Life Processes 03: The Variety of Living Organisms PDF

LIFE PROCESSES 03 THE VARIETY OF LIVING ORGANISMS 25 Uncorrected proof, all content subject to change at publisher discretion. Not for...

LIFE PROCESSES 03 THE VARIETY OF LIVING ORGANISMS 25 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 UNIT 1 ORGANISMS AND LIFE PROCESSES All living organisms are composed of microscopic units known as cells. These building blocks of life have a number of features in common, which allow them to grow, reproduce, and generate more organisms. In Chapter 1 we start by looking at the structure and function of cells, and the essential life processes that go on within them. Despite the fact that cells are similar in structure, there are many millions of different species of organisms. Chapter 2 looks at the diversity of living things and how we can classify them into groups on the basis of the features that they show. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 2 24/04/2017 18:09  ORGANISMS AND LIFE PROCESSES LIFE PROCESSES 3 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 1 LIFE PROCESSES There are structural features that are common to the cells of all living organisms. In this chapter you will find out about these features and look at some of the processes that keep cells alive. LEARNING OBJECTIVES ◼ Understand the characteristics shared by living ◼ Know that ATP provides energy for cells organisms ◼ Know the word equation and balanced chemical ◼ Describe cell structures and their functions, including symbol equation for aerobic respiration the nucleus, cytoplasm, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes and vacuole ◼ Know the word equations for anaerobic respiration ◼ Know the similarities and differences in the structures ◼ Investigate the evolution of carbon dioxide and heat of plant and animal cells from respiring seeds or other suitable living organisms ◼ Understand the role of enzymes as biological catalysts ◼ Understand the processes of diffusion, osmosis and in metabolic reactions active transport by which substances move into and ◼ Understand how temperature changes can affect out of cells enzyme function, including changes to the shape of the ◼ Understand how factors affect the rate of movement of active site substances into and out of cells ◼ Understand how enzyme function can be affected by changes in pH altering the active site ◼ Investigate diffusion in a non-living system (agar jelly) ◼ Investigate how enzyme activity can be affected by BIOLOGY ONLY changes in temperature ◼ Explain the importance of cell differentiation in the BIOLOGY ONLY development of specialised cells ◼ Investigate how enzyme activity can be affected by ◼ Describe the levels of organisation within organisms – changes in pH organelles, cells, tissues, organ systems ◼ Describe the differences between aerobic and BIOLOGY ONLY anaerobic respiration ◼ Understand the advantages and disadvantages of ◼ Understand how the process of respiration produces using stem cells in medicine. ATP in living organisms All living organisms are composed of units called cells. The simplest organisms are made from single cells (Figure 1.1) but more complex plants and animals are composed of millions of cells. In many-celled (multicellular) organisms, there may be hundreds of different types of cells with different structures. They are specialised so that they can carry out particular functions in the animal or plant. Despite all the differences, there are basic features that are the same in all cells. ▲ Figure 1.1 Many simple organisms have ‘bodies’ made from single cells. Here are four examples. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 3 24/04/2017 18:09  4 ORGANISMS AND LIFE PROCESSES LIFE PROCESSES Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 There are eight life processes which take place in most living things. Organisms: ◾◾ require nutrition – plants make their own food, animals eat other organisms ◾◾ respire – release energy from their food ◾◾ excrete – get rid of waste products ◾◾ respond to stimuli – are sensitive to changes in their surroundings ◾◾ move – by the action of muscles in animals, and slow growth movements in plants ◾◾ control their internal conditions – maintain a steady state inside the body ◾◾ reproduce – produce offspring ◾◾ grow and develop – increase in size and complexity, using materials from their food. CELL STRUCTURE This part of the book describes the cell structure of ‘higher’ organisms such