Biology Unit 5Topic 7 Run for your lifeFast and Slow Twitch Fibres
Sliding Filament Theory1. When the muscle is relaxed, the binding sites on the actin are covered by tropomyosin.
2. When the membrane of the muscle is depolarised, calcium ions are released from the sarcoplasmic reticulum and bind with the troponin which displaces tropomyosin from the binding site, exposing the myosin binding sites on the actin filaments.
3. The myosin head binds to the myosin binding sites on the actin, using energy from ATP and forming cross bridges.
4. As the myosin heads attach to the actin filaments, ADP and Pi on the myosin head are released.
5. The myosin changes shape, causing the myosin head to nod forward. The attached actin moves over the myosin.
6. An ATP molecule binds to the myosin head. This causes the myosin head to detach.
7. An ATPase on the myosin head hydrolyses the ATP, forming ADP and Pi.
8. This hydrolysis causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again.
Questions:1. Name 2 roles for ATP in the contraction of muscle fibres Disconnecting myosin head from actin Active transport of calcium ions back to the sarcoplasmic reticulum.
2. How does rigor mortis occur? Lack of ATP means cells are unable to use active transport to pump calcium ions back to the sarcoplasmic reticulum Myosin head unable to detach from actin
Joints and MovementsMuscles bring about movement at a joint. Muscles can only pull; they cannot push, so at least two muscles are needed to move a bone to and fro. Pair of muscles that work this way are described as antagonistic. A muscle that contracts to cause extension of a joint is called an extensor; the corresponding flexor muscle contracts to reverse the movement.Joint StructureJoints are separated by a cavity filled with synovial fluid, which enables them to move freely. It also acts as a lubricant.The bones are held in position by ligaments that control and restrict the amount of movement in the joint. Ligaments join bone to bone. They are strong and flexible.Tendons attach muscles to the bones, enabling the muscles to power joint movement.Cartilage protects bones within joints. They absorb synovial fluid and act as a shock absorber.Muscle cells have several nuclei (multinucleate) because a single nucleus cannot effectively control the metabolism of such a long cell. Reaction of aerobic respiration
In aerobic respiration, the hydrogen stored in glucose is brought together with oxygen to form water again. Overall, there is a release of energy and this can be used to generate ATP.ATP
Investigating respirationRate of respiration of an organism is an indication of its demand for energy. Respiration rate is the uptake of oxygen per unit time is measured by the means of a respirometer. A respirometer detects changes in pressure or volume of a gas.
Potassium hydroxide or soda lime absorbs carbon dioxide released by the organism. Therefore, only oxygen gas causes a change in volume. GlycolysisThe initial stages of carbohydrate breakdown known as glycolysis occur in the cytoplasm of cells, including the sarcoplasm of muscle cells. It starts with the breakdown of six-carbon glucose molecule into two molecules of pyruvate (3C). Glycolysis does not need oxygen. It is the first stage of aerobic and anaerobic respiration and is the only stage of anaerobic respiration. The first reactions of glycolysis need an input of energy from ATP to get things started, because glucose (a hexose sugar) is quite stable and unreactive.[Glucose is phosphorylated]. Two phosphate groups are added to the glucose from two ATP molecules, and this increases the reactivity of glucose. It can now be split into two molecules of 3-carbon (3C) compounds. Each 3C sugar is oxidised, producing a 3-carbon compound, pyruvate.Hydrogen is removed and transferred to the hydrogen acceptor NAD. Enough energy is released at this stage to make two molecules of ATP. Phosphate from the intermediate compounds is transferred to ADP, creating ATP. This is substrate level phosphorylation, because energy for the formation of ATP comes from the substrates.Glycolysis reactions yield a net gain of two ATPs, two pairs of hydrogen atoms and two molecules of 3-carbon pyruvate.
The link reactionIf oxygen is available, the 3C pyruvate created at the end of glycolysis passes into the mitochondria. There it is completely oxidised, forming CO2 and H2O.In the presence of oxygen, 3 things happen:1. The pyruvate is decarboxylated (a molecule of CO2 is removed released as a waste product).
