Biology Unit 1

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  • Created by: Suaad
  • Created on: 24-12-12 11:29

Diet & Exercise

What is needed for a balanced diet? 

  • Proteins - growth and repair (found in meat, eggs, fish & nuts)
  • Carbohydrates - provides energy (found in pasta, rice & bread)
  • Fats - store energy (found in butter, lard & oil)
  • Water - needed for chemical reactions
  • Fibre - helps prevent consitipation (found in fruite & veg)
  • Vitamins & Minerals - needed for general health

Malnutrition - Can be when a person is both overweight and underweight

Metabollism - chemical reactions

Factors that affect metabollism are:

  • Genetics
  • Exercise
  • muscle/fat ratio
  • Gender
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Diet & Exercise (continued)


Cholesterol is made in the liver and then stored in the gall bladder. There are two types of cholesterol.

HDLs (High-density lipoprotein) - These are the healthy types of cholesterol

LDLs (Low-density lipoprotein) - These are the bad kind of cholesterol and is the main reason for heart disease. 

If there is a high number of cholesterol in the blood it could then lead to blocked arteries which will then limit the amount of blood reaching the heart causing the person to then have a heart attack.

Factors that can make you prone to Type II Diabetes

  • High blood pressure
  • Obesity or overweight

Type II Diabetes cannot control blood sugar which is then a result of insulin not being recognised by the cells in the body to take the sugar out of the blood

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Defence against diseases (part 1)

Bacteria can produce toxins that harm the body cells. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes. The name of disease causing microorganisms are called pathogens. 

There are 3 main methods on how white blood cells can protect the body from infectious diseases.


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Defence against diseases (part 2)

White blood cells

The first method on how white blood cells can protect the body from infectious diseases is  that when they detect the pathogen they ingest it and destroy them. Once they ingest it the white blood cell produces enzymes which then destroy the pathogen. 

The second method is when the white blood cell produces antibodies that have a particular shape that can go around the antigens to destroy the pathogen.

The third and final method is when the white blood cell produces antitoxins that counteract the toxins which are released by the pathogens. The antitoxins neutralises the toxins which then eventually destroy the pathogen.


Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body.When injected into the body they stimulate white blood cells to produce antibodies against the pathogen. If the the person does get infected by the pathogen late, their body can respond in the same way as if they had the disease before. If a large proportion of the population is immune to a particular pathogen, the spread of that pathogen is greatly reduced

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Infectious Diseases

MMR - Measles, Mumps & Rubella

 Some scientists thought that this vaccination linked to autism. 

Ignaz Semmelweis

In the 19th century, Ignaz Semmelweiss realised the importance of cleanliness in hospitals. Semmelweiss insisted that doctors should wash their hands before examining patients, something that was not common at the time. This policy greatly reduced the number of deaths from infectious diseases in his hospital. Theses were the steps which he carried out:

  • Made observations - collected some date
  • Hypotheses - an idea or prediction
  • Experiment/investigation - wash your hands before dealing with patients
  • Results - the number of deaths reduced
  • Conclusion - washing your hands helped

Unfortunately, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

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Infectious Diseases (part 2)

Painkillers helps to relieve the symptoms of an infectious disease, but they do not kill the pathogens involved.  For example, paracetamol, aspirin and morphine block nerve impulses from the painful part of the body, or block nerve impulses travelling to the part of the brain responsible for perceiving pain.

Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses because they live and reproduce inside cells. It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

Antibiotic Resistance

If you take antibiotics too often the bacteria will develop antibiotic resistance. The antibiotic resistance is caused by a mutation - a random change in the DNA which affects the genes. 

Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.

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Culturing Microorganisms

Culturing Microorganisms

The action of antibiotics and disinfectants can be investigated using cultures of microorganisms (populations of microorganisms that have been grown for a purpose). It is important that the cultures are uncontaminated by other microorganisms, so sterile conditions are needed:

  • the Petri dishes, nutrient agar jelly and other culture media must be sterilised
  • the inoculating loops used to transfer microorganisms must be sterilised (usually by passing the metal loop through a Bunsen burner flame)
  • the lid of the Petri dish is sealed with sticky tape to stop microorganisms from the air getting in and contaminating the culture.

