Biology Revision

Biology revision


Genetic variation and its control


Genes carry information from parents. They are passed on in gametes (sex cells) and carried on chromosomes. They control our characteristics.


There are only a few features which are entirely inherited - eye colour, genetic diseases, blood group and hair colour. There are many environmental features such as skin colour, IQ, diseases, hairstyles, tatoos and scars.

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Genetic variation and its control

Sexual reproduction

In this type of reproduction there are two parents so therefore there is variety in the offspring. The plant gametes are the egg cell and pollen, and the animal gametes are the egg cell and sperm. An advantage of sexual reproduction is that the species can adapt to survive, but a disadvantage is that it relies on gametes meeting.

Asexual reproduction

This involves just one parent so there is no variety in the offspring. The offspring are known as clones. Asexual reproduction occurs mainly in bacteria, cells, small animals and plants such as the strawberry plant. An advantage is that it is very safe as there is only one parent, but a negative is that the species cannot adapt to survive.

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Genetic variation and its control

Advantages of Genetic Modification

  • Genetically engineered pest resistance
  • Genetically engineered herbicide tolerance
  • Extra nutritional crops
  • Withstanding drought or high salt
  • Cold tolerance
  • Genetically engineered disease resistance

Disadvantages of Genetic Modification

  • Costly for companies and customers
  • Insects may develop pest resistance
  • Genetically engineered herbicide tolerance may pass on to weeds
  • Allergy risks
  • Unknown effects on human health
  • May harm organisms that feed on it
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The Carbon Cycle

Carbon Cycle

Carbon dioxide is removed from the atmosphere by plants for photosynthesis. The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins, which make up the body of plants.

When plants respire, some carbon dioxide is released into the atmosphere. When plants are eaten by animals, which are eaten by other animals, some carbon becomes part of the fats and protein in their bodies. When the animals respire, carbon dioxide is released into the atmosphere.

When plants and animals die, some animals feed on their bodies. When these animals respire, some carbon dioxide is released.

By the time decay is complete on dead plants and animals, all the energy originally absorbed by the plants has been transfered, Burning wood and fossil fuels also produces carbon dioxide.

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The Carbon Cycle


carbon dioxide + water (+ light energy) -----> glucose + oxygen


glucose + oxygen -----> carbon dioxide + water (+ energy)

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Decay processes

Products of organisms

Animals produce droppings which return to the environment. Plants leaves fall off and are decomposed in the environment.


Decomposers are organisms which feed on waste products and dead animals. These can be bacteria or fungi. They release CO2, water and minerals as waste, and plants use these to grow and develop

Detritus Feeders

These eat dead animals and produce waste. When the detritus feeders die, they are eaten by decomposers, along with their waste products. Examples are maggots and some worms.

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Decay processes

Decay conditions

Decay is faster in warm conditions, but slows down and stops in cold conditions. If it is too hot, the enzymes don't work. Decay also needs moist, oxygenated conditions.

Importance of decay

Decay is vital for recycling natural resources. It helps us recycle waste, and rots material in compost heaps, which can then be used as plant fertiliser. It also breaks down bodily waste in the sewers, so it is safe to wash into the sea.

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Energy in Biomass


Biomass is mass of living material. It decreases along a food chain.

Pyramids of Biomass

An example:


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Environmental Change

Distribution of Organism

This is affected by:

  • changes in competitor and predator
  • outbreak of disease
  • change in habitat


Trees take in carbon dioxide and give out oxygen. This keeps the right level in the atmosphere. Forests also store water because the leaves slow down evaporation. Deforestation, for timber, paper, farming and building land, disrupts the level of gas and water.

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Environmental Change

Global Warming

Earth radiates energy towards space but it is absorbed by greenhouse gases instead.The atmosphere absorbs too much heat, so it is warmer. This increases the greenhouse effect, sea levels rise, and there are changes in wind patterns and rainfall.

Chemicals causing Eutrophication

Fertiliser is used to grow crops and is leached (drained) out of the soil and into rivers after heavy rain. This causes algae and weeds to die and rot, so that bacterial decomposers thrive in the river bed. The bacteria use oxygen so that other water animals die of oxygen starvation.


Chemicals e.g. DTT are used to kill insects and stop a decrease in crops. These are dangerous and don't break down in animals so they pass along the food chain. Therefore they stay in the atmosphere.

