GASEOUS EXCHANGE THEORY & QUESTION ANSWERS Chapter # 13

GASEOUS EXCHANGE

All living organisms require energy to perform various activities. They obtain energy by ATP. The continuous supply of ATP is made possible through respiration. Thus living organisms are always in need of gaseous exchange.

GASEOUS EXCHANGE IN PLANT:
Plants perform gaseous exchange during two processes.

• Photosynthesis
• Respiration

In plants, photosynthesis takes place during the day, where plants absorb CO₂ and release O₂. Respiration, however, takes place constantly, both day and night.

Gaseous Exchange in Unicellular Plants:
In unicellular and lower plants, gaseous exchange occurs through the cell membrane and moist body surface by diffusion.

Gaseous Exchange in Multi-cellular Plants:
In multicellular and higher plants, gaseous exchange occurs throughout the structure of leaves and stems. These plants have two types of respiratory structures:

• Stomata
• Lenticels

Gaseous Exchange through Stomata:
Stomata are microscopic pores located on both surfaces of leaves. Each stoma is surrounded by two bean-shaped cells called guard cells. Each guard cell has chloroplasts, with outer thinner and inner thicker walls. Guard cells control the opening and closing of stomata, allowing gaseous exchange to occur via diffusion.

Gaseous Exchange through Lenticels:
Woody stems have localized regions of loosely arranged cells with intercellular air spaces called lenticels. Through lenticels, respiratory gases can move freely in and out of the stem by diffusion.

Photorespiration:

Photorespiration is a process in which plants utilize oxygen and release carbon dioxide during daylight. Plants that perform photorespiration are biochemically classified as C₃ plants, examples of which include wheat, rice, maize, and sugarcane. 

MECHANISM:
During hot and dry days in summer, stomata are close in order to conserve water, and the rate of transpiration becomes reduced. The concentration of oxygen increases, then the carbon dioxide. In this condition, Ribulose biphosphate (RuBP) is combining with the oxygen instead of carbon dioxide in the presence of an enzyme ribulose biphosphate carboxylase / oxygenase or rubisco. It breaks into two compounds: phospho glyceric acid and phosphoglycolate.

$\text{RuBP} + \text{O}_2 \rightarrow \text{PGA} + \text{Phosphoglycolate}$

Phosphoglycolate is broken down to release $\text{CO}_2$.

$\text{Phosphoglycolate} \rightarrow \text{Serine} + \text{CO}_2$

Photorespiration is a useless process because there is no energy produced like in the process of respiration.

GASEOUS EXCHANGE IN ANIMALS:
Animals exchange gases during respiration. They take in oxygen and give out carbon dioxide constantly. Respiratory gases move across most respiratory surfaces by diffusion.

GASEOUS EXCHANGE IN HYDRA:
Hydra is a simple multicellular animal. It belongs to the Phylum Coelenterata. Hydra has a diploblastic body, i.e., the body consists of two cell layers.

• Ectoderm
• Endoderm

Respiratory organs are absent in hydra. It has aerobic respiration.

• Respiration Surface of Hydra:
  The ectoderm acts as the respiratory surface. The gaseous exchange in hydra takes place through this surface.

• Mechanism of Gaseous Exchange:
  In hydra, gaseous exchange takes place by diffusion. The ectoderm of hydra is permeable for oxygen and $\text{CO}_2$. During respiration, $\text{O}_2$ is diffused inside the body through ectoderm. Then $\text{CO}_2$ is also diffused throughout body cells by continuous diffusion. The resultant $\text{CO}_2$ exits out through ectoderm by diffusion.

GASEOUS EXCHANGE IN EARTHWORM:
The earthworm is a complex multicellular animal. It belongs to phylum Annelida. It has a cylindrical segmented body. The earthworm has aerobic respiration. The respiratory system is absent in the earthworm. The blood circulatory system of the earthworm helps in the transportation of gases in the body.

