Membranes Topic 1.3

Cell membrane structure

Membrane Structure Phospholipid bilayer Hydrophobic Hydrocarbon Tail Hydrophilic Phosphate Head Cholesterol Glycoproteins Integral proteins Peripheral Proteins

Cell Membrane Theory Gorter and Grendel extracted phospholipids from red blood cells and deduced that the membrane had 2 layers (bilayer) It did not explain where the proteins were Davson and Danielle deduced that the proteins were adjacent to the lipids on either side

Davson and Danielli Saw 2 dark lines with a lighter band in between Proteins usually look dark on micrographs and lipids look lighter

Cell Membrane Theory Singer and Nicolson (1966) Proteins occupy a variety of positions in the membrane They are also capable of moving their location Like tiles in a mosaic Fluid-Mosaic Model

Falsification of Davson and Danelli Membrane proteins were found to be different sizes and shapes. It is unlikely they would form a continuous layer on top of the membranes Freeze etching showed proteins dsipearsed throughout the membrane Fluorescent anti-body tagging of membrane proteins supported the freeze etching results

Falsification of Davson and Danelli

Phospholipids Partly hydrophilic and hydrophobic (amphipathic) Phospholipid phosphate groups are attracted to water where as their hydrocarbon tails are not. Hydrocarbon tails are attracted to one another just as the phosphate heads are attracted to other phosphate heads. This attraction makes for a strong but fluid membrane.

Structure of phospholipids

Fluid Mosaic Model Phospholipids in membranes are in a fluid state. This allows membranes to change shape. Proteins are seen to be sporadically placed in the membrane. Proteins float within the membrane as well as on the surface.

Membrane Proteins 4.Cell to Cell communication: 1. Hormone Binding Site: Site exposed on the outside of the membrane allows one specific hormone to bind thus transmitting a signal to the inside of the cell. 2. Enzymes: Enzymes located in the membranes either catalyze reactions inside or outside the cell. (this depends on where their active site is) 3. Cell Adhesion: Forms tight junctions between cells and tissues. 4.Cell to Cell communication: - Cell receptors receive signals via hormones or neurotransmitters

Membrane Proteins 5. Passive Transport Protein: Channels or passages through the center of the membrane proteins. Each channel is specific for certain substances. 6. Protein Pumps for Active Transport: Pumps release energy from ATP and use it to move specific substances across the membrane.

Membrane proteins

Cholesterol in membranes Found in animal cells Types of lipid called a steroid Mostly hydrophobic so it attaches to the lipid portion of the membrane It does have a small hydrophilic portion that is attached to the phosphate head It is usually found between phospholipids

Cholesterol in membranes

Role of cholesterol in membranes Controls the fluidity of the membrane Too fluid and it cannot control what comes in/out Not fluid enough and it restricts cell movement Cholesterol disrupts the packaging of the hydrocarbon tails (so not so rigid) Reduces the permeability of some ions Show video