Cell Membrane Structure Topic 1.3

Cell Membranes are Specialized  Cell membranes have a specialized structure to help them perform their function.  State the function of the cell membrane…

Phospholipid bilayer  Most of the plasma membrane is made of a phospholipid bilayer  Phospholipids are amphipathic – they possess both  In the membrane, they are arranged in a double layer.  What characteristics of the phospholipid allow this arrangement?

Phospholipids  Phospholipids have a hydrophilic head and a hydrophobic tail  The hydrophilic head is polar,  The hydrophobic tail is not polar, s

Phospholipids, continued  Because of their hydrophobic & hydrophilic parts, phospholipids will spontaneously form bilayers in solution –  The hydrophilic heads will face out, towards the solution, and the hydrophobic tails will face in, isolated from the solution by the heads

The Cell Membrane’s Phospholipid Bilayer ✴ The cell produces phospholipids to form the bilayer of phospholipids for its membranes ✴ The phospholipid bilayer is one way the cell ✴ Only small and uncharged molecules can fit between the phospholipids to squeeze through the membrane

The Cell Membrane’s Phospholipid Bilayer  Only small and uncharged molecules can fit between the phospholipids to squeeze through the membrane  Are small uncharged molecules the only ones the cell needs?  The membrane has more to it than the bilayer – other parts contribute to its regulation of movement in and out of the cell, and do other jobs as well

The Fluid Mosaic Model  We use the fluid mosaic model to describe the plasma membrane because it’s:  Fluid –  A mosaic - it contains a diverse number of molecules - phopholipids, glycolipids, proteins, glycoproteins, and cholesterol

The Fluid Mosaic Model, continued  Phospholipids are arranged as a bilayer  Proteins and cholesterol are embedded in the membrane or attached to the outside

Membrane proteins  We can see from the picture that membrane proteins can interact with the lipid bilayer.  How can proteins interact with the bilayer given that it’s made up of phospholipids?

Membrane proteins, continued  Proteins can be amphipathic just like phospholipids  This means that they can have both hydrophilic and hydrophobic parts.

Our current model of the cell membrane is called the Singer- Nicholson fluid mosaic model Key features: Phospholipid molecules form a bilayer - phospholipids are fluid and move laterally Peripheral proteins are bound to either the inner or outer surface of the membrane Integral proteins – span the entire membrane The membrane is a fluid mosaic of phospholipids and proteins Proteins can move laterally along membrane

Our current model of the cell membrane is called the Singer- Nicholson fluid mosaic model There is strong evidence for this model: Biochemical techniques Membrane proteins were found to be varied in size and globular in shape Such proteins would be unable to form continuous layers on the outside of the membrane. The membrane proteins had hydrophobic regions and therefore would embed in the membrane not layer the outside

Our current model of the cell membrane is called the Singer- Nicholson fluid mosaic model There is strong evidence for this model: Fluorescent antibody tagging Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell. This showed that membrane proteins are free to move within the membrane rather than being fixed in a layer. red or green fluorescent markers attached to antibodies which would bind to membrane proteins The membrane proteins of some cells were tagged with red markers and other cells with green markers. The cells were fused together.

Our current model of the cell membrane is called the Singer- Nicholson fluid mosaic model This model was first proposed in by Singer-Nicolson in 1972 Before then Davson-Danielli model was widely accepted …

Analysis of evidence from electron microscopy that led to the proposal of the Davson- Danielli model. The model: A protein-lipid sandwich Lipid bilayer composed of phospholipids (hydrophobic tails inside, hydrophilic heads outside) Proteins coat outer surface Proteins do not permeate the lipid bilayer Pore Proteins Phospholipids This explains: Despite being very thin membranes are an effective barrier to the movement of certain substances. The evidence: In high magnification electron micrographs membranes appeared as two dark parallel lines with a lighter coloured region in between. Proteins appear dark in electron micrographs and phospholipids appear light - possibly indicating proteins layers either side of a phospholipid core. Davson-Danielli Model

This technique involves rapid freezing of cells and then fracturing them. Interpreting the image: The fracture occurs along lines of weakness, including the centre of membranes. The fracture reveals an irregular rough surface inside the phospholipid bilayer The globular structures were interpreted as trans- membrane proteins. Falsification of the Davson-Danielli model – freeze fracturing Conclusion: This is contrary to the Davson-Danielli model which only involves proteins coating the surface of the membrane. A new model is needed to explain the presence of as trans- membrane proteins.

Membrane Proteins  Proteins embedded in the membrane can either be peripheral or integral. Peripheral proteins may either be on the outside or inside surface of the membrane; Integral membrane proteins span the width of the plasma membrane, coming in contact with both the cytoplasm & external environments

Functions of Membrane Proteins  Glycoprotein:  Integral Protein:  Peripheral Protein:  Phospholipids:  Cholesterol: Use your coloring sheet and your BZ book (pg 59) to fill out the functions of the differen t structures of the cell membrane