Protein diffusion question

[iceman132]

According to Examcrackers Biology page 102:

Since peptides are proteins they can't diffuse through the membrane so they bind to receptors on membrane and act through a second messenger.

Page 266
Amino acids are absorbed by FACILITATED DIFFUSION and ACTIVE TRANSPORT.

Amino acids are proteins so I don't understand how Amino acids can go through facilitated diffusion and active transport.

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[MT Headed]

Amino acids are proteins so I don't understand

Yeah, well, while a protein is a polymer of amino acids, I don't think I would go so far as to say that amino acids are proteins. A brick isn't a building. It isn't hard to build a channel or a pump to absorb the little amino acids. Your digestive system can absorb an amino acid, a dipeptide, and even some tripeptides. It wouldn't be able to absorb an entire protein intact.

Practice questions try to get you to think about the two different types of hormones. Lipid soluble hormones need carrier molecules like albumin to get transported around the blood, then they slip easily across the phospholipid bilayer (like dissolves like), and then are transported into the nucleus to affect transcription. Water soluble hormones float around in the blood independently, and since they can't slip through the phospholipid bilayer they instead attach to a membrane receptor, triggering a cascade of signals inside the cell.

I'm sure in the real world things behave differently, but dammit that's how mcat biology works and that's all I need to know for the next month!

If you know these two mechanisms by heart, know which hormones fall into which of these two categories (watch out for the tricky tyrosine derivatives!), where they are made, and what they do, you should be set.

 

[paul411]

Facilitated diffusion means that a substance passively diffuses across the membrane with the help of an integral membrane protein.

Amino acids (building blocks of proteins; not proteins themselves) passively diffuse across membranes via facilitated diffusion just like glucose in erythrocytes.

 

[whiteshadodw]

large peptides are more likely to be taken up via endocytosis as its pretty difficult for a large protein to just cross the membrane, even with a membrane protein.

 

[MD Odyssey]

Since peptides are proteins they can't diffuse through the membrane so they bind to receptors on membrane and act through a second messenger.

Page 266
Amino acids are absorbed by FACILITATED DIFFUSION and ACTIVE TRANSPORT.

I suspect that the question is trying to draw a distinction between peptide hormones which act upon a specific receptor on the membrane and steroid hormones, which have intracellular receptors that they can get at by diffusing through the membrane.

Something like a cholesterol is non-polar, so it won't have any issues diffusing through the lipid bilayer. An individual amino acid, even if it has a lipid side-chain, is going to be polar, charged, and most likely solvated by water molecules. It needs some sort of aid in getting across the membrane, which is why there are things like transporters which bring them into the cell. Small proteins and oligopeptides have an even harder time getting across the membrane - they're polar, often charged, and in general just too big to have any meaningful ability to diffuse through the bilayer. So, they have to be brought into the cytoplasm by something endocytosis or pinocytosis, as whiteshadodw pointed out.