• Respiratory Surface of Earthworm:
  The outer skin of the earthworm acts as the respiratory surface. It is permeable and provides a greater surface area for respiration. The skin of the earthworm has goblet glands, which secrete mucus. Mucous keeps the skin moist. Beneath the skin, the network of blood vessels is also present.

Mechanism of Gaseous Exchange:
In earthworm, gaseous exchange takes place through moist skin. During respiration, $\text{O}_2$ enters by diffusion inside the body through skin. Blood absorbs the $\text{O}_2$ and transports it throughout body cells by circulation. The resultant $\text{CO}_2$ exits through skin by a similar procedure.

GASEOUS EXCHANGE IN COCKROACH:
Cockroach is a complex multicellular animal. It belongs to the Phylum Arthropoda. It has externally segmented body and jointed legs. Cockroach has aerobic respiration. It has evolved a special type of system which is termed as tracheal system.

Respiratory System of Cockroach:
The tracheal system of cockroach consists of two organs.

• Spiracles
• Trachea

Cockroach has an open-type blood circulatory system, but the blood of cockroach does not help in the transportation of gases because hemoglobin is absent in blood.

• Spiracles:
  They are small openings which are present on both lateral sides of the body. Cockroach has 10 pairs of spiracles. According to the position, they are divided into two types:

  • Thoracic spiracles

  • Abdominal spiracles

  • Thoracic Spiracles:
    Those spiracles which are present at the thorax region are called thoracic spiracles. There are two pairs in number. Thoracic spiracles are present at the junction of thoracic segments. They have outward opening valves and help in expiration.

  • Abdominal Spiracles:
    Those spiracles which are present at the abdominal region are called abdominal spiracles. There are 8 pairs in number and are present at the 8 segments of the abdomen. They have inward opening valves and help in inspiration.

• Trachea:
  Trachea are tube-like structures in which spiracles are open. The wall of the trachea is composed of a single layer of epithelial cells. The wall also contains cartilaginous rings. The fine branches of trachea, called tracheoles, are filled with a fluid called tracheal fluid. Tracheoles are present near cells or tissues, and they are blind from the anterior end.

Mechanism of Gaseous Exchange:
The mechanism of gaseous exchange in cockroach is completed in two continuous steps.

• Inspiration
• Expiration

• Inspiration:
  The rushing of air inside the body is called inspiration. In cockroach, during inspiration, air enters through abdominal spiracles and finally reaches into tracheoles by trachea. Air is dissolved in tracheal fluid; then $\text{O}_2$ of air is diffused inside the cell by diffusion.

• Expiration:

        The outward movement of air from the body is called expiration. During expiration, the resultant  

${\text{ CO}}_2$CO2​ is diffused in air from cells by diffusion. By inward movement of sternum and tergum,            pressure is exerted on the trachea, and air is expelled out through thoracic spiracles.

GASEOUS EXCHANGE IN FISH:
Respirator organs in fish and other aquatic animals are gills.

• Structure of Gills:
  They are formed as an outgrowth of the pharynx and lie within the body. Each gill is a highly vascularized structure consisting of two rows of hundreds of filaments, which are arranged in a V-shape, supported by the cartilage called gill arc or gill bar. Each filament is folded into numerous plate-like structures called lamellae that give the gills a greater surface area and are provided with a network of blood capillaries. Each gill is covered by an operculum, and the cover either opens through gill slits.

• Mechanism of Gaseous Exchange:
  Water enters into the mouth, passes over the gill through the pharynx, and exits back through the opercula. Since the concentration of oxygen in water is low and water is denser than air, fish must use considerable energy to ventilate its gills.

• Counter Current Flow:
  Gaseous exchange in gills is also facilitated due to the counter-current flow of water and blood. The blood flows in a direction opposite to the movement of water in capillaries of lamellae across the gills. Thus, the most highly oxygenated blood is brought close to the water, just entering the gills, and that has even higher oxygen content than the blood. As the water flows over the lamellae, it gradually loses its oxygen to the blood; it encounters the blood, i.e., also increasingly in oxygen. In this way, the gradient encouraging oxygen to move from water into the blood is maintained across the lamellae. Counter-current flow of water is effective as it enables the fish to extract up to 80% - 90% of oxygen from the water that flows over the gills.