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Plant Competition

Plants compete for:

  • Light
  • Space
  • Water
  • Nutrients

Animal Competition

Animals compete for:

  • Food
  • Water
  • Mates
  • Territory
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An adaptation is a feature which enables an organism to survive in the conditions of its usual and natural habitat.


Extremophiles are organisms which live in extreme environments. This may mean:

  • High salt
  • High pressure
  • High temperature
  • Low temperature
  • High levels of radiation
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Animal adaptations

There may be adaptations to:

  • Legs
  • Teeth
  • Ears
  • Paws
  • Colour
  • Fur
  • Fat
  • Eyes
  • Permeability
  • Hair
  • Mouth
  • Storage
  • Nose
  • Water Loss
  • Size
  • Shape
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Plant Adaptations

There may be adaptations to:

  • Stem
  • Leaves
  • Cuticle
  • Roots
  • Shape
  • Surface
  • Colour
  • Products e.g. CO2
  • Growth
  • Seeds
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A drug

A drug is a substance or chemical which alters the way in which the body works.


Alcohol is a depressant. It affects:

  • Heart and circulation
  • Mental health
  • Lungs
  • Liver
  • Fertility
  • Stomach
  • Bones
  • Intestines
  • Pancreas
  • Kidneys

It can also cause brain damage, cancer and antisocial behavior.

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Nicotine is smoked in cigarettes which contain tar and other chemicals. It affects unborn babies, circulation, the brain, the lungs, skin, fingers and teeth, fertility and the immune system. It can cause emphysema, cancer and heart disease.

Types of drugs

Experimental - A drug is tried a few times to see what it is like

Recreational - A drug is used regularly but in a 'controlled' way

Dependent - A drug is addictive and your body needs it

Stimulant - Speeds reactions up

Depressant - Slows reactions down

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Control in Plants


The hormone which controls plant growth.

Phototropism (response to light)

Auxin is produced at the tip of the shoot. It is stimulated on the shady side of the root. The cells on this side elongate and the shoot bends towards the light.

Geotropism (response to gravity)

Auxin stimulates cells to elongate on lower side of shoot, which therefore bends up, away from gravity.

Auxin slows down cell growth on lower side of root, meaning cells on upper side grow quicker and root bends down, towards gravity.

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Control in Plants

Hydrotropism (response to moisture)

Auxin is produced on one side of the root so cells elongate to bend towards moisture. The side auxin is stimulated on can change according to where the moisture is.

Ripening with auxin

Auxin is sprayed on unpollenated flowers, which then ripen without any seeds or pips.

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The Nervous System and Control in the Body

Receptors and stimuli

  • Eyes - colour, light, shape, motion etc.
  • Ears - Pitch, vibration, amplitude, sound, pressure (balance)
  • Nose - Smell
  • Skin - Pain, temperature, pressure, texture
  • Tongue - taste

Reflex arc

The stimulus is detected by the receptor, which sends an impulse down the sensory neurone. the impulse passes across the synapse to the inter neurone, then down this across the next synapse. The impulse travels along the motor neurone to the effector, a muscle or a gland, which stimulates the response

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The Nervous System and Control in the Body


The impulse hits the synaptic vesicles, and changes into a chemical called a neurotransmitter. This diffuses across the gap and the receptors of the next neurone change it back into an electrical impulse.

Controlling internal conditions

Temperature - Maintain at 37C so enzymes work properly. Cool down by sweating and warm up by shivering.

Blood Sugar - Controlled by pancreas. Cells provided with constant supply of energy.

Ion Content - Lost when sweating and in urine.

Water Content - Lost in exhalation, sweating and from kidneys.

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How our bodies defend themselves against infection


Pathogens are microbes that cause diseases.

The three types are bacteria, virus and fungus.

They produce toxins - the symptoms that make us ill.


Viruses inject into our cells. When our cells reproduce, the virus is also reproduced. The virus then breaks out, killing ther cells in doing so. We cannot treat viruses with antibiotics because killing the virus would kill the cells.

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How our bodies defend themselves against infection


Antibiotics kill bacteria in the body which causes diseases. They destroys the bacteria but not our cells. They cure diseases like plagua and TB, but have no effect on viruses.

Defence Barriers

  • Mucus/ lining of airway
  • Skin/scabs
  • Stomach acid
  • White Blood Cells
    • They ingest pathogens
    • They produce antibodies to destroy each type of pathogen
    • they produce antitoxins to neutralise the toxins which make us ill
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The Nervous system

Parts of the Nervous System

The nervous system consists of...

