AQA Biology 2b


Enzymes are Catalysts

  • 1) Living things have thousands of different chemical reactions going on inside them all the time. These reactions need to be carefully controlled - to get the right amounts of substances.
  • 2) You can usually make a reaction happen more quickly by raising the temperature. This would speed up the useful reactions but also the unwanted ones too... not good. There's also a limit to how far you can raise the temperature inside a living creature before its cells start getting damaged.
  • 3) Living things produce enzymes that act as biological catalysts. Enzymes reduce the need for high temperatures and we only have enzymes to speed up the useful chemical reactions in the body.

A catalyst is a substance which increases the speed of a reaction, without being changed or used up in the reaction.

  • 4) Enzymes are all proteins and all proteins are made up of chains of amino acids. These chains are folded into unique shaes, which enzymes need to do their jobs.
  • 5) As well as catalysts, proteins act as structual components of tissues (e.g. muscles), hormones and antibodies.
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Enzymes Catalyse Reactions

1) Chemical reactions usually involve things either being spilt apart or joined together.

2) Every enzyme has a unique shape that fits onto the substance involved in a reaction.

3) Enzymes are really picky - they usually only catalyse one reaction.

4) This is because, for the enzyme to work, the substance has to fit its special shape. If the substance doesn't match the enzymes shape, then the reaction won't be catalysed.


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Right Temperature and pH

  • 1) Changing the temperature changes the rate of an enzyme-catalysed reaction.
  • 2) Like with any reaction, a high temperature increases the rate at first. But if it gets too hot, some of the bonds holding the enzyme together break. This destroys the enzyme's special shape and so it won't work anymore. It's said to be denatured.
  • 3) Enzymes in the human body normally work best at around 37 degrees C.
  • 4) The pH also affects enzymes. If it's too high or too low, the pH interferes with the bonds holding the enzyme together. This changes the shape and denatures the enzyme.
  • 5) All enzymes have an optimum pH that they work best at. It's often neutral pH 7, but not always - e.g. pepsin is an enzyme used to break down proteins in the stomach. It works best at pH 2, which means it's well-suited to the acidic conditions there.


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Digestive Enzymes

Not all enzymes work inside body cells - some work outside cells. For example, the enzymes used in digestion are produced by cells and then released into the gut to mix with food.

1) Starch, proteins and fats are BIG molecules.

2) Sugars, amino acids, glycerol and fatty acids are much smaller molecules. They can pass easily through the walls of the digestive system.

3) The digestive enzymes break down the BIG molecules into smaller ones.

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Amylase converts starch into sugars.

Starch -------------Amylase Enzyme-------------> Maltose

(and other sugars, e.g. dextrins)

Amylase is made in three places:

  • The salivary glands
  • The pancreas
  • The small intestine
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Protease converts proteins into amino acids.

Proteins -------------Protease Enzymes-------------> Amino Acids

Protease is made in three places:

  • The stomach (it's called pepsin there)
  • The pancreas
  • The small intestine
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Lipase converts lipids into glycerol and fatty acids.

Lipid -------------Lipase Enzymes-------------> Glycerol & Fatty Acids

Lipase is made in three places:

  • The pancreas
  • The small intestine
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Bile neautralises stomach acid and emulsifies fats.

1) Bile is produced in the liver. It's stored in the gall bladder before it's released into the small intestine.

2) The hydrochloric acid in the stomach makes the pH too acidic for enzymes in the small intestine to work properly. Bile is alkaline - it neutralises the acid and makes conditions alkaline. The enzymes in the small intestine work best in these alkaline conditions.

3) It emulsifies fats. In other words it breaks the fat into tiny droplets. This gives a much bigger surface area of fat for the enzyme lipase to work on - which makes its digestion faster.

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Breakdown of Food

1) Enzymes used in the digestive system are produced by specialised cells in the glands and in the gut lining.

2) Different enzymes catalyse the breakdown of different food molecules.

Salivary Glands
Gall Bladder
Large Intestine
Small Intestine

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The Digestive System


Salivary Glands

  • These produce amylase enzyme in the saliva.


  • (Oesophagus)


  • It pummels the food with its muscular walls.
  • It produces the protease enzyme, pepsin.
  • It produces hydrochloric acid for two reasons: to kill bacteria and to give the right pH fpr the protease enzyme to work (pH 2 - acidic)..
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The Digestive System


  • Where bile is produced. Bile neutralises stomach acid and emulsifies fats.

