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Chapter 3
Cell Organelles (Plasma
Membrane (Phospholipid)
Slide 46-67
Plasma Membrane
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protein
molecules
phospholipid
bilayer
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Cell/Plasma Membrane
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
Surrounds all types of cells.
Prokaryote & Eukaryotes


In a plant cell, it lies
beneath the cell wall
In animal cells, it is the
outer boundary (made of
cholesterol)

Provides cell with



Protection and supports the
cell
Control of movement of
materials in/out of cell
Maintains condition of cell
Cytoplasmic Membrane
• Surrounds the cell and
decides what comes in
and out
• Semi-permeable: allows
nutrients in and waste
products out
• Made of a phospholipid
bilayer
• Also called Plasma
Membrane
Cytoplasmic Membrane
Factory Part:
Gates or Doors
Found in:
Plant cells
Animal cells
Prokaryotic cells
Cell membrane

Encloses the cell contents, security gate

phospholipid bilayer with embedded proteins
Phospholipid bilayer
Membrane proteins
Cytoplasmic Membrane

The cytoplasmic/plasma/cell membrane
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In eukaryotic cells, as in prokaryotes
Fluid phospholipid bilayer embedded with
proteins and glycoproteins.
Phospholipid bilayer is arranged so that the polar
ends of the molecules (the phosphate and glycerol
portion of the phospholipid that is soluble in water)
form the outermost and innermost surface of
the membrane while the non-polar ends (the fatty
acid portions of the phospholipids that are
insoluble in water) form the center of the
membrane
Cytoplasmic Membrane

The cytoplasmic/plasma/cell membrane
 Contains glycolipids as well as complex lipids called sterols
 Cholesterol molecules found in animal cell membranes not found in
prokaryotic membranes(except for some mycoplasmas
 Sterols make the membrane less permeable to most biological molecules
 Help to stabilize the membrane
 Probably add rigidity to the membranes aiding in the ability of eukaryotic
cells lacking a cell wall to resist osmotic lysis.
 Proteins and glycoproteins in the cytoplasmic membrane are quite diverse and
function as:
 a. channel proteins to form pores for the free transport of small
molecules and ions across the membrane
 b. carrier proteins for facilitated diffusion and active transport of
molecules and ions across the membrane
 c. cell recognition proteins that identifies a particular cell
 d. receptor proteins that bind specific molecules such as hormones and
cytokines
 e. enzymatic proteins that catalyze specific chemical reactions.
Cellular membranes are fluid mosaics
of lipids and proteins

Phospholipids are the most abundant lipid in the plasma
membrane

Phospholipids are amphipathic molecules, containing
hydrophobic and hydrophilic regions
(having two different affinities, as a polar end that is
attracted to water and a nonpolar end that is repelled by
it.)

The fluid mosaic model states that a membrane is a
fluid structure with a “mosaic” of various proteins
embedded in it
Plasma Membrane
Contains cell contents
 Double layer of phospholipids & proteins

Phospholipids

Polar



Hydrophylic head
Hydrophobic tail
Interacts with water
AP1: BIOCHEMISTRY: PHOSPHOLIPIDS
https://www.youtube.com/watch?v=sHN7e52Gh4U
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BCLN - Phospholipids - Biology
https://www.youtube.com/watch?v=SH_LgL12k1g
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Cell/Plasma Membrane
http://legacy.hopkinsville.kctcs.edu/instructors/Jason-Arnold/VLI/Module%202/m2cellstructure/f4-05_a_model_of_the_pl_c.jpg
The Plasma Membrane

In animal cells, the plasma membrane has
four principal components:
1. A phospholipid bilayer.
2. Molecules of cholesterol interspersed within the
bilayer.
3. Proteins that are embedded in or that lie on the
bilayer.
4. Short carbohydrate chains on the cell surface,
collectively called the glycocalyx, that function
in cell adhesion and as binding sites on
proteins.
The Plasma Membrane
phospholipids
cholesterol
proteins
glycocalyx
cell
exterior
cytoskeleton
Phospholipid bilayer:
a double layer of
phospholipid
molecules whose
hydrophilic “heads”
face outward, and
whose hydrophobic
“tails” point inward,
toward each other.
peripheral
protein
Cholesterol molecules
that act as a patching
substance and that
help the cell maintain
an optimal level of
fluidity.
integral
protein
Proteins, which are
integral, meaning
bound to the
hydrophobic interior of
the membrane, or
peripheral, meaning not
bound in this way.
cell
interior
Glycocalyx: sugar
chains that attach to
proteins and
phospholipids, serving
as protein binding sites
and as cell lubrication
and adhesion
molecules.
Figure 5.1
The Phospholipid Bilayer

Phospholipids are molecules composed of
two fatty acid chains linked to a charged
phosphate group.

The fatty acid chains are hydrophobic,
meaning they avoid water, while the
phosphate group is hydrophilic, meaning it
readily bonds with water.
The Phospholipid Bilayer

Such phospholipids arrange themselves
into bilayers—two layers of phospholipids
in which the fatty acid “tails” of each layer
point inward (avoiding water), while the
phosphate “heads” point outward (bonding
with it).
The Phospholipid Bilayer

Phospholipids take
on this
configuration in the
plasma membrane
because a watery
environment lies
on either side of
the membrane.
The Phospholipid Bilayer

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As temperatures cool, membranes switch from a fluid state to a
solid state
The temperature at which a membrane solidifies depends on the
types of lipids
Membranes rich in unsaturated fatty acids are more fluid than
those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about
as fluid as salad oil
The steroid cholesterol has different effects on membrane fluidity
at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains
movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight
packing
Figure 7.8
Fluid
Unsaturated hydrocarbon
tails
Viscous
Saturated hydrocarbon tails
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol within the animal
cell membrane
Cholesterol
Cell membranes are made of
PHOSPHOLIPIDs


HYDROPHILIC heads
(water liking)
-Attracted to the water
called POLAR

HYDROPHOBIC tails
(water fearing)
-Not attracted to the
water
 called NON-POLAR
A Phospholipid
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