Cross Bridge Cycling

Question 1

What is the concentration of sodium and potassium ions in the cytoplasm?

Answer 1

[Na+] = 12mM; [K+] = 155mM

Question 2

What is the concentration of sodium and potassium ions in the extracellular fluid?

Answer 2

[Na+] = 145mM; [K+] = 4mM

Question 3

What is the most metabolically active form of ATP?

Answer 3

Magnesium salt

Question 4

What is the formula weight ATP(Mg)?

Answer 4

529

Question 5

How much work per mole of ATP can skeletal muscle produce?

Answer 5

24kJ

Question 6

What is released during the hydrolysis of i) ATP; ii) ADP; iii) AMP?

Answer 6

i) >24kJ/mol
ii) Pi + H+ -31kJ/mol
iii) -14kJ/mol

Question 7

What triggers cross-bridge formation?

Answer 7

When Ca2+ released from the SR binds to troponin

Question 8

What are the 4 steps of the cross-bridge cycle?

Answer 8

Cross-bridge formation; power stroke; cross-bridge detachment; reactivation of myosin head

Question 9

What happens during cross-bridge formation?

Answer 9

The activated myosin head binds to the actin’s exposed active site
Pi is released and the bond between myosin and actin becomes stronger

Question 10

What happens during the power stroke?

Answer 10

ATP is released

The myosin head pivots, sliding the myofilament to the centre of the sarcomere

Question 11

What happens during cross-bridge detachment?

Answer 11

Another ATP binds to myosin head
The link between actin and myosin weakens
Myosin head detaches

Question 12

What happens during the reactivation of the myosin head?

Answer 12

ATP is hydrolysed to ADP + Pi
The energy released during hydrolysis reactivates the myosin head, returning it to its cocked position
Cycle repeats (as long as active sites on G actins remain exposed) and the sarcomere shortens

Question 13

When does cross-bridge cycling end?

Answer 13

When Ca2+ are actively transported back to the SR

Question 14

When does peak force (P0) occur?

Answer 14

At 0 velocity

Question 15

When does peak velocity (Vmax) occur?

Answer 15

At 0 force

Question 16

What is isometric contraction?

Answer 16

Contraction without appreciable shortening in distance between origin and instertion

Question 17

What is concentric contraction?

Answer 17

Contracting resulting in shortening of muscle. Metabolically more demanding than eccentric contraction

Question 18

What is eccentric contraction?

Answer 18

Contraction resulting in elongation of a muscle. Performs negative work or to decelerate a body part

Question 19

What is force determined by?

Answer 19

The CSA and, therefore, number of crossbridges that generate tension

Question 20

What is velocity determined by?

Answer 20

The activity of myosin ATPase