2. The pyruvate is also dehydrogenated (hydrogen is removed). The hydrogen is transferred to the hydrogen acceptor NAD+ to form NADH + H+.
3. The resulting acetate (2C) combines with coenzyme A (CoA) to form the 2-carbon molecule acetyl coenzyme A (acetyl CoA), which then enters the Krebs cycle.Krebs cycleKrebs cycle takes place in the matrix of the mitochondria, where the enzymes that catalyse the reactions are located and includes the following reactions:Acetyl CoA (2C) combines with a 4-carbon compound to create one with six carbons.A series of reactions take place where the 6-carbon compound is both decarboxylised and dehydrogenated, recreating the 4-carbon compound.In addition, one of the steps in the cycle involves substrate level phosphorylation with direct synthesis of a single ATP.CO2 is released a waste product and the hydrogen acceptors NAD and FAD.As a result, the 4-carbon compound is regenerated to combine with more acetyl CoA.So after one turn of Krebs cycle, we have 3 molecules of reduced NAD The electron transport chain 1 molecule of reduced FAD 1 molecule of ATP 2 molecules of CO2Two molecules of acetyl CoA enter Krebs cycle for each molecule of glucose, so the cycle turns twice for each glucose molecule, so we get each of the products timed by two.The most important role of Krebs cycle is to provide hydrogen that can be used in the electron transport chain to provide energy for the formation of ATP.The electron transport chainThe electron transport chain provides the means by which the energy from the hydrogen atoms removed from the compounds in Krebs cycle, glycolysis and the link reaction by coenzymes can be used to make ATP. For most hydrogen produced, the coenzyme NAD is the hydrogen acceptor. But those released in one step of the Krebs cycle are accepted by the coenzyme FAD rather than NAD.Oxygen is required for this final stage of aerobic respiration. The reactions take place on the inner membrane of the mitochondria. When a coenzyme accepts the hydrogen with its electron, the coenzyme is reduced, becoming reduced NAD or reduced FAD. The reduced coenzyme shuttles the hydrogen atoms to the electron transport chain on the mitochondrial inner membrane. The electron transport chain involves a chain of carrier molecules along which hydrogen atoms and electrons are passed. The hydrogen atoms are passed on to other carrier molecules from the hydrogen carriers [reduced NAD and FAD] Reduced NAD is the first carrier in the chain; it passes its hydrogen on to FAD. The hydrogen atoms split into hydrogen ions (H+) and electrons.The electrons are transferred along a series of electron carriers. The hydrogen ions stay in solution in the space between the inner and outer membranes of the mitochondria. Protons move across the inner mitochondrial membrane creating high H+ concentrations in the intermembrane space. H+ diffuses back into the mitochondrial matrix down the electrochemical gradient. H+ diffusion allows ATPase to catalyse ATP synthesis. Finally, the electrons recombine with the hydrogen ions to form hydrogen atoms and are passed on to oxygen to form water. Oxygen is therefore the final electron acceptor. ATP is formed at 3 points along the chain, so for each reduced NAD, 3 ATP molecules are made. For each reduced FAD, 2 ATP molecules are made. Formation of ATP in this way is called oxidative phosphorylation.StageSite in CellNumber of ATPs made
GlycolysisCytoplasm2 [4 made, 2 used] per glucose
Link reactionMatrix of mitochondrionNone
Krebs cycleMatrix of mitochondrion2 [one per turn] per glucose
Electron transport chainInner membrane if mitochondrion34 per glucose
ATP synthesis by chemiosmosisThe cristae on the mitochondrion give the inner membrane a large surface area, so there is more room for electron carriers and ATP formation. The chemiosmosis theory provides a model to explain the synthesis of ATP in oxidative phosphorylation. The energy released by the electron transport chain is linked to pumping hydrogen ions from the matrix into the space between the two membranes of the mitochondrion. This results in a higher concentration of hydrogen ions in the intermembrane space than in the matrix of the mitochondrion: an electrochemical gradient is set up. The hydrogen ions pass back into the matrix through the stalked granules, along the electrochemical gradient. As they do, their electrical potential energy is sued to make ATP from ADP and Pi. ATPase catalyses the reaction.Anaerobic respirationThis is respiration in the absence of oxygen. In the absence of oxygen, only glycolysis can operate. As a result, the energy yield in anaerobic respiration is low. Without oxygen to accept the hydrogen ions and electrons, the electron transport chain ceases; the reduced NAD created during glycolysis, the link reaction and the Krebs cycle is not oxidised. Without a supply of oxidised NAD, most respiration reactions cannot continue.In muscle cells for example, glycolysis can occur, but the pyruvate is converted into lactate. The reduced NAD made in glycolysis again passes its hydrogen on to pyruvate, this time reducing it to lactate and an oxidised form of NAD is produced.Glucose lactate + 2ATP
