The plasma membrane consists of phospholipids arranged in a bilayer with embedded proteins and carbohydrates. It selectively permits the passage of ions and molecules in and out of the cell. The major components of the membrane are lipids including phospholipids, sphingolipids, sterols, and archaebacterial ether lipids. Membrane proteins come in integral, peripheral, and anchored forms and perform many functions. Carbohydrates attached to lipids and proteins form glycoproteins and glycolipids that serve important roles in cell recognition.

1. Chemical composition of plasma
membrane
D.INDRAJA

2. • plasma membrane is also known as cell membrane or cytoplasm membrane.
• It is the biological membrane, separates interior of the cell from the outside
environment.
• Selective permeable to Ions and organic molecules.
• Its basic function is to protect the cell from its surroundings.
• It consists of the phospholipids bilayer with embedded proteins.
• Cell membranes are involved in: cell adhesion, ion conductivity and cell signaling
and serve as the attachment surface for several extracellular structures

3. • The molecular structure of cell membrane is totally dependent on:
(a) Membrane Lipids
(b) Membrane Proteins
(c) Membrane Carbohydrate

4. Membrane lipids
• The cell membrane consists of three classes of amphipathic lipids: phospholipids,
sphingolipids, and sterols.
• The fatty chains in phospholipids and glycolipids usually contain an even number
of carbon atoms, typically between 16 and 20.
• The entire membrane is held together via non- covalent interaction of hydrophobic
tails.
• In animal cells cholesterol is normally found in the irregular spaces between the
hydrophobic tails of the membrane lipids, where it confers a stiffening and
strengthening effect on the membrane.
• The major lipids in the cell membrane is phospholipids.
• Each phospholipid molecule has hydrophilic (polar) head and a hydrophobic (non-
polar) tail.
• The hydrophilic heads interact with water while hydrophobic tails remain away
from it and in contact with each other.
• hydrophilic and hydrophobic molecules interact differently with water

10. Sphingo-phospholipids
• Sphingo-phospholipids Are Derivatives of Sphingosine
• Sphingo-phospholipids, the second large class of membrane lipids, also have a
polar head and two nonpolar tails, but unlike glycerophospholipids they contain no
glycerol.
• Sphingo-phospholipids are composed of one molecule of the long-chain amino
alcohol sphingosine (4-sphingenine) or one of its derivatives, one molecule of a
long-chain fatty acid, a polar head alcohol, and sometimes phosphoric acid in
diester linkage at the polar head group

13. Blood groups

14. Glycolipids

15. galactolipids
• In galactolipids the C3 of the glycerol moiety is connected to one or more
galactose residues by glycosidic linkages.
• The C1 and C2 of glycerol are esterified with fatty acids.
• Galactolipids are predominantly present in the membranes of plant cells.
• They are particularly abundant in thylakoid membranes of chloroplasts.
• Galactolipids constitute about 70 to 80% of plant membrane lipids and thus they
are probably the most abundant membrane lipids in the biosphere.

16. Sulfolipids:
• They are membrane glycolipids with sulfur containing functional groups.
• Sulfonated glucose is joined to the C3 of diacylglycerol in glycosidic linkage.
• Plant membranes are also rich in sulfolipids.
• The sulfonated head group of sulfolipid holds a negative charge like that of the
phosphate group in phospholipids.

17. Cerebrosides:
• It is a ceramide with single sugar residue at the C1-hydroxyl group. The sugar
residue of cerebrosides may be either glucose or galactose and thus there are two
categories of cerebrosides namely glucocerebrosides and galactocerebrosides.
• Cerebrosides lack the phosphate group and thus they do not hold any charge (non-
ionic).
• Cerebrosides are abundantly found in the cell membranes of nerves and muscles
of animals.
• Nerve cell membranes are particularly rich in galactocerebrosides whereas
glucocerebrosides are abundant tissues.

18. Globosides:
• They are sphingo-glycolipids with more than one sugar as the side chain of a
ceramide.
• They are different from gangliosides since they lack the sialic acid residues.
• The carbohydrate moieties are usually a combination of N-Acetyl-galactosamine,
D-glucose or D-galactose.

19. • Gangliosides:
• Gangliosides are membrane lipid with most complex structure.
• It is a glyco-sphingolipid with many carbohydrate moieties and one or more sialic
acids linked on the sugar chain. About 6% of brain lipids are gangliosides and they
are first isolated from the ganglion of brain cells.
• Gangliosides are also abundant in the lipid rafts of plasma membrane.

20. Sterols:
• Sterols are the third major category of membrane lipids and are usually present in
the membrane of eukaryotic cells.
• Structurally sterols consist of four fused carbon rings (A, B, C, D) and a
hydrocarbon chain (alkyl side chain).
• Rings A. B and C are six carbon rings whereas the ring D is a five carbons
structure.
• This fused ring structure is called the steroid nucleus.
• The steroid nucleus of all sterols is derived from a Cyclized derivative called
cyclopentanoperhydrophenanthrene.

