Plasma Membrane (Structure and Functions)

Plasma Membrane is a selectively permeable bio membrane that performs variety of functions for the survival of a living system !

1. Harjinder Singh
Dept. Of Botany
Meerut College, Meerut
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2. Faculty: Science
Department: Botany
Name: Harjinder Singh
Name of degree programme: B Sc. II
Course: Cytology, Genetics, Evolution and Ecology
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3. Plasma membrane or cell membrane is an outermost
envelope surrounding the cytoplasm of cell, that
separates and protect the cell contents from the
extracellular environment and provides the
connectivity between the cell and it’s external
environment.
It is living and selective permeable membrane
common to all cells made of phospholipids, proteins
and some conjugated molecules. It regulates the
materials that enter and exit the cell.
Cell Membrane can be divide into:
a) Cytoplsamic membrane: It controls the internal
environment of the cell.
b) Internal membrane: Itencloses some cell
organelles –mitochondria, chloroplast, nucleus etc.
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4. First time In 1672, Nehemiah Grew studying the cell made the description of “mass of
bubbles” in relation to plant parenchyma . In 1682 he reported that the cell membrane
resembled a lace-like material. His research established the fact that both plant cavities
and their fibers showed continuity and all plant organisms had membrane systems.
•In 1837,Schleiden(botanist) and in 1839, Schwann suggested that there was universal
mechanism for the development of cell.
•In 1844 C. Nageliconducted the membrane permeability studies.
•In 1854 N. Pringsheimshowed that there was a membrane around protoplast of which
permeability varied depend on conditions.
•In 1855, C. Nageliand K. Cramer further conducted membrane permeability tests of plant
cells and established the existence of cell membrane around protoplasm.
•In 1877 Wilhelm Pfefferproposed the Cell Membrane Theory that cell was seen to be
enclosed by a thin surface.
•In 1899 C. Overton showed that membrane was a lipid structure.
•In 1925 Gorter and Grandelstudied the RBCs that suggested that membrane was
consisted of lipids only ( Idea rejected).
•1n 1932 Harveyand Colefirst suggested that lipid membrane was surrounded by proteins.
•!n 1935 Daneilliand Davson proposed first classical (Sandwich) model.
•In 1959 Robetsonproposed famous Unit Membrane Concept( Trilaminar Model)
, with the help of electron microscope and showed thtplasma membrane consists of three
layers: outer and inner of proteins and middle of lipids.
•In 1972 Singerand Nicholsonproposed most accepted Fluid Mosaic Model describing
that plasma membrane are composed of phospholipids bilyer with various protein
molecules floating within it.
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5. Chemically plasma membrane consists of lipids,
proteins held together by non covalent bonds and
some carbohydrates are also attached to the lipids
and proteins
The ratio of lipid to protein in a membrane varies,
depending on the type of cellular membrane
(endoplasmic reticulum vs. Golgi apparatus), the
type of organism (bacterium vs. plant vs. animal),
and the type of cell.
Various kinds of enzymes are reported in different
biomembranes e.g. phosphatases, ATP-ase,
esterases, nucleases etc.
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6. Depending upon it’s characteristic lipid composition, each type of cellular
membrane (3-6 nm thick) differs from one another in the types of lipids, the
nature of the head groups, and the particular species of fatty acylchain.
Membranes contain a wide diversity of lipids, all of which are amphipathic; that
is, they contain both hydrophilicand hydrophobic regions. There are three
main types of membrane lipids: phosphoglycerides, sphingolipids, and
cholesterol (in animals) and sterol (plants).
The phospholipid bilayer has a polar hydrophilic (water attracting) headof
phospholipids facing outwards and their non-polar, hydrophobic (water
repelling) tailsfacing in towards the middle of the bilayer. membrane
phosphoglycerides have an additional group linked to the phosphate, most
commonly either choline(forming phosphatidylcholine, PC), ethanolamine
(forming phosphatidylethanolamine, PE), serine (forming phosphatidylserine,
PS), or inositol(forming phosphatidylinositol, PI). Each of these groups is small
and hydrophilic and, together with the negatively charged phosphate to which it
is attached, forms a highly water-soluble domain at one end of the molecule,
called the head group. The fatty acylchains are hydrophobic, unbranched
hydrocarbons approximately 16 to 22 carbons in length.
Hydrophilicportions of both proteins and phospholipids are maximally exposed
to water resulting in a stable membrane structure.