GASEOUS EXCHANGE IN FROG:
The frog is a complex multicellular animal. It belongs to phylum Chordate. Frog belongs to class Amphibia because it is found in both water and land. The frog has aerobic respiration. It has three types of respiration:

• Cutaneous respiration
• Buccal respiration
• Pulmonary respiration

• Cutaneous Respiration:
  Respiration takes place through the skin, called cutaneous respiration. This type of respiration is found in frogs when they are present inside the mud during the hibernation season.

• Buccal Respiration:
  Respiration takes place through the buccal cavity, called buccal respiration. This type of respiration is found in frogs when they are in water.

• Pulmonary Respiration:
  Respiration takes place through the lungs, called pulmonary respiration. This type of respiration is found in frogs when they are on land.

Pulmonary Respiratory System of Frog:
The pulmonary respiratory system of frogs consists of the following organs:

• Nostrils

• Bucco-pharyngeal Cavity

• Glottis

• Larynx

• Bronchus

• Lungs

• Nostrils:
  They are small openings. There are two pairs in number. One pair is present outside, while the other pair is present inside. Nostrils help as a passage of air.

• Bucco-pharyngeal Cavity:
  Buccal cavity and Pharynx are collectively called the bucco-pharyngeal cavity. The floor of this cavity is movable. It also helps as a passage of air.

• Glottis:
  It is a small opening. The respiratory system and digestive system are separated through the glottis. The opening of the glottis is guarded by a flap called the epiglottis.

• Larynx:
  It is a chamber-like structure. It is also known as the voice box. The walls of the larynx are composed of cartilage.

• Bronchus:
  They are hollow tube-like structures that open into the lungs. The passage of air takes place through the bronchus.

• Lungs:
  The lungs are balloon-shaped structures. A single pair of lungs is present. Lungs have a pinkish-reddish color. The outer surface of the lungs is smooth, while the inner surface is folded. Both the lungs are composed of thin-walled small chambers called alveoli or air sacs. At the upper surface of alveoli, blood capillaries are present. Alveoli help in the exchange of gases.

Mechanism of Gaseous Exchange:
During inspiration, air enters through nostrils into the bucco-pharyngeal cavity. At that time, the glottis is closed. Due to the air, the floor of the bucco-pharyngeal cavity moves downward. After that, the nostril is closed, and the glottis is open. Then the floor of the bucco-pharyngeal cavity moves upward, due to which air rushes into the lungs. Inside the lungs, $\text{O}_2$ is absorbed in the blood, and $\text{CO}_2$ comes out of the lungs from the blood. Due to the contraction of the lungs, air is expelled out with $\text{CO}_2$. Frogs have incomplete ventilation because lungs are not completely empty with air.

GASEOUS EXCHANGE IN BIRDS:
Birds are complex multicellular animals. They belong to Phylum Chordata. Birds have aerobic respiration. They have complete ventilation. The ventilation of air takes place in a unidirectional manner. Birds have evolved the most efficient respiratory system.

Respiratory System of Birds:
Birds have developed a complex respiratory system. The respiratory system consists of two organs.

• Lungs:
  Lungs are balloon-shaped structures. A pair of lungs is present inside the abdominal cavity at both sides of the vertebral column.

  Lungs are muscularized and vascularized structures because they consist of muscles and a large number of blood vessels. The outer surface of the lungs is smooth, while the inner surface is divided into numerous small, highly vascularized, thin membranous channels known as parabronchi.

• Air Sacs:
  Birds have 8–9 non-vascularized structures called air sacs. Air sacs are present around the lungs, and they also connect with parabronchi. Some air sacs are also found inside the bone.

