OCR Gateway Science B3 revision cards

Module B3 of the Gateway Additional Science revision cards to cover the eight areas covered


Molecules of life

Cell structure - animal cell

  • cell membrane - controls movement of substances in and out of cell
  • cytoplasm - where chemical reactions take place
  • nucleus - carries genetic information on chromosomes
  • mitochondion - where respiration takes place inside the cell

Chromosomes, Genes and DNA

  • chromosomes made up of DNA that form genes
  • genes carry code for proteins (protein synthesis)
  • DNA copies itself - DNA replication
  • Base code sequence (in 3 code sequence e.g. CAA or CGC)
  • Base codes join up in DNA replication C - G, and A - T)

DNA base codes form the basis for DNA fingerprinting

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Molecules of Life part 2

DNA replication and Enzymes

  • process of replication speeded up by use of enzymes
  • enzymes are biological catalysts
  • enzymes are not alive
  • catalysts speed up reactions yet remain unchanged themselves

Enzymes and rate of reaction

  • enzymes are substrate specific
  • active site is where reaction occurs
  • enzymes work in optimum conditions specific to each type of enzyme
  • enzymes can be denatured (changed shape of active site)
  • denaturing of active site prevents subtstrate from fitting perfectly so no product made
  • denaturing caused by incorrect pH
  • denaturing caused by temperature being too high
  • Low temperatures slow enzyme activity
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  • movement from an area of high concentration to an area of low concentration
  • diffusion occurs in many areas of the body - lungs, kidneys, small intestine.
  • diffusion occurs between placenta and mother
  • semi-permeable membrane

Aiding diffusion

  • increased surface are
  • thin layer between cells
  • moist surface
  • good blood supply
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Examples of diffusion


- gases in and out of blood
- amino acids, fatty acids, glycerol and glucose in and out of blood.
- nitrogenous waste in and out of blood.
- urea in and out of blood.
- potassium and sodium ions between nerve synapses.
- substances to and from the foetus via the placenta.
- carbon dioxide and oxygen in and out of leaf

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Keeping it moving

Circulatory system

  • Made up of - arteries, veins, capillaries, heart and blood.
  • Arteries - thick elastic and muscular wall to withstand high pressure.
  • Veins - large lumen (space) to help blood flow (low pressure) and valves to stop back flow.
  • Capillaries - thin permeable wall allows exchange of materials.
  • Heart - double pump.

Heart and Blood

  • Atria and ventricles (ABOVE).
  • Valves in the heart stop the blood flowing backwards.
  • Pulmonary artery takes to lungs, aorta to body, vena cava back from body, left side thicker wall.
  • Blood - red blood cells (carry oxygen), white blood cells (fight disease), plasma (dissolved food, waste and carbon dioxide), platelets (broken down cells help clotting)
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Divide and Rule - mitosis

Multicellular organisms grow large

When growing the organisms can differentiate cells

Human cells contain 23 pairs of homologus chromosomes

Mitosis - Fancy name for cell division
1. Cell resting.
2. Chromosomes in the nucleus are copied.
3. Spindle forms, chromosomes arranged along equator.
4. Chromosomes to poles of cells (opposite ends of cell).
5. Cell divides - two geneticaly identical diploid daughter cells formed.

This type of cell division occurs in growth and repair.

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Divide and rule - meiosis

Meiosis - Fancy name for sex cell division.

Haploid cells formed (gametes - sperm and ova.)
1. Homologous chromosomes pair up.
2. One from each pair goes to poles.
3. Strands of chromosomes move to opposite poles.
4. Four new haploid cells formed (half thenumber of chromosomes of normal cells).


Sperm cells - tail to swim, streamlined, mitochondria to power tail, acrosome on head to help pentrate ovum membrane.

Ovum - larger than sperm, large 'food' store

Variation occurs in sexual reproduction as genes shuffled in meiosis

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Growing up

Plant cells - same as animal cells with the addition of;

  • chloroplasts - contain chlorophyll for photosynthesis
  • vacuole - used for storage of sap and support of cell
  • cell wall - made of cellulose this strengthens the cell


  • animal grow by increasing in cell number
  • differentiation of cells occurs early in animal development
  • plant cells maintain ability to differentiate
  • plant cells grow by elongation
  • mitosis occurs at root tips and shoots.
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Growing up - stem cells

Stem cells

  • undifferentiated animal cells.
  • found in embryos.
  • can be found in adult bone marrow but only able to differentiate to a limited number of cells.

Stem cell research

  • investigating cancer.
  • drug testing.
  • transplants - grow new organs

Ethical issues involved because of the nature of stem cell origin.

  • right to life?
  • wasted embryos?
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Growing up - gestation

Gestation = pregnancy

  • Human gestation = 266 days (9 months) on average
  • Foetus grows quickly from embryo in the uterus.
  • Different body parts grow at different rates.
  • Head grows quickly.

Human growth

  • infancy - fastest % growth rate
  • childhood
  • adolescence
  • maturity
  • old age

Baby growth measured using mass and head size. Shown on graphs in exams

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Controlling plant growth

Plant hormones (auxins) control plant growth

  • auxins used in roots, shoots, flowering and fruit
  • auxins respond to different stimuli
  • phototropisms - light
  • geotropisms - gravity
  • roots are negative phototropisms and positive geotropisms.

Commercial use of hormones

  • Rooting hormones - encourages growth of roots. Used in cuttings.
  • Fruit ripening hormones - cause fruit to ripen during transport
  • Weedkillers - disrupt growth pattern of target plants
  • Control dormancy - speed up or slow down growth and development
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New Genes for Old

Characteristics - desired attribute of plant or animal
Yield - quantity needed increased
Selective breeding - produces plant or animal with desired characteristics
Cross breeding - selection of characteristics from two breeds
In-breeding - limited gene pool more chance of disease and problems
Mutation - corruption of DNA sequence that alters or changes protin production

Selective breeding

A desired characteristic is identified. Plants / animals showing characteristic bred until offspring show desired characteristic. Takes many generations

  • modern plants - tomato plant bred to get size, taste, colour, or shape
  • cattle - bred or quantity of milk, quality of milk or beef production.
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New genes for old - part 2

Genetic engineering - Method

  • Characteristic selected
  • Gene for characteristic isolated
  • Gene inserted into chromosome of different organism
  • Organism replicated to produce many copies
  • Enzyme used to 'cut' gene from chromosome


  • improved resistance to herbicides and disease
  • improve quality of food
  • to produce substance required

Example - insulin producing bacteria

Ethical Considerations - possible cross breeding, mutation from genetc modification

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More of the Same

Clones - genetically identical
Cloning - involves asexual reproduction (mitosis)

Cloning animals

  • Animal artificially inseminated
  • Fertilised egg divides to 8 cell embryo
  • 8 cell embryo split into 4 embryos of 2 cells
  • 2 cell embryos inserted into surrogate animals

2nd method involves removal of fertlised egg cell nucleus and replacement with donor nucleus (Dolly the sheep)

Plants can be cloned by taking a cutting and using rooting hormones

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More of the Same

Asexual reproduction in plants

  • Potato - tuber grows shoots and roots form clone of parent plant
  • Strawberries - runners from parent plant go along ground to from clone of parent plant
  • Spider plants - plantlets grow from parent plant
  • Cuttings - done by gardeners use rooting hormones

Advantages of cloning

  • lots of same organism produced
  • speed
  • ability to grow from organisms with poor breeding


  • genetically identical reduced gene pool
  • reduced variation
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great resources! A lot of time and effort has gone into these, Thanks =D



thanks sir its helping me loads :))



need more detail




needs to be more detailed

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