Explain how the body obtains energy from fat, carbohydrates and proteins


All living things requires energy to stay warm (mammals in this case) and to carry out other life process i.e. maintenance, growth, movement, daily activities etc. All of the dietary energy in humans is obtained from the main food sources including carbohydrates, fat and proteins. These major food types are also known as macronutrients and each has its own energy content that provides energy by breaking their chemical bond energy in food molecules. Sugars and fat generate higher energy levels than proteins in non photosynthetic organisms. Fat provide far more energy per gram than carbohydrate or protein for example carbohydrate and protein provides 16.8 KJ/g whereas fat provides 37.8 kJ of energy per gram.

Metabolism a set of chemical reaction plays an important role in providing energy that helps an organism to maintain life. Metabolic process is organised in different pathways that leads a chemical reaction to another through the help of enzymes and coenzymes. The breakdown of food molecules leads to a process known as oxidative phosphorylation that occurs in mitochondria. This process is essential for providing Adenosine triphosphate (ATP) is a primary source of energy for cellular activities. As the metabolic pathway is organised in to different stages, each stage should be explored in details to understand the process. Hence these stages will be explored later in the essay to answer the essay question in full.

Nutrients to Energy- Three Main Stages

The macronutrients presented in our food are the main source of energy for our body and all three nutrients must be broken down into smaller molecules before the cells can utilize them to produce energy. The breakdown of the larger molecules and oxidisation of those molecules are known as catabolism. The breakdown happens in digestion system where the breakdown is relatively similar for each nutrient. Specialised enzymes, a catalyst, digest specific polymers into monomers, for instant protease are specialised to catabolise proteins into amino acid and glycoside hydrolases turn polysaccharides into monosaccharides and fats are hydrolysed into fatty acids and glycerol by lipase. Oxidation of these molecules occurs once the small subunits are filtered into the cytosol of a cell through an active transport protein.

Glycolysis reaction, which happens under anaerobic conditions, is a metabolic pathway that takes placehttp://en.wikipedia.org/wiki/Glycolysis inside all living cells. Glycolysis breaks sugar molecules glucose, a 6 carbon atom, and fructose into two pyruvate molecules, that contains 3 carbon atoms in each molecule. A difference exists during the combustion of carbohydrate molecule that can occur anaerobically while this is not true for the other two macronutrients.

The transformation of glucose into pyruvate happens in 10 different stages. Each stage has a different enzyme to catalyse 10 different sugar molecules. In the first 5 stages, called preparatory phase, two molecules of ATP per each glucose molecule are used to provide energy to drive the reaction. At the start of last five stages known as pay off phase 2 NAD+ and GAPDH enzyme turn the NAD+ into a NADH molecule by pulling off a hydrogen molecule from GAPDH, two H+ are also produced at this stage. At the end of the stages two NADH are given and four ATP molecules are given from ADP plus P1. The resulted pyruvate proceeds to mitochondria from cytosol to lose two carbon dioxide molecules and change to two carbon acetyl group that joins with coenzyme A to produce acetyl CoA before it enters the citric acid cycle.

Triglycerides, main form of fat, are oxidised in order to break them into smaller units such as fatty acid and glycerol inside the cytoplasm. Fatty acids are activated in cytoplasm before they enter cytosol, a same medium for glucose to citric acid. The activation must be done before the oxidation of fatty acid begins. During the activation, fatty acids change to fatty acyl CoA and ATP turns into AMP. Glycerol is transmitted to the glycolysis while the fatty acids are oxidised through beta-oxidation inside the mitochondria. There are four main enzymes located in mitochondria, therefore a series of four stages occur that convert acyl CoA to acetyl CoA. Two molecules of carbon from an acyl CoA is shortened at each stage to create a molecule of acetyl CoA and a molecule of NADH and FADH2. The resulted acetyl CoA is passed to the citric acid cycle and NADH plus FADH is entered into the electron transport chain.

Proteins consist of carbon, hydrogen, oxygen and nitrogen. Although carbohydrates and proteins hold a similar structure but there is still a difference among their structure. Carbohydrates are made out of carbon, hydrogen, and oxygen while protein has an addition of nitrogen and sulfur. Nitrogen is responsible for the creation of essential amino acids. There are all together 20 essential amino acids that build all body cells in animals. Body cell metabolise amino acids into fats or glycogen if excessive proteins are consumed in human diet. The breakdown of proteins to amino acids through digestion opens the path to energy metabolism of proteins.

