Edexcel Categories Archives: Topic 7: Run for your Life

7.4 Health, exercise and sport

7.4 Health, exercise and sport


Benefits of exercise

  • Increasing arterial vasodilation lowers blood pressure and reduces the risk of cardiovascular disease and stroke
  • Increases the level of blood HDLs which transport cholesterol to the liver where it is broken down
  • Reduces the level of LDLs, reducing the development of atherosclerosis
  • Helps maintain a healthy weight – decreases risk of obesity as metabolic rate increases during exercise
  • Increased sensitivity of muscle cells to insulin improves blood glucose regulation and reduces the risk of type 2 diabetes
  • Increases bone density and reduces its loss during old age – delays the onset and slows the progress of osteoporosis
  • Reduces the risk of getting some cancers
  • Improves mental well-being
  • Moderate exercise – increased number and activity of natural killer cells in blood and lymph. Provide non-specific immunity, recognise glycoproteins on the surface of pathogens and secrete apoptosis-inducing molecules, causing cell lysis


Effects of too little exercise:

  • Increased risk of obesity
  • High blood pressure and high LDL levels, increasing the risk of CHD and stroke
  • High risk of cancer
  • Higher risk of type 2 diabetes – decreased sensitivity of liver and muscle cells to insulin – high blood glucose levels not returned to normal as fast as normal
  • Higher risk of osteoporosis


Risks of too much exercise

However people who exercise too much can cause damage to their body.

  • Joints become abnormally worn e.g. wear of cartilage, swelling of knees and other synovial joints due to a build up of fluid and damaged ligaments
  • Strenuous exercise can make the immune system less effective/immune suppression – more likely to suffer infections e.g. of the upper respiratory tract
  • High levels decrease the activity of the lymphocytes (phagocytes, B cells and T helper cells) and the ability of the immune system to destroy viruses and other pathogens. They also have increased exposure to pathogens.
  • The decrease in T helper cells reduces the amount of cytokines available to activate lymphocytes (B and T cells) so fewer antibodies are produced
  • Psychological stress (due to heavy training schedules) and physical stress cause secretion of the hormones adrenaline and cortisol from the adrenal glands which suppress the immune system
  • Overtraining à poor athletic performance, chronic fatigue, increased wear and tear on joints, immune syppression resulting in more frequent infections


Exercise and the joints

Keyhole surgery:

A common injury is a torn anterior cruciate ligament. This ligament supports the bones at the knee and can be torn due to repeated forces. It is normally treated with keyhole surgery (arthroscopy) – a small incision is made, through which an arthroscope (allows images to be observed by passing light rays along optical fibres) is fed. Surgical instruments are also passed through the holes. Tendons or ligaments taken from elsewhere in the body repair the damaged ligament.

+ Only small incisions – less blood loss and scarring

+ Less pain and recovery time – easier to return to normal activities



If a knee joint is badly damaged, the whole joint must be replaced with a prosthetic (artificial) joint. Prostheses can replace whole limbs or limb parts.

+ Make it possible for people with disabilities to participate in sport

+ Make it possible for people with injuries to play sport again

+ Variations in design for specific activities, can be articulated so they bend

However they are expensive as they must be made of high-quality materials that:

  • Are unaffected by body fluids
  • Can stand up to the high forces of the knee
  • Have the right combination of flexibility and strength for movement and support


Performance-enhancing drugs

Some athletes use performance-enhancing drugs, many of which are now banned from sport and athletes face penalties if found to have used them. There are various types that have different effects on the body:


  • Anabolic steroids – increase strength, speed and stamina by increasing muscle size and allowing athletes to train harder and increase aggression
  • Stimulants – speed up reactions, reduce fatigue and increase aggression
  • Narcotic analgesics – reduce pain so injuries don’t affect performance


Steroids increase protein synthesis so can increase muscle size and strength. Erythropoietin increases the rate at which red blood cells are made, increasing oxygen-carrying capacity of the blood.


Peptide hormones such as erythropoietin are protein chains. They cannot pass through cell membranes easily because they are charged, instead they bind to a receptor on the cell membrane. This activates a second messenger in the cytoplasm which causes chemical changes in the cell by affecting gene transcription.


