Bioenergetics + Metabolism

Some words to know

  • Bioenergetics is the flow of energy a biological system, and describes the conversion of macronutrients (carbs, protein, fat, which all contain chemical energy) into micronutrients (glucose, ATP) that can be used for energy.  The energy is harvested from the breakdowns of chemical bonds within the macronutrients.
  • Metabolism is the total sum of all chemical reactions.  Catabolism is the breakdown of large molecules into smaller molecules.  Anabolism is the opposite of that, building large molecules from smaller ones.
    • Catabolic reactions are typically exergonic reactions, meaning they are energy-releasing.  While anabolic reactions, as well as muscle contractions, are generally endergonic reactions which require energy.
  • Energy stored in the chemical bonds of ATP is used to power muscular activity.  The ATP stored in skeletal muscle is replenished by 3 basic systems - phosphagen, glycolytic, and oxidative.


ATP - Adenosine Triphosphate

ATP is required for all muscular activity and growth, therefore its important to understand how exercise affects ATP hydrolysis and resynthesis.  The breakdown of 1 ATP molecule to yield energy is called hydrolysis because it requires 1 molecule of water.  The catabolism of ATP is facilitated by the enzyme ATPase.  Notice how the chemical reaction has arrows going in both directions, which means this is the reaction for both the hydrolysis and resynthesis of ATP.

ATP + H2O ← via ATPase → ADP + Pi + H+ + energy

ATP and a water molecule, with the help of ATPase, will become adenosine di-phosphate, inorganic phosphate, hydrogen, and energy.

ATP Replenishment Intro

ATP is replenished in the muscle cells by 3 different processes - the phosphagen system, glycolysis, and the oxidative system.  The phosphagen system and glycolysis do not require oxygen, and are therefore considered anaerobic, and occur in the sarcoplasm of the cell.  While the oxidative system is aerobic and requires oxygen, and occurs in the mitochondria.   Out of the 3 macronutrients, only carbohydrates can be metabolized for energy without oxygen, therefore making it essential for anaerobic metabolism.

The way this section is taught doesn't totally highlight the fact that all 3 energy systems are active at any given time.  The magnitude of each system's contribution to overall work performance depends mostly on the intensity of the activity, and also on the duration of the activity.

Phosphagen System

Creatine Phosphate (CP or PCr) is an energy reserve for rapidly replenishing ATP, and is stored in limited amounts in the muscle cells.  Creatine kinase is the enzyme that catalyzes the reaction.  This process is active at the start of all exercise, during the first 0-6 seconds.  Mostly used for short-term, high intensity activities.  

PCr → via Creatine Kinase → Creatine + Pi + Energy

Energy + Pi + ADP → ATP

Creatine kinase breaks down creatine phosphate, steals the inorganic phosphate, and produces energy.  The energy then adds the phosphate to ADP to synthesize ATP.


Glycolysis is the breakdown of carbs to resynthesize ATP. There are two sources of carbs in the body used for glycolysis: muscle glycogen stores and glucose from the blood. The glycolysis process is driven by many enzymatic reactions, and therefore takes a longer time to resynthesize ATP than the phosphagen system, which only requires one enzyme.  But glycolysis has a much higher capacity to produce ATP because of the abundance of glycogen and glucose.  Both glycolysis and the phosphagen system occur in the sarcoplasm and are considered "anaerobic".  

Pyruvate is the end result of glycolysis, and can be dealt with in two different ways depending on the intensity of the exercise. Although the names suggest the two processes depend on oxygen availability, that's not true because glycolysis itself doesn't depend on it, and its confusing and mean that someone named it such.  Glycolysis itself is anaerobic, not depending on oxygen, but pyruvate is handled differently depending on the availability of oxygen and the energy demands of the exercise.

Anaerobic Glycolysis | Fast Glycolysis: high intensity exercise, high energy demand, insufficient oxygen = pyruvate is converted into lactate

Aerobic Glycolysis | Slow Glycolysis: lower intensity exercise, lower energy demand, sufficient oxygen = pyruvate is shuttled into the mitochondria, converted into Acetyl-CoA and enters the Krebs cycle, also known as the citric acid cycle.

If demand is high and intensity is high, energy must be transferred more quickly, the body will depend on anaerobic glycolysis and convert pyruvate into lactate.  On the other hand, if energy demands and exercise intensity are low enough and enough oxygen is available, the body can shuttle pyruvate to the mitochondria to be oxidized.

Lactate clearance?

Glycolysis Energy Yields

The net reaction for anaerobic glycolysis, when pyruvate is converted into lactate, yields 2 ATP per glucose molecule and a lot of lactate.  The amount of lactate that results makes this process of glycolysis less preferred.

Glucose + 2Pi + 2ADP → 2Lactate + 2ATP + H2O

The net reaction for aerobic glycolysis, when pyruvate is shuttled into the mitochondria, yields 2 ATP per glucose.  It also results in other molecules that can be further manipulated and converted into ATP or used to synthesize energy.

Glucose + 2Pi + 2ADP + 2NAD+ → 2Pyruvate + 2ATP + 2NADH + 2H2O


Oxidative Energy System

The oxidative energy system is the primary source of ATP at rest and during low intensity activities.  At rest prior to exercise, ATP is replenished by this energy system using 70% fats and 30% carbohydrates as fuel, then there is  a shift toward using more carbs and less fat at the onset of exercise.  But after prolonged, sub maximal, steady-state exercise (slow, long distance running, for example), there is a shift away from carbs and back to fats, and to a much lower extent, to protein as well.  When looking at the full picture of metabolic fuel sources, protein does not play a significant role.  Its contribution is only slightly increased during long bouts of exercise, typically longer than 90 minutes.