HomeostasisThe ability of an organism to control its internal environment e.g. core temperature, blood sugar level and pH of blood!Negative feedbackAn increase or a decrease is detected, a response is triggered, the level of the substance is brought back to normal.Positive feedbackProcess which results in a substance departing from its normal level becoming further from the normal.

The Endocrine systemHormones are chemicals produced by the endocrine system to carry information around the bodyEndocrine glands have no ducts but pour their secretions directly into the blood.Hormones may be protein or steroids/lipidMost endocrine glands secrete hormones in small amountsHormones mat be effective for a long timeHormones have target organs bearing specific receptor sites on their membrane.

Control on the oestrous cycleMenstrual CycleThis is the human equivalentDay 0-5 menstruating outer layer of endometrium removalDay 6-14 development of the Graafian follicle Day 14, ovulation (release of secondary oocyte)Day 15-28 development of endometrium and formation of corpus luteum

Hormonal control of the menstrual cycleThe menstrual cycle continues until menopause. It is controlled by four hormones; Follicle stimulating Hormone oestrogen, luteinising hormone Progesterone (FLOP)Oestrogen and progesterone are made in the ovaryFSH and LH released from the anterior pituitary gland (at the base of the brain)

Follicle Stimulating HormoneStimulates the development of a follicle containing the secondary oocyte into a graafian follicle. Stimulates oestrogen production from the follicle

Oestrogen Started the repair of the endometrium following menstruation. Also stimulates luteinising hormones.

Luteinising HormoneThis brings about ovulation i.e. the release of the secondary oocyte and corpus luteum formation. It also stimulates progesterone production from the corpus luteum ProgesteroneThickens the endometrium and causes secretion of mucus from its cells. Also progesterone inhibits FSH. Those high levels of progesterone inhibit FSH whole level drops The other hormone levels then drop in turn Without LH to stimulate it the corpus luteum degenerates The small amounts progesterone can no longer maintain the endometrium which sloughs off-mensuration The low level of progesterone remover FSH inhibits and the cycle begins again

Regulation of Blood glucose levelNormal level of blood glucose are 90mg/100cm cubed blood maintained to ensure a constant supply of glucose for respiration.An increase will lower the water potential of the blood leading to dehydration of the adjacent cells.A decrease will lead to inadequate ATP productions from respiration Pancreas as a receptorThe islets of Langerhans in the pancreas detect a change in the blood glucose level and respond by secreting the hormones insulin from its beta cells or glucagon from its alpha cells to re-establish normal values.Storage of glucoseIt is stored as glycogen in liver and skeletal muscle. The molecule is compact and insoluble in water therefore has no osmotic effect.Hormone control of blood glucose levelRaised blood glucose levelOccurs after eating carbohydrate, glucose is absorbed into the blood. If glucose is above the normal value, it is detected by Beta cells of the islets of Langerhans which secrete insulin.Insulin, a protein hormone, travels in the blood and binds to receptor sire on the surface membrane of all cells but especially liver and skeletal muscle cells.Insulin: Increase the permeability of cells to glucose by opening glucose channels in the membrane Increase the number of these carrier in the membrane It activates enzymes to stimulate the reaction of glucose to glycogen (glycogenesis) It may also activate enzyme to convert glucose into fatTherefore: Cell respiration increase to produce more ATP Increases glucagon in liver and muscles Increases fat storesThereby removing glucose from the blood.HyperglycaemiaThe water potential of the blood is lowered causing the movement of the water from the cells into the blood by osmosis.This-dehydrates the cells Increase blood pressureWhen the glucose reserves of glycogen have been used up the body responds by metabolising stored fat than proteins. The products produce toxins

Lowered blood glucose levelsBelow normal levels of blood glucose are detected by Alpha cells of the islets of Langerhans in the pancreas which then produce the protein hormone Glucagon and secrete it into the blood. Only liver and muscle cells have receptors on their plasma that bind to glucagon: It activates an enzyme to catalyse the conversion of glycogen into glucose (glycogenesis) Glucagon also stimulates enzymes to convert amino acids to glucose The raising of the blood glucose level causes the alpha cells to reduce the secretion of glucagon. This is negative feedback. The action of glucagon is antagonistic to that of insulin. The two are mutually exclusive when on is working the other is not. This allows very sensitive control of blood glucose levels about a set point.

