Lipids

1
Lipids Biological Membranes
Biological membranes that define the boundaries
of cells are composed largely of lipid molecules,
which form a permeability barrier allowing only
certain molecules to enter the cell. Lipids are
one of the four important classes of biomolecules
that have a variety of structural and functional
roles.

• Harini Chandra

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Master Layout (Part 1)
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This animation consists of 4 parts Part 1
Fatty acids Part 2 Membrane lipids Part 3
Proteins in membrane structures Part 4
Properties of cell membranes
b
w
2
2
w
1
3
a
Fatty acid general structure
3
w-3 double bond
Degree of unsaturation
Chain length
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Definitions of the componentsPart 1 Fatty
acids
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1. Fatty acid Fatty acids, more simply known as
fats, are the key components of lipids that play
an important role in signal transduction pathways
and as structural elements of membranes. They are
long hydrophobic chains of different lengths that
possess a carboxylate group at one end. Most
naturally occuring fatty acids have an even
number of carbon atoms with varying degrees of
unsaturation. 2. Degree of unsaturation The
number of double bonds present in a fatty acid
chain defines its degree of unsaturation. 3.
Chain length The total number of carbon atoms
present in a fatty acid chain is its chain
length. 4. w-carbon Fatty acids are numbered
starting from their carboxyl group with the
second and third carbon atoms being known as the
a and b carbons. The carbon atom that is furthest
away from the carboxylate group, at the distal
end of the chain is known as the w carbon.
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3
4
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Part 1, Step 1
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Fatty acids general structure
2
Degree of unsaturation
w-carbon
Chain length
Carboxylate group (C1)
3
Short chain length increased unsaturation
enhance fluidity decrease MP of fatty acids.
Saturated fatty acid
Unsaturated fatty acid
4
Action
Audio Narration
Description of the action
As shown in animation.
Fatty acids are long hydrophobic chains of
carbon atoms having different lengths and a
carboxylate group at one end. Fatty acids that do
not contain any double bonds are said to be
saturated while those possessing one or more
double bonds in their structure are unsaturated.
Most naturally occuring fatty acids have an even
number of carbon atoms with varying degrees of
unsaturation. The carboxylate group is numbered
as one and the last carbon atom that is furthest
away from the carboxylate group is known as the
omega carbon.
(Please redraw all figures.) First show the
figure on the left appearing with its labels from
the right end to the left in parts as depicted in
the animation. Next, show the figure on the right
with the labels appearing after the figure.
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Part 1, Step 2
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Systematic name is derived by replacing the final
e of the parent hydrocarbon with oic/oate.
For eg. C16 fatty acid is hexadecanoate (parent
hydrocarbon is hexadecane).
Fatty acids - nomenclature
Palmitic acid (C16, saturated)
2
3
Oleic acid (C18, monounsaturated)
Double bonds referred to by cis/trans- ?-number
at which the double bond is located in
superscript.
181 indicative of 18 carbon atoms with 1 double
bond.
Systematic name cis-?9-octadecenoate
4
Action
Audio Narration
Description of the action
As shown in animation.
(Please redraw all figures.) First show the
figure on top followed by the green callour
located above it. Next show the figure below
followed by the two text boxes and then the
violet callout shown.
The fatty acid names are derived from their
corresponding parent hydrocarbons by replacing
the e at the end with oic or oate. A
saturated fatty acid with 16 carbon atoms, for
instance, is known as hexadecanoate. If there is
one double bond, then it become deceneoate with
the position of the double bond being indicated
as a superscript after a delta symbol. For
instance, a 18 carbon fatty acid with one double
bond is known as ocatadecenoate while with two
double bonds, it is known as octadecadienoate.
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Master Layout (Part 2)
1
This animation consists of 4 parts Part 1
Fatty acids Part 2 Membrane lipids Part 3
Proteins in membrane structures Part 4
Properties of cell membranes
2
Cholesterol
Fatty acid
Sugar unit
3
Glycolipids
4
5
Phospholipids
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Definitions of the componentsPart 2 Membrane
lipids
1
1. Lipids These are water insoluble biomolecules
that readily dissolve in organic solvents like
chloroform and have a wide range of biological
functions. They are important components of
membranes, serve as fuel reserves and signalling
molecules. Three important membrane lipids
include phospholipids, glycolipids and
cholesterol. 2. Phospholipid Phospholipids are
composed of four components fatty acids, a
platform to which the fatty acid is attached,
phosphate residue and an alcohol attached to the
phosphate. The platform to which the fatty acids
are linked is commonly glycerol but in some
cases, a more complex alcohol known as
sphingosine may also be present. Phospholipids
containing glycerol are known as
phosphoglycerides, with two OH groups of glycerol
being esterified with the carboxylate groups of
fatty acids. The fatty acid chains form the
hydrophobic tail while the remaining components
constitute the hydrophilic head group. 3.
