had risen to 315 ppm and by 1980 it had increased still further, to 338 ppm. This increase is a result of human disturbance of the carbon cycle by the burning of fossil fuels and the destruction of forests. Moreover, the proportion of carbon dioxide in the air is continuing to increase. This continual increase has become a matter of concern because although carbon dioxide allows short-wavelength radiation from the Sun through to the Earth's surface, it absorbs some of the longer wavelength radiation that is re-radiated by the surface (water vapour and clouds also have this absorptive effect), giving rise to the "greenhouse effect". Thus carbon dioxide prevents the loss of radiation from the Earth, and the greater the amount of this gas the warmer it will become. An extreme example of the greenhouse effect occurs on Venus, where carbon dioxide makes up 95 per cent of the atmosphere and the average surface temperature is about 475 °C. On Earth an increase of the carbon dioxide concentration to 570 ppm could, according to one calculation, raise global temperatures by an average of 3 °C, which could have unforeseen and possibly disastrous ecological consequences.

Formation of Clouds

Function of the Atmosphere

The atmosphere, a thin shell of gases surrounding the Earth, is a very effective protective shield that reflects and absorbs harmful radiation and objects like meteorites. The atmosphere protects the Earth from receiving too much radiation from the Sun. The Sun is a vast ball of exploding gases – mostly hydrogen that is converted to helium through a nuclear reaction. At its surface, the Sun reaches a temperature of 6,000 °C at the surface. The Sun's heat and light travel almost 150 million kilometers (93 million miles) passing Mercury and Venus, until reaching the Earth's atmosphere. An extremely important layer in the atmosphere lies between 25 and 50 kilometers (15 and 31 miles) above the ground. This is rich in a gas called ozone. The ozone layer filters out ultraviolet light from the Sun. Below this, the atmosphere becomes much more mixed. The air that we breathe is made up of 78.09 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, with the remaining 0.03 percent consisting of carbon dioxide and many other gases. The progress of this energy coming from the Sun is hampered even further in the 10 kilometers (6 miles) just above the Earth's surface – the area in which our weather exists. Clouds, water surfaces, snow and ice reflect energy back from the Earth. Ultimately, only one in every 2,000 million parts of the Sun's energy reaches the Earth. Without an atmosphere, the Earth would be extremely hot during the day and extremely cold at night. It has been calculated that if the Earth's atmosphere disappeared, temperatures at the equator would reach 80 °C during the day and fall to –140 °C at night.

Particles in the Atmosphere

Even when the air seems to be completely clear, it is full of atmospheric particles – invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, bacteria and viruses. Some atmospheric particles are so large that you will feel them if they strike you. However, particles this large rarely travel far before they fall to the ground. Finer particles may be carried many miles before settling during a lull in the wind, while still tinier specks may remain suspended in the air indefinitely. The finest particles are jostled this way and that by moving air molecules and drift with the slightest currents. Only rain and snow can wash them out of the atmosphere. These tiny particles are so small that scientists measure their dimensions in microns – a micron is about one 25-thousandth of an inch. They include pollen grains, whose diameters are sometimes less than 25 microns; bacteria, which range from about 2 to 30 microns across; individual virus particles, measuring a very small fraction of a micron; and carbon smoke particles, which may be as tiny as two hundredths of a micron.

Particles We Breathe

Particles are frequently found in concentrations of more than a million per cubic inch of air. A human being's daily intake of air is about 450,000 cubic inches. This means that we inhale an astronomical numbers of foreign bodies. Particles larger than about 5 microns are generally filtered from the air in the nasal passages. Other large particles are caught by hairlike protuberances in the air passages leading to the lungs and are swept back toward the mouth. Most of the extremely fine particles that do reach the lungs are exhaled again – although some of this matter is deposited in the minute air sacs within the lungs. From these air sacs, particles may go into solution and pass through the lung walls into the bloodstream. If the material is toxic, harmful reactions may occur when it enters the blood. Fine particles retained in the lungs can cause permanent tissue damage, as with Coalworkers' pneumoconiosis (black lung disease), caused by buildup of coal dust in the lungs, and with silicosis, which is caused by the buildup of silicon dust.