- Brain

- Spinal Cord

- Spinal Nerves

- Receptors

It allows organismms to react to their surroundings and to co-ordinate their behaviour.

Information from receptors passes along neurones (nerve cells) to the brain which co-ordinates the response.

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The Nervous System

Nervous System Flow

The Nervous System goes from the....

  • Receptor,
  • Sensory Neurones,
  • Relay Neurones,
  • Spinal Cord,
  • Brain,
  • Motor Neurones,
  • Effector.
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The Nervous System


There are 3 different types of neurones:

  • Motor Neurone

Where impulse travels away from cell body

  • Sensory Neurone

Where impulse travels towards the cell body

  • Relay Neurone

Where impulse travels away and towards the cell body

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The Nervous System


Neurones are streched to make connections between parts of the body!

Neurones do not touch each other, there is a very small gap between them called a synapse.

When an electrical impulse reaches a synapse via 'Neurone A' a chemical transmitter is released which activates receptors on 'Neurone B' and causes a electrical impulse to be generated in 'Neurone B' .

The chemical transmitter is then destroyed.

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The Nervous System

Types of Receptor

Different receptors detect different stimuli...

- Eyes detect ... Light

- Ears detect ... Sound and Position/Balance

- Nose detects ... Smell

- Mouth detects ... Taste

- Skin detects ... Touch, Pressure, Temperature and Pain.

Reflex Action

Sometimes concious actions are too slow to prevent the body from harm...

Therefore, reflex actions take place, reactions that bypass the brain.

Reflex Actions are Automatic and Quick!

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The Nervous System


A 'synapse' is the junction or gap between two neurones.

When an impulse is sent from one neurone to another, in the synapses a chemical substance is secreted out of the neurone on it's way to the other neurone. This chemical substance is called 'Neurotransmitters'.

There are 3 main neurones in the brain and they are:

  • Sensory Neurone: Carries signals from the sense organ (receptor) to the Central Nervous System (CNS).
  • Relay Neurone: Carries messages to one part of the CNS to the other. E.g: The spinal cord to the brain, vice versa
  • Motor Neurone: Carries signals from the CNS to the effector.
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The Nervous System

Needed Knowledge

  • The CNS: The brain and the spinal cord.
  • Synapse: A junction between 2 neurones.
  • Electrical Impulses: How signals within a nerve cells.
  • Neurones: Are nerve cells that carry information as tiny electrical signals.
  • Neurotransmitter: Chemical substances that pass through the synapses.
  • Axon: Is the long part of the neurone that carries the signal.
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The Blood

Compostition of Blood

The Red Blood Cells:

  • Carry oxygen from the lungs to other parts of the body. It contains a red pigment called 'Haemoglobin' which gives blood it's red colour.


  • Are tiny fragments of cells that help clot blood when the skin is cut open.

White Blood Cells:

  • Are part of our Immune System, which help fight diseases in the body.


  • The liquid part of the blood. It's yellowy in colour and mostly made up of water and dissolved substances.
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The Bodies Defences

Spreading Diseases

  • Air borne: diseases spread through droplets in the air
  • Direct contact: touching an infected person
  • Indirect contact: using or sharing an object an infected has used
  • Vector borne: an animal or insects is carry the microbe

Vertical and Horizontal Infections:

1. Vertical: Is when a pregnant woman can pass on her diseases to the unborn child through the placenta.

2. Comin in contact with an infected person, by swap bodyliy fluids

E.g: Sexual intercourse, kissing etc...

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The Bodies Defences

Keeping Microbes Out

First Line of Defence:

  • Platelets
  • Hairs
  • Ear Wax
  • White Blood Cells
  • Watery Eye (Lysozyme)
  • Reflexes
  • Hydrochloric acid in the stomach
  • Mucus
  • Antibodies

3 ways of defending the body:

1. Physical and chemical barriers

2. Non-specific Immunity: White Blood Cell + Inflammation

3. Specific Immunity: Specialised White Blood Cell + The Immune System

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Darwin's Theory

Darwins Theory

-Charles Darwin :- there is much variation in a species

-more offspring are produced than the environment can support (food and breading ares)

-Only the ones best suited or adapted will survive and breed. this i known as natural selection or 'survial of the fittest'

-Those that survie passs their genes onto their offspring. eventually the species which is less suited will become extinct.

Dawins theory was accepted because :- religion had an important place in society. - it is difficult to prove. - many scientists didn't accept the theory. -attempts to demonstrate evolution through tests have failed.

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