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7. The structure of a phospholipid molecule: This phospholipid molecule
is composed of a hydrophilic head and two hydrophobic tails. The
hydrophilic head group consists of a phosphate-containing group attached
to a glycerol molecule. The hydrophobic tails, each containing either a
saturated or an unsaturated fatty acid, are long hydrocarbon chains.
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9. The back bone of the membrane is composed of amphiphilic phospholipid molecules.
The hydrophilic or water-loving areas of these molecules are in contact with the aqueous
fluid both extracellular and cytoplsamic sides the cell. Hydrophobic, or water-repelling
molecules, tend to be non-polar.
A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid
molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the
third carbon. This arrangement gives the overall molecule, a polar with negative charge,
and a tail (the fatty acids), with no charge.
They interact with other non-polar molecules in chemical reactions, but generally do not
interact with polar molecules. When placed in water, hydrophobic molecules tend to form
a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen
bonds with water and other polar molecules on both the exterior and interior of the cell.
Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic.
In contrast, the middle of the cell membrane is hydrophobic and will not interact with
water. Therefore, phospholipids form an excellent lipid bilayer cell membrane that
separates fluid within the cell from the fluid outside of the cell.
❖Lipid bilayer facilitates the regulated fusion or budding of membranes and has ability to
self-assemble. It is important for maintaining the proper internal composition of a cell, in
separating electric charges across the plasma membrane, and in many other cellular
activities.
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10. It is the membraneProteins, thatare
responsible for carrying out specific cell
membrane functions. Depending on the cell
type and the particular organelle within that cell,
a membrane may contain hundreds of different
proteins.
Each membrane protein has a defined
orientation relative to the cytoplasm, so that the
properties of one surface of a membrane are
very different from those of the other
surface(Sidedness)
continue……
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11. On the basis of their relationship(position) to the lipid bilayer
membrane proteins can be grouped into threedistinct classes,
though they perform various functions.
1. Integral proteins ortransmembrane proteins: They present
across the lipid bilayer thus have domains that protrude from both
the extracellular and cytoplsamic sides of the membrane.
2. Peripheral proteins: They are located entirely outside of the lipid
bilayer, on either the cytoplsamic or extracellular side, yet are
associated with the surface of the membrane by non covalent bonds.
3. Lipid-anchored proteins: They are located outside the lipid
bilayer, on either the extracellular or cytoplsamic surface, but are
covalently linked to a lipid molecule that is situated within the bilayer.
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12. 1.Integral proteins : They are amphipathicin nature
may act as
a) receptorsthat bind specific substances at the membrane
surface.
b) channels or transporters involved in the movement of ions
and solutes across the membrane.
c) agentsthat transfer electrons during the processes of
photosynthesis and respiration.
Integral or transmembrane contain one or more transmembrane
helices.
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13. •Integral proteins are embedded permanently in the
membrane by hydrophobic, electrostatic, and other non-covalent
interaction. The most common type of integral proteins are
transmembrane proteins, which span across the lipid bilayer.
•They usually adopt an α-helical configuration . Single-pass
membrane proteins cross the membrane only once, while the
multiple-pass membrane proteins are crossing the membrane
several times.
•Many of the integral membrane proteins function as ion channels
or transporters, regulating the influx of ions/molecules between the
extracellular and intracellular spaces.
•Cell surface receptors, linkers, enzymatic proteins, and proteins
responsible for cell adhesion are all classes of integral membrane
proteins .
•Recent studies have shown that activity of these proteins
depends on the lipid composition and membrane-protein
interactions.
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14. 2. Peripheral proteins : They are associated
with the membrane by weak electrostatic bonds .
These proteins provide mechanical support for the
membrane and function as an anchor for integral
membrane proteins. Some peripheral proteins on the
internal plasma membrane surface function as
enzymes, specialized coats or factorsthat transmit
transmembrane signals. They have a dynamic
relationship with the lipid membrane
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15. ❖Peripheral proteins temporarily bind to the surface of the
membrane with weak interactions. They have a unique amino acid
sequence which allows them to bind and congregate on the surface
of the membrane.
•There is no hydrophobic region of amino acids in peripheral
proteins structure, therefore they can attach to membrane surface
without being locked onto it.
•The primary role of peripheral proteins is to provide a point of
attachment for other components to the cell membrane. Both
membrane cytoskeleton and components of extracellular matrix are
linked to the cell membrane through peripheral proteins, thus they
help the cell to maintain its
shape while the membrane remains flexible to bend based on the
cellular functions.
