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2. CELL WALL
 Cell wall was first observed and named simply as a "wall" by Robert
Hooke in 1665.
 In 1804, Karl Rudolphi and J.H.F. Link proved that cells have
independent cell walls.
 Cell wall is comparatively thick and rigid envelope that surrounds
plant cells.
 It is considered as secretion of protoplasm the living portion of the
cell. It is laid down in the telophase stage of the cell division, when the
pectin rich vesicles of golgi bodied (about 2.0 m) in diameter, migrate
to the equatorial region of the cell.

3.  Cell wall is synthesized later to complete cytokinesis.
 The cell wall is a protective layer outside the cell membrance that
also provides support for the cell's structure.
 A cell wall refers to the rigid and semi permeable protective layer in
some types of cells.
 This outer covering is located next to the plasma membrane of the
cells of plants, algae, bacteria and fungi.

4. CELL WALL STRUCTURE

5. CELL WALL LAYER
 The plant cell wall is differentiated into three layers as follows.
1. Middle lammella,
2. Primary cell wall,
3. Secondary cell wall.

6. 1. MIDDLE LAMELLA
 Middle lamella is the cementing substance between two adjacent
cells.
 It is made of pectic compounds, chiefly calcium and magnesium
pectate. Pectic substances are the mixture of polygalacturon (D-
galacturonic acid) and polysaccharides (netural sugar).
 Sometimes lignin and hemicellulose also are found in the middle
lamella.
 In mature ripe fruits, the softening in due to the dissolution of middle
lamella by the action of pectolytic enzymes.

7. 2. PRIMARY CELL WALL
 Primary cell wall is formed when cellulose fibrils produced by
protoplasm, are accumulated in concentric rows on either side of
middle lamella.
 It is thin and elastic in young enlarging cells and becomes thick and
rigid during cell maturity.
It is capable of growth and expansion.
The backbone of primary cell wall is formed by the cellulose fibrils.
 The matrix is composed of hemicellulose, pectin compounds, lipids,
structural proteins.

8. STRUCTURE OF PRIMARY CELL WALL

9. 3. SECONDARY CELL WALL
 Secondary cell wall is laid down between the primary cell wall and
protoplast and is formed when the cell mature.
 It is formed by the deposition of cellulose on primary cell wall.
 It is inelastic and rigid.
 It is chiefly composed of cellulose and other non cellulosic
substances like lignin, fats and proteins.

10.  The secondary cell wall commonly has three layers S1,S2, and S3
e.g, outer layer, the middle layer and the inner layer.

11. STRUCTURAL ORGANIZATION OF CELL
WALL
The cell wall is structurally thought as a fine interwoven network of
cellulose strands.
According to siegel (1962) elementary fibrils or micelles are the
smallest structural units of the cell wall.
They are composed of appoximately 100 individual cellulosic chains
with a cross sectional area of about 3000Angsrom.
The matrix
It is the ground substance in which fibrils are embedded. It is
amorphous and is deposited with pectin, or lignin hemicelluloses.

12. The fibrils
The fibrils are composed of cellulose. The cellulose molecules are polymer
of disaccharide cellobiose having approx. 3000 glucose unit which are
arranged in linear order.
The cellulose molecules of the primary wall are arranged along the
longitudinal axis of the cells in definitely organized longitudinal bundles.
These bundles known as macrofibrils.
Plasmodesmata
 On maturation, certain fibrous structure develop called plasmodesmata.
Which the adjacent cell are in communication with each other.
Plasmodesmata are also called tonofibrils.

13. COMPOSITION OF CELL WALL
1.Cellulose:
It provides shape and strength to the cell wall. It composes 20-30 %
of the dry weight of primary wall and accounts 40-90% of the dry
weight of secondary wall.
2.Pectins:
They are group of polysaccharides, which are rich in galacturonic
acid, rhamnose, arabinose and galactose. Pectins are present in high
concentration in the middle lamella where they presumably serve the
function of cementing adjacent cells together.

14. 3. Hemicelluloses:
These are matrix polysaccharides built up of a variety of different sugars.
They differ in different species and in different cell types.
Xylan:
It typically makes up roughly 5% of primary cell wall and 20% of
secondary cell wall in dicots. This hemi cellulosic polysaccharide is linked
with xylose and arabinose.
4. Proteins:
Different varieties of protein are present in the cell wall, most of which
are linked with carbohydrate forming glycoprotein. The cell wall glycoprotein
extensin contains an unusual amino acid hydroxyproline (about 40%), which
is generally absent from the protoplast. Extensins are present in the primary
cell walls of dicots making up one to ten percent of the wall.