Steroid hormones are formed from lipids and have complex ring structures. The hormone-receptor complex functions as a transcription factor, switching enzyme synthesis on of off.

Ethical issues of performance-enhancing substances


  • Some performance-enhancing drugs are illegal
  • Competitions are unfair if some people take drugs – they gain an advantage rather than through training or hard work
  • There are serious health risks such as high blood pressure or heart problems
  • Athletes may not be fully informed of the health risks



  • It is up to each individual – athletes have the right to make their own decisions about drugs and whether they are worth the risk
  • Drug-free sport isn’t fair anyway – different access to training facilities, coaches, equipment etc
  • Hard to detect every drug so its hard to develop the technology
  • No ban on nutritional substances such as vitamins


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7.3 The heart, energy and exercise

7.3 The heart, energy and exercise


Controlling the heart

The heart is myogenic – it contracts and relaxes automatically without stimulation from nerves to pump blood around the body. It is specific and made of specialised tissue called cardiac muscle. Rhythmic contraction of the cardiac muscle is coordinated through electrical impulses passing through the cardiac tissue.


In the right atrium wall, muscle tissue is present called the sinoatrial node (SAN) which acts as a pacemaker. This has an intrinsic rate of contraction a bit higher than the rest of the heart muscle. As the SAN cells contract, they generate action potentials, sending a wave of depolarisation along the right atria wall, causing them to contract (atrial systole).


When the wave of depolarisation reaches the atroventricular node (AVN), it is delayed by 0.13 seconds to ensure the atria have finished contracting before the ventricles and the depolarisation then continues down between the ventricles, along fibres called the bundle of His and down the purkyne fibres, and up through the ventricle walls. This causes the ventricles to contract together from the bottom up (from the apex), slightly after the atria (ventricular systole). Blood is squeezed into the aorta and pulmonary artery.


There is then a short delay before the next wave of depolarisation in the SAN. During this time the heart muscles relax and repolarises, this is diastole.


Differences in resting heart rate are caused by different size, body size and genetic factors. A larger heart usually has a lower resting heart rate – it expels more blood with one beat so does not need to beat as frequently to keep blood circulation constant. Endurance training produces a lower heart rate because there is an increase in the heart size due to thickening of the muscle cell walls.


Electrocardiograms (ECG)

The electrical activity in a heart is monitored and recorded by an ECG which produces a trace. Electrodes are attached to a person’s chest and limbs to record the electrical currents produced during the cardiac cycle. When there is a change in polarisation of the cardiac muscle, there is a small electrical current that can be detected on the skin. An ECG is usually performed on a patient at rest.


  • P wave = contraction (depolarisation) of the atria
  • QRS complex = contraction (depolarisation) of the ventricles
  • T wave – relaxation/recovery (repolarisation) of the ventricles
  • PR interval – time for impulses to be conducted from SAN across the atria to the ventricles, through the AVN


You can work out the time for one complete cardiac cycle by multiplying the number of squares between the QRS complex by 0.2 then doing 60 divided by the answer.


Abnormal ECGs can be used to diagnose heart problems. Doctors compare patient’s ECGs with a normal trace to diagnose problems with the heart rhythm such as CVD.


For example:

  • Tachycardia – increased heart rate of more than 100bpm – sign of heart failure – cannot pump blood efficiently – can increase risk of heart attack
  • Bradycardia – heart rate of less than 60pbp
  • Fibrillation – irregular heartbeat – atria and ventricles have lost their rhythm. Atrial fibrillation – chest pains, fainting and increased risk of stroke. Ventricular fibrillation – heart attack
  • Ischaemia – heart muscle does not receive blood due to atherosclerosis causing blockage of coronary arteries – disrupts normal electrical activity and rhythm, arrhythmias is caused (irregular heart beat)


  • A longer delay between the P and R waves – electrical impulses cannot pass easily from the atria to the ventricles – e.g. damage to the bundles of His
  • A flat T wave – indicate lack of blood flow to cardiac muscle in the heart wall – no energy for contraction – could be an indication of CVD
  • Problems with AVN – atria are contracting but ventricles are not – some P waves not followed by a QRS complex


So ECGs can provide information on irregular heartbeats, areas of damage and inadequate blood flow.


