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By the end of this lesson you should be able to:

  • Define the term gaseous exchange
  • State the importance of gaseous exchange in plants
  • Explain three mechanisms of opening and closing of stomata
  • Describe the internal section of the root, stem and leaf of terrestrial plants.
  • Describe the internal section of the root, stem. and leaf of aquatic plants

  • Compare the internal cross section of the leaf, stem and root of terrestrial and aquatic plants
  • Identify the various respiratory structures in animals.
  • State how the characteristics of respiratory surfaces adapt them to their function.
  • Explain the mechanism of gaseous exchange in protozoa, insects, fish, frogs and mammals.
  • Explain the factors determining energy requirements in man.
  • State the causes, symptoms and preventive measures for respiratory diseases.


In this topic you will learn how plants and animals exchange oxygen and carbon dioxide between them and their environment. You will also learn about the various structures involved and their adaptatiions.

Background Information

During introduction to biology topic you learnt that gaseous exchange is one of the characteristics of living organisms. You learnt that some plants grow on land while others grow in water. You also learnt that during photosynthesis plants take in Carbon IV oxide and give out oxygen gas. Animals take in air rich in oxygen and give out air rich in carbon IV oxide. In this topic we are going to learn how plants take in useful gases and remove waste gases from the body. We will also learn how animals exchange gases in addition to diseases that affect the gaseous exchange system.

Definition of Gaseous Exchange

Gaseous exchange refers to the diffusion of respiratory gases across the respiratory surfaces. The respiratory gases are oxygen and carbon iv oxide gas. The respiratory surfaces vary from organism to organism. They are the actual sites where gases diffuse into and out of the body of the organism.

Importance of Gaseous Exchange

Gaseous exchange enables living organisms to obtain useful gases and remove the waste gases from their bodies. Plants and animals obtain oxygen which they need for respiration while giving out carbon IV oxide as a waste gas.Plants utilize the carbon IV oxide produced from respiration for the process of photosynthesis in presence of light.

Structure and function of guard cells and stomata

Guard cells are special epidermal cells found on leaves.

The illustration shows guard cells with surrounding epidermal cells

There are two guard cells for every stoma which are placed opposite each other. Their inner walls are thick while the outer walls are thin. They contain chloroplasts.

Mechanism of Closing and Opening of Stomata

There are three theories which have been put across to explain the mechanism of opening and closing of the stomata:

  • Photosynthetic theory,
  • pH theory
  • Potassium ion theory.

Photosynthetic theory

You learnt earlier during the topic nutrition that enzyme controlled reactions are reversible. Enzymes are protein and therefore sensitive to pH. pH can be alkaline, neutral or acidic.

We learnt that Carbon IV oxide dissolves in water to form acidic solutions.In presence of light photosynthesis occurs in the guard cells due to presence of chloroplast. Sugars are formed in the guard cells. The guard cells develop a higher osmotic pressure (tendency to draw in water by osmosis) compared to the neighboring epidermal cells which do not carry out photosynthesis. The guard cells draw in water from the epidermal cells by osmosis and swell becoming turgid.

The thin outer walls of guard cells stretch more than the thick inner walls. A space is left between the guard cells as they move apart and at this stage the stomata is said to be open.


In absence of light

In absence of light photosynthesis does not occur in the guard cells. Sugars formed in presence of light are converted into starch by enzymes in the guard cells. The guard cells lose their osmotic pressure. Water moves out of the guard cells into the epidermal cells by osmosis, and they become flaccid. The thick inner walls straighten up as the thin outer walls lose their stretch. The space between the guard cells becomes small and the stoma closes up. Note that the stoma does not completely close up. A small aperture is left to allow gaseous exchange to continue taking place in absence of light.


pH theory

In presence of light Carbon IV oxide is used up during the process of photosynthesis. Less carbon IV oxide is found in the guard cells. The pH of the guard cells rises i.e. it becomes less acidic. This favors the conversion of starch into sugars by enzymes in the guard cells. Presence of sugars makes the guard cells to develop an osmotic pressure. The guard cells gain water by osmosis from the surrounding epidermis cells to become turgid. The thin outer walls of the guard cells stretch more than the thick inner walls. The thick inner walls bulge away from each other and the space between the two guard cells increase. The stoma opens.

