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Energy comes from the three main nutrients carbohydrates, protein, and fats, with carbohydrates being the most important energy source. In cases where carbohydrates have been depleted, the body can utilise protein and fats for energy.
How Does The Body Produce Energy?
The 4 Methods To Create ATP (Adenosine Triphosphate) A Unit Of Energy
Energy is delivered to the body through the foods we eat and liquids we drink. Foods contain a lot of stored chemical energy; when you eat, your body breaks down these foods into smaller components and absorbs them to use as fuel.
Energy comes from the three main nutrients carbohydrates, protein, and fats, with carbohydrates being the most important energy source. In cases where carbohydrates have been depleted, the body can utilise protein and fats for energy. Your metabolism is the chemical reactions in the body’s cells that change this food into energy.
Most of the energy the body needs is for being at rest, known as the Basal Metabolism. This is the minimum amount of energy the body requires to maintain its vital functions such as breathing, circulation and organ functions. The rate at which energy is utilised for such functions is known as the Basal Metabolic Rate (BMR) and varies based on genetics, sex, age, height and weight. Your BMR drops as you get older because muscle mass decreases.
Optimal energy metabolism requires getting sufficient nutrients from our foods, otherwise our energy metabolism underperforms and we feel tired and sluggish. All foods give you energy and some foods in particular help increase your energy levels, such as bananas (excellent source of carbohydrates, potassium and vitamin B6), fatty fish like salmon or tuna (good source of protein, fatty acids and B vitamins), brown rice (source of fibre, vitamins and minerals), and eggs (source of protein). There are actually many foods that provide an abundant amount of energy, particularly those packed with carbohydrates for available energy, fibre or protein for a slow release of energy and essential vitamins, minerals and antioxidants.
Foods are metabolised at a cellular level to make ATP (Adenosine Triphosphate)by a process known as cellular respiration. It is this chemical ATP that the cell uses for energy for many cellular processes including muscle contraction and cell division. This process requires oxygen and is called aerobic respiration.
Glucose + Oxygen → Carbon dioxide + Water + Energy (as ATP)
Initially, large food macromolecules are broken down by enzymes into simple subunits in the process known as digestion.
Proteins are broken down into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol—through the action of specific enzymes. Following this process, the smaller subunit molecules then have to enter the cells of the body. They firstly enter the cytosol (the aqueous part of the cytoplasm of a cell) where the cellular respiration process begins.
Aerobic Respiration
There are four stages of aerobic cellular respiration that occur to produce ATP (the energy cells need to do their work):
Stage 1 Glycolysis (also known as the breakdown of glucose)
This occurs in the cytoplasm and involves a series of chain reactions known as glycolysis to convert each molecule of glucose (a six-carbon molecule) into two smaller units of pyruvate (a three-carbon molecule). During the formation of pyruvate, two types of activated carrier molecules (small diffusible molecules in cells that contain energy rich covalent bonds) are produced, these are ATP and NADH (reduced nicotinamide adenine dinucleotide).This stage produces 4 molecules of ATP and 2 molecules of NADH from glucose but uses 2 molecules of ATP to get there,- so it actually results in 2 ATP + 2 NADH and pyruvate. The pyruvate then passes into the mitochondria.
Stage 2 The Link reaction
This links glycolysis with stage 3 the Citric acid/ Krebs cycle, which is explained below. At this point, one carbon dioxide molecule and one hydrogen molecule are removed from the pyruvate (called oxidative decarboxylation) to produce an acetyl group, which joins to an enzyme called CoA (Coenzyme A) to form acetyl-CoA, which is then ready to be used in the Citric acid/Krebs cycle. Acetyl-CoA is essential for the next stage.
Stage 3 The Citric Acid/Krebs Cycle
Taking place in the mitochondria, the acetyl-CoA (which is a two-carbon molecule) combines with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule). The citrate molecule is then gradually oxidized, allowing the energy of this oxidation to be used to produce energy-rich activated carrier molecules. The chain of eight reactions forms a cycle because, at the end, the oxaloacetate is regenerated and can enter a new turn of the cycle. The cycle provides precursors including certain amino acids as well as the reducing agent NADH that are used in numerous biochemical reactions.
