Dietary Energy Production

1. Summary of Dietary Energy Production
   
   • Stage One
   • Stage Two - Glycolysis
   • Stage Three - Krebs Cycle
   • Stage Four - Oxidative Phosphorylation
2. Quiz

• The body uses the carbohydrates, fats and proteins in the diet to produce energy. These follow a broadly similar pathway involving 4 stages, although they join it at different points.
• Stage 1 is digestion: food is broken down into small building blocks (such as sugars, amino acids and fatty acids) that can be absorbed into cells. Stage 2 is glycolysis, the oxygen independent breakdown of glucose to pyruvate within cells, which makes a small amount of ATP. Stage 3 is the Krebs cycle, an oxidative pathway that takes place within the mitochondria. Stage 4 is oxidative phosphorylation, which uses the energy carried by NADH and FADH₂ (electron-carrying molecules made in the earlier stages) to make ATP.

This article gives a simple summary of the pathway that the carbohydrates, fats and proteins in the diet broadly follow to produce energy in the body (see the diagram below). For more detail on each step, see the specialised articles on the Metabolism Unit page. 

Diagram - Overview of the Dietary Production of Energy

SimpleMed original by Dr. Maddie Swannack

Stage One

• Basic digestion of the macromolecules (large molecules such as starch, proteins and fats) found in the diet occurs in this stage, breaking down food into forms that can be absorbed into cells.
• This occurs extracellularly (outside the cells) in the gastrointestinal (GI) tract, using enzymes (e.g. salivary amylase, which digests starch, or pepsin, which digests proteins).
• It is a catabolic process (one that breaks molecules down) because it takes complex molecules and breaks them down into simpler, building-block materials.
• There is no energy produced at this stage.

Stage Two - Glycolysis

• This stage is intracellular (inside the cells). Glycolysis (the breakdown of glucose to pyruvate) occurs in the cytosol (the fluid part of the cell), and the link reaction, which turns pyruvate into acetyl CoA (a two-carbon molecule that feeds the Krebs cycle), occurs in the mitochondria.  
• There are many pathways in stage two. For example, pyruvate can be converted to acetyl CoA, or it can be converted to lactate and then back to pyruvate before forming acetyl CoA. Not all of these pathways occur in all tissues.
• It is still a catabolic process, with the wider range of building block materials being simplified to less complex molecules like Acetyl CoA.
• This process is oxidative: the molecules being broken down are oxidised (they lose electrons) while the co-enzyme NAD⁺ (a carrier that picks up electrons and becomes NADH) is reduced. Glycolysis itself does not need oxygen.  
• It produces some energy (in the form of ATP with a net of 2; 4 molecules produced with 2 used).
• Parts of this stage are irreversible. Glycolysis itself does not produce CO₂, but the link reaction releases one molecule of CO₂ for each pyruvate it converts to acetyl CoA.

Stage Three - Krebs Cycle/Tricarboxylic Acid (TCA) Cycle

• This stage occurs in the mitochondria and is a single pathway called the Krebs cycle. It is the common final pathway for the oxidation of carbohydrates, fats and proteins, which feed into it as acetyl CoA or as other intermediates (the molecules that make up the cycle).
• It is an oxidative pathway (meaning it requires the co-enzymes NAD⁺ and FAD, which pick up electrons to become NADH and FADH₂). FAD stands for flavin adenine dinucleotide.
• Each turn of the cycle makes one GTP (guanosine triphosphate, an energy-carrying molecule similar to ATP; some tissues make ATP instead), three NADH and one FADH₂. Most of the energy from the cycle is held in NADH and FADH₂, which pass it on in stage four.
• This stage is effectively irreversible. Its first step releases a large amount of energy, and carbon is lost as CO₂: two molecules of CO₂ are given off in each turn, matching the two carbons of the acetyl group (the two-carbon unit of acetyl CoA) that enter the cycle.

Stage Four - Oxidative Phosphorylation

• This stage also occurs in the mitochondria, in the inner mitochondrial membrane, which separates the matrix (the inner compartment) from the intermembrane space (the gap between the inner and outer membranes).
• It involves electron transport and ATP synthesis.
• NADH and FADH₂ (electron carriers made in the earlier stages) are re-oxidised, producing H⁺ + e⁻ (protons and electrons) which are used to establish the proton motive force (the energy stored in the H⁺ gradient across the inner membrane) that powers ATP synthase (the enzyme that makes ATP).
• This step leads to the production of large amounts of ATP.
• This stage requires oxygen to be the terminal electron acceptor (the final acceptor of electrons at the end of the chain), which is reduced to H₂O.

Edited by: Dr. Ben Appleby and Dr. Marcus Judge

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