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Revision B3 - Evolution How did life start? • the Earth is about 4,500 million years old • there is evidence living things existed on Earth at least 3,500 million years ago • no-one was there to record how life began. • Over many millions of years these molecules joined with other molecules, becoming gradually more complex and dependent on each other. The process of evolution by natural selection eventually led to all of the different living things that we see on Earth today. Darwin’s finches • He noticed that the finches (songbirds) on the different islands were fundamentally similar to each other, • but showed wide variations in size, beaks and claws from island to island. • For example, their beaks were different depending on the local food source. • Darwin concluded that, because the islands are so distant from the mainland, the finches which had arrived there in the past had changed over time. Natural selection • Individuals in a species show a wide range of variation. • This variation is because of differences in their genes • Individuals with characteristics most suited to the environment are more likely to survive and reproduce. • The genes that allow these individuals to be successful are passed to their offspring. Lamarck His theory involved two ideas: • the law of use and disuse, and • the law of inheritance of acquired characteristics. Evidence for evolution: fossils • All the main stages of its evolution have been preserved in fossil form. • Over 60 million years, the horse evolved from a dogsized creature that lived in rainforests into an animal adapted to living on the plains, and standing up to 2 metres high. Evidence for evolution: rapid changes • Evolution is difficult to observe because it usually occurs over many years. This is one reason why the theory of evolution is still a theory, not a law • Before the Industrial Revolution in Britain, most peppered moths were of the pale variety • Moths with a mutant black colouring were spotted easily by birds and eaten • Airborne pollution in industrial areas blackened the birch tree bark with soot Antibiotic-resistant bacteria Microorganisms such as bacteria and viruses reproduce rapidly and can evolve in a relatively short time Extinction • changes to the environment, such as the climate • new diseases • new predators • new competitors The dodo was a large flightless bird that lived in the Mauritius, a group of islands in the Indian Ocean. These were uninhabited, and the dodo had no natural predators. Then Mauritius were colonised by the Dutch in 1638. Dodos were hunted for food and easy to catch, because they were not afraid of people. New competitors were brought to the islands, including pigs, cats and rats, which ate the dodos' eggs and young. Within 80 years, the dodo was extinct.. Evolution of humans • Humans did not evolve from apes such as gorillas and chimps. Instead, humans and apes share a common ancestor that lived millions of years ago. This diverged over time to form many species of hominid. All, including early humans, became extinct - except modern humans • The ability to stand upright, so predators and prey could be seen more easily. • Having a larger brain, providing the ability to plan ahead, work together, and eventually speak. Hormones • chemicals • secreted by glands • transported by the bloodstream to target organs. The nervous system • The nervous system uses electrical impulses to bring about fast, but shortlived, responses. It consists of the: • brain and spinal cord (which make up the central nervous system, or CNS) • neurones. • Nerve cells are also called neurones • Receptors • Effectors Reflex actions • When a receptor is stimulated it sends a signal to the central nervous system, and the brain co-ordinates a response. But sometimes a very quick response is needed, one that does not require the involvement of the brain. This is a reflex action. • Receptor detects a stimulus (change in the environment). • Sensory neurone sends a signal to a relay neurone. • Motor neurone sends a signal to an effector. • Effector produces a response. Control of internal conditions • Water content of the body This is controlled to protect cells by preventing too much water from entering or leaving them. Water content is controlled by water loss from • the lungs when we exhale • the skin by sweating, and • the body, in urine produced by the kidneys. Control of internal conditions • Blood sugar level This is controlled to provide cells with a constant supply of energy. The blood sugar level is determined by the release and storage of glucose, which is controlled by insulin Ion (salts) content of the body This is controlled to protect cells by stopping too much water from entering or leaving them. Ion content is controlled by the loss of ions from • the skin by sweating, and • the body in urine produced by the kidneys. Temperature of the body This is controlled to maintain the temperature at which enzymes work best. Body temperature is controlled by • controlling blood flow to the skin • sweating • shivering. C3 – Food, Plants and Health • Plants make their own food through photosynthesis • Carbohydrates Sugars are carbohydrates. They are small molecules containing carbon, hydrogen and oxygen • Proteins Amino acids are small molecules containing carbon, hydrogen and oxygen • To make the compounds they require, plants need these elements: • carbon • hydrogen • oxygen • nitrogen. Plant growth Recycling elements • As plants grow, they remove elements such as nitrogen, potassium and phosphorus from the soil through their roots. Over time the soil loses these elements and becomes less fertile • The elements needed by plants are returned to the soil when living organisms die and decay. These elements are also passed on when animals eat other living things. • Plants can take up and use nitrogen when it is in the form of nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation. • Lightning • Nitrogen fixing bacteria • Nitrogen compounds are returned to the soil through: • excretion and egestion by animals • the death and decay of plants and animals • denitrifying bacteria in the soil. The nitrogen cycle Intensive farming 1. 