Why does PASSIVE diffusion occur only in inactive organisms?

The only answer that I can formulate:
Passive diffusion only occurs in very small organisms as they have a very large surface area to volume ratio.
Inactive organisms have a low metabolic rate, and therefore respiration rate is also lower. Therefore they need less oxygen.

Based on these two facts, I can conclude that passive diffusion does not bring in as much oxygen into the body as compared with a specialised exchange surface in larger organisms.

Is this right? Anything missing?

I wouldn't say it only occurs in inactive organisms, but I'm willing to overlook this.

Your first sentence is partly true. The organism should have a large SA/Vol ratio is all it relies on is passive uptake of nutrients. It doesn't necessarily have to be a very small organism. I also think that they don't need less oxygen, but rather can survive on less oxygen as opposed to your active counterpart.

Your last sentence is true. I think you should also point out that passive diffusion will not get nutrients/oxygen quickly enough or in large quantities which is required for active organisms, as they most likely have higher metabolic rates and are actively reproducing/replicating and will require specialised transport systems.

Hope this helps

thanks!

im a little confused because my book also states that "the smaller the Sa/v, the more active an organism, the greater the need for an active transport system"

why does a small Sa/v mean a more active organism? Or is just one of those thing to accept?

The smaller the SA/V, the lesser the amount of exchange via diffusion. Therefore, to compensate for this, the organism will have to have an active transport system, for an efficient exchange of substances. Also, a smaller SA/V may or may not mean an active organism.