exam qs

• Damage to myelin sheaths of neurons can lead to problems controlling the contraction of muscles. Suggest one reason why. (2)

  • Action potentials travel more slowly
  • So delay in muscle contraction / muscles don’t contract / muscles contract slower
• Cannabinoid receptors are found on the pre-synaptic membrane of neuromuscular junctions. When a cannabinols binds to its receptors, it closes calcium ion channels. (4)

  • Prevents influx of calcium ions into pre-synaptic membrane
  • Synaptic vesicles don’t fuse with membrane / vesicles don’t release neurotransmitter
  • Neurotransmitter does not diffuse across synapse / does not bind to receptors on post-synaptic membrane
  • No action potential / depolarisation (of post-synaptic membrane) / sodium ion channels do not open / prevents influx of sodium ions.
• Describe the role of ATP in muscle contraction. (2)

  • to break actinomyosin bridges
  • to move / bend the myosin head
  • so actin filaments are moved inwards
  • for active transport of calcium ions into the sarcoplasmic / endoplasmic reticulum
• What is the role of ATP in myofibril contraction (2)

  • reaction with ATP allows the binding of myosin to actin / breaks actinomyosin bridge
  • provides energy to move myosin head
• Describe the role of calcium ions and ATP in muscle contraction (5)

  • Ca2+ uncovers binding site on actin
  • Ca2+ binding to troponin / moving tropomyosin
  • allows myosin heads to attach to actin filaments
  • allows sliding of actin and myosin filament
  • binding of ATP causes myosin head to detach from actin filament
  • hydrolysis of ATP releases energy
  • which changes the configuration of myosin head
• Use your knowledge of how myosin and actin interact to suggest how the myosin molecule moves the mitochondrion towards the presynaptic membrane. (2)

  • myosin head attaches to actin and performs power stroke
  • pulls mitochondria past/along the actin
  • next myosin head attaches to actin and performs power stroke
• Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse. (2)

  • mitochondria supply ATP / energy
  • to move vesicles / for active transport of ions / for myosin to move past actin
  • re-synthesis and absorption of neurotransmitter
• During intense exercise, the sarcoplasmic reticulum of muscle fibres becomes less efficient at reabsorbing calcium ions. Explain how this could affect muscle contraction and relaxation. (4 marks)

  • Calcium ions remain in the cytoplasm for longer, binding to troponin continuously.
  • Tropomyosin remains displaced, keeping actin-binding sites exposed.
  • This prolongs muscle contraction and delays relaxation.
  • Muscle fatigue occurs as the fibres cannot reset properly.
• Explain the importance of acetylcholinesterase in the neuromuscular junction. (2 marks)

  • Breaks down acetylcholine in the synaptic cleft.
  • Prevents continuous stimulation of the muscle fibre, ensuring muscle relaxation.
• Describe the sequence of events that occurs at the neuromuscular junction leading to muscle contraction. (4 marks)

  • Action potential arrives at the presynaptic membrane.
  • Voltage-gated calcium ion channels open, and calcium ions enter the presynaptic neuron.
  • Synaptic vesicles release acetylcholine into the synaptic cleft via exocytosis.
  • Acetylcholine binds to receptors on the sarcolemma, opening sodium ion channels, leading to depolarization of the muscle fibre.
• What is the role of calcium ions in muscle contraction?

  • They bind to troponin, causing tropomyosin to move.
• Which structure transmits an action potential deep into the muscle fibre?

  • T-tubules
• ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use.

  • releases a relatively small amount of energy / little energy lost as heat
  • releases energy instantaneously
  • phosphorylates other compounds, making them more reactive
  • can be rapidly re-synthesised
  • it is not lost from / does not leave cells
• Describe the roles of calcium ions and ATP in the contraction of a myofibril. (5)

  • Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum
  • (Calcium ions) cause movement of tropomyosin (on actin
  • (This movement causes) exposure of the binding sites on the actin
  • Myosin heads attach to binding sites on actin
  • Hydrolysis of ATP (on myosin heads) causes myosin heads to bend
  • (Bending) pulling actin molecules
  • Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites).
• Both slow and fast muscle fibres contain ATPase. Explain why. (2)

  • hydrolysis of ATP
  • muscle contraction requires energy / ATP
  • use of ATP by myosin
• What is the role of phosphocreatine (PC) in providing energy during muscle contraction? (2)

  • phosphocreatine provides phosphate / phosphorylates
  • to make ATP
• Describe the role played by tropomyosin in myofibril contraction (2)

  • moves out of the way when calcium ions bind
  • allowing myosin to bind to actin / cross-bridge formation
• Describe the role of myosin in myofibril contraction (2)

  • head of myosin binds to actin and pulls actin past
  • myosin detaches from actin and re-sets / moves further along actin
• Suggest and explain one advantage of the high glycogen content of fast muscle fibres (2)

  • glycogen broken down gives lots of glucose for glycolysis / anaerobic respiration
  • glycolysis / anaerobic respiration not very efficient / only yields 2 ATP per glucose
• Suggest and explain one advantage of the number of capillaries supplying slow muscle fibres (2)

  • many capillaries give lots of oxygen / shorter diffusion pathway for oxygen / large surface area
  • allows high rate of / more aerobic respiration OR prevents build-up of lactic acid / (muscle) fatigue
• As humans age, there is a decrease in body protein.

  Give the name of one body protein that could have resulted in:

  [2 marks]

  reduced muscle power : Actin / myosin / tropomyosin

  reduced immunity : Antibodies