Scanned Science Revision Booklet PDF

## Section Six - Cells and Cell Functions ### Cells After all that new-fangled cloning and fretting over the state of the environment, it must come as quite a relief to get back to some good, old-fashioned, boring science. No? Well, tough. ### Plant and Animal Cells Have Similarities and Differen...

## Section Six - Cells and Cell Functions ### Cells After all that new-fangled cloning and fretting over the state of the environment, it must come as quite a relief to get back to some good, old-fashioned, boring science. No? Well, tough. ### Plant and Animal Cells Have Similarities and Differences Most human cells (like most animal cells), have the following parts: - **Nucleus**: Contains genetic material that controls the activities of the cell. - **Cytoplasm**: Gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions. - **Cell membrane**: Holds the cell together and controls what goes in and out. - **Mitochondria**: Location for most of the reactions for respiration. Respiration releases energy that the cell needs to work. - **Ribosomes**: Location where proteins are made in the cell. Plant cells usually have all the bits that animal cells have, plus a few extra things that animal cells don't have: - **Rigid cell wall**: Made of cellulose; supports the cell and strengthens it. - **Permanent vacuole**: Contains cell sap, a weak solution of sugar and salts. - **Chloroplasts**: Location for photosynthesis which makes food for the plant. They contain a green substance called chlorophyll. ### Most Cells are Specialised for Their Function Similar cells are grouped together to make a *tissue*, and different *tissues* work together as an *organ*. Most cells are specialised for their function within a tissue or organ. Here are a couple of plant examples: #### **1) Palisade Leaf Cells are Adapted for Photosynthesis** - Packed with chloroplasts for photosynthesis. - Tall shape means a lot of surface area is exposed down the side for absorbing carbon dioxide from the air in the leaf. - Thin, so you can pack loads of them in at the top of a leaf. Lots of palisade cells make up palisade tissue where most of the photosynthesis happens. #### **2) Guard Cells are Adapted to Open and Close Pores** - Special kidney shape which opens and closes pores (stomata) in a leaf. - When the plant has plenty of water, guard cells fill with water and go plump. This makes the stomata open so gases can be exchanged for photosynthesis. - When the plant is short of water, guard cells lose water and go floppy, making the stomata close. This helps stop too much water vapour escaping. - Thin outer walls and thickened inner walls make the opening and closing work. - Sensitive to light and close at night to save water without losing out on photosynthesis. See also red and white blood cells page 63 and sperm page 71. There's quite a bit to learn in biology - but that's life, I guess. At the top of the page are typical cells with all the typical bits you need to know. But cells aren't all the same - they have different structures and produce different substances depending on the job they do. ## Section Seven - Organs and Systems 1 ### Respiration and Exercise Respiration happens in little tiny structures called mitochondria (see page 52). ### Respiration is **NOT** “Breathing In and Out” Respiration is really important - it releases the energy that cells need to do just about everything. - **Respiration** is the process of breaking down glucose to release energy, and it goes on in every cell in your body. (Glucose contains energy in the form of chemical bonds.) - **Respiration** happens in plants too. All living things respire; it's how they get energy from their food. **Respiration** is the process of breaking down glucose to release energy, which goes on in **every cell**. ### Respiration can be **Aerobic** or **Anaerobic** **Aerobic respiration** is respiration using oxygen ("aerobic" just means "with air"). It's the most efficient way to release energy from glucose. Word equation: ``` glucose + oxygen → carbon dioxide + water (+ ENERGY) ``` **Anaerobic respiration** happens when there's not enough oxygen available (e.g., when you're exercising hard). Anaerobic just means without air, and it's **NOT** the best way to release energy from glucose, but it's useful in emergencies. The overall word equation is: ``` glucose → lactic acid (+ ENERGY) ``` ### When You Exercise You Respire More - When you exercise, your muscles need more energy so you respire more. - You need to get more oxygen into the cells. Your breathing rate increases to get more oxygen into the lungs, and your heart rate increases to get this oxygenated blood around the body faster. - During really vigorous exercise (like sprinting), your body can't supply enough oxygen to your muscles quickly enough, so they start respiring anaerobically. - Anaerobic respiration produces lactic acid, which builds up in your muscles and causes pain. When you stop exercising, you'll have an oxygen debt. You have to keep breathing hard to repay the oxygen that you didn't manage to get to your muscles. The oxygen breaks down the lactic acid. - Athletes monitor their heart rate and breathing rate to help with their training. - The