Biology, B3

GCSE Biology, Gateway module B2

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Cells and DNA

Plant and animal cells have similarities and differences

Plant and animal cells: nucleus, cell membrane, cytoplasm, mitochondria

Only in plant cells: rigid cell walls, large vacuole, chloroplasts

  • The nucleus contains DNA
  • It is a double helix strand and the two strands are made up of nucleotides
  • There are 4 bases A,T, C, G

DNA can replicate itself:

  • DNA copies itself every time the cell divides
  • when it copies itself the double helix 'unzips' to form two single strands
  • as the DNA unwinds, new nucleotides join on where the bases fit making an exact copy of the DNA on the other strand
  • The result is two molecules of DNA that are identical
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DNA finger printing

=A way of comparing DNA samples,it is used in forensic science and paternity tests


  • isolate DNA from cells
  • special enzymes are used to cut the DNA into fragments
  • The DNA is seperated out in the process of chromotography and suspended in a gel and an electric current is passed through
  • The DNA is negatively charged so it moves towards the positive anode
  • The DNA is tagged with a radioactive marker and placed on a photographic film
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Protein Synthesis

Proteins are made by reading the code in DNA

  • DNA controls the production of proteins and a gene codes for a particular protein
  • Proteins are chains of animo acids
  • Every protein has a different number and order of amino acids giving them all certain shapes meaning they can have different functions
  • The order of the bases decides the order of amino acids
  • Each amino acid is coded for by a sequence of the three bases CGT

Enzymes are biological catalysts which increase the speed of a reaction

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  • Every enzyme has an active site- the part where it joins on to its substrate (the molecule changed)
  • Enzymes are very specific and usually only work with one substrate
  • this is because the substrate has to fit into the active site as it is a 'lock and key' mechanism

Enzymes like it warm but not too warm as they become denatured and the active site won't fit the substrate anymore, this is a permenant change

Enzymes also like the right pH as if they are too high or too low it will also denature the enzyme so its often pH 7 that works best but thats not alway the case e.g. pepsin in stomach prefers pH 2.

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Diffusion is the passive movemnt of particles from a region of higher concentration to a region of lower concentration

The rate of diffusion depends on three main things

  • distance- substances diffuse faster when they don't have as far to move
  • concentration difference- they diffuse faster when their is a bigger difference
  • Surface area- the more surface area the faster they move
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Diffusion in Cells

Small food molecules can diffuse into the blood

  • absorption happens in the small intestine, after big molecules like starch and proteins have been broken down into glucose and animo acids
  • can diffuse into blood from small intestine as the concentration is higher
  • When the blood reaches the cells that need the substances they can diffuse out easily.

Villi in the small intestine help with diffusion

  • The small intestine is adapted for the absorption of food
  • It is very long so there is lots of time for absorption to take place
  • There is a big surface area as the small intestine is covered ion nilli
  • each cell on the surface of a villus has its own microwilli
  • Villi have a single permeable layer of surface cells and a very good blood supply to assist quick absorption
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Diffusion in cells

Alveoli carry out gas exchange in the body

  • The lungs contain air sacs called alveoli where gas exchange happens
  • The blood passing next to the alveoli has just returned from the rest of the body so contains lots of carbon dioxide and very little oxygen
  • Oygen diffuses out of the alveolus into the blood
  • carbon dioxide diffuses from the blood into the alveolus
  • when the blood reaches body that needs the oxygen it is released from the red blood cells by diffusion
  • At the same time the carbon dioxide diffuses out of body cells into the blood

They are specialised for gas exchange:

  • Huge number of alveoli gives a large surface area
  • moist lining for gases to dissolve in
  • have thin walls which are permeable
  • have good blood supply maintaining the concentration gradient
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Diffusion in Cells

DIffusion happens in the placenta

  • There are villi in the placenta giving it a large surface area containing capillaires which have the foetus's blood in
  • spaces around the villi called sinuses become filled with the mother's blood allowing the mothers blood and the foetus's blood to flow very close together so that there is a short distance for diffusion
  • food and oxygen diffuse across from the mother's blood to the foetus's
  • Carbon dioxide and other waste products diffuse from the foetus to the mother

