Last Updated: April 26, 2024


Respiration is a vital biological process through which living organisms convert nutrients into energy, crucial for survival. This process involves the inhalation of oxygen and the exhalation of carbon dioxide, occurring at both cellular and organismal levels. In cellular respiration, cells break down glucose, releasing energy stored in its chemical bonds. This energy fuels various cellular activities, ensuring the organism’s growth, repair, and maintenance

What is Respiration?

Respiration is a critical biological process that allows living organisms to obtain energy from their environment. This process can be divided into two main types: cellular respiration and breathing.

Cellular respiration is a chemical process that occurs within the cells of organisms. In this process, cells convert nutrients, primarily glucose, into ATP (adenosine triphosphate), the energy currency of the cell. This conversion involves the breakdown of glucose in the presence of oxygen, a process known as aerobic respiration. Aerobic respiration is highly efficient and produces up to 38 ATP molecules from a single glucose molecule. However, in environments lacking oxygen, cells may undergo anaerobic respiration, which is less efficient and produces only 2 ATP molecules along with byproducts like lactic acid or ethanol.

Breathing is the physical process of inhaling oxygen and exhaling carbon dioxide from the atmosphere. In animals, this process involves respiratory systems, such as lungs in mammals or gills in fish, which facilitate the exchange of gases between the organism’s blood and the external environment.

Classifications of Respiration

Respiration is a critical biological process through which all living organisms extract energy from food. This process falls into two main categories: aerobic respiration and anaerobic respiration.

Aerobic Respiration

Aerobic respiration requires oxygen to proceed and represents the most efficient form of energy production in respiration. Most animals, plants, and many microorganisms commonly use this process.

  • Process: Glycolysis initiates aerobic respiration, breaking down glucose into pyruvate. This pyruvate then enters the mitochondria, where it undergoes the Krebs cycle and oxidative phosphorylation, producing a high yield of ATP.
  • Equation: The overall chemical equation for aerobic respiration is:𝐢6𝐻12𝑂6+6𝑂2β†’6𝐢𝑂2+6𝐻2𝑂+πΈπ‘›π‘’π‘Ÿπ‘”π‘¦ (𝐴𝑇𝑃)C6​H12​O6​+6O2​→6CO2​+6H2​O+Energy (ATP)
  • Energy Yield: Aerobic respiration can generate up to 36 ATP molecules per glucose molecule, making it highly efficient.

Anaerobic Respiration

Anaerobic respiration does not require oxygen and occurs in environments where oxygen is scarce, such as in certain bacterial environments, deep-sea vents, or human muscles during intense exercise.

  • Process: Like aerobic respiration, anaerobic respiration begins with glycolysis but follows different pathways after producing pyruvate, depending on the organism and conditions. In humans, the process converts pyruvate into lactic acid. In yeasts, it results in the production of ethanol and carbon dioxide.
  • Equation: The equations can vary, but a common example (fermentation in yeast) is:𝐢6𝐻12𝑂6β†’2𝐢2𝐻5𝑂𝐻+2𝐢𝑂2+πΈπ‘›π‘’π‘Ÿπ‘”π‘¦ (𝐴𝑇𝑃)C6​H12​O6​→2C2​H5​OH+2CO2​+Energy (ATP)
  • Energy Yield: Anaerobic respiration typically yields only 2 ATP molecules per glucose molecule, making it less efficient than aerobic respiration.

Comparing Aerobic and Anaerobic Respiration

  • Oxygen Requirement: Aerobic requires oxygen; anaerobic takes place in its absence.
  • Energy Efficiency: Aerobic respiration produces more ATP, making it more efficient.
  • By-products: Aerobic respiration results in carbon dioxide and water; anaerobic respiration may produce lactic acid, alcohol, and carbon dioxide.
  • Usage: Aerobic is typical in environments with ample oxygen, while organisms use anaerobic respiration in oxygen-poor environments or during temporary oxygen deficits.

Cellular Respiration

Cellular respiration is an essential process that all living organisms use to extract energy from food molecules and convert it into a usable form, ATP. This complex biochemical pathway is fundamental for survival, driving everything from basic cellular functions to complex behaviors in multicellular organisms.

The Equation

The overall chemical equation for cellular respiration can be summarized as follows:

C₆H₁₂O₆ + 6Oβ‚‚ β†’ 6COβ‚‚ + 6Hβ‚‚O + Energy (ATP)

This equation represents the breakdown of glucose (C₆H₁₂O₆), a simple sugar that is a key energy source in many organisms. During cellular respiration, glucose combines with oxygen (Oβ‚‚) to produce carbon dioxide (COβ‚‚), water (Hβ‚‚O), and energy in the form of ATP.

Stages of Cellular Respiration

Cellular respiration includes three primary stages: glycolysis, the Krebs cycle, and oxidative phosphorylation.

