Gaseous exchange in plants,Living organisms

This is the exchange of respiratory gases between the organism and the environment. It takes place across specialized surfaces called respiratory surfaces. Gaseous exchange helps an organism to get rid of CO2 produced during respiration within cells and at the same time obtain oxygen needed for aerobic respiration to occur.
Note: Breathing is an active process involving movement of air in and out of the body whereas gaseous exchange is a passive process involving passage of air through respiratory surfaces/gaseous exchange surfaces.

Characteristics of a good respiratory surface
Respiratory surfaces are sites where gaseous exchange takes place in the body of the organism. Respiratory surfaces possess the following characteristics:
1) They have a large surface area to volume ratio to enable rapid diffusion of gases. This is achieved by folding or branching of structures to form alveoli in lungs, gill filaments in the gills and tracheoles in insects.
2) They are moist to allow easy diffusion of gases.
3) They are thin walled to reduce on the distance over which diffusion has to take place.
4) They have a good network of blood capillaries for easy transportation of gases to the respiring tissues.
5) They are well ventilated to maintain a high concentration gradient that favours diffusion of gases.
Note; respiratory surfaces of insects are not supplied with a network of blood capillaries because the blood of insects does not transport gases. The gases are transported in the tracheole tubes.


Plants do not have a special respiratory surface for gaseous exchange. They use simple pores i.e. stomata of the leaves and lenticels of the stems for gaseous exchange.
Gases circulate in the plant by simple process of diffusion due to abundant large intercellular spaces that make diffusion faster.
Plants do not need special respiratory surfaces and blood transport system because:

  • They utilize CO2 produced by the plant cells for photosynthesis thus preventing accumulation.
  • Plants produce oxygen as a bi-product of photosynthesis which is then used in respiration.
  • Plants have numerous stomata and lenticels that favour fast gaseous exchange.
  • They have large intercellular spaces that favour fast circulation of gases without blood.
  • They have low demand for oxygen due to their low metabolic rate because they are less active since they are immobile.

Gaseous exchange in simple organisms

Small organisms like amoeba, paramecium, hydra and jellyfish have a large surface area to volume ratio. In such organisms gaseous exchange takes place over the whole body surface. Because of their small body volume, diffusion alone is enough to transport oxygen and Carbon dioxide into, around and out of their bodies.
Larger organisms such as insects and vertebrates have a small surface area to volume ratio. In these organisms, gaseous exchange takes place in a specialized region of the body known as a respiratory surface. The respiratory surface is part of the respiratory organ. It is the actual site where gaseous exchange takes place.

Surface area to volume ratio and gaseous exchange
Surface area to volume ratio is an important aspect in gaseous exchange. It is obtained by calculating the total surface area and dividing it by the volume of the object in question.
Consider two boxes A and B below

image 84

Box A is smaller than box B. we can work out the surface area to volume ratio of each box to prove that smaller objects have a larger surface area to volume ratio than big ones.
Starting with box A
Total surface area.

A = 2(2X1) + 2(1X2) + 2 (2X2)
A = 4 + 4 + 8
A = 16cm2
Volume of A
V = 2X1X2
V = 4 cm3

Surface area to volume ratio of A = 16/4

Box B
Total surface area.
A = 2(3X2) + 2(3X4) + 2(2X4)
A = 12 + 24 + 16
A = 52cm2
Volume of B
V = 4X2X3
V = 24 cm3
Surface area to volume ratio of B= 52/2.3

The surface area to volume ratio of A is larger than that of B.

  • Therefore the surface area to volume ratio of smaller organisms is larger than that of larger organisms. This facilitates a faster rate of diffusion to ensure that all body tissues are supplied with respiratory gases.
  • Smaller organisms also have a short diffusion distance i.e. it takes less time for gases to move to all parts of their body. Most of them are single celled and some have only one layer of cells.
  • Larger organisms on the other hand have a smaller surface area to volume ratio. This reduces the rate of diffusion and diffusion alone cannot meet the respiratory demands of their large bodies.
  • They also have a large diffusion distance because they have very many layers of cells. Due to this large organisms have developed mechanisms, which reduce on the diffusion distance and increase the surface area to volume ratio.
  • Mammals have developed a blood circulatory system, which transports blood containing respiratory gases through highly branched blood vessels to all cells of the body.
  • Insects have developed a tracheal system, which has finely divided tubes known as tracheoles, which carry respiratory gases to and from all cells in the body of the insect.

Examples of respiratory surfaces and corresponding respiratory organs

image 85

NB: the movement of gases and water to and from respiratory surface is called ventilation (breathing).

The respiratory organs of insects consist of a network of tubes known as tracheal tubes, which make up the tracheal system. These tubes reach all the body tissues like the capillaries.

The tracheal system of insects

image 86

Ventilation mechanism

When the abdominal wall expands, the internal pressure reduces and the volume increases.
This forces air containing oxygen in to the insect through the spiracles, to the trachea and then the tracheoles.
Between the tracheoles and muscles of the insect, gaseous exchange occurs with oxygen entering in to the tissues and CO2 released from tissues, diffusing into the fluid in the tracheoles


Abdominal wall contracts, internal volume decreases while pressure increases, forcing air with a high concentration of carbon dioxide in the tracheoles out of the insect through the spiracles.