Safety in the lab

Bacteria grow and reproduce more quickly when they are warm than when they are cold. It would be dangerous to incubate (keep and grow) cultures at temperatures close to body temperature (37°C) because doing so might allow the growth of pathogens harmful to health. So the maximum temperature used in school and college labs is 25°C. However, higher temperatures can be used industrially, and these produce faster growth.

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The Nervous System

The job of the nervous system is to detect stimuli so we could coordinate our behaviour. A stimuli is a change in the environment (e.g. change in light or sound)


  • Receptors found in eyes are sensitive to light
  • Receptors found in ears are sensitive to sound and they detect position (balance)
  • Receptors found in the tongue are sensitive to chemicals in food
  • Receptors found in the nose are sensitive to chemicals in the air
  • Receptors found in the skin are sensitive to touch, pressure, temprature and pain

CNS - Brain & Spine

Where two neurones meet there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical (neurotransmitters) diffuses across the gap and makes the next neurone transmit an electrical signal.

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Reflex Actions

Reflexes                                                                                                                        ➥ are responses that are designed to protect you                                                              ➥ they are designed to be quick                                                                                      ➥ it doesn't involve the brain but it does involve the spine.

When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates the response, but sometimes a very quick response is needed, one that does not involve the brain: this is a reflex action.

In a simple reflex action:

Receptors ➾ Sensory Neurone ➾ Relay Neurone ➾ Motor Neurone ➾ Effector ➾ Response

This is what happens during a reflex:

  • receptor detects a stimulus - a change in the environment
  • sensory neurone sends impulses to relay neurone
  • motor neurone sends impulses to the effector
  • effector produces a response
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The Endocrine System

Hormones are chemical substances that regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the bloodstream. Several hormones are involved in the female menstrual cycle. Hormones can be used to control human fertility and have advantages and disadvantages.

Control in humans

The internal environment of the body is controlled by the nervous system and hormones. The maintenance of a constant internal environment is called homeostasis. Below are some of the internal conditions that need to be controlled.

Water content in the body

All chemical reactions happen in a watery environment, if there isn't enough water in your body you start to dehydrate. Water enters the body by drinks and food. You lose water by breathing (water vapour is a product of respiration), sweating (depends on how active you are) and by urinating (via kidneys). Your body has control mechanisms to make sure you don't produce too much urine if you're dehydrated and you produce more if you have an excess of water in your body. The key organ that does this is the kidney.

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The Endocrine System (part 2)

Ion (salts) content in the body

Ion levels are controlled to protect cells from too much water entering or leaving them. Ion content is controlled by the loss of ions from:

  • the skin- through sweating
  • passing urine- produced by the kidneys.

Temprature of the body

This is controlled to maintain the temperature at which enzymes work best. Body temperature is controlled by sweating, shivering, and controlling blood flow to the skin.

Blood Sugar Levels

This is controlled to provide cells with a constant supply of energy. Blood sugar level is controlled by the release and storage of glucose controlled by insulin.

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The Menstrual Cycle

Several hormones control the menstural cycle which includes controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining. These hormones are secreted by the ovaries and pituitary gland.

Follicle Stimulating Hormone (FSH)

The hormone FSH is secreted (released) by the pituitary gland. FSH makes two things happen:

  • it causes an egg to mature in an ovary
  • it stimulates the ovaries to release the hormone oestrogen.


The hormone oestrogen is secreted by the ovaries. Oestrogen makes two things happen:

  • it inhibits (stops) FSH being produced - so that only one egg matures in a cycle
  • it stimulates the pituitary gland to release luteinizing hormone (LH), which triggers ovulation (the release of the mature egg from the ovary).
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Hormones in Controlling Fertility

Oral Contraceptives

Human fertility is controlled by hormones. This means that knowledge of hormones can be used to decide to increase, or reduce, the chances of fertilisation and pregnancy.

The oral contraceptive, commonly known as the pill, greatly reduces the chances of mature eggs being produced. It contains oestrogen or progesterone (another hormone). These hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries. 

Benefits and Risks

Oral contraceptives allow couples to choose the time they start a family, and choose the time they stop having children.