Homeostasis and responding to exercise

Homeostasis = The maintenance of a steady internal state in the body almost regardless of changes in either the external or internal conditions.


The core body temperature must be maintained in a narrow range at around 37oC for cells to function and stop enzymes becoming denatured. This is a dynamic equilibrium, matching the supply of oxygen and glucose to the changing demands, while removing carbon dioxide and maintaining an even temperature. Each condition has a norm value that the mechanism maintains.


Homeostasis involves coordination and control. Changes in the body are detected by a sensor/receptor. This sends a message to an effector which reverses or increases the change. Negative feedback systems provide a way of maintaining a condition – a change is registered by receptors and effectors are stimulated to restore the equilibrium. In a positive feedback system, effectors increase the effect which triggered the response.


The communication in a feedback system can be hormones (chemical messengers) or by nerve impulses (electrical messengers). Homeostasis plays a role in exercise when conditions change rapidly and demands are high.


Nervous control:

Heart rate is controlled by the cardiovascular control centre in the medulla of the brain. Chemical and stretch receptors in the blood vessels and heart chambers send nerve impulses to the cardiovascular centre. Two types of nerves of the autonomic system then carry impulses to the heart, controlling its rate:


  • Sympathetic nervous system: Usually excitatory – speeds heart rate – releases adrenaline and noradrenaline when stimulated
  • Parasympathetic nervous system: Usually inhibitory – slows heart rate – releases acetylcholine when stimulated


Nerve impulses travelling down the sympathetic nerve from the cardiovascular centre stimulate the SAN. This increases the frequency of the signals from the pacemaker region so the heart rate increases. In contrast nerve impulses in the parasympathetic nerve inhibit the SAN and slow the heart down.


Hormonal control:

Adrenaline is secreted from the adrenal glands above the kidneys during fear, shock or excitement, stimulating the SAN to increase its rate of contraction. This speeds the frequency of excitation, supplying extra oxygen and glucose for the muscles and brain in a fight or flight response. Adrenaline also causes dilation of the arterioles supplying skeletal muscles and constriction of arterioles going to the digestive system and other non-essential organs. This maximises blood flow to the active muscles. Adrenaline also causes an anticipatory increase in heart rate before exercise.


Responding to exercise

As the atria fill with blood at the start of the cardiac cycle, stretch receptors in the muscle walls of the heart send nerve impulses to the cardiovascular control centre. This sends more impulses along the sympathetic nerve to the SAN, increasing the heart rate. The increased stretching of the heart atrial muscle also makes the muscles contract harder, increasing the volume of blood expelled at each stroke.


Baroreceptors (sensitive to pressure) in the aorta and carotid arteries are stretched as blood pressure increases after exercise. They send nerve impulses to the parasympathetic system to slow the heart rate and cause vasodilation. This lowers the blood pressure.


The reverse happens when exercise starts – the blood vessels dilate in response to adrenaline and blood pressure falls, reducing the stretch of the baroreceptors. When the baroreceptors do not stimulate the cardiovascular control centre it sends signals along the sympathetic nerve to stimulate the heart rate and increase blood pressure.  


Increased heart rate caused by:

  • Increase in carbon dioxide
  • Decrease in oxygen
  • Decrease in blood pH
  • Increase in temperature


Breathing rhythms: Components of lung volume

Ventilation rate is the volume of air breathed in or out in a minute


Ventilation rate = tidal volume x breathing rate


  • Tidal volume is the volume of air in a normal breath (about 0.4dm3)
  • Breathing rate is how many breaths are taken in a minute


Cardiac output is the total volume of blood pumped by the left ventricle every minute.

Cardiac output increases during exercise because heart rate and stroke volume both increase as the heart pumps faster and harder.


Cardiac output (cm3/min) = stroke volume (cm3) x heart rate (bpm)


  • Stroke volume is the volume of blood pumped by the left ventricle each time it contracts


Cardiac output can be increased by:

  • Increasing strength of contraction which increases the rate oxygen enters the blood in the lungs and carbon dioxide leaves it
  • Increasing heart rate which increases the rate blood moves through the vessels, delivering oxygen to muscle tissues and removing carbon dioxide and lactate from them.