Adrenaline and blood glucose levelsAdrenalins is produced when the body is stressed. It : Activates an enzyme to catalyses the conversion of glycogen to glucose Inactivates the enzyme converting glucose to glycogen

Diabetes MellitusType 1Normally occurs early in the life and may be due to an autoimmune response whereby the bodys own immune system attacks its own beta cells of the pancreas. It develops quickly over a few weeksInsufficient insulin is produced. Blood sugar levels become high and stay high, but body cells do not receive enough glucose. Proteins and fats are broken down.Symptoms: weight loss and weakness. Glucose is excreted in the urine. The person is thirsty. Possible lack of consciousness. Treatment:Insulin injections 2-4 times a day but the dose must be accurate as if the does is too high hyperglycaemia results. Insulin cannot be taken orally as the protein hormone would be digestedType 2 Occurs in older people especially overweight. Usually produce lots of insulin but liver and muscle dont respond to it.Treatment:Exercise and diet to lose weight, may be supplemented by insulin injections. Drugs to stimulate insulin production may also be taken.

Second messenger model of hormone action (protein hormone)Insulin, glucagon and Adrenaline are all protein hormones The protein hormone combines with the receptor site on the target cell membrane to form an hormone-receptor complex This process release Adenyl Cyclase which catalyses the conversion of ATP to Cyclic AMP, with the release of a great deal of energy. A specific enzyme which does the job dictated by the protein hormone is the activated.

Thermoregulation There are 3 ways that the body gains or loses heat: Conduction- the heat is passed to particle, mostly in solids Convections- heat is carried on particles of gas or liquid as a current Radiation- energy transfer as electromagnetic waves.

Control of Body Temperature in Endotherms This is by the hypothalamus The hypothalamus has thermo receptors to monitor the temperature of Blood passing through it-core body temperature Receptors in the Skin send sensory impulses too. Any change in core temperature will cause the hypothalamus to bring about changes which conserve heat or lose heat

Heat LossVasodilation Hypothalamus send impulses to the skin arterioles, there muscle relaxes and the dilate to fill blood and supple more to the surface capillariesThis gives:1) A shorter diffusion distances for heat to reach the skin surface2) More heat at the skin surfaceHeat is rapidly Conducted through the short distance to the skin surface where it is Radiated away from the body.

SweatingMore sweat secreted onto skin surface. Body heat is used to evaporate waterPilorelaxtionImpulses cause the hair erector muscle to relax. Less air is trapped this reducing the insulating effect of body hair.Long term heat exposureBody accumulates by reducing the hormone thyroxin, to decrease the metabolic rate.Behavioural mechanismMany animals are nocturnal avoiding the hot part of the day. Human seek shade, a breeze and rest to have less muscle activity, less muscle respiration and therefore less heat produced. Heat GainsVasoconstriction In cold conditions the superficial arterioles contract, so reducing the quantity of blood reaching the skin surface in its capillaries. This way little heat is lost be radiation from the skin surfaceSweatingDecreased rate of sweatingShiveringInvoluntary contraction which produces metabolic heat through the extra respiration needed for energyInsulationIt is an effective means of reducing heat loss from the body e.g. a jumper adds a extr layer of air and other behavioural responses. Metabolic RateIncreased metabolic rate by making more of the hormones adrenalin and thyroxine.PiloconstrictionImpulses along the sympathetic nervous system from the hypothalamus and cause the hair erector muscle to contract. Ait is trapped, warming up and reduces the thermal gradient between the body and the external environment.

Protein SynthesisThe expression of a gene (DNA in a nucleus) involves the production of protein (on ribosomes or in a cytoplasm)RNA-ribonucleic acidRibonucleic acid mononucleotide is made of 3 main parts:1) Ribose sugar2) Nitrogen base; guanine cytosine, adiene and uracil 3) PhosphateIt is a single chain that is shorter then DNATypes of RNAThere are three types, messenger RNA, transfer RNA and ribosomal RNA1) Messenger RNA- a single strand, synthesised in the nucleus then leaving t direct protein synthesis2) Transfer RNA- a single strand folded into a clover leaf shape. It carries the amino acid needed for protein synthesis.3) Ribosonal RNA ( what ribosomes are made of)

Genetic CodeThis is the sequence of bases carrying genetic information. The RNA base works in triplets to make sufficient codes to stand for 20 amino acids.As there are 4 different bases; this means there are 64 3 letter codes to stand for 20 amino acids so: Some amino acids have more than on triplet- Degenerate A gene is a sequence of nucleotides, so they must never overlap. Gene MutationAny change in the DNA quantity or structure is called a Mutation.A mutation in a body cell is not passed on to the next generation. However, a mutation in a gamete is.A gene mutation is a change in one or more nucleotide bases. (Downs syndrome, an extra chromosome. 21 is a chromosome mutation.A sequence of DNA codons is transcribed onto mRNA then translated into a sequence of amino acids.Therefore a change in a DNA base could cause a change in the amino acid sequence.

