AQA A2 Biology Unit 5 Chapter 11: Muscle Contraction

Charlotte Lloyd
Flashcards by Charlotte Lloyd, updated more than 1 year ago
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AQA A2 Biology Unit 5 Chapter 11: Muscle Contraction

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Question Answer
Name 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
Describe the macroscopic structure of skeletal muscles Muscles 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
What are myofibrils? Tiny muscle fibres; Which share a nuclei and a cytoplasm, known as the sarcoplasm
Why are muscle fibres not made up of individual cells? Individual cells would be inefficient; Gaps between cells would be points of weakness
What two proteins/filanments are myofibrils made of? Actin and myosin
Describe actin: Thin proteins/filaments; Consit of two strands twisted around one another
Describe myosin: Thicker; Consist of long rod-shaped fibres with bulbous heads which project to the side
What is the isotropic (I) band? Also known as the I or light band; Where the actin and myosin filaments do not overlap
What are the anisotropic (A) bands? Also known as the A or dark bands; Where the actin and myosin filaments overlap
Describe the following structures: a) H-zone b) Z-line c) Sarcomere a) Light region at the centre of the A (dark) band b) Centre of the I (light) band c) distance between adjacent z-lines
What are the two types of skeletal muscles? Fast twitch and slow twitch
What is the function of slow twitch muscles? Contract slowly and less powerfully; For endurance
How 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
What is the function of fast-twitch muscles? Contract rapidly and powerfully; Used for intense exercise
How 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)
What is a neuromuscular junction? Point where a motor neurone meets a skeletal muscle fibre; Many junctions along a muscle fibre
What 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
How 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
How 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
What 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
Explain 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
What two proteins is the myosin filament made of? Fibrous protein arranged into filaments; Globular protein formed into two bulbous structures (the heads)
Describe the structure of actin Globular protein arranged into long chains that are twisted into a helical strand
Describe the structure of tropomyosin Forms long thin threads that are woven around the actin filaments
Summarise 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
How 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
How 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
How 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
How 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
How 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
How 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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