Fish uses water as a medium of gaseous exchange and their respiratory surface is the internal gill.
Fish absorb dissolved oxygen from water by use of gills. In most fish there is a pair of gills on each side of the body and in bony fish the gills are covered by a gill plate also called the operculum.

Structure of the gill

image 87

Parts of the gill:

  1. Gill bar: this provides an attachment and support to the gill filaments.
  2. Gill raker: These are hard projections from the gill bar.
    They trap food suspended in water.
    They protect the gill filament by filtering out sand particles in water before reaching the gill filament.
  3. Gill filaments:
    These are sites of gaseous exchange in the fish.
    They are finger-like projections that increase the surface area for gaseous exchange.
    They have a network of capillaries whose blood moves in the opposite direction with water (counter current flow) to maintain a high concentration gradient by carrying away the diffused gases.
    Filaments have a thin membrane
    They are well ventilated.
    They are numerous to increase the surface area.

Mechanism of ventilation in bony fish
Ventilation in bony fish occurs in two phases i.e. inhalation and exhalation.

Inward movement of water

  • This is the process by which water containing dissolved oxygen is allowed into the body of the fish.
  • The fish closes the operculum (gill cover) and opens the mouth.
  • It then lowers the floor of the mouth cavity. This increases volume of the mouth cavity and lowers its pressure below that of the surrounding water.
  • The mouth then opens to let in water into the mouth cavity (buccal cavity)
  • Water flows into the mouth cavity through the mouth.
  • It then closes the mouth and rises the buccal cavity to decrease the volume and increase the pressure in the buccal cavity.
  • Meanwhile the gullet is closed.
  • This makes the water current to flow into the gill chamber.
  • As water passes over the gill filament, gaseous exchange takes place i.e. oxygen diffuses into blood while CO2 diffuses from blood into the water.

Out ward movement of water:

  • For water to flow out after gaseous exchange, the operculum muscle relax then water flows out.
  • Meanwhile the buccal floor is still raised and the mouth is still closed.
  • The buccal floor then lowers to repeat the cycle.

a) Tad pole
Tad poles first use external gills and later internal gills as surface of gaseous exchange.
The tad pole takes in water through the mouth and the water passes over the gills and then out of the body through the gill slit.
The oxygen diffuses from the water into the blood while CO2 diffuses from blood into water.

b) Adult amphibians
In adults gaseous exchange takes place through the;

  1. Skin.
  2. Lining of the mouth cavity.
  3. Lungs.
    Amphibians depend mostly on their skin and buccal cavity for their gaseous exchange while they are in water. Lungs are only used when on land or when the water dries and the amphibian has to remain in mud.
  1. The skin
    The skin is thin walled, moist and has a good network of blood capillaries. The skin acts as a respiratory surface when the amphibian is in and out of water. It’s used when the oxygen need is low.
    On land, the atmospheric oxygen dissolves in the layer of moisture and then diffuses across the skin into the blood.
    At the same time, CO2 diffuses from the blood into the atmospheric air.
    In water, the oxygen dissolved in it, diffuses from the water across the skin into blood. CO2 diffuses from blood into water.
  1. The buccal cavity
    The buccal cavity has a thin lining which is kept moist. It also has a good network of blood capillaries. The cavity is ventilated in the following ways.
    During inhalation:
    The mouth floor lowers when it closes.
    This increases the volume of the buccal cavity reducing the pressure within.
    This forces the air from the atmosphere through the nostrils into the buccal cavity.
    Oxygen diffuses through the thin cavity membrane into blood while Carbon dioxide diffuses from blood into the buccal cavity.
    During exhalation:
    The muscles of the floor of the buccal cavity relax raising the floor of the mouth.
    This leads to a reduction in volume and an increase in pressure within the mouth cavity.
    Air then moves out to the atmosphere through the nostrils.
  1. The lungs
    The lungs consist of sacs supplied by a good network of blood capillaries.
    They have a large surface area.
    It is supplied with a lot of blood capillaries
    It is thin walled.
    Ventilation of the lungs occurs in the following stages;
    The mouth closes and the nostrils open.
    Muscles of the floor of the buccal cavity contract to lower the mouth floor. This increases the volume and reduces the pressure within the buccal cavity.
    Air enters through the nostrils into the buccal cavity.
    The nostrils close, the muscles of the floor of the buccal cavity relax to raise the floor of the buccal cavity, while those of the abdominal cavity contract.
    This causes the volume of the buccal cavity to reduce and that of the abdominal cavity to increase.
    Pressure in the buccal cavity increases and that in the lungs decreases.
    It opens the glottis and air moves from the mouth cavity into the lungs through the trachea.
    Oxygen diffuses from the lungs into blood and Carbon dioxide from the blood into the lungs.

For exhalation, the abdominal muscles relax to reduce the volume of the lungs while the floor of the mouth cavity is lowered to increase its volume.
This creates a higher pressure in the lungs and low pressure in the buccal cavity.
Waste air is forced from the lungs into the buccal cavity
The valve to the lungs (glottis) closes and nostrils open.
Muscles of the floor of the mouth cavity relax raising the floor and increasing pressure in the buccal cavity.
Waste air is forced from the cavity through the nostrils to the atmosphere.