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OCR Categories Archives: Module 5: Communication, Homeostasis and Energy

Respiration

Respiration

(a) outline why plants, animals and microorganisms need to respire, with reference to active transport and metabolic reactions

Respiration– the process whereby energy stored in complex organic molecules (carbohydrates, fats and proteins) is used to make ATP, occurring in living cells.

 

All living organisms need energy to drive their biological processes in order to survive. Metabolic reactions that need energy include:

  • Active Transport – moving ions and molecules across a membrane against a concentration gradient.
  • Secretion– large molecules made in some cells are exported by exocytosis.
  • Anabolism– synthesis of large molecules from smaller ones, e.g. proteins from amino acids, steroids into cholesterol and cellulose from β-glucose.
  • Replication of DNA and synthesis of organelles before a cell divides.
  • Endocytosis– bulk movement of large molecules into cells.
  • Movement– movement of bacterial flagella, eukaryotic cilia and undulipodia, muscle contraction and microtubule motors that move organelles around inside cells.
  • Activation of chemicals– glucose is phosphorylated at the beginning of respiration so that it is more unstable and can be broken down to release energy.

 

(b) describe, with the aid of diagrams, the structure of ATP

ATP stands for adenosine triphosphate and is a phosphorylated nucleotide. Each molecule consists of adenine, ribose and three phosphates.

 

(c) state that ATP provides the immediate source of energy for biological processes

ATP can be hydrolysed to ADP and Pi (inorganic phosphate), releasing 30.6 kJ energy per mol. So, energy is immediately available to cells in small, manageable amounts that will not damage the cell(enzymes and proteins can denature or membranes could become too fluid if too much energy is released), so it’s easier to harness the energy and it will not be wasted. The energy released from ATP hydrolysis is an immediate source of energy for biological processes, such as DNA replication and protein synthesis.

ATP is the ‘universal energy carrier’:

  • Found in all living cells.
  • Small and soluble – can move around the cell.
  • High energy bonds between phosphates – breaks down to release energy where required.
  • Produced where energy is released.

Immediate source of energy

Anabolic reactionsbuilding larger molecules from smaller molecules (hydrolysis).

Catabolic reactionsbreaking larger molecules to form smaller molecules (condensation).

Catabolic reactions release energy that the building of ATP uses. The hydrolysis of ATP releases energy that other anabolic reactions could use.

(d) explain the importance of coenzymes in respiration, with reference to NAD and coenzyme A

The Stages of Respiration:

  • Glycolysis– occurs in the cytoplasm which can take place in aerobic or anaerobic conditions. Glucose is broken down to two molecules of pyruvate.
  • The link reaction– occurs in the matrix of the mitochondria. Pyruvate is dehydrogenated and decarboxylated `and converted to acetate.
  • Krebs cycle– occurs in the matrix of the mitochondria. Acetate is dehydrogenated and decarboxylated.
  • Oxidative phosphorylation– occurs on the folded inner membrane (cristae) of mitochondria. This is where ADP is phosphorylated to ATP.

Coenzymes are needed to help enzymes carry out oxidation reactions, where hydrogen atoms are removed from substrate molecules in respiration. The hydrogen atoms are combined with coenzymes, so that they can be carried and can later be split into hydrogen ions and electrons, to the inner mitochondrial membranes where they will be involved in oxidative phosphorylation.

(e) state that glycolysis takes place in the cytoplasm

Glycolysis is a very ancient biochemical pathway, occurring in the cytoplasm of all living cells (prokaryotic and eukaryotic) that respire. It is an anaerobic metabolic pathway.

(f) outline the process of glycolysis beginning with the phosphorylation of glucose to hexose bisphosphate, splitting of hexose bisphosphate into two triose phosphate molecules and further oxidation to pyruvate, producing a small yield of ATP and reduced NAD

  1. One ATP molecule is hydrolysed and the phosphate group released is attached to the glucose molecule at carbon 6, called phosphorylation. The energy from the hydrolysed ATP molecule activates the hexose sugar and prevents it from being transported out of the cell.
  2. Glucose 6-phosphate is rearranged, using the enzyme isomerase, into fructose 6-phosphate.
  3. Phosphorylation occurs again forming hexose 1,6-bisphosphate.
  4. The hexose 1,6-bisphosphate splits into two molecules of triose phosphate.
  5. Each triose phosphate is oxidised, removing hydrogen atoms using dehydrogenaseenzymes.
  6. The coenzyme NADaccepts the hydrogen atoms and becomes reduced NAD.
  7. Two molecules of ATP are formed, calledsubstrate-level phosphorylation (the formation of ATP directly during glycolysis and the Krebs cycle only).
  8. The triose phosphate molecules are converted to pyruvate, which is actively transportedto the mitochondrial matrix. In the process, another two molecules of ADP are phosphorylatedto make twomolecules of ATP.

(g) state that, during aerobic respiration in animals, pyruvate is actively transported into mitochondria

During aerobic respiration in animals, the triose phosphate molecules are converted into pyruvate and are activelytransported into mitochondria.

(h)explain, with the aid of diagrams and electron micrographs how the structure of mitochondria enables them to carry out their functions

How does the structure of mitochondria enable them to carry out their functions?

(i) state that the link reaction takes place in the mitochondrial matrix

Pyruvate that is produced during glycolysis is transported across the inner and outer membrane to the mitochondrial matrix where the link reaction takes place.

(j) outline the link reaction, with reference to decarboxylation oh pyruvate to acetate and the reduction of NAD

  1. The pyruvate molecule is decarboxylated by the enzyme pyruvate decarboxylase, removing a carboxyl group which eventually becomes carbon dioxide.
  2. The pyruvate molecule is also dehydrogenated by the enzyme pyruvate dehydrogenase, removing hydrogenatoms forming acetate.
  3. The hydrogen atoms are accepted by the coenzyme NAD, becoming reduced NAD.
  4. The acetate combines with coenzyme A forming acetyl CoA.

2 pyruvate + 2NAD+ + 2CoA → 2CO2 + 2NADH + 2 acetyl CoA

(k) explain that acetate is combined with coenzyme A to be carried to the next stage

Coenzyme A (CoA) accepts acetate to become acetyl coenzyme A. The function of CoA is to carry acetate to the Krebs cycle.

(l) state that the Krebs cycle takes place in the mitochondrial matrix

The Krebs cycle takes place in the mitochondrial matrix. It produces one molecule of ATP by subtrate-level phosphorylation and reduces three molecules of NAD and one molecule of FAD.

