1.3 Membrane Structure

Question 1

Define hydrophilic

Answer 1

Substances which are attracted to water

Question 2

Define hydrophobic

Answer 2

Substances which are not attracted to water

Question 3

Define amphipathic

Answer 3

A molecule that is part hydrophilic and part hydrophobic

Question 4

Why are phospholipid molecules unusual?

Answer 4

Because they are amphipathic - part of the molecule is hydrophilic, and part is hydrophobic

Question 5

What is the hydrophilic part of a phospholipid molecule?

Answer 5

The phosphate group - or phosphate heads

Question 6

What is the hydrophobic part of a phospholipid molecule?

Answer 6

2 hydrocarbon chains - or hydrocarbon tails

Question 7

Define phospholipid bilayer

Answer 7

The 2 layers which are formed when phospholipid molecules are mixed with water - and the phosphate heads are attracted to the water, and the hydrocarbon tails are attracted to eachother - creating 2 layers

Stable structure

Question 8

What forms the basis of all cell membranes?

Answer 8

Phospholipid bilayer

Question 9

Who deduced that the cell membrane contained a bilayer?

Answer 9

Gorter and Grendel, 1920s

Question 10

When was the model proposed by Davson and Danielli?

Answer 10

1930s

Question 11

What was the model proposed by Davson and Danielli?

Answer 11

Sandwich model - layers of protein adjacent to the phospholipid bilayer, on both sides of the membrane.

Question 12

Why did Davson and Danielli propose the sandwich model?

Answer 12

They thought it would explain how membranes, despite being very thin, are a very effective barrier to the movement of some substances.

Question 13

What supported the Davson and Danielli model?

Answer 13

Electron micrographs from the 1950s showed a railroad track appearance - proteins appear dark in electron micrographs, and phospholipids appear light - fitted the model

Question 14

When was the model proposed by Singer and Nicholson?

Answer 14

1966

Question 15

What is the Singer and Nicholson model of membrane structure? (Fluid Mosaic Model)

Answer 15

Proteins occupy a variety of positions in the membrane

Peripheral proteins are attached to the inner and outer surface

Integral proteins are embedded in the phospholipid bilayer - some protruding out from one or both sides of the bilayer

Proteins are like tiles in a mosaic - phospholipid molecules are free to move in each of the two layers of the bilayer, proteins are also able to move

Question 16

Who proposed the fluid mosaic model?

Answer 16

Singer and Nicholson

Question 17

State what evidence falsified Davson and Danieli’s model

Answer 17

Freeze-etched electron micrographs
Structure of membrane proteins
Fluorescent antibody tagging

Question 18

Describe the freeze-etched micro graphs technique - Davson and Danieli model falsification

Answer 18

Rapid freezing of cells, and then fracturing them
The fracture occurs along the line of weakness - including the centre of membranes
Globular structures scattered through freeze-etched images of the centre of membranes were interpreted as transmembrane proteins

Question 19

Describe the structure of membrane proteins - Davson and Danieli model falsification

Answer 19

Improvements in biochemical techniques allowed proteins to be extracted from membranes.
They were found to be very varied in size and globular in shape- so were unlike the type of structural protein that would form continuous layers on the periphery of the membrane.
The proteins were hydrophobic on at least part of their surface so they would be attracted to the hydrocarbon tails of the phospholipids in the centre of the membrane.

Question 20

Describe fluorescent antibody tagging - Davson and Danieli model falsification

Answer 20

Red or green fluorescent markers were attached to antibodies that bind to membrane proteins.
The membrane proteins of some cells were tagged with red markers and other cells with green markers.
The cells were fused together. Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.
This showed that membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer.

Question 21

What is the currently accepted membrane model?

Answer 21

Singer and Nicholson - fluid mosaic model

Question 22

What is the main function of cell membranes?

Answer 22

To form a barrier through which ions and hydrophilic molecules cannot easily pass

Question 23

What are the 6 functions of membrane proteins?

Answer 23

1. Hormone binding sites (hormone receptors) - insulin receptors
2. Immobilised enzymes with the active site on the outside - in the small intestine
3. Cell adhesion to form tight junctions between groups of cells in tissues and organs
4. Cell-to-cell communication - receptors for neurotransmitters at synapses
5. Channels for passive transport to allow hydrophilic particles across by facilitated diffusion
6. Pumps for active transport which use ATP to move particles across the membrane

Question 24

What are the 2 groups of membrane proteins?

Answer 24

Integral proteins and peripheral proteins

Question 25

Describe integral proteins?

Answer 25

Integral proteins are hydrophobic on at least part of their surface
Embedded in the hydrocarbon chains in the centre of the membrane
Can be transmembrane - extend across the membrane, hydrophilic parts projecting through the regions of phosphate heads on either side

Question 26

Describe peripheral proteins

Answer 26

Peripheral proteins are hydrophilic on their surface, so are not embedded in the membrane

Most are attached to the surface of integral proteins - this attachment is often reversible

Some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the membrane surface

Question 27

What is the protein content of a very active membrane?

Answer 27

The more active a membrane, the higher the protein content

Question 28

What features do you need to include when drawing the fluid mosaic model of membrane structure?

Answer 28

Phospholipids
Integral proteins
Peripheral proteins
Cholesterol

Question 29

What are the two main components of cell membranes?

Answer 29

Phospholipids and proteins

Question 30

What is a key component of cell membranes in animals?

Answer 30

Cholesterol

Question 31

What group of substances does cholesterol belong to?

Answer 31

Cholesterol is a type of lipid - but is not a fat or oil

It belongs to a group of substances called steroids

Question 32

Describe the cholesterol molecule?

Answer 32

Most of a cholesterol molecule is hydrophobic, so it is attracted to the hydrophobic hydrocarbon tails in the centre of the membrane

But one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic. This is attracted to the phosphate heads on the periphery of the membrane.

Cholesterol molecules are therefore positioned between phospholipids in the membrane

Question 33

Describe the fluidity of cell membranes

Answer 33

The hydrophobic hydrocarbon tails usually behave as a liquid

The hydrophilic phosphate heads act more like a solid

Overall - the membrane is fluid as components of the membrane are free to move

Question 34

Why does the fluidity animal cell membranes need to be carefully controlled?

Answer 34

Because if the animal cell membranes are too fluid, they would be less able to control what substances pass through

If they were not fluid enough, the movement of the cell and substances within it would be restricted

Question 35

What is the function of cholesterol in animal cell membranes?

Answer 35

Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules - preventing them from crystallising and behaving as a solid

Restricts molecular motion, and therefore the fluidity of the membrane

Reduces the permeability to hydrophilic particles - such as sodium ions and hydrogen ions

Helps the formation of vesicles during endocytosis

Question 36

What does the shape of cholesterol do?

Answer 36

The shape helps membranes to curve into a concave shape, which helps in the formation of vesicle during endocytosis