The first birth-control pills contained higher amounts of oestrogen than the pills taken today. This caused women to have significant side effects, such as changes in weight, mood and blood pressure. Modern birth-control pills contain much less oestrogen. Some only contain progesterone, which also leads to fewer side effects.

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IVF (In vitro fertillisation)

Fertility Treatments

Fertility treatments increase a woman's chance of becoming pregnant, although the treatment may not always work. On the other hand, because the treatment boosts the production of mature eggs, multiple conceptions sometimes occur, with twins or triplets being expected. This increases the risk of complications in pregnancy and childbirth, and may lead to premature or underweight babies.


This is where the egg is fertilised outside the woman’s body and then implanted back into her uterus. As FSH can also be used to encourage the production of several mature eggs at once, it is used as part of IVF to increase the number of eggs available for fertilisation.

Designer babies

Some people worry about the ethical implications of IVF. They are concerned that couples may want 'designer babies' with 'desirable' qualities, so may only want certain fertilised eggs. For example, they may want a girl if they have lots of boys in the family, or they may wish to avoid producing a baby with an inherited defect.

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Control in plants

Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.

A tropism is a growth in response to a stimulus and an auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weed killers, rooting powder and to control fruit ripening.

Sensitivity in plants

Plants need light and water for photosynthesis. Plant responses - calledtropisms - help make sure that any growth is towards sources of light and water.

There are two main types of tropism:

  • positive tropism – the plant grows towards the stimulus
  • negative tropism – the plant grows away from the stimulus.

Phototropism is a tropism where light is the stimulus. A gravitropism (also called a 'geotropism') is a tropism where gravity is the stimulus. The roots and shoots of a plant respond differently to the same stimuli.

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Control in plants (part 2)


Auxins are a family of hormones found in plants. Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. They change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins:

  • cells in shoots grow more
  • cells in roots grow less.


In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light.

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the lit side. This causes the root to bend away from the light.

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Control in plants (part 3)


Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top sidem causing the root to bend in the direction of the force of gravity.

In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.

Weed killers

Selective weed killers work on some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat. Selective weed killers contain growth hormone that cause the weeds to grow too quickly and then die. The weed killer is absorbed in larger quantities by the weeds than the beneficial plants, which stay healthy.

Rooting powders

Rooting powder contains growth hormones to make stem cuttings quickly develop roots. 

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Drugs are chemicals that affect body chemistry. There are three different types of drugs, these are medical, recreational and performance enhancing drugs.

Medical drugs have to go through stages to make sure that they work.

  1. The drugs are tested using computer models and human cells grown in the laboratory. Many substances fail this test because they damage cells or do not seem to work.
  2. Drugs that pass the first stage are tested on animals. In the UK, new medicines have to undergo these tests, but it is illegal to test cosmetics and tobacco products on animals. A typical test involves giving a known amount of the substance to the animals, then monitoring them carefully for any side-effects.
  3. Drugs that have passed animal tests are used in clinical trials. They are tested on healthy volunteers to check they are safe. Very low doses of the drug are given to begin with. If there are no problems, further clinical trials are done to find the optimum dose for the drug.Clinical trials are not without risk. Sometimes severe and unexpected side effects occur. Most substances do not pass all of the tests and trials, so drug development is expensive and takes a long time.
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Medical drugs

Double blind trials

It is important to be certain that a drug really does have positive effects, rather than people feeling better simply because they expect to feel better if they take a medicine. This is called the placebo effect.

Double blind trials aim to minimise the placebo effect. Some patients are given the drug while others are given a placebo. A placebo is designed to appear exactly the same as the drug itself, but it does not actually contain any of the drug. The doctors and patients are not told who have received the drug and who have received the placebo until the trial is over. Doctors are not told which patient has the placebo because then the doctors could be biased.

Devoloping medical drugs takes at least 10 years and is very expensive.

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  • Thalidomide was initially designed to be a sleeping pill.
  • However it was also found to treat morning sickness in pregnant women.
  • Doctors didn't test to see what effect thalidomide had on pregnant women and the unborn child.
  • Over time, doctors found out that thalidomide did have an effect on the unborn child.
  • There were a lot of babies born with deformed limbs.
  • The drug was then banned.

