Lecture 3.pptx

2
Cell Membrane Structure and Function

3
Membranes and Cell Transport
• All cells are surrounded by
a plasma membrane.
• Cell membranes are
composed of a lipid bilayer
with globular proteins
embedded in the bilayer.
• On the external surface,
carbohydrate groups join
with lipids to form
glycolipids, and with
proteins to form
glycoproteins. These
function as cell identity
markers.

4
Fluid Mosaic Model
• In 1972, S. Singer and G. Nicolson proposed the Fluid
Mosaic Model of membrane structure
Extracellular fluid
Carbohydrate
Glycolipid
Transmembrane
proteins
Glycoprotein
Peripheral
protein
Cholesterol
Filaments of
cytoskeleton
Cytoplasm

5
Phospholipids
• In phospholipids, two of the –OH groups on glycerol are joined to
fatty acids. The third –OH joins to a phosphate group which joins,
in turn, to another polar group of atoms.
• The phosphate and polar groups are hydrophilic (polar head) while
the hydrocarbon chains of the 2 fatty acids are hydrophobic
(nonpolar tails).
Structural formula Space-filling model Phospholipid symbol
Hydrophilic
head
Hydrophobic
tails
Fatty acids
Choline
Phosphate
Glycerol

6
Phospholipids
• Glycerol
• Two fatty acids
• Phosphate group
Hydrophilic
heads
Hydrophobic
tails
ECF WATER
ICF WATER

7
Phospholipid Bilayer
• Mainly 2 layers of phospholipids; the non-polar tails
point inward and the polar heads are on the surface.
• Contains cholesterol in animal cells.
• Is fluid, allowing proteins to move around within the
bilayer.
Polar
hydro-philic
heads
Nonpolar
hydro-phobic
tails
Polar
hydro-philic
heads

8
The Fluidity of Membranes
• Membrane molecules are held in place by relatively weak hydrophobic
interactions.
• Most of the lipids and some proteins drift laterally in the plane of the
membrane, but rarely flip-flop from one phospholipid layer to the other.
• Membrane fluidity is influenced by temperature. As temperatures cool,
membranes switch from a fluid state to a solid state as the phospholipids
pack more closely.
• Membrane fluidity is also influenced by its components. Membranes rich in
unsaturated fatty acids are more fluid that those dominated by saturated
fatty acids because the kinks in the unsaturated fatty acid tails at the
locations of the double bonds prevent tight packing.
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)

9
Membrane Components
• Steroid Cholesterol
o Wedged between phospholipid molecules in the plasma
membrane of animal cells.
o At warm temperatures (such as 37°C), cholesterol restrains the
movement of phospholipids and reduces fluidity.
o At cool temperatures, it maintains fluidity by preventing tight
packing.
o Thus, cholesterol acts as a “temperature buffer” for the
membrane, resisting changes in membrane fluidity as
temperature changes.
Cholesterol

10
Membrane Components
• Membrane carbohydrates
o Interact with the surface molecules of other cells, facilitating cell-cell recognition
o Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cell from
another
• Membrane Proteins
o A membrane is a collage of different proteins embedded in the fluid matrix of the
lipid bilayer
o Peripheral proteins are appendages loosely bound to the surface of the membrane
o Integral proteins penetrate the hydrophobic core of the lipid bilayer
o Many are transmembrane proteins, completely spanning the membrane
Glycoprotein
Carbohydrate
Microfilaments
of cytoskeleton Cholesterol Peripheral
protein
Integral
protein
Glycolipid
Fibers of extracellular
matrix (ECM)
N-terminus
C-terminus
a Helix
CYTOPLASMIC
SIDE
EXTRACELLULAR
SIDE

11
Functions of Cell Membranes
• Regulate the passage of substance into
and out of cells and between cell
organelles and cytosol
• Detect chemical messengers arriving at
the surface
• Link adjacent cells together by membrane
junctions
• Anchor cells to the extracellular matrix