current UK government recommendation is to exercise for at least 30 minutes, five times a week in order to stay fit and healthy. But the official advice changes all the time - not so long ago the recommendation was 20 minutes, three times a week. ### Alveoli are Specialised for Gas Exchange - The huge number of microscopic alveoli gives the lungs an enormous surface area. - There's a moist lining for gases to dissolve in. - Alveoli have very thin walls - only one cell thick, so the gas doesn't have far to diffuse. - They have a great blood supply to maintain a high concentration gradient. - The walls are permeable - so gases can diffuse across easily. “Oxygen debt” - cheap to pay back Advice about exercise (and diet) is based on scientific evidence from many different surveys and studies. ## Section Nine - Plants and Energy Flow ### Photosynthesis Plants can make their own food - it's ace. Here's how... ### Photosynthesis Produces Glucose Using Sunlight 1) Photosynthesis is the process that produces "food" in plants. The "food" it produces is glucose. 2) Photosynthesis happens in the leaves of all green plants - this is largely what the leaves are for. 3) Photosynthesis happens inside the chloroplasts, which are found in leaf cells and in other green parts of a plant. Chloroplasts contain a substance called chlorophyll, which absorbs sunlight and uses its energy to convert carbon dioxide and water into glucose. Oxygen is also produced. ``` carbon dioxide + water → glucose + oxygen ``` ### Four Things are Needed for Photosynthesis to Happen: - **Light**: Sunlight beating down on the leaf provides the energy for the process. - **Chlorophyll**: The green substance which is found in chloroplasts and which makes leaves look green. Chlorophyll absorbs the energy in sunlight and uses it to combine carbon dioxide and water to make glucose. Oxygen is just a by-product of this reaction. - **Carbon dioxide**: Carbon dioxide diffuses into the leaf from the air around. - **Water**: Water comes up from the soil, up the roots and stem, and into the leaf via the veins. ### Plants Use the Glucose for Six Different Things: - **Respiration**: This releases energy that enables them to convert the rest of the glucose into various other useful substances which they use to build new cells and grow. - **Making fruits**: Glucose, along with another sugar called fructose, is turned into sucrose for storing in fruits. Fruits deliberately taste nice so that animals will eat them and spread the seeds all over the place in their poo. - **Making cell walls**: Glucose is converted into cellulose for making cell walls. - **Making proteins**: Glucose is combined with nitrates to make amino acids which are then made into proteins. - **Stored in seeds**: Glucose is turned into lipids (fats and oils) for storing in seeds. - **Stored as starch**: Glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like in winter. Starch is insoluble, which makes it much better for storing because it doesn't bloat the storage cells by osmosis like glucose would. ### I'm working on sunshine... woah o... Plants are pretty crucial in ensuring the flow of energy through nature. They are able to use the Sun's energy to make glucose - the energy source which humans and animals need for respiration (see p.60). Make sure you know the photosynthesis equation inside out - it's important later in the section too. ## Section Nine - Plants and Energy Flow ### Rate of Photosynthesis A plant's rate of photosynthesis is affected by the amount of light, the amount of carbon dioxide, and the temperature of its surroundings. Photosynthesis slows down or stops if the conditions aren't right. ### The Limiting Factor Depends on the Conditions - **Limiting factor**: Something which stops photosynthesis from happening any faster. The amount of light, amount of carbon dioxide, and the temperature can all be the limiting factor. - **The limiting factor depends on the environmental conditions.** E.g., in winter, cold temperatures might be the limiting factor, at night light is likely to be the limiting factor. ### There are Three Important Graphs for Rate of Photosynthesis #### **1) Not Enough Light Slows Down the Rate of Photosynthesis** Chlorophyll uses light energy to perform photosynthesis. It can only do it as quickly as the light energy is arriving. - If the light level is raised, the rate of photosynthesis will increase steadily, but only up to a certain point. - Beyond that, it won't make any difference because then it'll be either the temperature or the carbon dioxide level which is now the limiting factor. #### **2) Too Little Carbon Dioxide Also Slows It Down** Carbon dioxide is one of the raw materials needed for photosynthesis - only 0.04% of the air is carbon dioxide, so it's pretty scarce as far as plants are concerned. - As with light intensity, increasing the amount of carbon dioxide will only increase the rate of photosynthesis up to a point. After this, the graph flattens out, showing that carbon dioxide is no longer the limiting factor. - As long as light and carbon dioxide are in plentiful supply, then the factor limiting photosynthesis must be temperature. #### **3) The Temperature Has to be Just Right** Temperature