Gall Bladder

  • Where bile is stored, before its released into the small intestine.


  • Produces protease, amylase and lipase enzymes. It releases these into the small intestine.
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The Digestive System

Small Intestine

  • Produces protease, amylase and lipase enzymes to complete digestion.
  • This is also where the digested food is absorbed out of the digestive system into the blood.

Large Intestine

  • Where excess water is absorbed from the food.


  • Where the faeces (made up mainly of indigestible food) are store before it departs through the anus.
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Respiration involves many reactions, all of which are catalysed by enzymes. These are really important reactions, as respiration releases the energy that the cells needs to do just about everything.

1) Respiration is no breathing in and out, as you might think.

2) Respiration is the process of releasing energy from the breakdown of glucose - and it goes on in every cell in your body.

3) It happens in plants too. All living things respire. It's how they release energy from their food.

Respiration is the process of releasing energy from glucose, which goes on in every cell.

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Aerobic Respiration

1) Aerobic respiration is respiration using oxygen. It's the most efficient way to release energy from glucose. (You can also have anaerobic respiration, which happens without oxygen, but that doesn't release nearly as much energy.)

2) Aerobi respiration goes on all the time in plants and animals.

3) Most of the reactions in aerobic respiration happen inside mitochondria.

Glucose + Oxygen ---> Carbon Dioxide + Water + ENERGY

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Releases Energy

Four examples of what energy released by aerobic respiration is used for:

  • To build up larger molecules from smaller ones (like proteins from amino acids).
  • In animals, to allow the muscles to contract (which in turn allows them to move about).
  • In mammals and birds the energy is used to keep their body temperature steady (unlike other animals, mammals and birds keep their bodies constantly warm).
  • In plants, to build sugars, nitrates and other nutrients into amino acids, which are then built up into proteins.
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When you exercise, your body quickly adapts so that your muscles get more oxygen and glucose to supply energy.

1) Muscles are made of muscle cells. These use oxygen to release energy from glucose (aerobi respiration), which is used to contract the muscles.

2) An increase in muscle activity requires more glucose and oxygen to be supplied to the muscle cells. Extra carbon dioxide needs to be removed from the muscle cells. For this to happen the blood has to flow at a faster rate.

3) This is why physical activity:

  • increases your breathing rate and makes you breath more deeply to meet the demand for extra oxygen.
  • increases the speed at which the heart pumps.
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1) Some glucose from food is stored as glycogen.

2) Glycogen's mainly stored in the liver, buteach muscle also has its own store.

3) During vigorous exercise muscles use glucose rapidly, so some of the stored glycogen is converted back to glucose to provide more energy.

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Anaerobic Respiration

1) When you do vigorous exercise and your body can'tsupply enough oxygen to your muscles, they start doing anaerobivc repiration instead of aerobic respiration.

2) "Anaerobic" just means without oxygen. It's the incomplete breakdown of glucose, which produces lactic acid.

glucose ---> energy + lactic acid

3) This is NOT the best way to convert glucose into energy because lactic acid builds up in the muscles, which gets painful. It also causes muscle fatigue - the muscles get tired and they stop contracting efficiently.

4) Another downside is that anaerobic respiration does not release nearly as much energy as aerobic respiration - but it's useful in emergencies.

5) The advantage is that at least you can keep on using your muscles for a while longer.

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Oxygen Debt

1) After resorting to anaerobic respiration, when you stop exercising you'll have an "oxygen debt".

2) In other words you have to "repay" the oxygen taht you didn't get to your muscles in time, because your lungs, heart and blood couldn't keep up with the demand earlier on.

3) This means you have to keep breathing hard for a while after you stop, to get more oxygen into your blood. Blood flows through your muscles to remove the lactic acid by oxidising it to harmless CO2 and water.

4) While high levels of CO2 and lactic acid are detected in the blood (by the brain), the pulse and breathing rate stay high to try and rectify the situation.

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Biological Detergents

1) Enzymes are the 'biological' ingredients in biological detergents and washing powders.

2) They're mainly protein-digesting enzymes (protease) and fat-digesting enzymes (lipeases).