Glycolipid Lipids that have a sugar component in
them are known as glycolipids. They are made up
of a sphingosine backbone with the amino group
acylated by a fatty acid and one or more sugar
residues attached to the primary hydroxyl group.
The simplest glycolipid is known as cerebroside
which contains either glucose or galactose as its
sugar residue. 4. Cholesterol Cholesterol is a
steroid molecule whose structure is significantly
different from that of phospholipids and
glycolipids. Cholesterol is found in varying
quantities in animal membranes but is not present
in prokaryotes. It is composed of a hydrocarbon
chain linked to one end and a hydroxyl group at
the other end.
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3
4
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Part 2, Step 1
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Membrane lipids - phospholipids
GLYCEROL
Fatty acid
Fatty acid
Phosphatidate (Diacylglycerol 3-phosphate)
2
Alcohol
Phosphate
Glycerol backbone
Phosphate group
3
Fatty acids
Shorthand depiction
Ester linkage
Polar head group
Hydrophobic tail
4
Action
Audio Narration
Description of the action
(Please redraw all figures.) First show the
coloured block diagram structures on left top.
First the blue rectangle must appear followed by
the two ovals and then the green rectangle and
finally the violet parallelogram. Next, the
structure on bottom right must appear. First the
central region must appear marked glycerol
backbone. Next, the groups on the left marked
fatty acids must appear followed by the pink
group on the right. The green box must then
highlight the region as indicated with the
corresponding label. The figure on bottom left
must then appear with the labels.
As shown in animation.
Phospholipids are composed of four components
fatty acids, a platform to which the fatty acid
is attached, phosphate residue and an alcohol
attached to the phosphate. The platform to which
the fatty acids are linked may be glycerol or
sphingosine. Phospholipids containing glycerol
are known as phosphoglycerides, with two OH
groups of glycerol being esterified with the
carboxylate groups of fatty acids. The simplest
phospholipid, phosphatidate, is made up of only
the phosphate group and fatty acids attached to
the glycerol backbone.
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Part 2, Step 2
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Common phosphoglycerides
Glycerol backbone
2
Fatty acids
3
Phosphatidyl ethanolamine
Phosphatidyl serine
Phosphatidyl choline
Phosphatidyl inositol
Diphosphatidyl glycerol (cardiolipin)
4
Action
Audio Narration
Description of the action
The important phophoglycerides found in membranes
are derived from phosphatidate by esterification
of the phosphate group with the hydroxyl group of
various alcohols. The most commonly observed
phosphoglycerides include phosphatidyl serine,
choline, ethanolamine, inositol and
diphosphatidyl glycerol, also known as
cardiolipin.
As shown in animation.
(Please redraw all figures.) First show the blue
and green parts of the structure with their
labels. Next, the red groups must appear
sequentially one after another with their
corresponding names appearing below as shown in
animation. (The red groups are layered one over
another watch in slide show mode only)
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Part 2, Step 3
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Phospholipids - sphingomyelin
2
Sphingosine
Fatty acid
3
Choline
Sphingomyelin
4
Action
Audio Narration
Description of the action
Sphingosine is another amino alcohol backbone
that serves as a platform for attachment of fatty
acids and alcohols. Sphingomyelin, derived from
sphingosine , consists of a fatty acid linked to
the amino group via an amide bond and a choline
moiety attached to the primary hydroxyl group via
a phosphate group.
As shown in animation.
(Please redraw all figures.) First show the
figure on top with its label followed by the
figure below with its labels as shown.
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Part 2, Step 4
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Membrane lipids - glycolipids
Fatty acid
Sugar residue(s)
2
SPHINGOSINE
3
Cerebroside
4
Action
Audio Narration
Description of the action
Lipids that have a sugar component in them are
known as glycolipids. They are made up of a
sphingosine backbone with the amino group
acylated by a fatty acid and one or more sugar
residues attached to the primary hydroxyl group.