15. FUNCTION OF CELL WALL
They determine the morphology, growth and development of plant cells.
 They protect the protoplasm from invasion by viral, bacterial and fungal
pathogens.
They withstand the turgor pressure which develops within the cells due to
high osmotic pressure.
It helps the aerial portion of the land plants to tolerate gravitational forces.
It being permeable, helps in the transport of materials in and out of the cell.
It provides a definite shape to the cell and protects internal organelles from
external shocks.

16. ORIGIN OF CELL WALL
1) Formation of matrix
2) Synthesis and orientation of cellulose microfibrils

17. Growth of the plant cell wall
Daniel J. Cosgrove (2005). Nature reviews molecular cell biology. 6(11): 850-861.
Summary:
Plant cells encase themselves within a complex polysaccharide wall, which
constitutes the raw material that is used to manufacture textiles, paper,
lumber, films, thickeners and other products.
The plant cell wall is also the primary source of cellulose, the most
abundant and useful biopolymer on the Earth.
The cell wall not only strengthens the plant body, but also has key roles in
plant growth, cell differentiation, intercellular communication, water
movement and defence.
 Recent discoveries have uncovered how plant cells synthesize wall
polysaccharides, assemble them into a strong fibrous network and regulate
wall expansion during cell growth.

18. CELL MEMBRANE
The cell membrane (also called the plasma membrane or
plasmalemma) is a biological membrane separating the interior of a
cell from the outside environment.
It appears in thin sections with the electron microscope as structure
about 7.5-10 nanometers thick.
Term coined by C. Nageli and C. Cramer in 1855, and Plasmalemma
coined by J. Q. Plowe in 1931.

19. STRUCTURE OF CELL MEMBRANE

20. MODELS OF CELL MEMBRANE
Sandwich model:
It is proposed by Danielli and Dayson in 1935.
In this model a lipid bilayer was coated on its either side with
proteins.
Mutual attraction between the hydrocarbon chains of the lipids and
electrostatic force between the protein and the head of lipid were
thought to maintain the stability of the membrane.
They predicted the lipid bilayer to be about 6.0 nm in thickness, and
each of the protein layer of about 1.0 nm thickness, giving a total
thickness of about 8.0 nm.

22. Unit membrane model:
Using evidence from various electron micrographs, Robertson in
1960, proposed the unit membrane hypothesis.
This hypothesis states that all cellular membranes have an identical
trilaminar structure (or dark-light- dark or railway track pattern, see
Thorpe, 1984).
However, thickness of the unit membrane has been found to be greater
in plasma membrane (10 nm) than in the intracellular membranes of
endoplasmic reticulum or Golgi apparatus (i.e., 5 to 7 nm).

24. Fluid mosaic model:
S. J. Singer and G. L. Nicolson (1972) suggested the widely accepted
fluid mosaic model of biological membranes. According to this model,
the plasma membrane contains a bimolecular lipid layer, both surfaces
of which are interrupted by protein molecules.
Proteins occur in the form of globular molecules and they are dotted
about here and there in a mosaic pattern. Some proteins are attached at
the polar surface of the lipid (i.e., the extrinsic proteins); while others
(i.e., integral proteins) either partially penetrate the bilayer or span the
membrane entirely to stick out on both sides (called transmembrane
proteins).

25. Further, the peripheral proteins and those parts of the integral proteins
that stick on the outer surface (i.e., ectoproteins) frequently contain
chains of sugar or oligosaccharides (i.e., they are glycoproteins).
Likewise, some lipids of outer surface are glycolipids.
The fluid-mosaic membrane is thought to be a far less rigid than was
originally supposed. In fact, experiments on its viscosity suggest that it
is of a fluid consistency rather like the oil, and that there is a
considerable sideways movement of the lipid and protein molecules
within it.

26. On account of its fluidity and the mosaic arrangement of protein
molecules, this model of membrane structure is known as the “fluid
mosaic model” (i.e., it describes both properties and organization of
the membrane).

27. CHEMICAL COMPOSITION OF PLASMA
MEMBRANE
The basic structural framework is the lipid bilayer.
1)Lipids
2)Proteins
3)Carbohydrate.
1) MEMBRANE LIPIDS:
75% - phospholipids
20% - cholesterol
5% - glycolipids.