3) Because the enzymes break down animal and plant matter, they're ideal for removing stains like food or blood.

4) Biological detergents are also more effective at lower temperatures (e.g. 30 degrees C) than other types of detergents.

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Changing Foods

1) The proteins in some baby foods are 'pre-digested' using protein-digesting enzymes (proteases), so they're easier for the baby to digest.

2) Carbohydrate-digesting enzymes (carbohydrases) can be used to turn starch syrup into sugar syrup.

3) Glucose syrup can be turned into fructose syrup using isomerase enzyme. Fructose is sweeter, so you can use less of it - good for slimming foods and drinks.

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Enzymes in Industry

Enzymes are really useful in industry. They speed up reactions without the need for high temperatures and pressures.


- They'respecific, so they only catalyse the reaction you want them too.
- Using lower temperatures and pressures means a lower cost as it saves energy.
- Enzymes work for a long time, so after the initial cost of buying them, you can continually use them.
- They are biodegradable and therefore cause less environmental pollution.


- Some people can develop allergies to enzymes (e.g. in biological washing powders).
- Enzymes can be denatured by even a small increase in temperature. They're also susceptible to poisons and changes in pH. This means the conditions in which they work mus be tightly controlled.
- Enzymes can be expensive to produce.
- Contamination of the enzyme with other substances can affect the reaction.

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1) DNA stands for deoxyribonucleic acid.

2) It contains all the instructions to put an organism together and make it work.

3) It's found in the nucleus of animal and plant cells, in really long molecules called chromosomes.

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1) A gene is a section of DNA. It contain the instructions to make a specific protein.

2) Cells make proteins by stringing amino acids together in a particular order.

3) Only 20 amino acids are used, but they make up thousands of different proteins.

4) Genese simply tell cells in what order to put the amino acids together.

5) DNA also determines what proteins the cell produces, e.g. haemoglobin, keratin.

6) This in turn determines what type of cell it is, e.g. red blood cell, skin cell.

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Unique DNA

Almost everyone's DNA is unique. The only exceptions are identical twins, where the two people have identical DNA, and clones.

DNA fingerprinting (or genetic fingerprinting) is a way of cutting up a person's DNA into smaller sections and then sperating them. Every person's genetic fingerprint has a unique pattern (unless they're twins or clones). This means you can tell people apart by comparing samples of their DNA.

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DNA Fingerprinting

DNA fingerprinting is used in...

1) Forensic science - DNA (from hair, skin flakes, blood, semen etc.) taken from a crime scene is compared with a DNA sample taken from a suspect.

2) Paternity testing - to see if a man is the father of a particular child.

Some people would like there to be a national database of everyone in the country. That way, DNA from a crime scene could be checked against everyone in the country to see whose it was. But others think this is a big invasion of privacy, and they worry about how safe the data would be and what else it might be used for. There are also scentific problems - false positives can occur if errors are made in the procedure or if the data is misinterpreted.

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Mitosis - Growth & Repair

1) Body cells normally have two copies of each chromosome - one from the organism's 'mother', and one from its 'father'. So, humans have two copies of chromosome 1, two copies of chromosome 2, etc.

2) Human cells have 23 pairs of chromosomes. The 23rd pair are a bit different.

3) When a body cell divides it needs to make new cells identical to the original cell - with the same number of chromosomes.

4) This type of cell division is called mitosis. It's used when plants and animals want to grow or replace cells that have been damaged.

Mitosis is when a cell reproduces itself by splitting to form two identical offspring.

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  • In a cell that's not dividing, the DNA is all spread out in long strings.
  • If the cell gets a signal to divide, it needs to duplicate its DNA - so there's one copy for each new cell. The DNA is copied and forms X-shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other. (The left arm has the same DNA as the right arm of the chromosome.)
  • The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the new cell.
  • Membranes form around each of the sets of chromosomes.
  • These become the nuclei of the two new cells.
  • Lastly, the cytoplasm divides.
  • You now have two new cells containing exactly the same DNA - they're identical.


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Asexual Reproduction

1) Some organisms also reproduce by mitosis, e.g. strawberry plants form runners in this way, which become new plants.

2) This is an example of asexual reproduction.

3) The offspring have exactly the same genes as the parent - so there's no variation.

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1) During sexual reproduction, two cells called gametes (sex cells) combine to form a new individual.