Diffusion happens in the synapses

  • nerve impulses arrive at a synapse triggering the release of transmitter substances
  • the transmitter diffuses across the gap and binds to receptors
  • this stimulates a new nerve impulse
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Blood Plasma

Plasma is the liquid bit of blood- carries everything around the body

  • red and white blood cells and platelets
  • water
  • digested foods like glucose and animo acids
  • carbon dioxide from body cells to lungs
  • urea from liver to kidneys
  • hormones
  • antibodies and antitoxins
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Red and white blood cells

Red blood cells carry oxygen

  • They are small and have a biconcave shape giving them a large surface area for absorbing and releasing oxygen
  • contain haemoglobin which gives it the colour and ontains iron
  • in the lungs oxygen reacts with haemoglobin to become oxyhaemoglobin
  • no nucleus so there is more space to carry more oxygen
  • they are very flexible so can fit through tiny capillaries

White blood cells are used for fighting disease

  • main role is defense against disease
  • produce antibodies to fight microbes
  • produce antitoxins to neutralise the toxins produced by microbes
  • Have a flexible shape to engulf micro organisms and digest them
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Blood vessels

Arteries- carry blood away from the heart

capillaries- are involved in the exchange of materials at the tissues

Veins- carry blood to the heart

Arteries- carry blood under high pressure so the walls are strong and elastic, the walls are thick compared to size of the lumen and they contain thick layers of muscle to make them strong

  • cholesterol is a fatty substance caused by eating too much saturated fat, it causes a build up of plaque in the walls of the lumen which narrows the artery and restricts blood flow causing heart attacks or strokes
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Capillaries are really small and are branches from arteries

  • they carry blood really close to every cell
  • have permeable walls so substances can diffuse,
  • they supply food and oxygen and take away waste
  • the walls are one cell thick increasing the rate of diffusion as distance substances travel is decreased

Veins take blood back to the heart

  • capillaries join up to form veins
  • the blood is at lower pressures so the walls don't have to be as thick
  • have a bigger lumen to help bloof flow
  • have valves to help blood flow in the right direction
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Circulatory system- the heart

Mammals have a double circulatory system, the first connects the lungs to the heart, Deoxygenated blood is pumped to the lungs to take in oxygen

The second connects the heart to the rest of the body and the oxygenated blood in the heart is pumped round the body giving up its oxygen so returns to the heart as deoxygenated blood

Double circulatory system allows blood to be pumped at greater force so blood reaches every tissue

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The heart

  • The right atrium of the heart recieves deoxygenated blood from the body through the vena cava
  • The deoxygenated blood moves through the right ventricle which pumps it to the lungs via the pulmonary artery
  • the left atrium recieves oxygenated blood from the lungs via the pulmonary vein
  • the oxygenated blood then moves through the left ventricle which pumps it round the whole of the body via the aorta
  • the left ventricle has a much thicker wall than the right as the blood has to be pumped to the whole body rather than just the lungs
  • The semi lunar, tricuspid and bicuspid valves prevent backflow of blood
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Replacement hearts

  • Heart has a pacemaker- determines how fast the heart beats

if it tops working it can be dangerous so can be replaced with an artificial one, heart valves can also be replaced

  • Can get a heart transplant

the whole heart can be removed and replaced by another one from a human donor but it involves major surgery and lifetime of drugs and medical care

Need a donor, and there are a shortage of these as:

  • need to be relatively young and tissues must be similar
  • body weight must be similar
  • hearts are only usable for no more than 6 hours outside the body so the donor must have recently died or be brain dead
  • close relatives must give permission
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Replacement hearts

Transplants may be rejected

  • patients immune system may recognise the hear as 'foreign' and attack it (rejection)
  • doctors use drugs to supress the patients immune system making the patient more vunerable to disease

Mechanical parts aren't usually rejected as they are usually made of metal or plastic

Pacemakers need a battery which lasts about 10 years but it easily replaced

Artificial valves need more major surgery and the blood doesn't flow through them as smoothly so may cause blood clots so patients have to take drugs to thin blood

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Being multicellular has its advantages:

  • Can grow bigger
  • Travel further for nutrients
  • less things can eat or squash you
  • allows for cell differentiation
  • allows multicellular organisms to be more complex
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Mitosis makes new cells for growth and repair and is when a cell reproduces itself by splitting to form two identical offspring

