Return to site

IB Biology - Cell Membrane Structure

Chapter 1.3 - Membrane Structure

May 3, 2020
  • Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
  • Membrane proteins are diverse in terms of structure, position in the membrane and function.
  • Cholesterol is a component of animal cell membranes.
  • Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
  • Drawing of the fluid mosaic model.
  • Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
  • Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Components of Cell Membrane

The cell membrane is constituted by 3 major components, including phospholipid, cholesterol and protein.

Phospholipid

  • Amphipathic compound

Amphipathic compound possesses both hydrophilic (water-loving, polar) and lipophilic (also known as hydrophobic, water-hating, fat-loving, non polar) properties.

  • Hydrophilic group: Phosphate group
  • Lipophilic group: Fatty acid
  • The phosphate group and fatty acids are connected by the glycerol

Cholesterol

  • In the mammal cell, absent in plant cell (still remember the IB biology chapter 1.2, the list states the differences between prokaryotes and eukaryotes)
  • To increase the rigidness of the membrane
  • Prevent the water-soluble to pass through the membrane

Protein

  • 3 kinds of protein: integral proteins, peripheral protein, transported protein  
  • Peripheral proteins: glycoproteins (carbohydrate + protein), receiver to the hormones, communicator between cell to cell and adhesion
  • Integral proteins: enzyme, carrying out the metabolic reaction
  • Transport proteins: large, polar materials move across the membrane via transport protein to enter or leave the cell

The Development of cell membrane theory

How do the 3 components construct the cell membrane? This question bewildered the scientists for half of a century. No microscope can reveal the detail of the cell membrane, thus the scientist achieved the cell membrane theory by try and error.

  • In 1925 Gorter and Grendel: lipid bilayer theory, the cell membrane constituted by 2 layers of lipid 
  • 1935 Davson and Danielli: modified the "lipid bilayer theory" into "fat sandwich theory" as the lipid is hydrophobic, it cannot face the water contained external environment. It suggested there were 2 layers of protein and the phospholipid bilayer placed between the protein layers

  • This “fat sandwich model” widely accepted until the Singer and Nicolson abolished this model by “fluid mosaic model” in 1972

  • Singer and Nicolson found out the proteins in the cell membrane are not universally the same, different cells have different proteins

  • Moreover, the image from freeze-etching (IB Biology chapter 1.1) reveals the surface of the cell membrane is rough, meaning there are proteins intersecting in the cell membrane but not laying smoothly on the membrane

Fluid Mosaic Model

  • There is neither lipid bilayer nor protein bilayer, it is phospholipid bilayer
  • Both the external and internal environment of the cell contains water, so the phospholipid arranged themselves tail-to-tail
  • the hydrophilic phosphate heads always face the water environment while the hydrophobic lipid tails face each other.
  • so the lipid tail always avoid facing the polar environment, leaving it to the phosphate head, the bilayer can maintain intact 
  • The cholesterol spreads in the phospholipid layer. One end of the cholesterol connects to the phosphate head, the rest of the cholesterol embed in the lipid tail
  • The cholesterol solidifies the bilayer and also restricts the movement of polar molecules 
  • the movement of polar molecules rely on the transport protein which penetrating the bilayer and providing a channel or pump to move the material in or out (you will learn it in IB biology chapter 1.4)
  • The integral proteins penetrate the membrane for the enzymatic reaction 
  • The peripheral proteins lay on the bilayer, the carbohydrate part of the protein face outwards the external environment to receive signal
  • Different cells possess different transport proteins for facilitating specific material movement
  • the cell membrane is semipermeable, it permits small and non-polar molecules to bypass it but restricts the large and polar molecules due to the amphipathic property of the phospholipid (repel the polar) and the mosaic model of the membrane (small molecule can squeeze through it)  

Why it is called the fluid mosaic model?

  • Fluid: the whole membrane is flexible, and the phospholipid can move one position to another, even to the next layer membrane 
  • Mosaic: the whole membrane is made up of many separate phospholipids, proteins, and cholesterol molecules, just like the mosaic picture 

Drawing of the Model

  • drawing 2D image of the membrane 
  • should include phospholipid (1 circle and 2 tails), membrane proteins (the glycoprotein should be shown)

In this IB biology topic, you need to understand

  • the function of phospholipid, protein, and cholesterol and their arrangement in the cell membrane 
  • how the amphipathic property of phospholipid leads to the phospholipid bilayer
  • what is the meaning of "fluid" and "mosaic" in the fluid mosaic model 
  • the differences of "fat sandwich theory" and "fluid mosaic model" 
  • the drawing of the fluid mosaic model 

~That is the end of today topics~

Subscribe
PreviousNext
 Return to site
Leave a Comment