1.3 Membrane structure

Question 1

Diagram of a phospholipid membrane

Answer 1

Question 2

Who proposed the fluid mosaic model?

Answer 2

Singer and Nicolson

Question 3

When was the fluid mosaic model proposed?

Answer 3

1972

Question 4

According to the fluid mosaic model, biological membranes consist of ___

Answer 4

Phospholipid bilayers with proteins embedded in the bilayer, making the membrane look like a mosaic.

Question 5

What is a phospholipid?

Answer 5

A lipid where one of the fatty acids has been replaced by a phosphate group.

Question 6

What is a bilayer?

Answer 6

A bilayer (‘bi’ meaning two) is a structure consisting of two layers.

Question 7

What is a phospholipid bilayer?

Answer 7

A structure consisting of two layers of phospholipids that are arranged according to their properties.

Question 8

What parts of the phospholipid bilayer are hydrophilic and hydrophobic?

Answer 8

The phosphate heads are hydrophilic (that is, ‘water-loving’) because of their charge, while fatty acids, which are non-polar, are hydrophobic (‘afraid of water’).

Question 9

Diagram of the structure of a phospholipid (This figure includes the molecular structure of a phospholipid and a shorthand drawing of a phospholipid with the hydrophobic and hydrophilic areas shown.)

Answer 9

Question 10

What is an amphipathic molecule?

Answer 10

A molecule that has both a hydrophilic and a hydrophobic part.

Question 11

How do the amphipathic properties of phospholipids explain the formation of the lipid bilayer?

Answer 11

The hydrophilic phosphate heads face the watery environment (cell cytoplasm and extracellular fluid), while the hydrophobic fatty acid chains are sandwiched in between, completely isolated from the water.

Question 12

Diagram of a simple phospholipid bilayer

Answer 12

Question 13

What are membrane proteins?

Answer 13

-These are a group of proteins with diverse structures associated with the cell membrane.

-Though they all carry out different functions, they all support the plasma membrane in carrying out its distinctive function.

Question 14

How can all membrane proteins be categorized?

Answer 14

Either as integral or peripheral proteins depending on their position in the membrane.

Question 15

Integral proteins

Answer 15

-They are amphipathic (they have hydrophobic and hydrophilic properties) and are embedded in the plasma membrane.

-In most cases, they pass completely through the membrane.

Question 16

Peripheral proteins

Answer 16

They are polar (hydrophilic) and are attached to the outside of the plasma membrane.

Question 17

Examples of functions of membrane proteins

Answer 17

1) Channels

2) Carriers

3) Recognition

4) Receptors

5) Enzymes

Question 18

Channel proteins

Answer 18

Some proteins have a pore/channel that allows the passive transport (no energy required) of substances between the inside and outside of the cell.

Question 19

Carrier proteins

Answer 19

-These proteins bind to substances on one side of the membrane and then change shape to transport them to the other side.

-Carrier proteins that use energy to change shape are termed protein pumps.

Question 20

Recognition protein

Answer 20

Certain proteins help the cell in differentiating between self and non-self cells (important in triggering an immune response).

Question 21

Receptor proteins

Answer 21

These proteins usually span the whole membrane to relay information from the inside or outside of the cell.

Question 22

Enzymes

Answer 22

These are proteins that enhance the rate of reactions that happen at the membrane level.

Question 23

Diagram showing the structure and function of membrane proteins

Answer 23

Question 24

Examples of other parts of the cell membrane other than membrane proteins

Answer 24

-Glycolipids

-Cholesterol

Question 25

Glycolipids

Answer 25

• These are a phospholipid and a carbohydrate attached together.
• They are important in maintaining the structure of the cell membrane and in cells differentiating between self and non-self cells.

Question 26

Cholesterol

Answer 26

• This is a steroid and is only found in animal cell membranes.
• This is vital in helping to maintain the structure of the cell membrane

Question 27

Diagram of the fluid mosaic model

Answer 27

Question 28

When drawing the fluid mosaic model, you should ensure that ___

Answer 28

1) Individual phospholipid molecules are shown by using the symbol of a circle with two parallel lines attached.

2) A range of membrane proteins is shown, including peripheral and integral.

3) The following labels are included: phospholipid bilayer, phospholipid molecule, glycoprotein, glycolipid, integral and peripheral proteins, and cholesterol.

Question 29

How does the body get cholesterol?

Answer 29

• By making its own cholesterol
• By obtaining it from food

Question 30

Why do our bodies need cholesterol?

Answer 30

-To function properly

-Some important hormones (such as sex hormones) are synthesized from cholesterol.

Question 31

Proposal of the Davson Danielli model

Answer 31

• The first model that attempted to describe the position of proteins within the bilayer was proposed by Davson and Danielli in 1935
• Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer
• The model was described as a ‘lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers
• The dark segments seen under an electron microscope were identified (wrongly) as representing the two protein layers

Question 32

Problems with the Davson Danielli model

Answer 32

• It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
• It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
• It did not account for the permeability of certain substances (did not recognize the need for hydrophilic pores)
• The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Question 33

Falsification of the Davson-Danielli model

Answer 33

• Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
• Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane
• Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
• Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively
• When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
• This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)
• Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
• These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localized to the outside of the membrane structure

Question 34

Proposal of Singer Nicolson (fluid mosaic) model

Answer 34

• A new model was proposed by Singer and Nicolson in 1972
• According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers
• This model, known as the fluid mosaic model, remains the model preferred by scientists today (with refinements)

Question 35

Effect of presence of cholesterol

Answer 35

• The presence of cholesterol in the membrane restricts the movement of phospholipids and other molecules, thus reducing membrane fluidity.
• But, at low temperatures, it also disrupts the regular packing of the hydrocarbon tails of phospholipid molecules, which prevents the solidification of the membrane.
• This enables the membrane to stay more fluid at lower temperatures, allowing the membrane to function properly.
• Additionally, it reduces membrane permeability to hydrophilic molecules and ions such as sodium and hydrogen.

Question 36

Describe the structure of cholesterol

Answer 36

• Cholesterol is a steroid made up of a non-polar part comprising four ring structures, a hydrocarbon tail, and a polar hydroxyl group (hydrophilic).
• Since it has both a hydrophilic and a hydrophobic region, it is considered an amphipathic molecule.
• This property allows cholesterol to insert itself into cell membranes by interacting with the phospholipids (which are also amphipathic).

Question 37

MRI evidence for the fluid mosaic model

Answer 37

• Further studies treated the membranes to chemicals that removed almost 70% of the protein membranes.
• Freeze fracture then showed a smoother lipid bilayer as many of the integral proteins had been removed.
• The development of the MRI machine, which uses magnetic fields to study molecules, showed that the proteins in the cell membrane could move around.
• This was only possible with the fluid mosaic model, as the Davson–Danielli model made movement of proteins impossible, as the protein was one layer.

Question 38

Cholesterol has a role in making sure which type of protein is anchored to the membrane?

Answer 38

Peripheral

Question 39

How thick is the plasma membrane?

Answer 39

5 to 10 nm

Question 40

How are glycoproteins involved in cell adhesion?

Answer 40

They also help cells attach to and bind other cells, which is called cell adhesion.