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CELL MEMBRANES
Topic 2.2b
SPECIFICATION- TOPIC 2
 2 Explain how models such as the fluid mosaic model of cell
membranes are interpretations of data used to develop
scientific explanations of the structure and properties of cell
membranes.
INTRODUCTION TO CELL MEMBRANES
 What’s the cause of CF symptoms?
 Can all substances pass across a membrane?
 The cell membrane is composed of the phospholipid bilayer
containing proteins, glycoproteins, glycolipids and cholesterol.
 Phospholipids: like triglycerides but one fatty acid is replaced
by a phosphate group.
PHOSPHOLIPID
Read pages 64-65 and answer these questions
 Phosphate head is hydrophilic, why?
 Fatty acid tail is hydrophobic, why?
 What happens when triglycerides are added to water?
 What happens when phospholipids are added to water?
MEMBRANE STRUCTURE
1.
How do phospholipds arrange themselves in a
membrane. Explain why the phospholipids are arranged in
this way.
MEMBRANE STRUCTURE
2. Label the diagram below identifying the - peripheral
proteins, integral proteins, glycolipids, glycoproteins,
cholesterol, phospholipid heads, phospholipid tails
MEMBRANE STRUCTURE
3. What is the function of the phospholipid bilayer?
 Keeps aqueous solutions separate- forming
compartments. Only small non-polar substances can
pass through because they can dissolve in lipids
(because they have no charge)- e.g. oxygen, carbon
dioxide
4. What are the functions of the glycoproteins and
glycolipids?
 Involved in cell recognition- cells recognize other cells
with similarly shaped glycolipids or glycoproteins and
then stick together to form tissues Receptors on outside of a membrane- e.g. LDL
receptors, hormone receptors. LDL or hormone binds to
the receptor if the shape fits
MEMBRANE STRUCTURE
5. List the functions of proteins in a membrane.
 Transport across the membrane- channel proteins
and carrier proteins
 Structural- help build up cell membrane
 Enzymes on the surface of a membrane- catalyse
reactions
 Receptors on outside of a membrane- e.g. LDL
receptors, hormone receptors. LDL or hormone binds
to the receptor if the shapes fit.
 Proteins on the outside- involved in cell recognitioncells recognize other cells with similarly shaped
proteins and stick together to form tissues
6. Is the protein in the picture below a globular or fibrous
protein? Explain your answer.
7. Which part of this protein is made of mainly hydrophillic
amino acids? Hydrophobic amino acids? Explain how you
knew this.
THEORETICAL MODELS: CELL
MEMBRANES
Aims of the lesson
• When scientists produce theoretical models, they use
their imagination and creativity to think about data in new
ways. The theoretical models that they produce are
therefore more than careful descriptions of the data.
• Because the models go beyond the data, more than one
theoretical model can be supported by the available
evidence.
• In some cases new evidence is gathered which shows
one model to be better than another.
STRUCTURAL MODELS OF MEMBRANES
Task 1
You should have a copy of sheet B1 .3 ‘Lipid layer evidence’.
1.1 From looking at the data in the table, would you agree with the conclusions
of Gorter and Grendel?
1.2 What aspects of the membrane structure is there no evidence for in this
data?
Task 2
You should have been given sheet B2.1‘Electronmicrograph evidence’ and a
description of two dif ferent models.
2.1 Which model (Danielli and Davson’s, Robertson’s (or both) is supported by
the evidence so far? Explain your answer. Look at the “Timeline”
2.2 Describe what you think led to each model being devised.
STRUCTURAL MODELS OF MEMBRANES
Task 3
B3.1 ‘Freeze fracture electronmicrograph evidence’,
B3.2 ‘NMR and X-ray dif fraction evidence’ and
B3.3 ‘Singer and Nicholson’s model’.
The time line will help you see the order these pieces of
evidence and models came in.
3.1 How is each of the models, including Singer and
Nicholson’s, supported or undermined by all the evidence
now available?
3.2 Which do you think is the most useful model? Justify
your answer.
Look at the ‘Timeline’ to see how the models were
developed.
USEFUL ANIMATIONS
 https://youtu.be/Qqsf_UJcfBc
 https://youtu.be/jEY9Bie92aM- Mr. Pollock
 https://youtu.be/Pfu1DE9PK2w - rap
SINGER AND NICHOLSON’S FLUID
MOSAIC MODEL
8. The model above is based on Singer and Nicholson’s fluid mosaic model.
Explain why it is called the fluid mosaic model.
FEATURES OF THE MODEL AND EVIDENCE
USE ALL THE EVIDENCE INCLUDING THAT IN THE
TEXBOOK
Feature of the model
Evidence
Lipids present
Non-polar substances pass
through easily, charged don’t
Protein channels
(hydrophobic/philic regions)
Bilayer
Charged substance (ions and polar
covalent substances) can pass
through
• Phospholipid behaviour in
water
• Total surface area of
lipids one molecule thick
is 2 x surface area of cell
membrane
• Electronmicrographs (EM)
FEATURES OF THE MODEL AND EVIDENCE
Feature of the model
Evidence
Proteins scattered in mosaic
Some proteins integral/ some
peripheral
Freeze fracture EM
Fluid
• X ray diffraction
• NMR scanning
• Fusing 2 cells- proteins from
each cell intermixed
• Q 2.14
Carbohydrate found on outer
surface of the membrane only
Lectin (which naturally bind to
polysaccharides) only adheres to outer
surface
• Freeze fracture EM
• Some easily removed
• Some only removed if the
whole membrane is disrupted
MEMBRANE STRUCTURE
10.Why is the membrane more fluid with unsaturated fatty
acids than with saturated fatty acids?
 MORE «kinks»
 NOT as closely packed
 LESS interactions
 Move MORE freely.
EVIDENCE FOR FLUID MOSAIC MODEL OF
MEMBRANE
SPECIFICATION- TOPIC 2
 2 Explain how models such as the fluid mosaic model of cell
membranes are interpretations of data used to develop
scientific explanations of the structure and properties of cell
membranes.