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Within the muscle fibre tropomyosin is wrapped around actin filaments and it blocks the binding sites for the myosin-heads.
When calcium ion concentration increases, calcium ions bind to sites on the tropomyosin (troponin-c sites) and this causes a conformational change in shape.
This change in tropomyosin’s shape exposes the myosin-head binding sites on the actin filaments and allows cross-bridge binding to occur.
This then allows the actin and myosin fibres to slide over each other and shorten (contract) the muscle fibre. This is known as Sliding Filament Theory.see more
The sequence of bases (a gene) on DNA determines the order of amino acids that come together to form a polypeptide (protein) chain.
The DNA is 'unzipped' by enzymes called Helicases. This allows a single strand of DNA to be copied (transcription) to form a molecule of messenger RNA (mRNA).
The mRNA then leaves the nucleus through the nuclear pores and enters the cytoplasm.
In the cytoplasm the mRNA associates with a ribosome.
Free amino acids within the cytoplasm are brought together in the correct order (determined by the sequence of the mRNA) to form a polypeptide chain (protein).see more
At rest there is a difference in the concentration of sodium ions (Na+) across the membrane of the neurone.
Outside the neurone in the extracellular fluid (ECF) the concentration of Na+ is around 140 mEq/L whereas inside in the intracellular fluid (ICF) it is about 14 mEq/L.
This difference in concentration of Na+ creates a strong concentration gradient across the membrane.
When there is a change in voltage, the voltage-gated Na+ channels open. This allows the Na+ to flow through the membrane from outside to inside, down the concentration gradient.
Normally the inside of the neurone has a negative charge but, when large amounts of Na+ flood into the neurone when the channels open this makes the inside of the neurone have a positive charge.
This change in charge from negative to positive is knows as depolarisation and this generates an action potential that travels down the neurone (axon).see more