Energy systems in the human body

For a good performance is essential to know how our body produce the energy starting from the macronutrients (carbohydrates, proteins and fat).
A degree in biochemistry would be necessary for a very deep understanding but, in this case, my purpose is just a simple smattering.
This is a cell:

The main reactions occur in the cytosol and in the peripheral part of mytochondria
The basic structure is this one:
Stage 1 involves the digestion, absorption, and assimilation of relatively large food macromolecules into smaller subunits for use in cellular metabolism. This has been dealt with here
Stage 2 degrades amino acid, glucose, and fatty acid and glycerol units within the cytosol into acetyl-coenzyme A
(formed within the mitochondrion), with limited ATP and NADH production.
Stage 3 within the mitochondrion, acetyl-coenzyme A degrades to CO2 and H2O with considerable ATP production.

If we reason in a concrete way, these are the physical pathways througth the body


Now we can go on with the others:

  • Glycolysis: is the process of breaking down glucose into two molecules of pyruvate. It produces ATP and is the first stage of cellular respiration. It’s aerobic and uses the fermentation phenomenon (a metabolic process that produces chemical changes in organic substrates through the action of enzymes)
  • Pyruvate: it’s a jolly. Can be made from glucose through glycolysis, converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through a reaction with acetyl-CoA. It can also be used to construct the amino acid alanine and can be converted into ethanol or lactic acid via fermentation.
    Pyruvic acid supplies energy to cells through the citric acid cycle when oxygen is present (aerobic respiration), and alternatively ferments to produce lactate when oxygen is lacking (lactic acid fermentation.
  • Acetyl-CoA: CoA is acetylated to acetyl-CoA by the breakdown of carbohydrates through glycolysis and by the breakdown of fatty acids through β-oxidation. Acetyl-CoA then enters the citric acid cycle, where the acetyl group is oxidized to carbon dioxide and water, and the energy released captured in the form of 11 ATP and one GTP (essential to signal transduction) per acetyl group.
  • β-oxidation: catabolic process by which fatty acid molecules are broken down to generate acetyl-CoA, which enters the citric acid cycle, and NADH and FADH2 (involved in redox reactions, carrying electrons from one reaction to another), which are co-enzymes used in the electron transport chain.
  • Citric acid cycle: is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into ATP and carbon dioxide.
    The name of this metabolic pathway is derived from the citric acid that is consumed and then regenerated by this sequence of reactions to complete the cycle. The cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces carbon dioxide as a waste byproduct. The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation (electron transport) pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP.
  • Oxidative phosphorylation: inside the mitochondria, cells use enzymes to oxidize nutrients, releasing energy which is used to produce ATP. It is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.
    Electrons are transferred from electron donors to electron acceptors such as oxygen, in redox reactions. These redox reactions release energy, which is used to form ATP. These redox reactions are carried out by a series of protein complexes and are called electron transport chains.
    The energy released by electrons flowing through this electron transport chain is used to transport protons across the inner mitochondrial membrane, in a process called electron transport. This generates potential energy in the form of a pH gradient and an electrical potential across this membrane. The ATP synthase uses the energy to transform ADP into ATP, in a phosphorylation reaction. The reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary mechanical motor.
    Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging (senescence). The enzymes carrying out this metabolic pathway are also the target of many drugs and poisons that inhibit their activities.
    It is the terminal process of cellular respiration in eukaryotes and accounts for high ATP yield.

Basically, if we provide the precise quantity of Fats, CHO and Proteins our machine (body) will work in a perfect way.
But, if we provide, for example more fats and less proteins than required, some interconversions can happen. The lower part of the image shows how fats can be converted in amino acids if required.
Below we can see the “metabolic mill” of how the macronutrients can be transformed

This must not lead to a very precise (and uncomfortable) view of nutrition: the body is able to “reconfigure” it self in order to have always what he need. Of course we cannot eat only sugar every day because:

  • Cardinal… vous avez toujour perdrix… et moi toujours reine? (the first one because the meal must various in ordfer to be satisfactory)
  • The interconversion has limits and waste products during that action