For B.Sc Biotechnology Students

1. Ultra Structure of Cell Wall and
Plasma Membrane
By
DR. KRISHNA
Assistant Professor in Biotechnology
Tumkur University, Tumakuru

2. Introduction:
• The cell wall - rigid, semi-permeable protective layer in some cell types.
• Cell wall is positioned next to the cell membrane (plasma membrane) in
most plant cells, fungi, bacteria, algae, and some archaea.
• Animal cells however, do not have a cell wall.
• The cell wall conducts many important functions in a cell:
1. Protection, 2. Structure, 3. Support.
• Cell wall composition varies depending on the organism.
In plants - mainly of strong fibers of the carbohydrate polymer
cellulose.
Cellulose is the major component of cotton fiber and wood and is
used in paper production.

3. Plant Cell Wall Structure

4. • There are at least 8000 to 15,000 glucose
monomers per cellulose molecule and are
0.25 to 0.5 µm long.
• The molecules are flat and ribbon like,
and lie parallel to each other. Hydrogen
bonding occurs between the molecules,
thus crystallizing and producing
aggregates. These aggregates are called
microfibril.
• Each microfibril contains 40 to 70 chains,
which lie side by side, and these can be
seen in Electron micrographs.
• The spaces between the microfibrils are
filled up with lignin, cutin, pectic
substances, hemicellulose, water etc.
Thus, the microfibril gains considerable
strength.

5. • In primary cell wall, the orientation of microfibril is transverse to
the long axis, and during growth the arrangement may be
longitudinal.
• The orientation in secondary wall may differ from primary wall.
Tracheids and fibres show three layers in their secondary wall the
outer layer (S1), the central layer (S2) and the inner layer (S3), among
which the central (S2) is the thickest.
• The S1 and S3 layers lie adjacent to primary wall and cell lumen
respectively.
• These layers S1, S2 and S3 may be distinguished by their respective
orientation of cellulose microfibrils.
• In S1 and S3, the microfibrils are in the form of a lax helix and in S2, it
is a steep one

6. Primary (growing) plant cell wall:
• Major carbohydrates are cellulose,
hemicellulose and pectin.
• cellulose microfibrils are linked via
hemicellulosic tethers to form the cellulose-
hemicellulose network, which is embedded
in the pectin matrix.
• Common hemicellulose in the primary cell
wall is xyloglucan
• Outer part of the primary cell wall of the
plant epidermis is usually impregnated with
cutin and wax
Secondary cell walls –range of compounds that modify their mechanical properties and permeability. The major polymers
that make up wood (largely secondary cell walls) include:
•cellulose, 35-50%
•xylan, 20-35%, a type of hemicellulose
•lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose
and pectin components, driving out water and strengthening the wall.

7. Layers of Cell wall
•The primary cell wall, generally a thin,
flexible and extensible layer formed while
the cell is growing.
•The secondary cell wall, a thick layer
formed inside the primary cell wall after
the cell is fully grown. It is not found in all
cell types. Some cells, such as the
conducting cells in xylem, possess a
secondary wall containing lignin, which
strengthens and waterproofs the wall.
•The middle lamella, a layer rich in
pectins. This outermost layer forms the
interface between adjacent plant cells and
glues them together.

8. Plant Cell Wall Function
A major role of the cell wall is to form a framework for the cell to prevent over
expansion. Cellulose fibers, structural proteins, and other polysaccharides help to
maintain the shape and form of the cell. Additional functions of the cell wall include:
• Support - the cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
• Withstand turgor pressure - turgor pressure is the force exerted against the cell wall
as the contents of the cell push the plasma membrane against the cell wall. This
pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.
• Regulate growth - sends signals for the cell to enter the cell cycle in order to divide
and grow.
• Regulate diffusion - the cell wall is porous allowing some substances, including
proteins, to pass into the cell while keeping other substances out.
• Communication - cells communicate with one another via plasmodesmata (pores or
channels between plant cell walls that allow molecules and communication signals
to pass between individual plant cells).
• Protection - provides a barrier to protect against plant viruses and other pathogens.
It also helps to prevent water loss.
• Storage - stores carbohydrates for use in plant growth, especially in seeds.

9. The Cell Wall of Bacteria
• cell wall in prokaryotic bacteria is composed of peptidoglycan.
• Peptidoglycan is a polymer composed of double-sugars and amino
acids (protein subunits).
• This molecule gives the cell wall rigidity and helps to give bacteria
shape.
• Peptidoglycan molecules form sheets which enclose and protect
the bacterial plasma membrane.
• The cell wall in gram-positive bacteria contains several layers of
peptidoglycan. These stacked layers increase the thickness of the
cell wall.
• In gram-negative bacteria, the cell wall is not as thick because it
contains a much lower percentage of peptidoglycan.
• The gram-negative bacterial cell wall also contains an outer layer
of lipopolysaccharides (LPS).
• The LPS layer surrounds the peptidoglycan layer and acts as an
endotoxin (poison) in pathogenic bacteria (disease causing
bacteria).
• The LPS layer also protects gram-negative bacteria against certain
antibiotics, such as penicillins.