Base SubstitutionsNonsense mutation.If a base is changed (e.g. GTC ATC) instead of coding for glutamine we code for Full stop.A short non-functional protein is produced.Mis-sense mutation.If a base is changed (e.g. GTC GTG) instead of coding for glutamine we code for histidine and this will affect the 3D tertiary structure formed by hydrogen ions and disulphide bonds.Silent mutation Ia base is changed (e.g. GTC GTT) we still code for glutamine which has more than one codon (a consequence of the degenerate code) so there is no effect.Base DeletionsIf a single base is deleted, as bases are read in threes (codons) there will be a frame shift. The gene is now read in the wrong 3 base groups and the amino acid sequence will probably be totally different.Causes of mutationsThey happen spontaneously and randomly during mitosis or meiosis, with a set frequency (mutation rate) they are permanentThey are assisted by external influences called mutagens (e.g. high energy radiation, certain chemicals) which increase the mutation rate.Cancer and uncontrolled cell divisionThe rate of cell division is controlled by 2 genes:

Photo-oncogenes which speed up cell divisionTumour suppresser genes which inhibit cell division

A proto-oncogene stimulates cell division by:1) Producing growth factors at appropriate times.2) Producing the correct no. of receptors on the cell membrane with which the growth factors bind.3) The binding stimulates the production of relay proteins within the cell.4) The binding of relay proteins to DNA opens up the gene for cell division.

Mutation of a proto-oncogene into an oncogene1) Growth factore made in excessive amounts2) Abnormal receptors produced which are permanently activated They need no growth factors Tumour suppressor genes.These counter the effect of an oncogene and maintain normal rates of cell division ie prevent tumour formation.

If a tumour suppressor gene mutates it is inactivated.

The mutant cells formed are often incapable of sustaining themselves and they die but survivors can clone themselves, forming tumours.

Regulation of gene action. A gene is opened by a transcriptional factor in the cytoplasm.

The transcriptional factor has a binding site white binds at the start of the genes position on the DNA.

When it leaves the cytoplasm and enters the nucleus, the transcriptional factor binds to the dna and the double helix opens.

mRNA is transcribed

mRNA is translated on the ribosome in the cytoplasm

when the gene is off the transcriptional factor binding site to the DNA is blocked by an inhibitor.

The role of oestrogen in gene regulation.

-Oestrogen is lipid soluble and diffuses through the phospholipid bilayer (membrane)-The transcriptional factor has a receptor molecule attached.-the oestrogen binds to the complementary shaped site on the receptor-The receptor changes shape, causing the transcriptional factor to release its inhibitor from the DNA binding site.-The transcriptional factor can enter the nucleus and bind to DNA.-The gene is stimulated to open.

SiRNA (Small interfering RNA)This is involved in a process tha can cut up mRNA thereby preventing translation. So even if an open geen is transcribed, it can be prevented from having an effect.

NB (an enzyme cuts up double stranded RNA into SiRNA pieces. One strand combines with an enzyme that can cut up the mRNA)

DNA technologyGenetic engineeringThis is the way genes are altered or transferred from one organism to another organism.

The new altered genetic material is called recombinant DNA

Producing DNA fragments In some diseases individuals cannot make chemicals for themselves e.g. insulin (a protein normally produced from its gene by transcription and then translation). Instead of using other animals as a source (immunelogical rejection, expensive, microbial contamination) if the human gene can be isolated, cloned, and transferred to a microorganism, the microbe can make the pure insulin for you.

Restriction endonuclease-enzymes extracted from microbes-they cut DNA at specific nucleotide sequences called recognition sites

enzyme 1 makes a blunt endG T T A A CC A A T T G

Enzyme 2 makes a staggered cut, leaving two complimentary ends which are called sticky endsA A G C T TT T C G A A Hind III recognises the 6 base pairs (bp) sequence

Once the cut has been made, if the 4 unpaired bases on each end are read from left to right they are the opposite of each other i.e they are palindromic.

