Select all of the following statements that are true about membrane proteins.
a. membrane proteins are composed of the same 20 amino acids found in soluble proteins in cells
b. membrane proteins can be covalently attached to carbohydrate structures
c. membrane proteins tend to diffuse laterally (within the same lipid layer) in a membrane, unless they are specifically anchored
d. membrane proteins can be covalently attached to lipid structures

b. membrane proteins can be covalently attached to carbohydrate structures

c. membrane proteins tend to diffuse laterally (within the same lipid layer) in a membrane, unless they are specifically anchored

Explanation:

Glycoproteins are proteins that have carbohydrates or sugars covalently attached. This glycoproteins can be located in the membrane.

Following the fluid mosaic model for cell membrane, proteins can diffuse laterally navigating on the liquid bilipid layer.

The statements that are true about membrane proteins are:

b. membrane proteins can be covalently attached to carbohydrate structures

c. membrane proteins tend to diffuse laterally (within the same lipid layer) in a membrane, unless they are specifically anchored

d. membrane proteins can be covalently attached to lipid structures

Explanation:

The biological membrane is formed of a phospholipid bilayer which has a hydrophilic and a hydrophobic side. It is composed of proteins, carbohydrates and lipids.

The membrane proteins are not composed of the same 20 amino acids found in soluble proteins because its structure need to be made of hydrophobic (rejects water) amino acids. And the soluble proteins are form of hydrophilic (attracted to water) ones in order to be soluble in the aqueous environment.  

The amino acids in membrane proteins are hydrophobic because they form part of the hydrophobic section of the biological membrane, and the phospholipid layer functions as a solvent for them.

The membrane proteins can be covalently attached to carbohydrate and lipid structures. When they are joined to one or more lipids they form lipoproteins and this union keeps them anchored to the phospholipid bilayer.  

On the other hand, proteins being covalently attached with carbohydrates create glycoproteins. This union makes glycoproteins more hydrophilic than simple proteins, because of the -OH groups of the sugars. This hydrophilic nature of the molecule influence in the folding of the protein at the tertiary structure, in its stability and in its intracellular destiny.

The fluid mosaic model was created to describe the general structure of the biological membrane. The fluidity in this structure comes from the lipids and proteins, which are the hydrophobic part of it. This characteristic allows for the membrane to have a fluid motion of all its components. They can move in different ways within the same lipid layer, one of those movements is the lateral diffusion. Proteins and lipids in membrane are free to move laterally unless they have certain interactions with other proteins, lipids or sugars that might keep them anchored.

the correct answer is d