12
6 Major Functions Of Membrane Proteins
1. Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some of
these proteins hydrolyze ATP as an energy ssource to
actively pump substances across the membrane
2. Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as a
team that carries out sequential steps of a metabolic
pathway.
3. Signal transduction. A membrane protein may have a
binding site with a specific shape that fits the shape of a
chemical messenger, such as a hormone. The external
messenger (signal) may cause a conformational change
in the protein (receptor) that relays the message to the
inside of the cell.
ATP
Enzymes
Signal
Receptor

13
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
4.
5.
6.
Glyco-
protein
6 Major Functions Of Membrane Proteins

14
Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor
Enzyme
Cell surface identity
marker
Attachment to the
cytoskeleton
Cell adhesion
Functions of Plasma Membrane Proteins

15
Membrane Transport
• The plasma membrane is the boundary that
separates the living cell from its nonliving
surroundings
• In order to survive, A cell must exchange materials
with its surroundings, a process controlled by the
plasma membrane
• Materials must enter and leave the cell through the
plasma membrane.
• Membrane structure results in selective permeability,
it allows some substances to cross it more easily
than others

16
Membrane Transport
• The plasma membrane exhibits selective permeability
- It allows some substances to cross it more easily
than others

 Cell Membrane consists almost entirely of a lipid bilayer with protein
molecules,
 Protein molecules are for transporting substances.
 Most of these penetrating proteins, therefore, can function as transport
proteins. According to their function, they are of 2 types :
Channel proteins: they have watery spaces all the way through the molecule and
allow free movement of water, as well as selected ions or molecules.
Carrier proteins: they bind with molecules or ions that are to be transported; that
cause conformational changes in the protein molecules then move the substances
through the interstices of the protein to the other side of the membrane.
 Both the channel proteins and the carrier proteins are usually highly
selective for the types of molecules or ions that are allowed to cross the
membrane.

Transport through the cell membrane occurs by two basic processes:
Diffusion : random molecular movement of substances molecule by molecule,
from one side of membrane to another.
2 subtypes :-
Simple diffusion: kinetic movement of molecules or ions occurs through a
membrane opening or through intermolecular spaces without any interaction
with carrier proteins in the membrane.
2 pathways:
(1) through the interstices of the lipid bilayer if the
diffusing substance is lipid soluble and,
(2) through watery channels that penetrate all the
way through some of the large transport
proteins
The protein channels has two main characteristics:
 selectively permeable to certain substances,
 channels can be opened or closed by gates that
are regulated by electrical signals (voltage-gated
channels) or chemicals that bind to the channel
proteins (ligand-gated channels).
Facilitated diffusion: Facilitated diffusion requires interaction of a carrier protein.
The carrier protein aids passage of the molecules or ions through the membrane by
binding chemically with them and shuttling them through the membrane.

Gated channel:
Gating of protein channels actually controlling the ion permeability of the
channels. Opening or closing of the channel can be done by conformational
change in the shape of the protein molecule itself.
2 principal ways:
Voltage gating: The molecular conformation of the gate responds to the
electrical potential across the cell membrane.
For e.g. strong negative charge on the inside of the cell membrane - gates remain tightly closed; when
the inside of the membrane loses its negative charge - gates open - allow sodium to pass inward.
when the inside of the cell membrane becomes positively charged - potassium gates open.
This is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve
signals.

