Structure and function of cell membrane

2. CELL MEMBRANE
STRUCTURE AND FUNCTION

3. • Plasma membrane was discovered by Schwann
(1838)
• Named as cell membrane by Nageli and Cramer
(1855)
• Membrane was given the name of Plasmalemma
by Plowe (1931)
• Chemically a biomolecule consists of lipid (20-
40%), proteins (59-75%) and carbohydrates
(1-5%)
• Important lipids of the membrane is
Phospholipids (some 100 types), sterols (e.g.,
cholesterol), glycolipids, sphingolipids (e.g.,
sphingomyelin, cerebrosides).

4. • Carbohydrates present in the membrane are
branched or unbranched oligosaccharides,
e.g., hexose, fructose, hexosamine, sialic
acid, etc.
• Protein can be fibrous or globular, structural,
carrier, receptors or enzymatic.
• Several types of models explained structure
of plasma membrane:
• Lamellar models/ sandwich model (Danielli and
Davson, 1935)
• Robertson model (David Robertson, 1959)
• Fluid-Mosaic model (Singer and Nicolson, 1972)

5. Fluid-Mosaic model
• The currently accepted model for the structure of
the plasma membrane, called the fluid mosaic
model, was first proposed by Singer and Nicolson
in 1972.
• According to the fluid mosaic model, the plasma
membrane is a mosaic of components-primarily,
phospholipids, cholesterol, and proteins-that move
freely and fluidly in the plane of the membrane.
• The principal components of the plasma
membrane are lipids (phospholipids and
cholesterol), proteins, and carbohydrate groups
that are attached to some of the lipids and
proteins.

7. • A phospholipid is a lipid made of glycerol, two
fatty acid tails, and a phosphate-linked head group.
Biological membranes usually involve two layers of
phospholipids with their tails pointing inward, an
arrangement called a phospholipid bilayer.
• Cholesterol, another lipid composed of four fused
carbon rings, is found alongside phospholipids in
the core of the membrane.
• Carbohydrate groups are present only on the outer
surface of the plasma membrane and are attached to
proteins, forming glycoproteins , or lipids, forming
glycolipids.

9. • Phospholipids, arranged in a bilayer, make up the
basic fabric of the plasma membrane. They are
well-suited for this role because they are
amphipathic.
• The hydrophilic, or "water-loving," portion of a
phospholipid is its head, which contains a
negatively charged phosphate group as well as an
additional small group which may also or be
charged or polar.
• The hydrophilic heads of phospholipids in a
membrane bilayer face outward, contacting the
aqueous (watery) fluid both inside and outside the
cell. Since water is a polar molecule, it readily
forms electrostatic (charge-based) interactions with
the phospholipid heads.

10. • The hydrophobic, or "water-fearing," part of a
phospholipid consists of its long, nonpolar fatty acid
tails. The fatty acid tails can easily interact with
other nonpolar molecules, but they interact poorly
with water.
• The phospholipid bilayer formed by these
interactions makes a good barrier between the
interior and exterior of the cell, because water and
other polar or charged substances cannot easily
cross the hydrophobic core of the membrane.

12. • Proteins are the second major component of plasma
membranes. There are two main categories of
membrane proteins: integral and peripheral.
• The portions of an integral membrane protein found
inside the membrane are hydrophobic, while those
that are exposed to the cytoplasm or extracellular
fluid tend to be hydrophilic.
• Some integral membrane proteins form a channel
that allows ions or other small molecules to pass.
• Peripheral membrane proteins are found on the
outside and inside surfaces of membranes, attached
either to integral proteins or to phospholipids.

14. • Carbohydrates are the third major component of
plasma membranes. In general, they are found on
the outside surface of cells and are bound either to
proteins (forming glycoproteins) or to lipids
(forming glycolipids).
• These carbohydrate chains may consist of 2-60
monosaccharide units and can be either straight or
branched.
• Along with membrane proteins, these carbohydrates
form distinctive cellular markers, sort of like
molecular ID badges, that allow cells to recognize
each other.

