B3- life on earth

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Classification and species

Similarities and differences between living things on earth can help us put them into groups, this process is classification.

Species- A group of organisms that breed together to produce fertile offspring. e.g. horse and donkey breed together to produce a mule, which is infertile. Therefore horses and donkeys are different species.

A mule. Credit: Dorling Kindersley (http://www.bbc.co.uk/schools/gcsebitesize/science/images/21c_mule.jpg)

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Adaptations- Feature that help an organism survive in the environment it lives in.

Adaptations increase the organisms chance of survival. If chances of survival are increased so are  the chances of the organism sucessfully reproducing.


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Food webs

All of the food chains in a hapitat join up to form a food web.

Interdependance of living things - How the loss of one organism can effect the other organisms in the web.


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Organsims are dependent on their environment and other species for their survival. When one changes the organisms are not as well adapted.

Organisms that are less adapted to their environment are less likely to survive and reproduce than those who are well adapted. It is possible that poorly adaptedspecies will become extinct.

Causes of extinction include:

  • Rapid changes of the environment e.g. climate
  • New diseases
  • New predators
  • New competitors

Changes in one part of the web can have a big impact on other parts of the web e.g. if one food source disapears a predator will have to find another source of food in order to survive.

Fossil record show that since life on earth began many species have become extinct. Extinction is still occuring, partly because of human activity, because we compete with others for food, space and water and are very sucessful predators.

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The Guagga and the dodo

The last guagga alive in the wild was shot in the late 1870s. The guagga became extinct because of the direct activity of humans.

The dodo was a large flightless bird. The groups of island it lived on was uninhabited up until 1638, when it became colonised by the dutch. Before they had had no natural predators. The human hunted the birds, which were easy to catch and brang other species such as pigs, rats and cats, which hunted the dodos and ate their eggs. Together this resulted in the extinction of the Dodos. Humans were indirectly responsiblefor the dodos extinction.

The Dodo, a distant relative of the pigeons, is heavily-built, roughly the size of a swan and flightless (http://www.bbc.co.uk/schools/gcsebitesize/science/images/bidodo_226.jpg)

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The sun

Nearly all organisms are dependant on energy from the sun.

Plants harness light energy, from the sun for photosynthesis. By the process of photosynthesis compounds such as glucose are made from carbon dioxide and water, using the energy.

Plants only absorb a small percentage of the suns energy for photosynthesis. The energy is absorbed by chemicals that make up the plants cells.


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Energy transfer

Animals can't make their own food so they have to eat. This is one way in which energy is transferred between organisms in an ecosystem. The energy is used for a few life processes.

In a food chain only around 10% of the energy is passed onto the next level. The rest of the energy passes out of the chain by:

  • Heat energy
  • Life processes e.g movement
  • Uneaten parts that pass to decomposers
  • Excreted and passed to decomposers

As less energy is transferred at each level of the food chain, the number of organisms on each level gets smaller.

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Percentage efficiency of energy transfer example

Total energy that came into level of food chain: 5,000kj. Amount of energy transferred to the next level: 500kj. Calculate the efficency using the equation: (Energy transferred to next level ÷ Total energy in) X 100. (500kJ ÷ 5,000kJ) X 100 = 10%

The percentage efficiency of energy transfer between producers and primary consumers in the ecosystem below is 10%.

Energy flow through an ecosystem (http://www.bbc.co.uk/schools/gcsebitesize/science/images/21c_bio_energy-ecosystem.jpg)

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The carbon cycle

All cells, animal plant or bacteria contain carbon because they all contain proteins, fats and carbohydrates. e.g. plant cell walls are made of cellulose- a carbohydrate.

Carbon is passed from the atmosphere, as carbon dioxide, to living things, passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again. This is known as the carbon cycle.

Removing carbon dioxide from the atmosphere:

  • Photosynthesis-The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.

Returning carbon dioxide to the atmosphere:

  • Respiration- Organisms return carbon dioxide to the atmosphere. Respiring organisms include plants, animals and microorganisms.
  • Combustion
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The carbon cycle continued

Passing on carbon:

1.When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal.

2.Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants.

3.The carbon then becomes part of these microorganisms and detritus feeders.

4.Materials from living things decay because they are digested by microorganisms. This process happens faster in warm, moist conditions with plenty of oxygen. Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen

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Nitrogen cycle

Organisms need Nitrogen to make proteins. They cannot get nitrogen from the air, because nitrogen gas is fairly unreactive. Plants can take nitrogen compounds such as nitrates and ammonium salts from the soil.

Nitrogen fixation- making nitrogen compounds from nitrogen in the air, it happens in 3 ways:

  • The energy in lightening splits nitrogen molecules into individual atoms, which react with oxygen to form nitrogen oxides. Nitrogen oxides are washed to the ground by rain, where they form nitrates in the soil.
  • Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants, such as peas, beans and clover, fix nitrogen gas into nitrogen compounds.
  • The Haber process is used by industry to produce ammonia from nitrogen and hydrogen. Ammonia is used to make nitrogen compounds that are used as fertiliser by farmers.
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Nitrogen cycle continued

Nitrogen compounds in living things are returned to the soil through:

  • excretion and egestion by animals
  • the decay of dead plants and animals.

Denitrifying bacteria present in soil break down nitrogen compounds and release nitrogen gas into the air.

As a result of these processes, nitrogen is cycled continually through the air, soil and living things. This is called ‘the nitrogen cycle’.

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Environmental change

Environmental change can be measured using non-living indicators including nitrate levels, temperature and carbon dioxide levels.

Environmental change can be measured using living indicators including phytoplankton, lichens and aquatic river organismssuch as mayfly nymphs.

