Membrane Structure Subtopic 1.3

Biological membranes are fluid and Dynamic

The fluid mosaic model 1.3.1 Proposed by Singer and Nicholson in 1972. Biological membranes consist of phospholipid bilayers with proteins embedded in the bilayer.

Phospholipid bilayer All lipids are made up of one glycerol and three fatty acid molecules. However, a phospholipid is a lipid where one of the fatty acids has been replaced by a phosphate group. The phosphate heads are hydrophilic. The fatty acids being non-polar are hydrophobic.

Fatty acids Long carbon chains with a carboxylic group (COOH) Saturated fatty acids Solid at room temperature Tightly packed High melting point Unsaturated fatty acids Double bonds make it loosely packed Viscous (liquid) at room temperature Low melting point

Phospholipids are amphipathic molecules A molecule that has both a hydrophilic and a hydrophobic part.

Membrane Proteins Categorized either as integral or peripheral proteins depending on their position in the membrane. Integral proteins are amphipathic (hydrophobic and hydrophilic properties) and are embedded "in" the plasma membrane. Peripheral proteins are polar (hydrophilic) and are attached to the outside of the plasma membrane.

Membrane proteins

Membrane protein functions Channels - some proteins have a pore/channel that allows the passive transport of substances between the inside and outside of the cell.

Membrane protein functions Carriers - these proteins bind to substance on one side of the membrane and then changes shape to transport it to the other side. Carrier proteins that use energy to change shape are termed protein pumps.

Membrane protein functions Recognition - certain proteins help the cell in differentiating between self and non-self cells (important in triggering an immune response).

Membrane protein functions Receptors - these proteins usually span the whole membrane to relay information from the inside and outside of the cell. 

Membrane protein functions Enzymatic catalysis - these are proteins that enhance the rate of reactions that happens at the membrane level.

Drawing the fluid mosaic membrane model The following labels are included: phospholipid bilayer, phospholipid molecule, glycoprotein, glycolipid, integral and peripheral proteins, cholesterol and transport proteins.

cholesterol Your membranes require cholesterol to function properly, and some important hormones (such as sex hormones) are synthesized from cholesterol. It has both a hydrophilic and a hydrophobic region, it is considered an amphipathic molecule. This property allows cholesterol to insert itself into cell membranes by interacting with the phospholipids 

What is the big deal about cholesterol in the membrane? It restricts the movement of phospholipids and other molecules thus reducing membrane fluidity. At low temperatures, it disrupts the regular packing of the hydrocarbon tails of phospholipid molecules which prevent the solidification of the membrane. This enables the membrane to stay more fluid. It reduces membrane permeability to hydrophilic molecules and ions such as sodium and hydrogen. 

The Davson-Danielli membrane model 1.3.3 Proposed in 1935, then revised in 1954. The cell membrane comprises of a lipid bilayer where two layers of polar lipid molecules are arranged with their hydrophilic heads outward. The lipid bilayer is itself sandwiched between two protein layers on either side of the membrane.

The Davson-Danielli membrane model cnt. Evidence obtained from electron microscopy which showed the membrane as a trilaminar. Proteins normally appear dark in electron micrographs and phospholipids appear lighter, it was deduced that membranes comprised of two proteins layers, one on either side of a phospholipid core.

The Davson-Danielli membrane model shortcomings It assumed that all membranes had identical structures, which did not explain how different types of membranes could carry out different functions. Proteins are amphipathic, though largely non polar, which makes it quite improbable for them to be found in contact with the aqueous environment on either side of the membrane.

The Davson-Danielli membrane model is falsified A new model proposed, the fluid mosaic, supported that proteins are individually embedded in the phospholipid bilayer, rather than coating it on both sides. This allowed the hydrophilic portions of both proteins and phospholipids to be maximally exposed to water resulting in a stable membrane structure. At the same time, it ensured that hydrophobic portions of proteins and phospholipids are in the non-aqueous environment inside the bilayer. Additionally, evidence from freeze fracture techniques also confirmed that proteins are embedded in the membrane. 

Models are used to represent living systems but there may be alternative models, and theories can be falsified by new evidence. Better technology.

https://www. wisc-online https://www.wisc-online.com/learn/natural-science/life- science/ap1101/construction-of-the-cell-membrane