(m) outline the Krebs cycle, with reference to the formation of citrate from acetate and oxaloacetate and the reconversion of citrate to oxaloacetate (names of intermediate compounds are not required)

  1. The acetate is offloaded from coenzyme A and joins with oxaloacetate (4C), to form citrate (6C).
  2. Citrate is decarboxylated and dehydrogenated forming a 5-carbon compound. The pair of hydrogen atoms is accepted by the coenzyme NAD (hydrogen acceptor), which becomes reduced NAD.
  3. The 4-carbon compound is decarboxylated and dehydrogenated forming a 4-carbon compound and another molecule of reduced NAD.
  4. The 4-carbon compound is changed into another 4-carbon compound. During this reaction a molecule of ADP is phosphorylated to produce a molecule of ATPsubstrate-level phosphorylation.
  5. The second 4-carbon compound is changed into another 4-carbon compound. It’s dehydrogenated and the coenzyme FAD (hydrogen acceptor) accepts the hydrogen atoms, and becomes reduced FAD.
  6. The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate and forms another molecule of reduced NAD.

(n) explain that during the Krebs cycle, decarboxylation and dehydrogenation occur, NAD and FAD are reduced and substrate level phosphorylation occurs

(o) outline the process of oxidative phosphorylation, with reference to the roles of electron carriers, oxygen and mitochondrial cristae

  1. NADH is reoxidised to form NAD+ and 2 hydrogen atoms, aided by the enzyme NADH dehydrogenase which is attached to the first electron carrier. The hydrogen atoms split into protons and electrons.

NADH à NAD+ + 2H

2H à2H+ + 2e

  1. The electrons are passed along electron carriers in the electron transport chain and lose energy by doing this.
  2. The energy that was lost in the electron transport chain is used to pump protons into the intermembranespace creating a proton gradient – the protons will want to move back into the matrix from a high concentration to a low concentration.
  1. The H+ ions cannot diffuse through the lipid part of the membrane so they diffuse through protein channels that are associated with ATP synthase, which is linked to the synthesis of ATP. The flow of protons is chemiosmosis.
  2. The flow of protons through the protein channels creates a proton motive forcewhich drives the rotation of the ATP synthase enzyme attached to the protein channel. The rotation causes the phosphorylation of ADP to make ATP.

ADP + Pià ATP

  1. The electrons are passed from the last electron carrier in the chain to oxygen, which is the final electronacceptor. Hydrogenions also join forming water.

4H+ + 4e + O2à 2H2O

 

(p) outline the process of chemiosmosis, with reference to the electron transport chain, proton gradients and ATP synthase

Chemiosmosis – the flow of hydrogen ions through a partially permeable membrane, relating the synthesis of ATP. The flow creates a proton motive force that rotates the enzyme ATP synthase, joining ADP and Pi to form ATP.

  1. Electrons are passed along electron carriers in the electron transport chain and lose energy.
  2. The energy is used to pump protons into the intermembrane space, creating a proton gradient between the intermembrane space and the matrix.
  3. The hydrogen ions diffuse through the protein channels, creating a proton motive force which drives the rotation of the ATP synthase enzyme attached to the protein channel. The rotation causes the phosphorylation of ADP to make ATP.

(q) state that oxygen is the final electron acceptor in aerobic respiration

Oxygen is the final electron acceptor in aerobic respiration, which joins to hydrogen to form water.

4H+ + 4e + O2à 2H2O

(r) evaluate the experimental evidence for the theory of chemiosmosis

Chemiosmosis is the flow of hydrogen ions (protons) through a partially permeable membrane, relating to the synthesis of ATP. The flow of the hydrogen ions create a proton motive force. This rotates the enzyme ATP synthase joining ADP and Pi to form ATP.

(s) explain why the theoretical maximum yield of ATP per molecule of glucose is rarely, if ever, achieved in aerobic respiration

The 10 molecules of NAD can theoretically produce 26 molecules of ATP during oxidative phosphorylation (each NAD molecule can make 2.6 molecules of ATP). Together with the 4 ATP made doing glycolysis and the Krebs cycle, the total yield of ATP molecules, per molecule of glucose respired, should be 30. However this is rarely achieved for the following reasons:

  • Some protons leak across the mitochondrial membrane, reducing the number of protons to generate the proton motive force.
  • Some ATP produced is used to actively transport pyruvate into the mitochondria.
  • Some ATP is used for the shuttle to bring hydrogen from reduced NAD made during glycolysis, in the cytoplasm, into the mitochondria.

(t) explain why anaerobic respiration produces a much lower yield of ATP than aerobic respiration.

Anaerobic respirationis the process where ATP is produced by substrate-level phosphorylation during glycolysis in the absence of oxygen, in the cytoplasm of eukaryotic cells.

As anaerobic respiration occurs in the absence of oxygen, the electron transport chain cannot happen so the link reaction, Krebs cycle and oxidative phosphorylation cannot happen. Therefore only glycolysis can happen and only ATP can be produced via glycolysis. The reducedNAD has to be reoxidised so that it can keep accepting hydrogen atoms in glycolysis. There are two ways that NAD can be reoxidised – lactate fermentation and alcohol fermentation.

(u) compare and contrast anaerobic respiration in mammals and in yeast

(v) define the term respiratory substrate

Respiratory Substrate– an organic substance that can be used for respiration.

The more protons, the more ATP produced as most ATP is formed from the flow of protons through channel proteins during chemiosmosis. Therefore the more hydrogen atoms there are in a molecule of respiratory substrate, the more ATP can be generated when that substrate is respired. It also follows that if there are more hydrogen atoms per mole of respiratory substrate, then more oxygen is needed to respire that substance.

(w) explain the difference in relative energy values of carbohydrate, lipid and protein respiratory substrates

 

 

 

 

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Animal Behaviour

Animal Behaviour

  • explain the advantages to organisms of innate behaviour

Behaviour – the responses of an organism to its environment, which increase its chances of survival. An organism must be able to detect changes in the environment, which form stimuli, then carry out an appropriate response through the operation of effectors.

Innate Behaviour – animal responses, which does not involve learning. It is an inherited response, similar in all members of the same species and is always performed in the same way in response to the same stimulus.

Advantages of Innate Behaviour:

  • describe escape reflexes, taxes and kineses as examples of genetically-determined innate behaviours

 

  • describe, using one example, the advantages of social behaviour in primates

Social Behaviour – behaviour that is learnt from those in the rest of the group with relatively defined roles for each member of the group.

