Bio Applications

The MCAT is an interdisciplinary exam, and energy is one of its favorite crossover topics. A single passage might describe a person climbing stairs and ask you to connect three subjects:

• Physics: $W = mgh$ (the work needed to lift the body’s mass).
• Biology: ATP hydrolysis powers each muscle contraction.
• Biochemistry: glucose oxidation produces the ATP.

This section bridges those worlds. Once you see how the same conservation laws govern both an engine pushing a car and a muscle pulling a tendon, the crossover questions stop feeling like trick questions.

ATP: The Energy Currency

Adenosine triphosphate (ATP) stores energy in its phosphoanhydride bonds. When ATP is hydrolyzed to ADP + $P_i$, it releases roughly 30.5 kJ/mol under standard conditions. Inside actual cells (where concentrations differ from standard state), the real release is closer to 50–54 kJ/mol — significantly more.

Muscle Contraction as Work

When a bicep lifts a weight, the physics work is $W = mgh$. But the biological work involves millions of myosin heads pulling on actin filaments, each driven by one ATP hydrolysis cycle. The bridge between scales:

1. Glucose is oxidized in cellular respiration, producing ATP.
2. ATP binds to myosin heads in the muscle.
3. ATP hydrolysis triggers the myosin “power stroke” — a tiny applied force over a tiny distance (work at the molecular level).
4. Many millions of power strokes happening simultaneously shorten the muscle, producing macroscopic force and shortening.
5. The macroscopic muscle force × the distance the load moves = the physics work done on the load.

So $mgh$ at the gym scale and $\Delta G$ of ATP hydrolysis at the molecular scale are connected by an unbroken chain of energy transfers.

Metabolic Efficiency

The human body converts food energy to mechanical work with an efficiency of roughly 20–25%. The remaining 75–80% becomes body heat. That’s why you get hot during exercise — most of the metabolic energy you’re burning ends up as warmth, not motion.

The two stages multiply (cascading efficiency, §2.10): $0.40 \times 0.50 = 0.20 = 20\%$. Just like any machine, every conversion step shaves off some of the original energy as heat.

Coupled Reactions and Free Energy

In biochemistry, endergonic reactions (positive $\Delta G$, non-spontaneous) are made to proceed by coupling them to ATP hydrolysis (negative $\Delta G$, very spontaneous). The combined $\Delta G$ has to be negative for the coupled reaction to actually go.

Example: phosphorylating glucose costs energy ($\Delta G = +13.8$ kJ/mol — won’t happen by itself). Couple it to ATP hydrolysis ($\Delta G = -30.5$ kJ/mol):

• Combined: $\Delta G = +13.8 + (-30.5) = -16.7$ kJ/mol.
• Now the overall reaction is exergonic and proceeds spontaneously.

This is the biological equivalent of using a heavy falling weight (ATP hydrolysis) to lift a lighter weight (glucose phosphorylation) — the surplus energy from ATP more than pays for the costly reaction. Energy is still conserved; the bookkeeping just spans both reactions.

Energy Storage in the Body

The body stores energy in multiple forms, each with a different energy density and accessibility:

This hierarchy is why athletes train differently for different events. A 100 m sprint is mostly ATP + creatine phosphate (already stored, instantly accessible). A marathon is mostly fat oxidation (vast reserves but slower to mobilize). A 400 m run is the painful in-between zone — too long for ATP/creatine, too fast for full aerobic fat metabolism — so it relies heavily on anaerobic glycolysis, producing lactate.

Biomechanics: The Body as a Machine

The musculoskeletal system is a collection of levers (§§2.8–2.9). Most joints operate as third-class levers: the effort (muscle) is between the fulcrum (joint) and the load (weight in hand). That gives a mechanical advantage less than 1 — the muscle has to pull much harder than the load it’s lifting.

That sounds like bad engineering, but it’s a deliberate trade-off: speed and range of motion. A small contraction of the bicep (a few centimeters) produces a large, fast movement at the hand (tens of centimeters). The body sacrifices brute force for quickness — exactly the opposite of what a crowbar does. Throwing a baseball, swinging a tennis racket, kicking a soccer ball — none of them would be possible with high-MA leverage; you need the speed.