  1. Glycolysis – This initial stage takes place in the cytoplasm of the cell, where glucose is split into two molecules of pyruvate. This process generates two molecules of ATP and two molecules of NADH, a carrier of electrons.
  2. The Krebs Cycle – Also known as the citric acid cycle, this stage occurs in the mitochondria. Here, pyruvate is further broken down, releasing carbon dioxide and generating more electron carriers: NADH and FADH2. This cycle also produces two ATP molecules.
  3. Oxidative Phosphorylation – In the mitochondria, the NADH and FADH2 produced in earlier stages deliver electrons to the electron transport chain. The energy released as electrons pass through this chain is used to pump protons across the mitochondrial membrane, creating a gradient. This gradient powers ATP synthase, which synthesizes a large amount of ATP by converting ADP and phosphate.


Breathing, also known as ventilation, is a physiological process essential for life in many organisms, facilitating the exchange of gases with the environment. This process primarily involves two key actions: inhalation and exhalation.

Inhalation is the intake of air into the lungs, where oxygen is absorbed into the bloodstream. During inhalation, the diaphragm and intercostal muscles (located between the ribs) contract, expanding the chest cavity and creating a lower pressure inside the lungs compared to the atmosphere. This pressure difference drives air into the lungs.

Exhalation is the release of air from the lungs, expelling carbon dioxideβ€”a waste product of cellular respirationβ€”back into the environment. In this phase, the diaphragm and intercostal muscles relax, the chest cavity contracts, and the pressure inside the lungs increases, pushing air out.

Different animals have adapted various respiratory systems to optimize breathing based on their environment:

  • Mammals use lungs with a highly branched and dense network of alveoli, tiny sacs where gas exchange occurs. This structure provides a large surface area for effective gas exchange.
  • Birds possess a unique respiratory system with air sacs that provide continuous airflow through the lungs for more efficient gas exchange than in mammals.
  • Fish breathe underwater through gills, which absorb oxygen dissolved in water. Gills have thin walls and a large surface area, enabling efficient oxygen uptake and carbon dioxide release in the aquatic environment.
  • Amphibians, like frogs, use their lungs and skin for breathing. Their skin must remain moist to allow for the diffusion of gases directly through it.

Respiration in Humans

Respiratory System in Humans

External Respiration

External respiration involves the intake of oxygen from the atmosphere and the expulsion of carbon dioxide from the blood. This process mainly occurs in the lungs through the following steps:

  1. Breathing In (Inhalation):
    • Humans inhale air through the nose or mouth, which then travels down the trachea and into the bronchi and bronchioles, finally reaching the alveoli.
    • The alveoli are tiny air sacs where gas exchange takes place. Oxygen in the inhaled air diffuses across the thin walls of the alveoli into the surrounding capillaries due to the higher oxygen concentration in the alveoli compared to the blood.
  2. Gas Exchange:
    • Blood arriving at the alveoli contains a high concentration of carbon dioxide and a low concentration of oxygen, a byproduct of cellular metabolism.
    • Oxygen binds to hemoglobin in the red blood cells, and carbon dioxide diffuses from the blood into the alveoli to be exhaled.
  3. Breathing Out (Exhalation):
    • After the exchange, the now oxygen-rich blood travels back to the heart, where it gets pumped to the rest of the body.
    • Simultaneously, carbon dioxide, a metabolic waste gas, gets expelled from the alveoli during exhalation, completing the cycle of external respiration.

Internal Respiration

Internal respiration refers to the exchange of gases between the blood and the body’s cells. This process is essential for delivering oxygen to cells and removing carbon dioxide from them. The steps include:

  1. Oxygen Transport:
    • Oxygenated blood from the lungs travels via arteries to various tissues and cells throughout the body.
    • Oxygen dissociates from hemoglobin and diffuses into the cells, driven by the cells’ lower oxygen concentration.
  2. Cellular Respiration:
    • Within the cells, oxygen is used for metabolic processes, particularly in the mitochondria, to produce energy in the form of ATP (adenosine triphosphate), water, and carbon dioxide during aerobic respiration.
  3. Carbon Dioxide Removal:
    • Carbon dioxide, produced as a byproduct of cellular respiration, diffuses out of the cells into the blood due to its higher concentration in the cells compared to the blood.
    • This carbon dioxide-rich blood returns to the lungs via veins, completing the cycle of internal respiration.


What Is the Simplest Definition of Respiration?

Respiration is the process by which cells convert nutrients into energy and expel waste.

What Is the Process of Respiration?

Respiration involves glycolysis, the Krebs cycle, and oxidative phosphorylation to produce ATP.

What Happens During Respiration?

During respiration, cells break down glucose to produce energy in the form of ATP, releasing CO2 and water as byproducts.

What Is the Definition of Respiration and Why Is It Important?

Respiration is the biochemical process that converts food into energy, essential for all biological functions and survival.

What Is the Difference Between Respiration and Breathing?

Respiration is a cellular process for energy production, while breathing is the physical act of inhaling and exhaling air.

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