Life Processes PDF

CHAPTER 6 Life Processes ed H ow do we tell the difference between what is alive and what is not h alive? If we see a dog running, or a cow chewing cud, or a man...

CHAPTER 6 Life Processes ed H ow do we tell the difference between what is alive and what is not h alive? If we see a dog running, or a cow chewing cud, or a man pu T shouting loudly on the street, we know that these are living beings. What is if the dog or the cow or the man were asleep? We would still think that they were alive, but how did we know that? We see them breathing, and re R we know that they are alive. What about plants? How do we know that bl they are alive? We see them green, some of us will say. But what about plants that have leaves of colours other than green? They grow over E time, so we know that they are alive, some will say. In other words, we tend to think of some sort of movement, either growth-related or not, as common evidence for being alive. But a plant that is not visibly growing is be C still alive, and some animals can breathe without visible movement. So using visible movement as the defining characteristic of life is not enough. Movements over very small scales will be invisible to the naked eye – o N movements of molecules, for example. Is this invisible molecular movement necessary for life? If we ask this question to professional biologists, they will say yes. In fact, viruses do not show any molecular movement in them (until they infect some cell), and that is partly why © there is a controversy about whether they are truly alive or not. Why are molecular movements needed for life? We have seen in earlier classes that living organisms are well-organised structures; they can have tissues, tissues have cells, cells have smaller components in them, and so on. Because of the effects of the environment, this organised, ordered nature of living structures is very likely to keep breaking down over time. If order breaks down, the organism will no longer be alive. So living creatures must keep repairing and maintaining their structures. Since all these structures are made up of molecules, they must move molecules around all the time. tt What are the maintenance processes in living organisms? Let us explore. 6.1 WHA WHATT ARE LIFE PROCESSES? no The maintenance functions of living organisms must go on even when they are not doing anything particular. Even when we are just sitting in  class, even if we are just asleep, this maintenance job has to go on. The processes which together perform this maintenance job are life processes. Since these maintenance processes are needed to prevent damage and break-down, energy is needed for them. This energy comes from outside the body of the individual organism. So there must be a process to transfer a source of energy from outside the body of the organism, which we call food, to the inside, a process we commonly call nutrition. If the body size of the organisms is to grow, additional raw material will ed also be needed from outside. Since life on earth depends on carbon- based molecules, most of these food sources are also carbon-based. Depending on the complexity of these carbon sources, different organisms can then use different kinds of nutritional processes. The outside sources of energy could be quite varied, since the h environment is not under the control of the individual organism. These pu T sources of energy, therefore, need to be broken down or built up in the is body, and must be finally converted to a uniform source of energy that can be used for the various molecular movements needed for re R maintaining living structures, as well as to the kind of molecules the bl body needs to grow. For this, a series of chemical reactions in the body are necessary. Oxidising-reducing reactions are some of the most E common chemical means to break-down molecules. For this, many organisms use oxygen sourced from outside the body. The process of acquiring oxygen from outside the body, and to use it in the process be C of break-down of food sources for cellular needs, is what we call respiration. In the case of a single-celled organism, no specific organs for taking o N in food, exchange of gases or removal of wastes may be needed because the entire surface of the organism is in contact with the environment. But what happens when the body size of the organism increases and the body design becomes more complex? In multi-cellular organisms, © all the cells may not be in direct contact with the surrounding environment. Thus, simple diffusion will not meet the requirements of all the cells. We have seen previously how, in multi-cellular organisms, various body parts have specialised in the functions they perform. We are familiar with the idea of these specialised tissues, and with their organisation in the body of the organism. It is therefore not surprising that the uptake of food and of oxygen will also be the function of specialised tissues. However, this poses a problem, since the food and oxygen are