13.10 Sliding Filament Model Flashcards
What happens to part of the sarcomere when muscle contracts?
-Light bands (I-band) become narrower
-Dark bands (A-band) remain the same width
-H-zones get shorter
-Z lines move closer together, shortening the sarcomere
Why do dark bands remain the same width when muscle contracts?
As the myosin filaments themselves have not shortened, but now overlap the actin filaments by a greater amount
What is the structure of myosin?
-Globular heads that are hinged to allow them to move backwards and forwards
-Binding site on their head for each actin and one for ATP
-Tails of several hundred myosin molecules are aligned together to form the myosin filament
What is the structure of actin?
-binding sites for myosin heads called actin-myosin binding sites
-two stands twisted around each other
-proteins tropomyosin and troponin are found between actin filaments
When a muscle is in resting state what occurs at the actin-myosin sites?
The actin-myosin binding sites is blocked by tropomyosin which is held in place by troponin, so myosin heads cannot bind to actin, and the filaments can’t slide past each other
When a muscle is stimulated to contract what occurs according to the sliding filament theory
-The myosin heads form bonds with actin filaments known as actin-myosin cross bridges
-The myosin heads flex in unison, pulling the actin filament along the myosin filament
-Myosin then detaches from the actin and its head turns back to its original shape using ATP
-Myosin reattaches further along the actin filament, the process occurs again
What is a motor unit
All the muscle fibres which are supplied by a single motor neurone
What are the stages of the interaction of actin and myosin during muscle contraction?
- Tropomyosin molecules prevent myosin heads from attaching to the binding site on the actin molecule
- When an action potential stimulates a muscle cell, it depolarises the sarcolemma, this spreads down to the T-tubules which are in contact with the sarcoplasmic reticulum
- When the action potential reaches the sarcoplasmic reticulum it stimulates calcium ion channels to open and calcium ions diffuse down their concentration gradient into the scaroplasm
- Calcium ions bind to troponin causing it to change shape, this pulls on the tropomyosin moving it away from the actin-myosin binding sites
- Binding sites are now exposed and myosin heads bind to the actin filament forming an actin-myosin cross-bridges
- Once attached, the myosin head flexes, pulling the actin filament along. The molecule of ADP bound to the myosin head is released
- An ATP molecule can now bind to the myosin head, causing the actin-myosin cross bridge to break and it to detach from the actin filament
- Calcium ions present in the sarcoplasm activate enzyme ATPase which hydrolyses ATP into ADP + Pi and energy, allowing the myosin head to return to its normal position
- The myosin head can now reattach to a binding site further along the actin filament and the cycle is repeated
How is ATP generated from Aerobic respiration?
-Most ATP is generated from ADP during oxidative phosphorylation in the mitochondria
-Only works when there is oxygen present so is good for long periods of low intensity exercise like jogging, walking
How is ATP generated from Anaerobic respiration?
-Made rapidly by glycolysis as no oxygen is present
-Pyruvate is produced is converted to lactic acid which can quickly build up in muscles causing fatigue
-Used during short periods of high intensity exercise eg. 100m sprinting
How is ATP generated from creatine phosphate?
-ATP is made by phosphorylation ADP by adding creatine phosphate
-CP is stored inside cells and the ATP-CP system generates ATP very quickly
-CP runs out after a few seconds so it is used during short bursts of vigorous exercise
-Anaerobic system and alactic so doesn’t form any lactate
