Fluid Mosiac Model

Question 1

What does the cell membrane being partially permeable mean?

Answer 1

Small molecules can pass between the gaps in the phosolipds

Question 2

What is a phospholipid made of?

Answer 2

A tail made up of 2 fatty acids with a phosphate group head attached to it

Question 3

Is the phosphate head hydrophobic or hydrophillic and why?

Answer 3

Hydrophillic

The phosphate group heads are polar so they attract other polar molecules

Question 4

Are the lipid tails hydrophobic or hydrophillic and why?

Answer 4

Hydrophobic

The lipid tails are non-polar so they repel polar molecules

Question 5

How do the phospholipids molecules in cell membrane arrange themselves?

Answer 5

They automatically arrange themselves into a fluid bilayer

Question 6

Why is the phospholipid bilayer fluid?

Answer 6

The phospholipid molecules are constantly moving

Question 7

Describe the phospholipid bilayer structure

Answer 7

• The hydrophilic phosphate heads face out towards the water (either the aqueous tissue fluid/the aqueous cytoplasm) on either side of the membrane
• The hydrophobic lipid tails are on the inside, making the centre of the bilayer hydrophobic

Question 8

What does the centre of the bilayer being hydrophobic mean?

Answer 8

Membrane doesn’t allow water-soluble, charged or polar substances e.g ions through it

Question 9

Why do the phospholipids not form micelles in the membrane?

Answer 9

The 2 fatty acids in the tails are too bulky to fit in the micelle’s interior

Question 10

Where are the proteins in the bilayer?

Answer 10

They are scattered throughout the bilayer (like tiles in a mosaic)

Question 11

Why can some of the proteins move?

Answer 11

As the phospholipid bilayer is fluid

Question 12

Define an “integral” protein

Answer 12

Protein that are fully embedded within the phospholipids, some even spanning both layers

Question 13

Describe the structure of integral protein

Answer 13

• The regions at the end are polar hydrophillic amino acids

- The middle portion is made up of non-polar, hydrophobic amino acids

Question 14

Define a “peripheral” protein

Answer 14

Proteins loosely attached on the outside surface of the cell membrane

Question 15

What are the two types of protein?

Answer 15

• Integral

- Peripheral

Question 16

What are the different kinds of proteins found in the cell membrane?

Answer 16

• Glycoproteins
• Channel proteins
• Carrier proteins
• Enzymes

Question 17

What is a glycoprotein?

Answer 17

Proteins with a polysaccharide chain attached to them

Question 18

What is the function of a glycoprotein?

Answer 18

To act as a receptor for hormones by producing specific shapes

Question 19

What is the function of channel/carrier proteins?

Answer 19

Large molecules and ions can only pass through them

Question 20

What are glycolipids?

Answer 20

Lipids with a polysaccaride chain attached

Question 21

What is the function of a glycolipid?

Answer 21

To provide energy and be markers for cellular recognition

Question 22

What is the function of cholesterol in the cell membrane?

Answer 22

To fit in between the phospholipids, making bonds with them and therefore make the membrane more rigid

Question 23

How does the ratio of phosopholipds containing unsaturated fatty acids to those containing saturated fatty acids relate to the fluidity of the membrane?

Answer 23

The greater the ratio of phosopholipds containing unsaturated fatty acids to those containing saturated fatty acids, the more fluid the membrane

Question 24

How do unsaturated phospholipds ensure membrane fluidity?

Answer 24

The kinks in the hydrocarbon tails of the unsaturated phospholipids prevent them packing too closely together

Question 25

Describe the old model for the cell membrane

Answer 25

3 layers - a phospholipid layer between 2 continuous protein layers

Question 26

What was the evidence for the old membrane model?

Answer 26

Based on the EM image of 3 layers where the dark outer layers were thought to be proteins and the lighter region within was thought to be the phospholipid layer

Question 27

What 2 things did the old model not allow for?

Answer 27

Hydrophillic phosphate heads to be in contact with the water

Hydrophobic non-polar amino acids on the outside of the membrane proteins to be kept away from water

Question 28

How was the EM image re-interpreted for the fluid mosaic model?

Answer 28

• The darker outer layers are the more electron dense phosphate heads
• The inner lighter layer are the lipid tails

Question 29

Whats was the proof for peripheral and integral proteins?

Answer 29

Peripheral proteins could be disassociated from the membrane easily by increasing the ionic strength of a solution it was placed in whereas the integral proteins could only be removed by adding detergents

Question 30

How did freeze/fracture electron microscopy prove the existence of integral proteins?

Answer 30

• Frozen membrane sections were fractured along a weak point between the lipid layers
• The inner fractured surface was covered in a heavy metal
• Scanning electron microscopy gave a 3D image revealing a smooth mosaic-like surface interspersed by much larger molecules

Question 31

Describe the plant protein experiment and its results

Answer 31

Plant proteins (lectins) that bind to polysaccharides were labelled with feretin which is viewable under the EM

• These were mixed with membrane samples
• They only bound to the outer surface and never the inner of the membrane

Question 32

What did the plant protein experiment prove?

Answer 32

• That membranes were asymmetric (the outer surface was different to the inner surface)
• Showed that polysaccharide chains were only on the outside of the membrane
• Proved the invalidity of the protein sandwich model as this would have had lectin binding on both sides

Question 33

How was cell membrane fluidity proved?

Answer 33

• A mouse cell and a human cell were fused

- The mouse and human proteins completely mixed throughout the cell membrane

Question 34

Why does a temperature below 0 degrees mean a high membrane permeability? 4

Answer 34

• Phospholipids don’t have much energy, don’t move much
• Tightly packed together, membrane is rigid
• But channel proteins and carrier proteins in membrane deform, increase permeability
• Ice crystals pierce membrane, make it highly permeable when it thaws

Question 35

What happens to the membrane at temperatures between 0 and 45 degrees?

Answer 35

• Phospholipids move around, not as tightly packed together
• Partially permeable membrane
• Temperature increases, phospholipids move more, increases permeability of membrane

Question 36

What happens to the membrane at temperatures above 45 degrees?

Answer 36

• Phospholipid bilayer starts to melt and membrane becomes more permeable
• Water inside cell expands
• Putting pressure on membrane
• Channel proteins and carrier proteins deform - can’t control what enters/leaves