Mechanism of Gaseous Exchange:
Birds have complete ventilation or unidirectional flow of air. During exchange, birds first inspire air; inspiratic air passes through lungs and stores inside air sacs, then birds perform second inspiration. In the second inspiration, air pushes the first inspiratic air into the lungs, where gaseous exchange takes place through parabronchi. After expired air is removed, the one-way flow of air enables a bird to fly at very high altitudes without any storage of oxygen.

GASEOUS EXCHANGE IN MAN:
Man is a complex multicellular and social living organism. It belongs to Phylum Chordata. Man has developed a complex respiratory system that helps in gaseous exchange. It has aerobic respiration. Man has complete ventilation in two directions.

Respiratory System of Man:
Man has developed a complex respiratory system. The respiratory system of man consists of nine organs.

• External Nares or Nostrils

• Nasal Cavity

• Internal Nares

• Pharynx

• Glottis and Epiglottis

• Larynx

• Trachea

• Bronchus

• Lungs

• External Nares or Nostrils:
  These are small openings present at the terminal part of the face just above the mouth and are one pair in number. The wall of external nares is composed of elastic cartilage.

  • Function:
    During inspiration and expiration, the passage of air takes place through external nares.

• Nasal Cavity:
  Each external nare opens behind into a cavity called the nasal cavity. The nasal cavity and buccal cavity are separated by a plate called the palate. The wall of the nasal cavity is also composed of elastic cartilage. The inner wall is lined with ciliated epithelial cells, which secrete mucus. Inside the nasal cavity, small hairs are also present.

  • Function:
    The mucus of the nasal cavity keeps the nose moist. Small hairs filter the air of germs and dust particles. The nasal cavity also helps in the passage of air during inspiration and expiration.

• Internal Nares:
  These are one pair of small openings present just above the junction of the buccal cavity and pharynx. The wall of internal nares is composed of elastic cartilage.

  • Function:
    Passage of air takes place during inspiration and expiration through internal nares.

• Pharynx:
  It is a small tubular structure with muscular walls. The buccal cavity and nasal cavity both open into it. The pharynx is present inside the neck region.

• Function:
  The pharynx performs two functions:
• Passage of air during respiration
• Passage of food during digestion

• Glottis and Epiglottis:
  Glottis is a small opening present in the wall of the pharynx. The respiratory and digestive tracts are separated through the glottis. The opening of the glottis is guarded by a flap called the epiglottis. Movement of the epiglottis is involuntary.

  • Function:
    Passage of air takes place through the glottis.

• Larynx:
  It is a small chamber-like structure. The larynx is also present inside the neck. The wall of the larynx is composed of cartilage. The larynx is also known as the sound box. Inside the larynx, one pair of cartilaginous structures is present, called vocal cords.

  • Function:
    The larynx performs two functions:
    • Passage of air takes place through the larynx.
    • It helps in producing sound through vocal cords.

• Trachea:
  The trachea is a long, straight tube-like structure. It is also known as the windpipe. The trachea is present inside the chest cavity. The wall of the trachea is composed of ciliated epithelial cells. The wall also contains C-shaped cartilaginous rings that prevent it from collapsing during air drawing. The wall of the trachea also contains goblet cells that secrete mucus. Mucus keeps the wall of the trachea moist.

  • Function:
    Passage of air takes place during inspiration and expiration.

• Bronchus:
  Near the lungs, the trachea is divided into two branches; each branch is known as a bronchus. Each bronchus enters into the lungs and divides into fine and small branches called bronchioles. The walls of the bronchi and bronchioles are composed of ciliated epithelial cells.

  • Function:
    Passage of air takes place during inspiration and expiration through the bronchi.

• Lungs:
  Lungs are the main respiratory organs. They help in gas exchange.

• Shape:
  Man has sac-like or balloon-shaped lungs.

• Number:
  Man has one pair of lungs.

• Colour:
  Both lungs have a pinkish-red colour. This colour appears due to the presence of blood capillaries on the lungs.

• Location:
  Lungs are located inside the thoracic or chest cavity.

• Protection:
  Both lungs are protected inside the bony cage. The bony cage is produced by the sternum bone at the ventral surface, ribs at both lateral surfaces, and the vertebral column at the dorsal surface. At the lower surface, both lungs are supported by the diaphragm.