15. • Carbohydrates are the third major component of
plasma membranes. In general, they are found on
the outside surface of cells and are bound either to
proteins (forming glycoproteins) or to lipids
(forming glycolipids).
• These carbohydrate chains may consist of 2-60
monosaccharide units and can be either straight or
branched.
• Along with membrane proteins, these carbohydrates
form distinctive cellular markers, sort of like
molecular ID badges, that allow cells to recognize
each other.

16. TIGHT JUNCTION

17. • Tight junctions, also known as occluding junctions
or zonulae occludentes (singular, zonula
occludens) are multiprotein junctional complexes
whose general function is to prevent leakage of
transported solutes and water and seals the
paracellular pathway.
• Tight junctions may also serve as leaky pathways
by forming selective channels for small cations,
anions, or water.
• Tight junctions (TJ) are specialized membrane
structures found in cell-cell contact areas where the
membranes of the neighboring cells come into a
close proximity.
• Tight junctions are present only in vertebrates.

18. • Tight junctions are composed of branching network of
sealing strands, each started acting independently from
the others.
• Therefore, the efficiency of the junction in preventing
ion passage increases exponentially with the number
of strands.
• There are at least 40 different proteins composing the
tight junctions.
• These proteins consist of both transmembrane and
cytoplasmic proteins.
• The three major transmembrane proteins are occludin,
claudins, and junction adhesion molecule (JAM)
proteins.
Structure of tight junction

19. • They hold cells together.
• Barrier function, which can be further subdivided into
protective barriers and functional barriers serving
purposes such as material transport and maintenance
of osmotic balance:
• Aims to preserve the transcellular transport.
• Tight junctions prevent the passage of molecules
and ions through the space between plasma
membranes of adjacent cells, so materials must
actually enter the cells (by diffusion or active
transport) in order to pass through the tissue.
Function of tight junction

20. TRANSPORT MECHANISM

21. • The cell membrane is one the great multitasker of
biology .
• It provides structure for the cell, protects cytosolic
contents from the environment, and allow cells to
act as specialized unit.
• This phospholipid bilayer can determine that what
molecule can move into or out of the cell, and so is
in large part responsible for maintaining the delicate
homeostasis of each cell.
• Transport: Transport is any process in which
movement of matter and/or energy occurs from one
part of a system to another.

23. Diffusion
• Diffusion is the net movement of material from an
area of high concentration of that substance to an
area with lower concentration of that substance.
• Simple Diffusion means that kinetic movement of
molecules or ions occurs through a membrane
opening or through intermolecular spaces without
any interaction with carrier proteins in the
membrane.
• Simple diffusion of lipid – soluble substances can
take place through the lipid bilayer , its rate
dependent on how highly lipid soluble it is (E.g.
oxygen , carbon dioxide , nitrogen , alcohol ).

24. • Water & lipid -insoluble substances simply diffuse
through protein channels, the number and size of
openings available determining its rate.
• The protein channels involved in simple diffusion
are distinguished by 2 important characteristics:
• They are often selectively permeable to certain
substances.
• Many of the channels can be opened or closed by
gates.
• Facilitated diffusion is also called carrier-mediated
diffusion because a substance transported in this
manner diffuses through the membrane using a
specific carrier protein to help.

26. •Fick’s Law of Diffusion:
The net diffusion rate of a gas across a
fluid membrane is proportional to the
difference in concentration, to the surface
area of the membrane, to the permeability
of the membrane to the substance and
inversely proportional to the thickness of
the membrane and molecular weight of the
molecule.

29. • The osmotic pressure is defined to be the pressure
required to maintain equilibrium, with no net
movement of solvent .
• Osmotic pressure depends on the molar
concentration of the solute but not on its identity . It
is the exact amount of pressure required to stop
osmosis.
• The tonicity of a solution refers to the effect on cell
volume of the concentration of non – penetrating
solutes in the solution surrounding the cell.