Form 4 Chapter 3 - Movement of sub across the PM (Part 1)

The plasma membrane (PM)

• PM is a boundary that separates the living cells from its surrounding.
  
• Function: to regulate the movement of substances in and out of the cell.
  
• It is selectively permeable/semi-permeable. Because it allows some substances to move across the PM easily/freely while others cannot.
  
• It is made up of Phospholipids (PL) bilayer & Proteins.

The PL bilayer:
i) Each PL molecule has a polar head (phosphate) and a pair of non-polar fatty acid tails.

ii) The polar head has hydrophilic property (“like water/attract water”), therefore both the heads of the lipid bilayer point towards the aqueous environments. One head pointing to the external environment (consists of interstitial fluid/tissue fluid) while the other head pointing towards the intracellular cytoplasm.

iii) The non-polar tails hv hydrophobic property (“dislike water/repel water”). The fatty acid tails of the PL bilayer point towards each other. This part form the selective barrier of the PM.

The proteins:
i) A few different types of protein can be found on the PM. Which include transmembrane protein, integral protein, peripheral protein.

ii) Transport protein is a transmembrane protein.


iii) 2 types of transport proteins are the carrier protein & pore protein.

• Besides, the PM also has cholesterol which makes the bilayer stronger, more flexible, more fluid.
  
• Glycoproteins are branching carbohydrate chains that attached to protein molecule while glycolipids consist of branching carbohydrate chain that attached to lipid molecule.
  

Fluid-mosaic model

1. which is proposed by Singer & G. Nicolson is used to describe the PM.
   
2. The model describes both the “mosaic” arrangement of proteins embedded throughout the lipid bilayers as well as the “fluid” movements of lipids & proteins alike.
   
3. The PM form a dynamic, flexible & fluid structure.
   
   
   
   

The Size & polarity of a molecule determine whether a molecule can or can’t pass through the PM easily.

• What is Polar & Non-polar molecule?
  The arrangement or geometry of the atoms in some molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative charge. If this is the case, the molecule is called a polar molecule, meaning that it has electrical poles.
  Otherwise, it is called a non-polar molecule. Whether molecules are polar or non-polar determines if they will mix to form a solution or that they don't mix well together.
  Examples:
  i) Water & sucrose are both polar molecules, therefore they mix well to form sucrose solution.
  ii) Water (a polar molecule) & oil (a non-polar molecule) will not mix!
  To learn more about Polar & Non-polar molecules, click here.
  

What are the molecules that can move easily/ freely across the PM?
i) Lipid soluble molecules – eg: fatty acids, glycerols, steroids, vit. A,D,E,K.
ii) Water – small & polar
iii) Oxygen & carbon dioxide – small uncharged molecule.

What are the molecules that cannot move freely across the PM? It requires the use of transport proteins.
i) Large , polar & water-soluble molecule. eg: glucose, amino acid.
ii) Ionic/charged, small, water –soluble molecule. eg: sodium ions, calcium ions.

to be continued . . . see you again in the next lesson. take care.