Membrane Structure Q&A

BioKnowledgy Quick Quiz fror 1.3

Which of the following is NOT a function performed by a membrane protein?

Enzyme synthesis

What is the function of the cytoplasmic membrane of this bacterium? (Picture of a regular bacterium with a flagella and pili)

To control entry and exit of substances

Which of the following is NOT a function of cholesterol in cell membranes?
I. Increase fluidity
II. Increase flexibility
III. Decreases permeability of the membrane to polar molecules

I (Increase fluidity) only

Meaning that increasing flexibility and decreasing permeability of the membrane to polar molecules ARE some of the functions of cholesterol in cell membranes.

The diagram below shows a plasma membrane. What is molecule X? (Picture of a drawn inside of a cell. X points to a squiggle between two circles.)

Cholesterol

Draw a labeled diagram to show the fluid mosaic structure of a plasma membrane, indicating the hydrophilic and hydrophobic regions.

MUST include:
Phospholipid bilayer (line with heads and tails)
Labeled hydrophilic heads and hydrophobic tails
Integral protein (carrier protein, embedded in the layer)
Protein channel (also integral, clear channel/pore)
Peripheral protein (with part on the surface)
Glycoprotein (carbohydrate attached on the inside)
Cholesterol (embedded in bilayer)
Labeled thickness

Compare and contrast the Davson-Danielli and Singer-Nicholson models of cell membranes and describe the evidence that disproves the Davson-Danielli model.

Compare and contrast - look at both similarities and differences. Point them out.
Both models include a phospholipid bilayer and proteins.
Davson-Danielli have proteins on the outside of the bilayer while Singer-Nicholson's had them embedded.
DD model has the proteins fixed in place whereas in SN proteins are free to move laterally.
For evidence:
Freeze fracture EM images showed transmembrane proteins embedded in the bilayer oR biochemical techniques shows membrane proteins to be hydrophobic in part
Fluorescent antibody tagging showed that proteins are mobile within the membrane.

Explain how the properties of phospholipids help to maintain the structure of cell membranes.

Phospholipids have hydrophilic and hydrophobic regions. Hydrophilic (phosphate) heads attracted to water and hydrophobic (fatty acid) tails are not.
Phospholipid bilayer forms with heads in contact with water on both sides of membrane and hydrophobic tails in centre kept away from water.
This creates a stability in the membrane.
The stability is brought about by an attraction between hydrophobic tails and between the hydrophobic heads and water.