Proteins do not pass through plasma membranes because:

• A. they are too large and polar to diffuse through the hydrophobic lipid bilayer
• B. they are rapidly degraded by membrane enzymes
• C. they are electrically neutral and thus cannot interact with membrane lipids
• D. they are actively repelled by membrane carbohydrates

Answer: (A)

Verified step by step guidance

1. Understand the structure of the plasma membrane: The plasma membrane is primarily composed of a phospholipid bilayer, which has hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads. This structure creates a selective barrier for molecules based on size, polarity, and charge.
2. Analyze the properties of proteins: Proteins are macromolecules made up of amino acids. They are typically large in size and often have polar or charged regions due to the presence of amino acid side chains.
3. Consider the mechanism of diffusion: Diffusion through the plasma membrane occurs for small, nonpolar molecules that can easily pass through the hydrophobic lipid bilayer. Large, polar molecules like proteins cannot diffuse through this barrier because their size and polarity prevent them from interacting favorably with the hydrophobic core of the membrane.
4. Evaluate the incorrect options: Proteins are not rapidly degraded by membrane enzymes, as enzymes are specific to their substrates and do not indiscriminately degrade proteins. Proteins are not electrically neutral; they often have charged regions. Additionally, proteins are not actively repelled by membrane carbohydrates, as carbohydrates primarily function in cell recognition and signaling.
5. Conclude why proteins cannot pass through the plasma membrane: Proteins are too large and polar to diffuse through the hydrophobic lipid bilayer, which is the primary reason they cannot pass through the plasma membrane without assistance from specialized transport mechanisms like protein channels or carriers.