This blog post provides readers with the following objectives. The reader will be able to:

o     Explain the concept respiration.  
o     Explain the concept of gaseous exchange. 
o     Outline the breathing mechanism in humans.
o     Cellular respiration


Respiration is a sum total of chemical reactions which result in the breakdown of food substance to release energy with or without the use of oxygen. All organisms require energy to sustain life. Respiration takes place in the mitochondria inside living cells.

Respiration involves:

1.     External respiration: is the uptake of oxygen and simultaneous elimination of carbon dioxide and water. This is commonly referred to as breathing or gaseous exchange. 

2.  Internal respiration: is also known as cellular or tissue respiration. Internal respiration is a series of chemical reactions within the cell in which organic molecules are oxidized to release energy. 

Structure of Respiratory System in human

the lungs

Structure of the Respiratory System

The respiratory system is composed of the following parts.

o  Nasal cavity: is divided into two portions by a cartilaginous septum. It is lined by fine hairs that filter the dust particles from the air. It opens into the region called the pharynx.

o   Pharynx is common to both food and air. This allows passage of air in case the nose is blocked. Pharynx continues into glottis. Glottis is guarded by a flap of tissue called the epiglottis.

o   Larynx is also called the voice box. The vocal cords stretch and vibrate when the air passes through them. Larynx continues as the trachea after the cords.

o   Trachea is held open by C-shaped rings of cartilage. The cartilage keeps the trachea from collapsing. The trachea branch into two main branches called bronchi (sing: bronchus)

o   Bronchus is also supported by the cartilaginous rings. The trachea and the bronchi are lined with ciliated cells and secretory cells (goblet cells). The goblets cells secrete mucus which trap fine particles of dust or bacteria. The cilia sweep trap particles along with the mucus toward buccal cavity for elimination.

The bronchus branches into several bronchioles. The bronchioles end as fine tubules which opens into a sac called alveolus or air sac

o   Lung is situated in the thoracic cavity. It’s a spongy and elastic organ that is broad at the bottom and taper at the top. It consists of air sacs, (the alveoli). Each lung is enclosed by two membranes called pleural membrane.

Diaphragm: is a thick membranous structure present below the lungs. It separates the thoracic from the abdominal cavity.

Mechanism of Breathing

This is the process by which the lungs expand to take in air then contract to expel it. The cycle of respiration consists of two phases: Breathing in (Inspiration or Inhalation) and Breathing out (Expiration or exhalation)



External intercostal muscles contract.

External intercostal muscles relax.

Ribs and sternum move up and out

Rib and sternum move down and in

Width of chest increases

Width of chest diminishes

Diaphragm contracts                                            

Diaphragm relaxes 

Depth of chest increases

Depth of chest diminishes

Capacity of thorax is increased 

Capacity of thorax is decreased

Pressure between pleural surfaces is reduced

Pressure between pleural surfaces is increased

Elastic tissue of lungs is stretched

Elastic tissue of lungs recoils

Air is sucked into alveoli from atmosphere

Air is forced out of alveoli to atmosphere

Diagram illustration of breathing mechanism, Inhalation and Exhalation

rathing mechanis,

Gaseous Exchange

The main respiratory surface in humans is the alveolus. Alveolus is one-cell thick, highly vascularized and provide a moist and extremely large surface area for gas exchange to occur. 

Inhaled oxygen is able to diffuse into the blood capillaries from the alveoli, while carbon dioxide from the blood diffuses in the opposite direction into the alveoli. The oxygen combines with hemoglobin to form oxyhemoglobin which is transported in the plasma. Hemoglobin does not release all of its oxygen as it passes through the body tissues. It releases its oxygen when; the concentration of O2 is low; high concentration of CO2 and high temperature.


Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate ions (HCO3). This reaction is catalyzed by the carbonic anhydrase enzyme in red blood cells.

The hemoglobin picks up the H+, preventing the blood from becoming acidic. The bicarbonate ion diffuses into the plasma where it is transported.

In the lungs, bicarbonate ions enter red blood cells, hemoglobin releases its hydrogen ions, and CO2 is released. As blood passes through the lungs, HCO3- + H+ form H2CO3 which then forms CO2 + H2O. The waste carbon dioxide can then be exhaled out of the body.