Vital capacity is the maximum volume of air we can inhale and exhale

Aerobic capacity is the ability to take in, transport and use oxygen


Long periods of strenuous exercise depends on maintaining a constant ATP supply, this depends on aerobic capacity.

  • VO2 is the volume of oxygen we consume per minute
  • VO2 max is the maximum amount of oxygen we can consume per minute


During exercise, there are greater muscle contractions so more blood returns to the heart (venous return) – in diastole the heart fills with a larger volume of blood – the heart muscle is stretched more, increasing stroke volume and cardiac output.


Practical – using a spirometer to investigate ventilation rate

Spirometers can be used to measure tidal volume and breathing rate and can also be used to investigate the effects of exercise. A spirometer is a machine with an enclosed chamber containing oxygen, lying over water.


  1. A person breathes through a mouthpiece/tube
  2. As they breathe in, the lid moves As they breathe out, it moves up
  3. These movements are recorded by a pen attached to the chamber lid, drawing on a rotating drum, creating a spirometer trace
  4. The soda lime in the tube absorbs carbon dioxide breathes out during respiration – only oxygen is in the chamber to inhale from – total volume decreases.


By counting the number of traces over a known period of time, breaths per minute can be calculated.


Control and regulation

The medulla controls breathing rate. Rhythmic patterns of nerve impulses are sent from the ventilation centre in the medulla to the muscles in the diaphragm and intercostal muscles, which respond by contracting rhythmically. There are two ventilation centres:

  • Inspiratory centre – controls breathing in – sends nerve impulses to intercostal and diaphragm muscles to make them contract – increases volume and decreases pressure in lungs. These impulses inhibit action of the expiratory centre. As the lungs inflate, stretch receptors are stimulated and send nerve impulses back to the medulla, inhibiting the action of the inspiratory centre.
  • Expiratory centre – controls breathing out – no longer inhibited, sends nerve impulses to the diaphragm and intercostal muscles to relax – lungs deflate, expelling air. The stretch receptors become inactive then the cycle repeats



During exercise, there is an increase in carbon dioxide in the blood and a decrease in blood pH. This is detected by chemoreceptors in the medulla, aorta and carotid bodies which then send nerve impulses to the respiratory/ventilation centre. Impulses are sent to the breathing muscles (effectors) – intercostal muscles and diaphragm to change the breathing rate in a negative feedback system, removing the extra carbon dioxide and increasing oxygen uptake. The muscles contract harder and faster, increasing the rate and depth of breathing.


Temperature control and exercise

When energy is transferred, some is lost as heat, so respiration and muscle contraction produce heat. During vigorous exercise, heat is generated in muscles, causing the blood temperature to rise. Thermoregulation is an important aspect of homeostasis to prevent enzymes becoming denatured and ensuring that normal metabolic reactions can take place.


Human temperature regulation

Changes in core temperature are detected by thermoreceptors in the skin and sent to the hypothalamus in the brain. The hypothalamus then sends nerve impulses to effectors which respond:


When temperature rises:

  • Vasodilation: Smooth muscle in the arteriole walls delivering blood to the skin dilate so a greater volume of blood flows into the surface capillaries – allows heat to be lost by radiation from the blood through the skin surface
  • Sweat glands activate: Sweat flows up the sweat ducts onto the skin surface, where the water in it evaporates – takes heat from the skin
  • Erector pili muscles attached to hairs relax to let hairs lie flat so they do not trap a layer of insulating air.


When temperature falls:

  • Vasoconstriction: Arterioles constrict so less heat is lost from the skin surface, blood is diverted beneath the insulating fat layer beneath the skin
  • Sweat glands secrete little or no sweat
  • Erector pili muscles contract, pilling hairs up – traps a layer of insulating air next to the skin
  • Certain muscles contract and relax rapidly (shivering), generating heat which increases blood temperature


Heat gain processHeat loss process
Vasoconstriction – arterioles in skin constrictVasodilation – inhibits arteriole contraction
Hair erector muscles contractHair erector muscles relax
Sweat glands are inhibitedSweat glands are stimulated
Liver secretes hormones – increases metabolic rateLiver – decreases metabolic rate
Skeletal muscles contract – shivering, increased respirationSkeletal muscles relax – no shivering


This is negative feedback – a receptor detects a change in the normal state of a system – triggers events to reverse the change. However temperature does fluctuate because of the lag time between the sensory (hypothalamus) detecting the change and the effectors respond to it.