10. Cell Wall Structure of Bacteria:

11. Types of cell wall
1. Gram positive cell wall
• Cell wall composition of gram positive bacteria.
• Peptidoglycan
• Lipid
• Teichoic acid
2. Gram negative cell wall
• Cell wall composition of gram negative bacteria
• Peptidoglycan
• Outermembrane:
• Lipid
• Protein
• Lipopolysaccharide (LPS)

12. Composition of cell wall:
1. Peptidoglycan:
• Peptidoglycan is porous cross linked polymer which is responsible for strength of
cell wall.
• Peptidoglycan is composed of three components.
• Glycan backbone
• Tetra-peptide side chain ( chain of 4 amino acids) linked to NAM
• Peptide cross linkage
• Glycan backbone is the repeated unit of N-acetyl muramic acid (NAM) and N-
acetyl glycosamine (NAG) linked by β-glycosidic bond.
• The glycan backbone are cross linked by tetra-peptide linkage. The tetra-peptide
are only found in NAM.
• More than 100 peptidoglycan are known with the diversity focused on the
chemistry of peptide cross linkage and interbridge.
• Although the peptidoglycan chemistry vary from organism to organism the glycan
backbone ie NAG-NAM is same in all species of bacteria.

13. The aminoacids found in tetra-peptide are-
•L-alanine: 1st position in both gm+ve and
gm-ve bacteria
•D-glutamic acid: 2nd position
•D-aminopimelic acid/ L-lysine: 3rd position
(variation occurs)
•D-alanine: 4th position

14. Peptide cross linkage in Gram positive and Gram negative bacteria:
•In gram negative bacteria, peptide cross
linkage occur between Diaminopamilic
acid (3rd position) of one glycan back
bone and D-alanine of adjacent glycan
back bone.
•In gram positive bacteria, peptide cross
linkage occur by peptide interbridge. The
type and number of aminoacids in
interbridge vary among bacterial
species.

15. 2. Teichoic acid:
• Teichoic acid is water soluble polymer of
glycerol or ribitol phosphate.
• It is present in gram positive bacteria.
• It constitutes about 50% of dry weight of
cell wall.
• It is the major surface antigen of gram
positive bacteria.
3. Outer membrane
• It is an additional layer present in gram
negative bacteria.
• It is composed of lipid bilayer, protein
and lipopolysaccharide (LPS) layer
Function of outer membrane:
• Structure component of gram-ve cell
wall
• LPS is an endotoxin produced by gram –
ve bacteria
• Lipid-A is antigenic Figure: Outer membrane in cell wall of gram negative bacteria

16. 4. LPS
• LPS is attached to outer membrane by hydrophobic bond. LPS is synthesized in cytoplasmic
membrane and transported to outer membrane.
• LPS is composed of lipid-A and polysaccharide.
• Lipid-A: it is phosphorylated glucosamine disaccharide.
• Polysaccharide: it consists of core-polysaccharide and O-polysaccharide.
Gram positive bacteria
• Micrococcus
• Staphylococcus
• Streptococcus
• Leuconostoc
• Gram negative bacteria
• E. coli
• Salmonella
• Klebsiella
• Shigella
• Pseudomonas

17. Cell Membrane:
•The term was originally used by Nageli and Cramer (1855) for
the membranous covering of the protoplast. The same was
named plasmalemma by Plowe (1931).
•Plasmalemma or plasma membrane was discovered by
Schwann (1838).
Membranes also occur inside the cytoplasm of eukaryotic cells as
covering of several cell organelles like nucleus, mitochondria,
plastids, lysosomes, golgi bodies, peroxisomes, etc.

18. Models and Ultrastructure of Plasma Membrane
The top four historical models of Plasma Membrane.
The models are: 1. Lipid and Lipid Bilayer Models 2. Unit Membrane Model (Protein-Lipid
Bilayer-Protein) 3. Fluid Mosaic Model 4. Dannelli Model.
1. Lipid and Lipid Bilayer Model:
• This model to explain the structure of plasma membrane was given by Overton, Gorion
and Grendel. Previously only indirect information was available to explain the structure
of plasma membrane. In 1902, Overton observed that substances soluble in lipid could
selectively pass through the membranes. On this basis he stated that plasma membrane
is composed of a thin layer of lipid.
• Subsequently, Gorter and Grendel in 1926 observed that the extracted from erythrocyte
membranes was twice the amount expected if a single layer was present throughout the
surface area of these cells. On this basis they stated that plasma membrane is made up
of double layer of lipid molecules. These models of Gorter and Grendel could not explain
the proper structure of plasma membrane but they put the foundation of future models
of membrane structure.