Microorganisms

Once, physicians were taught that infectious microorganisms quickly settle out of the air and die. Today, the droplets ejected, say, by a sneeze, are known to evaporate almost immediately, leaving whatever microorganisms they contain to drift through the air. Only a relatively small fraction of the microorganisms that human beings breathe cause disease. In fact, most bacteria are actually helpful. Some, for example, convert atmospheric nitrogen into usable plant food. Pathogenic, or disease-producing, microorganisms, however, can be very dangerous. Most propagate by subdivision-each living cell splits into two cells. Each of the new cells then grows and divides again into two more cells. Provided with ideal conditions, populations multiply quickly. Fortunately microorganisms do not thrive very well in the air. Unless there is enough humidity in the air, many desiccate and die. Short exposure to the ultraviolet radiation of the Sun also kills most microorganisms. Low temperatures greatly decrease their activity, and elevated temperatures destroy them rapidly. Still, many microorganisms survive in the air, despite these hazards. Among the tiniest of airborne particles are viruses, which are on the borderline between living matter and lifeless chemical substances.

Thickness of Ozone Layer

The ozone layer is a layer in Earth's atmosphere that has a high concentration of ozone molecules. The average thickness of the ozone layer is 50km. Although, if it was compressed to near sea level conditions, the ozone layer would only be 3 mm thick on average. The ozone layer protects us from harmful sun radiation. Protection given by the ozone layer has decreased due to man-made compounds released such as CFCs. The concentration of ozone in the ozone layer has decreased by about 3 % due to man-made emissions.

Exercises

Find the missing word.

1. Of the radiation reaches the outer atmosphere, only about 46 per cent reaches the Earth's surface.	______
2. Heating long-wavelength radiation near ground level makes the air expand so that it becomes less dense than the overlying cold air.	______
3. In one respect, the composition of the atmosphere has changing in the last 200 years.	______
4. The continual increase of carbon dioxide concentration becomes a matter of great concern.	______
5. Carbon dioxide prevents the loss of radiation the Earth.	______
6. Up to a height of about six miles, the air is full suspended bacteria, fungal spores and pollen grains.	______
7. Air becomes less dense at higher altitudes, the composition of the atmosphere is the same at sea level and at great heights.	______
8. The Sun is a vast ball of exploding gases – mostly hydrogen that is converted to helium a nuclear reaction.	______
9. Clouds, water surfaces, snow and ice reflect energy from the Earth.	______
10. Finer particles may be carried many miles settling.	______
11. Only rain and snow can wash them of the atmosphere.	______
12. Some atmospheric particles are so large that you will feel them they strike you.	______
13. Each living cell splits two cells.	______
14. Many microorganisms survive in the air the hazards.	______
15. Viruses are on the borderline living matter and lifeless chemical substances.	______

Arctic Tundra

The arctic tundra, in the far northern reaches of the Earth, around the North Pole, is characterized by low temperatures and short growing seasons. The angle of the sun's rays is always low in the tundra. Therefore, it never receives much energy from the sun at any time.

Summer

In the tundra, summer lasts six to eight weeks. Summer days are very long. The long days allow enough heat from the sun to build up so that the upper layer of soil can thaw. The lower layer of soil, known as the permafrost, is always frozen. Because melting snow cannot drain into the permafrost, water collects on the surface. Thus, in the summer, the tundra becomes covered in marshes and ponds. During this short growing season, grasses and sedges become abundant. Low mats of lichens and mosses cover large areas. Some trees, such as birches and willows grow close to the ground. They rarely grow more than a few centimeters tall. The leaves of most plants are small. Flowers bloom and quickly develop seeds. Arctic hares and arctic foxes can be seen in brown coats. (Their coats turn white in winter.) Reindeer, also known as caribou, are plentiful. Lemmings are also active. Snowy owls and weasels, which prey on lemmings, can be seen when the lemming population is high.

