Unit 5: Section 1- Muscles and Respiration

A summary of the muscles and respiration topic in edexcel's Run for your Life

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Muscles and Movement

  • Skeletal muscle is used for movement. The muscles are attached to bones by tendons, and ligaments attach bones to bones. 
  • Muscles work together in antagonistic pairs (extensors and flexors) to move a bone. A flexor bends a joint when it contracts, and an extensor straightens a joint when in contracts
  • Muscles work in pairs because they can only pull when they contract- they can't push
  • Muscle is made up of bundles of muscle fibres. The cell membrane of muscle fibres is the sarcolemma. Bits of it fold inwards across the fibre and stick into the sarcoplasm.
  • These folds are transverse (T) tubules and help spread electrical impulses through the sarcoplasm
  • A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm. It stores and releases the calcium ions needed for muscle contraction
  • Have lots of mitochondria to release ATP. They are multinucleate (many nuclei).
  • Muscle fibres have myofibrils- long, cylindrical organelles which are specialised for contraction
  • Thick myofilaments- myosin, thin myofilaments- actin.
  • Under an electron microscope, myofibrils have dark bands (myosin) and light bands (actin)
  • Myosin and actin slide over each other to make the sarcomeres contract (they themselves don't contract. This is sliding filament theory.
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Muscle Contraction

  • Myosin filaments have hinged globular head, which have binding sites for actin and ATP.
  • Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
  • Troponin and tropomyosin (proteins) are found between actin filaments, and help myofilaments move past each other. In resting muscle, tropomyosin blocks the actin-myosin binding sites- held in place my troponin
  • 1. An action potential depolarises the sarcolemma and causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm. The Ca2+ binds to troponin, causing it to change shape and pull the tropomyosin to expose the actin-myosin binding site. This allows the myosin head to bind, forming an actin-myosin cross bridge.
  • 2. Ca2+ ions activate ATPase, which breaks down ATP to release energy. The energy is used to move the myosin head to pull the actin filament along
  • 3. ATP also provides energy to break the actin-myosin cross bridge. The myosin head re-attaches to a different binding site, repeating the cycle. Many cross bridges form and break rapidly, pulling the actin along and shortening the sarcomere- this causes muscle contraction
  • When the muscle stops being stimulated, the Ca2+ leave binding sites on troponin and move back to the sarcoplasmic reticulum (via active transport). The troponin returns to its original shape, so tropomyosin blocks the actin-myosin binding sites again. Actin slides to relaxed position- lengthens the sarcomere
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Aerobic Respiration 1

  • 1. Glycolysis- in the cytoplasm of the cell. The first stage of aerobic and anaerobic respiration. 
  • Stage 1- Phosphorylation- Glucose is phosphorylated by adding 2 Pi from 2 ATP -> 2 ADP and 2 triose phosphate
  • Stage 2- Oxidation- Triose phosphate oxidised to pyruvate, releasing H+ which is accepted by NAD, forming 2 reduced NAD. 4 ATP formed, but net gain of 2ATP
  • 2. Link reaction- in mitochondrial matrix. Occurs twice for every glucose molecule. 
  • Pyruvate is decarboxylated, releasing CO2. NAD is reduced, changing pyruvate into acetate. Acetate is combined with coenzyme A (CoA) to form acetly coenzyme A (acetyl CoA). No ATP produced
  • 3. Krebs Cycle- in matric of mitochondria. Each reaction controlled by a specific enzyme
  • Acetyl CoA combines with oxaloacetate to form 6C citrate. 
  • Citrate converted to a 5C molecule. Decarboxylation (CO2 removed) and dehydrogenation (H+ removed) occur, and the H+ is used to produce reduced NAD from NAD
  • 5C molecule converted to 4C molecule. Decarboxylation and dehydrogenation occur, producing 1 red FAD and 2 red NAD.
  • ATP is produced through substrate-level phosphorylation (when P transferred from an intermediate to ADP)
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Aerobic Respiration 2

  • 4. Oxidative phosphorylation- energy carried by e- (from reduced coenzymes) used to make ATP. 2 processes- the electron transport chain and chemiosmosis.
  • H atoms released from red NAD and FAD (as oxidised to NAD and FAD). H split into H+ and e-
  • Electrons move along electron transport chain (ETC) losing energy at each carrier. Energy used to pump protons from mitochondrial matrix into the intermembrane space
  • An electrochemical gradient is set up as there are more H+ in the intermembrane space
  • Protons move down the gradient into the mitochondrial matrix via ATP synthase. This drives the synthesis of ATP from ADP and Pi
  • At the end, H+, e- and oxygen combine to form water. Oxygen is the final electron acceptor
  • 32 ATP can be made from one glucose
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Respirometers and Anaerobic Respiration

  • Woodlice are placed into a tube (containing soda lime to absorb CO2) and connected to a manometer (with fluid set to a known level)
  • The apparatus is left for a period of time, in which there will be a decrease in the volume of air in the tube, due to O2 consumption by the woodlice
  • The decrease in the volume of air will reduce pressure in the tube and caused the coloured liquid in the manometer to move towards the test tube
  • The distance moved by the liquid in a given time is measured. The value can be used to calculate the volume of O2 consumed by the woodlice per minute
  • Temperature and volume of soda lime in the tube are controlled variables, and the test is repeated and mean volume of O2 calculated to produce more reliable results
  • Anaerobic respiration doesn't use oxygen and doesn't involve the link reaction, Krebs cycle or oxidative phosphorylation. Releases energy through lactate fermentation
  • Lactate fermentation- glucose converted to pyruvate via glycolysis. Reduced NAD transfers H to pyruvate, forming lactate and NAD. NAD can be reused in glycolysis. Glycolysis can therefore continue to produce small amounts of ATP
  • Lactic acid builds up, and needs to be broken down. Cells can convert lactic acid back to pyruvate (can enter Krebs cycle), or liver cells convert it to glucose (respired or stored)
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