Hierarchy – where individuals have a place in the order of importance within the group.

Mountain gorillas live in stable groups (called a troop) of around 10 individuals. It consists of:

  • One dominant silverback male:
  • Protects the other members of the group.
  • Leads them in search of food.
  • Is the only male that mates with the mature females.
  • Females:
  • Care of the young offspring – during the first 5 months the infant remains in constant contact with the mother, suckling at hourly intervals. By the age of 12 months, infants will venture as far as 5m from the mother.
  • Teaches the young the social and other skills necessary to live independently.
  • Offspring:
  • Play
  • Imitate adult behaviour of foraging for food.
  • As younger males reach sexual maturity they leave the group to live alone until they are mature enough to attract females.
  • As younger females mature they either stay with the same group or leave to join another group.

As with all primates, grooming for gorillas is an important social activity – one individual picks the parasites from the fur of another – reinforcing relationships between individuals. A variety of calls, displays and grants are used to signal danger to other members of the group, to issue threats to predators or other groups, and in play fighting displays as juveniles learn how to behave as adults. Facial expressions are also important in gorillas and other large primates, especially in terms of recognition of other members of the group.

Advantages of Social Behaviour:

  • Maternal care and group protection enhances survival rate of the young.
  • Learned behaviour increases survival.
  • Group security allows the slow maturation of the brain to not impede survival.
  • Knowledge and protection of food sources is shared with the group.
  • Group works deters predators
  • discuss how the links between a range of human behaviours and the dopamine receptor DRD4 may contribute to the understanding of human behaviour

Dopamine – acts as a neurotransmitter and a hormone. It is produced all over the body and controls the ‘pleasure and reward’ centre of our brain. It is a precursor molecule in the production of adrenaline and noradrenaline. Dopamine increases general arousal and decreases inhibition, leading to an increase in creativity in conjunction with cerebral activity.

Low brain levels of dopamine are associated with Parkinson’s disease, the treatment of which involves clinical administration of the dopamine precursor L-dopa, as dopamine cannot cross the blood-brain barrier (capillaries in the brain are ‘less leaky’). However, L-dopa can lead to changes in behaviour including addition, anxiety, hallucinations and sleepiness. High brain levels of dopamine has been linked to the development of mental health conditions such as schizophrenia.

There are five different dopamine receptors referred to as DRD1 to DRD5. Each of these is coded for by a separate gene. The range and variations within number of receptors means that dopamine has different effects on different people. Binding of dopamine to its receptor is involved in a number of processes, including the control of motivation and learning, and is linked to regulatory effects on other neurotransmitter release. A number of antipsychotic drugs work by blocking dopamine receptors.

The DRD4 Receptor Gene:

DRD4 – one of the five genes that code for dopamine receptor molecules.

There are currently over 50 known variants of the DRD4 gene. The variants differ in a specific sequence known as a variable number tandem repeat. This means there are numerous combinations of the genetics controlling the production of the receptor. It is thought that the inheritance of particular variants of the DRD4 gene affects the levels and action of dopamine in the brain, implicating in human behaviour.

Attention-deficit Hyperactivity Disorder (ADHD):

For example, people some combinations of alleles are shown to have a higher risk of becoming ADHD. Drugs such as methylphenidate (Ritalin) used to treat ADHD affect dopamine levels in the brain. In a number of studies, a particular dopamine receptor variant of DRD4, has been shown to be more frequent in individuals suffering from ADHD.

Addictive and Risk Behaviours:

Also, a number of studies have suggested that particular variants of DRD4 receptor gene are implicated in increased likelihood of addictive behaviours, including smoking and gambling. A study into the effects of administering L-dopa to one group of individuals and haloperidol (drug that blocks dopamine receptors) to another group showed not only a difference in general arousal, but also a significant difference in the risk-taking levels of the individuals.

Obsessive-compulsive Disorder (OCD):

OCD is thought to result from a deficiency in the levels of the neurotransmitter serotonin. In 2007, a genome-wide scan for DNA sequences related to OCD was carried out when the DNA from 1008 people from 219 families were analysed. Eight genetic markers that appear to be linked to OCD were found.

 

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Plant Responses

Plant Responses

  • explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress

Like animals, plants must also need to respond to external stimuli. This is important to:

  • Avoid predation.
  • Avoid abiotic (non-living) stress.
  • Maximise photosynthesis.
  • Obtain more light, water and minerals.
  • Ensure germination in suitable conditions/pollination.
  • Seed set/seed dispersal.
  • define the term tropism

Tropism – a directional growth response in which the direction of the response is determined by the direction of the external stimulus. Tropisms may be positive (a growth response towards the stimulus) or negative (a growth response away from the stimulus).

They include

  • Phototropism (light) – shoots grow towards light – they are positively phototrophic.
  • Geotropism (gravity) – roots grow towards the pull of gravity.
  • Chemotropism (chemicals) – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
  • Thigmotropism (touch) – shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
  • explain how plant responses to environmental changes are co-ordinated by hormones, with reference to responding to changes in light direction

Hormones, also referred to as plant growth regulators, coordinate plant responses to environmental stimuli. Like animal hormones, plant hormones are chemical messengers that can be transported away from their site of manufacture, by active transport, diffusion and mass flow in the phloem sap or in xylem vessels, to act at target cells or tissues of the plant. They bind to receptors on the plasma membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular cells. This specific binding makes sure that the complementary shapes on the membranes of particular cells.

The cell wall around a plant cell limits the cell’s ability to divide and expand. Therefore, growth in plants happens where there are groups of immature cells that are still capable of dividing – these places are called meristems.

  • evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation

Apical Dominance:

Apical dominance – when a growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot. So if you break the shoot tip (the source of auxin) off a plant, the plant starts to grow side branches from lateral buds that were previously dormant.

Auxin is constantly made by cells at the tip of the shoot. It is then transported downwards, from cell-to-cell. This auxin accumulates in the nodes between the lateral buds. Somehow, its presence here inhibits their activity. Two simple experiments provide evidence for this mechanism:

  1. If we cut the tip off two shoots and apply IAA (synthetic auxin) to one of them, the one with IAA will continue to show apical dominance and the side shoots will not grow. The one without IAA will branch out sideways.

If a growing shoot is tipped upside down, apical dominance is prevented and the lateral buds start to grow out sideways. This can be explained by the fact that auxin is not transported upwards against gravity, but only downwards. So in the upside-down shoot, the auxin produced in the apical meristem does not reach the lateral buds and therefore cannot affect them

Gibberellin and Stem Elongation:

Gibberellin – a group of plant hormones that stimulate cell elongation, germination and flowering.

