3.3.2 Gas exchange

Buckle up gas exchange is pretty long…

Gas exchange in larger animals:

As mentioned in 3.3.1, multicellular organisms have a range of adaptations to combat a large surface area to volume ratio. One adaptation I mentioned was a specialised gas exchange surface. Features of this include:

• A large surface area (to increase surface area to volume ratio)
• Thin (to decrease the distance of diffusion meaning materials cross faster)
• Selectively permeable (e.g our lungs are permeable to oxygen and carbon dioxide)
• A means of moving the environmental/external medium (e.g we inhale/exhale to move air to maintain a concentration gradient)
• A transport system of the internal medium (e.g capillaries to move our blood, again to maintain a concentration gradient)

Gas exchange in single celled organisms:

Single celled organisms are very small (duh). As mentioned in 3.3.1, small organisms have a large surface area to volume ratio. It follows that single celled organisms have a very large surface area to volume ratio. They absorb oxygen through their cell-surface membrane by diffusion and release carbon dioxide through their cell-surface membrane by diffusion.

Gas exchange in insects:

Yes, insects are fairly small. Yes, they have a large surface area to volume ratio. However, this increase in surface area conflicts with their conservation of water. Therefore, they have evolved an internal network of tubes known as tracheae and tracheoles (sort of like our bronchi/bronchioles). The trachea are supported by strengthened rings to stop them collapsing (so is our thorax!). Tracheoles are smaller than tracheae and are dead end tubules that extend throughout the body tissue of the insect. Because of this, oxygen is brought right to the respiring tissue, reducing the diffusion distance. 

Yeah, that makes sense, but how does the air actually get in and out of the tubes?

Trachea open at the surface of the insect forming spiracles. Spiracles act sort of like stomata on plant leaves and may be opened/closed by a valve. When spiracles open, gas exchange can occur (but water can also escape!!). Because of this water loss, insects often keep their spiracles closed and only open them when gases need to be exchanged.

Yeah, that makes sense too, but how does the air actually go through the tubes? Well, this occurs in three ways…

• Mass transport - Insects can contract muscles which squeeze the trachea moving air in and out (much like inhalation/exhalation in humans, for example).
• Along a concentration gradient - At the respiring tissue, oxygen is used up so there is little/no oxygen there. This causes gaseous oxygen (in atmospheric air) to diffuse along the trachea and tracheoles. Then quite the opposite occurs, co2 is produced at the respiring tissue which moves out of the insect.
• Diffusion through water - firstly, it is important to point out that diffusion through air is faster than diffusion through water. But, the ends of the tracheoles are filled with water. If anaerobic respiration occurs (e.g during periods of major activity), lactate is produced. Lactate is soluble and lowers the water potential of the cell, meaning the water at the end of the tracheoles moves into the cell by osmosis. This decreases the volume of water at the ends of the tracheoles drawing air further into them meaning the final diffusion pathway is in gaseous phase not liquid phase (which is faster). However, this leads to greater water evaporation.

Gas exchange in fish:

Much like our lungs, fish have gills which increase the surface area of the gas exchange surface. They are made up of gill lamellae which are stacked perpendicular to the gill filaments which increase the surface area of the gills. The flow of water over the gills is in the opposite direction to the flow of blood through the gills. This is known as countercurrent flow and ensures maximum uptake of oxygen from the water. The countercurrent principle ensures that:

• Blood (already partly saturated with oxygen) meets water which is at its maximum oxygen saturation (so diffusion down a concentration gradient, from water to blood, occurs)
• Blood (only a little bit saturated with oxygen) meets water which is at it’s almost minimum oxygen saturation (basically, it’s already lost lots of its oxygen to the other blood). This means that, again, diffusion down a concentration gradient, from water to blood, occurs.

This system maintains the diffusion gradient for the entire width of the gill lamellae, up taking about 80% in total of the oxygen available. Should water flow in the same direction as the blood of fish in gills (a principle known as parallel flow) only 50% of the available oxygen would be absorbed by the blood.

Gas exchange in plant leaves:

I did say that gas exchange was long…..

Okay so the major difference between animal and plant gas exchange is plants also photosynthesise, so also need to take up carbon dioxide (not just oxygen). The volume of gases exchanged by a plant often vary as sometimes the products of photosynthesis can be used as the substrate for respiration and vice versa.

Overall in a leaf there is a short diffusion pathway as leaves are very thin. Much like our alveoli/gills of a fish, air spaces inside the leaf create a very large surface area to volume ratio. Gases just move in and out of the plant by diffusion. Some adaptations of leaves that aid diffusion include:

• Thin leaves to decrease the diffusion distance
• Small pores known as stomata much like insect spiracles - decrease diffusion pathway as no cell is far from a stoma. Each stoma is surrounded by guard cells which open and close the stomatal pore controlling the rate of gas exchange. This is important because it means that plants can balance the conflicting needs of gas exchange and water loss by closing stomatal pores when water loss would be excessive (e.g in warm/very dry conditions).
• Interconnecting air spaces throughout the mesophyll so gases readily come into contact with mesophyll cells
• Large surface area for rapid diffusion