• Covering:
  Both lungs are covered by a double-layered membrane called the pleural membrane. Between both layers, a fluid is present called pleural fluid. Pleural fluid develops pressure called pleural pressure, which helps in inspiration and expiration. Pleural fluid also keeps the lungs moist.

MORPHOLOGY OF LUNGS:
Externally, lungs have a smooth surface. Lungs have a large number of blood capillaries on their surface. Externally, lungs are divided into lobes. The right lung consists of three lobes, while the left lung consists of two lobes.

Anatomy of Lungs:
Internally, lungs are folded in structure; they are spongy in nature. Lungs are muscularized and vascularized structures. Each lung consists of small sac-like structures called air sacs or alveoli.

Structure of Alveoli:
Alveoli are small sac-like structures; both lungs contain 700 million alveoli. The wall of the alveoli is composed of a thin layer of epithelial cells. Each alveolus has a branch of bronchiole. On the surface of the alveoli, a network of blood capillaries is also present, which helps in gaseous exchange.

Function of Lungs:
Lungs help in gaseous exchange between alveoli and blood.

• Mechanism of Gaseous Exchange:
  During inspiration, air enters inside the lungs. The oxygen of air is absorbed in the blood, which is present on the alveoli surface through diffusion. Absorption of oxygen takes place by the binding of Hemoglobin with oxygen. Similarly, carbon dioxide is released into the air of alveoli from blood by diffusion.

• Mechanism of Breathing:
  The process of breathing takes place as a result of two continuous processes.

  • Inspiration (Inhalation)

  • Expiration (Exhalation)

  • Inspiration (Inhalation):
    "The process in which air enters into the lungs from the environment through the respiratory tract called inspiration."

    Mechanism:
    During inspiration, the outer layer of intercostals and diaphragm muscles both contract, due to which ribs move in an upward direction while at the same time, the diaphragm moves in a downward direction. An area is produced inside the chest cavity to expand the lungs during inspiration. At the same time, pleural pressure is reduced on the lungs. Air enters inside the lungs through the respiratory tract, and the process of inspiration takes place.

  • Expiration (Exhalation):
    "The movement of air from lungs to the environment called expiration."

    Mechanism:
    Expiration is the process opposite to inspiration. During expiration, the inner layer of intercostal muscles contract, and diaphragm muscles are relaxed. Due to this, ribs move in an inward direction, and the diaphragm moves in an upward direction. At the same time, pleural pressure is increased on the lungs, resulting in air being expelled into the environment, and expiration takes place.

• Rate of Breathing:
  The process of breathing is voluntary and involuntary. Deep breathing, slow breathing, and the stoppage of breathing is under voluntary control, while continuous breathing is involuntary control. The rate of breathing is influenced by the concentration of CO₂ and H₄ in the blood. Concentration of CO₂ and H⁴ is controlled by Carotid and Aortic bodies, which are present in carotid and aortic arteries. The whole process is controlled by the last part of the brain called Medulla Oblongata.

• Lung Capacity:
  Both the lungs of man have a 5-liter air capacity, equivalent to 5000 cm³. There are three types of air volume found in human lungs.

• Tidal Volume:
  During normal breathing, air occupies 10% of total capacity, which is equal to 450-500 cm³. This capacity of the lung is called tidal volume.

• Vital Volume:
  During deep breathing, air enters the lungs and occupies a 4-liter capacity of the lungs. This capacity of the lungs is called vital volume.

• Residual Volume:
  The remaining one liter of air is already present inside the lungs. This capacity of the lungs is called residual volume. Residual volume prevents the lungs from collapsing during expiration.

RESPIRATORY TRACT DISORDERS:

• Tuberculosis (T.B):
  It is an infectious disease caused by a bacterium, Mycobacterium tuberculosis. It is a contagious disease that is transferred from one person to another by coughing, sneezing, or by using patient personal things. It is common in unhygienic areas. Its main symptoms are prolonged coughing, fever, loss of weight, loss of appetite, chest pain, difficulty in breathing, and spitting with blood.

