Why Do Cells Need Oxygen? Cellular Respiration Explained

All of us breathe, but not many of us know why or how, and that's why we're here with the details for anyone who wants to learn more than what meets the eye.

Many inquisitive minds have wondered about why we require oxygen and what breathing does exactly in our bodies. For all you curious cats, this article is here to help and break it down to the molecules to explain the science behind why our body cells need oxygen!

Although our body has several interdependent systems, however, none of them would function without the excellent job of our body cells, and the same applies to the process of respiration as well.

Oxygen, glucose, RBCs, or hemoglobin, it's all available, but our body would never be able to sustain without the aerobic cellular respiration along with the release of energy, which is a result of this process.

From glycolysis, the citric acid cycle, and the electron transport chain to the production of pyruvate, ATP molecules, and oxidative phosphorylation, we've got it all covered.

If your mind is a universe of random unanswered questions, you might want to get them answered by checking out why do cells divide, and why do we fall.

Why do cells need oxygen?

Our body requires oxygen to harness energy by breaking food molecules into a form that will be utilized by our body, and the main ingredients in this recipe are glucose and oxygen. Voluntary and involuntary muscle movements along with the functions of cells use the process of cellular respiration as the only source of energy.

Cells require oxygen to carry out aerobic cellular respiration, which again is a collection of three processes. It all starts with glycolysis, which literally means 'sugar splitting.'

This stage can proceed without oxygen, but the yield of ATP will be minimal. Glucose molecules break down into a molecule that transports NADH, called pyruvate, carbon dioxide, and an additional two ATP molecules.

The pyruvate formed after the glycolysis process is still a three-carbon molecule compound and needs to be broken down further. Now begins the second stage called the citric acid cycle, also known as the Krebs cycle.

Cells cannot carry out this process without oxygen because the pyruvate breaks down into loose hydrogen and carbon, which needs to go through oxidation to produce more ATP molecules, NADH, carbon dioxide, and water as a byproduct.

If this process were to take place without oxygen, the pyruvate would go through fermentation, and lactic acid is released. The third and final stage is oxidative phosphorylation which involves the electron transport change and cannot proceed without oxygen.

Electrons are taken to special cell membranes by transporters called FADH2 and NADH. The electrons are harvested here and ATP is produced.

Used electrons get depleted and cannot be stored in the body which is why they bind with oxygen and later with hydrogen to form water as a waste product. Therefore, oxygen in cells is important for all these stages to perform efficiently.

What is cellular respiration?

A chain of metabolic processes and reactions take place inside a cell to generate ATP molecules and waste. This process is called cellular respiration and takes place in three processes which convert the chemical energy in our body's nutrients and oxygen molecules to produce energy.

All the reactions that occur during cellular respiration have the sole purpose of generating energy, or ATP, by converting the energy from the food we eat.

Nutrients that are used up during respiration to produce energy include amino acids, fatty acids, and sugar while oxidation processes need oxygen in its molecular form because it provides the most amount of chemical energy. ATP molecules have energy stored in them, which can be broken down and used to sustain cellular processes.

Respiratory reactions are catabolic and involve breaking large, weak high-energy bond molecules, like molecular oxygen, and replacing them with stronger bonds to release energy.

Some of these biochemical reactions are either redox reactions, where the molecule undergoes reduction, while the other goes through oxidation.

Combustion reactions are a type of redox reaction that involves an exothermic reaction between glucose and oxygen during respiration to produce energy.

Although it may seem like ATP is the final required energy source for the cells, it isn't. ATP is further broken down into ADP which is a more stable product that can efficiently help carry out the processes that require energy in the cells.

If you are wondering which cell functions require aerobic respiration, they include molecule transportation or locomotion across cell membranes and biosynthesis to form macromolecules.

How does oxygen reach the blood?

By now, we have understood the overall importance of oxygen and how our cells used oxygen to function normally. One question still stands unanswered, and that's how does this oxygen reach the bloodstream in the first place.

As we breathe, oxygen, nitrogen, and carbon dioxide present in the air make their way into our lungs, and upon entering the alveoli, it diffuses into the blood. Of course, it's not as simple as it sounds, so let's understand it in detail.

Even though the human body depends on nutrition for energy, this source makes up only 10% of the energy stored in our body whereas oxygen makes up around 90%!

This oxygen is required by every cell in our body and is transported through blood via our vascular and respiratory systems, which include our nose, lungs, heart, arteries, veins, and eventually, the cells. It all begins with breathing because the respiratory organs are the gateway for oxygen to enter your body.

