Gas exchange is a biological process through which different gases are transferred in opposite
directions across a specialised respiratory surface.
Gases are constantly required and produced as a by-product of cellular and metabolic
reactions so an efficient system for their exchange is extremely important.
It is linked with respiration in animals, and both respiration and photosynthesis in
plants.
In respiration, oxygen is required to enter cells whilst waste carbon dioxide must be
removed – the opposite is true for photosynthesis, where CO2 enters plants and O2 is released.
The exchange of gases essentially occurs as a result of diffusion down a concentration
gradient – gas molecules moving from an area of high concentration to low concentration.
Diffusion Diffusion follows Fick’s Law.
It is a passive process affected by factors such as:
The surface area available The distance the gas molecules must diffuse
across The concentration gradient
Gases must first dissolve in a fluid in order to diffuse across a membrane therefore all
gas exchange systems require a moist environment.
In single-celled organisms diffusion can occur straight across the cell membrane; as organisms
increase in size so does the distance gases must travel across.
Their surface-area-to-volume ratio also decreases.
Diffusion alone is not efficient enough and specialised respiratory systems are required.
This is the case with humans and fish where circulatory systems have evolved: These are
able to transport the gases to and from the respiratory surface and maintain a continuous
concentration gradient.
In Humans
Both oxygen and carbon dioxide are transported around the body in the blood – through arteries,
veins and capillaries.
They bind to haemoglobin in red blood cells although this is more effective with oxygen.
Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate
ions (HCO− 3).
This reaction is catalysed by the carbonic anhydrase enzyme in red blood cells:
The main respiratory surface in humans are the alveoli.
Alveoli are small air sacs branching off from the bronchioles in the lungs.
They are one-cell thick and provide a moist and extremely large surface area for gas exchange
to occur.
Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli
- they are also extremely thin so the total distance gases must diffuse across is only
around 2-cells thick.
An adult male has about 300 million alveoli which range in size from 75 to 300 microns
in diameter.
Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while carbon
dioxide from the blood diffuses in the opposite direction into the alveoli.
The waste carbon dioxide can then be exhaled out of the body.
Continuous blood flow in the capillaries as well as constant breathing maintains a steep
concentration gradient.
Varying response During physical exercise excess carbon dioxide
is produced as a result of increased respiration: This must be removed and muscles and cells
require increased oxygen.
The body responds to this change by increasing the breathing rate, therefore maximizing the
rate of possible gas exchange.
In Plants Air diffuses directly into and out of plants
through pores on the underside of leaves known as stomata.
The stomata are controlled by guard cells.
When the guard cells are turgid, stomata are open.
When they are flaccid, stomata close and gas exchange cannot take place.
The main respiratory surface is the spongy mesophyll cells inside the leaf.
They have large air spaces and therefore provide a large surface area for gas exchange.
Leaves are also very thin so the diffusion distance is small.
During the day photosynthesis occurs at a faster rate than respiration so there is an
overall uptake of CO2 and release of O2.
At night this is reversed as photosynthesis stops and only respiration can take place.
In Fish Fish must extract oxygen dissolved in water,
not air, which has led to the evolution of gills and opercula.
Gills are specialised organs containing filaments and lamellae – the lamellae contain capillaries
and provide a large surface area and short diffusion distance as they are extremely thin.
Water is drawn in through the mouth and passes over the gills in one direction while blood
flows through the lamellae in the opposite direction – this counter current maintains
a steep concentration gradient.
O2 is able to continually diffuse down its gradient into the blood and CO2 into the water.
Summary of main systems Other examples
Insects such as crickets do not have an inner skeleton so exchange gases across structures
known as trachea and tracheoles: These are tubes that run directly into the body of the
insect.
Air enters the trachea through valves known as spiracles and diffusion can then occur
straight into the respiring tissues.
Amphibians are able to use their skin as a respiratory surface, as well as having lungs
and sometimes gills.
See also Respiratory system
Ventilation References