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7.2 Muscles and movement

7.2 Muscles and movement


Structure and function of muscles

Muscle tissue is made of specialised cells that use energy from the hydrolysis of ATP to become shorter by contraction.

Striated (skeletal/voluntary) muscle attaches to the bones by tendons and appears stripy under a microscope. Skeletal muscles contract and relax to move bones at a joint. The cells in striated muscle are highly specialised muscle fibres. Each fibre contains many nuclei (multinucleate), mitochondria (provides ATP for contraction) and sarcoplasmic reticulum (contains calcium ions). The cell membrane of a muscle fibre is the sarcolemma. Parts of the sarcolemma fold inwards across the fibres and stick into the sarcoplasm (cytoplasm containing organelles such as mitochondria). These folds are fibules and help spread electrical impulses through the sarcoplasm.


Muscle fibres are made of myofibrils, which are made of many short units called sarcomeres. The ends of sarcomeres have a Z line and the middle has an M line (attachment for myosin). They contain bundles of protein filaments (myofilaments) called actin and myosin which move past each other to make muscles contract.


They produce alternating patterns of light and dark bands:

  • Dark A bands contain the thick myosin filaments and some overlapping thin actin filaments
  • Light I bands contain thin actin filaments only
  • Around the M line is the H zone which contains myosin filaments only


When the muscle contracts, the dark band overlaps the intermediate band, shortening the length of the muscle and the sarcomere.


 Skeletal Cardiac Smooth
Function LocomotionPumping blood through heartLine blood vessels, digestive tract, uterus etc.
Cells StriatedSpecialised striatedUnstriated
Control VoluntaryInvoluntaryInvoluntary
Arrangement Regular – muscle contracts in one directionCells branch and interconnect – efficient transfer of impulses – simultaneous contractionNo regular arrangement – cells contract in different directions
Speed of contractionRapidIntermediateSlow
Length of time contractedShortIntermediateLong


Types of muscle fibres

Within striated muscle tissue, there are two types of muscle fibre. Fast twitch fibres are adapted for rapid contraction over a short period of time. Slow twitch fibres are adapted for less rapid contraction over longer periods of time (continuous activity).


Slow twitchFast twitch
Contract slowlyContract quickly
Muscles used for posture contain many slow fibresMuscles used for fast movement
Good for endurance activities e.g. long-distance runningGood for short bursts of speed and power e.g. sprinting
Fatigue resistantTire/fatigue easily (lactic acid)
Energy released through aerobic respirationEnergy released through anaerobic respiration (mostly glycolysis)
Contains many mitochondria and capillaries to supply muscles with oxygen and for Krebs cycle and ETCContain fewer mitochondria and capillaries
Reddish because of richness of myoglobin (red, oxygen storing protein)White due to lack of myoglobin (few reserves of oxygen)
Low glycogen contentHigh glycogen content
Low levels of creatine phosphateHigh levels of creatine phosphate
Less sarcoplasmic reticulumMore sarcoplasmic reticulum
Relatively narrow so oxygen can diffuse rapidlyRelatively wide


The sliding filament theory

Muscle contraction works as actin and myosin filaments slide between each other, causing the sarcomeres to become shorter, shortening the whole muscle fibre.

  1. A nerve impulse (action potential) arrives at the neuromuscular junction and depolarises the sarcolemma
  2. Calcium ions (Ca2+) are released from the sarcoplasmic reticulum and diffuse across the sarcoplasmic reticulum into the muscle fibre
  3. Calcium ions bind to tropopin, pulling the attached tropomyosin, exposing the myosin binding sites on the actin
  4. Myosin head binds with the actin filaments, forming cross-bridges
  5. ATP on the myosin head breaks down into ADP + Pi, providing energy needed for muscle contraction
  6. Myosin head nods forward, pulling the actin towards the centre of the sarcomere
  7. An ATP molecule binds to the myosin head, breaking the cross-bridge, the myosin head detaches from the actin filament
  8. An ATPase molecule on the myosin head hydrolyses ATP into ADP and Pi
  9. The myosin heads move back to their original upright position
  10. This process repeats as long as the action potentials continue to arrive


When the muscle is not longer being stimulated, calcium ions are moved back into the sarcoplasmic reticulum by active transport (using ATP).  The troponin molecules move back to their original shape and the tropomyosin blocks the actin binding sites. This means the myofilaments cannot slide past each other because the myosin heads cannot bind to the actin. The actin filaments slide back to their relaxed position, lengthening the sarcomeres.