affects the rate of photosynthesis - because it affects the enzymes involved. - As the temperature increases, so does the rate of photosynthesis - up to a point. - If the temperature is too high (over about 45 °C), the plant's enzymes will be denatured (destroyed), so the rate of photosynthesis rapidly decreases. - Usually though, if the temperature is the limiting factor, it's because it's too low, and things need warming up a bit. ### No, no... no, no, no, no... no, no, no, no... no, no, there's no limit... You can create the best conditions for photosynthesis in a greenhouse. Farmers use heaters and artificial lights and they can also increase the level of carbon dioxide using paraffin burners. By keeping plants in a greenhouse, they're also keeping out pests and diseases. The plants will grow much more quickly. ## Section Nine - Plants and Energy Flow ### Transpiration If you don't water a house plant for a few days, it starts to go all droopy. Then it dies, and the people from the *Society for the Protection of Plants* come round and have you arrested. Plants need water. ### Transpiration is the Loss of Water from the Plant 1) Transpiration is caused by the *evaporation* and *diffusion* (see page 56) of water from inside the leaves. 2) This creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the *xylem vessels* (see next page) to replace it. 3) This in turn means more water is drawn up from the roots, and so there's a constant *transpiration stream* of water through the plant. Transpiration is just a side effect of the way leaves are adapted for *photosynthesis*. They have to have *stomata* in them so that gases can be exchanged easily (see previous page). Because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata. The transpiration stream does have some benefits for the plants: - The constant stream of water from the ground helps to keep the plant cool. - It provides the plant with a constant supply of water for *photosynthesis*. - The water creates *turgor pressure* in the plant cells, which helps support the plant and stops it wilting (see next page). - Minerals needed by the plant (see page 83) can be brought in from the soil along with the water. ### Transpiration Rate is Affected by Four Main Things - **Light intensity**: The brighter the light, the greater the *transpiration rate*. Stomata begin to close as it gets darker. *Photosynthesis* can't happen in the dark, so they don't need to be open to let carbon dioxide in. When the stomata are closed, very little water can escape. - **Temperature**: The warmer it is, the faster transpiration happens. When it's warm, the water particles have more energy to evaporate and diffuse out of the stomata. - **Air movement**: If there's lots of air movement (wind) around a leaf, transpiration happens faster. If the air around a leaf is very still, the water vapour just surrounds the leaf and doesn't move away. This means there's a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn't happen as quickly. If it's windy, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly, from an area of high concentration to an area of low concentration. - **Air humidity**: If the air around the leaf is very dry, *transpiration* happens more quickly. This is like what happens with air movement. If the air is humid, there's a lot of water in it already, so there's not much of a difference between the inside and the outside of the leaf. Diffusion happens fastest if there's a really high concentration in one place, and a really low concentration in the other. ### Transpiration -- The Plant Version of Perspiration... One good way to remember those four factors that affect the rate of transpiration is to think about drying washing. Then you'll realize there are far more boring things you could be doing than revision, and you'll try harder. No, only joking - it's the same stuff: sunny, warm, windy and dry. ## Section Nine - Plants and Energy Flow ### The Evolution of the Atmosphere For 200 million years or so, the atmosphere has been about how it is now: 78% nitrogen, 21% oxygen, and small amounts of other gases, mainly carbon dioxide and noble gases. There can be a lot of water vapour too. But it wasn't always like this. Here's how the past 4.5 billion years may have gone: <start_of_image> This is the first two billion years - **Earths surface**: Originally molten. It was so hot that any atmosphere just "boiled away" into space. - **Volcanoes**: Gave out lots of gas, including carbon dioxide, water vapour, and nitrogen. This was how the oceans and atmosphere were formed. - **Early atmosphere**: Probably mostly carbon dioxide, with virtually no oxygen. - **Oceans formed**: When the water vapour condensed. ### Phase 2 - Green Plants Evolved and Produced Oxygen - **Green plants**: Evolved over most of the Earth. They were quite happy in the carbon dioxide atmosphere. - **Carbon dioxide dissolved**: Much of the early carbon dioxide dissolved into the oceans. The green plants also removed carbon dioxide from the air and produced oxygen by photosynthesis. - **Sedimentary rocks**: When plants died and were buried under layers of sediment, the carbon they had removed from the air became "locked up" in sedimentary rocks as insoluble