2) Gametes only have one copy of each chromosome. This is so that you can combine one sex cell from the 'mother'and one sex cell from the 'father' and still end up with the right number of chromosomes in body cells. For example, human body cells have 46 chromosomes. The gametes have 23 chromosomes each, so that when an egg and sperm combine, you get 46 chromosomes again.

3) The new individual will have a mixture of two sets of chromosomes, so it will inherit features from both parents. This is how sexual reproduction produces variation.

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Meiosis - Two Divisions

To make new cells which only have half the original number of chromosomes, cells divide by meiosis. In humans, it only happens in the reproductive organs (e.g. ovaries and testes).

Meiosis produces cells which have half the normal number of chromosomes.

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  • As with mitosis, before the cell starts to divide, it duplicates its DNA - one arm of each chromosome is an exact copy of the other arm.
  • In the first division in meiosis (there are two divisions) the chromosome pairs line up in the centre of the cell.
  • The pairs are then pulled apart, so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell.
  • In the second division the chromosomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosome are pulled apart.
  • You get four gametes each with only a single set of chromosomes in it.

After two gametes join at fertilisation, the cell grows by repeatedly dividing by mitosis.


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Embryonic Stem Cells

1) Differentiation is the process by which a cell changes to become specialised for its job. In most animal cells, the ability to differentiate is lost at an early stage, but lots of plant cells don't ever lose this ability.

2) Some cells are undifferentiated. They can develop into different types of cell depending on what instructions they're given. These are called STEM CELLS.

3) Stem cells are found in early human embryos. They're exciting to doctors and medical researchers because they have the potential to turn into any kind of cell at all. This makes sense if you think about it - all the different types of cell found in a human being have to come from those few cells in the early embryo.

4) Adults also have stem cells, but they're only found in certain places, like bone marrow. These aren't as versatile as embryonic stem cells - they can't turn into any cell type at all, only certain ones.

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Stem Cells

1) Medicine already uses adult stem cells to cure disease. For example, people with some blood diseases (e.g. sickle cell anaemia) can be treated by bone marrow transplants. Bone marrow contains stem cells that can turn into new blood cells to replace the faulty ones.

2) Scientists can also extract stem cells from very early human embryos and grow them.

3) These embryonic stem cells could be used to replace faulty cells in sick people - you could make beating heart muscle cells for people with heart disease, insulin-producing cells for people with diabetes, nerve cells for people paralysed by spinal injuries, and so on.

4) To get cultures of one specific type of cell, researchers try to control the differentiation of the stem cells by changing the environment they're growing in. So far, it's still a bit hit and miss - lots more research is needed.

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Stem Cell Research

1) Some people are against stem cell research because they feel that human embryos shouldn't be used for experiments since each one is a potential human life.

2) Otehrs think that curing patients who already exist and who are suffering is more important that the rights of embryos.

3) One fairly convincing argument in favour of this point of view is that embryos used in the researchare usually unwanted ones from fertility clinics which, if they weren't used for research, would probably just be destroyed. Campaigners for the rights of embryos want this banned too.

4) These campaigners feel that scientists should concentrate more on finding and developing other sources of stem cells, so people could be helped without having to use embryos.

5) In some countries stem cell research is banned, but it's allowed in the UK as long as it follows strict guidelines.

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Your Chromosomes

There are 22 matched pairs of chromosomes in every single human body cell. The 23rd pair are labelled ** or XY. They're the two chromosomes that decide whether you turn out male or female.

All men have an X and a Y chromosome: XY
The Y chromosome causes male characterisitics.
All women have two X chromosomes: **
The ** combination allows female characteristics to develop.

When making sperm, the X and Y chromosome are drawn apart in the first division in meiosis. There's a 50% chance each sperm cell gets an X-chromosome and 50% chance it gets a Y-chromosome.

A similar thing happens when making eggs. But the original cell has two X-chromosomes, so all the eggs have one X-chromosome.

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Genetic Diagrams

  • 1) To find the probability of getting a boy or a girl, you can draw a genetic diagram.
  • 2) Put the possible gametes from one parent down the side, and those from the other parent along the top.
  • 3) Then in the middle square you fill in the letter from the top and side that line up with that square. The pairs of letter in the middle show the possible combinations of gametes.
  • 4) There are two ** results and two XY results, so there's the same probability of getting a boy or a girl.
  • 5) Don't forget that this 50:50 ratio is only a probability at each pregnancy. If you had four kids they could all be boys.