The simplest glycolipid is known as cerebroside
which contains either glucose or galactose as its
sugar residue.
As shown in animation.
(Please redraw all figures.) First show the box
diagrams shown on top left. The green rectangle
must appear first follwed by the blue
parallelogram and then the brown oval. Next show
the structure below in which the blue region must
appear first followed by the green region and
finally the pink labeled box.
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Part 2, Step 5
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Membrane lipids - cholesterol
2
Alkyl side chain
3
D
C
B
Polar head group
A
Steroid nucleus
4
Action
Audio Narration
Description of the action
As shown in animation.
(Please redraw all figures.) Show the appearance
of the structure above. First ring A must appear
followed by ring B, then ring C and then ring D.
All the other groups attached at the various
positions must appear after all four rings have
appeared.
Another group of structural membrane lipids is
the sterols, found in most eukaryotic cells. The
steroid nucleus consists of four fused rings that
are oriented in a planar manner. Cholesterol is
an amphipathic molecule with a polar hydroxyl
head group and a non-polar steroid nucleus and
hydrocarbon side chain. In addition to having a
structural role in membranes, sterols are
precursors for several products such as steroid
hormones and bile acids.
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Part 2, Step 6
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Micellar arrangement
Membrane formation by phosphoglycerides
Hydrogen bonding, electrostatic attraction
Polar head group
Hydrophobic tail
2
Hydrophobic, Van der Waals interactions
3
Random orientation
Bilayer arrangement
4
Action
Audio Narration
Description of the action
The amphipathic nature of the phosphoglyceride
molecules consisting of a polar head group and a
hydrophobic tail enables them to rearrange
themselves in an aqueous environment. When they
come in contact with aqueous surroundings, they
can either reorient to form a micellar structure
or a bilayer arrangement. In these arrangements,
the polar head groups are in contact with water
by means of hydrogen bonding while the
hydrophobic tails interact with each other
through hydrophobic and Van der Waals
interactions. The lipid bilayer arrangement is
more favoured for phospholipids and glycolipids.
As shown in animation.
(Please redraw all figures.) First show the green
structures oriented randomly. Next they must be
surrounded by water (blue clouds). When this
happens, the green structures must reorient
themselves in the two arrangements shown on the
right after the arrows. The arrows and green
ovals with the text must then appear.
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Master Layout (Part 3)
1
This animation consists of 4 parts Part 1
Fatty acids Part 2 Membrane lipids Part 3
Proteins in membrane structures Part 4
Properties of cell membranes
Glycoprotein
2
Outside
Glycolipid
Integral protein
3
Lipid bilayer
Phospholipids
4
Sterol
Inside
Peripheral protein
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Definitions of the componentsPart 3 Proteins
in membrane structures
1
1. Lipid bilayer The flat membrane sheets that
form a barrier around cells consisting of two
layers of lipid molecules is known as the lipid
bilayer. The hydrophobic tail regions are
sequestered within the bilayer, away from the
aqueous environment while the polar heads face
outward and interact with the surrounding
molecules. The bilayer is also embedded with
proteins that perform specific functions for the
cell. 2. Integral proteins Those proteins that
span the membrane and are embedded within the
lipid bilayer are known as integral proteins.
They interact extensively with the hydrophobic
chains of lipids and cannot be easily dissociated
from the membrane. 3. Peripheral proteins
Peripheral membrane proteins, however, are only
bound to the membrane surfaces by means of
electrostatic and hydrogen bond interactions with
the polar head groups of the lipids. They can be
easily dissociated from the membrane with mild
agents such as salts, acids or alkali since they
are not embedded within it. 4. Glycoprotein
Carbohydrate groups are often covalently attached
to proteins to form glycoproteins. The sugar
residues are typically attached to the amide
nitrogen atom of the aspargine side chain or to
the oxygen atom of the serine or threonine side
chain. These glycoproteins are components of cell
membranes and have a variety of functions in cell
adhesion processes.
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3
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Part 3, Step 1
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Integral proteins - Bacteriorhodopsin
Amino acid sequence of membrane protein
2
3
Residues of the 7 membrane-spanning helices
(largely non-polar)
Membrane spanning a-helices
Charged residues
4
Action
Audio Narration
Description of the action
As shown in animation.