In absence of light photosynthesis does not take place in the guard cells. Carbon IV oxide released during respiration accumulates in the guard cells and dissolves forming an acidic solution.

A low pH (acidic) favors the conversion of sugars into starch by enzymes in the guard cells. The guard cell develops higher osmotic pressure compared to the surrounding epidermal cells. As a result they lose water by osmosis and become flaccid. The thick inner walls straighten up as the thin outer walls lose their stretch. The space between the guard cells becomes small and the stomata close.

Potassium ion theory

According to this theory the stomata opens when potassium ions are actively pumped from the neighbouring epidermal cells into the guard cells. Water moves from the epidermal cells into the guard cells by osmosis. The guard cells become turgid and the stomata open. The stomata close when the potassium ions diffuse from the guard cells to the epidermal cells and as a result the guard cells lose water by osmosis

Gaseous Exchange in Terrestrial Plants

Terrestrial plants can be grouped into two types:

  • Xerophytes
  • Mesophytes


Xerophytes are plants that grow in areas that subjects them to harsh climatic condition i.e. little water and extreme temperature. They are described as xerophytic plants e.g. Acacia, Aloe and pine. In this session we will learn how the structures in Acacia plants adapt it to carry out gaseous exchange.

Internal structure of the leaf of an arid or semi arid habitat plant

The leaf has the following features:

  • Few stomata and of small size
  • Stomata are only on the lower epidermis
  • Small intercellular air spaces in the spongy mesophyll region

A small percentage of gaseous exchange occurs in the lenticels

These are small openings on the stem. Cells in the lenticel are thin walled, and are loosely packed to create air spaces for gaseous exchange. Oxygen diffuses in while carbon IV oxide diffuses out.


Mesophytes are plants that thrive in areas that have moderate climatic conditions.

Leaves of mesophytes are broad. They have many stomata on both upper and lower epidermis. The stomata are large in size. Their spongy mesophyll tissues have large air spaces.

Stems of mesophytes

Their stems have numerous lenticels for gaseous exchange as in xerophytes.


The root hair cells are thin walled for faster rates of gaseous exchange. They hae a projection, the root hair, which increases the surface area or gseous exchange. Their epidermal cells are thin walled for faster rates of gaseous exchange.

Aquatic Plants

Aquatic plants are adapted to grow in water. They are divided into two:

  • Hydrophytes: plants growing in fresh water.
  • Halophytes: Plants growing in saline water

These plants grow under low oxygen and carbon IV oxide concentration conditions. Some aquatic plants are submerged, others are emergent and still others are floating.

In this lesson we are going to learn how the stems, roots and leaves of aquatic plants are adapted for gaseous exchange.

Cross section shows the following features:

Numerous stomata on upper epidermis only

Large intercellular air spaces in spongy mesophyll region

Presence of aerenchyma tissues

Presence of air bladder

Broad leaves to increase surface area for gaseous exchange.

Thin leaves to reduce the distance over which diffusion takes place

The stomata are large in size

A cross section of a Floating Hydrophyte Stem

Note that the stems have large aerenchyma tissues. Conducting tissues are at the center of the stem to create space for aerenchyma tissues. Roots are fibrous to create a large surface area for gaseous exchange

Gaseous Exchange in Emergent Plants

These are plants that grow in water logged soils. Their leaves and stems match those of mesophytes. The root of emergent saline water plants such as mangroves have special breathing roots called pneumatophores for gaseous exchange.

Gaseous Exchange in Submerged Plants

These are plants that grow inside water. An example is Elodea.Their leaves are generally thin to increase the rate of diffusion of respiratory gases. The leaves are deeply dissected in some species to increase the diffusion surface area. They have large and numerous aerenchyma tissues which are sites for gaseous exchange.They use the epidermis for gaseous exchange. These plants however lack guard cells for gaseous exchange.The submerged plants also tend to lack stems and roots.