Each turn of the cycle produces two molecules of carbon dioxide, three molecules of NADH, one molecule of GTP (guanosine triphosphate) and one molecule of FADH2 (reduced flavin adenine dinucleotide).
Because two acetyl-CoA molecules are produced from each glucose molecule utilised, two cycles are required per glucose molecule.
Stage 4 Electron Transport Chain
In this final stage, the electron carriers NADH and FADH2, which gained electrons when they were oxidizing other molecules, transfer these electrons to the electron transport chain. This is found in the inner membrane of the mitochondria. This process requires oxygen and involves moving these electrons through a series of electron transporters that undergo redox reactions (reactions where both oxidation and reduction take place). This causes hydrogen ions to accumulate in the intermembrane space.
A concentration gradient then forms where hydrogen ions diffuse out of this space by passing through ATP synthase. The current of hydrogen ions powers the catalytic conversion of ATP synthase, which, in turn, phosphorylates ADP (adds a phosphate group) therefore producing ATP. The endpoint of the chain occurs when the electrons reduce molecular oxygen, which results in the production of water.
Although there is a theoretical yield of 38 ATP from the breakdown of one glucose molecule, realistically it is thought 30-32 ATP molecules are actually generated.
This process of aerobic respiration takes place when the body requires sufficient energy just to live, as well as to carry out everyday activities and perform cardio exercise. While this process yields more energy than the anaerobic systems, it is also less efficient and can only be used during lower-intensity activities.
So, if you have SLOW and STEADY energy requirements, your NET ENERGY PRODUCTION from aerobic respiration equals 30-32 Molecules of ATP.
Glucose + Oxygen → Carbon dioxide + Water + Energy (as 30-32 ATP)
The body releases carbon dioxide and water in this process. This will theoretically burn the highest number of calories.
Under other physiological conditions the body can still acquire its energy in other ways:
There are further energy processes the body uses to create ATP, they depend on the speed at which the energy is required and whether they have access to oxygen or not.
Anaerobic Respiration
Human muscle can respire anaerobically, a process that does not require oxygen. The process is relatively inefficient as it has a net energy production of 2 molecules of ATP.
This is effective for vigorous exercise of between 1-3 minutes duration, such as short sprints. If the intense exercise requires more energy than can be supplied by the oxygen available, your body will partially burn glucose without oxygen (anaerobic). Without the presence of oxygen, the electron transport chain cannot work. Therefore, the usual number of ATP molecules cannot be made. The anaerobic pathway uses pyruvate, the final product from the glycolysis stage.
Pyruvate is reduced to lactic acid by NADH, leaving NAD+ after the reduction. This reaction is catalysed by an enzyme (lactate dehydrogenase) and leads to the recycling of NAD+. This then allows the process of glycolysis to continue.
This glycolysis pathway yields 2 molecules ATP, which can be used for energy to drive muscle contraction. Anaerobic glycolysis occurs faster than aerobic respiration as less energy is produced for every glucose molecule broken down, so more has to be broken down at a faster rate to meet demands.
Lactic acid (the by-product from anaerobic respiration) builds up in the muscles causing the “burn” felt during strenuous activity. If more than a few minutes of this activity are used to generate ATP, lactic acid acidity increases, causing painful cramps. The extra oxygen you breath in following intensive exercise, reacts with the lactic acid in your muscles, breaking it down to make carbon dioxide and water.
So, summing up: Exercises that are performed at maximum rates for between 1 and 3 minutes depend heavily on anaerobic respiration for ATP energy. Also, in some performances, such as running 1500 meters or a mile, the lactic acid system is used predominately for the “kick” at the end of a race.
Therefore, if you are doing VIGOUROUS EXERCISE for 1-3 minutes, there will be NO TISSUE OXYGEN AVAILABLE so you will see a NET ENERGY PRODUCTION from anaerobic respiration equal to 2 molecules of ATP.