2. 3. 4. • • • To grow crops successfully, farmers need to be able to: Replace elements in the soil that are removed by growing plants. Stop weeds from competing with their crops for space, water, minerals and light. Stop pests such as insects and fungi from damaging crop plants. Stop the spread of disease by insects and fungi. Intensive farming methods achieve these things by using: artificial fertilisers herbicides (chemicals that kill weeds) pesticides (chemicals that kill pests). • • • Removing hedgerows to make large fields destroys animals' habitats, and reduces species diversity. Some of the lost species would have eaten pests. So pest numbers grow, and more chemical pesticides are needed to control them. Growing the same crop year after year makes the soil less fertile, so more chemical fertilisers are needed Organic farming People may choose organic food because they: • believe it is a healthier • think it tastes better • feel it is kinder to livestock and the environment. Organic farmers avoid using: • artificial fertilisers (choosing natural ones such as manure and compost instead) • pesticides (they use hand-weeding and biological pest controls, such as ladybirds that eat aphids) • hormones and other additives in livestock feed Food additives • Processed foods, including vegetable oils, may have chemicals added to them. These additives have different roles, including extending a product’s shelf life and improving its taste and appearance Type of additive Example Typical use colouring Tartrazine (E102) orange colouring for soft drinks, sweets and sauces emulsifier Lecithin (E322) allows oil and water to mix to make margarine, ice cream and salad cream preservative Benzoic acid (E210) used in many foods to stop harmful microorganisms from growing sweetener Aspartame (E951) in low-calorie drinks and food Food safety 1 • Food may contain chemicals that could be toxic or harmful. These include: • natural chemicals • chemicals formed during food processing or cooking • herbicides and pesticides used by farmers. Moulds Moulds may grow on crops before harvesting, or while they are being stored. They may release harmful chemicals. For example, nuts and cereals can contain aflatoxin - released by a common fungus called Aspergillus. Aflatoxin is poisonous and can cause liver damage. Food safety 2 • Overcooking or burning food produces PAHs (polycyclic aromatic hydrocarbons), which are suspected of causing cancer. • Frying food to make crisps and chips produces acrylamide, a chemical known to cause cancer if given to laboratory rats in high doses. Herbicides and pesticides Farmers may use herbicides to kill weeds, and pesticides to kill insects and other pests. These chemicals remain on the food unless they are washed away by rain. So it is important to wash fruit and vegetables before use. You may wish to view this funny BBC News video from 2006 about the need for accurate food labelling. Digestion • Digestion • Our bodies cannot use most of our food directly. First, it must be broken down into small molecules that are soluble (they can dissolve). This is called digestion, and occurs in the digestive system. • Starch is broken down into glucose (a sugar). • Proteins are broken down into amino acids. Amino acids and proteins • Cell growth Cells grow by making new proteins from amino acids that are transported in the bloodstream. Amino acids join together end to end to make proteins such as haemoglobin Proteins are also found in: • Hair • Muscle • Skin • Tendons. Excretion Your body cannot store excess amino acids. The liver breaks them down to form urea. This is removed from the blood by the kidneys, and lost when you urinate Diabetes • Too little glucose causes confusion, unconsciousness, even coma. • Too much glucose causes exhaustion and blurred vision. • Processed foods often contain a lot of sugar. This is absorbed easily into the blood, causing a rapid increase in the blood glucose level. Insulin is a hormone that reduces the blood glucose level. Type 1 diabetes Type 2 diabetes How it works The pancreas stops making enough insulin The body no longer responds to its insulin How it is controlled Injections of insulin Exercise and appropriate diet Occurs Childhood Later in life P3 – Energy and Radioactivity Atomic structure and radiation • The protons and neutrons are found in the nucleus at the centre. The electrons are arranged in energy levels, or shells, around the nucleus. • Radiation Radioactive elements give out ionising radiation from their nuclei. This happens all the time, whatever is done to the substance. • Alpha radiation Alpha radiation consists of alpha particles. These are identical to the nucleus of a helium atom, which comprises two protons and two neutrons. • Beta radiation Beta radiation consists of high-energy electrons that are emitted from the nucleus. These do not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. • Gamma radiation Gamma radiation is very short wavelength (high frequency) electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and