Gas Exchange in Insects (A-level Biology)

Gas Exchanges in Unicellular Organisms:

• In unicellular organisms, gas exchanges occur via the cell membrane. Oxygen and carbon dioxide are small enough to pass through cell membranes without any trouble.
• Gas exchanges across membranes are carried out through simple diffusion. Oxygen, which is in higher concentrations in the environment, diffuses across the membrane into the cell. Carbon dioxide, which is in higher concentrations in the cell, diffuses into the environment. Both gases diffuse across a concentration gradient.
• Amoeba proteus is an example. Amoeba proteus is a single celled organism which uses this system for gas exchange. Try to remember this organism in case you are asked to provide an example of a unicellular organism which uses its cell membrane to carry out gas  exchange.

Gas Exchanges in Insects

• Insects don’t have a transport system. As oxygen needs to be transported directly to tissues which are undergoing respiration, they have a specialised gas exchange system.
• Most insects are land organisms with a tough exoskeleton. Insects have an exoskeleton, which helps to prevent the insects from losing water from their bodies, as well as protecting them from their environment.
• Exoskeletons are too thick for gas exchange. Because of their structure, exoskeletons are usually too thick to allow for simple diffusion to occur for gas exchanges.
• Gas exchanges in insects occur via the tracheal system. Gas exchange occurs via a special organ system known as the tracheal system. It is a very simple respiratory system found in many insects.
• Mechanical ventilation is used by some active insects. Occasionally, ventilation of active insect’s respiratory systems can happen via mechanical ventilation (pumping the abdomen) or air reserves, sometimes both simultaneously.
• There are multiple ways of diffusing gases. Diffusion is used in the trachea, whereas mass transport is used as a result of muscle contraction, and volume changes in the tracheoles.

Functions of Tracheal System Structures

• Spiracles – tiny holes able to let air enter the body and prevent water loss. They are controlled by specialised muscles to be opened or closed. Spiracles are used to pass air to the trachea.
• Trachea – a tube lined with chitin, which branches into smaller tubes known as tracheoles.
• Tracheoles – they deliver oxygen to the cells and tissues of the insect.
• Chitin – impermeable rings which work to stop structures collapsing. As they are impermeable, it stops diffusion.
• Lactic acid – oxygen dissolves into this fluid, where it reaches individual cells to start simple diffusion. The carbon dioxide from this process gets released into the environment through the spiracles.

Pathway of Oxygen in Tracheal System

1. Air enters into the body of insects through spiracles.
2. Spiracles transfer the air to thin tubes called the tracheae.
3. The tracheal system contains a special fluid for carrying oxygen.

Sometimes, water builds up at the bottom of the tracheoles, causing slower diffusion. To remove this, lactic acid begins to build up in the cells, decreasing their water potential, where water can move back into the cells.