Biology Unit 2

On all subjects to do with biology unit 2 at GCSE

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Diffusion- The movement of molecules from high concentration to a low concentration.

  • Diffusion happens when the particles are free to move. e.g The smell of cooking travels around the house from the kitchen.
  • The larger the difference in concentration, the faster the rate of diffusion.

Examples of diffusion in living cells:

Diffusion of:

  • Oxygen into the cells of your body from bloodstream as cells are respiring.
  • Carbon dioxide into photosynthesising plant cells
  • Simple sugars and amino acids from gut through cell membranes.
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Osmosis- The movement of water across a partially permeable membrane from a dilute solution to a more concentrated solution.(Osmosis is a special case of Diffusion)

  • Water is needed to support cells and because chemical reaction take place in solution
  • Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid (stiff) so they that able to hold the plant upright.

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Photosynthesis- The process where green plants use water, carbon dioxide and sunlight to produce carbohydrates(stoned energy) and release oxygen (as a by-product)



  • Plants absorb water through their roots and carbon dioxide through their leaves.
  • Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.
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Plants and Minerals

Three most important elements plants need to take in to stay alive:

  • Carbon- Comes from atmosphere
  • Hydrogen- This come from water in the soil
  • Oxygen- Water in soil/ atmosphere

Two important mineral ions needed by plants are:

  • nitrate - for making amino acids, which are needed to make proteins
  • magnesium - for making chlorophyll

If a plant does not get enough minerals, its growth will be poor. It will suffer from deficiency symptoms:

  • deficient in nitrate - it will suffer from stunted growth
  • deficient in magnesium - it's leaves will turn yellow
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Energy Transfer- Food Chains

Energy Transfer

  • Energy is transferred along food chains from one stage to the next.
  • 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:

  • 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
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Efficiency of food production

Efficiency of food production can be improved by reducing the amount of energy lost to the surroundings. You can do this by:

  • preventing animals moving around too much
  • keeping their surroundings warm

Calculating efficiency:

A cow has eaten 100KJ of stored energy (grass), excreted 63KJ and energy stored in body tissues is 4KJ. How much has been used up by repsiration?

100 - 63 - 4 = 33 kJ- Used up by respiration

Only 4 kJ of the original energy available to the bullock is available to the next stage, which might be humans.

efficiency = 4100 × 100 = 4%

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Carbon Cycle

All cells contain Carbon because they all contain proteins, fats and carbohydrates.

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:- Green plants remove carbon dioxide from the air by photosynthesis

Returning:- Organisms return carbon dioxide to the atmosphere by respiration.

Passing from one organism to next:- An animal eats a plant, carbon from the plant becomes part of the animals fats and proteins. Microorganisms and some animals feed on waste material & remains of dead animals and plants. The carbon then becomes part of these microorganisms and detritus feeders.

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Enzymes are biological catalysts- they speed up reaction

  • Enzymes are also proteins that are folded into complex shapes that allow smaller molecules to fit into them.- The place where the fit is called the active site

If the shape of the enzyme changes, its active site may no longer work- it has been denatured They can be denatured by high temperatures or extremes of pH.

  • As the temperature increases, so does the rate of reaction.

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Enzymes and Respiration

Aerobic Respiration Respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

glucose + oxygencarbon dioxide + water (+ energy)

  • The process mostly takes place in the mitochondria

The energy released is used to:

  • build larger molecules from smaller ones
  • enable muscle contraction in animals
  • maintain a constant body temperature
  • build sugars, nitrates and other nutrients in plants into amino acids and then proteins
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Enzymes in digestion

Digestion involves the breakdown of large, insoluble molecules into smaller soluble molecules.

  • Amylase- produced by salivary glands, pancreas and small intestine- Catalyses digestion of starch in mouth and small intestine.
  • Protease- produced by stomach, pancreas and small intestine- catalyses breakdown of proteins in amino acids in stomach and small intestine
  • Lipase- produced by pancreas and small intestine- Catalyses breakdown of lipids(fats and oils) to fatty acids and glycerol.

Speeding up digestion:

  • Protease enzymes work best in acid conditions
  • Amylase and lipase work best when conditions are slightly alkaline
  • The liver produces bile-stored in the gall bladder- it is squirted into the small intestine and neutralises stomach acid. It makes the conditions slightly alkaline.
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The conditions inside you body must be controlled strictly- this is called Homeostasis

This conditions include: water content, ion content, body temperature and blood glucose concentration.

  • We must remove the waste products through chemical reactions from the body.

Carbon dioxide- waste product of respiration- excreted through lungs

amino acids- converted into urea by liver- excreted by kidney in urine

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Controlling internal conditions

Controlling body temperature- The thermoregulatory centre of the brain(which controls the body's response) and receptors in the skin detect temperature change,

Body temperature rises- blood vessels near skin dilate- more blood flows through-heat lost by radiation.

Sweat glands produce more sweat - this evaporates from skin's surface- energy required for evaporation comes from skin's surface- so we cool down

Body temp falls- blood vessels constrict- less blood flows through skin capillaries -less heat radiated.

We 'shiver'- muscles contract- requires respiration- some energy produced is released as heat.

Controlling blood sugar- The pancreas monitors and controls the level of sugar in our blood. If there is too much sugar-pancreas produces insulin- excess sugar then stored in liver a glycogen. If the pancreas is not producing enough insulin, diabetes is caused.

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Cell Division and growth

  • Body cells need to divide to produce new cells for growth or repair
  • Mitosis is the type of cell division that produces identical new cells

Stem Cells

  • Stem cells are not specialised but can develop into many different types of cell when required.- They are found in the embryo and in adult bone marrow.
  • there are ethical issues surrounding use of stem cells.- Many embryonic cells we carry research out on are aborted embryos or 'spare' from fertility treatment.

Cell division in sexual reproduction

  • Sex cells(gametes) are produced by meiosis
  • Four cells are produced from each parent. They are all different.
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DNA, genes and chromosomes

DNA - carry the genetic code that determines the characteristics of a living thing.

Genes -A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.

Chromosomes -The cell’s nucleus contains chromosomes made from long DNA molecules.- Human body cells have 23 pairs of chromosomes.

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Alleles are different forms of a gene. They can be dominant or recessive.

  • Genetic diagrams help us to understand the possible outcomes when parents produce offspring.

Huntington’s disease is a disorder of the nervous system that is caused by a dominant allele.Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.

  • the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
  • the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair.
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Danni Lawson-Kirby


You are a legend, they will really help me thanks :D

Emily Wall


No problem! Good luck with your exam




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