Gaseous exchange is needed in both respiration and photosynthesis. It is the exchange of oxygen and carbon dioxide between a living thing and its environment.
In tiny organisms, e.g. single celled animals and plants or bacteria, the two gases just pass across the cell membrane into or out of the living thing.
For respiration, oxygen must go into the cell and carbon dioxide must leave. It is the opposite in photosynthesis, where carbon dioxide is needed and oxygen has to be removed.
In larger organisms the simple diffusion of gases through the surface of the organism is just not fast enough to keep pace with either respiration or photosynthesis. Bigger organisms need large surface areas for gaseous exchange; we have lungs. There are billions of tiny alveoli in our lungs. Although each one is tiny and has a tiny surface area, there are so many of them that we can get oxygen into our bodies and get rid of carbon dioxide as fast as we need to.
Volume and Surface Area
Imagine trying to build a human body twice as big as an average human being. If it was twice as big in each dimension it would have a volume EIGHT (2 x 2 x 2) times bigger, so it would need 8 times as much oxygen and would make 8 times as much carbon dioxide. Unfortunately, it would have a respiratory surface that was only FOUR (2 x 2) times bigger so if it was able to, it would have to breathe in and out TWICE as fast as a normal human.
I will be re-writing and expandingthis page soon.
You can have a lot of fun with this surface area/volume idea. Think about a mouse growingto the size of an elephant. It would be hundreds of times bigger in every dimension. 100 x 100 x 100 = 1,000,000 so the mouse would be a million times heavier.
Yes, its legs would be very long, but thickness of its legs would be only 10,000 bigger than a normal mouse. Its legs would only be ten thousand times stronger; not enough to support its weight. For the same reason it would have difficulty with gaseous exchange.
Oh yes, there is a solution. Give the giant mouse much thicker legs so that it can stand. Give it much bigger lungs so that it can get oxygen and get rid of carbon dioxide fast enough. By the time you have made all these changes to your giant mouse, it will look much more like an elephant than a mouse.