Bacteriorhodopsin is an archaeal integral
membrane protein that plays a role in energy
transduction, using light energy for the
transport of protons from inside to outside the
cell. It is made of seven membrane-spanning alpha
helices that are oriented perpendicular to the
plane of the membrane. Determination of the amino
acid sequence of this protein revealed that most
of the residues within the membrane are
non-polar, thereby allowing favorable
interactions with the lipid hydrocarbon chains.
Very few charged residues were found in the
structure.
(Please redraw all figures.) First show the
figure on the left. The red box must then appear
which must be zoomed into to show the figure on
the right with the highlighted regions as
depicted along with all the labels and the key
shown below.
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Part 3, Step 2
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-
-
-
Integral proteins Porins
-
Hydrogen bonded b-strands
Amino acid sequence of porin from
Rhodopseudomonas blastica
2
Hydrogen bonds
3
Hydrophobic residues (on surface of structure)
Hydrophilic residues (buried inside)
4
Action
Audio Narration
Description of the action
Porins are another class of integral membrane
proteins that form channels within the membrane.
They are composed entirely of b-strands with
essentially no alpha helices in their structure.
These beta strands are hydrogen bonded to each
other to form a beta sheet which folds to form a
hollow cylindrical structure. The folding occurs
such that the polar amino acid residues line the
inside of the cylinder, thereby making it
hydrophilic. This allows the channel to be filled
with water and also allows passage of small ions
and charged molecules. The non-polar residues
facing outside interact hydrophobically with the
lipid chains of the membrane.
As shown in animation.
(Please redraw all figures.) First show the
structure on top with its label. Then show the
dotted arrows and the figure on right top. The
brown circles must appear and pass through the
blue cylinder. This must happen continuously
throughout this animation. Simultaneously, the
green box must appear and this region must be
zoomed into and the figure below must be shown
with its labels and the key on the left.
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Part 3, Step 3
1
Peripheral proteins
Acid/alkali added change in pH
Dissociation
Integral proteins
2
Peripheral membrane proteins
3
Dissociation
4
Action
Audio Narration
Description of the action
As shown in animation.
Peripheral membrane proteins are attached to
either the outside or inside surface of the
membrane via electrostatic and hydrogen bond
interactions with either the lipid heads of the
membrane or with other integral proteins. These
polar interactions can be easily disrupted by
addition of acids or alkali which modify the pH
or by addition of salts.
(Please redraw all figures.) First show the
figure in the middle with the yellow and blue
shapes labeled a-e. Next show the blue cloud
appearing on the blue shapes with the
corresponding label. The blue shapes, d e, must
then dissociate from the figure as shown.
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Part 3, Step 4
1
Each amino acid is associated with a free energy
change for its transfer from a hydrophobic to
aqueous environment.
Prediction of transmembrane helices Hydropathy
index
Threshold value for helix detection
2
3
20 amino acid residues
30 Ao
Free energy calculations are made for transfer of
every 20 amino acid residues (i.e. 1-20, 2-21,
3-22 etc.) from hydrophobic to aqueous
environment. This is plotted as a hydropathy plot.
4
Action
Audio Narration
Description of the action
As shown in animation.
(Please redraw all figures.) First show the long
chain like structure shown in the centre with the
labels. Next show the dialogue box on the right
top followed by the dialogue box at the bottom.
Once this is shown, the graph on the right must
appear with the arrow mark and text box.
It is possible to predict transmembrane helix
regions of a protein by calculating the free
energy changes associated with the transfer of
residues from a hydrophobic to aqueous
environment. The width of a membrane is typically
around 30Ao, which can fit approximately 20 amino
acid residues. Therefore the free energy change
for hypothetical alpha helices formed every 20
residues, from residue1 to 20, 2 to 21, 3 to 22
and so on are calculated until the end of the
sequence is reached. These free energy changes
are plotted against the first amino acid residue
of every 20-residue window ito obtain a
hydropathy plot. A peak above 84 kJ/mol is
indicative of a likely membrane spanning helix.
This, however does not detect membrane spanning b
sheets.
