Here's why:
* Passive transport means the movement of substances across the cell membrane without requiring energy from the cell. This is driven by the concentration gradient (from high to low concentration) or the pressure gradient.
* Protein carriers are integral membrane proteins that bind to specific molecules and facilitate their movement across the membrane.
Examples of substances transported by passive-transport protein carriers:
* Glucose: Although small, glucose is polar and cannot easily pass through the hydrophobic cell membrane. Facilitated diffusion using a glucose transporter protein allows glucose to enter cells.
* Amino acids: These are the building blocks of proteins and are transported via specific amino acid carrier proteins.
* Water: While water can move across the membrane through simple diffusion, aquaporins (protein channels) increase water permeability and facilitate water movement.
Key point: Although the movement of substances across the membrane is passive, the presence of a protein carrier is essential for these specific molecules.