1. DNA replicates itself

2. DNA coils into double armed chromosomes- exact dupliacates

3. The chromosomes line up and are pulled by a fibre to the ends of the cell where a membrane forms around them

4. The cytoplasm divides into two new cells containing exactly the same genetic material

end result is two new cells with exactly the same genetic mixture (clones)

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Meiosis makes gametes (sperm and egg which are haploid)

Meiosis involves 2 divisions

1. DNA replicates itself

2. Chromosomes arrange themselves in pairs one from dad one from mum

3. First division- pairs split up and form 2 new cells with no pairs

4. Second division- (like mitosis) chromosome splits is half

result is 4 new cells which are all genetically different

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Sexual reproduction

Sexual reproduction involves the fusion of male and female gametes because there are two parents, the offspring contains a mixture of their parents genes

Variation as inherit features from both parents

Sperm cells are adapted to their function-

Transport the males DNA to females egg so the DNA can combine

  • small and long tails so can swim
  • have lots of mitochondria to produce energy
  • acrosome at front of head which produces enzymes that digest the membrane of the egg cell
  • produced in large numbers
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Animals stop growing when reach full growth but plants keep growing

Animals grow by cell division, plants mainly by elongation

Stem cells can turn into different types of cell

  • differentiation is the process by which a cell changes, animals lose ability to differentiate at an early stage
  • most cells are specialised for a particular job
  • some cells are undifferentiated- STEM CELLS
  • Stems cells are found in early embryos and adult bone marrow
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Stem cells

Can be used to cure diseases and disorders

  • medicine uses stem cells to cure blood disorders
  • early human embryos contain lots of stem cells which can be extracted and grown
  • Tissues from stem cells can be used for drug testing and development

Some people are against stem cell research

  • they think embryos shouldn't be used as it harms potential human life but most of the ones used are going to be destroyed anyway
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Growth in humans

Mammals give birth to live young so growth begins inside the womb- gestation

Gestation length is related to the size of the animal

the human body doesn't grow evenly and certain organs (brain) grow faster than others

Babys head size increases in proportion with its body weight

Human life span has 5 stages:

  • Infancy- first year of life, rapid growth
  • Childhood- between infancy and puberty, 2-13, development of brain
  • Adolescence- begins with puberty, 13-19
  • Maturity/Adulthood- cell division for growth stops
  • Old age- between age 65 and death
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Auxins are plant hormones that control the growth at tops of roots and shoots

Auxin produced at the top diffuses backwards to stimulate the cell elongation process

If the tip of the shoot is removed there is no auxin so plant stops growing

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Auxin in shoots

Shoots are positively phototropic (grow towards light)

  • When a shoot tip is exposed to light it auxin accumulates on the shaded side making the cells elongate faster so shoot bends towards light

Shoots are negatively Geotropic (grow away from gravity)

  • when a shoot is growing sideways gravity produces an unequal distribution of auxin in the tip with more auxin on the lower side, the lower side grows faster bending the shoot upwards
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Auxin in roots

Roots are positively geotropic (grow towards gravity)

  • A root growing sideways will have more auxin on the lower side but in a root extra auxin inhibits growth so the top will elongate faster meaning the root bends downwards

Roots are negatively phototropic

  • more auxin accumulates on the shaded side the auxin inhibits cell elongation so the root bends downwards back to the ground
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Commercial use of plant hormones

Selective weedkillers

  • kill broad leafed plants by using hormones to disrupt normal growth patterns

Growing cuttings from rooting powder

  • If you put cuttings in rooting powder containing plant hormones the plant will produce roots

Controlling the ripening of fruit

  • can be controlled for transporting fruit, ripening hormone is added to unripe food when it arrives at is destination

Controlling dormancy

  • seeds won't germinate unless in certain conditions so are dormant, adding gibberellin breaks the dormancy
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A mutation is a change in the DNA base sequence

Most are harmful- in reproductive cells may cause offspring to develop abnormally or dies, or can lead to cancer

Some are beneficial-

  • might give an organism a survival advantage over the rest of the population, this mutation is then passed onto the offspring- natural selection
  • an example is bacterium resistant to antibiotics
  • Blue budgies are a mutation of yellow budgies

