Cell Biology
Lecture 2
Structure of Plasma membrane
Phospholipidbilayer: protein in embedded in 2
layers of lipid.
 All the cell membranes (organelles) have similar
structure
 In 1925, two Dutch scientists (E. Gorter and R.
Grendel) extracted the membrane lipids from a
known number of red blood cells and provided
the first evidence that biological membranes
consist of lipid bilayers by calculating the surface
area of lipids.
 The principal components of cell membranes,
consisting of two hydrocarbon chains
(usually fatty acids) joined to a polar head group
containing phosphate.
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The lipid bilayer of the plasma membrane is less than
5nm but the individual leaflets of the bilayer can be
resolved. Electron microscopic image
Types of phospholipids
phosphatidylcholine, phosphatidyletha
nolamine, phosphatidylserine,
and sphingomyelin)
 A fifth phospholipid, phosphatidylinositol,
is also localized to the inner half of the
plasma membrane. Although
phosphatidylinositol is a quantitatively minor
membrane component, it plays an important
role in cell signaling.

The outer leaflet consists predominantly of
phosphatidylcholine, sphingomyelin, and glycolipids (their
carbohydrate portion exposed on the cell surface), whereas the
inner leaflet contains phosphatidylethanolamine,
phosphatidylserine, and phosphatidylinositol.
 Cholesterol is distributed in both leaflets and is present in about
the same molar amounts as the phospholipids. The net negative
charge of the head groups of phosphatidylserine and
phosphatidylinositol is indicated.
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Function of Plasma Membrane
Mechanical Support
 Cell Signaling
 Selective permeability
 Active transport
 Bulk Transport
 Metabolic activity

Mechanical Support
1. Defines/encloses the cells: maintain the
physical integrity by enclosing the cytoplasm and
organelles. Forms a barrier between intra and
extracellular environment.
 2. Maintains the cytoskeleton: Keeps the
organelles and proteins in an appropriate position
 3. Extracellular Matrix: cell membrane of one cell
interacts with the cell membrane of adjacent cell.
 4. Protection: protects the cell from harmful
chemicals/ infections present in the extracellular
environment
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Cell Signaling
 Cells can not live in an isolated environment.
 Prokaryotes communicate with each other, other
organisms and surrounding environment.
 Eukaryotes e.g yeasts, slime molds, and
protozoans mate, differentiate and respond to the
environment by secreting pheromones
Cells are able to receive and process signals.
Individual cells receive many signals
simultaneously, and they then integrate the
information they receive into a unified action
plan.
 They also send out messages to other cells both
near and far.
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What kind of signals do cells receive?
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Most cell signals are chemical in nature.
Prokaryotic organisms have sensors that detect
nutrients and help them navigate toward food
sources.
In multicellular organisms, growth factors,
hormones, neurotransmitters, and extracellular
matrix components are some of the many types
of chemical signals cells use.
These substances can exert their effects locally,
or they might travel over long distances.
Some cells also respond to mechanical stimuli.
For example sensory cells in skin and ear.
General features of phospholipid
bilayer
First, the structure of phospholipids is responsible for the
basic function of membranes as barriers between two
aqueous compartments. Because the interior of
the phospholipid bilayer is occupied by hydrophobic fatty
acid chains, the membrane is impermeable to water-soluble
molecules, including ions and most biological molecules.
 Second, bilayers of the naturally occurring phospholipids are
viscous fluids, not solids. The fatty acids of most natural
phospholipids have one or more double bonds, which
introduce kinks into the hydrocarbon chains and make them
difficult to pack together. The long hydrocarbon chains of
the fatty acids therefore move freely in the interior of the
membrane, so the membrane itself is soft and flexible. In
addition, both phospholipids and proteins are free to diffuse
laterally within the membrane—a property that is critical
for many membrane functions.

Signaling in plants and animals
In plants and animals, extra cellular
signaling molecules control
 Metabolism
 Growth and differentiation of tissues
 Synthesis and secretion of proteins
 Composition of intracellular and
extracellular fluids
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General principle signaling
1.
2.
3.
4.
5.
6.
Synthesis of signaling molecules by the
signaling cells
Release of signaling molecules
Transport of the signal to the target cell
Detection of a signal by a specific receptor
protein present on the target cell
A change in cellular metabolism, function
or development triggered by the receptorsignal complex
Removal of the signal, which often
terminate the cellular response
1
3
2
4
5
6
Signaling cell
Cellular responses due to cell
signaling
Changes in the activity or function of
specific enzymes and other proteins
present in the cells
 Changes in the amount of protein
produced by a cell e.g. modification of
transcription factors that stimulate or
repress gene expression

Types of signaling
Gap Junctions Allow Signaling Information
to Be Shared by Neighboring Cells
Signals are passed to the neighboring cells through
gap junctions.
 These are specialized cell-cell junctions that can
form between closely apposed plasma membranes,
directly connecting the cytoplasms of the joined
cells via narrow water-filled channels.
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(a)
(b)
(c)
(d)
(c1)
(c2)
(c3)
(c4)
(c5)
Receptor protein exhibit ligand
binding effect
Receptor present on Plasma or nuclear
membrane has ligand binding sites
 Signaling molecules (hormones,
pheromones or neurotransmitters) act as
ligands
 Confirmational change occurs in the
receptor that initiate a sequence of
chemical reactions
 Receptor proteins are specific for each
horomone
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Different cells have different sets of receptor for
the same ligand and each of which induces a
different response
Different cells respond in a variety of way to the
same ligand (e.g. acetylcholine)
Different ligands can induce the same cellular
response in some cells (glucagon/epinephrine)
In most receptor-ligand system, the ligand do not
have any function except to bind to receptor
Upon binding it changes the properties of
receptor which then produce signals to the cell
that a specific product is present
Target cells often degrade or modify the ligand to
terminate or modify their response
The same signaling molecule can
induce different responses in different
target cells
Lecture prepared from
The Cell: A Molecular Approach. 2nd
edition
http://www.ncbi.nlm.nih.gov/books/NBK989
8/
 Molecular Cell Biology, Lodish and co
5Edition, Chapter 15
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