The net yield is just 2 ATP molecules per glucose molecule. The end product of anaerobic respiration is lactate, which builds up in the muscles and must be disposed of later. As lactate accumulates, [lactic forms lactic acid in solution] the pH of cell falls, inhibiting enzymes that catalyse the glycolysis reactions. They glycolysis reactions and the physical activity depending on them cannot continue.As pH reduces, substrate may no longer be able to bind to enzymes active site. After a period of anaerobic respiration, most of the lactate is converted back into pyruvate. It is oxidised directly to CO2 and H2O via the Krebs cycle, thus releasing energy to synthesise ATP. As a result, oxygen uptake is greater than normal in the recovery period after exercise. This excess oxygen requirement is called the oxygen debt. It is needed to fuel the oxidation of lactate.When oxygen becomes available again, the lactate is broken down. First it is carried in the bloodstream to the liver where it is converted back into pyruvate. About 1/5 is used to release energy in aerobic respiration and the rest is converted into glycogen. The oxygen required to break down the lactate is called the oxygen debt.At the end of the activity, the oxygen debt is repaid by continuous deep and rapid breathing.
Cardiac Output
Blood comes back from the body and it enters into the heart via the vena cava into the right atrium. As it enters into the right atrium, the atrium contracts, which pushes the blood into the right ventricle. Once the blood gets into the right ventricle, the right ventricle contracts and that pushes the blood through the semilunar valves into the pulmonary artery. That blood goes to the lungs and picks up oxygen, because you are breathing in oxygen and that oxygenates the blood. Both processes happen simultaneously, blood is being pumped to the lungs at the same time it's being pumped to the rest of the body.The volume of blood pumped in one minute is called the cardiac output. This increases during exercise. Cardiac output depends on the volume of blood ejected from the left ventricle (the stroke volume) and the heart rate:Cardiac output = Stroke volume x Heart rateStroke volumeThe stroke volume is the volume of blood pumped out of the left ventricle each time the ventricle contracts. How much blood the heart pumps out with each contraction is determined by how much blood is filling the heart, that is, the volume of blood returning to the heart from the body.In diastole, during exercise the heart fills with a larger volume of blood. The heart muscle is stretched to a greater extent, causing it to contract with a greater force and so more blood is expelled. This increases stroke volume and cardiac output. When the body is at rest, the ventricles do not completely empty with each beat; approximately 40% of the blood volume remains in the ventricles after contraction.Heart rateDifferences in resting heart rate are caused by many factors. For example, our hearts differ in size, owing to body differences in body size and genetics. A larger heart will usually have a lower resting heart rate. It will expel more blood with each heart beat and so, other things being equal, does not have to beat as frequently to circulate the same volume of blood around the body.Control of heart rateThe heart is myogenic; it can contract and relax without having to receive impulses from the nervous system. The cardiac cycle is started by specialised cardiac muscle tissue in the wall of the right atrium called the sinoatrial node SAN. The cells of the SAN set the rhythm at which all the other cardiac muscle cells beat and so can control the speed of the cycle.SAN sends out electrical impulses to the rest of the atria. These impulses spread out in a wave of depolarisation over the atrial walls. The cardiac muscle in the walls of both atria contract in rhythm with the impulses from the SAN, so both right and left atria contract at the same time. The impulse travels to some specialised cells called the atrioventricular node AVN. From here, the impulse is conducted to the ventricles after a delay of about 0.13 seconds. The delay ensures that the atria have finished contracting and that the ventricles have filled with blood before they contract. After this delay, they signal reaches the Purkyne fibres. These are large, specialised muscle fibres that conduct impulses rapidly to the apex of the ventricles. There are left and right bundles of fibres and they are collectively called the bundle of His. The first ventricular cell to be to be depolarised is at the apex of the heart, so that contraction begins at this point and travels upwards towards the atria. This produces a wave of contraction moving up the ventricles, pushing the blood into the aorta and the pulmonary artery.Electrocardiograms ECGThe electrical activity of the heart can be detected and displayed on an electrocardiogram ECG, a graphic record of the electrical activity during the cardiac cycle. The ECG is a useful diagnostic tool in that it can detect these irregularities. In an ECG, electrodes are taped to the persons chest and limps to record the electrical currents produced during the cardiac cycle. When there is a change in polarisation of the cardiac muscle, a small electrical current can be detected at the surface of the skin. This is what an ECG measures.