PlasmidsA plasmid is a small loop of DNA inside the bacterial (prokaryote) cell in addition to the bacterial chromosome.It can be removed, cut open, have an etra gene put in, and sewn back up again. In this way it is used as a vector (carry extra gene into the host cell).

Performing genetic engineering-remove the plasmid from a bacterial cell.-cut it open using a specific restriction endonuclease (e.g. Hind III) to form sticky ends This will occur at the recognition site.-cut out the required gene (e.g. for human insulin) from human DNA using the same restriction endonuclease. (the required gene will then have complementary bases making its sticky ends compared with the plasmid)- Add the required gene to the opened plasmid.- The complementary bases will form a weak link with hydrogen bonds.-Use the enzyme DNA ligase to permanentl bond the gene and the plasmid. It will bond the sugar-phosphate back bones (phospholipidiester bonds formed by condensation reactions) -Temperature shock the bacterium and bombard it with Ca++, this will transform the bacterium and allow it to take up the recombinant plasmid.

Reverse transcriptase to make a gene-mRna for human insulin is extracted from a cell, islets of Langerhans.-Treated with reverse transcriptase to produce a single strand of DNA. DNA nucleotides have to be supplied.-Treated with DNA polymerase to form a double helix of DNA (synthetic gene) again DNA nucleotides have to be supplied.-The ends of the synthetic gene are cut, producing sticky ends using restriction endonuclease.-The same restriction endonuclease is used to open the plasmid as was used to cut the synthetic gene.-DNA ligase is used to incorporate the gene into the plasmid (forms phosphodiester bonds).

Marker Genes Antibiotic resistance markersTo check if a bacterial call has received the plasmid containing the new gene a gene for resistance to an antibiotic e.g. tetracycline can be spliced alongside it. If the cells are grown on a medium containing tetracycline, only the ones with the antibiotic resistance gene will grow (they also have the new gene).

Flourescent markersA certain species of jellyfish has the gene to produce a green fluorescent pigment. This gene can be engineered out of the jellyfish and incorporated into a bacterial plasmid; it will be obvious that the bacterium has taken up the plasmid because the bacterium will fluoresce green. If a useful gene (e.g. human insulin) is spliced in alongside you can quickly separate any that have not taken up the plasmid.

Enzyme markersWhen the lactase is secreted it will change the colour of a particular indicator blue.If a required gene is spliced within the lactase gene and the plasmid bearing them is successfully taken up by a bacterium, it will not be able to make lactase and the medium will stay colourless.Untransformed bacteria with the original entire lactase gene would still produce the lactase and turn the medium blue.

The polymerase-chain reactionDetective work may rely on minute quantities of DNA (blood, sperm, skin cell) used as evidence. PCR allows a way of copying fragments of DNA many times thereby increasing the quantity of DNA that can be analysed.1) A piece of DNA is targeted2) Primers are made. These are short lengths of single stranded DNA (20-30 nucleotides) artificially synthesised to be complementary to one end of each of the two original DNA strands.3) Buffer is added and the DNA is heated to 95 degrees for 20s to separate the strands in the target DNA by breaking the hydrogen bonds.4) The primers are added and the solution allowed to cool to 55degrees so that hydrogen bonds can form between the DNA and the complementary primers.a. The primers-i. Provide the starting point for the DNA polymerase as it can only build up from an existing sequenceii. Prevent the two separated strands from rejoining.5) DNA nucleotides (deoxyribose sugar, phosphate and base) and DNA polymerase are added. The enzyme is extracted from heat tolerant bacteria living in thermal springs ie. It is a thermostable enzyme.

The solution is heated to 72 degrees for 30s. This is the optimum temperature for the enzyme and it allows two new strands of DNA to form alongside the originals.6) The process is repeated (every 2 minutes).

In vitro (PCR) an In Vivo (Mitosis) cloningIn VitroIn Vitro is very rapidIn Vitro does not require living cells (complex culturing)

In VivoIn Vivo is useful to transform another organism (plasmid used as a vector to deliver the gene)In Vivo has almost no risk of contamination as restriction endonucleases are so specific (rogue DNA unlikely to have the sticky ends to enter the plasmid).In Vivo is accurate. Mutations very rare, but PCR sometimes copies incorrectly.In Vivo is precise (cuts out specific genes with no extra DNA).In Vivo transformed bacteria can be grown on to make many gene products.

