Question

b. how can some large molecules and charged ions get through the cell membrane?

Explanation:

The cell membrane is selectively permeable. Large molecules (like glucose) and charged ions (like Na⁺, K⁺) can't pass through the lipid bilayer directly. They use two main mechanisms: 1. Facilitated Diffusion: For polar/charged substances (ions, some sugars) or large molecules, integral membrane proteins (channel or carrier proteins) help. Channel proteins form pores (e.g., ion channels for Na⁺), carrier proteins bind and change shape (e.g., glucose transporters). This is passive (no energy, down concentration gradient). 2. Active Transport: For moving against concentration gradients (low to high), energy (ATP) is used. Protein pumps (e.g., Na⁺-K⁺ pump) move ions. Also, Endocytosis/Exocytosis for very large molecules (e.g., proteins, bacteria): endocytosis (cell engulfs via vesicles) and exocytosis (vesicles fuse with membrane to release contents).

Answer:

Large molecules and charged ions cross the cell membrane via:

• Facilitated Diffusion: Through channel (e.g., ion channels) or carrier proteins (e.g., glucose transporters) (passive, down concentration gradient).
• Active Transport: Via protein pumps (e.g., Na⁺-K⁺ pump) using ATP (moves against gradient).
• Endocytosis/Exocytosis: For very large molecules (e.g., proteins), the cell engulfs (endocytosis) or releases (exocytosis) via vesicles.