Gas exchange in Humans, Fish and Insects

• Single celled organisms have a high surface area to volume ratio, which allows for the exchange of substances to occur via simple diffusion
• The large surface area of an organism allows for maximum absorption of nutrients and gases and secretion of waste products, the small volume means the diffusion distance to all organelles is short
• In larger organisms, there is less surface area for the absorption of nutrients and gases and secretion of waste products, the greater volume results in a longer diffusion distance to the cells and the tissues of the organism
• Large multicellular organisms have evolved adaptations to facilitate the exchange of substances between their environment, having a large variety of specialised cells, tissues, organs and systems
• Multicellular organisms need specialised systems for efficient supply of oxygen and removal of waste products
• Why is an efficient supply of oxygen needed?

  Organisms require ATP in order to carry out the biochemical processes required for survival, the majority of ATP is produced through aerobic respiration, which requires oxygen
• Why is the removal of carbon dioxide required in organisms?

  Carbon dioxide is a toxic waste product of aerobic respiration, and i it accumulates in cells and tissues, it can alter the pH
• The time taken for oxygen to diffuse from the cell surface membrane to the tissues in larger organisms takes too long
• What do effective exchange surfaces in organisms have?

  A large surface area, short diffusion pathways and a large maintained concentration gradient
• The adaptations of insects allows gases to be exchanged directly between cells and the atmosphere, therefore meaning that there is no need to transport them
• All insects posses a rigid exoskeleton, which surrounds the insects body and is covered by a waxy cuticle that is impermeable to gases
• What is the waxy cuticle surrounded insects made of?

  Layers of chitin
• The layers of chitin forming the waxy cuticle surrounding a insects exoskeleton forms a waterproof barrier
• How does gas exchange occur in an insect?

  Air enters the trachea through spiracles and flows into the trachea and tracheoles, which then directly diffuses into the cells
• What are spiracles?

  Pores on the surface of the exoskeleton that air enters
• Carbon dioxide and oxygen will diffuse either in or out of the spiracles down the concentration gradient
• Spiracles can be open or closed
• Why do insects keep spiracles closed?

  To prevent water loss from occuring
• When do spiracles open?

  When allowing for gas exchange
• What are trachea?

  Tubes within the insect ventilation system which lead to narrow tubs called tracheoles
• Tracheoles run between cells and into the muscle fibres
• Tracheoles in insects are the site of gas exchange
• The walls of trachea and tracheoles are reinforced with stiffened bands of chitin
• What is the function of chitin in the walls of trachea and tracheoles?

  Keeps them open and prevents collapse due to the air pressure inside them constantly fluctuating
• Tracheoles have thin walls, which shortens the diffusion distance of gases to cells
• Tracheoles are highly branched, which increases the surface area for gas exchange
• There are fluids in the ends of the trachea where it joins the tissues, this can be withdrawn into the body fluid to increase the surface area of the tracheole exposed to the air
• Muscles in insects can pump the body and force air in and out, thus maintaining the concentration gradient for gases
• What are some adaptations of tracheoles?

  They have thin walls and are highly branched
• Through contracting muscles between each body segment, the insect can compress the trachea and thus pumps gases in and out of its body
• Abdominal pumping raises pressure in the body and forces air out of the spiracles down the pressure gradient
• Abdominal pumping can be done to increase the removal of carbon dioxide when energy demands increase and respiration levels are highest, for example during intense movement
• During flight, the production of lactate in the respiring muscles lowers the water potential of muscle cells, causing water found at the end of the tracheoles to be pulled into the muscle through osmosis, allowing gases to diffuse across more rapidly
• Fish need to have an internal gas exchange system to allow for efficient gas exchange to occur
• Why do fish need internal gas exchange systems?

  Their scales are waterproof and they too small a surface area to volume ratio to exchange gases over their body surface
• As oxygen dissolves less readily in water, fish are adapted to directly extract it from the water
• Where are gills located?

  Behind a fishes head
• What is the structure of a gill?

  Made up of thin plates called gill filaments, which are attached to a bony gill arch
• Gill filaments create a large surface area for water to flow over
• Gill filaments are covered in tiny folds called the lamella, which further increases the surface area of the gills