Overview
This lecture explains how myosin and actin, with the help of ATP, enable muscles to contract by converting chemical energy to mechanical energy.
Myosin and Actin Structure
- Myosin is a protein with two interwound strands, classified as an ATPase enzyme.
- Actin is a filamentous protein that myosin "walks" along during muscle contraction.
Muscle Contraction: The Cross-Bridge Cycle
- Step 1: ATP binds to the myosin head, causing it to release from actin.
- Step 2: ATP is hydrolyzed to ADP and phosphate, cocking the myosin head into a high-energy position.
- Step 3: Release of phosphate triggers the power stroke, pushing the actin filament and generating movement.
- Step 4: ADP is released, and the myosin head remains attached to actin until another ATP binds.
ATP’s Role in Mechanical Movement
- ATP provides energy by hydrolysis, allowing myosin to change shape and move along actin.
- The conversion of chemical energy in ATP to mechanical motion is fundamental for muscle contraction.
Core Principles
- Muscle contraction involves repeated cycles of myosin heads binding, moving, and releasing actin, powered by ATP.
- Energy from ATP hydrolysis is stored as conformational (shape) changes in the myosin head.
Key Terms & Definitions
- Myosin — Motor protein that uses ATP to move along actin filaments and produce muscle contraction.
- Actin — Protein filament that serves as a track for myosin movement in muscle cells.
- ATP (Adenosine Triphosphate) — The main energy currency molecule in cells.
- ATPase — Enzyme that catalyzes the breakdown of ATP into ADP and phosphate, releasing energy.
- Power Stroke — Force-generating step where myosin pushes on actin, causing movement.
- Cross-Bridge Cycle — Sequence of events in muscle contraction where myosin heads bind, move, and release actin.
Action Items / Next Steps
- Review animations or diagrams showing the cross-bridge cycle.
- Prepare to learn how nerves stimulate muscle contractions in the next lesson.