This is the second in a series about energy. If this is your first read, having some background info is useful so check out the link below.
Part 1: Energy: What is it?
We know energy is the capacity to do work and work is essentially a measure of movement. Thus, energy creates movement. Quick, what’s the tissue in our body that contracts to create movement? If you guessed muscle, you would be correct. Mammalian muscle is a metabolically active soft tissue that comes in three forms: smooth (think intestine), cardiac (heart) and skeletal (such as biceps). Because this page is meant for athletics, we’ll keep the focus on skeletal muscle but all three function similarly.
So, how do our muscles contract? Where does the energy for contraction come from? To start, our muscles are made of individual units which then contain increasingly smaller subunits. For simplicity, it’s best to think of a whole muscle such as your biceps as being made up of fibers (units) which are then made up of smaller filaments (subunits). The proteins actin and myosin are the two main filaments. When a muscle contracts, it shortens and as this happens, the two filaments slide past each other in what’s known as the …wait for it… Sliding Filament Theory. But what actually causes actin and myosin to slide past each other?
There is a fancy chemical molecule called Adenosine Triphosphate (ATP) which serves as the energy for this sliding mechanism. There is one adenosine molecule (A) bound to three phosphate molecules (TP). Here’s the method by which chemical energy in the form of ATP transfers to mechanical energy when a muscle contracts which then ultimately becomes kinetic energy, or movement:
1. Myosin filaments have numerous ‘heads’ which branch off the main strand and serve as the connection to actin. This connection is called a cross-bridge
2. ATP comes along and binds to a specific site on the myosin head which releases it from actin. The key here is that myosin is able to split the ATP molecule (which is the energy) in half through a process called hydrolysis (literally meaning ‘splitting with the help of water’). The reaction looks like this:
ATP + H2O → ADP + Pi + H
3. ATP was split into one A bound to two phosphate molecules (DP) one free phosphate (Pi) and a Hydrogen atom (H) floating around.
4. This splitting of ATP changes the angle of the myosin head which was previously bound to actin, tilting the head towards the center of the filament in what’s called a power stroke (sounds awesome). This allows myosin to ‘pull’ itself a tiny bit along the actin filament. Thus, the splitting of ATP has given the myosin head the energy it needed to move along the actin filament.
5. After the power stroke, ADP and Pi are released from the myosin head and can re-form ATP at another location. The myosin head returns to its original angle, ready to bind a new actin molecule if the signal is there.
6. This process of splitting ATP, the power stroke, and subsequent return to baseline happens over fractions of a second and can occur who-knows-how-many times before the muscle can no longer contract. Consider also, that each myosin filament contains numerous heads and there are thousands of filaments. Thus, as ATP is split, myosin is able to pull itself along the actin filament contracting and shortening the muscle.
Works Cited:
(1) Kenney et al. Physiology of Sport and Exercise. Seventh Edition. Human Kinetics, 2020.
(2) Nelson and Cox. Principles of Biochemistry. Fifth Edition. Sara Tenney, 2008