carbonates and fossil fuels. - **Carbon released "locked up":** When we burn fossil fuels today, this "locked-up" carbon is released and the concentration of carbon dioxide in the atmosphere rises. ### Phase 3 - Ozone Layer Allows Evolution of Complex Animals - **Oxygen build up**: Killed off some early organisms that couldn't tolerate it, but allowed other, more complex organisms to evolve and flourish. - **Ozone layer**: The oxygen also created the ozone layer which blocked harmful rays from the sun and enabled even more complex organisms to evolve - us, eventually. - **Virtually no carbon dioxide left**: Now. ### 4 billion years ago, it was a whole other world... We've learned a lot about the past atmosphere from Antarctic ice cores. Each year, a layer of ice forms and bubbles of air get trapped inside it, and it's buried by the next layer. So the deeper the ice, the older the air, and if you examine the bubbles in different layers, you can see how the air has changed. The measurements have to be ultra-precise though as the changes between layers can be very very tiny. ## Section Five - The Earth and the Atmosphere ### Atmospheric Change Evidence for how the atmosphere evolved has been found in rocks and other sources. But no one was there to record the changes as they happened. So our ideas about the atmosphere are still *theories*. ### There are Competing Theories About Atmospheric Change As well as the theories on page 36, there are other theories about how the Earth's atmosphere changed millions of years ago. Ultimately, all the theories have to be judged on the evidence. For example, one theory says that the water on Earth came mainly from comets rather than volcanoes. When this theory was first suggested, it seemed far-fetched. However, space science research soon suggested that lots of small icy comets really are hitting the Earth every day. So far so good. But studies of comets found that the water in them isn't the same as the water on Earth (it's got more "heavy water" in it). So current thinking is that most of Earth's water probably didn't come from comets. ### The Atmosphere Changes All the Time - **Graph of carbon dioxide and global temperature data**: Shows carbon dioxide levels rising rapidly over the last few thousand years, and a global temperature rise that's been more or less keeping up. - **Huge changes in the climate before**: These changes are small beer compared with the changes described on page 36, when the entire composition of the atmosphere was changing... but they're still pretty big. - **Ice ages**: There have been several ice ages (times when large areas of the Earth's surface are covered with ice) over the last few million years. These happen for various reasons (e.g., things to do with the Earth's orbit, movement of continents, carbon dioxide in the atmosphere, and so on). - **Changes in the Earth's temperature**: They happen all the time. However, we've recently become aware that the planet is warming faster than before, and what this could mean for us. ### The Atmosphere is Still Changing - **Burning fossil fuels**: Releases carbon dioxide, and as the world has become more industrialised, more fossil fuels have been burnt in power stations and in car engines. The level of carbon dioxide in the atmosphere has increased by about 25% since 1750. - **Carbon dioxide**: A greenhouse gas - it traps heat from the sun (see page 38 for more info). ### Over the last 50 years, the amount of ozone in the ozone layer has decreased... - **Holes in the ozone layer**: Form over Antarctica and the Arctic each year. - **Ozone is broken down by man-made gases**: Called CFCs, widely used as aerosol propellants and fridge coolants between the 1930s and the 1980s. - **Ozone layer protects**: Us from harmful UV radiation which can cause skin cancer. It's difficult to test whether changes in the ozone layer are to blame for increases in skin cancer, though. Other factors affect skin cancer - people sunbathe more and have more beach holidays abroad, so they expose themselves to more UV radiation anyway. ### The atmosphere's evolving — shut the window will you... Whether people believe scientific theories or not depends on the evidence that people produce to support them. Without evidence, a theory goes nowhere. Quite right too, I say. ## Section Five - The Earth and the Atmosphere ### Populations and Competition Organisms have to compete for resources in the environment where they live. ### Population Size is Limited by Available Resources Population size is limited by: - The total amount of food or nutrients available (plants don't eat, but they get minerals from the soil). - The amount of water available. - The amount of light available (this applies only to plants really). - The quality and amount of shelter available. Animals and plants of the same species and of different species will **compete** against each other for these resources. They all want to survive and reproduce. Similar organisms will be in the closest competition - they'll be competing for the same *ecological niche*. **Example: Red and Grey Squirrels** - These two different species like the same kind of habitat, same kind of food, type of shelter, etc. - Grey squirrels are better adapted