unit 1.3

head

- choline, phosphate, glycerol - towards water hydrophilic- water loving ( attracted to water) polar molecules ( neg charged molecules)

tails

- fatty acid chains - away from water - hydrophobic ( repels, not attracted to water) - Non-polar (no charge) molecules - either saturated or unsaturated

saturated hydrocarbon tails

- have as many hydrogen present as carbon ( lack double bonds) - causes a straight chain - decreases fluidity

unsaturated hydrocarbon tails

- have fewer hydrogens present that can bond with carbons( so there are double bonds between the carbons) - causes kinked chains which increases fluidity.

The hydrophobic hydrocarbon tails usually behave a liquid. Hydrophilic phosphate heads act more like a solid. Though it is difficult to determine whether the membrane is truly either a solid or liquid it can definitely be said to be fluid. It is :

-It needs to be fluid enough that the cell can move. -It needs to be fluid enough hat the required substances can move across the membrane -If too fluid however the membrane could not effectively restrict the movement of substances across itself.

why called a fluid?

-phospholipid bilayer is flexible, allowing for cellular shape changes. - membrane lipids and ( some protein can drift)

why called a mosaic?

A mosaic of proteins is embedded in the membrane

Cholesterol is a type of lipid, but it not a fat or oil. What group does it belong to?

Cholesterol is a steroid.

The presence of cholesterol in the membrane restricts the movement of phospholipids and other molecules. How does the affect the physical properties of the membrane.

Cholesterol makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane. It reduces the membrane fluidity and permeability to hydrophilic, water soluble molecules and ions such as sodium and hydrogen.

What chemical properties of the membrane are affected by cholesterol.

Cholesterol reduces the permeability to hydrophilic particles, such as, sodium ions and hydrogen ions.

State the function of glycoproteins found in the plasma membrane.

Glycoproteins play an important part in hormone function.

Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Interpreting the image: -The fracture occurs a long lines of weakness including the center of membranes. -The fracture reveal an irregular rough surface inside the phospholipid bilayer. -The globular structures.

Cholesterol also encourages the membrane to take on a concave shape. What structures and processes does this aid?

It helps in the formation of vesicles during endocytosis.

Integral proteins

Span the membrane

Fluorescent antibody tagging used red or green fluorescent markers attached to antibodies which would bind to membrane proteins on the surface of the cell membrane. What did experiment with the markers find out and what conclusions could be drawn from them?

The experiments showed that proteins are actually free to move around in the membrane and the conclusion made is that the proteins were not fixed in a ???? layer like in the Davson-Danielli model.

Improvements in biochemical techniques allowed proteins to be extracted from membranes. How did insights gained from these techniques undermine the Davson-Danielli model?

The insight gained was that the proteins actually vary in size and are globular in shape and that they occupy a variety of positions in the membrane. Also, because the phospholipid molecules are free to move around, the proteins can move too. This is unlike the Davson-Danielli model that shows structured protein that forms continuous and similar layers.

The presence of cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules. What impact does this have on the physical properties of the membrane.

This increases the flexibility as it prevents the tails from crystallizing and behaving as a solid. It restricts molecular motion and therefore enables the membrane to stay fluid and maintain its strength and integrity.

Cholesterol makes up around about 20% of the mass of cell membranes. This percentage varies greatly and some membranes, e.g. bacteria, do not contain it at all.

a. Where in the plasma membrane can cholesterol be found? Positioned between phospholipids in the membrane. b. What properties cause it to be located in the position? CANT UNDERSTAND THE ANSWER

what components of the cell membrane contribute to the fluid quality of the cell membrane?

cholesterol, temperature, phospholipids, proteins, and fatty acids

effect on membrane fluidity with warm temp

from increasing fluidity

Describe the evidence from electron microscopy, such as the image to the right, that supported this model.

he evidence: In high magnification electron micrographs, membranes appeared as two dark parallel lines with a lighter colored region in between. Proteins appear dark in electron micrographs and phospholipids appear light possibly indicating proteins layers either side of a phospholipid core. This explain: Despite being very thin membranes ?? an effective barrier to the movement of certain substances.

peripheral proteins

not embedded, attached to membrane surface

effect on membrane fluidity with cold temp

prevents lower temps from inhibiting fluidity

role of cholesterol with cold temp

prevents tight packing

which two molecules are the principle components of membranes?

protein and a lipid

role of cholesterol with warm temp

restrains movement of phospholipids

explain how the structure and properties of phospholipids help maintain the structure of cell membranes

tails and heads

what is the role of carbohydrates on the surface of the cell membranes?

to act as "receptors" for cell to cell recognition