•They are also involved in various other functions including cell
communication, energy transduction, and molecule
transfer across the membrane
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16. 3.Lipid-Anchored Membrane Proteins : Various
types of lipid-anchored membrane proteins present on the external
face of the plasma membrane . They are bound to the membrane by a
small, complex oligosaccharide linked to a molecule of
phosphatidylinositolandcalled GPI (glycosyl-phosphatidylinositol)-
anchored proteins.Theymay be also present on the inner side of
membrane and anchored to the lipids by hydrocarbon chains
They act as receptors, enzymes, andcell-adhesion proteins.
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17. The plasma membranes of eukaryotic cells also contain
carbohydrates in the form of conjugated molecules.
Depending on the species and cell type, the carbohydrate content of
the plasma membrane ranges between 2 and 10 percent.
More than 90 percent of the membrane’s carbohydrate is covalently
linked to proteins to form glycoproteinsand remaining
carbohydrate is covalently linked to lipids to form glycolipids.
Glycoproteins are almost found in all living organisms and serve a
number of important roles as structural molecules, immunologic
molecules, transport molecules, receptors, enzymes, and hormones.
Along with carbohydrates some molecules of
cholesterol(Animal)/ Sterol(Plant) are also embedded in the lipid
bilayer to provide fluidity to the membrane.
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20. 1.Gorterand Grendelmodel (1925)
2.Bilayer Model or Classical Sandwich Model
by Danielliand Davson(1935)
3. Unit Membrane Concept (Trilaminar Model)
by J D Robertson(1959)
4.Fluid Mosaic Model by J Singer and G
Nicolson(1972)
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21. They studied on RBCs. of various mammals and
proposed that cell membrane is composed of
lipids only.
Hydrophobic ends toward the interior and
hydrophilic ends point outward of the membrane.
They showed that molecules could make single
or double layers.
They opened the door to the identification of the
molecular structures of the membrane
Though model was imperfect because lipids are
impermeable to the molecules.
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22. After the findings of Harveyand
Colethat lipids layer are
surrounded by proteins, Danielli
and Davsoncame up with a
sandwich membrane model or
protein--protein model.
According to this model, there are
four layers as P-L-L-P.
The surfaces were surrounded by
a thin layer of protein on both
sides just like sandwich..
Lipids molecule are amphipathic
with hydrophilic heads towards
protein and hydrophobic tails
towards centre.
But due to lack of the facility of
higher resolution power of
electron microscope the actual
assembly of protein and lipids
could not revealed.
---2nm
--3.5nm
--2nm
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23. The amount and types of vary greatly
between different cells.
It was unable to describe how the proteins
would allow the membrane to change shape
without bonds being broken.
Membrane proteins are largely hydrophobic
and therefore should not be found where the
model positioned them.
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24. In 1959, Robertson ,observed the structure in
electron micrograph staining the tissues with
osmium and saw Trilaminar appearance of
membranes.
He saw no space for the pores in the micrograph.
Osmium binds preferentially to the polar head
groups of the lipid bilayer, producing the
trilaminar.
The transverse sections of the membranes
revealed the three-layer membrane structure
known as the ‘railway’. In this view, which would
later be called “Unit membrane”, there was a
third layer between the two dense layers which
was less dense pattern.
Robertson suggested that the dark regions were
protein layers, while the open area in the middle
was equivalent to the lipid layer
Though this model provided great insight into the
structure of PM but functioning could not
understand perfectly.
In 1964, Brady and Tramsreported that
membrane are composed of lipids and proteins,
where proteins entered the membrane and the
lipid components are fluid, that paved the way to
development of Fluid Mosaic Model 24H. SINGH

25. With the development of freeze-breaking techniques
and immuno-electron microscopy techniques, scientists
identified isolated membrane proteins and membrane-
embedded proteins through antibodies.
On the basis of their findings Singerand Nicolsonintroduced most
accepted and famous FLUID MOSAIC MEMBRANE MODEL in
1972.
“This model describe P M as a flexible boundary of
the cell. It states that it is a lipid bilayer in which
proteins occur as a ‘mosaic’ of discontinuous
particles that penetrate irregularly deep or even
through the lipid bilayer. Phospholipids molecules
present in a fluid state and capable to move and
rotate freely”.