as animals, plants and fungi. The cells of bacteria are simpler in structure and will be described in Chapter 2. Most cells contain certain parts such as the nucleus, cytoplasm and cell membrane. Some cells have structures missing, for instance red blood cells are unusual in that they have no nucleus. The first chapter in a biology textbook will usually present diagrams of ‘typical’ plant and animal cells. In fact, there is really no such thing as a ‘typical’ cell. Humans, for example, are composed of hundreds of different kinds of cells – from nerve cells to blood cells, skin cells to liver cells. What we really mean by a ‘typical’ cell is a general diagram that shows all the features that you will find in most cells (Figure 1.2). However, not all these are present in all cells – for example the cells in the parts of a plant that are not green do not contain chloroplasts. animal cell plant cell cell wall nucleus cell membrane (inside cell wall) vacuole cell membrane cytoplasm mitochondria mitochondria cytoplasm chloroplasts 10 μm 1 nucleus (1 μm = 1000 mm) ▲ Figure 1.2 The structure of a ‘typical’ animal and plant cell. The living material that makes up a cell is called cytoplasm. It has a texture rather like sloppy jelly, in other words somewhere between a solid and a liquid. Unlike a jelly, it is not made of one substance but is a complex material made of many different structures. You can’t see many of these structures under an ordinary light microscope. An electron microscope has a much higher magnification and can show the details of these structures, which are called organelles (Figure 1.3). M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 4 24/04/2017 18:09  ORGANISMS AND LIFE PROCESSES LIFE PROCESSES 5 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 mitochondria The largest organelle in the cell is the nucleus. Nearly all cells have a nucleus. The few types that don’t are usually dead (e.g. the xylem vessels in a stem, Chapter 11) or don’t live for very long (e.g. red blood cells, Chapter 5). The nucleus controls the activities of the cell. It contains chromosomes (46 in human cells) which carry the genetic material, or genes. Genes control the activities in the cell by determining which proteins the cell can make. The DNA remains in the nucleus, but the instructions for making proteins are carried out of the nucleus to the cytoplasm, where the proteins are assembled on tiny structures called ribosomes. A cell contains thousands of ribosomes, but they are too small to be seen through a light microscope. One very important group of proteins found in cells are enzymes. Enzymes control the chemical reactions that take place in the cytoplasm. All cells are surrounded by a cell membrane, sometimes called the cell cytoplasm nucleus surface membrane to distinguish it from other membranes inside the cell. ▲ Figure 1.3 The organelles in a cell can be This is a thin layer like a ‘skin’ on the surface of the cell. It forms a boundary seen using an electron micropscope. between the cytoplasm of the cell and the outside. However, it is not a complete barrier. Some chemicals can pass into the cell and others can pass out. We say that the membrane is partially permeable. The membrane can go further than this and actually control the movement of some substances – it is selectively permeable. One organelle that is found in the cytoplasm of all living cells is the mitochondrion (plural mitochondria). In cells that need a lot of energy such as muscle or nerve cells, there are many mitochondria. This gives us a clue to their function. They carry out some of the reactions of respiration (see page 12) releasing energy that the cell can use. Most of the energy from respiration is released in the mitochondria. PLANT CELLS All of the structures you have seen so far are found in both animal and plant cells. However, some structures are only ever found in plant cells. There are three in particular – the cell wall, a permanent vacuole and chloroplasts. The cell wall is a layer of non-living material that is found outside the cell membrane of plant cells. It is made mainly of a carbohydrate called cellulose, although other chemicals may be added to the wall in some cells. Cellulose is a tough material that helps the cell keep its shape and is one reason why the ‘body’ of a plant has a fixed shape. Animal cells do not have a cell wall and tend to be more variable in shape. Plant cells absorb water, producing an internal pressure that