Winter

When winter arrives, ponds and lakes freeze. With short days and low temperatures, there is little energy available. Food is scarce. Living things become dependent on stored energy for survival. Almost all tundra plants are dormant in winter. Many insects remain in the egg or larval stage. Reindeer migrate southward. Many birds also travel south. Lemmings and other small animals burrow under sheltered spots in the snow, where they consume seeds and parts of plants that they stored during the summer.

Climatic Change Causes

The causes of climatic fluctuations have not yet been fully elucidated, although many different theories have been proposed. Some scientists believe that small variations in the Earth's orbit around the Sun (which would affect the intensity of solar radiation reaching the Earth) are the principal cause. But others have hypothesized that minute alterations in the Earth's tilt on its axis may cause the climatic belts to shift, thus changing the climate as a whole. It has also been suggested that long- and short-term fluctuations in the Sun's activity – those caused by the 11-year sunspot cycles, for example – may affect the climate. Changes may also occur following prolonged volcanic activity. Volcanic dust can reduce the amount of solar radiation reaching the surface, causing changes in the weather. After the eruption of Krakatoa in 1883, for example, dust stayed in the atmosphere for three years; during this period a 10 per cent fall in solar radiation was recorded in southern France. There is also some concern that major climatic changes may result from man's activities, such as deforestation and pollution of the atmosphere.

Historical Climatic Changes

The existence of coal seams in Antarctica and of dinosaur fossils in Spitsbergen (which is within the Arctic Circle) demonstrates that climates have changed radically during the millions of years of the Earth's history. According to recent findings it appears that the Northern Hemisphere had a warmer climate between ad 900 and 1300 than it does today. It was in the tenth century that Norsemen founded a settlement in Greenland, where average temperatures were estimated to be 1–4 °C higher than they are today, but this settlement had disappeared by the end of the fifteenth century, probably because of the gradually worsening climate.

In Europe the period 1450–1850 is often called the Little Ice Age. Although no precise figures exist before the invention of meteorological instruments, there is much evidence for the Little Ice Age from historical documents (including records of crop failures and paintings of frozen rivers which never freeze today), and from modern analyses of such factors as seed and pollen counts in soils and deposits dating from that period. From 1850 the climate became warmer, although a few decades ago there seems to have been a certain amount of cooling – as evidenced by the fact that in 1968 Arctic ice reached as far south as north-eastern Iceland, the first time this had occurred for 40 years.

The Year Without a Summer

In the year 1816, an abnormally cold summer had a disastrous effect on human beings, particularly those in southeastern Canada, the northeastern United States and northern Europe. June began as usual in North America, with temperatures in the northeastern US and southeastern Canada rising into the 80s (Fahrenheit) during the day.

Then, on Wednesday, June 5, a cold blustery wind swept out of Hudson Bay and drove down across the St. Lawrence Valley and on into New England. Heavy rains whipped by strong winds lashed the land all afternoon and night. Each hour the temperature dropped. By the next morning, thermometers registered in the low 40s and were still going lower when the snow began. At Bennington, Vermont, snow fell that day from just after daylight until midafternoon. When finally, the storm stopped, the snow was 12 inches deep in Quebec city, and many parts of New England lay under a 6-inch blanket of snow. A farmer remarked in his journal that it was "the most gloomy and extraordinary weather ever seen". Day after day, the winter weather gave no signs of warming. Instead, it got worse. No thermometer climbed above 50 degrees, and most were in the low 30s. Tender crops that the hopeful farmers had put out earlier in the month were killed by the unseasonable frost, and the whole land looked as though it had been seared by a scorching fire. Through most of July and August, the days started with temperatures in the 40s. By late August, early morning temperatures were in the 30s. On the few successively warm days, people tried gardening again. Farmers planted corn and other crops, hoping that somehow they might still get a harvest before winter. Nevertheless, time after time their gardens and fields were devastated by frost and hidden by snow. The killing frost that came shortly after mid-September was the first of the new winter. It was slightly earlier than usual. The cold weather resulted in many deaths, including deaths from starvation. Although the winter of 1816–17 was an especially severe one, spring in 1817 came as usual. The summer of 1817 was normal.