Composition of Air


Breathed In

Breathed Out







Carbon Dioxide



Tracer Gases



Water vapour



Check out Electric Human Respiratory System Model for Teaching 

Respiratory System Model

Cellular Respiration

There are two types of cellular respiration: Aerobic and Anaerobic respiration.

Aerobic Respiration                                                                                 

This is the breakdown of glucose in living cells to provide energy in the presence of oxygen. It’s carried out by the vast majority of organisms. 

Aerobic respiration takes place in three stages glycolysis, Krebs cycle or tricarboxylic acid (TCA) cycle and electron transport chain.

Anaerobic Respiration        

This occurs in some organisms when glucose is broken down to release energy in absence oxygen. In humans, muscle cells respire anaerobically and the by-product is lactic acid. Plant and yeast cells respire anaerobically, producing alcohol as a by-product. Lactic acid fermentation and alcoholic fermentation are two types of anaerobic fermentation.

Anaerobic respiration reaction or fermentation

Difference between Aerobic and Anaerobic Respiration

Aerobic respiration

Anaerobic respiration

Water is produced as waste product

Alcohol or Lactic acid is produced as waste product

Oxygen is used up

Oxygen is not required

Complete breakdown of glucose

Partially breakdown of glucose

More energy released

Less energy is released

Takes place in mitochondria

Takes place in cytoplasm

more efficient

less efficient

Respiratory Quotient

During aerobic respiration, O2 is consumed and CO2 is released. The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

RQ  =  Volume of CO2 evolved
           Volume of O2 consumed

The respiratory quotient depends upon the type of respiratory substrate used during respiration. When carbohydrates are used as substrate and are completely oxidized, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below:

determine respiratory quotients on carbohydrates

When fats are used in respiration, the RQ is less than 1. Calculations for a fatty acid, oleic, tripalmitin, if used as a substrate is shown:

determine QR of fat and oil (lipids)

When proteins are respiratory substrates, the ratio would be about 0.9. In living organisms, respiratory substrates are often more than one; pure proteins or fats are not used as respiratory substrates.


Glycolysis is the anaerobic catabolic reaction, where a molecule of glucose is converted into two molecules of pyruvic acid or pyruvate. In the eukaryotic cells, it occurs in the cytosol. It yields a pyruvate molecule, four molecules of ATP and two NADP molecules. Both ATP and NADP molecules are energy-rich and are used in other cell reactions.

Steps in the Glycolysis

1. Phosphorylation of glucose by enzymes called hexokinases to form glucose-6-phosphate.

2. Glucose-6-phosphate is then rearranged into fructose-6-phosphate by glucose phosphate isomerase.

3. Fructose-6-phosphate is phosphorylated by ATP to form fructose 1, 6-disphosphate by phosphofructokinase

4. Fructose 1, 6-disphosphate molecule split by aldolase into two 3C sugars (isomers), namely dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Dihydroxyacetone phosphate changes to glyceraldehyde 3-phosphate to form 2 molecules of glyceraldehyde 3-phosphate.  

5. Phosphate is added to glyceraldehyde 3-phosphate to form 1, 3-bisphosphoglycerate by enzyme dehydrogenase.  Hydrogen atoms released and picks up by NAD to form NADH2  

6. Enzyme phosphoglycerate kinase converts 1, 3-disphosphoglycerate to glycerate-3-phosphate.

7. Glycerate-3-phosphate is then converted into glycerate 2-phosphate by Phosphoglycerate mutase

8. Glycerate-2-phosphate is then converted to phosphoenolpyruvate by Enolase. H2O is released as a result.

9. Phosphoenolpyruvate is hydrolyzed to form pyruvate with the synthesis of ATP, catalyzed by the enzyme pyruvate kinase

Fate of Pyruvate

The fate of pyruvate depends on the following factors; the cell in which the pyruvate was produced (animal cell, plant cell or bacterial cell) and whether there is presence of oxygen or not.

1.  Lactic Acid Fermentation: In this process, the pyruvate is converted to lactate. Formation of lactate is catalyzed by lactate dehydrogenase.

a.      This process occurs in the bacteria and involved in making of yogurt

fate of pyruvate

b.   In animal cells, during exercise (strenuous muscular activity), when oxygen is inadequate for cellular respiration, pyruvate is reduced to lactate by lactate dehydrogenase. The building up of lactate causes drop in pH which result in energy deprivation and cell death. The symptoms are muscle pain and fatigue. During recovery, lactate is transported to the liver where it can be reconverted to pyruvate or glucose.

