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How Cells Get Energy from Food

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Cellular respiration is the process by which the chemical energy of food molecules is released and partially captured in the form of ATP.  ATP (adenosine triphosphate) is the molecule used by the body to transport chemical energy within cells for metabolism - it is the energy source that powers our body. Carbohydrates, fats, and proteins can all be used as fuels in cellular respiration, but glucose is most commonly used as an example to examine the reactions and pathways involved.  The inhabitants of a cell that power the rest of the cell are called the mitochondria.  The more mitochondria in a cell, the more energy it uses.  For example, the heart and other muslces have more mitochondria per cell, using more energy, than a skin and hair cell.


We can divide cellular respiration into three metabolic processes: glycolysis, the Krebs cycle, and oxidative phosphorylation. Each of these occurs in a specific region of the cell.
1. Glycolysis (pathway converting glucose to pyruvate) occurs in the cytosol (the liquid found inside cells).
2. The Krebs cycle (sequence of reactions by which cells generate energy) takes place in the mitochondria.
3. Oxidative phosphorylation (metabolic pathway by which mitochondria reform ATP) via the electon transport chain is carried out on the inner mitochondrial membrane.

In the absence of oxygen, respiration consists of two metabolic pathways: glycolysis and fermentation. Both of these occur in the cytosol.
In glycolysis, glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (a  3-carbon molecule). This change is accompanied by gaining 2 ATP molecules and 2 NADH molecules.  NADH (Nicotinamide Adenine dinuclotide) is a coenzyme found in all living cells.  An enzyme is a molecule that causes triggers chemical reactions in the body.  A coenzyme is a nonprotein compound needed for an enzyme to function.
Kreb's Cycle
The Krebs cycle occurs in the mitochondria and generates a pool of chemical energy (ATP, NADH, and FADH2) from the oxidation of pyruvate, the end product of glycolysis.
Pyruvate is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA (a 2-carbon molecule). When acetyl-CoA is oxidized to carbon dioxide in the Krebs cycle, chemical energy is released and captured in the form of NADH, FADH2, and ATP. 
FADH2 (flavin adenine dinucleotide) is a redox cofactor involved in metabolic chemical reactions.  It can.  It can exist in different redox states which it converts between by accepting or donating electrons.  Redox is addition or subtraction of electrons or oxygen to a molecule, atom or ion. 
Oxidative Phosphorylation via the Electron Transport Chain
Oxidative phosphorylation is the pathway by which mitochondria reform ATP.  Tbis pathway, the electron transport chain, is a series of compounds that transfer electrons from donors to acceptors across a membrane. The electron transport chain consists of a series of molecules, mostly proteins, embedded in the inner mitochondrial membrane.The electron transport chain allows the release of the large amount of chemical energy stored in reduced NAD+ (NADH) and reduced FAD (FADH2). The energy released is captured in the form of ATP (3 ATP per NADH and 2 ATP per FADH2).

NADH + H+ + 3 ADP + 3 Pi + 1/2 O2 → NAD+ + H2O + 3 ATP

FADH2 + 2 ADP + 2 Pi + 1/2 O2 → FAD+ + H2O + 2 ATP
All cells are able to synthesize ATP via glycolysis. 
Fermentation complements glycolysis and makes it possible for ATP to be continually produced in the absence of oxygen. By oxidizing the NADH produced in glycolysis, fermentation regenerates NAD, which can take part in glycolysis once again to produce more ATP. NAD (nicotinamide adenine dinucleotide) is a coenzyme in cells.  It has an oxygen molecule can convert to a reduced form, NADH, which does not contain oxygen.
Glucose and Energy
The chemical energy stored in glucose generates far more ATP in aerobic respiration than in respiration without oxygen (glycolysis and fermentation).  Each molecule of glucose can generate 36-38 molecules of ATP in aerobic respiration but only 2 ATP molecules in respiration without oxygen (through glycolysis and fermentation). 
So in chronic health conditions with mitochondrial disfunction, which can include autism, there is an insufficiency of energy because of insufficient oxygen.  Supplements that help to transport oxygen such as sulfur and selenium, can improve energy levels and overall health.