SdYTiM2yuj.pdf - Biology Past Paper PDF

How did the scientific method disprove the idea of spontaneous generation? What about the belief that rotting meat produces ies?How could you disprove that by using the scienti cmethod? Well, in 1668 an Italian biologist, Francesco Redi, did just that. Many scientists consider this to be the rsttrue...

How did the scientific method disprove the idea of spontaneous generation? What about the belief that rotting meat produces ies?How could you disprove that by using the scienti cmethod? Well, in 1668 an Italian biologist, Francesco Redi, did just that. Many scientists consider this to be the rsttrue ‘experiment’. He used wide-mouth jars containing meat. Some jars were le open to the air. Others were covered with a piece of gauze. A erseveral days, maggots and then iescould be seen in the open jars, but none appeared in the closed jars.Redi hypothesised that only iescould produce more iesand predicted that, in his experiment, ieswould be found in the open jars, but not in the covered jars. He maintained all the jars under the same conditions and so he controlled many variables. By choosing to cover some jars with gauze rather than an impermeable seal, he allowed air to enter all the jars – again he controlled a variable that could have a ectedthe outcome of the experiment. His results matched his prediction and when other people tried the experiment, they too got the same results. Redi was able to conclude that iescannot be produced from rotting meat. He also went on to say that it was unlikely that any form of spontaneous generation was possible. Most people accepted this for larger organisms, but, at round about this time, the microscope had been invented and the whole world of microbiology was opened up. Many people still believed that micro-organisms could arise by spontaneous generation. It took the work of Louis Pasteur to disprove this. In 1859, Pasteur carried out experiments to show that the micro-organisms that caused wine and broth to go cloudy came from the air and were not made from the broth itself. He used special ‘swan-necked asks’like that shown in Figure 1.4. Pasteur boiled broths in swan-necked asksto kill any microorganisms that might be in them. eboiling forced steam and air out of the asks.When the boiling stopped and the broth cooled, air was sucked back into the asks.Some contained a lterto prevent all solid particles from getting into the growth medium from the air. Others had no lterbut, in these, the dust (and the microorganisms) in the air settled in the lowest part of the neck of the ask.All the askswere kept under the same conditions in Pasteur’s laboratory. Pasteur found that the broths stayed clear for months. At the end of this time, he treated the asksin one of three ways: He le some of them as they were. He broke the necks on some. He tilted others to allow the dust in the low part of the neck to mix with the broths.What do we mean by cause and effect? Scienti c experiments try to establish cause and e ect. ismeans that they try to prove that a change in one factor brings about a change in another factor. efactor that the scientist changes, or manipulates, is called the independent variable (or IV for short). efactor that the scientist measures to see if it changes when the IV is changed is called the dependent variable (or DV for short). escientist will want to ndout if changes in the independent variable produce changes in the dependent variable. In the example on pages 5 and 6, the independent variable was the presence or absence of tomato juice. edependent variable was the number of tomato seeds germinating. To prove cause and e ect– to prove that it is changes in the IV (and nothing else) that are causing changes in the DV – we musttake all the steps we can to ensure that the experiment is a fair test. We must make sure that any other factors which could a ectthe results are the same for the di erentconditions we set up. In the tomato seed example, if one group of seeds had been at a higher temperature than the other group, this could have made them germinate faster. We wouldn’t have known whether it was the tomato juice a ectingthe results or the temperature. Our experiment would not be valid. So we must keep constant anything other than the IV that might in uencethe results. eseare controlled variables. In the tomato seed experiment, the controlled variables were: temperature lighting conditions number of seeds per dish, and volume of liquid added (water or tomato juice). Occasionally, there is a variable that might in uencethe results that you can’t control. Such a variable is a confounding variable. isis because it ‘confounds’ the interpretation of the results. You couldn’t be certain that it was the IV producing the changes in the DV because of the presence of the confounding variable. For example, if you measure the carbon dioxide uptake by wheat plants as the light intensity changes over the day, you cannot control the e ectof change in temperature. It could be a confounding variable.Accuracy Accuracy refers to how precisely you measure or count something. For example, you could measure time with a clock, a wristwatch or a stop-clock accurate to 0.01 seconds. elevel of accuracy you choose must re ectthe magnitude of what you are measuring. You don’t always need the most accurate measuring instrument. For example, if