In Japan, a plant disease called Bakanae is caused by a fungus and makes rice grow very tall. Attempts to isolate the fungal compounds involved identified a family of compounds called gibberellins. One of these was gibberellic acid (GA3). Scientists began applying GA3 to dwarf varieties of plants (e.g. maize, peas), which made these plants grow taller. These results seem to suggest that gibberellic acid is responsible for plant stem growth, but such a conclusion is too hasty.

Scientists compared GA1 concentrations of tall pea plants (homozygous for the dominant Le allele), and dwarf pea plants (homozygous for the recessive le allele), which were otherwise genetically identical. They found that plants with higher GA1 concentrations were taller. However, to show that GA1 directly causes stem growth, the researches needed to know how GA1 is formed. They worked out that the Le allele was responsible for producing the enzyme that converted GA20 to GA1.

They also chose a pea plant with a mutation that blocks gibberellin production between ent-Kaurene and GA12-aldehyde. Those plants produce no gibberellin and only grow to about 1cm in height. However, if you graft a shoot onto a homozygous le plant (which cannot convert GA20 to GA1), it grows tall. The shoot has no GA20 of its own, but it has the enzyme to convert GA20 to GA1 – this confirmed that GA1 caused stem elongation. Dwarf varieties of plants lack the dominant allele for an enzyme needed for synthesis of gibberellins.

Further studies have shown that gibberellins cause growth in the internodes by stimulating cell elongation (by loosening cell walls) and (by stimulating production of a protein that controls the cell cycle). Internodes of dwarf peas have fewer cells and shorter cells than those of tall plants, and mitosis in the intercalary meristems of deep-water rice plants increases with gibberellin treatment.

  • describe how plant hormones are used commercially

 

 

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Excretion

Excretion

(a) define the term excretion

Excretion – the removal of metabolic waste from the body, of by-products or unwanted substances from normal cell processes.

Functions of the Liver:

  • control of: blood glucose levels, amino acid levels, lipid levels
  • synthesis of: red blood cells in the fetus, bile, plasma proteins, cholesterol
  • storage of: vitamins A, D and B12, iron, glycogen
  • detoxification of: alcohol, drugs
  • breakdown of hormones
  • destruction of red blood cells

 

(b) explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body

Carbon Dioxide:

Carbon dioxide is produced by every living cell in the body as a result of respiration. It’s passed from the cells of respiring tissues into the bloodstream where it is transported, mostly in the form of hydrogencarbonate ions, to the lungs where the CO2 diffuses into the alveoli to be excreted as we breathe out.

  • Affecting Haemoglobin:
  • CO2 is mostly transported in the form of hydrogencarbonate ions which forms hydrogen ions.The hydrogen ions combine with haemoglobin and they compete with oxygen for space, reducing oxygentransport.
  • CO2 can also directly combine with haemoglobin to form carbaminohaemoglobin, which has a loweraffinityfor oxygen than normal haemoglobin.
  • Respiratory Acidosis:
  1. Excess CO2 dissolves in the blood plasma and combines with water to form carbonic acid.

CO2 + H2O à H2CO3

  1. The carbonic acid dissociates to release hydrogen ions.

H2CO3àHCO3 + H+

  1. The H+ ions lower the pH and make the blood more acidic.
  2. If the change in pH is small then the extra hydrogen ions are detected by the respiratory centre in the medullaoblongata of the brain. Breathing rate increases to help remove excess CO2.
  3. If the blood pH drops below 7.35 it results in slowed or difficult breathing, headache, drowsiness, restlessness, tremor, confusion, rapid heart rate and changes in blood pressurerespiratory acidosis. It can be caused by diseases or conditions that affect the lungs themselves such as emphysema, chronic bronchitis, asthma or severe pneumonia, as well as blockage of airways due to swelling, a foreign object or vomit.

 

Nitrogenous Compounds (Urea):

Urea is produced in the liver from excess amino acids bring broken down, called deamination (the removal of the amine group from an amino acid to produce ammonia). It’s passed into the bloodstream to be transported to the kidneys where the urea is removed from the blood to become part of the urine.

  • The body can’t store proteins or amino acids, but they contain almost as much energy as carbohydrates so it would be wasteful to excrete excess amino acids.
  1. Excess amino acids are transported to the liver and the potentially toxic amino acid is removed (deamination). The amino group initially forms the very soluble and highly toxic compound, ammonia.

Deamination: amino acid + oxygen → keto acid + ammonia

  1. The ammonia is converted to a less soluble and less toxic compound called urea, which can be transported to the kidneys for excretion.

Formation of Urea: ammonia + carbon dioxide → urea + water

2NH3 + CO2 → CO(NH2)2 + H2O

  1. The remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.

(c) describe, with the aid of diagrams and photographs, the histology and gross structure of the liver

Hepatic Artery– supplies liver with oxygenated blood.

Hepatic Veindeoxygenated blood leaves the liver which rejoins the vena cava and the blood returns to normal circulation.

Hepatic Portal Vein–has capillaries at both ends deoxygenated blood travels to the liver from the digestive system. The blood is rich in the products of digestion.

Bile Ducttransports bile from the liver to the gall bladder where it is stored until required to aid the digestion of fats in the small intestine. (Bile emulsifies fat, producing a larger surface area, so they are broken down more easily.)

Arrangement of Cells in the Liver:

The liver is divided into lobes, which are further divided into cylindrical lobules.

As the hepatic artery and hepatic portal vein enter the liver they split into smaller and smaller vessels, which run between, and parallel to, the lobules and are known as inter-lobular vessels. Blood from the two vessels is mixed and passed along a sinusoid which is lined by liver cells. The sinusoids empty into the intra-lobular vessel, a branch of the hepatic vein. The branches of the hepatic vein from different lobules join together to form the hepatic vein, which drained from the liver.

As blood flows along the sinusoid it is in very close contact with the liver cells. They are able to remove molecules from the blood and pass molecules into the blood.

The liver manufactures bile and is released into the bile canaliculi (small canals) which join together to form the bile duct and transports the bile to the gall bladder.

The Different Cells in the Liver:

  • Liver cells (hepatocytes) have a simple cuboidal shape with many microvilli on their surface. Their metabolic functions include protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification, etc. = cytoplasm is very dense and is specialised in the amounts of certain organelles that it contains. For example, they contain lots of mitochondria as they need ATP.
  • Kupffer cells are specialisedmacrophages – move about in the sinusoids and are involved in the breakdown and recycling of old red blood cells. The breakdown of haemoglobin produces bilirubin, which is excreted as part of the bile and in faeces giving the brown pigment.