The P wave shows depolarisation of the atria, leading to atrial contraction (atrial systole)The PR interval shows the time taken for impulse to be conducted from the SAN across the atria to the ventricles, through the AVN.QRS complex shows the wave of depolarisation resulting in contraction of the ventricles (ventricular systole)T wave shows repolarisation of the ventricles during the hearts relaxation phase (diastole).ECG can be used to measure heart rate. A heart rate less than 60bpm is known as bradycardia. Tachycardia is heart rate greater than 100bpm.ECG can provide information about abnormal heartbeats, areas of damage and inadequate blood flow.Breathing and exercise Oxygen diffuses from the alveoli into the blood and carbon dioxide diffuses out of the blood capillaries into the alveoli. The volume of air we breathe in and out at each breath is our tidal volume. Ventilation rate is the total volume of air taken into the lungs in 1 minute.Ventilation rate = Breathing rate x Tidal volumeNervous control of heart rateHeart rate is under the control of the cardiovascular control centre located in the medulla of the brain. Changes in cardiac output are controlled by the autonomic nervous system. There are 2 distinct parts to the autonomic nervous system: The sympathetic SNS and the parasympathetic nervous system PNS. Stimulation of the sinoatrial node SAN by the sympathetic nerve causes an increase in heart rate [increases cardiac output] by: Increasing the heart rate Increasing the stroke volumeStimulation of the sinoatrial node SAN by the parasympathetic nerve causes a decrease in heart rate [decreases cardiac output] by: Decreasing the heartbeat rate Decreasing the stroke volumeThese 2 opposing systems work on a negative feedback principle, involving 2 centres in the medulla of the brain.During vigorous exercise, the CO2 level in the blood increases as a result of increased respiration. This causes a lowering of the blood pH, so it becomes more acidic. The cardiovascular control centre detects accumulation of CO2 and lactate in the blood, reduction of oxygen in the blood and increased temperature. Mechanical activity in muscles and impulses are sent to the cardiovascular control centre. This changes result in a higher heart rate [increase in cardiac output]. Blood flow to the lungs increase, so the extra CO2 is removed.As the sound of a pistol, skeletal muscles contract and stretch receptors in the muscles and tendons are stimulated. They send impulses to the cardiovascular control centre. This in turn raises the heart rate via the sympathetic nerve. There is an increase in venous return, which leads to a rise in the stroke volume. The elevated heart rate and stroke volume result in higher cardiac output, thus increasing transporting oxygen and fuel to muscles more quickly.Stretch muscles in the walls of the aorta can detect changes in blood flow to them, due to blood pressure. To prevent it rising too far, pressure receptors in the aorta and in the carotid artery send nerve impulses back to the cardiovascular control centre. Inhibitory nerve impulses are then sent from here to the sinoatrial node. In this way, an excessive rise in blood pressure is avoided through negative feedback, which prevents further rise in heart rate.Hormonal effects on heart rateFear, excitement and shock cause the release of the hormone adrenaline into the bloodstream from the adrenal glands located above the kidneys. Adrenaline has an effect on the heart rate similar to stimulation by the sympathetic nerve. It has a direct effect on the sinoatrial node, increasing the heart rate to prepare the body for any likely physical demands.The control of breathingThe ventilation centre in the medulla oblongata of the brain controls breathing. InhalationThe ventilation centre sends nerve impulses ever 2-3 seconds to the external intercostal muscles and diaphragm muscles. Both of these sets of muscles contract causing inhalation.ExhalationAs the lungs inflate, stretch receptors in the bronchioles are stimulated. The stretch receptors send inhibitory impulses back to the ventilation centre. As a consequence, impulses to the muscles stop and the muscles relax, stopping