The cell membrane is selectively permeable, meaning it allows certain molecules to pass through while restricting others. Large, polar, and charged molecules typically cannot pass easily through the cell membrane. Here’s why:
- Large Molecules: Large molecules, such as proteins and polysaccharides, are too big to pass through the small pores of the cell membrane. They require specialized transport mechanisms like endocytosis or exocytosis.
- Polar Molecules: Polar molecules, such as water and glucose, have an uneven distribution of charge. The hydrophobic interior of the cell membrane repels these molecules, making it difficult for them to pass through without the help of transport proteins.
- Charged Molecules: Charged molecules, such as ions (e.g., Na+, K+, Cl-), are repelled by the hydrophobic core of the cell membrane. They require ion channels or carrier proteins to facilitate their transport across the membrane.
In summary, large, polar, and charged molecules face significant barriers when trying to pass through the cell membrane and often require specialized transport mechanisms to cross it.