(d) describe the formation of urea in the liver, including an outline of the ornithine cycle

Most people in the developing world eat more than the 40-60g of protein we need every day. Excess amino acids cannot be stored, as the amine groups make them toxic. However, the amino acid molecules contain a lot of energy so it would be wasteful to excrete the whole molecule. In order for the amino component to be excreted, it must undergo the processes of deamination and the orithine cycle.

(e) describe the roles of the liver in detoxification

The liver is able to detoxify many compounds, e.g. hydrogen peroxide which is produced in the body or alcohol which is consumed as part of our diet. Toxins can be rendered harmless by oxidation, reduction, methylation or combination with another molecule.

(f) describe, with the aid of diagrams and photographs, the histology and gross structure of the kidney

(g) describe, with the aid of diagrams and photographs, the detailed structure of a nephron and its associated blood vessels

(h) describe and explain the production of urine, with reference to the processes of ultrafiltration and selective reabsorption

Ultrafiltration:

The afferent arteriole is wider than the efferent arteriole which is wider than the capillaries. Therefore there is a higher blood pressure in the capillaries to push fluid from the blood into the Bowman’s capsule.

 

The barrier between the blood in the capillary and the lumen of the Bowman’s capsule consists of 3 layers:

  1. The endothelium of the capillaries – has narrow gaps(pores) between its cells that blood plasma and the substances dissolved in it can pass.
  2. The basement membrane – consists of a fine mesh of collagen fibres and glycoproteins. They act as a filter to prevent the passage of molecules with a relative molecular mass of greater than 69000 (proteins and erythrocytes).
  3. The epithelial cells of Bowman’s capsule, called podocytes (finger-like projections). These ensure that there are gaps between cells.

 

Selective Reabsorption:

The cells lining the proximal convulated tubule are specialised to achieve reabsorption:

  • Microvilli– increases the surface area for reabsorption.
  • Cotransporter proteins– transport glucose or amino acids, in association with sodium ions from the tubule into the cell – facilitated diffusion.
  • Sodium-potassium pumps– pump sodium ions out the cell and potassium ions into the cell.
  • Many mitochondria– indicates an active or energy-requiring process, which needs lots of ATP.

 

  1. At the capillary end of the proximal convulated tubule cell, sodium ions are actively transported out of the cells into the blood by sodium-potassium pumps, reducing the concentration of sodium ions in the cell.
  2. At the filtrate end of the proximal convulated tubule, sodium ions move into the cell along with glucose or amino acid molecules by facilitated diffusion using co-transporter proteins, and the levels of sodium ions increase in the cell.
  3. The glucose and amino acids in the cells are able to just diffuse into the tissue fluid and into the blood, from a high concentration inside the cell to a low concentration in the capillaries.
  4. The movement of sodium ions, glucose and amino acids reduces the water potential in the cells so water will enter the cell by osmosis. The blood in the capillaries has an even lower water potential so water moves into the capillaries by osmosis.
  5. Larger molecules (e.g. small proteins that may have entered the tubule) will be reabsorbed by endocytosis.

(i) explain, using water potential terminology, the control of water content of the blood, with reference to the roles of the kidney, osmoreceptors in the hypothalamus and the posterior pituitary gland

Water Reabsorption:

The role of the loop of Henle is to create a low (very negative) water potential in the tissue of the medulla. This ensures that even more water can be reabsorbed from the fluid in the collecting duct.

 

As the fluid in the descending limb gets deeper into the medulla, the water potential becomes lower (more negative) due to the increasing concentration of sodium and chloride ions down the descending limb.

  • The wall of the descending limb is permeable to water so water is lost by osmosis to the surrounding tissue.
  • Sodium and chloride ions can diffuse into the tubule from the surrounding tissue.

 

As the fluid in the ascending limb moves back up towards the cortex, the water potential becomes higher (less negative) due to the decreasing concentration of sodium and chloride ions up the ascending limb.

  • At the base of the tubule, sodium and chloride ions diffuse out of the tubule into the tissue fluid.
  • Higher up the tubule, sodium and chloride ions are actively transported out into the tissue fluid.
  • The wall of the ascending limb is impermeable to water so the fluid in the tubule loses salts but not water as it moves up the ascending limb.

The arrangement of the loop of Henle is known as a hairpin countercurrent multiplier system. This is where one part of the tubule passes close to another part of the tubule with the fluid flowing in opposite directions, allowing exchange between the contents. The arrangement increases the efficiency of salt transfer from the ascending limb to the descending limb, and causes a build-up of salt concentration in the surrounding tissue fluid.

Animals that live in drier habitats, have longer loop of Henles. It provides them with a longer countercurrent mechanism that can increase the salt concentration in the medulla more than in other mammals. Therefore more water can be reabsorbed, which is important as there’s not much water is available for them to drink.

Osmoregulation:

  1. When there’s a decrease in water potential, the osmoreceptors in the hypothalamus of the brain, loses water by osmosis. They shrink and stimulate neurosecretory cells in the hypothalamus.
  2. The neurosecretory cells are specialised neurones that produce and release ADH. The ADH is manufactured in the body of the cells. ADH flows down the axon to the terminal bulb in the posterior pituitary gland – stored there until it is needed.
  3. The ADH enters the blood capillaries and transported around the body to the collecting duct. They bind to the complementary receptors on the walls of the collecting duct causing a chain of enzyme controlled reactions forming vesicles containing water-permeable channels (aquaporins), making the walls of the collecting duct more permeable.
  4. More water is reabsorbed by osmosis into the blood and less urine is produced – negative feedback.

(j) outline the problems that arise from kidney failure and discuss the use of renal dialysis and transplants for the treatment of kidney failure

Common causes of kidney failure are:

  • hypertension – high blood pressure can damage the small blood vessels in your kidneys and stop them working properly.
  • diabetes mellitusblocks the small blood vessels of your kidney and makes them leaky so you kidneys work less efficiently.
  • Infection.

Once the kidneys fail completely the body is unable to remove excess water and certain waste products from the blood. This includes urea and excess salts. It is also unable to regulate the levels of water and salts in the body. This will rapidly lead to death.

Dialysis is the most common treatment for kidney failure. It removes wastes, excess fluid and salt from blood by passing the blood over a dialysis membrane, which is partially permeable, allowing the exchange of substances between the blood and the dialysis fluid.

