Which of the following statements about transmembrane proteins is correct?

a. the signal sequences are always cleaved off.
b. they are always inserted with the amino terminus on the lumenal side and the carboxyl terminus in the cytosol.
c. they cross the membrane only once.
d. they usually have one or more î± helices spanning the membrane bilayer.

The correct option is D.

Explanation:

There are different types of proteins in cells, one of these is integral proteins. Integral protein perform diverse functions in the cells such as transporters, linkers, ion channels, receptors, etc. Trans-membrane proteins are integral proteins, they are a kind of proteins that  are permanently attache to the membranes of the cells and they span the entirety of the cells they are attached to.

The properties of trans membrane protein include the following:

1. They typically have one or more alpha helices spanning the membrane layer.

2. They have both hydrophobic and hydrophilic regions.

3. Each of the proteins have 3 distinct domains, which are intracellular, extracelllular and inside the lipid bilayrs.

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