to deciduous woodland, so when they were introduced into Britain, red squirrels disappeared from many areas - they just couldn't compete. ### Populations of Prey and Predators Go in Cycles In a community containing prey and predators (as most of them do, of course): - The population of any species is usually limited by the amount of food available. - If the population of the prey increases, then so will the population of the predators. - However, as the population of predators increases, the number of prey will decrease. More grass means more rabbits. More rabbits mean more foxes. But more foxes mean less rabbits. Eventually less rabbits will mean less foxes again. This up-and-down pattern continues... ### Parasites and Mutualistic Relationships The survival of some organisms can depend almost entirely on the presence of other species. - **Parasites**: Live off a host. They take what they need to survive, without giving anything back. This often harms the host, which makes it a win-lose situation. - Tapeworms absorb lots of nutrients from the host, causing them to suffer from malnutrition. - Fleas are parasites. Dogs gain nothing from having fleas (unless you count hundreds of bites). - **Mutualism**: A relationship where both organisms benefit - so it's a win-win relationship. - Most plants have to rely on nitrogen-fixing bacteria in the soil to get the nitrates that they need. But leguminous plants carry the bacteria in nodules in their roots. The bacteria get a constant supply of sugar from the plant, and the plant gets essential nitrates from the bacteria. - "Cleaner species" are fantastic. E.g., oxpeckers live on the backs of buffalo. Not only do they eat pests on the buffalo, like ticks, flies and maggots (providing the oxpeckers with a source of food), but they also alert the animal to any predators that are near, by hissing. ### Revision stress - Don't let it eat you up... In the exam you might get asked about the distribution of any animals or plants. Just think about what the organisms would need to survive. And remember, if things are in limited supply, then there's going to be competition. And the more similar the needs of the organisms, the more they'll have to compete. ## Section Three - Adaptation and Evolution ### Classification Scientists classify species so that anyone who reads their research knows exactly which organism it's about. ### Classification is Organising Living Organisms into Groups - Nowadays, scientists classify organisms into groups based on genetic similarities. For example, bats, whales, and humans might seem quite different, but they have a similar bone pattern in their forelimbs, and they're all genetically related; the classification system reflects these similarities. - Living things are divided into kingdoms e.g., the *animal kingdom*, the *plant kingdom*, etc. Kingdoms are then subdivided into smaller and smaller groups. - *Genus*: A group of closely-related species. - *Species*: A group of closely-related organisms that can breed to produce fertile offspring (see next page). - **Binomial system**: Used to name organisms uses the Latin names of the *genus* and the *species* they belong to. E.g., humans are *Homo sapiens*. "Homo" is our genus name, and "sapiens" is our species. ### Living Things Can be Plants, Animals or Something Else - To be a member of the plant kingdom, organisms must contain chloroplasts and therefore be able to make their own food by photosynthesis (see page 78). - Members of the animal kingdom move about from place to place and have compact bodies (unlike plants, which spread out to catch as much light and water as possible and can't move about freely). Animals can't make their own food so they have to find things to eat, such as plants or other animals. - Other organisms, like fungi and bacteria, don't have animal or plant features and are put in other kingdoms. - Some single-celled organisms have features of both plants and animals. Euglena can move from place to place by thrashing its flagellum but also has chloroplasts which allow it to make its own food. It's put into a kingdom called Protoctista, along with some other single-celled organisms. ### Vertebrates Have Backbones The animal kingdom is divided into vertebrates and invertebrates. Vertebrates are animals with a backbone and an internal skeleton. Invertebrates don't have these structures - some do have an external skeleton though. Vertebrates are divided into five groups, called *classes* - fish, amphibians, reptiles, birds and mammals. - **Fish**: Live in water. They have scales, and gills for gas exchange. - **Amphibians**: Exchange gas partly through their skin, so gases must be able to move in and out - their skin's got to be permeable and moist. - **Reptiles**: More adapted to live on the land. They've got a dry, scaly skin which stops them losing too much water. - **Birds**: Most can fly and they've got feathers to help them do this. You'll also find a beak - useful for cracking seeds or catching prey. - **Mammals**: Have fur covering their bodies to keep them warm. They give birth to their young (rather than laying eggs like other vertebrates) and produce milk to feed them. The rules of the classification system were made up using the