In a kidney transplant the old kidneys are left in place unless they are likely to cause infection or are cancerous. The donor kidney can be from a living relative who is willing to donate one of their healthy kidneys or from someone who has died. A kidney transplant is a major surgery. While the patient is under anaesthesia, the surgeon implants the new organ into the lower abdomen and attaches it to the blood supply and the bladder.

 

(k) describe how urine samples can be used to test for pregnancy and detect misuse of anabolic steroids

Substances or molecules with a relative molecular mass of less than 69 000 can enter the nephron. If these substances are not reabsorbed further down the nephron they can be detected in the urine

 

 

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Communication

 Communication

  • outline the need for communication systems within multicellular organisms, with reference to the need to respond to changes in the internal and external environment and to co-ordinate the activities of different organs

All living things to maintain a certain limited set of conditions inside their cells as cellular activity relies on the action of enzymes which only work efficiently at optimum conditions:

  • a suitable temperature – optimum for enzyme.
  • a suitable pH – optimum for enzyme.
  • an aqueous environment that keeps the substrates and products in solution – enzyme-substrate complexes can easily form as the molecules are quite close together.
  • freedom from toxins and excess inhibitors – inhibit the enzyme.

  • state that cells need to communicate with each other by a process called cell signalling

Cells communicate with each other by the process of cell signalling. This is where one cell releases a chemical that is detected by another cell and will respond to the signal released by the first cell.

  • state that neuronal and hormonal systems are examples of cell signalling

There are two major systems of communication that work by cell signalling:

  • Neuronal system – an interconnected network of neurones that signal to each other across synapse The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.
  • Hormonal system – uses the blood to transport its signals. Cells in an endocrine organ release the signal (a hormone) directly into the blood, where it is carried all over the body but only recognised by specific target cells. The hormonal system enable longer-term responses to be coordinated.

  • define the terms negative feedback, positive feedback and homeostasis
  • explain the principles of homeostasis in terms of receptors, effectors and negative feedback

Negative feedback – a process that brings about the reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.

For negative feedback to work effectively there must be a complex arrangement of structures that are all coordinated through cell signalling.

There are a number of structures required for this pathway to work:

  • Sensory receptors – e.g. temperature receptors or glucose concentration receptors. They are internal and monitor conditions inside the body. If they detect a change they will be stimulated to send a message.
  • A communication system – e.g. nervous system and hormonal system. This acts by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells. The message may or may not pass through a coordination centre such as the brain.
  • Effector cells – e.g. liver cells or muscle cells. These cells will bring about a response that reverses the change detected by the receptor cells.

Positive feedback – a process that increases any change detected by the receptors. It tends to be harmful and does not lead to homeostasis.

When positive feedback occurs the response is to increase the original change. This destabilises the system and is usually harmful, e.g. when the body gets cold – below a certain core body temperature, enzymes become less active meaning the exergonic reactions that release heat are slower and release less heat, allowing the body to cool further and slows down the enzyme-controlled reactions even more, so that the temperature spirals downwards.

Positive feedback can also be beneficial. It can be used to stimulate an increase in a change, e.g. at the end of pregnancy to bring about dilation of cervix – as the cervix begins to stretch the change is signalled to the anterior pituitary gland, stimulating the secretion of the hormone oxytocin, leading to increased uterine contractions which stretch the cervix more, causing the secretion of more oxytocin, until the cervix is fully dilated and the baby can be born.

Homeostasis – the maintenance the internal environment in a constant state despite external changes, so enzymes can work at an optimum.

  • describe the physiological and behavioural responses that maintain a constant core body temperature in endotherms and endotherms, with reference to peripheral temperature receptors, the hypothalamus and effectors in skin muscles

Ectotherms – organisms that relies on external sources of heat to regulate its body temperature.

Endotherms – organisms that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.

There are 4 ways that heat moves from one place to another:

  1. Conduction – when heat transfers from one surface tp another by direct contact (solids).
  2. Convection – when hot objects heat the particles around them, causing the heat energy to be carried up and away (liquids and gases).
  3. Radiation – where rays of infrared radiation transfer heat from any hot object to a cooler one (waves – vacuum).
  4. Evaporation – where heat energy is used to evaporate a liquid. The gaseous molecules then carry the energy away with them as they disperse.

The thermoregulatory centre in the hypothalamus monitors blood temperature and detects any changes in the core body temperature. An early warning of the body temperature could help the hypothalamus respond more quickly and avoid too much variation in the core body temperature.

The peripheral temperature receptors in the skin monitor the temperature in the extremitiesearly warning for the core body temperature. This information is fed to the thermoregulatory centre in the hypothalamus – negative feedback.

 

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Hormones

Hormones

(a) define the terms:

(b) explain the meaning of the terms first messenger and second messenger, with reference to adrenaline and cyclic AMP (cAMP)

First messenger – the hormone that transmits a signal around the body.

Second messenger – molecules called cAMP that transmits a signal inside the cell.

Adrenaline:

The adrenaline receptor on the outside of the cell surface membrane has a shape complementary to the shape of the adrenaline molecule. The receptor is associated with an enzyme on the inner surface of the cell surface membrane. The enzyme is called adenyl cyclase.

 

  1. Adrenaline in the blood binds to its specific and complementary receptor on the cell surface membrane. The adrenaline molecule is the first messenger.
  2. When it binds to the receptor is activates the enzyme adenyl cyclase, which converts ATP to cyclic AMP (cAMP – a second messenger).
  3. The cAMP can then cause an effect inside the cell by activating enzyme action.

 

(c) describe the functions of the adrenal glands

The adrenal glands are found lying just above the kidneys – one of each side of the body. Each gland can be divided into a medulla region and a cortex region.

 (d) describe, with the aid of diagrams and photographs, the histology of the pancreas, and outline its role as an endocrine and exocrine gland

The pancreas is a small organ lying below and behind the stomach. It is an unusual organ in that it has both exocrine and endocrine functions.

Pancreatic duct – a tube that collects all the secretions from the exocrine cells in the pancreas and carries the fluid to the small intestine.

α cells – found in the islets of Langerhans and secrete the hormone glucagon.

β cells – found in the islets of Langerhans and secrete the hormone insulin.

Glucagon – the hormone that causes blood glucose levels to rise.

Insulin – the hormone that causes blood glucose levels to fall.

(e) explain how blood glucose concentration is regulated, with reference to insulin, glucagon and the liver

The cells in the islets of Langerhans monitor the concentration of glucose in the blood. The normal concentration of glucose is 80-120mg per 100cm3 of blood, or 4-6 mmol dm-3.