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Genetic Diagrams

The other type of genetic diagram looks a bit more complicatd, but it shows exactly the same thing.

  • 1) At the top are the parents.
  • 2) The middle circles show the possible gametes that are formed. One gamete from the female combines with one gamete from the male (during fertilisation).
  • 3) The criss-cross lines show all the possible ways the X and Y chromosomes could combine. The possible combinations of the offspring are shown in the bottom circles.
  • 4) Remember, only one of these possibilities would actually happen for any one offspring.


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Mendel - Genetic Experiments

On his garden plot, Gregor Mendal noted how characteristics in plants were passed on from one generation to the next. The results of his research were published in 1866 and eventually became the foundation of modern genetics.

Genetic Diagram:

First Cross:                                                                      Second Cross:

Mendal had shown that the height characteristics in pea plants was determined by separately inherited "hereditary units" passed on from each parent. The ratios of tall and dward plants in the offspring showed that the until for tall plants, T, was dominant over the unit for dward plants, t.

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Mendal - Conclusions

Mendel reached these three important conclusions about herefity in plants:

1) Characteristics in plants are determined by "hereditary units".

2) Hereditary units are passed on from both parents, one unit from each parent.

3) Hereditary units can be dominant or recessive - if an individual has both the dominant and recessive unit for a characteristic, the dominant characteristic will be expressed.

We now know that the "hereditary units" are of course genes. But in Mendel's time nobody knew anything about genes or DNA, and so the significance of his work  was not to be realised until after his death.

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Possible Genes of Offspring

1) Alleles are different versions of the same gene.

2) In genetic diagrams letters are usually used to represent alleles.

3) If an organism has two alleles for a particular gene th same, then it's homozygous. If its two alleles for a particular gene are different, then it's hetrozygous.

4) If the two alleles are different, only one can determine what characteristic is present. The allele for the characteristic that's shown is called the dominent allele (use a capital letter for dominant alleles - e.g. 'C'). The other one is called recessive (and you show it with small letters - e.g. 'c').

5) For an organism to display a recessive characteristic, both its alleles must be recessive (e.g. cc). But to display a dominant characteristic the organism can be either CC or Cc, because the dominant allele overrules the recessive one if the plant/animal/other organism is hetrozygous.

  • Remember, gametes only have one allele, but all the other cells in an organism have two.
  • Genotype means what alleles you have. Phenotype means the actual characteristic.
  • When you cross two parents to look at just one characteristic it's called monohybrid cross.
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Cystic Fibrosis

Cystic fibrosis is a genetic disorder of the cell membranes. It results in the body producing a lot of thick sticky mucus in the air passages and in the pancreas.

1) The allele which causes cystic fibrosis is a recessive allele, 'f', carried by about 1 person in 25.

2) Because it's recessive, people with only one copy of the allele won't have the disorder - they're known as carriers.

3) For a child to have the disorder, both parents must be either carriers or sufferers.

4) There's a 1 in 4 chance of a child having the disorder if both parents are carriers.


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Polydactyly is a gentetic disorder where a baby's born with extra fingers or toes. It doesn't usually cause any other problems so isn't life-threatening.

1) The disorder is caused by a dominant allele, 'D', and so can be inherited if just one parent carries the defective allele.

2) The parent that has the defective allele will be a sufferer too since the allele is dominant.

3) There's a 50% chance of a child having the disorder if one parent has the D allele.


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Embryonic Screening

1) During in vitro fertilisation (IVF), embryos are fertilised in a labratory, and then implanted into the mother's womb. More than one egg is fertilised, so there's a better chance of the IVF being successful.

2) Before being implanted, it's possible to remove a cell from each embryo and analyse its genes.

3) Many genetic disorders could be detected in this way, such as cystic fibrosis.

4) Embryos with 'good' alleles would be implanted into the mother - the ones with 'bad' alleles destroyed. 

There is a huge debate raging about embryonic screening

Many people think that embryonic screening isn't justified for genetic disorders that don't affect a person's health, such as polydactyly.

