AP Bio Crash Course for Spring 2021 🧬

Get unit reviews, practice essays, and live support!

ap bio study guides

2.4 Plasma Membranes

#membrane

⏱️  2 min read

written by

Tejas Bhartiya

tejas bhartiya

June 11, 2020


The plasma membrane is made up of a phospholipid bilayer. These have 2 parts, a hydrophobic (water-hating) part, and a hydrophilic (water-loving) part. 

The hydrophilic heads, comprised of a phosphate group, face the outside and inside of the cell, where water is present. The hydrophobic tails, comprised of fatty acids, face inward and do not interact with water.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fmembrane.jpg?alt=media&token=0b0bc56a-3ed1-44ae-8102-ba75d4c7895f

Image courtesy of Wikimedia Commons

As seen below, there are proteins embedded into the plasma membrane. These proteins can be hydrophobic, hydrophilic, charged, uncharged, polar, or non-polar depending on the configuration of the amino acids in the protein.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Flipid%20bilayer.jpg?alt=media&token=79cabb40-a7e6-43a7-8950-f1c0bb26ebde

Image courtesy of Wikimedia Commons

The cell membrane also has glycoproteins and glycolipids attached to it along with steroids. These groups help with cell signaling and the attachment of the cell to other structures.

Models of the plasma membrane are given the name: fluid mosaic model. This represents the fact that the membrane is fluid and somewhat moveable. The proteins embedded in the membrane, which serve a variety of functions, create the mosaic portion of the name.

🎥 Watch AP Biology - Plasma Membranes

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.

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.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fdiffusion.png?alt=media&token=c477859c-2a65-4acf-b12f-f95da8eba9fc

Image courtesy of Wikimedia Commons

continue learning

Slide 1 of 12
Fiveable Logo

2550 north lake drive
suite 2
milwaukee, wi 53211

92% of Fiveable students earned a 3 or higher on their 2020 AP Exams.

*ap® and advanced placement® are registered trademarks of the college board, which was not involved in the production of, and does not endorse, this product.

© fiveable 2020 | all rights reserved.