Cross Bridge Cycle

Question 1

Myosin cross bridges are the __ ___ sites.

Answer 1

force generating

Question 2

Contraction does not necessarily cause ___ of fiber.

Answer 2

Shortening

Question 3

Contraction is defined as

Answer 3

generation of force

Question 4

In the relaxed state, thin and thick filaments overlap only at

Answer 4

ends of A band

Question 5

Sliding filament model of contraction 1: During contraction, thin filaments slide past ___ filaments so that actin and myosin filaments overlap to a greater degree.

Answer 5

thick

Question 6

Sliding filament model of contraction: When does sliding begin?

Answer 6

When the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments.

Question 7

Sliding filament model of contraction: For sliding to begin, the ___ heads on the thick filaments latch onto __ __ ___ on actin in the thin filaments.

Answer 7

myosin,

myosin-binding sites

Question 8

Sliding filament model of contraction 2: These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the ___ of the sarcomere.

Answer 8

center

Question 9

Sliding filament model of contraction: What happens when cross bridge attachments form and break several times during a contraction?

Answer 9

It generate tension and propels the thin filaments toward the center of the sarcomere.

Question 10

Sliding filament model of contraction 3: What is the end result of the generated tension that propels the thin filaments toward the center of the sarcomere?

Answer 10

Causes the muscles to shorten

Question 11

Sliding filament model of contraction: Notice that as the thin filaments slide centrally, the Z discs to which they attach are pulled __ the M line.

Answer 11

toward

Question 12

Overall, as a muscle cell shortens, what are the 4 things that occur?

Answer 12

1- I bands shorten
2-Distance between Z discs shorten
3-H zones disappear
4-A bands move closer together; length does not change

Question 13

Cross bridges occur when

Answer 13

myosin heads bind to actin

Question 14

Overall, as a muscle cell shortens, what are the 4 things that occur?

Answer 14

1- I bands shorten
2-Distance between Z discs shorten
3-H zones disappear
4-A bands move closer together; length does not change

Question 15

During muscle shortening, the ___ is pulled toward the M line.

Answer 15

Z discs

Question 16

Contraction ends when the ___ ___ become inactive, ___ declines, and the muscle fiber ___.

Answer 16

cross bridges
tension
relaxes

Question 17

A contraction is triggered by a series of molecular events known as the

Answer 17

Cross bridge cycle

Question 18

The formation of a cross bridge is initiated when ___ ___ released from the sarcoplasmic reticulum bind to troponin.

Answer 18

Calcium ions

Question 19

The formation of a cross bridge is initiated when calcium ions released from the sarcoplasmic reticulum bind to ___. This binding causes ___ to change shape.

Answer 19

troponin, troponin

Question 20

____ moves away from the myosin binding sites on actin allowing the myosin to bind actin and form a cross bridge.

Answer 20

Tropomyosin

Question 21

The myosin head must be activated before cross bridge cycle can begin. This occurs when ___ binds to the head and is hydrolyzed to ADP and inorganic phosphate.

Answer 21

ATP

Question 22

The myosin head must be activated before cross bridge cycle can begin. This occurs when ATP binds to the head and is hydrolyzed to _________.

Answer 22

ADP and inorganic phosphate.

Question 23

The energy liberated from the __ _ __ activates the myosin head, forcing it into the cocked position.

Answer 23

hydrolysis of ATP

Question 24

Cross bridge cycle maybe divided into four steps.

Step 1: __ __

Answer 24

Crossbridge formation

Question 25

Step 1: Cross bridge formation
The activated myosin head binds to actin, forming a cross bridge. ___ ___ is released and the bond between myosin and actin become stronger.

Answer 25

Inorganic phosphate

Question 26

What is released to cause the bond between myosin and actin become stronger?

Answer 26

Inorganic phosphate

Question 27

What is Step 2?

Answer 27

Step 2: The power stroke

Question 28

Step 2: The power stroke

__ is released and the activated myosin head pivots, sliding the thin myofilament toward the center of the sarcomere.

Answer 28

ADP

Question 29

What is Step 3?

Answer 29

Step 3: Cross bridge detachment

Question 30

Step 3: Cross bridge detachment

When another ___ binds to the Myosin head, the link between the myosin head and actin __ and the myosin head detaches.

Answer 30

ATP, weakens

Question 31

What is Step 4?

Answer 31

Step 4: Reactivation of myosin head

Question 32

Step 4: Reactivation of myosin head

ATP is hydrolyzed to __ __ ___ __. The energy released during hydrolysis reactivates the myosin head, returning it to the cocked position.

Answer 32

ADP and inorganic phosphate

Question 33

As long as the binding sites on __ remain exposed, the cross bridge cycle will repeat.

Answer 33

actin

Question 34

And as the cycle repeats, the thin myofilaments are pulled ___ each other and the sarcomere ___. This shortening causes the whole muscle to contract.

Answer 34

Toward, shortens

Question 35

Cross bridge cycling ends when calcium ions are actively transported back into the ___ ___.

Answer 35

sarcoplasmic reticulum

Question 36

___ returns to its original shape allowing tropomyosin to glide over and cover the myosin binding site on actin.

Answer 36

Troponin

Question 37

Polypeptide strand which is a rod-shaped protein and spirals about the actin core to help stiffen and stabilize it.

Answer 37

Tropomyosin

Question 38

Successive tropomyosin molecules are arranges end to end along the actin filaments, and in a relaxed muscle fiber, block __ __ __ __ __ so that myosin heads on the thick filaments cannot bind to thin filaments.

Answer 38

myosin-binding sites on actin

Question 39

Troponin is a globular 3-polypeptide complex. One of its polypeptides is an inhibitory subunit that binds to

Answer 39

actin

Question 40

Another type of troponin binds to tropomyosin and helps position it on

Answer 40

actin

Question 41

The third type of troponin binds

Answer 41

calcium ions