28. Mostly lipids are Amphipathic having both polar & nonpolar parts.
Acts as permeability barriers. Essential for the maintenance of fluidity
of membranes.
Phospholipids
"Head" - Polar part-phosphate group
"Tail" - Non polar part - long chain
fatty acids

29.  Cholesterol molecules are weakly amphipathic & are interspersed
among the other lipids in both layers of the membrane.
 Glycolipids present only in the membrane layer that faces the
extracellular fluid.

30. MEMBRANE PROTEIN
Two types of proteins are present in membrane
1) Integral proteins 2) peripheral proteins
Integral proteins extend through the lipid bilayer are firmly
embedded in it.
Most integral proteins are transmembrane proteins.
 Peripheral proteins are not embedded in the membrane.
They associate more loosely with the polar heads of membrane lipids
at the inner or outer surface of the membrane.
Many membrane proteins are glycoproteins.

31. Membrane Proteins (functions)
 Ion channels
 Carriers (transporters)
 Receptors
 Enzymes
 Cell identity markers (glycoproteins & glycolipids)

32. MEMBRANE CARBOHYDRATE
Carbohydrate are covalently bound to lipids to form glycolipids and to
the protein to form glycoproteins.
These are mostly
• Glucose
• Galactose
• Mannose
• N-acetyl glucosaine
• N-acetyl galactosamine

33. TRANSPORT ACROSS CELL
MEMBRANE
1) Passive transport
Simple diffusion
Facilitated diffusion
2) Active transport
Primary Active Transport
Secondary Active Transport
3) Bulk Transport
Exocytosis
Endocytosis

34. Active Transport
1) Primary Active Transport.
 Transport against concentrate gradient in which energy is used
directly. It is carrier mediated.
Example -sodium-potassium pump, calcium pump.
2) Secondary Active Transport
Transport against concentrate gradient in which energy is used
indirectly.
The transport of two or three molecules are coupled.
This transport is coupled with Na-KATPase, that requires the ATP. It
occurs by symport and antiport.

35. Bulk Transport
 Also called transport by vesicle formation. It involves formation of
membrane bound vesicles. It involves transport of macromolecule.
1) Exocytosis
 Expulsing out of molecules from the cell.
2) Endocytosis
Engulfing large molecules by the cell.

36. FUNCTION OF CELL MEMBRANE
 Keeps a cell intact
 Protective barrier
Regulate transport in & out of cell (selectively permeable)
Small lipid-soluble molecules, e.g. oxygen and carbondioxide can pass easily
Water can freely cross the membrane
Ions and large molecules cannot cross without assistance
Allow cell recognition
Provide anchoring sites for filaments of cytoskeleton
Provide a binding site for enzymes
Interlocking surfaces bind cells together (junctions)
Contains the cytoplasm (fluid in cell)

37. ORIGIN OF CELL MEMBRANE
All cellular membranes grow from pre- existing membranes which act
as templates for the addition of new precursors.
All cells divide, daughter cells receive a full complement of
membrane systems which undergo growth until the next division, to be
passed on to subsequent progeny.
Meanwhile the molecules within the membrane undergo continuous
replacement.

38. The basic structure and dynamics of cell membranes: An update of
the Singer–Nicolson model
Felix M. Goni (2014). Biochimica et Biophysica Acta 1838 (2014) pp.1467–1476.
Summary
The fluid mosaic model of Singer and Nicolson (1972) is a
commonly used representation of the cell membrane structure and
dynamics. However a number of features, the result of four decades of
research, must be incorporated to obtain a valid, contemporary version
of the model. Among the novel aspects to be considered are:
(i) the high density of proteins in the bilayer, that makes the bilayer a
molecularly “crowded” space, with important physiological
consequences;
(ii) the proteins that bind the membranes on a temporary basis, thus
establishing a continuum between the purely soluble proteins, never in
contact with membranes, and those who cannot exist unless bilayer-
bound;

39. (iii) the progress in our knowledge of lipid phases, the putative
presence of non-lamellar intermediates in membranes, and the role of
membrane curvature and its relation to lipid geometry,
(iv) the existence of lateral heterogeneity (domain formation) in cell
membranes, including the transient microdomains known as rafts, and
(v) the possibility of transient and localized transbilayer (flip-flop)
lipid motion.

40. REFERENCES
Verma, P. S. and Agarwal, V. K. (2005). CELL BIOLOGY, GENETICS,
MOLECULAR BIOLOGY, EVOLUATION AND ECOLOGY.
Page no. 112-153.