topic 1.3- membrane structure

why are membranes described as being fluid?

membranes are described as being fluid because the phospholipids and many proteins are free to move around in their monolayer. Being fluid allows exchange to occur.

describe structure of plasma membranes.

All plasma membranes are phospholipid bilayers. The width of a plasma membrane is 7-10nm; very narrow.

what is the function of plasma membranes in eukaryotic cells?

Plasma membranes control the exchange between the cytoplasm and the organelle.

what is the function of plasma membranes at the surface of cells?

Plasma membranes control which molecules/ions enter or leave a cell, controlling the exchange between the cell and its environment. It acts as a barrier to many polar molecules and ions.

how is the fluidity of plasma membranes controlled?

The fluidity is controlled by:

why do phospholipids form bilayers?

The inside and outside of a cell are aqueous environments. This means the polar hydrophilic group heads project (extend) outwards as they are attracted to the polar water molecules, and the non-polar hydrophobic fatty acid tails are repelled by water and therefore extend inwards towards the centre of the bilayer.

Describe the structure of a phospholipid.

A phospholipid consists of 1) a polar (hydrophilic) phosphate group head, 2) a glycerol molecule, and 3) 2 non-polar (hydrophobic) fatty acid tails.

explain davson and daniellis sandwich model

Davson and Danielli proposed that there were layers of protein adjacent to the phospholipid bilayer on both sides of the plasma membrane. They created the sandwich model because they thought it would explain how membranes, despite being very thin, are an effective barrier to the movement of some substances. High magnification electron micrographs of membranes were made in the 1950s, which showed a railroad track appearance - two dark lines with a lighter band between. In electron micrographs: proteins appear dark, phospholipids appear light. This fits Davson and Danielli's sandwich model.

what are the problems with the sandwich model? what did this lead to?

In the 1950s/60s some experimental evidence didn't fit with the model; freeze etched electron microscopes: involves rapid freezing of cells and fracturing them. Fractures occur along the lines of weakness; centres of membranes. Globular structures scattered through image of centre of membrane were interpreted as transmembrane (integral) proteins. Structure of the membrane proteins: Extracted proteins that were found varied in size and globular shape, unlike structural proteins that form continuous layers. On the periphery of the membrane, proteins were hydrophobic so are attracted to the non-polar fatty acid tails in the centre of the membrane. Fluorescent antibody tagging: red or green fluorescent markers are attached to antibodies that bind to membrane proteins. Proteins in some cells tagged red marker and some green marker; cells fused together. In 40 minutes, red and green was mixed throughout the membrane fused cell. This shows that membrane proteins are free to move rather than fixed in a peripheral layer.

explain singer and nicholsons model

Singer and Nicholson found that proteins occupy a variety of positions in the membrane; Peripheral proteins are attached to the surface of the phospholipid bilayer. Integral proteins are embedded in the phospholipid bilayer; in some cases, the protein protrudes out the bilayer on both sides. From above the proteins look like a mosaic formed by protein molecules scattered throughout the bilayer, and the phospholipid molecules are free to move between each of the two layers, which means that the proteins are also free to move. This gives the model the name: "the fluid mosaic model".

explain gorter and grendels model.

•Gorter and Grendel calculated that the area the phospholipids occupied when in a monolayer was twice as large as the area of the plasma membrane. They deduced that the membrane contained a bilayer of phospholipids. There were several errors in methods, but these errors cancelled each other out. There is now strong evidence for cell membranes being based on phospholipid bilayers.