Thalidomide today

Thalidomide is now used as a treatment for leprosy and bone cancer. Its use is heavily regulated, however, to prevent a repeat of the problems it caused in the last century.

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There are two types of recreational drugs. There are legal recreactional drugs which include caffiene, nicotine and alcohol. Also, there are illegal recreational drugs which include drugs such as cocaine, heroin, cannabis and ecstasy.

Why do people take recreational drugs?

  • To improve their mood and to make them happier.
  • To make them become more relaxed. 
  • To socialise


The alcohol in alcoholic drinks such as wines, beer and spirits is ethanol, which is a depressant, meaning that it slows down signals in the nerves and brain.

Small amounts of alcohol help people to relax, but greater amounts lead to a lack of self-control. Drinkers of alcohol may not realise how much they are consuming, and fall ill as a result. They may become unconscious, and may even fall into a coma.

Alcohol is addictive. Long-term effects of alcohol include damage to the liver and brain, and it is often the cause of weight gain. 

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About 114,000 people die every year as a result of smoking-related illnesses. All cigarettes sold now carry a prominent health warning.

Nicotine is the addictive substance in tobacco smoke. It reaches the brain within 20 seconds and creates a dependency so that smokers become addicted. Smoking increases the risks of:

  • heart disease and strokes
  • miscarriage, premature birth and low birth weight
  • lung cancer, mouth cancer and throat cancer.

Nicotine and alcohol are addicitive, you start to come dependant on their effects. If you stop taking them you suffer from withdrawal effects.

Cocaine, heroin and other illegal recreational drugs have much more sever withdrawal symptons such as nausea, headaches, physical pain etc...

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Illegal recreactional drugs

Heroine, cannabis and ecstasy have adverse effects on heart and circulatory system.

There is evidence that suggests that cannabis may lead to mental illnesses. Some people describe cannabis as being a gateway drug. Gateway drugs are drugs that are taken but leads on tomore powerful and harder drufs such as cocaine and heroin. 

Recreational drugs, both legal and illegal, may also damage our health indirectly. For example, buying the drugs reduces the amount of money available to buy food, and may place users in dangerous situations.

Injecting a drug using a needle and syringe that someone else has used may lead to a number of diseases from infected blood, including HIV and hepatitis. Users of illegal drugs may turn to crime to pay for their habit, and this affects the lives of other people.

Many more people use legal drugs, both prescribed and non-prescribed, than use illegal drugs. This means that the overall impact of health from legal drugs is greater than the overall impact of illegal drugs.

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Legal recreational drugs such as nicotine and alchol have a larger impact on the society than illegal drugs because more people take them. The long term effects cause health issues and these cost money to deal with and therefore it is a much bigger impact on the society.

Performance enhancing drugs

Athletes are often under great pressure to perform well. Some drugs are capable of artificially improving their performance; these include:

  • stimulants – these boost heart rate and other body functions
  • anabolic steroids – these stimulate the growth of muscles.
  • beta blockers – these help people stay calm and steady (can be used in sports such as snooker and shooting)

Some of these drugs are banned by law. Others are legally available on prescription or even in some medicines available from the pharmacy. However, the use of performance enhancing drugs is widely seen as unfair. They may also damage the athlete’s body.

Sporting regulations ban the use of performance enhancing drugs, and athletes are regularly tested to make sure that they are not using them.

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Each organism has certain characteristics that allow it to live sucessfully in its habitat. These features are called adaptations. Organisms within a community compete with each other for limited resources, including water and space. Plants also compete with each other for light and minerals.

Ecological words

  • Environment - all the conditions that surround a living organism
  • Habitat - the place where an organism lives
  • Population - all the members of a single species that live in a habitat
  • Community - all the populations of different organisms that live together in a habitat
  • Ecosystem - a community and its habitat

Organisms live in different habitats need different adaptations. Animals and plants may have specific features that adapt them to their environment: these include barbs and spines, poisons and warning colours that deter predators and herbivores. Some harmless species may even resemble a poisonous or dangerous species to increase their chances of survival.

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Habitats have limited amounts of the resources needed by living organisms and organisms can only survive if they can get enough of these resources, so they must compete for resources with other organisms. If they are unsuccessful and cannot move to another habitat, they will die.