(f) outline how insulin secretion  is controlled, with reference to potassium channels and calcium channels in beta cells

When glucose levels are too high:

(g) compare and contrast the causes of Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetes mellitus

Diabetes mellitus – a disease in which blood glucose concentrations cannot be controlled effectively, by the body.

Hyperglycaemia – the state in which the blood glucose concentration is too high (hyper = above, glyc = gucose, aenmia = blood).

Hypoglycaemia – the state in which the blood glucose concentration is too low (hypo = under).

(h) discuss the use of insulin produced by genetically modified bacteria, and the potential use of stem cells, to treat diabetes mellitus

 

Producing Insulin from Genetically Modified Bacteria:

Insulin used to be extracted from the pancreas of animals – usually from pigs as this matches human insulin most closely. However, more recently insulin can be produced by bacteria that have been genetically engineered to manufacture human insulin. Advantages of using genetically engineered insulin include:

  • It’s an exact copy of human insulin – faster acting and more effective.
  • Less chance of developing tolerance to the insulin.
  • Less chance of rejection due to an immune response.
  • Lower risk of infection.
  • Cheaper to manufacture the insulin than to extract it from animals.
  • The manufacturing process is more adaptable to demand.
  • Some people are less likely to have moral objections to using the insulin produced from bacteria than to using that extracted from animals.

Treatment of Diabetes:

Type I diabetes is treated using insulin injections. The blood glucose concentration must be monitored and the correct dose of insulin must be administered to ensure that the glucose concentration remains fairly stable.

Scientists have recently found precursor cells in the pancreas of adult mice. These cells are capable of developing into a variety of cell types and may be true stem cells, which can be used to treat type I diabetes.

Type II diabetes is treated by careful monitoring and control of the diet. Care is taken to match carbohydrate intake and use. This may eventually be supplemented by insulin injection or use of other drugs which slow down the absorption of glucose from the digestive system.

(i) outline the hormonal and nervous mechanisms involved in the control of heart rate in humans

The heart pumps blood around the circulatory system. Blood supplies the tissues with oxygen and nutrients such as glucose, fatty acids and amino acids. It also removes waste products, such as carbon dioxide and urea, from the tissues so that they do not accumulate and inhibit cell metabolism.

How does the heart adapt to supply the body with more oxygen and glucose?

  • Increase heart rate – increase in the number of beats per minute.
  • Increase the strength of contraction.
  • Increase the stroke volume – increase the volume of blood pumped per beat.

Control of Heart Rate:

The cardiac muscle is myogenic – the heart will contract and relax by itself. The heart contains its own pacemaker called the sinoatrial node (SAN). The SAN is a region of tissue that can initiate an action potential, which travels as a wave of excitation over the atrial walls, through the atrioventricular node (AVN) and down the Purkyne fibres to the ventricles, causing them to contract.

The heart is supplied by nerves from the medulla oblongata of the brain, which connect to the SAN. These do not initiate a contraction, but they can affect the frequency of the contractions.

  • Action potentials sent down the sympathetic nerve increases the heart rate.
  • Action potentials sent down the vagus nerve decreases the heart rate.

Adrenaline made in the medulla of the adrenal glands can also increase the heart rate.

Under resting conditions the heart rate is controlled by the SAN. This has a set frequency at which is initiates a wave of excitation. The frequency of excitation waves can be controlled by the cardiovascular centre in the medulla oblongata. There are many factors that affect the heart rate:

  • Movement of limbs is detected by stretch receptors in the muscles. These send impulses to the cardiovascular centre informing it that extra oxygen may soon be needed – increases heart rate.
  • The carbon dioxide produced when exercising reacts with water in the blood plasma and reduces the pH. The change in pH is detected by chemoreceptors in the carotid arteries, the aorta and the brain. The chemoreceptors send impulses to the cardiovascular centreincreases heart rate.
  • Adrenaline is secreted in response to stress, shock, anticipation or excitement to help prepare the body for activity – increases heart rate.
  • When we stop exercising the concentration of carbon dioxide in the blood falls, reducing the activity of the sympathetic pathwaydecreases heart rate.
  • Blood pressure is monitored by stretch receptors in the walls of the carotid sinus. If blood pressure is too high, the stretch receptors send signals to the cardiovascular centredecreases heart rate.

 

 

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Nerves

Nerves

 

  • outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses

Sensory receptors – specialised cells that can detect changes in our surroundings. They are energy transducers that convert one form of energy to another. A transducer is adapted to detect changes in a particular form of energy.

Stimulus – a change in energy levels in the environment. Whatever the stimulus the sensory receptors convert the energy into a form of electrical energy called nerve impulse.

  • describe, with the aid of diagrams, the structure and functions of sensory and motor neurones

There is a number of different neurones, including:

  • Sensory neurone – carry action potentials from a sensory receptor to the central nervous system.
  • Relay neurone – connect sensory and motor neurones.
  • Motor neurone – carry action potentials from the central nervous system to an effector, e.g. muscle or gland.

  • describe and explain how the resting potential is established and maintained

Resting potential– the potential difference or voltage across the neurone cell membrane while the neurone is at rest. It is about -60mV inside the cell compared with the outside. The membrane is said to be polarised.

  1. The sodium-potassium pumps in the membrane of the axon continually pump 3 Na+ ions out the cell for every 2 K+ ions moved into the cell against a concentration gradient, using energy provided by a large number of mitochondria found in the neurone.
  2. Due to the concentration gradients, Na+ ions tend to leak back into the cell, and K+ ions tend to leak out.
  3. K+ ions leaks out at a faster rate (1.5 times) than the rate of Na+ ions leaking back in, meaning that there are more positive ions (cations) outside the cell than inside the cell, and more negative ions (anions) in the cytoplasm.
  4. This all results in a potential difference of -60mV between the inside and outside of the cell. The difference is called the resting potential. The axon membrane is said to be polarised when it is in this state.
  • describe and explain how an action potential is generated

Action potential– the depolarisation of the cell membrane so that the inside is more positive than the outside, with a potential difference across the membrane of +40mV.