now taken up at one place in the body of the organisms, while all parts of the body tt need them. This situation creates a need for a transportation system for carrying food and oxygen from one place to another in the body. When chemical reactions use the carbon source and the oxygen for no energy generation, they create by-products that are not only useless for the cells of the body, but could even be harmful. These waste by- products are therefore needed to be removed from the body and discarded outside by a process called excretion. Again, if the basic rules for body 94 Science design in multi-cellular organisms are followed, a specialised tissue for excretion will be developed, which means that the transportation system will need to transport waste away from cells to this excretory tissue. Let us consider these various processes, so essential to maintain life, one by one. Q U E S T 1I O N S ? 1. Why is diffusion insufficient to meet the oxygen requirements of multi- ed cellular organisms like humans? 2. What criteria do we use to decide whether something is alive? 3. What are outside raw materials used for by an organism? 4. What processes would you consider essential for maintaining life? h pu T 6.2 NUTRITION is When we walk or ride a bicycle, we are using up energy. Even when we re R are not doing any apparent activity, energy is needed to maintain a state of order in our body. We also need materials from outside in order bl to grow, develop, synthesise protein and other substances needed in E the body. This source of energy and materials is the food we eat. How do living things get their food? be C The general requirement for energy and materials is common in all organisms, but it is fulfilled in different ways. Some organisms use simple food material obtained from inorganic sources in the form of carbon o N dioxide and water. These organisms, the autotrophs, include green plants and some bacteria. Other organisms utilise complex substances. These complex substances have to be broken down into simpler ones before they can be used for the upkeep and growth of the body. To © achieve this, organisms use bio-catalysts called enzymes. Thus, the heterotrophs survival depends directly or indirectly on autotrophs. Heterotrophic organisms include animals and fungi. 6.2.1 Autotrophic Nutrition Carbon and energy requirements of the autotrophic organism are fulfilled by photosynthesis. It is the process by which autotrophs take in substances from the outside and convert them into stored forms of energy. This material is taken in the form of carbon dioxide and water tt which is converted into carbohydrates in the presence of sunlight and chlorophyll. Carbohydrates are utilised for providing energy to the plant. We will study how this takes place in the next section. The carbohydrates which are not used immediately are stored in the form of starch, which no serves as the internal energy reserve to be used as and when required by the plant. A somewhat similar situation is seen in us where some of the energy derived from the food we eat is stored in our body in the form of glycogen. Life Processes 95  Let us now see what actually happens during the process of photosynthesis. The following events occur during this process – (i) Absorption of light energy by chlorophyll. (ii) Conversion of light energy to chemical energy and splitting of water molecules ed into hydrogen and oxygen. (iii) Reduction of carbon dioxide to carbohydrates. These steps need not take place one after the other immediately. For example, desert h plants take up carbon dioxide at night and pu T prepare an intermediate which is acted upon is by the energy absorbed by the chlorophyll during the day. re R Let us see how each of the components of the above reaction are necessary for bl photosynthesis. E If you carefully observe a cross-section of a leaf under the microscope (shown in Fig. 6.1), you will notice that some cells contain green be C dots. These green dots are cell organelles called chloroplasts which contain chlorophyll. Let us Figure 6.1 do an activity which demonstrates that Cross-section of a leaf chlorophyll is essential for photosynthesis. o N Activity 6.1 © Take a potted plant with variegated leaves – for example, money plant or crotons. Keep the plant in a dark room for three days so that all the starch gets used up. Now keep the plant in sunlight for about six hours. Pluck a leaf from the plant. Mark the green areas in it and trace them on a sheet of paper. Dip the leaf in boiling water for a few minutes. After this, immerse it in a beaker containing alcohol. Carefully place the above beaker in a water-bath and heat till the alcohol begins to boil. tt What happens to the colour of the leaf? What is the colour of the solution? Now dip the leaf in a dilute solution of iodine for a few minutes. Take out the leaf and rinse off the iodine solution. no Observe the colour of the leaf and compare this with the tracing of Figure 6.2 the leaf done in the beginning (Fig. 