The resources that animals compete for include:

  • food
  • water
  • space


Remember that plants make their own food using photosynthesis, so they do not compete for food. The resources that plants compete for include:

  • water
  • space
  • mineral salts.
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Cold Climates

The Arctic is cold and windy with very little rainfall. Plants in the Arctic often grow very close to the ground and have small leaves. This helps to conserve water and to avoid damage by the wind.

The polar bear

Polar bears are well adapted for survival in the Arctic. Their adaptations include:

  • a white appearance as camouflage from prey on the snow and ice
  • thick layers of fat and fur for insulation against the cold
  • a small surface area to volume ratio, to minimise heat loss
  • a greasy coat that sheds water after swimming
  • large furry feet to distribute their load and increase grip on the ice.
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Hot climates


  • large, flat feet to spread their weight on the sand
  • thick fur on the top of the body for shade, and thin fur elsewhere to allow easy heat loss
  • a large surface-area-to-volume ratio to maximise heat loss
  • the ability to go for a long time without water - they don't store water in their humps, but they lose very little water through urination and perspiration
  • the ability to tolerate body temperatures up to 42ºC
  • slit-like nostrils and two rows of eyelashes to help keep out sand.

Desert plants

Cacti are well adapted for survival in the desert. Their adaptations include:

  • stems that can store water
  • widespread root systems that can collect water from a large area.

In addition, cacti have spines instead of leaves. These minimise the surface area and so reduce water loss by transpiration. The spines also protect the cacti from animals that might eat them.

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Extremophiles are organisms that live in very extreme environments and can survive conditions that would kill most other organisms. The extreme conditions can include:

  • high temperatures
  • high concentrations of salt in water
  • high pressures.

Certain bacteria can live in hot springs or around deep-sea hydrothermal vents, where the water can be very hot. Certain plants grow well in salt marshes where the salt concentration is too high for most plants. For example, samphire looks a bit like a dandelion but can grow close to the sea shore.

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Environmental changes

Environmental change

Animals and plants are exposed to environmental change. These changes may be caused by living factors, such as a change in a predator, a food source or a competitor. Environmental changes may also be caused by non-living factors, such as a change in temperature or rainfall.

Changes in the environment affect the distribution and behaviour of living organisms. For example, humans have been cutting down trees for thousands of years. We do this to clear land for farming and building, and for wood to use as a fuel or building material. This deforestation can have local effects, such as a reduction in food and shelter for animals. It can also have more widespread effects, such as changes in rainfall and temperature. These changes may change the distribution of bird species, for example.

Predators and prey

If the prey population grows, predator numbers will respond to the increased food supply by increasing as well. Growing predator numbers will eventually reduce the food supply to the point where it can no longer sustain the predator population, and the number of predators will go down.

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Indicators of pollution

Air pollution

The most common source of air pollution is the combustion of fossil fuels. This usually happens in vehicle engines and power stations. Sulfur dioxide is released if the fuel contains sulfur compounds. This gas contributes to acid rain. Lichens can be used as air pollution indicators, especially of the concentration of sulfur dioxide in the atmosphere.

Lichens are plants that grow in exposed places such as rocks or tree bark. They need to be very good at absorbing water and nutrients to grow there. Rainwater contains just enough nutrients to keep them alive. Air pollutants dissolved in rainwater, especially sulfur dioxide, can damage lichens, and prevent them from growing. This makes lichens natural indicators of air pollution. For example:

  • bushy lichens need really clean air
  • leafy lichens can survive a small amount of air pollution
  • crusty lichens can survive in more polluted air.

In places where no lichens are growing, it is often a sign that the air is heavily polluted with sulfur dioxide.

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Measuring environmental changes


Rainfall is measured using a rain gauge. The depth of rain is usually measured daily, for example at the same time every morning.

A simple rain gauge consists of a funnel that empties into bottle. The daily contents of the bottle are poured into a measuring cylinder. This is calibrated so that it reads the depth of rainfall in millimetres.