pushes against adjacent cells, giving the plant support (see Chapter 11). Without a cell wall strong enough to resist these pressures, this method of support would be impossible. The cell wall is porous, so it is not a barrier to water or dissolved substances. We call it freely permeable. Mature (fully grown) plant cells often have a large central space surrounded by a membrane, called a vacuole. This vacuole is a permanent feature of the KEY POINT cell. It is filled with a watery liquid called cell sap, which is a store of dissolved Nearly all cells contain cytoplasm, sugars, mineral ions and other solutes. Animal cells do contain vacuoles, but a nucleus, a cell membrane and they are only small, temporary structures. mitochondria. As well as these Cells of the green parts of plants, especially the leaves, contain another very structures, plant cells have a cell wall and a permanent vacuole, and plant important organelle, the chloroplast. Chloroplasts absorb light energy to cells that photosynthesise contain make food in the process of photosynthesis (see Chapter 10). They contain a chloroplasts. green pigment called chlorophyll. Cells from the parts of a plant that are not green, such as the flowers, roots and woody stems, have no chloroplasts. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 5 24/04/2017 18:09  6 ORGANISMS AND LIFE PROCESSES LIFE PROCESSES Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 Figure 1.4 shows some animal and plant cells seen through the light microscope. (a) 10µm (b) 25µm ▲ Figure 1.4 (a) Cells from the lining of a human cheek. (b) Cells from the photosynthetic tissue of a leaf. ENZYMES: CONTROLLING REACTIONS IN THE CELL KEY POINT The chemical reactions that take place in a cell are controlled by a group of The chemical reactions taking place proteins called enzymes. Enzymes are biological catalysts. A catalyst is a in a cell are known as metabolic chemical which speeds up a reaction without being used up itself. It takes reactions. The sum of all the metabolic part in the reaction, but afterwards is unchanged and free to catalyse more reactions is known as the metabolism reactions. Cells contain hundreds of different enzymes, each catalysing of the cell. The function of enzymes is a different reaction. This is how the activities of a cell are controlled – the to catalyse metabolic reactions. nucleus contains the genes, which control the production of enzymes, which then catalyse reactions in the cytoplasm: KEY POINT You have probably heard of enzymes genes → proteins (enzymes) → catalyse reactions being involved in digestion of food. In the intestine enzymes are secreted Everything a cell does depends on which enzymes it can make, which in turn onto the food to break it down. These depends on which genes in its nucleus are working. are called extracellular enzymes, which What hasn’t been mentioned is why enzymes are needed at all. They are means they function ‘outside cells’. necessary because the temperatures inside organisms are low (e.g. the human However, most enzymes stay inside body temperature is about 37 °C) and without catalysts, most of the reactions cells and carry out their function there; they are intracellular. You will find out that happen in cells would be far too slow to allow life to go on. The reactions about digestive enzymes in Chapter 4. can only take place quickly enough when enzymes are present to speed them up. KEY POINT It is possible for there to be thousands of different sorts of enzymes because Secretion is the release of a fluid or they are proteins, and protein molecules have an enormous range of structures substances from a cell or tissue. and shapes (see Chapter 4). M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 6 24/04/2017 18:09  ORGANISMS AND LIFE PROCESSES LIFE PROCESSES 7 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 The molecule that an enzyme acts on is called its substrate. Each enzyme has a small area on its surface called the active site. The substrate attaches to the active site of the enzyme. The reaction then takes place and products are formed. When the substrate joins up with the active site it lowers the energy needed for the reaction to start, allowing the products to be formed more easily. Enzymes also catalyse reactions where large molecules are built up from smaller ones. In this case, several substrate molecules attach to the active site, the reaction takes place and the larger product molecule is formed. The product then leaves the active site. The substrate fits into the active site of the enzyme like a key fitting into a lock. Just as a key will only fit one lock, a substrate will only fit into the active site of a particular enzyme. This is known as the lock and key model of enzyme action. It is the reason why enzymes are specific, i.e. an enzyme will only catalyse one reaction. 