6.2). Variegated leaf (a) before What can you conclude about the presence of starch in various areas and (b) after starch test of the leaf? 96 Science  Now, let us study how the plant obtains carbon dioxide. In Class IX, we had talked about stomata (Fig. 6.3) which are tiny pores present on the surface of the leaves. Massive amounts of gaseous exchange takes place in the leaves through these pores for the purpose of photosynthesis. But it is important to note here that exchange of gases occurs across the surface of ed stems, roots and leaves as well. Since large amounts of water can also be lost through these stomata, the plant Figure 6.3 (a) Open and (b) closed stomatal pore closes these pores when it does not need carbon dioxide for photosynthesis. The opening and closing of the h pore is a function of the guard cells. The guard cells swell when water pu T flows into them, causing the stomatal pore to open. Similarly the pore is closes if the guard cells shrink. re R Activity 6.2 bl Take two healthy potted plants E which are nearly the same size. Keep them in a dark room for three days. be C Now place each plant on separate glass plates. Place a watch-glass containing potassium hydroxide by the side of one of o N the plants. The potassium hydroxide is used to absorb carbon dioxide. Cover both plants with separate (a) (b) © bell-jars as shown in Fig. 6.4. Use vaseline to seal the bottom Figure 6.4 Experimental set-up (a) with potassium of the jars to the glass plates so hydroxide (b) without potassium hydroxide that the set-up is air-tight. Keep the plants in sunlight for about two hours. Pluck a leaf from each plant and check for the presence of starch as in the above activity. Do both the leaves show the presence of the same amount of starch? What can you conclude from this activity? Based on the two activities performed above, can we design an tt experiment to demonstrate that sunlight is essential for photosynthesis? So far, we have talked about how autotrophs meet their energy requirements. But they also need other raw materials for building their no body. Water used in photosynthesis is taken up from the soil by the roots in terrestrial plants. Other materials like nitrogen, phosphorus, iron and magnesium are taken up from the soil. Nitrogen is an essential element used in the synthesis of proteins and other compounds. This is Life Processes 97  taken up in the form of inorganic nitrates or nitrites. Or it is taken up as organic compounds which have been prepared by bacteria from atmospheric nitrogen. 6.2.2 Heterotrophic Nutrition Each organism is adapted to its environment. The form of nutrition differs depending on the type and availability of food material as well as how it is obtained by the organism. For example, whether the food ed source is stationary (such as grass) or mobile (such as a deer), would allow for differences in how the food is accessed and what is the nutritive apparatus used by a cow and a lion. There is a range of strategies by which the food is taken in and used by the organism. Some organisms h break-down the food material outside the body and then absorb it. pu T Examples are fungi like bread moulds, yeast and mushrooms. Others is take in whole material and break it down inside their bodies. What can be taken in and broken down depends on the body design and re R functioning. Some other organisms derive nutrition from plants or bl animals without killing them. This parasitic nutritive strategy is used by a wide variety of organisms like cuscuta (amar-bel), ticks, lice, E leeches and tape-worms. 6.2.3 How do Organisms obtain their Nutrition? be C Since the food and the way it is obtained differ, the digestive system is different in various organisms. In single-celled organisms, the food o N may be taken in by the entire surface. But as the complexity of the organism increases, different parts become specialised to perform different functions. For example, Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over © the food particle forming a food-vacuole (Fig. 6.5). Inside the food- vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out. In Paramoecium, which is also a unicellular organism, the cell has a definite shape and food is taken in at a specific spot. Food is moved to this spot by the movement of cilia which cover the entire surface of the cell. tt Figure 6.5 Nutrition in Amoeba 6.2.4 Nutrition in Human Beings The alimentary canal is basically a long tube extending from the mouth no to the anus. In Fig. 6.6, we can see that the tube has different parts. Various regions are specialised to perform different functions. What happens to the food once it enters our body? We shall discuss this process here. 98 Science  Activity 6.3 Take 1 mL starch solution (1%) in two test tubes (A and B). Add 1 mL saliva to test tube A and leave both test tubes undisturbed for 20-30 minutes. Now add a few drops of dilute iodine solution to the test tubes. In which test tube do you observe a colour change? What does this indicate