An ordinary thermometer can be used to measure the temperature in an environment. Traditional maximum and minimum thermometers have a U-shaped tube. Each side contains a pin which moves inside with the liquid: one pin records the maximum temperature; and the other pin records the minimum temperature. After readings have been taken, the pins are reset using a magnet.

A digital thermometer connected to a data logger allows an almost continuous measurement of temperature over time. It also has the advantage that no one needs to be there to take a reading.

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Energy in biomass

Food chains

A food chain shows what eats what in a particular habitat. Pyramids of biomass reveal the mass of living material at each stage in a chain. The amount of material and energy decreases from one stage to the next. The arrows between each item in a food chain always point in the direction of energy flow - in other words, from the food to the feeder.

Radiation from the Sun is the ultimate source of energy for most communities of living things. Green plants and algae absorb some of the Sun’s light energy and transfer this energy to chemical energy. This happens during photosynthesis, and the chemical energy is stored in the substances that make up the cells of the plants or algae. The other organisms in a food chain are consumers, because they all get their energy and biomass by consuming (eating) other organisms.

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Words and their meanings

Common words and their meanings

  • Producers - green plants and algae, they make food by photosynthesis
  • Primary consumers - usually eat plant material - they are herbivores for example rabbits, caterpillars, cows and sheep
  • Secondary consumers - usually eat animal material - they are carnivores for example cats, dogs and lions
  • Predators - kill for food they are either secondary or tertiary consumers
  • Prey - the animals that predators feed on
  • Scavengers - feed on dead animals for example, crows, vultures and hyenas are scavengers
  • Decomposers - feed on dead and decaying organisms, and on the undigested parts of plant and animal matter in faeces
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Energy transfer

Energy is transferred along food chains from one stage to the next but not all of the energy available to organisms at one stage can be absorbed by organisms at the next one. The amount of available energy decreases from one stage to the next.

Some of the available energy goes into growth and the production of offspring. This energy becomes available to the next stage, but most of the available energy is used up in other ways. For example:

  • energy released by respiration is used for movement and other life processes, and is eventually lost as heat to the surroundings
  • energy is lost in waste materials, such as faeces.

All of the energy used in these ways returns to the environment, and is not available to the next stage. The animation shows how the level of available energy goes down as it is transferred through a temperate forest food chain.

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Pyramids of biomass

Biomass means the mass of living material at a stage in a food chain. Biomass decreases from one stage to the next, just like the amount of energy.

A pyramid of biomass is a chart, drawn to scale, showing the biomass at each stage in a food chain. The bars become narrower as you reach the top. This pyramid of biomass is for the food chain:

oak tree → caterpillar → blue *** → sparrowhawk

Note that you do not need to draw the organisms but you must draw your pyramid of biomass to scale. Each bar should be labelled with the name of the organism. In your examination, you will not be asked to draw a pyramid of numbers.

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The carbon cycle

Decay processes

Decay is an essential life process that digests food or waste matter and recycles materials. Materials from living things decay because they are digested (broken down) by microorganisms. These microorganisms cause decay by releasingenzymes that break down compounds to be absorbed by their cells.

 Bacteria and fungi are the main groups of decomposer.

Factors affecting decay

The factors that affect the rate of decay include moisture, temperature and amount of available oxygen.


Some of the substances released during decay are needed by plants for healthy growth. In a stable community of living things, processes that return substances to the environment (such as decay) are balanced by the processes that remove and use substances. In this way, the substances are continuously recycled.

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The carbon cycle

The carbon cycle shows how carbon moves from the atmosphere, through various animals and plants, then back into the atmosphere again.

Removing carbon dioxide from the atmosphere

Green plants and algae remove carbon dioxide from the atmosphere by photosynthesis. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants and algae.

Returning carbon dioxide back to the atmosphere

Organisms return carbon dioxide to the atmosphere by respiration. It is not just animals that respire. Plants, algae and microorganisms do too. Carbon dioxide is also released into the atmosphere when fossil fuels such as coal and oil, and wood, are burned.

Passing carbon from one organism to the next

When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these microorganisms and detritus feeders.

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Why organisms are different

The genetic information passed from parent to offspring is contained in genes carried by chromosomes in the nucleus. Sexual reproduction produces offspring that resemble their parents, but are not identical to them. Offspring resemble their parents because they contain genetic information passed on to them by their parents.

