1.3 Membrane Structure

The structure of biological membranes make them both fluid and dynamic

Lipid where one fatty acid is replaced by a phosphate group, in membranes characterized by phosphate (polar) head and lipid (non polar) tails

Polar/ charged unit that is attracted to water

Non polar unit that is repelled by water

When put into water, an emergent property is that phospholipids self-organize to keep heads wet and tails dry

Molecule made form bio the hydrophilic and hydrophobic subunits

Phospholipids flow past each other laterally but can’t move vertically

Permanently embedded proteins that go all the way through membranes. They are

Protein thats permanently embedded in the cell and goes all the wya through, penetrating one surface (mono topic), or many surfaces (poly-topic). Integral proteins are amphipathic.

Proteins with oligosaccharide (few sugar) chain attached, used for cell recognition by immune system and as hormone receptors

Temporary association with cell membrane, polar molecules that reside on the outside of the membrane

Transport, receptors, anchorage, cell recognition, intercellular joining, enzymatic activity

Makes phospholipids pack more tightly and regulates fluidity and flexibility of the membrane by providing structure and rigidity as well as regulating permeability to hydrophilic molecules

Hydroxyl group makes the head of the molecule hydrophilic and attracted to phosphate heads on membrane periphery. The hydrophobic tail attracts to lipid tails within the membrane

Maintained in mammalian membranes by cholesterol as the hydrophobic tails behave as a liquid while the phosphate head behaves as a solid, making the membrane neither solid nor liquid but fluid

Fluidity required for cell movement and material exchange, however overly fluid membranes cannot effectively regulate and restrict the movement of substances in and out of the cell

Presence of cholesterol in membranes restricts phospholipid movement and therefore reduces fluidity

The presence of cholesterol prevents crystallization and solidification at cold temperatures, therefore increasing fluidity by disrupting regular packing of hydrocarbon tails

Considered the most accurate model to date. Phospholipids form a bilateral that is fluid and moves laterally. The membrane also consists of proteins that can be either peripheral or integral. The membrane is a fluid mosaic of phospholipids and proteins. Proposed in 1972.

Membranes proteins were found to be globular and varied in size so they would be unable to form a continuous layer on the membranes periphery. Membrane proteins also has hydrophobic tendencies, suggesting they would embed within the membrane as opposed to outside of it.

A phospholipid membrane surrounded by a layer of protein on either side

High magnification electron microscopes show membranes appearing as two dark lines with lighter space in-between, leading to the belief that proteins surrounded the phospholipid bilayer as proteins are darker beneath the microscope.

Freeze fracturing that demonstrated fractures along lines of weakness including the center of the membrane. The fracture also revealed an irregular and rough surface within the bilayer with globular structures that were then interpreted as trans-membrane proteins

Red and green fluorescent markers attached to the antibodies that bind to membrane proteins. Within 40 minutes the red and green proteins had mixed throughout the membrane, showing that proteins were not fixed to the peripheral layer