1. Cell membrane composition Lecture NO:1, 1st MBBS
Dr Muhammad Ramzan

2. Cell membrane – the definition
Is a biological membrane that separates the interior of all cells from the outside environment
Also called as plasma membranes
Protects and maintains the integrity of a cell

3. Cell membrane – the background
Is selectively permeable to ions and organic molecules
Controls the movement of substances in and out of the cells.
The fundamental structure consists of Phospholipid Bilayer with embedded proteins
It is thin, Pliable and 7-10nms thick and envelops the cell

4. Cell membrane composition Proteins, Fats and CHOs
Cell membrane consists of:
Proteins (55%)
Phospholipids (25%),
Cholesterol (13%)
Other lipids (4%)
And Carbohydrates (3%).

5. RBC membrane composition – A model special characteristics
Plasma membrane of the Human Erythrocyte is a model for membrane studies as it has :
No nuclei and internal membranes
Can easily be isolated for biochemical analysis
Studies of the RBC plasma membrane provided the first evidence of the existence of Phospho lipid Bilayer 3

6. Plasma membrane – electron micrograph Rail road track
The plasma membrane is referred to as a rail road track as it appears as:
Dense lines on electron micrographs that are separated by an intertwining space
(space between the two lines appears divided)

7. Cell membrane - Rail road track

8. Membrane proteins – the major component
Proteins are the major component of the plasma membrane and consist of 50-55% by weight
Proteins are much larger molecules than lipids
one mole of protein = moles of lipids

9. Membranous proteins – the synthesis the Ribosomes
Membranous proteins are synthesized in the Rough Endoplasmic Reticulum ( RER) of all the cells
Are transported to the Golgi apparatus and further to the plasma membrane
CHOs groups are added to the polypeptide chains both in RER and Golgi apparatus
Most of the Trans membrane proteins are Glycoprotein with their oligosaccharides exposed on cellular surface

10. Membrane proteins – Types Two (2)
Major membranous proteins are two - 2
Based on the nature of the membrane + protein interactions
Integral/ Tran membrane/intrinsic proteins
Peripheral membrane/Extrinsic proteins

11. Membrane proteins- Integral proteins
These proteins cross the entire thickness of the membrane and is its structural component
Cannot be removed without disrupting the cell
Integral proteins are the abundant type of Glycoproteins to span the lipid Bilayer and :
May include Ion channel and Transport proteins, (GPCR) Protein Coupled Receptors (GPCR) and LDL receptors

12. Peripheral proteins – do not pass through lipid Bilayer
Are Indirectly associated with the membrane through protein - protein interactions or Ionic bonds or
Directly to the surface of integral proteins, either on the Cytoplasmic or extracellular side
They do not cross through the hydrophobic lipid Bilayer
they can be removed from the membrane through polar agents without disrupting the phospholipids Bilayer

13. Integral and Peripheral proteins

14. Tran membrane/Integral proteins of RBC the types
Transmembrane proteins cross the lipid Bilayer and have lateral mobility in the membrane
Major ones are Glycoproteins, Glycophorins and Band 3
Aquaporin
RH(Rhesus) proteins
The red cell glucose transporters (GLUT-1)
Band 3 is an ion transporter and is a Dimmer/tetramer
Band 3 Tetramer; crosses the lipid Bilayer.
Its Cytoplasmic domain interacts with the cytoskeleton via :
Ankyrin and Spectrin

15. Trans membrane Proteins of RBC

16. Tran membrane proteins of RBC
Glycophorins, general proteins that contribute to Glycosalytion
Glycophorin C crosses the Bilayer to interact with Protein 4.1 with in the junctional complex of Cytoskeleton
Aquaporin is a membrane water channel protein
RH (Rhesus) proteins are associated with Band 3 and is one of the most immunogenic human blood group system
The red cell Glucose transporter(GLUT1) is passive one and is present in other tissues also.

17. Integral proteins of RBC – Two (2)

18. Integral proteins of RBC – Band 3
Band 3 is a large glycoprotein with 929 amino acids and 14 spanning α regions/sites
It is an anion transporter of HCO3¯ (bicarbonates) and Cl¯ for red blood cell membrane.
It has 2 globular structure containing internal channels :
for anions to travel across the lipid Bilayer

19. Integral protein of RBC- Role of Band 3 anion transporters

20. Membranous lipids – 3 types
The cell membrane consists of 3 classes of amphepathic lipids:
Phospholipids, Glycolipids and Cholesterol
The amount of each depends upon the type of cell
In majority of cases phospholipids are the most abundant
In RBC studies 30% of the plasma membrane is lipid

21. Plasma membrane composition - the lipids

22. Membranous lipids/lipid Bilayer - self assembled
According to fluid mosaic model, the biological membranes can be considered as a 2 dimensional liquid :
Where all lipids and proteins molecules diffuse more or less easily .
Lipid Bilayer go through self assembly process in the formation of membranes in Endoplasmic Reticulum

23. Phospholipid Bilayer – the composition
It primarily consists of a thin layer of amphepathic phospholipids on both the :
Cytoplasmic and extracellular surfaces resulting in Bilayer
The head is composed of Phospholipids and
Hydro phobic tail is in the middle having CH and Fas
It is maintained by the hydrophobic and non covalent inter-
actions and H2 bonds

24. Lipid Bilayer and Membrane fluidity
The FAs in lipid Bilayer are usually unsaturated; have kinks, unable to pack and therefore they :
move freely in the interior of the membrane
So that the membrane is soft and flexible
It also allows the movements of the molecules laterally, and prevents vertical movements

25. Phospholipid Bilayer

26. lipid Bilayer – the significance 2 aspects
Lipid Bilayer is critical for membrane function:
Firstly, as barrier B/W the 2 aqueous compartments as interior is occupied by hydrophobic moles FAs
Phospholipid Bilayer is viscous fluid not solids

27. Membranous Carbohydrates
Carbohydrates form a layer on the extracellular surface of the cell membrane – the Glycocalyx
It protects the cell from physical and chemical injury and act as adhesions to attach the cells together
Provides attachments to the agents like Bacteria/Virus

28. Significance of carbohydrates
Carbohydrates are attached to the Lipids as Glycolipids on the outer cell surface as :
blood group antigens- blood transfusions
Glycoproteins are Proteins to which CHO are attached
These serve as cell surface receptors and hormones
(G- Protein Coupled receptors(GPRC) – All Pituitary hormones)