AQA biology unit 2

bite size revision chunks for anyone studying biology gcse, only unit 2.

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  • Created by: india
  • Created on: 13-01-09 21:08


  • cells are living things, they constantly have to replace substances which are used up (e.g. food and oxygen) and remove other substances which would accumulate (e.g. carbon dioxide, waste products)
  • Diffusion takes place automatically without the need for energy.
  • Diffusion is the spreading of the particles of gas which results in a net movement from a higher concentrated area to a lower concentrated area.
  • The greater the difference in concentration, the faster the rate of diffusion.
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  • Osmosis is the movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
  • The membrane allows the water molecules through but not the solute molecules as they are too large.
  • The effect of Osmosis is to gradually dilute the solution. A good example is hair root cells.
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  • Green plants make their food using sunlight, this is called photosynthesis.
  • Four things needed are:

- light from the sun

- carbon dioxide diffused from the air

- water from the soil

- chlorophyll in the leaves

  • Two things produced:

- glucose - for biomass and energy

- oxygen - released into the atmosphere as a by-product.

  • Word equation for photosynthesis:

carbon dioxide + water --> glucose + oxygen

  • Light energy is absorbed by green chlorophyll. Some of the glucose is used immediately by the plant to provide energy via respiration. Most is converted into insoluble starch which is stored in the stem, leaves or roots.
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Factors Affecting Photosynthesis

  • Temperature:

- As temperature rises, so does the rate of photosynthesis.

- As the temperature approaches 45C the enzymes are destroyed -rate drops.

  • Carbon dioxide concentration:

- As the carbon dioxide concentration rises, so does the rate of photosynthesis

- Rise in carbon dioxide leads to have no effect.

  • Light intensity:

- As the light intensity increases so does the rate of photosynthesis.

- Rise is light intensity leads to have no effect.

All of these, at a particular time, may be the limiting factor.

Photosynthesis (therefore, growth) can be controlled in a greenhouse, where temperatures are increased, carbon dioxide levels are increased and light intensity is increased.

Plants also require nitrates and magnesium (mineral ions) for healthy growth.

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Food chains

Radiation from the sun is the source of energy for all communities on living organisms.

  • In green plants, photosynthesis captures a small fraction of solar energy which reaches them.
  • Energy is stored in the substances which make up the cells of the plant and can be passed onto organisms which eat the plant.

Grass --> Rabbit --> Stoat --> Fox

Pyramids of Biomass

  • The mass of living material (biomass) at each stage of the food chain is less than it was at the previous stage.
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Transfer of Energy and Biomass

  • Biomass and energy are lost at every stage of the food chain - mostly in waste.
  • Energy released through respiration is used up in movement and lost as heat energy.
  • Particularly true in warm-blooded animals, whose bodies must be kept at a constant temperature.

Improving the Efficiency of Food Production

  • We can improve the efficiency of food production by reducing the number of stages in a food chain or by limiting an animals movement and controlling its temperature.

grass --> cow --> human

is not as efficient as

vegetables --> human

Exam tip! : may have to evaluate the pros and cons of rearing animals or the use of battery farming.

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Recycling the Materials of Life

  • Living things remove material from the environment for growth, but when they excrete waste or die, these materials are returned to the environment.
  • This is helped by microorganisms which break down the waste and dead bodies.
  • This decay process releases substances used by plants for growth.
  • Microorganisms digest materials faster in warm, moist conditions where there is plenty of oxygen.
  • Humans also use microorganisms in

- sewage works to break down human waste

- compost heaps to break down plant material waste.

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The Carbon Cycle

  • The processes which remove materials are balanced by processes which return materials.
  • This constant recycling of carbon is called the Carbon Cycle.
  • Carbon dioxide is removed from the atmosphere by green plants. Some is returned to the atmosphere by the plants during respiration.
  • The carbon obtained by photosynthesis is used to make carbs, fats and proteins in plants. When plants are eaten this carbon becomes carbs, fats and proteins in animals.
  • When plants/animals ie, other microorganisms feed on their bodies causing them to break down.
  • As the detritus feeders and microorganisms eat the dead plants, they release carbon dioxide into the atmosphere.
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  • Enzymes are biological catalysts.
  • They increase the rate of chemical reactions in an organism.
  • Enzymes are protein molecules made up of long chains of amino acids.
  • Each enzyme has a different substrate, which fits onto the complimentary active site. When it joins, it is known as an enzyme substrate complex. This lowers the activation energy of the reaction.
  • Different enzymes work best at certain pH levels and temperatures.

High temperatures destroy most enzymes' special shape, this is why it is dangerous for a humans temperature to go above 37C.

Inside living cells

Enzymes in living cells catalyse (speed up) processes such as respiration, protein synthesis and photosynthesis.

Energy released during respiration is used to build larger molecules, enable muscles to contract, maintain a constant temperature in colder surroundings and make proteins in plants from amino acids.

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Aerobic Respiration

  • Aerobic respiration releases energy through the breakdown of glucose molecules by combining them with oxygen inside living cells.
  • The written equation for aerobic respiration is:

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

Outside Living Cells

  • Digestive enzymes are produced by specialised cells in glands in the digestive system.
  • The enzymes pass out of the cells into the digestive system where they come into contact with food molecules.
  • Three enzymes - protease, lipase and amylase - are produced in four separate regions of the digestive system. they digest proteins, fats and carbs to produce smaller molecules which can be absorbed.

Amylase - digests starch - prodcues sugar molecules - happens in: salivary glands, pancreas and small intestine.

Protease - digests proteins - produces amino acids - happens in: stomach, pancreas and small intestine.