Beta Oxidation/Gluconeogenesis or Fat Burning (Aerobic Lipolysis)
A fat molecule consists of a glycerol backbone and three fatty acid tails. They are called triglycerides. In the body, they are stored primarily in fat cells called adipocytes making up the adipose tissue. To obtain energy from fat, the triglyceride molecules are broken down into fatty acids in a process called ‘Lipolysis’ occurring in the cytoplasm. These fatty acids are oxidized into acetyl- CoA, which is used in the Citric acid/Krebs cycle. Because one triglyceride molecule yields three fatty acid molecules with 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body (over 100 molecules of ATP generated per molecule of fatty acid).
Therefore, when glucose levels are low, triglycerides can be converted into acetyl-CoA molecules and used to generate
ATP through aerobic respiration.
This need arises after any period of not eating; even with a normal overnight fast, mobilization of fat occurs, so that by the morning most of the acetyl-CoA entering the Citric acid/Krebs cycle comes from fatty acids rather than from glucose.
Following a meal, however, most of the acetyl-CoA entering the Citric acid/Krebs cycle comes from glucose from food, with any excess glucose being used to replenish depleted glycogen stores or to synthesize fats.
This is a SLOW, NOT IMMEDIATE ENERGY SOURCE but has a NET ENERGY PRODUCTION of over 100 molecules of
ATP.
ATP Phosphocreatine (ATP-PC)
This energy system consists of ATP (all muscle cells have a little ATP in them) and phosphocreatine (PC), which provide immediate energy from the breakdown of these high energy substrates.
Firstly, ATP that is stored in the myosin cross-bridges (within the muscle) gets broken down producing adenosine diphosphate (ADP) and one single phosphate molecule. Then, an enzyme, known as creatine kinase, breaks down phosphocreatine (PC) to creatine and a phosphate molecule. This breakdown of phosphocreatine (PC) releases energy, which allows the adenosine diphosphate (ADP) and phosphate molecule to re-join forming more ATP. This newly formed ATP can then be broken down to release energy to fuel activity. This will continue until creatine phosphate stores are depleted.
Short, sharp explosive bursts of exercise (10-30 secs) use this system. It doesn’t require oxygen but is very limited to short periods of explosive exercise, such as a sprint or weight/power lifting. This is why creatine supplementation helps this sort of exercise, ensuring there is adequate creatine phosphate to provide those required phosphates. The ATP-CP system usually recovers 100% in 3 mins; so, the recommended rest time in between high intensity training is 3 minutes.
In short, for sharp explosive bursts of exercise needing FAST, IMMEDIATE energy this system produces COPIUS
AMOUNTS OF ATP until the creatine phosphate in muscles runs out.
Different forms of exercise use different systems to produce ATP
For short distance sprinters/ weight lifters the energy system used would be ATP-PC as its fast and only few seconds
During intense, intermittent exercise and throughout prolonged physical activity the energy system used would typically be via the glycogen route (fat burning /no oxygen)
In endurance events like marathon running or rowing etc., which lasts for unlimited time would use the energy process of aerobic respiration.
Role of gut bacteria in energy regulation
Gut bacteria plays an important role in nutrient and energy extraction and energy regulation. The bacteria makes a multitude of small molecules (known as metabolites) that can act as signals that can modulate appetite, energy uptake, storage and expenditure, something which is explored in the review article Gut Microbiota-Dependent Modulation of
Energy Metabolism.
Gut bacteria influences the bioavailability of polysaccharides and how this occurs is unclear but it is an increasing area of research, with this 2016 paper, on the causality of small and large intestinal microbiota in weight regulation and insulin resistance, investigating the subject at length.
Side effects with low energy levels
Not properly managing your energy levels can result in both physical and cognitive functions being affected.
Physical signs can include: reduced stamina, reduced strength and less ability to recover from exercise.
Performance related effects can include: loss of focus, slow reaction times, low mood, poor working memory, poor decision making and decreased reaction times.
POSTED BY: ALISON ASTILL SMITH
2nd February 2022
Healthy Cells in Body
