Question about Extracellular Circulation

[Jay20155555]

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For some reason its just not clicking for me. What is it referring to when it says the external environment? Just anything outside the cells? And how does this allow for movement of food that are outside the cell.....

Also from the second one about circulatory system. Does that mean our cells bodies dont transport food and oxygen from the environment to cell cells by diffusion?

Actually while typing this i think I get it. Is it trying to say if cells are close to the "external environment" they can just passively diffuse into our cells? While if they are too far from the "external environment" a circulatory system is needed to pump food/oxygen in the cells? Why cant they just do it by endocytosis..... I could be completely wrong haha

 

[Ari Rezaei]

Yeah, you almost got it! The external environment is anything outside of the organism, like the air around you right now. In order for an organism to survive, it has to gather nutrients and resources from its external environment, and evolution has created a couple of different ways of doing that. One of them is just simple diffusion, which for example the jellyfish uses. The skin of a jellyfish is so thin that oxygen from the external environment can just diffuse and supply the cells inside.

In contrast, humans are much thicker and cannot absorb oxygen from the external environment via diffusion. So we have a circulatory system where we breathe in the oxygen, the oxygen gets transferred into our blood, and then it gets carried via our circulatory system to the tissues that need it.