GCSE Biology 2b

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• Enzymes

1. Enzymes are catalysts produced by living things
   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 amount of substance
   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. so ... living things produce enzymes that act as a biological catalysts. Enzymes reduce the need for high temperatures and we only have enzymes to speed up the useful chemical reactions in the body.
   5. Enzymes are all proteins and all proteins are make up of chains of amino acids. These chains are folded into unique shapes, which enzymes are needed to do their jobs.
   6. As well as catalysts, proteins act as a structural compounds of tissues (e.g. muscles), hormones and antibodies.
2. Enzymes have special shapes so they can catalyse reactions
   1. Chemical reactions usually involve things either being split apart or joining 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 enzyme's shape, then the reaction won't be catalysed.
3. Enzymes need the right temperature and pH
   3. Changing the temperature changes the rate of an enzyme-catalysed reaction
   4. Like with any reaction, a higher 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 activation zone (special shape) and so it won't work any more. It's said to be denatured.
   5. 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.
   6. Enzymes in the human body normally work best at 37°C
   7. All enzymes have an optimum pH that they work beat 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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• Enzymes and Digestion

1. Digestive enzymes break down big molecules into smaller ones
   1. Starch, proteins and fats are BIG molecules. They're too big to pass through the walls of the digestive system.
   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 BIG molecules into smaller ones.
2. Bile neutralises the 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 alkaline conditions.
   3. It emulsifies fats. In other words it breaks the fat into tiny droplets. This gives a much bigger surface are of fat for the enzyme lipase to work on - which makes its digestion faster.
3. The breakdown of food is catalysed by enzymes
   2. Enzymes used in the digestive system are produced by specialised cells in glands and in the gut lining.
   3. Different enzymes catalyse the breakdown of different food molecules.

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• Enzymes and Respiration

1. Respiration is not breathing in and out
   1. Respiration involves many reactions, all of which are catlysed by enzymes. There are really important reactions, as respiration releases the energy that all cells need to do just about everything.
   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.
2. Aerobic respiration needs plenty of oxygen
   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. Aerobic respiration goes on all the time in plants and animals.
   3. Most of the reactions in aerobic respiration happens inside mitochondria.
3. Respiration releases energy in all kinds of things

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• Exercise

1. Exercise increases the heart rate
   1. Muscles are make of muscle cells. these use oxygen to release energy from glucose (aerobic respiration), which is used to contract the muscle.
   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:
      1. Increases your breathing rate and makes you breathe more deeply to meet the demands for extra oxygen.
      2. Increases the speed at which your heart pumps.
2. Glycogen is used during exercise
   1. Some glucose from food is stored as glycogen.
   2. Glycogen's mainly stored in the liver, but each muscle has its own store.
   3. During various exercise muscles use glucose rapidly, so some of the glycogen is converted back to glucose to provide more energy.
3. Anaerobic respiration is used if there's not enough oxygen
   1. When you do vigorous exercise and your body can't supply enough oxygen to your muscles, they start doing anaerobic respiration instead or aerobic respiration.
   2. "Anaerobic" just means "without oxygen". It's the incomplete breakdown of glucose which produces lactic acid.
   3. It's not the best way of converting glucose into energy because lactic acid builds up in the muscles, which gets painful. It also causes muscle fatigue - the muscle gets tired and they stop contracting efficienty.
   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 advantages is that at least you can keep using your muscles for a while longer.
4. Anaerobic respiration leads to an oxygen debt
   1. After resorting to anaerobic respiration, when you stop exercising you'll have an "oxygen debt".
      1. In other words you have to "repay" the oxygen that you didn't get to your muscles in time, because your lungs, heart, and blood couldn't keep up with the demand earlier on.
         1. This means you have to keep breathing hard for a while after you stop, to get oxygen into your blood. Blood flows through your muscles to remove lactic acid by oxidising it to harmless carbon dioxide and water.
   2. While high level 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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• Uses of Enzymes

