Notes
DP Biology
Cell Membranes
2.4.1 Draw and label a diagram to show the structure of membranes (1).
Notes
DP Biology
Cell Membranes
Notes
DP Biology
Cell Membranes
2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to
maintain the structure of the cell membranes (3)
Explain means to give a detailed account of causes, reasons
or mechanisms.
This model of the bilayer's has the proteins removed for
clarity.
 The 'head's have large phosphate groups, thus they
are hydrophilic (attract water) or polar. These section are
suited to the large water content of the tissue fluid and
cytoplasm on opposite sides of the membrane.
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 The fatty acid tails are non-charged, hydrophobic
meaning they repel water. This creates a barrier between the
internal and external 'water' environments of the cell. The
'tails' effectively create a barrier to the movement of charged
molecules
 The individual phospholipids are attracted through
their charges and this gives some stability. They can however move around in this plane
The stability of the phospholipid can be increased by the presence of cholesterol molecules.
Notes
DP Biology
Cell Membranes
2.4.3 List the functions of membrane proteins (1).
List means to give a sequence of names or other brief answers with no explanation.
Notes
DP Biology
Cell Membranes
2.4.4 Define diffusion and osmosis (1).
define means to give the precise meaning of a word, phrase or physical quantity.
Diffusion: passive movement of particles from a region of high concentration to a region of
low concentration.
Osmosis is the passive movement of water molecules from a regions of lower solute
concentration to a region of higher solute concentration
DIffusion ideas:
The movement of particles is caused by the kinetic energy possessed by the particle.
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The direction of movement is random.
Observing groups of particles it emerges that they move from regions of high concentration to
regions of low concentration.
Alternatively the statement can be in terms of pressure. Movement from a region of high
pressure to a region of low pressure.
However, most biological diffusion takes place through membranes and involves sources, sinks
and diffusion gradient.
The movement of particles is caused by the kinetic energy possessed by the particles
The direction of movement is random
Observing groups of particles it emerges that they move from regions of high concentration to
regions of low concentration.
Alternatively the statement can be in terms of pressure. Movement from a region of high
pressure to a region of low pressure.
However, most biological diffusion takes place through membranes and involves sources, sinks
and diffusion gradient.
Osmosis ideas:

water moves(because they have kinetic energy) through plasma membranes pores called
aquaporin.
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Water molecules have kinetic energy like other molecules.
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Water molecules move randomly and will if they come into contact with the membrane pass
straight through.
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The tendency is for water to pass from lower solute (left) to higher solute (right) concentrations.
Notes
DP Biology
Cell Membranes
2.4.5 Explain passive transport across membranes by simple diffusion and facilitated
diffusion (3).
Explain means to give a detailed account of causes, reasons or mechanisms.
The passive movement implies that there is no expenditure of energy in moving the
molecules from one side of the membrane to the other:
However the molecules themselves possess kinetic energy which accounts for why they are in
movement.
The membrane therefore 'allows' the molecules to pass through without needing to add any
additional energy to the kinetic energy already possessed by the particles.
Particles will in fact pass in both directions but over all the emerging pattern is that
molecules move from a region of their high concentration to a region of their low
concentration.
 Some molecules are so small that they pass through the
membrane with little resistance
 This includes Oxygen and Carbon Dioxide
 Lipid molecules (even though very large) pass through
membranes with very little resistance also.
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Larger molecules (red) move passively through the
membrane via channel proteins
 These proteins(grey) have large globular structures
and complex 3d-shapes
 The shapes provide a channel through the middle of
the protein, the 'pore'
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The channel 'shields' the diffusing molecule from the non-charged/ hydrophobic/ non-polar
regions of the membrane.
Notes
DP Biology
Cell Membranes
2.4.6 Explain the role of protein pumps and ATP in active transport across
membranes(3).
Explain means to give a detailed
account of causes, reasons or
mechanisms.
Molecules are moved against the
concentration gradient from a region
of their low concentration to a region
of their high concentration.
Active mean that the membrane
protein 'pump' requires energy (ATP)
to function
The source of energy is ATP is
produced in cell respiration
Transported molecules enter the
carrier protein in the membrane.
The energy causes a shape change in the protein that allows it to move the molecule to the other side of
the membrane.
 The sodium-potassium, pump that creates electrochemical gradient across the cell membrane of all cells.
 Cells are -ve charged on the inside relative to the
outside.
 This pump is modified in the nerve cell to create
some of the electrochemical phenomena seen in nerve
cells.
Notes
DP Biology
Cell Membranes
2.4.7 Explain how vesicles are used to transport materials within a cell between the
rough endoplasmic reticulum , Golgi apparatus and plasma membrane(3).
Cells will manufacture molecules for secretion outside of the cell.
Some of these secretion molecules are complex combinations of proteins, carbohydrates and lipids.
The base protein is coded for by a gene whose expression begins the process.
Protein is synthesized and moved to the endoplasmic reticulum
The protein is move through the rER and modified
A spherical vesicle is formed from the end of the rER with the protein inside
The vesicle migrates to the golgi apparatus
Vesicle and golgi membranes fuse. The protein is released into the lumen of the golgi apparatus.
The golgi modifies the protein further by adding lipid or polysaccharides to the protein.
A new vesicle is formed from golgi membrane which then breaks away. The vesicles migrates to the
plasma membrane.
The vesicle migrates to the plasma membrane fuses and secretes content its contents out of the cell. A
process called exocytosis.
2.4.8 Describe how the fluidity of the membrane allow s it to change shape, break and
re-form during endocytosis and exocytosis(2).
a) Exocytosis: vesicle membrane fuses with the plasma
membrane.
b) Endocytosis:a vesicle is formed by the infolding of the
plasma membrane
In each of the cases above the membranes are able to
form and break without loss of the continuity of the
plasma membranes. The process is very similar to the
childhood game of playing with bubbles of detergent.
Bubbles are produced then they can be watched readily
joining together or splitting apart.
Notes
DP Biology
Cell Membranes
Membrane fluidity:
(a) The phospholipid molecules can change places in the
horizontal plane. This creates the so called fluid property of
the membrane.
(b) Molecule exchange in the vertical plane does not occur.
This maintains the integrity of the membrane.
(c) Cholesterol embedded in the membrane reduces its
fluidity.
Mechanism for the making and breaking
of the cell membrane.
The Li Yang and Huey Huang model has shown
that it is the proportion of other molecules in
the plasma membrane that will determine
whether is opens or closes.
It is suggested that the molecules that initiate
membrane fusion or breakage will
be:lipids;proteins and cholesterol.
They derived their model by a serendipitous
observations whilst performing other
experiments.
With x-ray diffraction patterns they showed
how the phospholipids will form an hourglass
shape at the point of contact. (click image for
diffraction x-rays)
The sequence of diagrams shows how a
membrane might fuse or split and yet self-seal
during a wide variety of biological situations.
Notes
DP Biology
Cell Membranes
In the model
(a) Membranes approach.
(b) Touching membranes note how the phospholipid heads flow together starting the process of fusion.
As noted above this requires the presence of additional molecules.
(c) At the point of contact there is a single lipid bilayer.
(d) The pore is open and the membranes are now continuous