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Master Layout (Part 4)
1
This animation consists of 4 parts Part 1
Fatty acids Part 2 Membrane lipids Part 3
Proteins in membrane structures Part 4
Properties of cell membranes
Bleach
Fluorescence Recovery After Photobleaching (FRAP)
2
Recovery
Recovery
Bleach
3
Lateral diffusion
Very fast
1 mm/s
4
Transverse diffusion (flip-flop)
Very slow
t1/2 in days
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Definitions of the componentsPart 4
Properties of cell membranes
1
1. Fluorescence Recovery After Photobleaching
(FRAP) This is a technique by which a cell
surface component is first labelled by means of a
fluorescent molecule and a small region of the
cell surface is viewed by means of fluorescence
microscopy. The fluorescent molecules in the
region being viewed are destroyed by a laser
pulse, a process known as bleaching. Once this
occurs, the time required for fluorescence to
reappear in this region is plotted against the
fluorescence intensity. This helps in
understanding the movement of molecules across
the cell surface. 2. Lateral diffusion The
process by which membrane components move
laterally from one region to another in the same
plane. This is a quick process and takes place in
a matter of microseconds. Proteins exhibit
varying degrees of lateral mobility, with some
being as mobile as lipids and others being almost
immobile. 3. Transverse diffusion (flip-flop)
This is a process by which molecules in the
membrane transition from one surface of the
membrane to the other. The time required for
transverse diffusion is significantly more than
that for lateral diffusion and can be measured by
electron spin resonance techniques. This process
is made quicker by the enzyme flippase. 4.
Fluid Mosaic Model The overall organization and
properties of biological membranes were proposed
by Jonathan Singer and Garth Nicolson in 1972 as
the Fluid Mosaic Model. They proposed that
membranes are two-dimensional solutions of
oriented lipids and globular proteins, with the
lipids serving as a solvent for integral
membrane proteins and functioning as a
permeability barrier. They also hypothesized that
membrane proteins undergo lateral diffusion
freely but not transverse diffusion.
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3
4
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Part 4, Step 1
1
Lateral diffusion of membrane components - FRAP
Bleach
2
Laser
Recovery
Bleaching
Recovery
3
Region being viewed through microscope
Cell surface components labelled with fluorescent
molecule
4
Action
Audio Narration
Description of the action
As shown in animation.
(Please redraw all figures.) First show the blue
figure with the green spots on it and the
corresponding labels. Followed by the red box
and its label. Next, show the laser and its
light falling on the green spot at the bottom.
Once this happens, the green spot must change
color to grey and the label bleaching must
appear. Then the laser must be removed and the
grey spot should move down and disappear and
simultaneously the green spot on top must move
into the red box as shown in animation. When
bleaching occurs, the downward slope of the graph
must be shown and when the green spot on top
enters the red box, the upward curve must be
shown to appear.
Lateral diffusion of membrane components can be
proved using fluorescence recovery after
photobleaching technique. A cell surface
component is first labelled by means of a
fluorescent molecule and a small region of the
cell surface is viewed by means of fluorescence
microscopy. The fluorescent molecules in the
region being viewed are destroyed by a laser
pulse, a process known as bleaching. Fluorescence
however reappears in the region after a certain
time that is dependent on the diffusion
coefficient of the molecules.
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Part 4, Step 2
1
Lateral diffusion Vs Transverse diffusion
Lateral diffusion
Very fast
2
1 mm/s
Flippase
3
Transverse diffusion (flip-flop)
Very slow
Very fast
t1/2 in seconds
t1/2 in days
4
Action
Audio Narration
Description of the action
The Fluid Mosaic model explains the lateral
diffusion of membrane components but not the
transverse diffusion. Lateral diffusion is a
rapid process taking place in the range of
microseconds. However, transverse diffusion, also
known as the flip flop reaction takes place
very slowly over a period of several hours. This
reaction is facilitated by the enzyme flippase,
which carries out transverse diffusion in the
time range of few seconds.
As shown in animation.
(Please redraw all figures.) First show the green
figures on top with the title lateral
diffusion. The blue shape must move as indicated
by the arrow and reach the position indicated on
the right. This must occur quickly. Next show the
figure on left bottom with the title transverse
diffusion. The blue shape alone must flip very
slowly in the direction indicated by the arrow to
reach the position shown on the right. Next, the
brown oval must appear with its label. When this
happens, the flipping must take place quickly in
the same way as the previous diagram.
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Part 4, Step 3
1
Length of fatty acyl chain longer chain, higher
Tm
Factors determining membrane fluidity
Presence of cholesterol greater cholesterol,
higher Tm
2
3
Degree of unsaturation Saturated fatty acids
increase Tm
4
Action
Audio Narration
Description of the action
As shown in animation.
