F215:04:09 The sliding filament model

Question 1

Where are sarcomeres?

Answer 1

they are the span between one Z-line to the next

Question 2

How long is the sarcomere in a relaxed state?

Answer 2

2.5um

Question 3

Why are Z lines close together when a muscle is contracted?

Answer 3

because the lengths of the I-band and H-zone are reduced

But the A band does not change in length during contraction

Question 4

Describe how the different zones change in the sarcomere when muscles contract

Answer 4

the lengths of the I-band and H-zone are reduced

But the A band does not change in length during contraction

Question 5

What are the 2 types of filament involved in contraction?

Answer 5

actin and myosin

Question 6

Describe actin

Answer 6

Two strands coiled around each other like a twisted double string of beads
Each strand is composed of of G-actin subunits
Then Tropoyosin molecules coil round the F actin to reinforce it

Question 7

What is the purpose of Tropomyosin and where is it found?

Answer 7

Found coiled round actin

Acts as a reinforcement

Question 8

What is tropomyosin?

Answer 8

a rod shaped protein found coiled round actin

It has a troponin complex attached to each tropomyosin molecule

Question 9

What is troponin made up of?

Answer 9

3 polypeptides, one binds to actin, one to tropomyosin and the other to calcium ions

Question 10

Why is it important that troponin binds to tropomyosin and actin?

Answer 10

Ti keep the tropomyosin in place around the actin

Question 11

What are thick filaments bundles of?

Answer 11

the protein myosin, whose heads stick out from opposite ends of the filament

Question 12

What does each myosin molecule consist of?

Answer 12

a tail and 2 protruding heads

Question 13

what is the power stroke?

Answer 13

When the head groups bend causing the thin filament to be pulled along and so overlap with more thick filament.
Its part of muscle contractions and ADP and Pi are released

Question 14

What are the 4 stages of muscle contraction?

Answer 14

1. Myosin head groups attach to the surrounding actin filaments forming a cross-bridge
2. The head group then bends, causing the thin filament to be pulled along and so overlap more with the thick filament (power stroke) ADp and Pi are released
3. The cross bridge is then broken as new ATP attaches to the myosin head
4. The head group moves backwards as the ATP is hydrolysed to ADP and Pi. It can then for a cross bridge with the thin filament further along and start again

Question 15

What is a cross bridge?

Answer 15

When the myosin head groups attach to the surrounding actin filaments forming the cross bridge

Question 16

How is the whole muscle shortened during the contraction of a muscle?

Answer 16

several million cross bridges are being continuously made and broken causing the thin filaments to slide past the thick filaments to shorten the sarcomere, which then shortens the whole muscle

Question 17

How is calcium involved in muscle contraction

Answer 17

As binding sites for the myosin head group on the actin fibre are covered by tropomyosin subunits, they cannot attach to myosin head groups.
When an action potential arrives, calcium ions are released from the sarcoplasmic reticulum and bind to the troponin molecules, which changes it shape and uncovers the binding sites for actin-myosin

Question 18

when happens when the nervous stimulation stops in terms of calcium?

Answer 18

The calcium ions are actively transported back into the sarcoplasmic reticulum by carrier proteins on the membrane which leads to muscle relaxation

Question 19

Why is ATP needed in the power stroke/.

Answer 19

As when myosin head groups have attached to the actin binding site and bends, the molecules are in their most stable form, so ATP is needed to break this and reset the myosin head forwards

Question 20

Is a muscle fibre a cell or an organelle?

Answer 20

A cell

Question 21

Are myofibrils cells or organelles?

Answer 21

organelles

Question 22

How much ATP is available in a muscle fibre at any one time?

Answer 22

Enough to support 1-2 seconds of contraction

Question 23

How is continued contraction maintained?

Answer 23

ATP must be generated as quickly as it is used up, this is carried out by 3 mechanisms:

• aerobic respiration
• Anaerobic respiration
• Transfer from creatine phosphase in the muscle cell sarcoplasm

Question 24

How does aerobic respiration produce ATP to allow contraction to be maintained?

Answer 24

Aerobic respiration takes place in muscle cell mitochondria
The level at which this process can regenerate ATP is dependent on the supply of oxygen to the muscles and the availability of respiratory substrates

Question 25

How does anaerobic respiration produce ATP to allow contraction to be maintained?

Answer 25

It takes place in the muscle cell of the sarcoplasm
Its quite a quick process but leads to a build up on lactic acid which is toxic
The lactic acid enters the blood and stimulated an increase in blood supply to the blood

Question 26

What are the pros and cons of using anaerobic respiration to maintain muscle contraction

Answer 26

Pros: 
quick
The lactic acid enters the blood and stimulated an increase in blood supply to the blood 
Cons: 
Lactic acid is toxic

Question 27

How does the transfer from creatine phosphase in the muscle cell sarcoplasm produce ATP to allow contraction to be maintained?

Answer 27

As the phosphate group from creatine phosphate can be transferred to ADP to form ATP very quickly by the action of the enzyme creatine phosphotransferase
The supply of creatine phosphate is sufficient to support muscular contraction for a further 2-4 seconds

Question 28

How long will the supply of creatine phosphate is sufficient to support muscular contraction for?

Answer 28

a further 2-4 seconds

Question 29

What enzyme ensures the phosphate group from creatine phosphate is transferred to ADP to form ATP very quickly?

Answer 29

creatine phosphotransferase

Question 30

List 3 uses of ATP in muscle cells

Answer 30

Contraction of sarcomeres in the power stroke

Protein synthesis

Question 31

Suggest why anaerobic exercise such as weight lifting does not help weight loss

Answer 31

As it leads to the production of lactate during explosive exercise
On completion of exercise, most of the lactate is converted back into pyruvate.
Anaerobic respiration is carried out during anaerobic exercise which can only use glucose as a respiratory substrate, to respire fat/fatty acids, aerobic respiration and exercise must be carried out

Question 32

Suggest which process for ATP regeneration is most important for a marathon runner

Answer 32

Aerobic respiration

Question 33

Suggest which process for ATP regeneration is most important for a 100m sprinter

Answer 33

creatine phosphate regeneration of ATP