Tissues of the skeletal system

Striated muscles are attached to the skeleton by strong, inelastic tendons. Tendons are made of long fibres of the protein collagen, and small amounts of elastin. When a muscle contracts it pulls on the tendons which transmit a force.

Bones are connected at joints such as finger joints, elbow joints and synovial joints. Ligaments hold these bones in synovial joints together. The hip, knee and ankle are synovial joints. They are where the bones in the joint are separated by a cavity filled with synovial fluid, secreted by the synovial membrane. This acts as a lubricant and allows for free movement.

Ligaments also contain collagen and elastin but have a higher proportion of elastin, so they can stretch more than tendons. Ligaments control and restrict the amount of movement in the joint.

Cartilage is firm and elastic, it protects the bones within the joints. Cartilage absorbs synovial fluid and acts as a shock absorber.

Muscles produce a force when they contract. When they relax, they stay in the same position unless pulled to their lengthened state. For example the major muscle causing the arm to bend at the elbow is the biceps, known as the flexor muscle. The triceps are the extensor muscle, causing the arm to straighten when it contracts. As the arm bends, the biceps contract and the triceps relaxes, although it may contract slightly to control the movement. Muscles can only pull they cannot push, two work together for movement, they are antagonistic muscle pairs. When one muscle is contracting, the other is relaxing.




  • Flex: When muscles contract to bend joints
  • Extend: When muscles relax to straighten joints
  • Flexor: A muscle that contracts to flex a muscle e.g. hamstrings (knee)
  • Extensor: A muscle that contracts to extend a muscle e.g. quadriceps (knee)
  • Joint: Where muscles bring about movement
  • Antagonistic: A pair of muscles that work together that pull to extend or flex a joint
  • Synovial joints: Bones of a joint separated by a cavity of synovial fluid
  • Synovial fluid: A fluid that enables joints to move freely, acting like a lubricant
  • Ligaments: Joins bones to bones, they are strong and flexible
  • Tendons: Joins muscle to bone
  • Cartilage: Protects bones at joints, absorbs synovial fluid and acts as a shock absorber
  • Muscle fibre: Bundles of muscle fibres make up muscles and each is a single muscle cell
  • Multinucleate: Many nuclei to control the metabolism of the cell
  • Myofibrils: Are made up of a series of contractible sarcomere units
  • Sarcomeres: Made of 2 proteins called actin and myosin
  • Actin: Thin filament in a sarcomere
  • Myosin: Thicker filament in a sarcomere
  • Troponin: Protein molecule on actin filament, calcium ions bind to it
  • Tropomyosin: Protein molecule on actin, they move and expose myosin binding sites on the actin
  • Sliding filament theory: Myosin and actin slide over each other causing muscle contraction and movement
  • Sarcoplasmic reticulum: Membrane-bound sacs around the myofibrils which release calcium ions for muscle contraction.
  • Sarcoplasm: Cytoplasm of a muscle cell
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7.1 Cellular respiration

Topic 7: Run for your life:

7.1 Cellular respiration


Respiration = The chemical process of releasing energy from organic compounds (respiratory substrates) such as glucose through oxidation. The energy released is used to combine ADP with inorganic phosphate to make ATP (energy). Respiration is a long series of enzyme-controlled reactions.


  • Aerobic respiration: Requires oxygen to fully oxidise the organic molecule. This releases a lot of energy
  • Anaerobic respiration: The breakdown of the molecule without oxygen. This releases much less energy.


ATP carries energy around the cell to where it is needed by diffusion. It is a molecule made from the nucleotide base adenine and 3 phosphate groups.

ATP is synthesised from ADP and Pi from an energy releasing reaction, such as the breakdown of glucose in respiration. The energy is stored as chemical energy in the phosphate bond. The enzyme ATPase catalyses the reaction.

