Which of the following cannot pass through the membrane without the help of a membrane protein? a. carbon dioxide; b. water; c. oxygen; d. charged molecules

Verified step by step guidance

1. Identify the types of molecules listed: carbon dioxide, water, oxygen, and charged molecules.
2. Recall the properties of the cell membrane, which is primarily composed of a phospholipid bilayer with hydrophobic (water-repelling) interior.
3. Understand that small nonpolar molecules like carbon dioxide and oxygen can diffuse through the lipid bilayer without assistance.
4. Recognize that water, although polar, can pass through the membrane slowly due to its small size, but often uses aquaporins (specialized proteins) for faster transport.
5. Conclude that charged molecules, due to their polarity and charge, cannot pass through the hydrophobic core of the lipid bilayer without the help of membrane proteins such as channels or carriers.

Key Concepts

Cell Membrane Structure

The cell membrane is a phospholipid bilayer that serves as a barrier to most substances. It is selectively permeable, allowing certain molecules to pass while restricting others. Small, nonpolar molecules like oxygen and carbon dioxide can diffuse freely, while larger or charged molecules require assistance from membrane proteins.

Membrane Proteins

Membrane proteins are integral or peripheral proteins that facilitate the transport of substances across the cell membrane. They play crucial roles in transporting ions and larger polar molecules that cannot pass through the lipid bilayer directly. These proteins can function as channels or carriers, enabling the movement of charged molecules and other substances.

Diffusion and Osmosis

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, while osmosis specifically refers to the diffusion of water across a semipermeable membrane. Both processes are passive and do not require energy. However, charged molecules cannot diffuse freely and typically need membrane proteins to facilitate their movement.