How The Body Makes And Uses Energy

Whether you are physically active or not, your body has to carry out various processes to stay alive and carry out day-to-day activities. But the energy does not come out of nowhere.

LAW OF ENERGY

According to the first law of thermodynamics, energy cannot be created. But, it must be converted from one form to another. Like an automobile that only runs on gas, your body runs on one kind of energy – chemical energy. Specifically, your body can only run on one form of chemical energy – Adenosine Triphosphate (ATP) – for its biological processes. Think of ATP as the gas in your tank.

ESSENTIAL ENERGY PRECURSORS FOR YOUR BODY

Your body is not capable of making the energy it needs out of nothing. Instead, it converts the chemical energy stored in your food into the type of energy that your body can utilize. Therefore, for your energy production to be optimal, you need to get enough essential nutrients (energy precursors) from your food.

The foods and drinks you consume contain lots of molecules that your body needs to produce energy. So, when you eat, your body breaks down the food into smaller absorbable components and uses these components to make ATP.

Enzymes act on the food you consume to break down large macromolecules into simple subunits in a process called digestion. With the help of certain enzymes, carbohydrates are broken down into glucose. Fats are also broken down into fatty acids and glycerol, while proteins are broken down into amino acids.

Carbohydrates are the largest and most important energy source. However, when your carbohydrates have been depleted, your body can use fats and proteins for energy production.

HOW YOUR BODY MAKES ENERGY

Though food is a source of energy for your body, your body does not straight up absorb energy from that burger you ate. Instead, the food is converted into ATP, which then fuels various body processes.

Aerobic Respiration

Your body cells can produce ATP by metabolizing glucose in your food into ATP in a process known as cellular respiration. This metabolism of glucose into ATP requires oxygen. Hence the name aerobic respiration and can be represented as follows:

            Glucose + Oxygen → ATP (Energy) + Carbon dioxide + Water

Aerobic respiration occurs in 4 stages:

• Glycolysis: glucose is broken down into pyruvate.
• Link Reaction: pyruvate is converted into acetyl-CoA, which is essential for stage 3.
• Citric AcidKrebs Cycle: the acetyl-CoA undergoes a further transformation.
• Electron transport chain: this stage requires oxygen to transport electrons through certain transporters to produce ATP as the final product.

Aerobic respiration usually generates a lot of ATP molecules – at least 32. Furthermore, aerobic respiration happens when your body requires just enough energy to stay alive, carry out daily activities, and engage in cardiovascular exercises like running.

Though this process is efficient and yields a lot of energy, it is not the most efficient because it does not produce the highest amount of ATP. Furthermore, it only occurs when you’re engaged in low-intensity activities.

Anaerobic Respiration

Your body can also produce ATP without oxygen – hence the name anaerobic respiration. It is an inefficient process that only generates about 2 molecules of ATP. Anaerobic respiration usually kicks in when your body undergoes intense exercise, which requires more energy than can be provided by the available oxygen. The intense activity causes your body to partially burn glucose to produce energy without oxygen.

Anaerobic respiration is inefficient because oxygen is essential for the electron transport stage. And the absence of oxygen means fewer ATP molecules are produced. Anaerobic breakdown of glucose happens quicker than aerobic respiration because less energy is produced per glucose molecule. So, your body has to burn more glucose faster to make sufficient energy to meet your body’s demands during intense activities.

Another downside is the muscle pain that often comes with anaerobic respiration. A significant by-product of anaerobic respiration is lactic acid which builds up in your muscles and causes the burn you feel during strenuous activities. If your body has to undergo more than a few minutes of anaerobic respiration for ATP production, more lactic acid is produced, which can result in painful muscle cramps. Fortunately, the extra oxygen you inhale following the intense activity reacts with the lactic acid. This results in a breakdown of the lactic acid into carbon dioxide and water, thereby reducing or eliminating the pain.

Aerobic Lipolysis

This is also known as Beta Oxidation, Gluconeogenesis, or Fat Breakdown because your body can obtain energy by burning or breaking down fat.

A fat molecule consists of triglycerides which are made of a glycerol backbone and 3 fatty acid tails. These fat molecules are primarily stored in fat cells known as adipocytes. The adipocytes make up your adipose tissue.

For your body to obtain energy from fat, it breaks down the triglycerides into fatty acids in a process called lipolysis. The fatty acids are then converted into acetyl-CoA. The acetyl-CoA is then used in the Krebs cycle to generate ATP through aerobic respiration.

Fatty acid molecules typically have more carbons than carbohydrates. Therefore, each fat molecule yields more energy than carbohydrates. Fat is an important energy source for your body because each molecule of fatty acids generates at least 100 ATPs. So, whenever your glucose levels are low, your body can make energy from triglycerides by converting them to acetyl-CoA, which is converted into ATP through aerobic respiration.

Your body feels the need to produce energy through lipolysis after a period of not eating. This happens even with usual overnight fasting. So, after an overnight fast, most of the acetyl-CoA entering the Krebs cycle will be from fatty acids instead of glucose. However, once you have your first meal, most of the acetyl-CoA will come from the glucose in your food. Your body then stores excess glucose as glycogen or fat.

Though aerobic lipolysis is slow, it generates the highest amount of energy.

HOW YOUR BODY USES ENERGY

Your body needs energy for everything it does every day, including walking, sleeping, breathing, and even eating and digesting the food.

Basal Metabolism

Even while at rest, your body requires energy to carry out essential processes. In fact, most of the energy your body uses daily – 50 to 80 percent – is used while at rest. Your body requires this energy at rest to maintain vital functions like breathing, blood circulation, and many organ functions.

This resting energy requirement is known as Basal Metabolism. And the minimum amount of energy your body needs for these basal functions is known as the Basal Metabolic Rate (BMR).

BMR varies from one individual to another, and it depends on various factors, including:

• Age
• Sex
• Genetics
• Height
• Weight

Increasing your physical activity level can help you maintain a good BMR and even increase the number of calories you burn daily.

Food Processing - Eating, Digestion, and Metabolism.

Your body uses at least 10 percent of its energy to eat, digest, and metabolize (convert food to energy) the food you eat.

During digestion, your body uses energy to break down the food – mechanically and chemically – into smaller components that can be absorbed into your bloodstream. The small components are then absorbed (using energy) across your intestinal wall. Undigested food and waste products have to be removed from your body in a process known as elimination, which also requires energy.

Your body also uses energy to convert small molecules like fatty acids and amino acids into complex vital ones like glycogen and hormones. These complex molecules are essential for the maintenance and growth of your body’s tissues and cells.

Burning Calories during Physical Activity

If you are physically active, your body will need about 20 percent of your energy to burn calories during physical activity.

CONCLUSION

While your body requires food as an energy source, it does not automatically use the chemical energy present in your food to fuel its processes. Instead, the food is first converted into an energy form that your body can use – ATP. The produced ATP can then be used to fuel any of your body’s processes.

Your body can make ATP in various ways – aerobic respiration, anaerobic respiration, and aerobic lipolysis. Of all these, aerobic lipolysis yields the highest amount of energy. But it does not kick in until your blood glucose level is low.

Most of the energy your body produces fuels essential functions like blood circulation, breathing, and organ functions. The produced energy is also used to power your body through other activities like eating, digestion, and physical activities.