1. Enzymes are used in biological detergents
   1. Enzymes are the 'biological' ingredients in biological detergents and washing powders
   2. They're mainly protein-digesting enzymes (proteases) and fat-digesting enzymes (lipases)
   3. Because the enzymes break down plant and animal matter, they're ideal for removing stains like food or blood
   4. Biological detergents are also more effective at working at low temperatures (e.g. 30 °C) than other types of detergents
2. Enzymes are used to change food
   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 to turn starch syrup into sugar syrup
   3. Glucose syrup can be turned into fructose syrup using an isomerase enzyme. Fructose is sweeter, so you can use less of it - good for slimming foods and drinks
3. Using enzymes in industry uses a lot of control
   1. Enzymes are really useful in industry. They speed up reactions without the need for high temperatures and pressures. You need to know the advantages and disadvantages of using them
      1. Advantages
         1. They're specific, so they only catalyse the reaction you want them to
         2. Using lower temperatures and pressures means a lower cost and it saves energy
         3. Enzymes work for a long time, so after the initial cost of buying them you can continually use them
         4. They are biodegradable and therefore cause less environmental pollution
      2. Disadvantages
         1. Some people can develop allergies to the enzymes (e.g. in biological washing powders
         2. 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 must be tightly controlled.
         3. Enzymes can be expensive to produce
         4. Contamination of enzymes with other substances can affect the reaction

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• DNA

1. Chromosomes Are Really Long Molecules of DNA
   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 cells, in really long molecules called chromosomes
2. A gene codes for a specific proteins
   1. A gene is a section of DNA. It contains 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. Genes simply tell cells in what order to put the amino acids together
   5. DNA also determines what proteins the cell produces, e.g. hemoglobin, keratin
      1. That intern decides what type of cell it is, e.g. red blood cell, skin cell
3. Everyone has unique DNA
   1. Almost everyone's DNA is unique. The only exception are identical twins or clones
   2. DNA fingerprinting is a way of cutting up a person's DNA into small sections and then separating them. Every person's genetic fingerprint had a unique pattern. This means that you can tell people apart by comparing their DNA
   3. DNA fingerprinting is used in...
      1. Forensic science - DNA (from hair, skin, blood etc.) taken from a crime scene is compared with DNA samples taken from a suspect
      2. Paternity testing - to see if someone is the father of a particular child

1. Mitosis

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   • Cell Division - Mitosis
   1. Mitosis makes new cells for growth and repair
      1. Body cells normally have 2 copies of each chromosome - one from the organism's mother and one from the father
      2. The diagram shows 23 pairs of chromosomes from a human cell. the 23rd pair is 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 to replace cells that have been damaged
   2. Mitosis is when a cell reproduces itself by splitting to form two identical offspring
      2. In the cell that's not dividing, the DNA is all spread out in long strings
      3. 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
         1. The left arm has the same DNA as the right arm of the chromosome
      4. The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The 2 arms of each chromosome go to opposite ends of the cell
      5. Membranes form around each of the sets of chromosomes. These become the nuclei of the 2 new cells
      6. Lastly the cytoplasm divides
      7. You now have 2 new cells containing exactly the same DNA - they'r identical
   3. Asexual reproduction also uses mitosis
      1. Some organisms also reproduce by mitosis e.g. strawberry plants form runners in this way
         1. This is an example of asexual reproduction
      2. The offspring have the exact same genes as the parent - there is NO VARIATION
2. Meiosis

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   • Cell Division - Meiosis
   1. Gametes have half the usual number of chromosomes
      1. During sexual reproduction, two ells called gametes (sex cells) combine to form a new individual
      2. Gametes only have one copy of each chromosome
         1. 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 the body cell.
            1. For example, human body cells have 46 chromosomes. The gametes have 23 chromosomes each, so that when the 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 feature from both parents
         1. This is how variation is produced
   2. Meiosis involves two divisions
      1. To make new cells which 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)
         1. As with Mitosis, before the cell starts to divide, it duplicates it's DNA - one arm of each chromosome is an exact copy of the other arm
         2. In the first division in meiosis (there are 2 divisions) the chromosome pairs line up in the centre of the cell
         3. The pairs are pulled apaet, so each new cell only has one copy of each chromosome. Some of the father's chromosomes (shown in blue) and some of the mother's chromosomes (shown in red) go into each new cell
         4. 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
         5. You get 4 gametes each with on;y a single set of chromosomes in it. After 2 gametes join at fertilisation, the cell grows by repeatedly dividing by mitosis