1 2 substrate enters enzyme enzyme's active site 4 3 reaction takes place products formed, which leave active site ▲ Figure 1.5 Enzymes catalyse reactions at their active site. This acts like a ‘lock’ to the substrate ‘key’. The substrate fits into the active site, and products are formed. This happens more easily than without the enzyme – so enzymes act as catalysts. After an enzyme molecule has catalysed a reaction, the product is released from the active site, and the enzyme is free to act on more substrate molecules. FACTORS AFFECTING ENZYMES A number of factors affect the activity of enzymes. The rate of reaction may KEY POINT be increased by raising the concentration of the enzyme or the substrate. Two other factors that affect enzymes are temperature and pH. ‘Optimum’ temperature means the ‘best’ temperature, in other words the temperature at which the reaction TEMPERATURE takes place most rapidly. The effect of temperature on the action of an enzyme is easiest to see as a graph, where we plot the rate of the reaction against temperature (Figure 1.6). DID YOU KNOW? Enzymes in the human body have evolved to work best at body temperature Kinetic energy is the energy (37 °C). The graph in Figure 1.6 shows a peak on the curve at this temperature, an object has because of its which is called the optimum temperature for the enzyme. movement. The molecules As the enzyme is heated up to the optimum temperature, the rise in of enzyme and substrate are temperature increases the rate of reaction. This is because higher temperatures moving faster, so they have give the molecules of the enzyme and the substrate more kinetic energy, so more kinetic energy. they collide more often. More collisions means that the reaction will take place more frequently. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 7 24/04/2017 18:09  8 ORGANISMS AND LIFE PROCESSES LIFE PROCESSES Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 rate of enzyme-catalysed optimum reaction / arbitrary unit temperature 0 10 20 30 40 50 60 70 temperature / ºC ▲ Figure 1.6 Effect of temperature on the action of an enzyme. However, above the optimum, temperature starts to have another effect. Enzymes are made of protein, and proteins are broken down by heat. From 40 °C upwards, the heat destroys the enzyme. We say that it is denatured. You can see the effect of denaturing when you boil an egg. The egg white is made of protein, and turns from a clear runny liquid into a white solid as the heat denatures the protein. Denaturing changes the shape of the active site so that the substrate will no longer fit into it. Denaturing is permanent – the enzyme molecules will no longer catalyse the reaction. Not all enzymes have an optimum temperature near 37 °C, only those of animals such as mammals and birds, which all have body temperatures close to this value. Enzymes have evolved to work best at the normal body temperature of the organism. Bacteria that always live at an average temperature of 10 °C will probably have enzymes with an optimum temperature near 10 °C. pH The pH around the enzyme is also important. The pH inside cells is neutral (pH 7) and most enzymes have evolved to work best at this pH. At extremes of pH either side of neutral, the enzyme activity decreases, as shown in Figure 1.7. The pH at which the enzyme works best is called its optimum pH. Either side of the optimum, the pH affects the structure of the enzyme molecule and changes the shape of its active site, so that the substrate will not fit into it so well. optimum pH rate of enzyme-catalysed reaction / arbitrary units KEY POINT Although most enzymes work best at a neutral pH, a few have an optimum below or above pH 7. The stomach produces hydrochloric acid, which makes its contents very acidic (see Chapter 4). Most enzymes stop working at a low pH, but the stomach makes an enzyme called pepsin which 5 6 7 8 9 has an optimum pH of about 2, so pH that it is adapted to work well in these unusually acidic surroundings. ▲ Figure 1.7 Most enzymes work best at a neutral pH. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 8 24/04/2017 18:09  ORGANISMS AND LIFE PROCESSES LIFE PROCESSES 9 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 ACTIVITY 1 ! ▼ PRACTICAL: AN INVESTIGATION INTO THE EFFECT OF Safety Note: Wear eye protection and TEMPERATURE ON THE ACTIVITY OF AMYLASE avoid skin contact with the liquids. Amylase is hazardous to the eyes. The digestive enzyme amylase breaks down starch into the sugar maltose. If the speed at which the starch disappears is recorded, this is a measure of the activity of the amylase. Figure 1.8 shows apparatus which can be used to record how quickly the starch is used up. transfer sample every 30 seconds spots of iodine solution water spotting tile amylase starch starch and amylase solution suspension mixture ▲ Figure 1.8 Investigating the breakdown of starch by amylase at different temperatures. Spots of iodine solution are placed into the dips on the spotting tile. Using a syringe, 5 cm3 of starch suspension is placed in one boiling tube, and 5 cm3 of amylase solution in another tube, using a different syringe. The beaker is filled with water at 20 °C. Both boiling tubes are placed in the beaker of water for 5 minutes, and the temperature recorded. The amylase solution is then poured into the starch suspension, leaving the tube containing the mixture in the water bath. Immediately, a small sample of the mixture is removed from the tube with a pipette and added to the first drop of iodine solution on the spotting tile. The colour of the iodine solution is recorded. A sample of the mixture is taken every 30 seconds for 10 minutes and tested for starch as above, until the iodine solution remains yellow, showing that all the starch is used up. The experiment is repeated, maintaining the water bath at different temperatures between 20 °C and 60 °C. A set of results is shown in the table below. M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 9 24/04/2017 18:09  10 ORGANISMS AND LIFE PROCESSES LIFE PROCESSES Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 Colour of mixture at different temperatures / (°C) Time / min 20 30 40 50 60 0.0 Blue-black Blue-black Blue-black Blue-black Blue-black 0.5 Blue-black Blue-black Brown Blue-black Blue-black 1.0 Blue-black Blue-black Yellow Blue-black Blue-black 1.5 Blue-black Blue-black Yellow Blue-black Blue-black 2.0 Blue-black Blue-black Yellow Brown Blue-black 2.5 Blue-black Blue-black Yellow Brown Blue-black 3.0 Blue-black Blue-black Yellow Brown Blue-black 3.5 Blue-black Blue-black Yellow Yellow Blue-black 4.0 Blue-black Blue-black Yellow Yellow Blue-black 5.5 Blue-black Blue-black Yellow Yellow Blue-black 6.0 Blue-black Brown Yellow Yellow Blue-black 6.5 Blue-black Brown Yellow Yellow Blue-black 7.0 Blue-black Yellow Yellow Yellow Blue-black 7.5 Blue-black Yellow Yellow Yellow Brown 8.0 Blue-black Yellow Yellow Yellow Brown 8.5 Brown Yellow Yellow Yellow Yellow 9.0 Brown Yellow Yellow Yellow Yellow 9.5 Yellow Yellow Yellow Yellow Yellow 10.0 Yellow Yellow Yellow Yellow Yellow The rate of reaction can be calculated from the time taken for the starch to be fully broken down, as shown by the colour change from blue-black to yellow. For example, at 50 °C the starch had all been digested after 3.5 minutes. The rate is found by dividing the volume of the starch (5 cm3) by the time: 5.0 cm3 Rate = _______ ​​   ​​= 1.4 cm3 per min 3.5 min Plotting a graph of rate against temperature should produce a curve similar to the one shown in Figure 1.6. Try this, using the results in the table. Better still, you may be able to do this experiment and provide your own results. If the curve doesn’t turn out quite like the one in Figure 1.6, can you suggest why this is? How could you improve the experiment to get more reliable results? M01_EDIB_SB_IGCSE_85084_CH01-02_002-037.indd 10 24/04/2017 18:09  ORGANISMS AND LIFE PROCESSES LIFE PROCESSES 11 Uncorrected proof, all content subject to change at publisher discretion. Not for resale, circulation or distribution in whole or in part. ©Pearson 2019 BIOLOGY ONLY ACTIVITY 2 ! ▼ PRACTICAL: AN INVESTIGATION INTO THE EFFECT OF pH Safety Note: Wear eye protection ON THE ACTIVITY OF CATALASE and avoid contact with the mixture. Catalase is hazardous to the eyes. Buffer solutions are solutions of salts that resist changes in pH. Different buffer solutions can be prepared for maintaining different values of pH. Buffer solutions are useful for finding the effect of pH on enzyme activity. Hydrogen peroxide (H2O2) is a product of metabolism. Hydrogen peroxide