Oxygen absorption present in the air is facilitated by the nose, mouth, trachea, diaphragm, lungs, and alveoli.

The basic process involves oxygen entering the nose or mouth, passing through the larynx and into the trachea.

Here, the air is prepared to suit the environment inside our lungs. Minute capillaries are found in abundance in the nasal cavity, and the warmth from this blood gets transferred to the cold air that enters our noses.

Then, the cilia present in the larynx and pharynx trap any dust particles or foreign bodies to avoid them from reaching the lungs.

Lastly, the goblet cells in the nasal cavity and respiratory tract secrete mucus which moistens the air along the way.

All these functions perform together so that our lungs get direct air without allowing any particles to get trapped in the lungs. After the air passes through the bifurcating bronchial tubes, the air is led into a network of around 600 million small sacs with a membrane that has pulmonary blood capillaries, these are called alveoli.

Due to the low concentration of oxygen in the blood and higher concentration in the lungs, the oxygen diffuses into the pulmonary capillaries.

Once the oxygen enters the bloodstream, it binds itself to the hemoglobin in red blood cells.

These capillaries transport the oxygen-rich blood into the pulmonary artery, from where it enters the heart. The heart synchronizes the respiration process by filling up with blood before each heartbeat and contracting to expel blood into the arteries to be taken to its respective zones.

The left ventricle and auricle of the heart pump oxygenated blood to the body while the right ventricle and auricle send deoxygenated blood from the body back to the lungs for the production and release of carbon dioxide.

With every beat, the arteries carry around 1.1 gal (5 l) of oxygenated blood away from the heart and into the systems throughout the body.

Whereas the veins are responsible for taking blood containing carbon dioxide back to the heart and into the lungs. Humans would never exist without this intricate process that is required for the production of energy.

Oxygen is a key component to generate energy for our cells in the form of ATP, which is essential to carry out various functions such as replacing old muscle tissue, building new muscle tissue or cells, and dispose the waste from our system.

How does cellular respiration happen?

As mentioned earlier, cellular respiration in humans is a system of three stages, four if you count one tiny step; glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation. The entire process ultimately involves using oxygen to generate energy for the cells in the form of the produced ATP molecule.

However, there are two types of cellular respiration, aerobic and anaerobic, the energy produced in the latter is does not need to use oxygen.

Glycolysis is the first step of aerobic cellular respiration that takes place in the cytosol, in which a six-carbon molecule of glucose is split into two three-carbon molecules which are phosphorylated by ATP to add a phosphate group to each of those molecules.

The second batch of the phosphate group is added to these molecules. Later, the phosphate groups are released from the phosphorylated molecules to form two pyruvate molecules and this final split produces releases energy that creates ATP by adding phosphate groups to ADP molecules.

From the cytosol, cellular respiration carries on into the mitochondria by letting pyruvate and oxygen penetrate through its external membrane, and without oxygen, further steps are incomplete.

In case of oxygen absence, the pyruvate goes through fermentation.

In humans, homolactic fermentation is observed during which an enzyme converts the pyruvate into lactic acid to prevent NADH accumulation and allow glycolysis to continue producing small amounts of ATP.

Next in the cellular respiration process comes the Krebs Cycle. When the three-carbon pyruvate enters the membrane of the mitochondria, it loses on carbon molecule and forms a two-carbon compound and carbon dioxide.

These byproducts are oxidized and bind with an enzyme called coenzyme A to form two molecules of acetyl CoA, linking carbon compounds to a four-carbon compound and generating six-carbon citrate.

Throughout these reactions, two carbon atoms are released from the citrate forming three NADH, one FADH, one ATP, and carbon dioxide molecules.

The FADH and NADH molecules perform further reactions in the internal membrane of the mitochondria to facilitate the electron transport chain. The last step of cellular respiration is the electron transport chain which has four complex proteins and begins when NADH electrons and FADH electrons are passed on to two of these proteins.

These protein complexes carry the electrons through the chain with a set of redox reactions during which energy is released and protons are pumped by the protein complex into the inter-membrane space of the mitochondria.

After the electrons go through the last protein complex, oxygen molecules bind with them.

Here an oxygen atom combines with two hydrogen atoms to form molecules of water.

Then, the higher concentration of protons in the intermembrane space attracts them inside the inner membrane, and the ATP synthase enzyme offers passage for these protons to penetrate the membrane. During this process, ADP is converted to ATP after the enzyme uses the proton energy, providing stored energy in the ATP molecules.

Even though a cell does not directly eat food, this entire respiration process helps it produce energy and stay alive.

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