When energy is required by a cell, ATP is broken back down into ADP and Pi, releasing energy from the phosphate bond. ATPase catalyses this reaction. The more ATPs used, the more energy is released.


In aerobic respiration, energy is released by splitting glucose into carbon dioxide (released as a water product) and hydrogen (combines with oxygen to produce water).  For aerobic respiration to occur, the cells must have mitochondria. The energy is used to phosphorylate ADP to ATP, providing energy for biological processes in a cell.


Aerobic respiration involves:

  • Glycolysis – splitting of sugar to form pyruvate – in cytoplasm
  • Link reaction – mitochondrial matrix
  • Krebs cycle – removal of hydrogen from pyruvate – mitochondrial matrix
  • Electron transport chain/oxidative phosphorylation – using hydrogen to produce ATP – inner mitochondrial membrane


Glycolysis is the first stage of respiration and occurs in the cytoplasm. Glycolysis makes pyruvate (3C) from glucose (6C). Glycolysis is the first stage in both anaerobic and aerobic respiration and doesn’t require oxygen to take place.


  1. Glycogen in muscle or liver converted to glucose
  2. A glucose molecule is phosphorylated as 2 ATPs donate phosphate to it. This produces 2ADP and 2 molecules of a 6C molecule
  3. The 6C molecule is reactive and so splits into 2 3C phosphates
  4. Hydrogen is removed (oxidised) and taken up by 2 NADs which become reduced. 2ATPs are made as phosphate groups are added to ADP (substrate-level phosphorylation). This forms 2 molecules of 3C pyruvate


Overall, 2 ATP are used and 4 are made from one glucose molecule – net gain of 2ATP.


Link reaction:

If oxygen is available, the pyruvate moves to the mitochondrial matrix, where the link reaction and Krebs cycle occurs. The link reaction converts pyruvate into acetyl CoA. The link reaction does not produce any ATP.


  • Carbon dioxide is removed from pyruvate (decarboxylation) and diffuses out of the mitochondria and out of the cell
  • Hydrogen is removed from pyruvate (dehydrogenation/oxidation) – accepted by NAD, producing reduced NAD
  • This converts pyruvate into a 2C molecule which immediately combines with coenzyme A to form the 2C compound acetyl coA


The link reaction and following Krebs cycle occurs twice for every glucose molecule.


Krebs cycle

The Krebs cycle (or citric acid cycle) also occurs in the mitochondrial matrix, where the enzymes that catalyse the reactions are located.


Acetyl CoA combines with a 4C compound to produce a 6C compound. This is converted back to the 4C compound to be used again through a series of enzyme-controlled steps. During this process, more carbon dioxide is released (decarboxylation) and diffuses out of the cell. More hydrogen is released through oxidation reactions (dehydrogenation) and picked up by NAD and FAD, producing reduced NAD and reduced FAD. ATP is also produced by substrate-level phosphorylation.


For each cycle, the products are:

  • 2 carbon dioxide
  • 3 reduced NAD
  • 1 reduced FAD
  • 1 ATP


Electron transport chain

Oxidative phosphorylation involves two processes – the electron transport chain and chemiosmosis. The hydrogens picked up by NAD and FAD are split into electrons and protons (hydrogen ions). The electrons are passed along the electron transport chain, on the inner membrane of the mitochondria.


As they move along the chain in a series of redox reactions, they lose energy which is used to transport hydrogen ions from the mitochondrial matrix across the inner membrane and into the intermembrane space. This causes a high concentration of hydrogen ions in this space, forming an electrochemical gradient. The hydrogen ions diffuse back into the matrix through protein channels in stalked particles, working as ATPases. This movement of the H+ ions provides energy to cause ADP and Pi to combine to make ATP. The active transport and diffusion of hydrogen ions is chemiosmosis or the chemiosmotic theory.


At the end of the chain, the electrons combine with oxygen to produce water. Oxygen is required in aerobic respiration as it acts as the final electron acceptor for the hydrogens.