about the presence or absence of starch in the two test tubes? What does this tell us about the action of saliva on starch? ed We eat various types of food which has to pass through the same digestive tract. Naturally the food has to be processed to generate particles which are small and of the same texture. This is achieved by h crushing the food with our teeth. Since the lining of the canal is soft, the food is also wetted to make its passage smooth. When we eat something pu T we like, our mouth ‘waters’. This is actually not only water, but a fluid is called saliva secreted by the salivary glands. Another aspect of the food we ingest is its complex nature. If it is to be absorbed from the alimentary re R canal, it has to be broken into smaller molecules. This is done with the bl help of biological catalysts called enzymes. The saliva contains an E enzyme called salivary amylase that breaks down starch which is a complex molecule to give sugar. The be C food is mixed thoroughly with saliva and moved around the mouth while chewing by the muscular tongue. o N It is necessary to move the food in a regulated manner along the digestive tube so that it can be processed properly in each part. The lining of © canal has muscles that contract rhythmically in order to push the food forward. These peristaltic movements occur all along the gut. From the mouth, the food is taken to the stomach through the food-pipe or oesophagus. The stomach is a large organ which expands when food enters it. The muscular walls of the stomach help in mixing the food Figure 6.6 Human alimentary canal tt thoroughly with more digestive juices. These digestion functions are taken care of by the gastric glands present in the wall of the stomach. These release hydrochloric acid, a protein digesting enzyme called no pepsin, and mucus. The hydrochloric acid creates an acidic medium which facilitates the action of the enzyme pepsin. What other function do you think is served by the acid? The mucus protects the inner lining of the stomach from the action of the acid under normal conditions. We Life Processes 99  have often heard adults complaining about ‘acidity’. Can this be related to what has been discussed above? The exit of food from the stomach is regulated by a sphincter muscle which releases it in small amounts into the small intestine. From the stomach, the food now enters the small intestine. This is the longest part of the alimentary canal which is fitted into a compact space because of extensive coiling. The length of the small intestine differs in various animals depending on the food they eat. Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Meat is easier ed to digest, hence carnivores like tigers have a shorter small intestine. The small intestine is the site of the complete digestion of carbohydrates, proteins and fats. It receives the secretions of the liver and pancreas for this purpose. The food coming from the stomach is h acidic and has to be made alkaline for the pancreatic enzymes to act. pu T Bile juice from the liver accomplishes this in addition to acting on fats. is Fats are present in the intestine in the form of large globules which makes it difficult for enzymes to act on them. Bile salts break them down into re R smaller globules increasing the efficiency of enzyme action. This is similar bl to the emulsifying action of soaps on dirt that we have learnt about in Chapter 4. The pancreas secretes pancreatic juice which contains E enzymes like trypsin for digesting proteins and lipase for breaking down emulsified fats. The walls of the small intestine contain glands which secrete intestinal juice. The enzymes present in it finally convert the be C proteins to amino acids, complex carbohydrates into glucose and fats into fatty acids and glycerol. The digested food is taken up by the walls of the intestine. The inner o N lining of the small intestine has numerous finger-like projections called villi which increase the surface area for absorption. The villi are richly supplied with blood vessels which take the absorbed food to each and every cell of the body, where it is utilised for obtaining energy, building © up new tissues and the repair of old tissues. The unabsorbed food is sent into the large intestine where more villi absorb water from this material. The rest of the material is removed from the body via the anus. The exit of this waste material is regulated by the anal sphincter. More to Know! Dental caries tt Dental caries or tooth decay causes gradual softening of enamel and dentine. It begins when bacteria acting on sugars produce acids that softens or demineralises the enamel. Masses of bacterial cells together with food particles stick to the teeth to form dental no plaque. Saliva cannot reach the tooth surface to neutralise the acid as plaque covers the teeth. Brushing the teeth after eating removes the plaque before the bacteria produce acids. If untreated, microorganisms may invade the pulp, causing inflammation and infection. 100 Science