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Against Embryonic Screening

1) There may come a point where everyone wants to screen their embryos so they can pick the most 'desirable' one, e.g. they want a blue-eyed, blond-haired, intelligent boy.

2) The rejected embryos are destroyed - they could have developed into humans.

3) It implies that people with genetic disorders are 'undesirable' - this could increase prejudice.

4) Screening is expensive.

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For Embryonic Screening

1) It will help to stop people suffering.

2) There are laws to stop it going to far. At the moment parents cannot even select the sex of their baby (unless it's for health reasons).

3) During IVF, most embryos are destroyed anyway - screening just allows the selected one to be healthy.

4) Treating disorders costs the Government (and the taxpayers) a lot of money.

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Fossils are the remains of plants and animals.

Fossils are the remains of organisms from many years ago, found in rocks.

Fossils provide the evidence that organisms lived ages ago.

Fossils form in rocks in one of three ways.

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Gradual Replacement By Minerals

(Most fossils happen this way.)

1) Things like teeth, shells and bones etc., which dont decay easily, can last a long time when buried.

2) They're eventually replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part.

3) The surrounding sediments also turn to rock, but the fossil stays distinct inside the rock and eventually someone digs it up.

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Casts & Impressions

1) Sometimes, fossils are formed when an organism is buried in a soft material like clay. The clay later hardens around it and the organism decays, leaving a cast of itself. An animal's burrow or a plant's roots can be preserved as casts.

2) Things like footprints can be pressed into these materials when soft, leaving an impression when it hardens.

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Preservation in places where no decay happens.

1) In amber (a clear yellow 'stone' made from fossilised resin) and tar pits there's no oxygen or moisture so decay microbes can't survive.

2) In glaciers it's too cold for the decay microbes to work.

3) Peat bogs are too acidic for decay microbes.

(A fully preserved man they named 'Pete Marsh' was found in a bog.)

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How Life Began

Fossils show how many of today's species have evolveed (changed and developed) over millions of years. But where did the first living things come from...

1) There are various hypotheses suggesting how life first came into being, but no one really knows.

2) Maybe the first life forms came into existence in a primordial swamp (or under the sea) here on Earth. Maybe simple organic molecules were brought to Earth on comets - these could have then become more complex organic molecules, and eventually very simple life forms.

3) These hypotheses can't be supported or disproed because there's a lack of valid and reliable evidence.

4) There's a lack of evidence because scientists believe many early organisms were soft-bodied, and soft tissue tends to decay away completely. So the fossil record is incomplete.

5) Plus, fossils that did form millions of years ao may have been destroyed by geological activity, e.g. the movement of tectonic plates may have crushed fossils already formed in the rock.

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The fossil record contains many species that don't exist any more - these species are said to be extinct. Dinosaurs and mammoths are extinct animals, with only fossils to tell us they existed at all.

Species become extinct for these reasons:

  • The environment changes too quickly (e.g. destruction of habitat).
  • A new predator kills them all (e.g. humans hunting them).
  • A new disease kills them all.
  • They can't complete with another (new) species for food.
  • A catastrophic event happens that kills them all (e.g. a volcanic erruption or a collision with an asteroid).
  • A new species develops (this is called speciation).

Dodos are now extinct. Humans not only hunted them, but introduced other animals which ate all their eggs, and we destroyed the forest where they lived.

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Speciation is the development of a new species.

1) A species is a group of similar organisms that can reproduce to give fertile offspring.

2) Speciation is the development of a new species.

3) Speciation occurs when populations of the same species become so different that they can no longer breed together to produce fertile offspring.

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Isolation & Natural Selection

Isolation is where populations of a species are separated. This can happen due to a physical barrier. E.g. floods and earthquakes can cause barriers that geogrphically isolate some individuals from the main population. Conditions on either side of the barrier will be slightly different, e.g. they may have different climates. Because the environment is different on each side, different characteristics will become more common in each population due to natural selection:

1) Each population shows variation because they have a wide range of alleles.

2) In each population, individuals with characteristics that make them better adapted to their environment have a better chance of survival and so are more likely to breed successfully.

3) So the alleles that control the beneficial characteristics are more likely to be passed on to the next generation.

Eventually, individuals from the different populations will have changed so much that they won't be able to breed with one another to produce fertile offspring. The two groups will have become separate species.

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