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27. According to this model,
It is ‘ fluid’ because phospholipid
are free to move. Phospholipids in
the lipid bilayercan either move
rotationally, laterally in one bilayer, or
occasionally undergo
transversemovement (flip flop)
between bilayers They can only move
from side to side, however, not
through the membrane.
▪It is called a ‘ mosaic’ because the
proteins are embedded in the
phospholipid bilayer (like mosaic
tiles embedded in mortar).
Hydrophilicportions of both proteins
and phospholipids are maximally
exposed to water resulting in a stable
membrane structure.
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29. Transport:
The plasma membrane regulates physically
transporting substances from one side of the
membrane to another, often from a region where
the solute is at low concentration into a region of
much higher concentration.
It allows a cell to accumulate substances for
metabolism and building macromolecules.
A protein that spans the membrane may provide a
hydrophilic channel ( aquaporins) across the
membrane that is selective for a particular solute
or porins for selected molecules.
-
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30. There are four broad categories of molecules found in the cellular environment. Some
of these molecule can cross the membrane and some of them need the help of other
molecules or processes.
hydrophilic Molecules are capable of forming bonds with water and other
hydrophilic molecules, called polar molecules.
hydrophobic are called nonpolar molecules. Here are the 4 types:
Small, nonpolar molecules(ex: oxygen and carbon dioxide) can pass through the
lipid bilayer and do so by squeezing through the phospholipid bilayers. They don't
need proteins for transport and can diffuse across quickly.
Small, polar molecules (ex: water): water molecules diffuse through the membrane
without the help of proteins but it slower process because the interior of the
phospholipid bilayer is made up of the hydrophobic tails.
Large, nonpolar molecules (ex: carbon rings): These rings can pass through but it is
also slow process.
Large, polar molecules (ex: simple sugar -glucose) and ions: The charge of an ion,
and the size and charge of large polar molecules pass through the nonpolar region of
the phospholipid membrane with the help of transmembrane proteins
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31. 1. Passive Transport:
a) Simple Diffusion.
b) Facilitated Diffusion
2. Active Transport: It may be -
a) Primary active transport.
b) Secondary Active transport
3. Bulk TRANSPORT:
a) Endocytosis
b) Exocytosis
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32. (A) Passive Diffusion:
Diffusion of nonpolar,
hydrophobic molecules like
Lipids from high to low
concentration gradient /or
Simple diffusion through
an aqueous channel
(aquaporins)formed within
an integral membrane
protein or a cluster of such
proteins. As in a, movement
is always down a
concentration gradient.
It does not required energy
or membrane transport
proteins.
e.g. transport of gases, ions
and lipid soluble molecules
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33. ▪Diffusion of molecules down the
concentration gradient which
required carrier proteins but
does not require energy.
▪Diffusion of hydrophilic
molecules ,in which solute
molecules bind specifically to a
membrane protein carriers
(facilitate transporters).
▪Based on the direction of
molecules it may be;
▪a) Uniport: Transport of single
type of molecule in one direction.
▪B) Antiport: transport of
molecules in opposite direction.
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34. It is diffusion against concentration gradient
from Low to high.Active transport by means
of a protein transporter (protein pumps)
with a specific binding site that undergoes a
change through energy driven by ATP .
It may be:
a) Primary Active transport: Transport against
gradient in which energy(ATP) used directly.
It requires carriers. e. g. sodium-potassium
pump, calcium pump.
b) Secondary Active Transport:Transport
against gradient in which energy (ATP) is
used indirectly. Here tow or three molecules
coupled e.g.NA-K and ATPase
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36. Also called transport by vesicles formation, which
involves formation of membrane bound vesicles.
It facilitates transport of macromolecules.
Two Types:
1. Endocytosis: is when a cell ingests relatively larger
contents than the single ions or molecules that
pass through channels. It may be:
a) Phagocytosis: engulfing of solid particles.
b) Pinocytosis: engulfing of liquid particles.
2. Exocytosis : expulsing out of the molecules from the
cell thecell membrane.
The shape of the membrane itself changes to allow
molecules to enter or exit the cell.
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37. Passive osmosis and diffusion: transports
gases (such as O
2and CO
2)and other small
molecules and ions
Transmembrane protein channels and
transporters: transports small organic
molecules such as sugars or amino acids
Endocytosis: transports large molecules (or
even whole cells) by engulfing them
Exocytosis: removes or secretes substances
such as hormones or enzymes
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38. Some glycoproteinserve
as identification tagsthat
are specifically recognized
by other cells.
Cells are able to recognize
other through these tags.