1. Embryonic stem cells can turn into any type of cell
   1. You know the differentiation is the process by which a cell changes to become specialised for it's job. In most animal cells, the ability to differentiate is lost at an early stage, but lots of plant cells don't ever loose this ability
   2. Some cells are undifferentiated. They can develop into different types of cells depending on what instructions they are 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 come from those few cells in the early embryo
2. Stem cells may be able to cure many diseases
   1. Medicine already uses adult stem cells to sure disease. Foe 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 old ones.
   2. Scientists can also extract stem cells from very early human embryos and grow them
      1. 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
   3. To get cultures of one specific type of cell, researchers try to control the differentiation of the stem cell by changing the environment they're growing in. So far, it's still a bit hit and miss - lots more research is needed
3. Some people are against stem cell research
   1. Some people are against stem cell research because they feel the human embryos should't be used for experiments since each one is a potential human life
      1. These campaigners feel that scientists should concentrate more on finding and developing other sources of stem cells, so people could help without using embryos
   2. Others think that curing patients who already exist and who are suffering is more important than the rights of the embryos
      1. One fairly convincing argument in favour of this point of view is that the embryos used in the research are usually unwanted ones from fertility clinics which, if they weren't used for research, would probably just be destroyed. But of course, campaigners for the right of the embryo usually want this banned too
   3. 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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• X and Y Chromosomes

1. Your chromosomes control whether you're male or female
   1. There are 22 matched pairs of chromosomes in every human body cell. The 23rd pair are lablled XX or XY. They're two chromosomes that decide whether or not you are male or female
      1. All biological males have an X and a Y chromosome: XY
         1. The Y chromosome caused male characteristics
      2. All biological females have two X chromosomes: XX
         1. The XX combination allowed female characteristics to develop
   2. When making sperm, the X and Y chromosomes are drawn apart in the first division of meiosis. There is a 50% chance of of each cell getting an X or Y chromosome
      1. A similar thing happens when making eggs. But the original cell has two X chromosomes, so all eggs have one X chromosome
2. Genetic diagrams show the possible combinations of gametes
   1. To find the probability of getting a girl or a boy, you can draw a genetic diagram.
      1. Put the possible gametes from one parent down the side, and those from the other parent along the top
         1. Then inside each middle square you fill in the letters from the top and side that line up with the square. The pares of letters in the middle show the possible combinations of the gametes.
   2. The other type of genetic diagram looks a bit more complicated, but it shows exactly the same thing
      1. At the top are the parents
         1. The middle circles show the possible gametes that are formed. One gamete from the female combines with one gamete from the male (during fertilisation)
            1. 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
               1. Remember only one of the possibility could actually happen for any one offspring

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• The Work of Mendel

1. Mendel did genetic experiments with pea plants
   1. Gregor Mendel was an Australian monk who trained in mathmatics and natural history at the university of Vienna. One his garden plot at the monastert, Mendel noted how characteristics in plants were passed on from one generation to the next
      1. The results of his research were published in 1866 and eventually became the foundation of modern genetics
         1. These diagrams show two crosses for height in pea plants that Mendel carried out
            3. Mendel had shown that the height characteristics in pea plants were determined by separately inherited "hereditary units" passed on from each parent. The ratios of tall and dwarf plants in the offspring chowed that the unit for tall plants, T, was dominant over the unit for dwarf plants, t
2. Mendel researched three important conclusions
   1. 1) Characteristics in plants are determined by "hereditary units"
   2. 2) Hereditary units are passed on from both parents, one unit from each parent
   3. 3) Hereditary units can be dominant or recessive - if an individual has both the dominant and the recessive unit for a characteristic, the dominant characteristic will be expressed

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• Genetic Diagrams

1. Genetic diagrams show the possible genes of offspring
   1. 1) alleles are different versions of the same gene
   2. 2) In genetic diagrams letters are usually used to represent alleles
   3. 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 dominant allele (the capital letter) the other one is called the recessive (the lowercase letter)
   4. 3) If an organism has two alleles for a particular gene that are the same, then it's homozygous. If its two alleles for a particular gene are different, then it;s hetrozygous
   5. 5) For an organism to display a recessive characteristic, both its alleles much be recessive (both lower case). But to display a dominant characteristic the organism has to have at least one upper case allele because the dominant allele overrules the recessive one
   6. Genotype is the allele you have
   7. Phenotype is the characteristic you have