animals and plants that were known about at the time. Sometimes newly discovered species don't really fit into any of the categories. These can be living species or fossil ones, such as *archaeopteryx,* which had reptilian teeth, clawed hands and a long bony tail, like a *dinosaur,* but also had wings and flight feathers, like a bird. ### I'm not a vertebrate - I'm completely spineless... There are loads of different types of organisms out there - so, no wonder the classification system gets a bit unwieldy. This makes life no easier for you, I'm afraid - you've just got to learn it... ## Section Three - Adaptation and Evolution ### Evolution We've identified about 1.5 million different species, and there's loads more. So how did they all get here...? ### No One Knows How Life Began We know that living things come from other living things - that's easy enough. But where the first living thing came from... that's a much more difficult question. - There are various theories suggesting how life first came into being, but no one really knows. - Maybe the first life forms came into existence in a primordial swamp (or under the sea) here on Earth. Maybe simple organic molecules were brought to Earth on comets - these could have then become more complex organic molecules, and eventually very simple life forms. - These ideas (and others) have their supporters. But we just don't know - the evidence was lost long ago. All we know is that life started somehow. And after that, we're on slightly firmer ground... ### The Fossil Record Shows That Organisms Have Evolved 1) **Fossil**: Any evidence of an animal or plant that lived ages ago. 2) **Fossils form in rocks**: As minerals replace slowly decaying tissue (or in places where no decay happens) and show features like shells, skeletons, soft tissue (occasionally), footprints, etc. They show what was on Earth millions of years ago. They can also give clues about an organism's habitat and food. 3) **Layers of rock**: Where fossils are found were made at different times. This means it's possible to tell how long ago a particular species lived. 4) **Similarities and differences**: Between fossils in rocks of different ages, we can see how species have evolved (changed and developed) over billions of years. 5) **Family tree**: You could put all species on a "family tree" - where each new branch shows the evolution of a new species. Then you could easily find the most recent common ancestor of any two species. The more recent the common ancestor, the more closely related the two species. 6) **Few organism turn into fossils**: When they die - most decay away completely. This leaves gaps in the fossil record, which means there are many species that we'll never know about. ### The Evolution of the Horse The fossil record of the horse provides strong evidence for the theory of evolution, but things are a little more complicated than we first thought. - If you stick all the fossil bones in order of age, they seem to show the modern horse evolving gradually from a creature about the size of a dog, with the middle toe slowly getting bigger to form the familiar hoof. - At first, some fossils didn't seem to fit. But now we know that several now-extinct kinds of horse evolved at the same time, and it all makes sense. ### There are Other Views About the Fossil Record Some people interpret the fossil evidence differently. E.g., *creationists* believe that each species was created separately by God and will never evolve into new species. They don't think the fossil record is evidence for gradual *evolution,* but simply shows a lot of different organisms, some of which are now extinct. ### Cell, blob, toad, monkey, me - what a fine family tree... The fossil record provides good evidence for *evolution*, but it can't prove it. But proving a theory of something that happens over millions of years was never going to be straightforward, I guess. ## Section Three - Adaptation and Evolution ### Evolution The theory of evolution (see last page) states that one of your (probably very distant) ancestors was a blob in a swamp somewhere. Something like that, anyway. ### Make Sure You Know the Theory of Evolution 1) **Evolution**: States that all the animals and plants on Earth gradually "evolved" over millions of years, rather than just suddenly appearing. 2) **Life on Earth began**: As simple organisms from which all the more complex organisms evolved. And it only took about 3,000,000,000 years. ### There are Lots of Modern Examples of Evolution - **Peppered Moths Adapted Their Colour**: Peppered moths are often seen on the bark of trees. Until the 19th century, the only ones found in England were light in colour. Then some areas became polluted and the soot darkened the tree trunks. A black variety of moth was found. The moths had adapted to stay camouflaged. - **Bacteria Adapt to Beat Antibiotics**: The "survival of the fittest" (see next page) affects bacteria just the same as other living things. They adapt to become resistant to our bacterial-fighting weapons - antibiotics. - If someone gets ill, they might be given an antibiotic which kills 99% of the bacteria. - The 1% that survive are resistant – so if they're passed on to somebody else, the antibiotic won't help them. - Nowadays bacteria are getting resistant at such a rate, the development of antibiotics can't keep up. - **Rats Adapt to Beat Poison**: The poison *warfarin* was widely used to control the rat population. However, a certain gene gives rats resistance to it, so rats which carry it are more likely to survive and breed. This gene has become more and more frequent in the rat population, so *warfarin* isn't as much use anymore. ### Environmental Change Can Cause Extinction 1) If a species can't evolve quickly enough, it's in trouble. 