Chromosomes, found in the cell nucleus, contain many genes. A gene is a section of DNA, which carries coding for a particular protein. Different genes control the development of different characteristics of an organism. Many genes are needed to carry all the genetic information for a whole organism.

The number of genes and chromosomes varies from species to species. For example, cells in human beings have 46 chromosomes that carry about 30,000 genes in each cell; and cells in fruit flies have 8 chromosomes that carry about 13,600 genes.

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Inherited variation

Variation due to genetic causes is inherited variation. For example, children usually look a little like their father, and a little like their mother, but they will not be identical to either of their parents. This is because they get half of their inherited features from each parent.

Here are some examples of inherited variation in humans:

  • eye colour
  • hair colour
  • skin colour.
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Environmental factors

Environmental variation

Characteristics of animal and plant species can be affected by factors such as:

  • diet
  • accidents
  • culture
  • lifestyle
  • climate.

For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller as it tries to reach more light.

Variation caused by the conditions is called environmental variation. Here are some other examples of features that show environmental variation:

  • your language
  • your religion
  • flower colour in hydrangeas (as these plants produce blue flowers in acidic soil and pink flowers in alkaline soil).
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Combined genetic and environmental factors

Some features vary because of a combination of genetic causes and environmental causes. For example, identical twins inherit exactly the same genetic information from their parents. But if you take a pair of twins, and twin A is given more to eat than twin B, twin A is likely to end up heavier.

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Sexual reproduction produces offspring that resemble their parents, but are not identical to them. Asexual reproduction produces offspring - clones - which are genetically identical to their parents.

Plants can be cloned artificially using cuttings or tissue culture. Animals can be cloned using embryo transplants or fusion cell cloning. Genetic information from one species can be transferred to another species using genetic modification.

Sexual reproduction

Organisms have sex cells called gametes. In human beings, the male sex cells are called s and the female sex cells are called eggs or ova.

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Sexual reproduction


Sexual reproduction happens when a male gamete and a female gamete join. This fusion of gametes is called fertilisation. Sexual reproduction allows some of the genetic information from each parent to mix, producing offspring that resemble their parents but are not identical to them. In this way, sexual reproduction leads to variety in the offspring. Animals and plants can reproduce using sexual reproduction.

In human beings, each gamete contains 23 chromosomes, half the number found in the other cells of the body. When the male and female gamete fuse, the new embryo contains the full 46 chromosomes – half from the father and half from the mother.

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Asexual reproduction

Asexual reproduction only needs one parent, unlike sexual reproduction, which needs two parents. Since there is only one parent, there is no fusion of gametes and no mixing of genetic information. As a result, the offspring are genetically identical to the parent and to each other: in other words, they are clones.


Asexual reproduction in plants can take a number of forms. Many plants develop underground food storage organs that later develop into the following year’s plants: potato plants and daffodil plants are examples of this.


Asexual reproduction in animals is less common than sexual reproduction but does happens in sea anemones and starfish, for example.

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Artificial cloning in plants

People may want to clone a plant deliberately, that is produce lots of identical new plants. Cloning of plants has many important commercial implications: it allows a successful variety of a plant to be produced commercially and cheaply in a short space of time and on a massive scale.


The simplest way to clone a plant involves taking a cutting. A branch from the parent plant is cut off, its lower leaves removed and the stem planted in damp compost. Plant hormones are often used to encourage new roots to develop. The cutting is usually covered in a clear plastic bag at this stage to keep it moist and warm. After a few weeks, new roots develop and a new plant is produced. The method is easy enough for most gardeners to do successfully.

Tissue culture

Another way of cloning plants is by tissue culture, which works not with cuttings but with tiny pieces from the parent plant. Sterile agar jelly with plant hormones and lots of nutrients is needed. This makes tissue culture more expensive and difficult to do than taking cuttings.

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Artificial cloning in animals

Embryo transplants

A developing embryo is removed from a pregnant animal at an early stage, before the embryo’s cells have had time to become specialised. The cells are separated from one another. They are then grown for a while in a laboratory and transplanted into host mothers.