If amino acids are used to generate energy it must be done through deamination process where amino acids are broken into their constituent parts. Vitamin B6 associate with its enzyme in transamination cause nitrogen to transfer to a kito acid causing amino acid to lose its nitrogen and amino group. Ammonia is synthesised when amino acid in transformed to L glutamate through transamination process. Ammonia produces urea that travels through the blood to the kidney and excreted in urine.

Now that urea is removed from the process the carbon skeleton of amino acids can be used in different ways i.e. for protein synthesis or ATP formation. Carbon skeleton can also be stored, mainly in livers, as glucose by gluconeogenesis. This starts by converting carbon skeleton into acetyl CoA so that the coenzyme can be transmitted to the citric acid cycle where acetyl CoA is oxidised to generate ATP. Gluconeogenesis (a metabolic pathway) aims to form glucose from using non carbohydrate carbon substrate including glycerol, glycogenic amino acid. The resulted glucose can be converted to glucose 6 phosphates from phosphoenolpyruvate. The end product is pyruvate; notice the end product of glucose in glycolysis is same. The process requires energy in order to provide energy during starvation in fasting or extreme exercise.

Citric acid cycle (also known as Kerb’s cycle) is a chain of eight reaction taking place in mitochondria. It is true for each macronutrient to go through this chain of cycle and the oxidation on all of the acetyl CoA carbons entered from different nutrients is similar. This is an important stage as most of the energy produced in mitochondria happens after this cycle is completed to produce molecule carrying electrons. The carbon present in acetyl CoA is fully oxidised to a CO­2 molecule during this reaction. Acetyl CoA filters its two carbon molecules to critic acid cycle and a reaction between acetyl and oxaloacetate produce citrate in the first chain of the cycle. Activated carrier molecules are generated from the oxidation of citrate molecules. Every cycle generates 3 NADH molecules, 1 GTP molecule and 1 FADH2 molecule. Two molecules of CO­2 are given off as waste. The NADH and FADH2 molecules carry hydrogen and electrons which then proceeds to an oxidative phosphorylation process. The oxidative phosphorylation provides most of the energy in the whole system. The cycle does not require oxygen to carry out the process but the oxidisation of pyruvate requires oxygen. Hence the cycle works under the aerobic condition.

The next and final step occurs along an electron transport chain in the mitochondrion inner membrane. The electron transport chain structure in four different proteins consists of five complexes. The high energy electrons from reduced electron carriers, NADH and FADH2, are bombarded to the electron transport chain where the electron moves from an electron donor to a terminal electron acceptor. These electrons are added to the NADH and FADH2 molecules in the citric acid cycle.

The electrons from NADH enters complex I where it’s oxidised back to NAD+. Therefore one electron is captured and joins a proton to form a Hydrogen atom and one electron is lost during NADH losses its hydrogen. The electron from the hydrogen carries onto next stage while the proton moves back the inner membrane after the production of FMN to FMNH2. The electron in last complex embeds to the molecules of O2 gas and combines to two H+ to produce water H2O. While the electrons travel through these four complexes and provides enough energy to pump H+ ions (protons) outside the inner membrane.

The concentration gradient of H+ is gained due to the movement of these protons. This gradient stores energy that is sufficient for the production of ATP by phosphorylation of ADP. This process is known as oxidative phosphorylation where the electron is in its lowest form of energy therefore all the energy from the food molecules are oxidised to synthesis enormous amount of ATP. There are approximately 30 molecules of ATP gained after the complete oxidation per molecule of glucose or fatty acids or amino acids to H2O and CO2. Complete combustion of proteins also produces NH3 as waste products.


As the essay reaches its conclusion we can suggest that these macronutrients follow a similar pathway to generate ATP. Although the means of getting to the citric acid cycle for each macronutrient is different i.e. fat must be activated before it enters cytosol whereas protein goes through deamination process, not true for either glucose or fat. Also the function of glucose and protein is quiet different glucose only provide energy to the cells but proteins can participate in protein synthesis to formation of enzymes and carry important materials through the body etc.

Molecular Biology of the Cell 4th edition, Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002.

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