2) The dinosaurs and woolly mammoths became extinct, and it's only fossils that tell us they ever existed. There are three ways a species can become extinct: - The environment changes more quickly than the species can adapt. - A new predator or disease kills them all. - They can’t compete with another (new) species for food. 3) As the environment changes, it'll gradually favor certain characteristics (see next page). 4) Over many generations those features will be present in more of the population. 5) But if the environment changes too fast the whole species may become extinct. ### Did you know exams evolved from the Spanish Inquisition... ...well, that's not really true. But it is true that humans and apes both evolved from a common ancestor. Scientists reckon that 5-8 million years ago the species separated, and one population evolved into humans, and the other into apes - so you're a distant relation (250,000th cousin, maybe) of a chimp. ## Section Three - Adaptation and Evolution ### Natural Selection Charles Darwin developed a theory about how evolution actually happened. He called it the theory of natural selection. This is how he came up with it... ### Darwin Made Four Important Observations... 1) **All organisms produce more offspring than could possibly survive** (e.g., only a few frogspawn survive and become frogs). 2) **Population numbers tend to remain fairly constant over long periods of time**. 3) **Organisms in a species show wide variation in characteristics.** 4) **Some of the variations are inherited, and so passed on to the next generation.** ### ...and Then Made These Two Deductions: 1) **Most offspring don't survive**: All organisms must have to struggle for survival. Being eaten, disease and competition, cause large numbers of individuals to die. 2) **Survival and reproduce better**: The organisms who have characteristics that allow them to survive and reproduce better (i.e. the most useful adaptations to the environment) will pass on these characteristics. This is the famous "survival of the fittest" statement. Organisms with slightly less survival value will probably perish first, leaving the fittest to pass on their genes to the next generation. ### Organisms with Certain Characteristics Will Survive Better Here's an example... once upon a time maybe all rabbits had short ears and managed OK. Then one day out popped a rabbit with big ears who could hear better and was always the first to dive for cover at the sound of a predator. Pretty soon, he's fathered a whole family of rabbits with big ears, all diving for cover before the other rabbits, and before you know it there are only big-eared rabbits left - because the rest just didn't hear trouble coming quick enough. This is how populations adapt to changes in their environment (an organism doesn't actually change when it's alive - changes only occur from generation to generation). Over many generations, the characteristic that increased survival becomes more common in the population. If members of a species are separated somehow, and evolve in different ways to adapt to different conditions, then over time you can end up with totally different species. ### Darwin's Theory Wasn't Popular at First 1) **Charles Darwin's theory caused some trouble**: It was the first plausible explanation for our own existence without the need for a "Creator". This was bad news for the religious authorities of the time, who tried to ridicule old Charlie's ideas. The idea that humans and monkeys had a common ancestor was hard for people to accept, and easy to take the mick out of. 2) **Some scientists weren't keen either, at first**: Darwin didn't provide a proper explanation of exactly how individual organisms passed on their survival characteristics to their offspring. 3) **Later, the idea of genetics was understood**: Which did explain how characteristics are inherited. ### Natural selection - **Sounds like vegan chocolates...** This is a good example of how scientific theories come about - someone observes something and then tries to explain it. Their theory will then be tested by other scientists using evidence - if the theory passes these tests, it gains in credibility. If not, it's rejected. Natural selection hasn't been rejected yet. ## Section Three - Adaptation and Evolution ### Elements, Compounds and Mixtures There are only about 100 or so different kinds of atoms, which doesn't sound too bad. But they can join together in loads of different combinations, which makes life more complicated. ### Elements Consist of One Type of Atom Only Quite a lot of everyday substances are elements: - Copper - Aluminium - Iron - Oxygen - Nitrogen ### Compounds are Chemically