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AQA A2 Biology Unit 5 Chapter 11: Muscle Contraction
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- 1Name three types of muscles and state where they are found:Cardiac muscle - found exclusively in the heart
Smooth muscle - found in the walls of the blood vessels and the gut
Skeletal muscles - attached to bond - 2Describe the macroscopic structure of skeletal musclesMuscles are made of many tiny muscle fibres called myofibrils;
Many myofibrils are grouped into a single muscle fibre;
Several muscle fibres are grouped into a bundle which group to form a muscle - 3What are myofibrils?Tiny muscle fibres;
Which share a nuclei and a cytoplasm, known as the sarcoplasm - 4Why are muscle fibres not made up of individual cells?Individual cells would be inefficient;
Gaps between cells would be points of weakness - 5What two proteins/filanments are myofibrils made of?Actin and myosin
- 6Describe actin:Thin proteins/filaments;
Consit of two strands twisted around one another - 7Describe myosin:Thicker;
Consist of long rod-shaped fibres with bulbous heads which project to the side - 8What is the isotropic (I) band?Also known as the I or light band;
Where the actin and myosin filaments do not overlap - 9What are the anisotropic (A) bands?Also known as the A or dark bands;
Where the actin and myosin filaments overlap - 10Describe the following structures: a) H-zone b) Z-line c) Sarcomerea) Light region at the centre of the A (dark) band
b) Centre of the I (light) band
c) distance between adjacent z-lines - 11What are the two types of skeletal muscles?Fast twitch and slow twitch
- 12What is the function of slow twitch muscles?Contract slowly and less powerfully;
For endurance - 13How are the slow-twitch muscles adapted for their function?Adapted for aerobic respiration:
Large store of myoglobin (red molecule which stores oxygen);
Large supply of glycogen (source of metabolic energy);
Rich supply of blood vessels (to deliver oxygen and glucose);
Many mitochondria - 14What is the function of fast-twitch muscles?Contract rapidly and powerfully;
Used for intense exercise - 15How are fast-twitch muscles adapted for their function?Thicker and more numerous myosin filaments (allow for powerful contractions);
High concentration of enzymes involved in anaerobic respiration;
Stores of phosphocreatine (rapidly generates ATP from ADP in anaerobic conditions) - 16What is a neuromuscular junction?Point where a motor neurone meets a skeletal muscle fibre;
Many junctions along a muscle fibre - 17What is a motor unit, and what do they allow?All muscles fibres supplied by a single motor neurone are known as a motor unit;
They allow control over the force of a contraction;
Powerful contraction stimulate many motor units - 18How does acetylcholine transmit an impulse across a neuromuscular junction?A nerve impulse causes the synaptic vesicles to fuse with the presynaptic membrane releasing acetylcholine;
Acetylcholine diffuses across the synaptic cleft to the postsynaptic neurone;
This alters the membrane's permeability to sodium (Na+) ions;
Na+ diffuses into muscle depolarising the membrane - 19How is acetylcholine 'recycled' and why?Acetylcholine is broken down by acetylcholinesterase;
The resulting ethanoic acid and choline diffuses back across the synaptic cleft into the presynaptic neurone where it is recombined into acetylcholine;
This ensures the muscle isn't overstimulated - 20What is the evidence for sliding filament theory?Theory = actin and myosin filaments slide over each other during contraction;
If correct in a contracted muscles there will be more overlapping filaments;
I band will become narrower;
Z lines are more close together (sarcomere shortens);
H zone becomes narrower - 21Explain how the A band helps support sliding filament theory:A (dark) band remains the same width as its length is determined by the length of the myosin filaments;
Thus discounting the theory that contraction is due to filaments shortening - 22What two proteins is the myosin filament made of?Fibrous protein arranged into filaments;
Globular protein formed into two bulbous structures (the heads) - 23Describe the structure of actinGlobular protein arranged into long chains that are twisted into a helical strand
- 24Describe the structure of tropomyosinForms long thin threads that are woven around the actin filaments
- 25Summarise sliding filament theory:Impulse stimulated tropomyosin to move, exposing the myosin binding sites;
Bulbous heads of the myosin filament form cross bridges with a binding site on the actin filament;
Filaments flex in unison pulling the actin filament along the myosin filament;
Cross-bridge breaks (using ATP) and heads return to their original position;
Heads can now re-attach further along the actin filament - 26How is a muscle stimulated?An action potential reaches many neuromuscular junctions simultaneously;
Ca2+ (calcium) ion channels open so Ca2+ diffuses into the synaptic knob;
Ca2+ ions cause synaptic vesicles to fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft;
Diffuses to, and alter the permeability of the postsynaptic neurone to Na+,
Na+ diffuse into and depolarise the membrane - 27How does an action potential cause calcium ions (Ca2+) to leave the sarcoplasmic reticulum?Action potential travel into fibres through a system of t-tubules that branch throughout the sarcoplasm;
T-tubules in contact with the sarcoplasmic reticulum which has absorbed Ca2+;
Action potential opens Ca2+ ion channels on the sarcoplasmic reticulum so Ca2+ flood into the sarcoplasm down a diffusion gradient - 28How do calcium ions (Ca2+) in the sarcoplasm allow cross-bridges to be formed?Ca2+ cause the tropomyosin molecules, that were blocking the binding site on the actin filament, to pull away;
ADP molecules attached to the myosin heads means they can bind to the actin filaments, forming a cross-bridge - 29How does the cross-bridge cause the actin filaments to move?Once attached to the actin filaments the myosin heads change their angle;
Pulling the actin filament along;
Releasing a molecule of ADP - 30How does the cross-bridge break, allowing the myosin heads to return to their original position?ATP molecule attaches to the myosin head, causing it to become detached from the actin filament;
Calcium ions activate ATPase which hydrolyses ATP to ADP providing energy for the myosin heads to return to their original position - 31How do muscles relax?When nervous stimulation ceases Ca2+ ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP;
This reabsorption of Ca2+ ions allows tropomyosin to block the myosin binding sites on the actin filaments again;
Myosin is now unable to bind to the actin filaments so contraction ceases
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- AQA A2 Biology Unit 5 Chapter 11: Muscle Contraction
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- Charlotte Lloyd
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