Most transmembrane proteins have regions facing the intra- and extracellular fluid, and ___ regions embedded in the phospholipid bilayer. a.hydrophilic; hydrophilic b. hydrophilic; hydrophobic c. hydrophobic; hydrophobic d. hydrophobic; hydrophilic e. lipophilic; lipophobic
The Correct Answer and Explanation is :
The correct answer is: b. hydrophilic; hydrophobic.
Explanation:
Transmembrane proteins are integral membrane proteins that span the entire lipid bilayer of a cell membrane. They have distinct regions with specific characteristics that allow them to interact with both the aqueous environment and the hydrophobic interior of the membrane.
Hydrophilic Regions
- Facing the intra- and extracellular fluid:
These regions of the protein are hydrophilic (“water-loving”) because they are in contact with the aqueous environment inside (cytoplasm) and outside (extracellular fluid) the cell. Hydrophilic regions are composed of amino acids with polar or charged side chains, enabling interactions with water molecules and other polar substances.
Hydrophobic Regions
- Embedded in the phospholipid bilayer:
The interior of the phospholipid bilayer is hydrophobic (“water-fearing”), formed by the nonpolar fatty acid tails of phospholipids. To maintain stability within this environment, the portions of the transmembrane protein that are embedded in the bilayer consist of hydrophobic amino acids with nonpolar side chains. These hydrophobic regions are often organized in α-helices or β-barrels that anchor the protein in the membrane.
Functional Importance
The arrangement of hydrophilic and hydrophobic regions is critical for the protein’s structure and function:
- Transport: Many transmembrane proteins, such as ion channels and transporters, have hydrophilic pores that allow polar molecules or ions to pass through.
- Signaling: Receptor proteins have extracellular hydrophilic domains to bind signaling molecules and transduce signals inside the cell.
- Stability: The hydrophobic interactions between the protein and the lipid bilayer stabilize its position in the membrane.
This amphipathic nature (having both hydrophilic and hydrophobic parts) enables transmembrane proteins to bridge the aqueous and lipid environments, ensuring their functional integration into cellular membranes.