- Animal cells...
- Nucleus - contains DNA in the form of chromosomes.
- Cell membrane - holds the cell together and controls what goes in and out.
- Ribosome - where proteins are synthesised.
- Cytoplasm - where most cells chemical reactions happen.
- Mitochondria - where most of the reactions involved in respiration take place.
- Plant cells...
- Have most of the above +
- Cell wall - made of cellulose. It supports the cell.
- Vacuole - a large structure that contains cell sap, a weak solution of sugar and salts.
- Chloroplasts - where photsynthesis happens.
- Bactrial cells are different...
- They have no true nucleus, instead they have a single strand of DNA that floats in the cytoplasm.
- They don't have chloroplasts or mitochondria.
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- Chromosomes are long molecules of coiled up DNA which is divided up into short section called genes.
- DNA is a double helix. Each of the two DNA strands is made up of lots of small groups called nucleotides.
- Each nucleotide contains a small molecule called a base.
- DNA has four different bases A, C, G, and T.
- A pairs with T and C with G.
- Francis Crick and James Watson were the first scientists to build a DNA model in 1953.
- They used x-ray data that showed DNA is a double helix and other data that showed bases were in pairs.
- By putting together this infomation they were able to build a model showing what DNA looked like.
- DNA copies itself when a cell divides.
- To do this the DNA double helix unzips to form two single strands.
- New nucleotides then join using complemantary base-pairing (A with T and G with C).
- The result is two double-stranded molecules of DNA.
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- DNA controls the production of proteins in a cell.
- A section of DNA that codes for a paticular protein is called a gene.
- Protiens are made up of chains of molecules called amino acids. Each different protien has its own particular order and number of them.
- This means each different protein has a different shape therefore function.
- It's the order of the bases in a gene that decides the order of amino acids.
- Each amino acid is coded for by three bases in the gene.
- Each gene contains a different sequence of bases.
- Proteins are made in the cell cytoplasm by ribosomes.
- The ribosomes use the code in the DNA to make proteins however the DNA can't move from the nucleus.
- To solve this a molecule called mRNA copys the code from the DNA the takes it to the ribosome.
- The proteins in a cell affect it's functions.
- Different types of cell have different fuctions because of the proteins.
- Genes that are switched on determine a cells function.
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Functions of protein.
- There are hundreds of different proteins that all have different fuctions.
- Carrier molecules - used to transport smaller molecules e.g. haemoglobin binds to oxygen molecules and transports them around the body.
- Hormones - used to carry messages around the body e.g. Insulin.
- Structural proteins - are physically strong e.g. collagen is a structural protein that strengthens connective tissues.
- Cells have lots of chemical reactions inside which need to be controlled.
- You can increase a reaction speed by raising the temperature.This would speed up the useful reactions but also the unwanted ones.
- Enzymes solve this - they act as a biological catalysts.
- Enzymes reduce the need for high temperature and we only have enzymes which speed up the useful chemical reactions.
- Substrate is the molecule changed in a reaction.
- Every enzyme has an active site - the part where it joins on to it's substarte to catalse the reaction.
- Enzymes only work with one substrate - high specificity for their substrate
- This is because for an enzyme to work the substrate has to fit the active sight - lock and key mechanism.
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More on enzymes.
- Changing the temperature changes the rate of an enzyme-catalysed reaction.
- Enzymes have an optimum temperature.
- If during a reaction it gets too hot some bonds holding enzymes together will break.
- This makes an enzyme lose its shape which means its active site doesn't fit the substrate anymore. This means it doesn't catalyse anymore.
- The enzyme is now denatured. This is an irreversible reaction.
- The pH also has an effect on enzymes if it is too low or high it will denature
- Often the optimum pH is 7 but it can change.
- The Q10 value shows how much the rate changes when the temperature is raised by 10*c.
- Q10 = rate at higher temperature/ rate at lower temperature.
- The reaction data you get in an exam could be in a graph or table.
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- A mutation is a change in the DNA base sequence.
- If a mutation occurs it could stop the production of the protein the gene usually codes for or a different protein is made.
- If a mutation occurs in reproductive cells the the offspring might develop abnormally.
- If a mutation happens in body cells the mutant cells can start to multiply and spread. This is cancer.
- Sometimes a different protein is produced through mutations which is benificial.
- This gives the organism a survival advantage. It then passes on the mutation to its offspring and soon the mutation becomes common.
- This is natural selection and evolution.
- A good example is a mutation in bacterium that makes it resistant to antibiotics, so the mutant gene lives on creating a resitant strain.
- Chances of mutation increase due to...
- Ionising radiation such as X-rays, UV and from radioactive substances.
- Chemicals known as mutagens - if they produce cancer they are called carcinogens.
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Multiplying cells - being multicellular.
