Edexcel SNAB A2 Topic 7: Run For Your Life. Revision summary


Muscle contraction:

·         Impulse arrives at neuromuscular junction. Causes Ca ions present in sarcoplasmic reticulum to be released.

·         Ca ions diffuse through the sarcoplasm. They bond to troponin, which causes the tropomyosin to move and expose the myosin binding sites on the actin.

·         Myosin heads bond to the binding site and cause the actin to slide over them, causing contraction.

·         ATP bonds to the myosin head, causing it to detach from the binding site.

·         ADP and Pi are released from the myosin head from the enzyme ATPase, causing the head to change shape and go back to its original position.

·         This process will carry on repeating as long as impulses continue to arrive.


Slow twitch muscle fibres:

·         Are adapted for aerobic respiration - slow contractions over a long time period.

·         Good blood supply from capillaries

·         Large stores of myoglobin

·         Relatively thin so oxygen can diffuse though them easily

·         Contain many mitochondria


Fast twitch muscle fibres:

·         Adapted for anaerobic respiration - quick contractions over a short time period

·         Not many capillary connections

·         Small stores of myoglobin

·         Few mitochondria

·         Relatively thick


Tendons - attach muscle to bone

Ligaments - attach bone to bone




This occurs in the cell cytoplasm. Glucose is obtained from its relatively intert storage compound, glycogen.

·         It is made more reactive by adding Pi from 2ATP to it to form phosphorylated glucose. This then spits from a 6C compound to two 3C compounds.

·         Hydrogen is also released and taken up by the coenzyme NAD to form reduced NAD (NADH). The phosphate groups are also removed and used to form 2ATP, which leaves two pyruvate molecules.

·         Two ATPs are used initially and four are made, leaving a net gain of 2ATP per molecule of glucose.


Link reaction


During this process each pyruvate molecule is decarboxylated and dehydrogenated (producing carbon dioxide and reduced NAD). This takes place after the pyruvate has moved from the cytoplasm to the mitochondria. The product then combines with coenzyme A to form acetyl coenzyme A (a 2C molecule).


Krebs cycle


During this process, acetyl CoA combines with a 4C compound, then is dehydrogenated to form NADH, as well as decarboxylated. The remaining 5C compound is also decarboxylated, and then hydrogenated to form three molecules of NADH and one FADH. This leaves the remaining 4C compound to combine with another molecule of acetyl CoA.


Electron transport chain


All the FADHs and NADHs have been collected for this moment. They reach the electron carriers on the inner membrane of the mitochondrion, where the H atoms split into electrons and H+. Electrons pass down the carriers, oxidising them and reducing them as they go along. As they do this, H+ can pass through the carrier into the intermembrane space, increasing




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