Gaseous exchange in Protozoa

Gaseous exchange in unicellular organisms such as amoeba is by simple diffusion across the respiratory surface which is the cell membrane. This is because of their large surface area too volume ratio. Usually the water surrounding the unicellular organism has a higher concentration of oxygen compared to inside of the unicellular organism. The difference in concentration gradient causes oxygen to diffuse from the water into the amoeba while carbon IV oxide which is high in concentration in the amoeba diffuses into the surrounding water.

An illustration of surface area to volume ratio using a large and a small chalk cube.

A large cube of chalk and a small cube of chalk are put in colored ink for a few minutes and then removed. A cross section af both pieces is made. It is observed that in the small cube ink has spread to all parts but in the large cube of chalk a section of the cube has no ink.
This explains the why diffusion used for gaseous exchange can be sufficient in small organism as compared to large ones.

Gaseous Exchange in Insects

You learnt in form one that all living organisms carry out gaseous exchange. Insects have a well developed breathing system called the tracheal system. We shall study the breathing mechanism in insects


In insects air enters the body through small opening on the thorax and abdomen called spiracles. The spiracles opens up to a system of branching tubes called the tracheal system. These tubes are of two types:

  • Trachea: Are wide and have rings chitin
  • Tracheole-narrow and lack rings of chitin
    • These tubes are permeable to gases. They are moist and have one cell thick (thin) wall. They are fluids filed for gases to dissolve and diffuse. The spiracles have hair around the opening and valves. The hairs prevent lose of moisture from the tracheal system and also prevent entry of dust particles into the tracheal system. The valves control the entry and exit of the respiratory gases.

      Mechanism of Gaseous Exchange in Insects

      Gaseous exchange occurs during inhalation and exhalation


      This involves air being drawn in through the thoracic spiracles. This occurs when the abdominal muscles relax causing an increase in the abdominal volume and decrease in the pressure. The valves in the thoracic spiracles open air is drawn in. they then close and due to muscle movement air is forced along the tracheal system. Oxygen dissolves in tracheal fluid and diffuses into the tissues due to diffusion gradient. Carbon IV oxide diffuses from the tissues to the trachea fluid due to diffuse gradient.


      During exhalation air is drawn out through the abdominal spiracles. This occurs when the abdominal muscles contract causing an increase in the abdominal pressure and a reduction in the volume. The valves on the abdomen spiracles open while those of thorax close air moves. Animation showing abdominal muscles contract and subsequent shortening of the abdomen.

      Gaseous Exchange in Fish
      Fish use gills for gaseous exchange

      During gaseous exchange the fish opens its mouth. Muscular contractions bring about the lowering of the floor of the mouth. The volume of the mouth cavity increases while the pressure decreases, water flows into the mouth. The operculum on both sides of the head bulges outwards causing reduction in pressure in the gill cavity. Water containing dissolved oxygen flows from the mouth cavity to the gill chamber. Oxygen diffuses from the water flowing over the gills into the blood capillaries in the gill filaments This is due to diffusion gradient. Carbon IV Oxide diffuses from the blood capillaries to the water in the opercula cavity as a result of diffusion gradient.

      After gaseous exchange in the gills the water rich in Carbon IV oxide is expelled. The fish close its mouth relaxation of the muscles causes the floor of the mouth to be raised involving the pressure and reduce the volume forcing any remaining water in the mouth cavity to flow towards the gill chamber and out of the gill chamber through the free edge of the operculum. As water flows out blood in the gill filaments vessels flow in the opposite direction a process called counter-current mechanism which increases efficiency of gaseous exchange.

      Gaseous Exchange in Amphibians

      Amphibians e.g. toads, frogs and newts use the the skin, the buccal cavity and the lungs for gaseous exchange

      Gaseous exchange over the Skin

      Gaseous exchange through the skin is also called cutaneous gaseous exchange. The skin of a frog is thin and highly vascularised. It iskept moist at all times for gases to dissolve. The atmosphere has more oxygen than the blood in the skin of frog. Oxygen diffuses from the air across the skin and capillary membranes and into the blood. Carbon IV oxide is at high concentration in the blood of the skin of the frog as compared to the atmosphere. It consequently diffuses out from the blood , through the capillaries and through the skin membranes into the atmosphere.