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• Genetic Disorders

1. Cystic Fibrosis is caused by a recessive allele
   1. Cystic fibrosis is a genetic disorder of the cell membranes. It results in the body producing a lot of thick mucus in the air passages and in the pancreas
      1. 1) the allele that causes the cystic fibrosis is a recessive allele
      2. 2) Because it's recessive, people with only one copy of the allele won't have the disorder - they're known as carriers
      3. 3) For a child to have the disorder, both parents must be either a carrier or a sufferer
      4. 4) As the diagram shows there's a 1 in 4 chance of a child having the disorder if bother parents are carriers
2. Polydactyly is caused by a dominant allele
   1. Polydactyly is a genetic disorder where a baby's born with extra fingers or toes. It doesn't usually cause any other problems so it isn't life-threatening
      1. 1) The disorder is caused by a dominant allele, D, and so can be inherited if just one parent carries the defective allele
      2. 2) The parent that has the defective allele will be a sufferer as it is dominant
      3. 3) As the genetic diagram shows, there's a 50% chance of the child having the disorder is one of the parents have a D allele
3. Embryos can be screened for genetic disorders
   1. 1) During in vitro fertilisation (IVF), embryos are fertilised in a lab, 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. 2) Before being implanted, it's possible to remove a cell from each embryo and analyse its genes.
   3. 3) Many genetic disorders could detected this way, such a cystic fibrosis
   4. 4) Embryos with "good" alleles would be implanted into the mother - the ones with "bad" alleles are destroyed
   5. There is a huge debate rang about embryonic screening. Here are some arguments for and against
      1. Against
         1. There may come a point where everyone wants to screen their embryos so they can pick the most 'desirable' one
         2. The rejected embryos are destroyed - they could have developed into humans
         3. It implies that people with genetic problems are 'undesirable' - this could increase prejudice
         4. Screening is expensive
      2. For
         1. It will hep to stop people suffering
         2. There are laws to stop it going too far. At the moment parents can't can't even select the sex of their baby (unless it is for health reasons)
         3. During IVF, most of the embryos are destroyed anyway- screening just allows the selected ones to be healthy
         4. Treating disorders costs the Government (and the taxpayers) a lot of money

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• Fossils

1. Fossils are the remains of plants and animals
   1. 1) from gradual replacement by minerals
      1. !) Things like teeth, shells, bones etc., which don't decay easily can last a long time when buried
      2. 2) They're eventually replaced by minerals as they decay forming a rock-like substance shaped like the original hard part
      3. 3) The surrounding sediment also turns to rock, but the fossil stays distinct inside the rock and eventually someone digs it up
   2. 2) from casts and impressions
      1. 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. 2) Things like footprints can be pressed into these materials when soft, leaving an impression when it hardens
   3. 3) From preservation in places where no decay happens
      1. 1) In amber and tar pits there's not oxygen or moisture so decay microbes can't survive
      2. 2) In glaciers it's too cold for the decay microbes to work
      3. 3) Peat bogs are too acidic for decay microbes.
2. But no one knows how life began
   1. Fossils show how many of today's species have evolved over millions of years. But where did the first living thing come from?
      1. 1) There are various hypotheses suggesting how life first came into being, but no one really knows.
      2. 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 by comets - these could have then become more complex organic molecules, and eventually very simple life forms
      3. 3) These hypotheses can't be supported or disproved because there's a lack of valid or reliable evidence
      4. 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. 5) Plus, fossils that did form millions of years ago may have been destroyed by geological activity, e.g. the movement of tectonic plates may have crushed fossils already formed in the rocks

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• Extinction and Speciation

1. Extinction happens if you can't evolve quickly enough
   1. The fossil record contains many species that don't exist anymore - these species are said to be extinct. Dinosaurs and mammoths are extinct, with only fossils to tell us they existed at all. Species become extinct for these reasons
      1. The environment changes too quickly (e.g. destruction of habitats)
      2. A new predator kill =s them all (e.g. humans hunting them)
      3. A disease kills them all
      4. They can't compete with other (new) species' for food
      5. A castastrophic event happens to kills them all (e.g. a volcanic eruption)
      6. A new species develops (this is called speciation)
2. Speciation is the development of a new species
   1. A species is a group of similar organism that can reproduce to give fertile offspring
   2. Speciation is the development of a new specie
   3. Speciation occurs when population of the same species become so different that they can no longer breed together to produce fertile offspring
3. Isolation and natural selection lead to speciation
   1. 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 geographically 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. 1) Each population shows variation because they have a wide range of alleles
      2. 2) In each population, individuals with characteristics that make them better adapt to their environment so they have a better chance of survival and so are more likely to breed successfully
      3. 3) So the alleles that control the beneficial characteristics are more likely to be passes on to the next generation
      4. Eventually, individuals from the different populations will have changed so much that they won't be able to breed with the other to produce fertile offspring. The two groups will have become separate species