X-rays, which can travel long distances. Penetrating properties of radiation Half-life There are two definitions of half-life, but they mean essentially the same thing: • The time it takes for the number of nuclei of the isotope in a sample to halve. • The time it takes for the count rate from a sample containing the isotope to fall to half its starting level. • Isotopes All the atoms of a given element have the same number of protons. The number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element. Radiation doses Doses of radiation are measured in sievert (Sv) - the amount of possible harm it could do to the body. The dose is based on: • the amount of radiation • the type. If the radioactive source is inside the body, perhaps after being swallowed or breathed in: • Alpha radiation is the most dangerous, because it is absorbed easily by cells. • Beta and gamma radiation are not so dangerous, as they are less likely to be absorbed by a cell and usually just pass right through it. If the radioactive source is outside the body: • Alpha radiation is not as dangerous, because it is unlikely to reach living cells inside the body. • Beta and gamma radiation are the most dangerous sources, as they can penetrate the skin and damage the cells inside. Radiotherapy Although ionising radiation can cause cancer, high doses can be directed at cancerous cells to kill them. This is called radiotherapy. About 40% of people with cancer undergo radiotherapy as part of their treatment. It is administered in two main ways: 1. From outside the body using X-rays or the radiation from radioactive cobalt. 2. From inside the body by putting radioactive materials into the tumour, or close to it. Some normal cells are also damaged by the radiation, but they can repair themselves better than the cancer cells are able to. Sterilising Surgical instruments are sterilised using high doses of gamma radiation. Food can also be sterilised by gamma radiation from radioactive cobalt. The radiation kills microbes, preserving the food for longer. Monitoring radiation • The human senses cannot detect radiation, so we need equipment to do this. • Photographic film goes darker when it absorbs radiation, like when it absorbs visible light. The more radiation the film absorbs, the darker it is when developed. People who may be exposed to radiation regularly include: • medical staff • workers at nuclear power stations • research scientists. Dose in sievert (Sv) Sterilising surgical instruments 25000 Typical radiotherapy dose 60 Legal dose limit for a worker 0.02 Mean annual dose from natural radiation 0.002 Typical chest X-ray 0.00002 Flying from the UK to Spain 0.00001 Electricity – a secondary energy source Coal, oil and natural gas are primary energy sources. Electricity is a secondary energy source, because we use primary energy sources to produce it. Electricity is convenient because: 1. It is transmitted easily over distance (through electricity cables). 2. It can be used in many ways (think of electric lamps, heaters, motors, and so on). Generating electricity • Generators are the devices that transfer kinetic energy into electrical energy. They can be turned directly, for example, by: • wind turbines • hydroelectric turbines • wave and tidal turbines. Efficiency of energy transfer Electric lamps Most of the electrical energy is transferred as heat rather than light energy. This is the Sankey diagram for a typical filament lamp Modern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energysaving lamp Calculating efficiency • The efficiency of a device such as a lamp can be calculated using this equation: • efficiency = useful energy transferred / energy supplied × 100 • The efficiency of the filament lamp is 10 ÷ 100 × 100 = 10%. This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy). Different energy resources • Our renewable energy resources will never run out. Their supply is not limited. There are no fuel costs, either. And they typically generate far less pollution than fossil fuels. • • • • • Renewable energy resources include: wind energy water energy (wave machines, tidal barrages and hydroelectric power) geothermal energy solar energy biomass energy (for example, energy released from wood). The fuel for nuclear power stations is relatively cheap. But the power stations themselves are expensive to build. It is also very expensive to dismantle old nuclear power stations or store radioactive waste, which is a dangerous health hazard Nuclear power stations • The main nuclear fuels are uranium and plutonium, both of which are radioactive metals. Nuclear fuels are not burnt to release energy. Instead, they are involved in nuclear reactions in the nuclear reactor which leads to heat being released. Category Examples Disposal Low level Contaminated equipment, materials and protective clothing Put in drums and surrounded by concrete Intermediate level Components from nuclear reactors, radioactive sources used in medicine or research Mixed with concrete, then put in a stainless steel drum in a purposebuilt store High level Used nuclear fuel and chemicals from reprocessing fuels Stored in a purpose-built store where air can circulate to remove the heat produced Nuclear fission (Higher Tier) • Nuclear power stations use the heat released by nuclear reactions to boil water to make steam. The type of nuclear reaction used is called nuclear fission. In nuclear fission: • a neutron collides with an uranium nucleus (which is large and unstable) • the uranium nucleus splits into two similar-sized smaller nuclei • more neutrons are released • these neutrons can then collide with more uranium nuclei.