  1. A stimulus causes sodium ion channels in the axon membrane to open, allowing Na+ ions to move into the cell, down an electro-chemical gradient (more positive to more negative).
  2. The movement of Na+ ions increases the potential difference resulting in voltage-dependent gates opening, which allow even more Na+ ions in. The large number of positive ions cause the potential difference to rise to +40mV.
  3. The sodium ion channels close and the potassium ions channels open, allowing K+ ions to move out of the cell, to try to restore the balance of charges either side of the membrane – this is repolarisation.
  4. So many K+ ions leave the cell causing the potential difference to drop below the –60mV (called hyperpolarisation), which causes the potassium ions channels to close.
  5. The membrane’s sodium-potassium pumps will quickly restore the potential difference to the normal resting potential figure of -60mV.

Refractory period– the short time when the membrane is hyperpolarised. In this time it is impossible to stimulate the cell membrane to reach another action potential.

Threshold potential– The potential difference across the membrane of about         –55mV. If the depolarisation of the membrane does not reach the threshold potential then no action potential is created.

 

 

 

  • describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage-gated sodium ion and potassium ion channels

  1. When an action potential occurs, the sodium ion channels open at a particular point along the neurone, allowing Na+ ions to diffuse across the membrane from high concentration outside the neurone to low concentration inside the neurone.
  2. The movement of Na+ ions into the neurone upsets the balance of ionic concentrations created by the sodium-potassium pumps.
  3. The concentration of sodium ions inside the neurone rises at the point where the sodium ion channels are open, causing the Na+ ions to diffuse sideways, away from this region of increased concentration.
  4. This is called a local current – the movements of ions along the neurone. The flow of ions is caused by an increase in concentration at one point, which causes diffusion away from the region of higher concentration.

 

The Myelin Sheath:

The myelin sheath is an insulating layer of fatty material, made of Schwann cells, which is impermeable to Na+ and K+ ions. Therefore, the ionic movements that create an action potential cannot occur at the parts of axon with the myelin sheath. Instead, the action potentials only occur between the Schwann cells. These gaps are called nodes of Ranvier. The action potentials appear to jump from one node to the next. This is called saltatory conduction. This speeds up the transition of the action potential. Myelinated neurones conduct action potentials more quickly than non-myelinated neurones.

Saltatory conduction‘jumping conduction’ refers to the way that the action potential appears to jump from one node of Ranvier to the next.

  • interpret graphs of the voltage changes taking place during the generation and transmission of an action potential

  1. The neurone is at resting state (-60mV) and the membrane is said to be polarised.
  2. A stimulus causes sodium ion channels to open and Na+ ions move into the cell causing the membrane potential to increase to the threshold potential (-55mV).
  3. The membrane becomes depolarised and the cell becomes positively charged inside compared to outside.
  4. The membrane potential reaches +40mV. The sodium ion channels close and the potassium ion channels open and K+ ions diffuse out of the cell.
  5. The membrane becomes repolarised as the potential difference goes back to negative inside compared with outside.
  6. The membrane becomes hyperpolarised as the potential difference overshoots slightly from too many K+ ions move out the cell.
  7. The original potential difference is restored so that the cell returns to its resting state.

 

  • outline the significance of the frequency of impulse transmission

When a stimulus is at a higher intensity the sensory receptor will produce more generator potentials. This will cause more frequent action potentials in the sensory neurone. When these arrive at the synapse they will cause more vesicles to be released. Therefore this creates a higher frequency of action potentials in the postsynaptic neurone. Our brain can determine the intensity of the stimulus from the frequency of signals arriving. A higher frequency of signals means a more intense stimulus.

 

Summation – refers to the way that several small potential changes can combine to produce one larger change in potential difference across the membrane.

Low-level signals can be amplified by a process called summation:

  • Temporal summation
  • If a low-level stimulus is persistent it will generate several action potentials in the presynaptic neurone releasing many vesicles which will produce an action potential in the postsynaptic neurone.
  • Several impulses arriving at the same neurone via the same synapse.
  • Spatial summation
  • Several presynaptic neurones each release small numbers of vesicles into one postsynaptic neurone.
  • Several impulses arriving at the same neurone via several synapses.
  • compare and contrast the structure of myelinated and non-myelinated neurones

  • describe, with the aid of diagrams, the structure of cholinergic synapse

Synapse – the junction between two or more neurones, where neurones can communicate with, or signal to, another neurone.

Cholinergic synapse – those that use acetylcholine as their transmitter substance.

Neurotransmitter – this is the transmitter substance which is a chemical released by the presynaptic neurone, that diffuses across the synaptic cleft (the gaps between two neurones) to transmit a signal to the postsynaptic neurone.

The presynaptic neurone ends in a swelling called the synaptic knob. There’s a number of specialised features:

·         many mitochondria – indicating active processes that need ATP.

·         large amount of smooth endoplasmic reticulum present.

· vesicles of a chemical called acetylcholine – the transmitter substance that will diffuse across the synaptic cleft.

· voltage-gated calcium ion channels in the membrane.

 

  • outline the role of neurotransmitters in the transmission of action potentials
  1. An action potential arrives at the presynaptic neurone. The voltage-gated calcium ion channels open and Ca+ ions diffuse into the synaptic knob.
  2. The Ca+ ions causes the vesicles containing acetylcholine to move to and fuse with the presynaptic membrane. Acetylcholine is released by exocytosis.
  3. Acetylcholine diffuse across the synaptic cleft and they bind to the receptor sites on the sodium ion channels in the postsynaptic neurone, causing the sodium ion channels to open. Na+ ions diffuse across the membrane and move into the postsynaptic neurone and an action potential is created.
  4. Acetylcholinesterase, found in the synaptic cleft, hydrolyses acetylcholine to ethanoic acid and choline so that they can be recycled and to stop the action potential in the postsynaptic neurone. Ethanoic acid and choline re-enter the presynaptic knob by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria.

  • outline the roles of synapses in the nervous system

The main role of synapses is to connect two neurones together so that a signal can be passed from one to the other. Other functions include:

  • Several presynaptic neurones converging to one postsynaptic neurone – allows several signals from different parts of the nervous system to create the same response.
  • One presynaptic neurone diverging to several postsynaptic neurone – allows one signal to be transmitted to several parts of the nervous system to create different responses. This is useful in a reflex arc – one postsynaptic neurone elicits the response while another informs the brain.
  • Ensures signals are transmitted in one direction – only the presynaptic neurone contains vesicles of acetylcholine.
  • Acclimatisation – after repeated stimulation a synapse may run out of vesicles containing the transmitter substance. The synapse is said to be fatigued, which explains why we soon get used to a smell or a background noise. It may also help to avoid overstimulation of an effector, which could damage it.
  • Creation of specific pathways – thought to be the basis of conscious thought and memory.
  • Allows temporal and spatial summation.

 

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