Selective Permeability of Plasma Membranes (College Board AP Biology)

• The phospholipid bilayer carries out the main function of the plasma membrane
  • To house molecules that control the movement of substances into and out of the cell
  • Most of these functions are carried out by proteins in the membrane
• Plasma membranes are globular proteins

These proteins are grouped into two categories:

• Integral; these are partially hydrophobic and therefore are embedded in the phospholipid bilayer (either in both layers or just one)
• Peripheral; these are hydrophilic and so are temporarily attached to either the surface of integral proteins (inside or outside the cell) or connected to the plasma membrane via a hydrocarbon chain
• The protein content of membranes can vary depending on the function
• Membranes of the mitochondria and chloroplasts have the highest protein content because they house many electron carrier proteins

Permeability of the Plasma Membrane

• The structure of cell membranes gives them selective permeability
• One role of cell membranes is to compartmentalize the internal environment of the cell from the external environment
• Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model

Fluid Mosaic Model of Membrane Structure Diagram

Examples of membrane proteins that contribute to selective permeability

• Small nonpolar molecules, including N₂, O₂, and CO₂, can pass freely across the membrane
  • By squeezing between the phospholipid molecules in the bilayer
• Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channel and transport proteins 
• Polar, uncharged molecules, including H₂O, pass through the membrane in small amounts
  • Water passes through channels called aquaporins
• Cell walls provide a structural boundary, as well as a permeability barrier for some substances to the internal environments
• Cell walls of plants, prokaryotes, and fungi are composed of complex carbohydrates

The Role of the Cell Wall

• The cell wall of plants plays an indirect role in selective permeability of the cell
• Most dissolved substances can pass through the large gaps in its structure
• Plant cell walls are composed of fibers of carbohydrates cellulose and hemicellulose
• The cell wall provides structural strength to a plant
• Therefore it allows a cell to retain its shape and integrity even when its surroundings are hypertonic or hypotonic
• The cell wall therefore limits permeability of the membrane to water, for example when a cell becomes turgid in a hypotonic solution and no further water can diffuse into the cell by osmosis

Plant Cell Wall Structure Diagram

Cell Walls of Prokaryotes and Fungi

• Most bacterial cell walls are made of peptidoglycan (a polymer of sugars and polypeptides)
• Other prokaryotes eg archaea have cell walls made of differing polysaccharides and proteins but not peptidoglycan
• Fungal cells walls are made of chitin or cellulose
• Many of these organisms' cell wall are coated with a sticky capsule, made of polysaccharide or protein
  • Capsules allow cells to stick together and also provide a barrier to permeability eg, as a way of preventing dehydration