My 2nd lecture about biological membranes, especially focusing on cell membrane. Lecture delivered on 19-Jan-2018 to First year MBBS students at Bannu Medical College.

1. BIOLOGICAL MEMBRANES
BY: Dr. Tehmas Ahmad Khan
Demonstrator-Biochemistry Department

2. COMPOSITION
• 1. Phospholipids form bilayer.
• 2. Certain lipids determine fluidity of plasma membrane:
• Cholesterol
• Saturated and unsaturated fatty acids (components of phospholipids)
• 3. Provide permeability barrier for water soluble molecules.

3. CHEMICAL COMPOSITION
• Membranes are composed of lipids, proteins and carbohydrates
• Typically it contains, 40 per cent of the dry weight is lipids, about 60 per cent
proteins and 1 to 10 per cent carbohydrates.
• Lipids are the basic structural components of cell membranes
• Lipid molecules have a ‘polar’ or ionic head hence hydrophilic and the other end
is a ‘nonpolar’ and hydrophobic tail. Hence they are amphipathic

4. TYPES OF LIPIDS
• Fatty acids50%
• Mostly Oleic acid and the degree of unsaturated fatty acids determine the fluidity
of membrane.
• Glycerophospholipids
• Sphingolipids Mostly found in CNS Cells
• Cholesterol also helps in decreasing fluidity of cell membrane.

5. PROTEINS
• Integral membrane proteins Intrinsic Proteins
• Peripheral membrane proteins Extrinsic Proteins
• Transmembrane proteins receptors for hormones, neurotransmitters, tissue
specific antigens, growth factors, etc.

6. CARBOHYDRATES
• Approximately 5 per cent of the weight of cell membranes is carbohydrate, in the
form of glycoproteins and glycolipids.
• The carbohydrate chains of many glycoproteins show structural variation from one
molecule to another, a phenomenon known as microheterogeneity

7. CARBOHYDRATES
• No free CHOs in membrane
• Occur almost invariably in combination with proteins or lipids in the form of
glycoproteins or glycolipids.
• In fact, most of the integral membrane proteins are glycoproteins & about 1/10th
of the membrane lipid molecules are glycolipids.
• One or several oligosaccharides are joined covalently to proteins.

8. FUNCTIONS OF MEMBRANE
• Membranes define the external boundaries of cells & regulate the molecular traffic
across that boundary
• Membranes divide the internal space in to discrete compartments to regulate
processes & compounds in eukaryotic cells
• Organize complex reaction sequences e.g. electron transport chain
• Involved in energy conservation

9. • Membranes have transporters & ion channels
• Involved in cell to cell communication
• Membranes possess receptors for hormones, neurotransmitters & growth
factors

10. FLUID MOSAIC MODEL OF
MEMBRANES
• Singer and Nicholson in 1972
• Intrinsic proteins are deeply embedded
• Peripheral proteins loosely attached
• Float in an environment of fluid phospholipid bilayers.
• It can be compared like icebergs floating in sea water.
• Within the plane of the membrane, different molecules move around one another, one
molecule of phospholipid can move several micrometers per second.

11. • the fluidity of the membrane are highly dependent upon the lipid composition of the
membrane.
Curtsey: Textbook of Medical
Biochemistry by MN Chatterjea

12. FORMATION OF LIPID BILAYER
• Membrane glycerophospholipids and sphingolipids spontaneously form bilayers
• Lipid bilayers are oriented with their hydrophobic tails inside the bilayer while
hydrophilic ‘polar’ heads are in contact with the aqueous solution on each side.
• A lipid bilayer can form only when the cross-sectional areas of the hydrophobic tail
and hydrophilic polar head are about equal.
• The hydrophobic effect and the solvent entropy provide the driving force for the
formation of lipid bilayer.

13. TRANSITIONAL TEMPERATURE
• The temperature at which the structure undergoes the transition from ordered to
disordered state.
• the hydrophobic side chains undergo a transition which is more gel like or
crystalline to a disordered state, taking on a more liquid like or fluid arrangement.
• The More the saturation of Fatty acid, more will be the transitional temperature.

14. EFFECTS OF FLUIDITY OF
MEMBRANE
• The fluidity of membrane significantly affects its functions:
• As membrane fluidity increases, its permeability to water and other small hydrophilic
molecules also increases.
• As fluidity increases, the lateral mobility of integral proteins also increases.

15. SPECIALTY OF RBC MEMBRANE
• Two major integral proteins
• Glycophorin (For blood group antigens)
• Band-3-Protein (helps in transport of HCO3)
• 3 major Peripheral Proteins
• Spectrin (maintains the biconcave disc)
• Actin (forms actin filaments and helps in membrane flexibility)
• Ankyrin (helps integration of above mentioned proteins)

16. QUESTION # 1
• Fluid Mosaic Model favors that cell membrane contains proteins which are
embedded in fashion of:
a) Zigzag
b) criss-cross
c) mosaic
d) eplic

17. ANSWER
• Fluid Mosaic Model favors that cell membrane contains proteins which are
embedded in fashion of:
a) Zigzag
b) criss-cross
c) mosaic
d) eplic

18. QUESTION # 2
• Cell membrane is chemically composed of 40-60 % of
a) Fats
b) lipids
c) sugar
d) cytosol

19. ANSWER
• Cell membrane is chemically composed of 40-60 % of
a) Fats
b) lipids
c) sugar
d) cytosol

20. QUESTION # 3
• All of the following are found in membranes except:
a) nucleic acids
b) Phospholipids
c) Glycoproteins
d) Glycolipids
e) Steroids

21. ANSWER
• All of the following are found in membranes except:
a) nucleic acids
b) Phospholipids
c) Glycoproteins
d) Glycolipids
e) Steroids

22. QUESTION # 4
• Distribution of intrinsic proteins in cell membrane is
a) Symmetrical
b) Asymmetrical
c) Random
d) Uniform

23. ANSWER
• Distribution of intrinsic proteins in cell membrane is
a) Symmetrical
b) Asymmetrical
c) Random
d) Uniform

24. LAST LECTURE’S ASSIGNMENT
• Movie Named “Lorenzo’s Oil”
• Lorenzo suffers from “Adrenoleukodystrophy”
• Peroxisomes are not oxidizing the fatty acids and thus Long chain Fatty acids
accumulates inside cell and myelination of nerves does not occurs.
• At end they develop a cure, certain type of oil that helps to improve the condition
of Lorenzo.

25. THANKS A LOT
• Resources used:
• Textbook of Medical Biochemistry by M.N.CHATTERJEA
• Basic Medical Biochemistry by Lieberman and MARK
• Textbook of biochemistry for medical students by Sreekumari and DM Vasudevan