For each reduced NAD, 3 ATP molecules are made

For each reduced FAD, 2 ATP molecules are made



  • ATPs made directly = 4
  • ATPs made from reduced NAD = 10 x 3
  • ATPs made from reduced FAD = 2 x 2


Total ATPs = 38 (under most favourable conditions)


Anaerobic respiration

If oxygen is not available, the link reaction and Krebs cycle stop and oxidative phosphorylation cannot occur as there is no final electron acceptor. Glycolysis can still continue as long as the pyruvate can be removed and the reduced NAD can be converted back to NAD. This does not produce as much energy, the net yield is 2 ATP per glucose molecule.


In animals this is done by converting pyruvate to lactate (lactate fermentation) in the cytoplasm. Reduced NAD from glycolysis transfers H to pyruvate to form lactate and NAD so glycolysis can continue. The lactate built up in muscles diffuses into the blood and is carried in solution as lactic acid in the blood plasma to the liver, where liver cells convert it back to pyruvate. This requires oxygen, this is the oxygen debt.


When oxygen is available again after exercise and oxygen uptake is greater than normal, some of the pyruvate in the liver cells is oxidised through the link reaction, Krebs cycle and electron transport chain. Some pyruvate is reconverted to glucose in the liver cells and this is released into the blood or converted into glycogen to be stored.


In plants and some microorganisms such as yeast, pyruvate is reduced to ethanol and carbon dioxide using the hydrogen from reduced NAD. This recreated oxidised NAD, allowing glycolysis to continue. This is called alcoholic fermentation.


Supplying instant energy

At the start of exercise, immediate ATP is regenerated using creatine phosphate. This is a substance stored in muscles that can be hydrolysed to release energy. This energy regenerates ATP from ADP and Pi, the phosphate is given by the creatine phosphate. Creatine phosphate breaks down as exercise begins.


Creatine phosphate à creatine + Pi


These reactions do not require oxygen and provide energy for 6 – 10 seconds.


Measuring the rate of respiration


The rate of oxygen uptake is measured using a respirometer. Organisms (woodlice) are placed into a tube and the same mass of a non-living material is placed in the other. Soda lime (or potassium hydroxide/KOH solution) in each tube absorbs the carbon dioxide. Cotton wool prevents contact of the soda lime with the organisms. A syringe is used to set the coloured fluid into the manometer at a known level and flows into the capillary tube without air bubbles to give the same quantity. The rubber bungs are fitted to make the tubes airtight. The control tube is exactly the same but without the organisms to make sure the results are due to respiration.

As the organisms respire, they take in oxygen and give out carbon dioxide. The removal of oxygen from the tube reduces the volume and pressure, causing the manometer fluid to move towards the organisms to fill the pressure space. The respired carbon dioxide is absorbed by the soda lime.


The distance moved by the liquid in a given time is measured. The mean volume of liquid = length = pi r^2. This gives the volume of oxygen absorbed per minute.


Temperature must be controlled.


Respiration key words:


  • Glycolysis: The splitting of glucose into a 3C compound, pyruvate. It is anaerobic and produces 2ATP, 2 reduced NAD and 2 pyruvate molecules
  • Pyruvate: A 3C compound produced in glycolysis
  • Coenzyme NAD: Accepts hydrogen atoms, becoming reduced NAD
  • Substrate-level phosphorylation: The synthesis of ATP by combining ADP and Pi through energy from the substrates of a reaction
  • Link reaction: Turns pyruvate into acetyl coA for the Krebs cycle by releasing carbon dioxide and 2 hydrogens.
  • Decarboxylation: Carbon dioxide is released as a waste product
  • Dehydrogenation: Hydrogens removed and taken up by coenzymes
  • Acetyl coA: First step in Krebs cycle, last step in the link reaction
  • Krebs cycle: A cycle that starts with acetyl coA and produces 2 carbon dioxide, 1 ATP, 3 reduced NAD and 1 reduced FAD
  • Electron transport chain: Produces the most of ATP for a cell. Hydrogens and electrons are received from NAD and FAD
  • Chemiosmotic theory: Because the outside of the mitochondria is more positive, it attracts positive H+ ions into it and produces ATP
  • Oxidative phosphorylation: Electrons and hydrogens combine and join with oxygen on stalked particles on the cristae of mitochondria, producing ATP
  • Lactate: In anaerobic respiration, NAD is reduced during glycolysis. The pyruvate is reduced to lactate and the oxidised form of NAD is reduced


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