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Lesson Note
Subject: Biology
Topic: Digestive System In Invertebrates.
Lesson Objectives: By the end of the lesson, the learners should be able to explain the following:

1. Respiratory System in Mammals
2. Mechanism of Respiratory System;
3. Process of Inhalation / Inspiration in man
4. Process of Expiration in Man
5. Internal Aerobic Respiration
6. Respiration in Plants (Stomata & Lenticels)
7. Mechanism of Gaseous Exchange in Plants.

The respiratory system of mammals is the most complex of all respiratory systems. It comprises of a pair of lungs enclosed in the thorax and connected to the outside by series of branched air tube.

In human beings air can be drawn in through the mouth or nose. Both lead into the pharynx a short passage way which branches at the end into two directions. One leads to the digestive tract while the other leads to the larynx (voice box) and the lower air pathway. The entrance of the larynx is called the glottis and it is covered by cartilagenous flap (epiglottis) which prevents food from entering the wind pipe. For air to enter larynx, the glottis must remain open.

    Respiratory System in Man. Image credit: buzzle.com

The trachea (wind pipe) branches into two bronchi. The presence of cartilagenous rings in the trachea and bronchi prevents them from collapsing when the air pressure in them is low. Each bronchus leads to the lung where it branches into small tubes called bronchioles. The alveoli are richly supplied with blood capillaries and are sited or surface where gaseous exchange takes place.

As oxygen follows this pathway from the outside to the lungs, C02 is released out from the lungs to the outside via same pathway.

This entails two phases namely external and internal respiration

External Respiration (Breathing)
This is the taking in of oxygen (inspiration or inhalation) and giving out of C02 and water vapour (expiration or exhalation).


  • The intercostals muscle contract.
  • The rib are moved upward and outward.
  • The diaphragm becomes flattened.
  • There is an increase in the volume of thoracic cavity.
  • Consequently air is drawn from the nostril to the trachea, bronchi, bronchioles, and finally to the alveoli.


  • The intercostals muscle relax
  • The ribs are moved downward and inward
  • The diaphragm becomes dome shape
  • There is decrease in the volume of thoracic cavity.
  • Consequently air is drawn from the alveoli, bronchioles, bronchi, and trachea.
  • Consequently air containing waste products like CO2 and water vapour from inside the alveoli or lungs are forced out through bronchioles, bronchi, trachea, and finally to the exterior through the nose.

Inhaled and exhaled air are made up of the following:
Air Component                       Inhaled Air            Exhaled Air
Oxygen                                        21%                          16%
Carbon dioxide                          0.03%                        4%
Nitrogen                                      78%                          78%
Water vapour                            variable            saturated (higher)

The oxidation of glucose to release energy is known as cellular respiration and it occurs in the mitochondria (power house) of all living cells.
Within the cytoplasm of the cells, one molecule of 6-carbon sugar is broken down into two molecules of 3-carbon pyruvate catalysed by the enzymes in the cytoplasm. This process does not require oxygen. Each pyruvic acid is further oxidized completely to carbon dioxide and water in the mitochondria. The breakdown of glucose to pyruvic acid is termed glycolysis whileseries of chemical reactions occurring within the mitochondrion, responsible for the final breakdown of food molecules to form carbon dioxide, water, and energy carried out by seven enzymes is known as the Krebs cycle (citric acid cycle). Most of the ATPs are generated in the Krebcycle (36 ATP). A total of 38 ATP molecules are formed when one molecule of glucose is completely oxidized.

                                  KREB CYCLE

In most cells, cellular respiration takes place in the presence of oxygen and this is known as aerobic respiration. The largest amount of ATP possible is generated through it from one molecule of glucose (38 ATP).
In some other organisms, the cells gets energy from breaking down glucose in the absence of oxygen, this is known as anaerobic respiration. Only two ATPs are produced. Lactic acid often results from anaerobic respiration instead of pyruvic acid in animals which make it useful in the production of yoghurt. In plants, alcohol and carbon(IV)oxide are produced.

There is no special respiratory organ in plant. Gases move in and out of the plant through the stomata and lenticels.
Stomata: They are tiny pores in the lower epidermis of leaves. Each stoma is enclosed within two bean shaped cells known as guard cells. It regulates the opening and closing of the stomata,
Lenticels: These are breathing pores or tiny opening found in the bark of older stems. Lenticels consist of a loose mass of small thin-walled cells which permits easy diffusion of gasses in and out of the plant.

Respiration in Plants. Image Credit: shutterstock.com

Oxygen, carbon dioxide, and water vapour are released by simple diffusion process in plants. Oxygen diffuses into plants through the stomata and lenticels while CO2 and water vapour diffuses out of the plant through the same opening. This is facilitated by the difference in the concentration gradient of these gases. The plant takes in oxygen mostly during the night and gives out carbon dioxide and water vapour during the day due to photosynthetic activities of the plant. Oxygen is the by-product of photosynthesis.

The opening and closing of the stomata is regulated by the guard cells. When the guard cell is turgid, the stomata open but when the cells become flaccid the stomata are closed.

Done studying? See all previous lessons in biology.
Take a quick test for this lesson.

1. List the two phases of respiration in a named mammal.
2. Discuss briefly the process of external respiration.

3. Differentiate between inhalation and exhalation in mammals.
4. Write a short note on ‘gaseous exchange in man’.
5. What is internal respiration?
6. Differentiate between aerobic and anaerobic respiration.7. State the organs of respiration in plant
8. Describe the gaseous exchange in plants.
9. What is (a) glycolysis (b) Krebs’s cycle?

Questions answered correctly? Bravo!!

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