Electrophoresis The radioactive DNA (*DNA) fragments have an equal negative charge. A voltage is applied across agar gel. The smaller fragments are attracted to the anode fastest as they have less mass. After a set period of time the fragments have become separated by mass. X-ray film is placed over the gel for several hours and the radioactivity exposes the film showing where it is on the gel. A pattern of bands develop.

Gene ProbesProduced as follows: Make a piece of DNA with the complementary base sequence to the gene. Label it radioactivity

The DNA to be tested s digested by a restriction endonuclease into smaller pieces. Pieces separated by gel electrophoresis into different sizes (=charge diff mass) Alkali added makes test DNA single stranded Test single stranded DNA transferred to a membrane Gene probe added to membrane and left to bind complementary sequence. If the gene probe washed away it has not bound to DNA on membrane no gene. If *gene probe has bound there will be fogging on photographic film.Genetic fingerprintingNon-coding DNA is used (introns). Introns contain repetitive DNA called sequences. The number and length of the core sequences vary uniquely with individuals. Identical twins (identical genotype) have identical introns. All the DNA is extracted from blood, sperm, root hair cells etc. Its quantity can e magnified by the polymerase chain reaction (PCR) Restriction endonucleases are used to cut the DNA (at specific points called recognition sites determined by the base sequence) close to the core sequence. DNA segments are separated by gel electrophoresis. The gel is immersed in alkali to make the DNA single stranded. The gel is covered with a nylon membrane and a piece of absorbent paper. The DNA is drawn up onto the membrane by capillary action and fixed in position by UV light. Radioactive gene probes (with a complementary base sequence to the core sequences) are added different probes for different core sequences. The probe will bind to the sections being looked for I they are present, if they are not the gene probe will wash away (it has not bonded) X-ray film is placed over the membrane. The remaining DNA will develop a pattern of bonds, each one corresponding to the position of the DNA fragments.Interpreting resultsIf there seems to be a match, a machine auto-scans the fingerprint, and calculates the odds of someone else having the same print.

DNA sequencing Modify the 4 bases A, G, T, & C so that they cannot attach to the next base in the sequence terminator nucleotides. Put identical single strands (clones) of the DNA to be tested in each of 4 test tubes. Add the nucleotides A G T & C AAdd a small quantity of A terminator to test tube 1, and G T & C terminators to test tubes 2 3 & 4 Add a *primer to start DNA synthesis DNA polymerase is also added to catalyse the DNA synthesis. The normal A competes with A* in test tube 1, randomly- so different sized fragments form all ending in A. All the *lengths of DNA are collected.Restriction MappingA piece of DNA is cut with a series of known restriction endonucleases. Each enzyme will make the cut at its own recognition site.

For a piece of DNA that has only one recognition site for each enzyme, each will make only one cut.

If they are used in pairs, the cut lengths will differ depending on the pairing chosen each time.

UsesFor 2 similar DNA where one DNA sequences is known; the degree of overlap can be determined, meaning that only some of the DNA sample has unknown base sequence far less work.

Ideal exam answerstRNA has an anticodon of complementary bases;tRNA caries a specific amino acid; Amino acids joined by peptide bonds by condensation reaction;

Plasmid = Small; circular piece of DNA; in prokaryote; that carries resistance genes;

RNA polymerase; ataches to start of gene; H0bonds break one DNA strand is used as template; it has complementary base pairings; the DNA base sequence is copied into mRNA; introns are removed;

Oestrogen inhibits production of FSH; so preventing follicle development; therefore ovulation;

Negative feedback = process that returned a certain level back to normal e.g. returning body temp back to normal when cold or hot. Totipotency & Cell specialisationAll cells contain the same genes, meaning that every cell can produce everything that the body makes potentially.-Some genes are permanently switched on e.g. for enzymes involved in respiration.-some genes are permanently switched off, e.g. insulin production gene in the heart muscle cell.

Zygote CellsA fertilized egg (zygote) and the first few cells produced from it have the ability to develop into any type of cell in the body.They are called totipotent cells.

Later these cells differentiate and become specialised for different functions. (e.g. muscle clells to contract, bone cells for strength) ie the genes coding for proteins to make the specialisation are switched on, extraneous genes are turned off.

Turning off genesThere are two main ways of doing this.-Preventing transcription and hence the production of mRNA.-Breaking down the mRNA before its genetic code is translated.

can specialised cells ever develop into a different type of cell?-Xylem, red blod cells have no nuclei no they have lost their DNA and have no genes at all.-Mature specialised cell lost their totipotency.- Adult mammals have a few totipotent cells ie undifferentiated cells capable of dividing e.g. skin, inner lining of small intestine, bone marrow. Bone marrow cells make blood cells. Also embryonic stem cells.-Under certain conditions stem cells can deveop into other types of cells. They can be used to treat some disorders e.g. sickle cell anaemia.