When the offspring are born, they are identical to each other and genetically related to the original pregnant animal. They are not related to their host mothers because they contain different genetic information.

Adult cell cloning

  1. the nucleus is removed from an unfertilised egg cell and discarded
  2. the nucleus is removed from an adult body cell and injected into the egg cell
  3. an electric shock is applied to make the egg cell begin to divide to form an embryo
  4. while it is still a ball of cells, the embryo is inserted into the womb of an adult female
  5. the embryo continues to grow and develop

The new individual is genetically identical to the animal that donated the nucleus from one of its body cells.

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Genetic modification

Genetic modification is different from cloning because in cloning they produce exact copies and in genetic modification they produces a unique set of genes. Also in cloning the genes are copied within the same species and in genetic modification the genes can be swapped across species.

How it works

Certain enzymes can cut pieces of DNA from one organism, and join them into a gap in the DNA of another organism. This means that the new organism with the inserted genes has the genetic information for one or more new characteristics. For example, the organism might produce a useful substance, or be able to carry out a new function. We say that the organism has been genetically modified.

Genetic modification works in animals, plants and microorganisms. For example, new genes can be transferred to crop plants to make GM crops. Some GM crops are resistant to certain herbicides (weed killers) while others are resistant to insect pests.

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Problems with genetic modification

There are strong arguments for and against genetic modification of crop plants. Genitically modified crops generally have increased yields, useful for feeding a growing population. Tobacco plants that glow in the dark when they need watering have even been produced.

However, some people are excited by the almost limitless possibilities of genetic modification, while others believe the process is unethical and should be banned. There are concerns about the effect of GM crops on wild flowers and insects, and whether eating GM food may harm human health.

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Darwin's theory of evolution explains how life on Earth has changed over geological time. Scientists believe this is the reason why all living things on Earth exist today. The theory is supported by evidence from fossils, and by the rapid changes that can be seen to occur in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past and the extinction of species continues to happen.

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Theories of evolution

Darwin's theory

Charles Darwin is famous for his theory of evolution, but he was not the only person to develop such a theory.

Darwin's theory caused controversy among his contemporaries and his ideas were only gradually accepted, although some people still do not believe them today. The reasons for skepticism include:

  • Darwin’s theory conflicted with religious views that God had made all the animals and plants on Earth
  • Darwin did not have enough evidence at the time to convince many scientists
  • it took 50 years after Darwin’s theory was published to discover how inheritance and variation worked.
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Theories of evolution

Lamarck's theory

Jean-Baptiste Lamarck was a French scientist who developed an alternative theory of evolution at the beginning of the 19th century. His theory involved two ideas:

  1. A characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears
  2. Any feature of an organism that is improved through use is passed to its offspring.

However, we now know that in most cases this type of inheritance cannot happen.

Lamarck's theory cannot account for all the observations made about life on Earth. For instance, his theory implies that all organisms would gradually become complex, and simple organisms disappear. On the other hand, Darwin's theory can account for the continued presence of simple organisms.

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Natural selection

The theory of evolution states that evolution happens by natural selection. The key points are that:

  • 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.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed on to the next generation. Given enough time, a species will gradually evolve.

You need to remember that variation can be caused by both genes and the environment. But it is only variation caused by genes that can be passed on to the next generation.

Life on Earth today exists because of the conditions that were present when life was evolving. If in the past the conditions had been different, evolution by natural selection could have produced some very different results. For example, if the Earth had been hotter, colder, bigger or smaller, our bodies would have changed to suit those conditions.

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Evidence for evolution: rapid changes

Mutations cause changes in genes. When new forms of a gene appear because of mutation, a rapid change in a species may happen if the environment changes. Such rapid changes support the theory of evolution by natural selection.

Peppered moths

Before the industrial revolution in Britain, most peppered moths were of the pale variety, meaning that they were camouflaged against the pale birch trees that they rest on. Moths with a mutant black colouring were easily spotted and eaten by birds. This gave the white variety an advantage and they were more likely to survive to reproduce.

Airborne pollution in industrial areas then blackened the birch tree bark with soot. This meant that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. The black variety now had the advantage and were more likely to survive and reproduce. Over time, black peppered moths have become far more numerous in urban areas.

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