      Gaseous exchange over the Buccal Cavity

      Gaseous exchange also takes place over the membranes of the buccal cavity. This is also known as a mouth cavity. The walls are lined with a thin membrane and kept moist. They also have an extensive network of blood capillaries.




      During Inhalation the mouth is closed and nostrils are open. The floor of the mouth cavity is lowered causing an increase in volume and lowering of pressure. This causes the air to enter through the nostrils into the mouth cavity, oxygen diffuses into the moist skin membranes and capillary membranes and is transported to all parts of the body.


      Ventilation in buccal cavity


      Carbon iv oxide diffuses from blood a cross the membrane of the capillaries and skin into the air in the mouth cavity .To expel the air the floor of the mouth cavity is raised causing the pressure to increase and the volume decreases. Meanwhile the mouth is closed and the nostrils open, hence air is expelled.

      Gaseous exchange over the Lungs

      Frogs have a pair of lungs hanging in the cavity. Air from the buccal cavity is received into the lungs through a structure called glottis. It open into a larynx which is connected to a trachea. The trachea branches into short tubes called bronchi (singular bronchus) which extends into each lung .The lungs comprises of airspaces referred to as alveoli. They have a film of moisture and a rich network of capillaries to allow gases to dissolve and increase the surface area for gaseous exchange.

      Ventilation in the lungs.

      During inhalation the nostrils open while mouth and glottis are closed. The floor of the mouth is lowered increasing the volume of the mouth cavity while the pressure is reduced. Air rich in oxygen enters through nostrils into the mouth cavity. The nostrils and mouth then closes. The floor of the mouth cavity is raised forcing open the glottis and air is forced into the lungs. Oxygen diffuses into the blood across the membranes of the alveoli and blood capillaries. Oxygen is used up in respiration and carbon iv oxide is produced and diffuses out of capillaries into the alveolar space.


      Occurs when nostrils are closed and air is sucked from the lungs into the mouth cavity .

      The glottis closes, nostrils open while the floor of the mouth is raised forcing air out of the mouth cavity into the atmosphere through the nostrils.

      Gaseous exchange in Mammals

      Structure Of Breathing System in Man

      Breathing system in man consists of the following structures: nostrils, trachea, lungs, diaphragm and chest cavity made of ribs and intercostal muscles.



      The trachea is a tube made up of rings of cartilage which ensures that it does not collapse during breathing. The lumen of the trachea is lined with ciliated epithelium. The cilia beat in waves and move the mucus and foreign particles towards the pharynx away from the lungs. As the trachea enters the lungs it divides into two branches called bronchi(singular bronchus)


      Lungs are found in the chest cavity. They are enclosed in a double membrane known as the pleural membrane. One part of the membrane adheres tightly to the lungs and other covers the inside of the thoracic cavity. The space between those membranes is known as the pleural cavity. It is filled with pleural fluid which reduces friction making the lungs move freely in the chest cavity. Within the lungs, each bronchus divide into small tubes called bronchioles in groups of tiny air sacs called alveoli (singular alveolus) hence the spongy nature of the lungs. Alveolus is covered by a fine network of blood capillaries.

      Mechanism of Breathing
      During inhalation the external intercostals muscle contract while internal intercostals muscles relax.
      The ribs contract and flatten. Volume of thoracic cavity increases

      Chest cavity during inhalation and exhalation


      The air enters through the nostrils into the trachea and bronchus and bronchioles. As the air passes through the nostrils the hairs trap dust particles; air is moistened and warmed making it advantageous to breath in through the nose.

      As air passes through this trachea, bronchus the cilia and mucus produced by the epithelium cells trap dust and bacteria and carries them away towards the esophagus. The tracheal branches have rings of cartilage which keeps them open during ventilation.

      Gaseous exchange in alveolus

      The alveoli are thin single celled air sacs found at the terminal end of bronchioles. They are vascularized, moist and numerous in number. Gaseous exchange at the alveolus takes place by diffusion.

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