Exchange of Gases 

Exchange of gases is an essential biological process: it takes up oxygen (O₂), which is required for cellular respiration, and releases carbon dioxide (CO₂). This gas exchange obviously occurs by diffusion across specialised respiratory surfaces in both animals and plants.

1.0Exchange and Regulation of Gases

Exchange of Gases in Plants

1. During Photosynthesis

During Photosynthesis, plants absorb carbon dioxide (CO₂) and release oxygen (O₂).

• Happens in cells that have chloroplasts, which are mostly found in leaves.
• CO₂ comes in through the stomata, and O₂ goes out through the stomata.
• The process only happens when the sun is shining.

Equation of Photosynthesis:
[$6C{O}_2+6H_{2}O\underset{\text{chlorophyll}}{\overset{\text{light}}{\to }}{C}_6{H}_{12}{O}_6+6O_2$]

2. During Respiration

Plants also perform respiration, where oxygen is taken in and carbon dioxide is released.

• Occurs both day and night.
• Involves the breakdown of glucose to release energy.

Equation of Respiration:
[$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{Energy (ATP)}$]

3. Sites and Pathways of Gas Exchange

• Stomata: Microscopic openings on leaf surfaces; major sites for gas exchange.
• Lenticels: Small pores on stems and woody surfaces for gaseous exchange.
• Root hairs: Allow gas exchange with soil air.

4. Mechanism of Gas Exchange in Plants

• Gases diffuse through stomata, lenticels, and intercellular spaces.
• The process depends on concentration gradients and environmental conditions like light, humidity, and temperature.
• During the day, both photosynthesis and respiration occur, but photosynthesis dominates, leading to net oxygen release.
• At night, only respiration occurs, so CO₂ is released.

Exchange of Gases in Animals

Animals require oxygen for aerobic respiration, which releases energy to perform life processes. The exchange of gases ensures a continuous supply of O₂ and removal of CO₂.

1. Gas Exchange in Simple Organisms

Gas exchange in organisms such as Amoeba, Hydra, or Flatworms occurs by simple diffusion directly through their body surfaces. Because they have a high surface area to volume ratio, diffusion is adequate to support them.

2. Gas Exchange in Complex Animals

As animals evolved larger and more complex bodies, specialized respiratory organs developed for efficient gas exchange.
These include:

• Skin – in earthworms and frogs (cutaneous respiration)
• Gills – in fish and aquatic organisms (branchial respiration)
• Tracheae – in insects (tracheal respiration)
• Lungs – in terrestrial vertebrates like reptiles, birds, and mammals (pulmonary respiration)

Exchange of Gases in Humans

Humans exhibit pulmonary respiration, where the lungs are the main organs of gas exchange.

1. Structure Involved in Human Respiration

The human respiratory system includes:

• Nostrils and Nasal cavity – filter and warm the air
• Pharynx and Larynx – passage for air movement
• Trachea and Bronchi – conduct air into the lungs
• Bronchioles and Alveoli – sites of gas exchange

2. Mechanism of Gas Exchange in Humans

Gas exchange occurs mainly in the alveoli of the lungs.

• Oxygen obtained from the air that is inhaled moves through the capillary membranes and into the blood. 
• Carbon dioxide from the blood moves into the alveolar air to be exhaled. This process is based on the partial pressure differences of the gases, oxygen and carbon dioxide, across the alveolar and capillary membranes.

3. Steps Involved

1. Pulmonary Ventilation (Breathing): Air movement into and out of the lungs.
2. Diffusion of Gases: Exchange of O₂ and CO₂ between alveoli and blood.
3. Transport of Gases: Oxygen is carried by hemoglobin; CO₂ is carried as bicarbonate or dissolved gas.
4. Internal Respiration: Exchange between blood and body cells.

2.0Mechanism of Gaseous Exchange

Diffusion as the Main Mechanism

The exchange of gases across membranes occurs by diffusion, based on differences in partial pressures:

• O₂ moves from regions of high to low oxygen concentration.
• CO₂ moves in the opposite direction.

This passive process does not require energy but depends on:

• Permeability of membranes
• Surface area of exchange
• Thickness of the membrane
• Pressure and concentration gradients

Factors Affecting Gas Exchange

• Temperature: Higher temperature increases diffusion rate.
• Surface area: Larger surfaces allow faster exchange (e.g., alveoli in lungs).
• Moisture: Gases dissolve in a thin film of water before diffusing across membranes.
• Ventilation rate: Determines oxygen availability and CO₂ removal efficiency.

3.0Comparison: Gas Exchange in Plants vs Animals