2.  Ethanol Fermentation: In this process, the pyruvate is converted to acetaldehyde and carbon dioxide, then to ethanol. It occurs in some organism such as yeast and plants. Formation of ethanol is catalyzed by two enzymes; Pyruvate decarboxylase catalyzes the first irreversible reaction to form acetaldehyde:

fate of pyruvate

Ethanol fermentation is used during wine-making.

Lactic acid fermentation and ethanol fermentation can occur in the absence of oxygen.

3.      Aerobic conditions: In the presence of O2 pyruvate is converted to Acetyl-CoA which enters Citirc acid cycle and gets completely oxidized to CO2, water, and 36 ATP.

Citric Acid Cycle

Citric acid cycle is also known as Krebs cycle or tricarboxylic acid cycle (TCA cycle).  It takes place in the mitochondrial matrix. Pyruvate is decarboxylated (a carboxyl group is removed) to form 2-C, acetate catalyzed by enzyme dehydrogenase. Two electrons are removed from pyruvate and accepted by NAD to form NADH2.

The acetate is converted into Acetyl-CoA by combining with co-enzymes A (CoA) and enters the citric acid cycle.

Steps of the Krebs Cycle

o   The unstable bond of acetyl CoA breaks and the 2-C acetyl group (acetate) combines to the four-carbon oxaloacetate to form citrate (6-C).

o   The citrate then goes through a series of chemical transformations to form isocitrate.

o   Isocitrate is oxidized to α-ketoglutarate (5-C). Electrons are released and accepted by NAD to form NADH.

o   The α-ketoglutarate oxidized and decarboxylated to form succinate (4-C).

o   The succinate is oxidized to malate. The electrons released are accepted by FAD and reduced to FADH2.

o   The malate molecule is oxidized by a NAD to form NADH. Oxaloacetate is regenerated to begin the cycle again.

Electron Transport Chain

At the end of the Krebs cycle, most of the energy extracted from glucose is in coenzymes (NADH and FADH2). These reduced coenzymes are oxidized giving up protons (H+) and electrons. The electrons are carried through electron transport chain.

The electron transport chain is made of electron carrier molecules located in the inner mitochondrial membraneThe hydrogen ions (electrons) are transported from one carrier molecule to another and are finally used to reduce oxygen to water.

During this transfer of electrons, lot of energy is released which is in the form of ATP by a process called oxidative phosphorylationATP is thus an energy rich molecule and can be called the energy currency of the cell.

Sequence of Electron Transfers and ATP Synthesis

1.  NADH is oxidized by passing high energy electron to FAD.  FAD is reduced to FADH2.

2.  FADH2 is oxidized as it passes electrons to an iron-sulfur protein (cytochrome).

3.  Iron-sulfur protein is oxidized as it passes electrons to ubiquinone (another form of cytochrome).

4.  Ubiquinone passes electrons on to a succession of electron carriers, most of which are cytochromes.

5.  The last cytochrome passes electrons to molecular oxygen, O2. As molecular oxygen is reduced it picks up two protons to form water.

6.  The electron transport chain does not make ATP directly. It generates a proton gradient across the inner mitochondrial membrane, which stores potential energy that can be used to phosphorylate ADP.

7.  The inner mitochondrial membrane has many copies of a protein complex, ATP synthase. This complex is an enzyme that makes ATP. 

Comparison between Photosynthesis and Cellular Respiration


Cellular Respiration

Requires energy

Releases energy as ATP molecules

Produce O

Ois used up

Occurs only in the presence of sunlight.

Occurs at all times.

Occurs in Chloroplasts

Occurs in mitochondrial, cytoplasm.

6CO+ 12H2O + light → C6H12O+ 6O+ 6H2O

C6H12O6 + 6O→ 6CO+ 6H2O + energy

Produces food and captures energy.

Breakdown of food, releases energy.

2 stages - Light dependent reaction and Light independent reaction.

4 stages - Glycolysis, Pyruvate oxidation, Kreb's cycle and Electron transport chain

Click Here for WAEC Past Questions and Answers on Respiration in Mammals

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