you were timing a reaction that was likely to last a few minutes at most, the stop-clock would be the best choice. But if you were timing something that lasts several hours, you just don’t need that level of precision and it might even be a hindrance – by measuring the seconds accurately, you might lose track of the hours! To measure volume, you could use: a syringe a measuring cylinder a pipette a buretteWhat are carbohydrates and why do we need them? All carbohydrates contain the elements carbon, hydrogen and oxygen. ehydrogen and oxygen atoms in a carbohydrate molecule are present in the ratio of two hydrogen atoms to one oxygen atom (for example, glucose, C6 H12O6 , and maltose, C12H22O11). Carbohydrates range from very small molecules containing only 12 atoms, to very large molecules containing thousands of atoms. Carbohydrates have a range of functions: eyare used to release energy in respiration – glucose is the main respiratory substrate of most organisms. Carbohydrates are a convenient form in which to store chemical energy; storage carbohydrates include: – starch in plants – glycogen in animals Some carbohydrates are used to build structures; structural carbohydrates include: – cellulose, which is the main constituent of the primary cell wall of plants – chitin, which occurs in the cell walls of fungi and in the exoskeletons of insects – peptidoglycan, which occurs in bacterial cell wallsWhat different types of carbohydrates are there? Monosaccharides are the simplest carbohydrates. A monosaccharide molecule can be thought of as a single sugar unit. Other, more complex, carbohydrates have two or more monosaccharide units joined together. Monosaccharides can be classi edaccording to how many carbon atoms are present in the molecule. A triose monosaccharide has three carbon atoms – formula C3 H6 O3. Glycerate phosphate is a triose important in photosynthesis A pentose monosaccharide has vecarbon atoms – formula C5 H10O5. Ribose is found in RNA nucleotides. A hexose monosaccharide has six carbon atoms – formula C6 H12O6. Glucose is the hexose produced in photosynthesis and used in respiration. ereare several di erent trioses, pentoses and hexoses. Each triose has the same number of each kind of atom (hence the formula C3 H6 O3 ), but the atoms are put together in a di erentway. eyare isomers of each other. esame is true for the pentoses and hexoses. Monosaccharides can be classi edin a di erentway – according to the functional group that they possess. ereare two functional groups in monosaccharides: the aldehyde group with the formula CHO (monosaccharides with this group are aldoses), and the ketone group, with the formula C=O (monosaccharides with this group are ketoses). emain signi canceof this di erenceis the ability to polymerise. Nearly all the polysaccharides found in living things are polymers of aldose monosaccharides. Figure 2.16 shows examples of each type of sugar according to both classi cations. estraight chain form of glucose can produce two di erentring forms – α-glucose and β-glucose. ereis only one di erencebetween these two. Can you see it? estraight chain form of fructose produces only one ring form. In these structural diagrams, the carbon atoms in the molecules are numbered according to their positions in the molecule. You do not have to know the position of every carbon, hydrogen and oxygen atom in these molecules. However, if you can remember the simpli edstructures shown in gure2.19 this will help you to understand how more complex carbohydrates are formed. esesimpli eddiagrams show the overall shape of the molecule, the position of each carbon atom and the hydrogen and oxygen atoms attached to carbon atoms 1 and 4. Disaccharide carbohydrate molecules are made by two monosaccharide molecules joining together. For example, a molecule of: maltose is derived from two α-glucose molecules sucrose is derived from an α-glucose molecule and a fructose molecule lactose (milk sugar) is derived from a β-glucose molecule and an -galactose molecule. In each of these examples, two hexose monosaccharides have reacted to form a disaccharide molecule. As the formula of a hexose is C6 H12O6 , you might expect the formula of the disaccharides to be C12H24O12. In fact, the formula is C12H22O11. A molecule of water (H2 O) is formed from a hydroxyl group from one monosaccharide and a hydrogen atom from the other ( gure2.21). isallows a bond to be formed between the two monosaccharide units to make a disaccharide. eprocess shown in Figure 2.21 is condensation. ebond that holds the two monosaccharide units together is a glycosidic bond. It is formed between carbon atom 1 of one α-glucose molecule and carbon atom 4 of the other α-glucose molecule. efull name of the bond is, therefore, a α-1,4-glycosidic bond. Amylopectin also has a linear ‘backbone’ of α-glucose molecules joined by α-1,4-glycosidic bonds. But in amylopectin, there are also side branches. eseoccur at certain points along the chain when a glucose molecule forms an α-1,6-glycosidic bond with another glucose molecule as well as the usual α-1,4-glycosidic bond. isresults in amylopectin having a branching structure as shown in gure2.25. ebranched nature of amylopectin means that there are many ‘ends’ to the molecule. is allows it to be quickly hydrolysed by enzymes acting at the ends of the chains to release glucose for respiration.