Question

1. Many different molecules need to pass in and out of a cell across the cell membrane and from the outside of the body to the inside. Some of these molecules are hydrophobic and some are hydrophilic. Define these two terms and explain why they differ in their ability to pass through cell membranes. What part of a cell membrane makes it so tricky for some molecules to get through? How do these two types of molecules pass through cell membranes?

Answer 1

Answer

The property of cell membrane that makes it selectively permeable and makes it so tricky for some molecules to get through is phospholipid bilayer.
- Cell membrane is made up of phospholipid. There are two layers of phospholipids containing hydrophilic head and hydrophobic tail.

• Hydrophilic molecules : The molecules that can easily dissolve in water are hydrophilic. As these molecules are not made up of lipids they cannot pass through cell membranes.
  - Hydrophilic molecules require channel or carrier proteins to facilitate their transport.
  - If these molecules pass through a channel, it is said to be facilitated diffusion. It is a passive transport as no energy (ATP) is used in the process.
  - If they are pumped by a carrier protein using energy, it is said to be an active transport.
  
  
  
• Hydrophopic molecules : The molecules that cannot dissolve in water but repel water are hydrophobic. As these molecules are made up of lipid substances, they can easily pass through cell membranes.
  - Hydrophobic molecules cross the membrane by simple diffusion. It is a passive transport as no energy (ATP) is used in the process.