Why does PASSIVE diffusion occur only in inactive organisms? Watch

ps1265A
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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?
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Eloades11
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(Original post by ps1265A)
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
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ps1265A
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(Original post by Eloades11)
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?
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Dynamo123
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(Original post by ps1265A)
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.
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ps1265A
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(Original post by Dynamo123)
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.
I've somehow managed to link these ideas together:

Inactive means lower metabolic rate which means less oxygen taken up by cells which means a lower volume which means a large surface to volume ratio which allows inactive organisms to allow exchange over their surfaces

Active organisms like mammals who need to maintain a high body temperature have a high metabolic rate which means a lot of oxygen is transported into cells, and also they have a lot more cells because of their size, and therefore the volume is large compared to their surface area and so passive diffusion cannot occur in them
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