inhalation and allowing exhalation.Exhalation is caused by the elastic recoil of the lungs and by gravity helping to lower the ribs. During exercise, the rate of respiration increases in your cells, so more oxygen is used up and more CO2 is produced. It is the rise in CO2 that triggers the changes in your breathing.Controlling breathing rate and depthA small increase in blood CO2 concentration causes a huge increase in ventilation. CO2 dissolves in the blood plasma making carbonic acidCarbonic acid lowers pH of the blood.Chemoreceptors sensitive to hydrogen ions are located in the ventilation centre of the medulla oblongata. They detect the rise in hydrogen ion concentration.Impulses are sent to other parts of the ventilation centre. Impulses are sent from the ventilation centre to stimulate the muscles involved in breathing.Increasing CO2 and decrease in pH leads to an increase in rate and depth of breathing, through the more frequent and stronger contraction of the appropriate muscles. The more frequent and deeper breaths maintain a steep concentration gradient of CO2 between the alveolar air and the blood. This in turn ensures the removal of CO2 and uptake of oxygen. The opposite response occurs with a decrease in CO2 [negative feedback]. Controlling breathing during exerciseThe motor cortex of the brain controls movement. As soon as exercise begins, impulses from the motor cortex have a direct effect on the ventilation centre in the medulla, increasing ventilation sharply.SpirometerA spirometer is used to measure the volume of air that moves in and out of the lungs. It is basically a clear, plastic box, filled at the bottom with water. The space inside the spirometer contains oxygen.When you breathe out, the box moves up (pen moves up) and when you breathe in, the box moves down (pen moves down).With most spirometers, the exhaled air passes through a container of soda lime, a chemical that absorbs CO2.Tidal volume is the volume of air breathed in and out during a single breath.Vital capacity is the maximum volume of air that can be breathed in or out of the lungs.Negative feedbacksIn humans, if cells are to function properly, the bodys internal conditions must be maintained within a narrow range of cells optimum conditions. The maintenance of this stable internal environment is called homeostasis.Each condition that is controlled has a norm value or set point that the homeostatic mechanisms are trying to maintain. Receptors are used to detect deviations from the norm. These receptors are connected to a control mechanism, which turns on or off effectors to bring the condition back to the norm value. A deviation from the norm results in a change in the opposite direction back to the norm, the process is known as negative feedback. These corrections mean that the actual value fluctuates in a narrow range around the norm. Homeostasis Temperature controlThermoregulation is the control of body temperature.Each control system must have: A receptor [sensor] which detects a stimulus. A stimulus is a change in the level of the factor being regulated. A coordinator, which receives and controls information from the receptor and triggers the action that will correct the change. An effector, which carries out the action that brings about the change.Our core body temperature is very stable at 37.50C. This body temperature allows enzyme controlled reactions to occur at a reasonable rate. At lower temperatures, the reactions would occur too slowly for the body to remain active; at higher temperatures the enzymes would denature.In humans, temperature is maintained by a negative feedback system. This system involves receptors that detect changes in the blood temperature. These receptors are located in a structure in the brain called the hypothalamus. The hypothalamus is the control mechanism and acts as a thermostat, turning on the effectors necessary to return the temperature to the norm.There are also thermoreceptors in the skin that detect temperature changes. If the skin is warm, then impulses are sent to the hypothalamus initiating the heat-loss responses and inhibiting heat-gain responses. If the skin is cold, the opposite happens.
