2.5 Membrane Permeability

Skills you’ll gain in this topic:

• Explain how plasma membrane structure contributes to selective permeability.
• Describe how molecules move across membranes based on size, charge, and polarity.
• Differentiate between passive and active transport processes.
• Analyze how membrane properties affect ion and water movement.
• Predict changes in cell conditions based on permeability and concentration gradients.

Membrane Permeability

Thanks to the structure of the membrane, with the hydrophobic tails and hydrophilic heads, the cellular membrane has selective permeability. This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.

The membrane acts like a barrier separating the inside of the cell from the external environment of the cell.

Small, non-polar molecules are able to freely cross the cell membrane, while polar or charged molecules require transport proteins to cross. 

If a molecule is small, polar, and uncharged (like water!) it may be able to pass through the membrane in small quantities but requires a transport protein to move across in any larger quantities. Transport proteins that are specifically designed to let the water molecules cross through the cell membrane are called aquaporins. 

Small non-polar molecules, including N2, O2, and CO2, can freely cross the membrane without any transport protein. Hydrophilic substances, on the other hand, can't cross the membrane freely. This is because the tail part of the membrane (hydrophobic) won't let hydrophilic stuff come by. So only hydrophobic stuff can go through the membrane. 

The hydrophobic fatty acid tails are what controls the movement of substances described above. They repel charged and polar molecules and make it very challenging for them to come across.

Diffusion Across the Plasma Membrane

Image courtesy of WikiMedia Commons

Cell Walls

Cell walls are important for structure and function of the cell. It acts as a structural boundary for the cell. 

Image Courtesy of BYJU's

Cell walls are made up of cellulose, which is a form of sugar. It's made up of carbohydrates, and we humans don't have cell walls. Plant, protists, fungi, and bacteria cells all have cell walls. Cell walls are usually made of chitin, which is a version of polysaccharide.