IB Biology 2016 syllabus

1. 7. MEMBRANE STRUCTURE
1.3 – Phospholipids form bilayers in
water due to the amphipathic
properties of phospholipid molecules
1.3 – Membrane proteins are diverse
in terms of structure, position in the
membrane and function
1.3 – Cholesterol is a component of
animal cell membranes

2. SOME DEFINITIONS
o Some substances are attracted to water – they are hydrophilic
o Other substances are not attracted to water – they are hydrophobic
o Some substances (such as phospholipids) are large enough to have
part of the molecule being hydrophilic and the other part of the
molecule being hydrophobic. These substances are called
amphipathic.

3. SOME HISTORY
o Scientists have known for a long time (since 1915) that the structure
of cell membranes included proteins and lipids. Further research
established that the type of lipids were phospholipids.
o In 1935 Hugh Davson and James Danielli proposed the Davson-
Danielli model – suggests that the phospholipids formed a bilayer
which was covered on both sides by a thin layer of globular protein.
o They proposed this because they thought it would explain how
membranes, despite being very thin, are a very effective barrier to the
movement of some substances
o Initial electron micrographs in the 1950s seemed to support the
Davson-Danielli model.

4. EVIDENCE THAT SUPPORTS THE
DAVSON-DANIELLI MODEL
o Chemical analysis of membranes showed that they were composed
of phospholipid and protein
o Evidence suggested that the plasma membrane of red blood cells
has enough phospholipids in it to form an area twice as large as the
area of the plasma membrane, suggesting a phospholipid bilayer
o Experiments showed that the membranes form a barrier to the
passage of some substances, despite being very thin and layers of
protein could act as the barrier
o Early electron micrographs showed membranes appearing as two
dark lines separated by a lighter band. This seemed to fit the model
as proteins usually appear darker than phospholipids in electron
micrographs – it was thought that these dark lines were the two
layers of protein on either side of the membrane.

6. PROBLEMS
WITH THE
DAVSON-
DANIELLI
MODELo The Davson-Danielli
model of membrane
structure was
accepted by most cell
biologists for about
30 years.
o In the 1950s and
1960s some
experimental
evidence
accumulated that did
not fit with the
Davson-Danielli
model. (taken from
Allott pg 27)

8. SOME HISTORY
o Another model of membrane structure was proposed in 1966 by
Singer and Nicholson
o In this model the proteins occupy a variety of positions in the
membrane. Peripheral proteins are attached to the inner or outer
surface. Integral proteins are embedded in the phospholipid bilayer.
They believed the proteins were ‘floating’ in a fluid layer of
phospholipid
o This model formed the basis of the currently accepted model of
membrane structure: The Fluid Mosaic Model.

9. THE FLUID MOSAIC MODEL
o The current agreed model for the cellular membrane is the fluid
mosaic model. All cellular membranes, whether plasma membranes
or organelle membranes, have the same general structure.

10. FLUID MOSAIC MODEL - BILAYER
o The ‘fluid’ part is composed of 2 layers of phospholipids (phospholipid
bilayer) – not a rigid structure
o Each phospholipid has a hydrophobic tail (water hating) positioned inwards.
WHY?
o Each phospholipid has a hydrophilic head (water loving) positioned
outwards. WHY?
o Proteins are interspersed throughout the bilayer – the ‘mosaic’ part of the
model

11. FLUID MOSAIC MODEL -
PHOSPHOLIPIDS
o The ‘backbone’ of the membrane is a bilayer produced
from huge numbers of molecules called phospholipids.
Phospholipids are amphipathic – they have a hydrophilic
head and a hydrophobic tail.
o The hydrophilic part of the phospholipid is the
phosphate group. The hydrophobic part is the fatty acid
chains.
o Hydrophobic and hydrophilic regions cause
phospholipids to align as a bilayer if there is water
present. This means that membranes have two distinct
areas when it comes to water solubility.
o The phosphate heads are attracted to the water and
face outwards and the hydrophobic tails are attracted to
each other but not water and so they face inwards
towards each other and away from water

12. Note: The fluid mosaic model can be drawn in 2D rather than 3D and phospholipids
should be drawn with a circle and two parallel lines attached.