Lipase - digests lipids - prodcues fatty acids and glycerol - happens in: small intestine and pancreas.

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The Function of Bile

  • Bile is produced in the liver and then stored in the gall bladder before being released into the small intestine.
  • Bile has two functions:
    • It neutralises the acid which is added to food in the stomach, to produce alkaline conditions in which the enzymes work best.
    • It emulsifies fats, i.e. breaks down large drops of fat into small droplets to increase their surface area. this enables the lipase to work much faster.

Use of Enzymes in the Home and Industry

In the home biological detergents may contain

  • protein-digesting (protease) enzymes to break down blood and food stains
  • fat digesting (lipase) enzymes to break down oil and grease stains.

In Industry

  • protease's are used to pre-digest protein in baby foods.
  • carbohydrates are used to convert starch into sugar syrup
  • isomerase converts glucose syrup into fructose syrup which is even sweeter. It can be used in smaller quantities which makes it ideal for use in slimming foods.
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Controlling Conditions

  • The water content, ion content, temperature and blood sugar levels of the human body have to be controlled so that it can function properly.
  • If the water or ion content of the body is wrong, too much water may move into or out of the cells by osmosis causing damage. Water and ions enter the body through food and drink.
  • Blood glucose concentration is monitored and controlled by the pancreas which secretes the hormone insulin.
  • Insulin converts glucose into insoluble glycogen and lowers blood glucose.
  • The level of insulin in the pancreas affects what happens in the liver. The pancreas continually monitors the body's sugar levels and adjusts the amount of insulin released to keep the body's sugar levels as close to normal as possible.
  • If the pancreas does not produce enough insulin, a person's blood glucose concentration may rise to a fatally high level. This is a condition called diabetes, which is related by
    • careful attention to diet
    • injecting insulin into the blood.
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Human Characteristics

Body cells contain 46 chromosomes arranged as 23 pairs. Chromosomes are made up of large molecules of DNA. A gene is a section of DNA.

Gametes - female eggs and male sperm - have 23 chromosomes. The fusion of these two cells produces a zygote with 46 chromosomes in total (23 pairs).

Inheritance of Sex

Of the 23 pairs of chromosomes in the human body, one pair is the sex chromosomes.

  • In females, these are identical and called the X chromosomes.
  • In males, one is much shorter than the other. The shorter one is the Y chromosome and the longer one is the X chromosome.

Like all pairs of chromosomes, offspring inherit one sex chromosome from the mother and one from the father.

Ultimately, the sex of the individual is decided by whether the ovum is fertilised by an X-carrying sperm or a Y-carrying sperm.

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Cell Division


  • This is the division of body cells to produce new cells. This occurs for growth and repair (also in asexual reproduction),
  • Before the cell divides, a copy of each chromosome is made so the new cells has exactly the same genetic information. This means that the cells of asexually reproduced offspring contain the same genes as the parents.


  • When gametes join at fertilisation, one chromosome comes from each parent and a single body cell with new pairs of chromosomes is formed.
  • This then divides repeatedly by mitosis to form a new individual, giving rise to variation.
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  • Some genes have different forms - these are called alleles.
  • The gene for eye colour there are two alleles - blue or brown.
  • In a pair of chromosomes, the alleles for a gene can be the same or different. if they are different one allele will be dominant and one allele will be recessive.
  • The dominant allele will control the characteristics of the gene. A recessive allele will only control the characteristics of the gene if it is present on both chromosomes in a pair.

Monohybrid Inheritance

  • genes exist in pairs, one on each chromosome in a pair, we call these pairs of genes 'alleles'.
  • When a characteristic is determined by just one pair of alleles then simple genetic crosses can be performed to investigate the mechanism of inheritance.
  • This type of inheritance is called monohybird inheritance.
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Inheritance of Eye Colour

  • In genetic diagrams we use capital letters for dominant alleles and lower case for recessive alleles. B for brown eyes and b for blue eyes.
  • If one parent has two dominant genes then all the offspring will inherit that characteristic.
  • if both parents have one recessive gene then this characteristic may appear in the offspring (1 in 4 chance)
  • If one parent has a recessive gene and the other has two recessive genes, then there is a 50% chance of that characteristic appearing.

these are only probabilities!

Differentiation of Cells

When cells develop a specialised structure to carry out a specific function, this is differentiation.

  • Most plant cells can differentiate throughout life.
  • Animal cells differentiate at an early stage so quickly become muscle, nerves etc.

Mature cells usually divide for repair and replacement.

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Genes, Chromosomes and DNA continued

Stem Cells

  • Stem cells are cells in human embryos and adult bone marrow, which have yet to differentiate.
  • They can be made to differentiate into many different types of cells, e.g. a nerve cell, so treatment with these cells may help conditions such as paralysis.

Genes, Chromosomes and DNA

In normal human cells, there are only 23 pairs of chromosomes. They consist of long, coiled molecules of DNA.

  • The DNA molecule itself consists of two strands which are coiled to form a double helix.
  • Genes are sections of DNA which code for a particular inherited characteristic e.g. blue eyes.
  • Each person has unique DNA (apart from identical twins which have the same DNA) which means it can be used for DNA fingerprinting (identification)
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Genetic Disorders

Embryos can be screened for the genes that cause genetic disorders.

  • Huntington's Disease

Disorder of the nervous system caused by a dominant allele. It can be passed on by only one parent who has the disorder.

  • Cystic Fibrosis

Disorder of the cell membranes. It must be inherited from both parents, who can be carriers without having the disorder themselves. It is caused by a recessive allele.

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