Topic 2.4 - Plasma Membranes

Lipids:

• Hydrophilic head is hydrophilic b/c phosphate has a charge and water is polar.
• Hydrophobic tail is hydrophobic b/c the fatty acids are non-polar.

Plasma membrane separates internal cell from environment from the outside

Semi permeable membrane lets things selectively come in and out.

• Composed primarily of phospholipids.
• Amphipathic: has polar and nonpolar sides) (hydrophobic tails and hydrophilic heads.
• Bilayer (two layers).

Hydrophilic heads get oriented towards the aqueous surfaces, hydrophobic tails are oriented away

Fluid Mosaic Model

Model describes the structure of the cell membrane. 

• Fluid: Membrane is held together by weak, hydrophobic interactions and can move/shift

• Mosaic: Composed of a variety of macromolecules

  • Composed of phospholipids:

  • Heads polar, tails non-polar

  • Will move around, which is what provides flexibility to the membrane

  • Temperature affects fluidity

  • It’s like butter– if it's too hot it gets too fluid, and too cold makes i too solid

  • Unsaturated hydrocarbon tails help maintain fluidity at low temperatures. 

  • Tail makes it difficult for things to stay packed together due to the kink in it

  • Cholesterol helps to maintain fluidity at both high and low temps

  • Can act as a barrier to keep lipids apart when it gets too cold.

  • Can act as a glue to keep phospholipids connected when it gets too hot.

  • Two major categories of proteins

  1. Integral / Transmembrane proteins

     1. Proteins embedded into the bilayer (going through it)
     2. Amphiphatic
     3. Involved in various transit methods.

  2. Peripheral Proteins

     1. Proteins not embedded into the bilayer (On the outside of it)
     2. Loosely embedded into surface on the hydrophilic part
     3. Have a lot of polar side chains
     4. Can signal to other cells and extracellular reactions
     5. Can act as enzymes, help w/ cell structure, etc.

  The proteins and lipids also have carbohydrates attached to them. There are:

  • Glycolipids
    • Carbohydrates bonded to lipids
  • Glycoproteins
    • Carbohydrates bonded to proteins
    • Most common

Plant cells also have a cell wall. It provides:

• Shape/structure
• Protection
• Regulation of water intake

The cell wall is composed of cellulose.

• Thicker than membranes
• Contains plasmodesmata
  • Hole-like structures in the cell wall filled with cytosol that connect adjacent cells
  • Makes it up for the fact that they are not permeable.