Radiation (x-rays and ultraviolet light) and certain chemicals (cigarette smoke, carcinogens) cause mutations

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Selective breeding

organisms can be selectively bred to combine the best features:

  • Maximum yield, good health, disease resistant,
  •  animal- qualities like temperate, fertility, good mothering
  • plants- attractive flowers, nice smell

selective breeding=artificial selection

1. from existing stock chose ones with best/desired characteristics

2. breed them together

3. select the best offspring and breed them together

4. continue over several generations and the desirable trait gets stronger and stronger

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Disadvantages of selective breeding

  • reduces the number of different alleles so there is less chance of animals becoming resistant and there is nothing to selectively breed from
  • more chance of developing genetic disorders
  • can be problems if new disease appears as it might wipe out all the animals
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Genetic engineering

  • produce organisms with new and useful features
  • risk that it may have unexpected harmful effects


1.gene responsible for desirable characteristic is selected

2. it is then 'cut' from the DNA using enzymes and isolated

3. the useful gene is inserted into the host DNA of a bacterium

4. host bacterium is cultivated

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Genetic engineering

3 examples:

  • rice has been genetically engineered to contain beta carotene in places where rice is a staple food and vitamin A deficiency is common
  • human insulin production has been put into bacteria
  • plants have become resistant to herbicides, frost damage, and disease

Moral and ethical issues

  • people worry won't just stop with plants and animals
  • evolutionary consequences are unknown
  • fears we are 'playing god'
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Cloning- Embryo transplants

Clones are genetically identical organisms

embryo transplants in cows:

1. sperm are taken from the prize bull

2. selected prize cows are given hormones to make produce lots of eggs

3. cows are artificially insemminated

4. fertilised eggs divide to give a ball of genetically identical cells which develops to an embryo

5.embryos are taken from prize cows and sex is checked and screened for genetic defects

6. embryos split into seperate cells and each cell grows into a new embryo which is a clone of the original,  the offspring are clones of each other not their parents

7. the embryos are implanted into other cows 'surrogate mothers' to grow

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Advantages and disadvantages of embryo transplants


  • hundreds of 'ideal' offspring can be produced every year
  • the original prize cow can keep producing prize eggs all year round


  • same alleles keep appearing so greater risk of genetic disorders
  • disease could wipe out whole population
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Adult Cloning

Dolly the sheep was the first mammal to be successfully cloned

1. nucleus of a sheep's egg cell was removed

2. another nucleus was inserted in its place- a diploid nucleus from another sheep and had all its genetic information

3. cell was stimulated so that it started dividing by mitosis as if it were a normal fertilised egg

4. the dividing cell was implanted into the uterus of another sheep

5. the result was dolly the sheep

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Benefits and risks of cloning


  • animals that can produce medicines in their milk could be cloned
  • animals that have organs suitable for organ transplantation into humans could be developed by genetic engineering and cloned
  • study of animal clones could lead to greater understanding of the development of the embryo
  • cloning could help preserve endangered species


  • cloned animals might not be as healthy
  • might have consequences not yet aware of
  • worried that humans might be produced by cloning

ethical issues with cloning people: involve lots of women donating eggs, need surrogate mothers, still births, and miscarriages, may not be as healthy, 'playing god' may be pspsychologically damaging for the clone

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Cloning Plants- Asexual Reproduction

Plants produce exact genetic copies of themselves without involving another plant

Gardeners are used to taking cuttings from good parent plants then planting them to produce identical copies of the parent plant

cloning plants is easier than cloning animals because plants keep their ability to differentiate

Commercial use of cloned plants, pros and cons:

  • you can be sure you'll get good plants
  • its possible to mass produce
  • if plants suffer from disease they all will
  • lack of genetic variation
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Commercial cloning

Commercial cloning- Tissue culture

1. choose plant based on characteristics

2. remove a small amount of tissue from parent plant (cells removed from tip of plant)

3. grow the tissue in a medium containing nutrients and growth hormones, done under aseptic conditions to prevent growth of microbes

4. as the tissues produce shoots and roots they are moved to potting compost

Hundreds of clones can be made from one plant

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