• Asthma (Deficiency in Breathing):
  It is a serious respiratory tract disorder. It is simply called an attack of breathlessness. It is characterized by wheezing when breathing out. It may be due to allergic reactions because of pollen grain, dust, or fur. In some conditions, it may be hereditary. Serious attacks of asthma may lead to fatal consequences.

• Lung Cancer (Abnormal Cell Division):
  It is a condition of rapid division of cells.

  • Cause: Smoking (active or passive)
  • Toxics: Nicotine, SO₂, etc.
  • Damage: Due to the damage of cilia of epithelial cells of the respiratory tract.
  • Effect: Dust and germs settle down inside the lungs.
  • Abnormality: Abnormal nuclei are developed and penetrate other tissues (disturbing normal cell division and causing cancer).

• Emphysema (Burst Alveoli):
  It is the condition of enlargement of alveoli of lungs.

  • Cause: NO₂, SO₂, CO inhaled by air.
  • Damage: Elasticity of lungs decreases.
  • Abnormality: Alveoli are ruptured and lungs become harder.
  • Symptoms: Supply of oxygen to brain and tissues disturbed, problem in breathing, sluggishness.

TRANSPORT OF GASSES:
During respiration, transportation of oxygen and carbon dioxide takes place. The transportation of O₂ and CO₂ takes place by means of a fluid called blood. So blood is the vehicle for the transportation of gases.

• Transportation Of Oxygen:

  • Role of Hemoglobin:
    Hemoglobin is an iron-containing protein found in the blood. Each hemoglobin has four molecules of iron, known as Heme part. Hemoglobin has a tendency to combine with oxygen at the surface of the lung. Oxygen of air is bound with the Hemoglobin of blood and produces an unstable compound called oxyhemoglobin.

    $$Hb + 4O_2 \rightarrow Hb (O_2)_4$$

    This unstable compound is transported towards the cells and tissues through blood circulation. Near the cells and tissues, oxyhemoglobin dissociates, and oxygen and Hemoglobin are separated. Then oxygen is diffused inside the cell by diffusion. Inside the cell, oxidation of food takes place as a result of which CO₂, H₂O, and energy are produced.

  • Role of Myoglobin:
    Myoglobin is also an iron-containing protein found in the muscles. They are smaller than Hemoglobin. Myoglobin tightly binds with oxygen, due to which the red color of muscles appears.

• Transportation Of Carbon Dioxide:
  Carbon dioxide is a respiratory by-product produced as a result of the oxidation of food in tissues to the lungs. All the CO₂ produced in body tissues reaches the lungs by three means.

• Transportation Of 35% CO₂:
  35% CO₂ is transported through Hemoglobin of blood. It combines with the amino group of Hemoglobin and produces an unstable compound called Carbaminohemoglobin. At the surface of the lung, this compound dissociates into Hemoglobin and CO₂. So in this way, 35% CO₂ reaches the lungs from tissue.

• Transportation Of 60% CO₂:
  60% CO₂ reaches the lungs through water of R.B.C. The following reactions take place during the transportation of this CO₂.

  $$CO₂ + H₂O(R.B.C) \leftrightarrow H₂CO₃$$ $$H₂CO₃ \leftrightarrow H⁺ + HCO₃⁻$$ $$K⁺ + HCO₃⁻ \leftrightarrow KHCO₃$$

• Transportation Of 5% CO₂:
  The remaining 5% CO₂ is transported through the water of plasma. The following reactions take place during the transportation of this CO₂.

  $$CO₂ + H₂O(plasma) \leftrightarrow H₂CO₃$$ $$H₂CO₃ \leftrightarrow H⁺ + HCO₃⁻$$ $$Na⁺ + HCO₃⁻ \leftrightarrow NaHCO₃$$

These all reactions take place in the presence of various enzymes such as Carbonic anhydrase present in Red Blood Cells.