Energy from Food
Conversion of Food into Energy
P.K.Ghatak, MD
No.40.
Maintaining good health and happiness is dependent on a regular supply of nutrients, food, and water. Over the centuries, the food habits of humans have changed as societies adapted to the choice of food based on the available food production and supply. The current generation in the affluent countries is faced with a vexing problem – unsolicited advice and push to follow a certain diet and avoid certain other food items in the name of maintaining good health and avoiding illness.
The fundamental human biochemistry remains the same irrespective of the positions these so-called specialists advocate.
Humans are omnivorous – there is nothing on this earth that grows or moves that humans have not eaten. Irrespective of our desire or the cost of food, the food we eat belongs to carbohydrates, fats, and proteins.
Our body is prudent and comprehensively uses food substances. Like we use cash for immediate transactions and withdraw money from our checking, savings, and investment accounts as needed, the body also follows the same pattern. Glucose is like cash and the immediate source of energy. It is readily available and used first. In continued demand, like strenuous exercise, glycogen from the liver and skeletal muscles is used next. Glycogen store is limited and it can maintain energy demand for only a limited hours. Fat is broken down next, and the end product of the fatty acids is Ketone bodies. Ketones are used by the brain and liver. The liver produces ketones but is unable to use it as fuel. In starvation and illnesses preventing swallowing, protein from the skeletal muscles is used.
Proteins are used mainly for repair, replacement of cells that die or are shed from the skin, GI, and GU tracts. Growth and development from newborn to adulthood, in pregnancy, and fetal growth in utero. Daily production of hormones, enzymes, cytokines, and secretion of various glands.
Animal and plant proteins that we eat must be broken down, and human proteins are synthesized chiefly in the liver. The hormones and enzymes, etc, are produced in the respective glands and organs.
Glucose, fatty acids, and amino acids during the oxidation process end in Acetyl-CoA, a molecule that is a common end product in all three and also a way to produce one category from the other among these three food categories.
Energy from Glucose:
Glucose produces 17kJ/gm
Each living cell must generate its own energy locally and use that energy for all biological activities. The biochemical reactions are guided by an assortment of enzyme systems, and unlike any chemical reactions, the living cells cannot utilize heat as a source of energy.
The sites of energy generation in a cell are 3.
Cytosol.
Mitochondria.
Membrane of the Endoplasmic Reticulum.
In the Cytosol.
Glucose is phosphorylated to glucose 6-phosphate and goes through 10 steps of enzymatic processes and is converted to 2 molecules of Pyruvate. There are several paths operative, but the common one is the Embden-Meyerhof pathway.
Pyruvate is converted to glyceraldehyde-3-phosphate requires ATP; further degradation results in a 2-carbon Acyl residue, which combines with coenzyme CoA to form acetyl-CoA, and 2 ATP. Acetyl CoA then enters the Mitochondria.
In the Mitochondria.
Acetyl CoA combines with alpha-keto glutamate and becomes Citric acid, a six-carbon compound and begins the citric acid cycle of Krebs, as represented in the following diagram.
At the end, every pyruvate molecule produces 2 molecules of CO2. And 2 molecules of ADP.
In the membrane of the Endoplasmic Reticulum.
In this stage, most of the ATP molecules are generated. The process is known as the Electron Transfer Chain. The substrate is oxidized by enzymatic stripping of H(+) ion, and another molecule acts as a H(+) receptor, accepts H (+), and becomes reduced, which in the next step hands over the H(+) to the next H(+)ion receptor and returns to the initial step. And the process continues like passing a baton in a relay race. Cytochrome enzymes take up H(+) ion in the final step. Cytochrome Oxidase transfers the H(+) ion to Oxygen and produces H2O and energy.
The balance sheet of ATP.
At the end of using and generating ATP, a net gain of 16 molecules of ATP are produced for each molecule of pyruvate. That is 32 ATPs for each molecule of glucose.
Fatty Acids.
Fatty acids produce 37 kJ/gm
In the Cytosol, long-chain fatty acids are broken down into shorter chains at the sites of double bonds. Short and medium-chain fatty acids diffuse into mitochondria; any remaining long-chain fatty acids have to be transported into the mitochondria by an enzyme.
In Mitochondria.
Beta Oxidation.
The two carbon terminals of the carboxyl end of fatty acids undergo successive oxidation and are ultimately left with an Acyl residue, which becomes acetyl-CoA. Omega and Alpha oxidation of the fatty acids also occur at a slower rate in certain locations of the cells and in certain tissues.
In the Membrane of Reticulo Endothelium.
The Acetyl-CoA generated from fatty acids is utilized in the same manner as mentioned under glucose.
Amino Acids.
An amino acid produces 17 kJ/gm
In the initial step, the Nitrogen group is removed from the amino acids by deaminase enzymes. The carbohydrate molecules so produced then enter the Krebs' cycle and produce energy.
Essential Fatty Acids and Essential Amino Acids.
Linoleic and Linolenic acids must be supplied from outside because the body is unable to produce them and are known as Essential fatty acids.
Out of 21 Amino Acids, 9 amino acids are essential amino acids.
The adverse effects of a deficiency in essential amino acids manifest quickly, and various symptoms and illnesses develop. The deficiency of essential Fatty Acids produces less dramatic but serious symptoms, to mention a few - dry and scaly skin, loss of hair and brittle hair. Poor growth in infants and children, susceptible to repeated infections.
When Glucose supply is limited, the Krebs Cycle slows down, and the body tries to maintain energy supply by increasing the use of fat. The excess Ketones produced are utilized by the brain cells and liver; however, if production of ketone bodies is in excess of the capacity to utilize them, the accumulated ketones alter the pH of the body to the acidic side, and the condition is called Acidosis.
Enzyme systems and metabolic functions of the body are pH dependent. In mild acidosis, nausea, vomiting, and anorexia develop. In moderate to severe acidosis, severe illness develops. High serum Potassium develops as a consequence of acidosis and can produce major abnormalities of heart rate and rhythm and if treatment is delayed, fatality will follow.
Because a minimum amount of carbohydrate is essential for adequate function of the Krebs cycle in order to utilize fat, it says fat burns in the fire of carbohydrate.
Footnote:
ATP – Adenosine Tri Phosphate. A high-energy molecule, the body uses ATP in processes where energy must be supplied.
ADP - Adenosine Diphosphate
NADP – Nicotinamide Adenine Dinucleotide Phosphate.
FADP – Flavin Adenine Dinucleotide -3 -Phosphate
NADP and FADP are coenzymes, take part in oxidation and reduction reactions.
Essential Amino Acids: Histidine, Isoleucine, Leucine, Lysine, Phenylalanine / Tryptophan, Threonine, Valine.
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