The fluidity of any biological membrane is
dependent on the properties of the fatty acid
chains present in it. Transition of the membrane
from a rigid state to a fluid state occurs
abruptly as the temperature is increased and
crosses the melting temperature, Tm. This melting
temperature is a function of the length of fatty
acyl chains present and their degree of
unsaturation. Increase in length of fatty acyl
chain increases the Tm while increase in the
degree of unsaturation decreases the Tm. In other
words, greater number of double bonds disrupts
the packing order achieved by saturated fatty
acids thereby decreasing the Tm. In animals, the
cholesterol content is another regulator of
fluidity. Greater the amount of the bulky
steroid, higher is the Tm.
(Please redraw all figures.) The graph must
appear gradually. As the graph is appearing in
the centre, the text around it must appear
sequentially as shown.
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Interactivity option 1Step No 1
1
Membrane lipids are useful for designing lipid
vesicles known as liposomes, which consist of
small aqueous compartments surrounded by a lipid
bilayer. These liposomes are increasingly being
used as drug delivery systems in hydrophobic
environments. Shown below is an example for
formation of a glycine-containing liposome. Click
on the green phospholipid layer to view liposome
formation and then answer the question below.
2
Glycine in water
Gel filtration
Sonication
3
Phospholipid (Click here)
4
Interacativity Type Options
Results
(Please redraw all figures.) User must click on
the green layer at the bottom of the first figure
to view the animation after which the question
with 4 options must appear and user must be
allowed to choose any 1 option.
When the user clicks on the green layer, the
animation must be shown. The green layer shown at
the bottom must gradually form green circles as
shown in the middle panel and must surround a few
red dots. Once this happens, the arrow saying
gel filtration must be shown and the red dots
must disappear leaving only the green circles
enclosing the red circles. The user must then
answer the question shown in the next slide.
Correct answer is C. If user gets it right,
correct answer must be displayed otherwise
wrong answer must be displayed.
Click to view experiment then choose the
correct answer.
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Interactivity option 1Step No 1
1
What property of membrane lipids allows them to
form such liposome vesicles?
2
A) Their low melting temperature
B) The presence of cholesterol
C) Their self-sealing nature
3
D) The presence of glycerol in the phospholipids
4
Interacativity Type Options
Results
(Please redraw all figures.) User must click on
the green layer at the bottom of the first figure
to view the animation after which the question
with 4 options must appear and user must be
allowed to choose any 1 option.
When the user clicks on the green layer, the
animation must be shown. The green layer shown at
the bottom must gradually form green circles as
shown in the middle panel and must surround a few
red dots. Once this happens, the arrow saying
gel filtration must be shown and the red dots
must disappear leaving only the green circles
enclosing the red circles. The user must then
answer the question shown in the next slide.
Correct answer is C. If user gets it right,
correct answer must be displayed otherwise
wrong answer must be displayed.
Click to view experiment then choose the
correct answer.
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27
Questionnaire
1

• 1. How many double bonds would be present in a
  fatty acid having the systematic name
  all-cis-?9, ?12, ?15-Octadecatrienoate?
• Answers a) 1 b) 2 c) 3 d)? 4
• 2. Which of the following is a saturated fatty
  acid with 18 carbon atoms?
• Answers a) cis- ?9-Octadecenoate b)
  Octadecanoate c) Eicosanoate
  d)? Tetradecanoate
• 3. Which of the following components is not
  present in Phosphatidyl inositol?
• Answers a) Sphingosine b) Glycerol c)
  Phosphate d)? Inositol
• 4. If the degree of unsaturation of fatty acyl
  chains increases, what happens to the Tm?
• Answers a) Tm increases b) Tm remains same
  c) Tm decreases d)? None of the above
• 5. The threshold value of hydropathy index for
  detection of alpha helices is?
• Answers a) -22 kJ/mol b) 22 kJ/mol c) 67
  kJ/mol d)? 84 kJ/mol

2
3
4
5
28
Links for further reading

• Books
• Biochemistry by Stryer et al., 6th edition
• Biochemistry by A.L.Lehninger et al., 4th edition
• Biochemistry by Voet Voet, 3rd edition
• Research papers
• Singer, S. J. Nicolson, G. L. The Fluid Mosaic
  Model of the Structure of Cell Membranes. Science
  1972, 175 (4023), 720-731.