PlantsMature plants have many totipotent ells. E.g. a carrot root cell on nutrient medium with appropriate hormones added it will form small carrot plants. This is In vitro culturing and the new plant is a clone of the original plant.

The nervous systemStimulus & responseBoth plants and animals respond to a stimulus in order to increase survival changes.Taxes (animalsA simple response whose direction is determined by the direction of the stimulus e.g. positive phototaxis (move towards lights), negative geotaxis (move away from gravity)

Some bacteria move towards specific chemicals-positive chemotaxis

Kinesis (animals)A response where the organism moves neither towards nor away from the stimulus but moves randomly faster as the intensity of the stimulus increases.

E.g. if a woodlouse finds itself in too dry an area, this activity increases the likelihood of finding moisture if they do their movements slow and there are fewer changes in direction.

Tropisms (plants)A growth movement in response to a directional stumulusShoots Positively phototropicRoots Positivel geotropic and negatively phototropic and positively hydrotropic

Plant Growth FactorsIAA (INDOLELACETIC ACID)A shoot grows towards light as follows:-cells in shoot tip make IAA- it diffuses down the shoot evenly.- on the light side of the shot the light destroys the IAA- The concentration on the shaded side is higher.- the IAA here (darkside) causes these cells to elongate more than those in the light.- The shaded site grows larger cells.

RootsIncreased IAA = decreased growthThe Nervous SystemThe nerve impulseIn neurones an influx of Na+ across the neurone membrane causes it to create an impulse or action potential.

The Resting Potential-neurones have a charge across theirh membranes. This is called the resting potential.-The inside of the neurone is negative with respect to the outside. It is said to be polarised.-This charge different is a result of the distribution of four ions, K+, Na+, Cl-, and COO- (protein).-The protein is made inside cells and cant easily escape.-Th ecell membrane contains sodium pumps using ATP energy for their active transport. 3x Na+out of the cell, only 2K+ in. This creates a more negative charge inside the cell.-K+ concentration inside the cell and associate with protein (COO-) to try to maintain electrochemical neutrality.-Cl- accompanies Na+ ion gates within the membrane which provides a route whereby K can escape along its diffusion greadient as they are not completely closed. Relatievly few K+ escape however because they are still attracted to overall negative charge inside the cell (also Na+ gates but they are completely closed.)-This means slight positive charge is set up outside the cell membrane making the inside more negative with respect to the ouside i.e. we have resting potential, charge different of -70mV.

Na+ gate

K+ gateCOO-

Na+ pumpCl+2xK+3xNa+

The action potential-Depolarisation takes place. This is when the charge acros the membrane is reversed to give a positive charge inside the cell.- The proteins making the ion channels change shape and open or close depending upon the voltage across the membrane (voltage gated channels)-The energy of the stimulus causes sodium ion channels to open and massive amounts of sodium ions diffuse in along their concentration gradient such that the positive charge they carry first neutralised the negatively charged interior then reverses it ie depolarises it.-Once a few sodium ions (Na+) gates have opened, and Na+ started to diffuse in, many more Na+ gates open to positive feed back.-When some Na+ have entered a generator potential has been produced. Eventually so many Na+ have entered, and the interio is so positive with respect to outside that the threshold value has ben passed and an action potential is formed.-The action potential is at around +40mV.-Cl- is left behind with its negative charge thus making a potential difference between the inside and outside of the cell which is the reverse of the resting potential.

COO- Cl- K+ Na+ huge amounts

Action Potential ^

Repolarisation-Na+ gates close now.- K+ gates open and there is a diffusion out of K+ along its diffusion and electrical gradient, from the interior of the neurone. This causes more K+ gates to open and a massive outward diffusion of K+ so that the cell interior is once again negative. -The is a temporary K+ over shoot which causes the membrane potential to become slightly more negative in the cell then at resting potential. This is called Hyperpolarisation.-K+ gates close now. Na+ pumps which exchange sodium for K+ will re-establish the normal Na and K distributions once more. This is the resting potential.

NB-For a short time after an action potential a further action potential cannot be generated. The Na+ and K+ channels have to reshape themselves. This is called the refractory period.Repolarisation

2xK+ 3xNa+ K+ HugeAmounts