21. • Cholesterol is a major sterol of the membrane of animal cells.
• They constitute about 30-40% of all membrane lipids in animals.
• Similar to other membrane lipids, cholesterol is also amphipathic with single polar
hydroxyl (-OH) head and non-polar bydrocarbon tail
• The OH group and hydrocarbon chain are attached to the C3 and C17 of the
steroid nucleus respectively.
• Cholesterol can esterify with long chain fatty acids to form cholesterol esters such
as cholesteryl sterate

22. • In mammals, cholesterol also acts as the metabolic precursor of steroid hormones
testosterone.
• Cholesterol is generally absent in plant such as cell membrane.
• However other sterols do occur in plants. Sterols of plants are commonly known
as phytosterols.
• Stigmasterol is an important membrane sterol of plant cells.
• Campesterol and sitosterol are other plant membrane sterols.
• Sterols are also present in the membranes of fungi. Ergosterol is the most
common fungal sterol.
• In animals and fungi, lanosterol acts as the precursor of sterols whereas as in
plants. eveloartenol is considered as the sterol precursor.
• Both Lanosterol and Cycloartenol are derived from the cyclization of triterpenoid-
squalene
• Bacteria are unable to synthesize any of the sterols and thus bacterial membranes
are generally free of sterols. However some bacteria can incorporate exogenous
sterols in to their membrane.

23. Archaebacterial ether lipids:
• They are the fourth major category of membrane lipids.
• They are special membrane lipids of archaebacteria and are absent in
prokaryotes and eukaryotes.
• Majority of archaebacteria lives in harsh and extreme environmental
conditions such as high temperature, high salinity or high pH.
• Thus they require more strong and durable membrane lipids than
prokaryotes and eukaryotes.
• Archaebacterial ether lipids consists of long (32C) branched hydrocarbon
chains linked at both end to glycerol.
• The linkage of hydrocarbon chains to the glycerol is through ether bonds (R-
O R) rather than the usual ester bonds.

24. • This may be due to the fact that ether bonds are more stable than ester bonds in
harsh environmental conditions such as high temperature and pH.
• Due to the presence of long hydrocarbon chain, archaebacterial ether lipids are
large and twice the length of phospholipids and sphingolipids.
• They span the entire width of surface membrane since they have polar groups at
both ends.
Fatty acid
chains
Isoprene units

25. • At each end of the molecule were the two glycerol moieties are present are further
linked to phosphates or sugar residues.

26. Membrane Proteins
• The cell membrane has large content of proteins, typically around 50% of
membrane volume.
• large variety of protein receptors and identification proteins, such as antigens, are
present on the surface of the membrane.
• Functions of membrane proteins can also include cell–cell contact, surface
recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting
substances across the membrane.

28. Integral Proteins
These penetrate the lipids bilayer (transmembrane protein)
Integral membrane proteins are permanently attached to the membrane. Such
proteins can be separated from the biological membranes only
using detergents, nonpolar solvents, or sometimes denaturing agents
They can be classified according to their relationship with the bilayer:
1. As a single α helix
2. as multiple α helices,
3. as a rolled-up β sheet (β-barrel)
4. Membrane proteins exposed to cytosolic side but anchored into cytosolic
monolayer an amphiphilic α helix

30. anchored Proteins
• That located outside the lipid bilayer, on the either the extracellular or cytoplasmic
surface, but are covalently linked to a lipid molecule that is situated within the
bilayer.
• The extra cellular proteins are connected to the plasma membrane by a small
complex oligosachharide linked to a molecule of phosphotidyl inositol that is
embeded in the outer leaflet of lipid bilayer these are called as GPI anchored
proteins

31. Peripheral Proteins
• These are located entirely outside of the lipid bilayer, on the cytoplasmic or
extracellular side, yet are associated with the surface of the membrane by
noncovalent bonds
• Peripheral membrane proteins are temporarily attached either to the lipid
bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and
other non-covalent interactions. Peripheral proteins dissociate following treatment
with a polar reagent, such as a solution with an elevated pH or high salt
concentrations

32. Membrane Carbohydrates
• Carbohydrates are the third major component of plasma membranes.
• The extracellular surface of the cell membrane is decorated with carbohydrate
groups attached to lipids and proteins.
• These carbohydrate chains may consist of 2-60 monosaccharide units and can be
either straight or branched.
• Carbohydrates are added to lipids and proteins by a process called glycosylation,
and are called glycolipids or glycoproteins.
• These short carbohydrates or oligosaccharides are usually chains of 15 or fewer
sugar molecules.
• Oligosaccharides give a cell identity (i.e., distinguishing "self" from "nonself")
and are the distinguishing factor in human blood types and transplant rejection.
• Along with membrane proteins, these carbohydrates form distinctive cellular
markers, that allow cells to recognize each other.

33. • These markers are very important in the immune system, allowing immune cells
to differentiate between body cells, which they shouldn’t attack, and foreign cells
or tissues, which they should.
• Types of carbohydrate in Membrane
Glycoprotein – Covalently bound to protein
i). Exoplasmic face of plasma membrane.
ii). They functions to increase the proteins’ solubility.
iii). Improper for proper folding.
Glycolipid – Covalently bound to lipid.
i). Found on exoplasmic leaflet.
ii). Carbohydrate portion faces the outside.
iii). Glucosylcerebroside is an example for glycolipid.
iv). Gangliosides in membrane of many nerve cells.
Blood group antigens are glycolipids or glycoproteins.