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The earth began approximately 3500 million years ago.

The main theory is that living things developed from molecules that could replicate, or copy, themselves, rather like DNA does.

There are two possible origins for these replicating molecules:

  • they were produced by the conditions on Earth at the time
  • they came from somewhere else, such as another planet in our Solar System, or further out in space.
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Darwins theory of evolution

His ideas caused alot of controversy because the conflicted with religious views.

  • All species have evolved from simple forms
  • Evolution happens by natural selection
  • Individuals in a species show a wide range of variation because of differences in their genes
  • Individuals with characteristics most suited to their environment are most likely to survive and reproduce
  • The characteristics that allowed them to succeed are passed to their offspring

Poorly adapted individuals are less likely to survive and reproduce, so their genes are less likely to be passed onto the next generation. In time a species will evolve.

Variation can be caused by both genes or environment but only genetic variation can be passed on to the next generation.

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Selective breeding

Selective breeding only occurs when humans intervene.

  • We choose the characteristics we want in an animal or plant.
  • We breed together a male and a female showing these characteristics.
  • From the offspring produced we select those that show the characteristic the most and breed them together.
  • This process is repeated over generations, each time selecting and breeding the animals or plants that have the chararectoristics we are looking for.

Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.Image showing dogs sitting in a line. They are all bred from a common ancestor (http://www.bbc.co.uk/schools/gcsebitesize/science/images/21c_theoryevolution_dogs.jpg)

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Mutations are changes that occur in genes, they are random and can be caused by background radiation or chemicals we come into contact with.

Sometimes they can be severe and the cell dies, sometimes the cell can divide uncontrollably and become cancerous, sometimes the changes are small and the cell survives. Rarely, the changes produce characteristics that are beneficial to us.

If the changes occur in normal body cells, they are lost when we die. But if they occur in sex cells, there is the possibility that the changes in the gene will be passed onto the next generation. Natural selection ensures that they are selected if they are useful, or disapear from the gene pool if they are not.

The combined effect of mutations, environmental changes and natural selection can sometimes produce changes that are so great that a new species is produced. This is rare and only occurs when the mutated organism can no longer breed with the original species and is capable of producing fertile offspring.

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Explanations for evolution

Some people find it hard to accept that such complexity in living organisms could have evolved from natural selection. Some religious people believe that all living things on earth were made by 'god', or that life was begun by 'god' but then evolved through natural selection .


  • The law of use and disuse
  • The law of inheritance and aquired characteristics

His theory stated that characterisitcs used more by an organism become bigger and stronger and those not used disapear. Any improved features are passed onto offspring.

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Lamarck vs Darwin


  • A giraffe stretches its neck to reach food high up.
  • The giraffes neck gets longer because it is used a lot.
  • The giraffes offspring iherit its long neck.


  • A giraffe with a longer neck can reach food higher up.
  • It is more likely to get enough food to survive and reproduce.
  • The giraffes offspring inherit its long neck.

Lamarcks theory does not account for all of the obesevations made about life on earth. For example his theory predicted that all organisms gradually become complex and simple organisms disapear.

Darwins theory can account for the continued presence of simple organisms.

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Most of the evidence for evolution comes from the fossil record. They show how much or how little fossils have changed over time. One problem with the fossil record is that it contains gaps, not all organsims follilise well and many fossils are destroyed by the movements of the earth or have simply not been discovered.


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Organisms are classified into different groups. This classification is done according to similarities and differences in characteristics including:

  • physical features
  • DNA.

Organisms are classified at different levels. These levels can be arranged in an order, progressing from:

  • a large group containing many organisms with a small number of characteristics in common, such as a kingdom
  • a small group containing fewer organisms with more characteristics in common, such as a species.

This classification of both living and fossil organisms helps scientists to make sense of the diversity of organisms on Earth, as well as to suggest evolutionary relationships between them.

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Biodiversity- having a wider range of different species as possible.

Maintaining biodiversity is an important part of using the environment in a sustainable way. Indiscriminate use of the environment, for example cutting down large areas of the rain forest to grow crops such as soya, results in a large number of species becoming extinct and reduces biodiversity.

It is only now that we are beginning to realise the potential medicines and crops that we can obtain from a wide range of different organisms. Only by maintaining biodiversity can we be sure that these benefits will be available for future generations.

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Biodiversity- having a wider range of different species as possible.

Maintaining biodiversity is an important part of using the environment in a sustainable way. Indiscriminate use of the environment, for example cutting down large areas of the rain forest to grow crops such as soya, results in a large number of species becoming extinct and reduces biodiversity.

It is only now that we are beginning to realise the potential medicines and crops that we can obtain from a wide range of different organisms. Only by maintaining biodiversity can we be sure that these benefits will be available for future generations.

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Continuous production of one type of crop, that is often genetically uniform.

Doing this in a large field means harvesting by machinery can be done efficiently.

There are disadvantages to monoculture. If pests and disease attacked the crop it could harm it easily, so farmers use a lot of chemical pesticides. This can harm the environment and so is not viewed as sustainable. Using large fields and pesticides reduces the variety of species. This hinders biodiversity.

Another disadvantage is that if a natural disaster were to occur, the whole crop could be wiped out.

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Improving sustainability

Packaging can be a major environmental problem. The use of biodegradable packaging is an option to tackle this problem. When put in a landfill site it should decompose quickly, unlike a lot of currently used packaging.

However, to decompose in a way that only produces carbon dioxide oxygen needs to be present, but in landfill oxygen is often not present. This results in the production of a powerful greenhouse gas methane being produced if decay occurs at all.

Another possible solution is to recycle the packaging. The materials and energy used in production, energy used in transport and pollution created are all considerations when trying to improve sustainability.

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