13. MEMBRANE PROTEINS
o Cell membranes have a wide range of functions. The primary function is to
form a barrier through which ions and hydrophilic molecules cannot easily
pass.
o This is carried out by the phospholipid bilayer. Almost all other functions
are carried out by proteins in the membrane. Some of the functions of these
proteins include: sites for hormone-binding, enzymatic action, cell adhesion,
cell-to-cell communication, channels for passive transport, pumps for active
transport.
o Because of these varied functions, membrane proteins are very diverse in
structure and in their position in the membrane. Based on their position they
can be divided into two groups:
o INTEGRAL PROTEINS
o PERIPHERAL PROTEINS
o Note: The amount of protein in a typical membrane varies depending on
how active the membrane is – in plasma membranes protein content is
approx. 50% where as in mitochondrial and chloroplast membranes its

14. INTEGRAL PROTEINS
o These are hydrophobic on at least part of their surface and they are
therefore embedded in the fatty acid chains in the centre of the
membrane
o Many integral proteins are transmembrane (they extend across the
membrane from one side to the other) with hydrophilic parts
projecting through the phosphate heads on either side

15. PERIPHERAL PROTEINS
o Peripheral proteins are hydrophilic on their surface so are not
embedded in the membrane.
o Most of them are attached to the surface of integral proteins
o Some of them have a single hydrocarbon chain attached to them
which is inserted into the membrane, anchoring the protein to the
membrane surface

16. YOU SHALL NOT PASS!
o Note: all molecules with low permeability have to be “carried” across the
membrane by carrier proteins
Low Permeability High Permeability
- Anything that can dissolve in
water (water soluble)
- Anything with an electrical
charge
- Anything large in size
- Anything that can dissolve in
lipids (lipid soluble)
- Anything with a no charge (e.g.
CO2 and O2)
- Water (because of pores)

17. MORE STRUCTURES IN THE
MEMBRANE
o Some proteins act as pores, others as carrier systems for transport
and some are used in cell recognition.
o Glycolipids – Cell membrane lipids with a carbohydrate group
attached to them. They play a role in cell recognition (and therefore
have a role in the immune response)
o Glycoproteins – Peripheral proteins with a carbohydrate attached to
the surface. They play a role in cell-to-cell communications and in
transport across the membrane.

18. MORE STRUCTURES IN THE
MEMBRANE
o Channel proteins – Contain channels that span the entire membrane
providing a passage for substances to be transported (aka facilitated
diffusion channels). They are passive (i.e. don’t‘ require cellular
energy to work) and they can be opened or closed according to the
needs of the cell
o E.G. - Aquaporins - Because water is a charged molecule it needs to move through
membrane pores (aquaporins)
o Carrier proteins – Bind to the specific substance to be transported
and undergo a series of conformational changes to transfer the
substance across the membrane. This can occur either passively (no
energy input) or actively (requires energy input)
o E.G. Membrane pumps (such as the sodium-potassium pump) – these proteins
actively pump substances across the membrane and require energy to do so.

21. CHOLESTEROL
o The two main components of cell membranes are phospholipids and
proteins. Animal cells also contain cholesterol.
o Cholesterol is a type of lipid called a steroid (i.e. not a fat or oil).
o Most of a cholesterol molecule is hydrophobic so it is attracted to
the hydrophobic tails of the phospholipids but there is one part of the
molecule that is hydrophilic. Cholesterol molecules are therefore
positioned between phospholipids in the membrane
o Cholesterol stiffens and
strengthens the
membrane, thereby
helping to regulate its
fluidity

22. CHOLESTEROL – ROLE (TAKEN FROM ALLOTT)

23. ESSENTIAL IDEA
THE STRUCTURE OF BIOLOGICAL MEMBRANES MAKES THEM FLUID
AND DYNAMIC

24. DRAWING THE FLUID
MOSAIC MODEL
Skill 1.3 – Drawing of the
fluid mosaic model
(activities booklet)