Chemical (ligand) gating: Binding of a chemical substance (a ligand)
with the protein; this causes a conformational or chemical bonding change in
the protein molecule that opens or closes the gate.
e.g. Acetylcholine opens the gate of the protein channel that allows uncharged
molecules or positive ions smaller than this diameter to pass through. This gate is
exceedingly important for the transmission of nerve signals from one nerve cell to
another and from nerve cells to muscle cells to cause muscle contraction.
Membrane Potential of Nerve

Osmosis Across Selectively Permeable Membranes—Water transport
 Most abundant substance that diffuses through the cell membrane is water.
 Water ordinarily diffuses in the two directions is balanced so precisely that
zero net movement of water occurs. Therefore, the volume of the cell remains
constant.
 However, under certain conditions, water movement occur across the cell
membrane, causing the cell either to swell or shrink, depending on the
direction of the water movement. This process of net movement of water
caused by a concentration difference of water is called osmosis.
 Osmotic Pressure: The exact amount
of pressure required to stop osmosis
is called the osmotic pressure.
The osmotic pressure exerted by
particles (molecules or ions) in a
solution, is determined by the number
of particles per unit volume of fluid, not
by the mass of the particles.

Active transport: movement of ions or other substances across the membrane in
combination with a carrier protein in such a way that the carrier protein causes the
substance to move against an energy gradient, such as from a low-concentration
state to a high-concentration state.
Active Transport of Substances Through Membranes
According to the source of the energy used to cause the transport:
primary active transport: the energy is derived directly from breakdown of adenosine
triphosphate (ATP) or of some other high-energy phosphate compound.
secondary active transport: the energy is derived secondarily from energy that has
been stored in the form of ionic concentration differences of secondary molecular or ionic
substances between the two sides of a cell membrane, created originally by primary active
transport.

Primary Active Transport
Sodium-Potassium (Na+ -K+ ) pump
 a transport process that pumps sodium ions outward through the cell membrane
of all cells and at the same time pumps potassium ions from the outside to the
inside.
 This pump is responsible for maintaining the sodium and potassium concentration
differences across the cell membrane, as well as for establishing a negative
electrical voltage inside the cells.
 Na+ -K+ ATPase pump can run in reverse.
 Na+ -K+ Pump is important for controlling Cell Volume.
 Electrogenic Nature of the Na+-K+ Pump: The fact that the Na+-K+ pump moves
three Na+ ions to the exterior for every two K+ ions to the interior means that a net
of one positive charge is moved from the interior of the cell to the exterior for each
cycle of the pump. This creates positivity outside the cell while negativity on the
inside. Thus, it creates an electrical potential across the cell membrane.

The carrier protein has two separate globular proteins: a larger one called the
α subunit, and a smaller one called the β subunit.
Larger protein has 3 specific features that are important for the functioning of
the pump:
 It has three receptor sites for binding
sodium ions on the inside portion of
the cell.
 It has two receptor sites for potassium
ions on the outside.
 The inside portion of this protein near
the sodium binding sites has ATPase
activity.
When 2K+ bind on the outside of the
carrier protein and 3 Na+ bind on the
inside, the ATPase function of the
protein becomes activated that cleaves
one ATP to adenosine diphosphate
(ADP) and liberating a high energy
phosphate bond of energy.

Secondary Active Transport—
Co-Transport and Counter-Transport
excess sodium outside the cell membrane is always attempting to diffuse to the
interior. Under appropriate conditions, this diffusion energy of sodium can pull
other substances along with the sodium through the cell membrane. This
phenomenon is called co-transport.
Sodium-glucose co-transport mechanism:
A special property of the
transport protein is that a
conformational change to allow
sodium movement to the interior
will occur until a glucose
molecule also attaches, and
the sodium and glucose both are
transported to the inside of the
cell at the same time.

Counter-transport
this time, the substance to be transported is on the inside of the cell and must be
transported to the outside. Once both proteins bound, a conformational change
occurs, and energy released by the sodium ion moving to the interior causes the
other substance to move to the exterior.
Two important counter-transport mechanisms (transport in a direction opposite
to the primary ion) are:
Sodium-calcium counter-transport occurs
through all or almost all cell membranes, with
sodium ions moving to the interior and calcium
ions to the exterior.
Sodium-hydrogen counter-transport occurs
in several tissues. When sodium ions moving to
the interior and it can transport large numbers
of hydrogen ions, thus making it a key to
hydrogen ion control in the body fluids.

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