19. 2. Unit Membrane Model (Protein-Lipid Bilayer-Protein):
• This is also known as unit membrane model. This model was
proposed by Davson, Daniell and Robertson. When surface tension
measurements made on the membranes, it suggests the presence of
proteins. After the existence of proteins the initial lipid bilayer model
proposed by Gorter and Grendel was modified. It was suggested that
surface tension of cells is much lower than what one would expect if
only lipids were involved.
• It may also be observed that if protein is added to model lipid water
system, surface tension is lowered. This suggested indirectly the
presence of proteins. On this basis Davson and Danielli proposed that
plasma membrane contained a lipid bilayer with protein on both
surfaces.

20. • Initially they supposed that proteins existed as covalently bonded globular
structures bound to the polar ends of lipids. Subsequently they developed
the model in which the protein appears to be smeared over the hydrophilic
ends of the lipid bilayer. This model makes its popularity for a long time.
• With the availability of electron microscope later, fine structure of plasma
membrane could be studied. Definite plasma membrane of 6 nm to 10 nm
(10nm = 100 Å; 1 nm = 10_6mm) thickness was observed on surface of all
cells, and plasma membranes of two adjacent cells were found to be
separated by a space, 1-15nm wide.
• It was also observed that the plasma membrane of most of the cells
appeared to be three layered. Two outer dense layers were about 2.0nm
thick and the middle layer about 3.5nm. The early ideas of Gorter and
Grendel and those of Davson and Danielli were first formalized by
Robertson in 1959 in the form of his unit membrane concept.

21. • This concept of unit membrane with three layers (two protein layers
and one lipid bilayer) only supported the concept proposed earlier by
Davson and Danielli. In this unit membrane the less dense middle
layer corresponded to hydrocarbon chains of lipids. Thickness of
unit membrane (10nm) was found to be greater in plasma membrane
than in intracellular membranes of endoplasmic reticulum or golgi
complex.

22. 3. Fluid Mosaic Model:
• To explain the structure plasma membrane various models have been put
forward from time to time. But none was universally accepted. In this
relation Gorter and Grendel, Davson and Danielli, etc. proposed model for
plasma membrane after that Fluid Mosaic Model for plasma membrane
was proposed which was universally accepted.
• It was proposed by Singer and Nicholson (1912). This model postulates that
lipid and integrated proteins are disposed in a sort of mosaic pattern and
all the biological membranes have a quasi-fluid structure where both lipid
and protein components are able to perform transitional movement within
lipid bilayer.
• In this model, lipid molecules may exhibit intra-molecular movement or
may rotate about their axis or may display flip-flop movement including
transfer from one side of bilayer to the other. Thus this concept implies
that main components of the membrane, i.e., lipids, proteins and
oligosaccharides are held together by means of non-covalent interactions
as suggested by Gitler (1972). A term amphipathy was coined by Hartley
(1936) to the molecules having both hydrophilic and hydrophobic groups.
Thus lipids and integrated proteins are amphipatic in nature.

23. 4. Dannelli Model:
• (i) Lipid and intrinsic proteins are present in a mosaic arrangement
and
• (ii) Biological membranes are semifluid so that lipids as well as
intrinsic proteins are able to make movements within the bilayer.
This concept of fluidity implies that lipids, proteins and
oligosaccharides are held in their positions by means of non-covalent
interactions. On this basis, it can be stated that components can be
dispersed by solvents or detergents without breaking any bonds.
Intrinsic proteins are also intercalated to greater or lesser extent into a
continuous lipid bilayer. Such proteins may be in contact with an
aqueous solvent on both sides of the membrane.

24. Sandwich Model (Lamellar Models) of Plasma Membrane
• Denielli and Davson have proposed that in the plasma membrane a double layer
lipid molecule is sandwiched between the two different layers of protein
molecules. The lipid layers are found internally and the protein molecules are
found externally. The inner ends of lipid molecules are non-polar and
hydrophobic where as outer ends are polar and hydrophilic.
James Danielli and Hugh Davson in 1935

25. Unit Membrane Model of Plasma Membrane
• Robertson in the year 1953 proposed the unit membrane model
consisting of three layers of the plasma membrane and it is
complimentary to the sandwich model. According to this there are
two outer layers of protein molecules each with 20 angstroms thick,
embracing a central layer of lipid molecules of about 35 angstroms
thick for a total thickness of 75 angstroms.
Robertson Model:
J. David Robertson
(1959) modified the
model of Danielli and
Davson by proposing
that the lipid bilayer is
covered on the two
surfaces by extended or
(3-protein molecules)