- Being multicellular has some important advantages...
- You can be bigger so you can travel further and get your nutrients in different ways.
- Being multicellular allows for cell differentiation. This means your cells can be specially adapted to do one job.
- Multicellular organisms are more complex.
- However being multicellular means that an organism has to have specialised organ systems inculuding...
- A system to communicate between different cells.
- A system to supply cells with nutrients they need.
- A system that controls the exchange of substances with the environment.
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Multiplying cells - mitosis
- Mitosis is when a cell reproduces itself by splitting to form two identical offspring
- This happens when you want identical cells.
- Before mitosis starts the DNA in the cell is replicated.
- Then at the beginning of mitosis, the DNA coils into double armed chromosomes. These arms are exact copies of each other - they contain the same DNA.
- The chromosomes line up at the centre of the cell and then divide as cell fibres pull them apart.
- The two arms of each chromosome go to opposite ends of one cell. Membranes form round each of these two different sets of chromosomes.
- The cytoplasm divides and you get two new cells containing exactly the same genetic material.
- You have ended up with two new cells that are genetically identical.
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Gametes and fertilisation.
- Gametes are fromed by meiosis in the ovaries and testes. Gametes are the sex cells.
- The body cells of mammals are diploid which means that each of the organisms body cells has two copies of each chromosome in it's nucleus. One from the mum and one from the dad.
- However gametes are different because they are haploid - they have only one copy of each chromosome. This is so that when the egg and sperm combine they'll form a diploid cell.
- At fertilisation male and female gametes combine to form a diploid cell.
- This cell is called a zygote.
- The characteristics of the zygote are controlled by the combination of genes on it's chromosmes.
- A sperms function is to transport the male's DNA to the females egg they are adapted to do this job...
- They're small and have long tails so they can swim to the egg.
- They have lot's of mitochondria to provide energy needed to swim.
- They also have acrosome at the front of the head which can release enzymes they need to digest their way through the membrane of an egg cell.
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- Meiosis involves two divisions.
- Meiosis starts the same way as mitosis the DNA replicates and forms double armed chromosmes.
- Next the chromosomes arrange themselves into pairs.
- In the first division these pairs split up - the chromosomes in each new pair move to opposite ends of the cell .
- In each of the two new cells there is a mixture of you're mum and dads chromosomes, but only half the usual number of chromosomes.
- The second division of meiosis is like mitosis each chromosome splits in half.
- You now have four new cells.
- The cells are genetically different from each other because the chromosmes all get shuffled up during meiosis.
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Stem cells, differentiation and growth.
- Plants grow continuosly by cell enlargement
- Animals grow untill they reach finite size by cell division. Cell division in plants happens in the areas of plants called meristems (roots + shoots).
- Differentiation - where a cell changes to become specialised for its job.
- In most animal cells the abilty to differentiate is lost at an early stage but in plants it is never lost.
- Some cells are undifferentiated e.g. Stem cells are found in early human embryos and have the potential to turn into any cell in the body.
- Adults also have stem cells but they're not as versatile as embryonic stem cells. They can only turn into certain types of cells.
- They are just found in certain places like bone marrow.
- Medicine already uses stem cells to cure diseases e.g. people with blood disorders like sickle cell anaemia and leukaemia can be cured by bone marrow transplants.
- Early embryos contain lots of stem cells which can be extracted and used.
- Some people are against stem cell reserach because they believe embros are a potential human life, but some are in favour of using stem cell because they can help stop people suffering.
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- There are different ways to mesure growth all have advantages and disadvantages...
- Lenght - easy to measure but it doesn't tell you about the changes in width, diameter and number of branches.
- Wet mass - easy to measure but very changeable.
- Dry mass - Its not affected by the amount of water in a plant or animal but you have to kill the organism to measure it.
- Humans go through five main phases of growth...
- Infancy - roughly the first two years of life. Rapid growth.
- Childhood - Period between infancy and puberty. Steady Growth.
- Adolescence - Begins with puberty and continues untill body development and growth are complete. Rapid growth
- Maturity/ adulthood - Peroid between adolescence and old age. Growth stops.
- Old age - usually between 65 and death.
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- Resperation is the process of releasing energy from glucose.
- The energy from reperation can't be used directly by cells so it's used to make ATP which acts as an energy source for many cell processes and transports energy.
- Resperation is controlled by enzymes so temperature and pH affect it.
- Aerobic resperation happens when there is lots of oxygen.
- Glucose + oxygen ----> carbon dioxide + water (+ energy).
- C6H12O6 + 6O2 ----> 6CO2 + 6H2O (+ energy).
- When you do lots of exercise your body can't supply enough oxygen to your muscles so it uses anerobic reperation which means without oxygen.
- In this the glucose is only partly broke down and lactic acid is produced.