Exercises

Find the missing word.

1. Living things become dependent stored energy for survival.	______
2. Snowy owls and weasels, which prey lemmings, can be seen when the lemming.	______
3. population is high.	______
4. This effect is counteracted by the warm air blankets many urban areas.	______
5. The assumption is false because any period used to assess climatic averages may turn to be abnormal.	______
6. Tundra receivesmuch energy from the sun at any time.	______
7. The killing frost came shortly after mid-September was the first of the new winter.	______
8. The causes of climatic fluctuations have not fully elucidated yet.	______
9. Global warming has caused the world's sea level rise.	______
10. Global warming is a change in climate has been resulting in higher temperatures.	______
11. The settlement had disappeared the end of the fifteenth century.	______
12. A few decades ago there seems have been a certain amount of cooling.	______
13. There was the belief that the weather is changeable, the climate is fixed and predictable.	______
14. Short days and low temperatures, there is little energy available.	______
15. It has also suggested that long- and short-term fluctuations in the Sun's activity may affect the climate.	______

Module 5: RAINFORESTS

Ants and Plants

Some of the plants of the tropical rainforest have mutually beneficial, or symbiotic, relationships with ants. A symbiotic relationship occurs between Cecropia trees and Azteca ants in the Central and South American rainforests. The umbrella-shaped Cecropia trees often colonize abandoned farmlands, disturbed roadsides and old clearings. They are small, quick-growing trees that increase in height by at least 6 feet (1.8m) each year and live for 20 years at most. The trunk and branches are hollow and divided by partitions, with thin patches in the outer wall. Azteca ants bite their way into the trunk through these thin patches and establish colonies. These ants introduce small plant-sucking insects into the chambers. The ants care for these small insects themselves, feeding on the sugary solution which the insects excrete. The plant produces small outgrowths from the leaf bases, which are rich in food materials. The ants eat these outgrowths and feed them to their larvae. Azteca ants are fierce, but do not sting. They attack anyone who disturbs or tries to cut the Cecropia in which they live. They also bite through the tips of climbers, which could otherwise smother the Cecropia plant. In Africa, the plant Barteria fistulosa provides shelter for large colonies of ants which, in turn, appear to defend it. This small tree grows up to 50 feet (15m) tall and is most abundant in old clearings and disturbed places, but also occurs in undisturbed forest. Ants occupy its horizontal branches, which are hollow. The ants clear other plants from the area around the tree base by biting off their tips. They attack any animal that disturbs or breaks the tree. Their sting is painful, and its effects can last for a couple of days. In Southeast Asia two members of the family Rubiaceae, Myrmecodia and Hydnophytum; a member of the family Asclepiadaceae, Dischidia; and two ferns, Phymatodes and Lecanopteris, provide ants with shelter, while the ants supply the plants with food. All are epiphytes and, except for Dischidia, have swollen rhizomes, or stem bases, which contain hollows. The hollows form whether or not ants are present. However, these plants are almost always colonized by ants, which fill some of the cavities with debris, including insect remains. The plants probably absorb the nutrients from this debris as the debris decomposes.

Kinds of Rainforests

Subtropical Rainforests

In southeast Brazil, on the Asian continent, in Burma and Assam and the eastern coast of Indochina, on the eastern coast of Australia and on the islands of the southwest Pacific, tropical rainforest changes steadily to subtropical rainforest as the latitude increases. Subtropical rainforests have different plants and animals than tropical rainforests. The trees in subtropical rainforests resemble types that can be found in the Earth's more temperate regions. Epiphyte and climber species also change. The change in plant species between tropical and subtropical rainforests happens gradually, according to the decrease in average temperature and changes in the number of hours of daylight