Provide mechanisms for a
cell to recognize itself and
other cells of its particular
individual organisms vs.
non-self (foreign
materials).
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39. Membrane proteins of adjacent cell may
be hooked together in various kinds of
junction:
Plasmodesmata are junctions between plant
cells, whereas animal cell contacts are carried
out through tight junctions, gap junctions, and
desmosomes.
A plasmodesma is a channel between the cell
walls of two adjacent plant cells that allow
materials to pass from the cytoplasm of one
plant cell to the cytoplasm of an adjacent cell.
A tight junction is a watertight seal between
two adjacent animal cells, which prevents
materials from leaking out of cells.
Desmosomes connect adjacent cells when
cadherins in the plasma membrane connect to
intermediate filaments.
Gap junction (communicating junction),
provide cytoplsamic channel for ions,
nutrients and other substances that allow the
cells to communicate.
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40. Enzymatic activity
▪Several enzymes in a membrane are ordered
as a team that carries out sequential steps in
metabolic pathway.
Signal transduction
The plasma membrane plays a critical role in
the response of a cell to external stimuli such
as a hormone, a process known as signal
transduction.
Membranes possess receptor proteins that
combine with specific molecules (ligands) or
respond to other types of stimuli such as light
or mechanical tension.
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41. Microfilaments or other elements of the cytoskeleton
may bonded to membrane proteins, a function that
helps maintain cell shapeand fixes the location
of certain membrane proteins .
Protein that adhere to the ECM can coordinate
extracellular and intracellular changes. (integrate
changes occurring outside and inside the cell)
Fibres of
extracellular matrix
(ECM)
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42. Plasma Membranes are intimately involved in the
processes by which one type of energy is converted
to another type (energy transduction).
The most fundamental energy transduction occurs
during photosynthesis when energy in sunlight is
absorbed by membrane-bound pigments, converted
into chemical energy, and stored in carbohydrates.
Membranes are also involved in the transfer of
chemical energy from carbohydrates and fats to ATP.
In eukaryotes, the machinery for these energy
conversions is contained within membranes of
chloroplasts and mitochondria.
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43. The structure ofPhospholipid responsible for the
basic function of membranes as barriers between
two aqueous compartments. Because the interior of
thephospholipid bilayeris occupied by
hydrophobic fatty acid chains, the membrane is
impermeable to water-soluble molecules, including
ions and most biological molecules. This protects all
the components of the cell from the outside
environment and allows separate activities to occur
inside and outside the cell.
It also provides structural support to the cell. It
tethers thecytoskeletonwhich is a network of
protein filaments inside the cell that hold all the
parts of the cell in place.
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44. Plasma membranes are selectively permeable (or
semi-permeable), meaning that only certain
molecules can pass through them.
Water, oxygen, and carbon dioxide can easily
travel through the membrane.
Generally, ions (e.g. sodium, potassium) and polar
molecules cannot pass through the membrane;
they must go through specific protein channels or
pores in the membrane instead of freely diffusing
through.
This way, the membrane can control the rate at
which certain molecules can enter and exit the cell.
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45. The plasma membrane may have extensions,
such as whip-like flagella or brush-like cilia, that
give it other functions.
In single-celled organisms, these membrane
extensions may help the organisms to move.
In multicellular organisms, the extensions have
different functions.
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46. Plasma Membrane acts as physical barrier b/t extracellular
environment and cytoplasm of the cell.
Gives the shape to the cell and protects the cell from external
environment.
It is selectively permeable and hence regulates the transport of
molecules. Lipid soluble molecules like O2 and CO2 and water can
easily diffuse.
Different kinds of membrane proteins catalyze respective enzymatic
activities
Participates in absorption, excretion and secretion.
It is responsible for intercellular communication.
It helps in maintaining the turgidity of the cell.
It serves as a receptor for various chemical stimuli such as amino
acids, hormones, and sugars.
In certain unicellular organisms like amoeba, plasma membrane
performs the function of ingestion of food (endocytosis) and
locomotion too. It also facilitate exocytosis process

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47. Cell membranes are dynamic (not static),
fluidstructure which held togetherby
hydrophobic interactions.
Membrane fluidity can be affected by:
a)The movement of phospholipids and some
proteins.
b) Phospholipidswith unsaturated hydrocarbon
tailsmaintain membrane fluidity at lower
temperature.
c) Length of fatty acylchain.
d) Presence of cholesterol/ sterol(plants).
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49. THANKS…….
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