- Glucose ----> Lactic Acid (+energy).
- After this you have oxygen debt meaning you need extra oxygen to break down the lactic acid.
- The lactic acid is taken to the liver to be broken down so the heart rate stays high.
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The Heart - double circulatory system.
- Mammals have a double circulatory system.
- The first system connects the heart to the lungs.
- Deoxygenated blood is pumped to the lungs to take in oxygen, it then returns to the heart.
- The second system connects the heart to the rest of the body.
- Oxygenated blood is pumped to the rest of the body.
- It gives up its oxygen then the deoxygenated blood returns to the heart to be pumped out to the lungs.
- The advantages of this are...
- Blood can be pumped at a high pressure
- This increases the rate of blood flow.
- This means more oxygen can be delivered.
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- Selective breeding is when humans artificially select the plants or animals that they're going to breed and have their genes remain in the population.
- Organisms are selectively bred to develop the best features...
- Maximum yield.
- Good health and disease resistant
- Other qualities like temperament, speed atractiveness etc.
- The basic process is...
- From your existing stock select the ones with the best characteristics then breed them with each other. Select the best of the offspring then breed them together.
- As you continue this process over generations the desirable trait gets stronger.
- The main problem is that it reduces the gene pool.
- Inbreeding ( breeding of closely related organisms) can cause genetic diseases. This is because lots of genetic disease are recessive.
- There is also problems if a new disease appeared because there isn't much variation. This means if one organism dies it is likely the rest will to.
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- Genetic engineering is moving genes from one organism to another.
- The main advantage is you can produce organisms with new and useful features.
- The main risk is that the inserted gene could have unexpected harmful effects.
- The stages of genetic engineering...
- First the gene that's responsible for producing the desirable characteristics is selected.
- It's then cut from the DNA using enzymes and isolated.
- The useful gene is then inserted into the DNA of another organsim.
- The organism then replicates and soon there are loads of similar organisms.
- In some places that rely on rice for food people have a vitamin A defiency.
- To solve this scientists have taken a gene that controls beta-carotene production from carrot plants and put it into rice plants.
- The gene for human insulin has been put into bacteria.
- Some plants are resistant to things like herbicides, frost damge and disease. This can be cut and put into useful plants such as crops
- However there are moral and ethical issues involved.
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Gene therapy and cloning animals.
- Gene therapy involves altering a person's genes in an attempt to cure genetic disorders. There are two types...
- The first would involve changing the genes in body cells.
- The second type of gene therapy would involve changing the genes in gametes. This is currently illegal.
- It could have unexpected effects which could cause problems for future generations.
- There are fears that it could lead to parents choosing their baby's genes.
- Cloning is making an exact copy of another organsim.
- Clones are genetically identical organisms.
- The first successfully cloned adult cell was a sheep called Dolly. Dolly was produced using a method called nuclear transfer.
- The nucleus of a sheep's egg cell was removed - this left the egg cell without any genetic infomation.
- Another nucleus was inserted in its place. This was a diploid nucleus from the sheep being cloned.
- The cell was given an electric shock so it started dividing by mitosis.
- The dividing cell was put into the uterus of a surrogate mother to develop.
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Uses and risks of cloning.
- The benifits are..
- It allows you to mass produce animals with desirable characteristics, e.g.
- Animals that produce medicine in their milk due to genetic engineering.
- Animals with useful human genes in them.
- Animals like pigs that have organs suitible for transplants.
- Human embryos could be produced by cloning adult body cells. The embryos could then be used to supply stem cells. These would have the same genetic infomation as the patient so would reduce the risk of rejection.
- There are risks to...
- Cloned animals might not be as healthy as normal ones.
- Cloning is new and might have unknown consequences.
- There are lots of ethical issues to do with cloning humans...
- There would have to be lots of surrogate pregnancies, probably with high rates of miscarrige and still birth.
- Clones of other mammals have been unhealty which means human clones might be.
- It could cause psychological damage to the clone knowing that they are a clone of another human.
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- Cloning plants is easier than cloning animals because many plant cells keep their ability to differentiate.
- Commercial cloning involves tissue culture....
- First you chose the plant that you want to clone.
- You remove several small pieces of tissue from the parent plant. You get the best results if you take it from the fast growing root and shoot tips.
- You grow the tissue in a growth medium containing nutrients and growth hormones.
- This is done under aseptic (sterile) conditions to prevent growth of microbes.
- As the tissues produce shoots and roots they can be moved to potting compost to carry on growing.
- Commercial use of cloned plants has good and bad points.
- You can be fairly sure of the characteristics of the plant.
- It's possible to mass produce plants that are hard to grow from seeds
- However if the plants suffer from a disease or a change in the environment they all have the same problems.
- There is also the usual problems of lack of variation (shown on card 15).
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