The document summarizes key aspects of cell membranes and transport across membranes. It discusses that membranes are flexible and selectively permeable barriers composed of lipids, proteins, and carbohydrates. It also describes the fluid mosaic model of membrane structure and the different classes of membrane proteins. Furthermore, it summarizes the different mechanisms of membrane transport including passive diffusion, facilitated diffusion, and active transport, noting that active transport requires energy to transport molecules against a concentration gradient.

1. CELL MEMBRANE /
PLASMA MEMBRANE
Department of Biochemistry, KMC, DuwakotTuesday, April 21,
2015
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2. Tuesday, April 21, 20152
All membrane shares
common features: flexibility,
self-sealing, selective
permeability, all appear
trilaminar in appearance and
are mostly 3-8 nm thick.

3. Overview of cell membrane
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 Are flexible, self-sealing, and selectively permeable to
polar solutes.
 Both prokaryotes and eukaryotes have same classes of
chemical components.
 Similar in structural organization.
 Major differences in: lipids, proteins and carbohydrate but
not in physiochemical interaction.
 Have trilaminar appearance under electron microscope.

4. Cell membrane
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Cell membranes act as selective barrier

5. Membrane forms compartments
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6. Plasma membrane
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Plasma membrane is involved in cell communication, import and export of
materials and cell growth and cell motility.
Function of cell membrane inside the cell Vs on the surface?
Role of cell inside the cell Vs outside the cell?

7. Membranes composition
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 Membrane structure
composed of lipids,
proteins and
carbohydrates.
 Major lipids in membranes
are Phospholipids,
Glycosphingolipids, and
Cholesterol.
 Each type of membrane have
characteristic lipids and
proteins.

8. Each type of membrane have
characteristic lipids and proteins
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 Relative proportion of lipids and proteins vary with type of
membrane.
 E.g.: Myelin sheath of neurons: primarily of lipids.
 PM of bacteria, mitochondria, chloroplast: more protein than lipids.
 Cholesterol & Cardiolipin content in Plasma membrane Vs
Inner mitochondrial membrane?
GLYCOPHORIN
So, what is the function of sugar moiety attached to
these surface glycoproteins?

9. Fluid mosaic model for membrane structure
according to Singer and Nicolson
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All membranes share some fundamental properties.

10. Amphipathic lipid aggregates that form
in water
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Depending on the precise conditions and the nature of lipids
three types of lipid aggregation are formed when mixed with
water.
The lipid bilayer is 3nm (30 Å) thick
Lysophospholipids, Glycerophospholipids, sphingolipids & sterols

11. Amphipathic compounds in aqueous
solution
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12. Proteins of cell membrane
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 Form channels for movements of ions and small
molecules
 Serve as transporters for large molecules
Types: 1. Peripheral, 2. Integral protein, 3.
Amphitropic protein

13. Peripheral, Integral and amphitropic
proteins
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Membrane proteins can be
distinguished by the
condition required to
disassociate them from the
membrane.

14. Many membrane proteins span lipid
bilayer
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Transbilayer disposition of
glycophorin in the erythrocyte.

15. Types of transmembrane protein
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Type I
E.g.: LDL-
receptor
Type II
E.g.: Transferrin
Type III
E.g.: Glucose
transporter

16. Types of transmembrane protein
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17. Transmembrane protein
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Bacteriorhodopsin

18. Fluidity of the membrane
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 Fluidity of the membrane depends on its lipid
composition
 Longer and saturated chain  less fluid
 Unsaturated fatty acid chain  increases fluidity
 Cholesterol modifies fluidity of the membrane

19. Transport of materials and
information across the membrane
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20. Classification of transporter
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21. Membrane transport of small
molecules
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Passive Active
Simple Facilitated
Via various
transporter
via ion channels

22. Active Vs Passive transport
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23. In Summary
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 The net diffusion of substance depends on the
following:
 1. Its concentration gradient across the membrane.
 2. The electric potential across the membrane
 3. Permeability coefficient of the substance for the
membrane.
 4. The hydrostatic pressure gradient across the
membrane.
 5. Temperature.

24. Transport pathways through cell membrane,
and basic mechanism of transport
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25. Three general classes of transport system
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26. Schematic representation of types of
transport system
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27. Facilitated diffusion Vs Active
transport
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 Similarities
 Involves carrier proteins, have specificity for ions, sugars and
amino acids.
 Resembles substrate-enzyme reaction.
 Have specific binding site for the solute.
 Carrier is saturable, so it has maximum rate of transport
(Vmax)
 There is binding constant for solute, so whole system have
Km
.
 Structurally similar inhibitor blocks transport.

28. Facilitated diffusion Vs Active
transport
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 Differences
 Facilitated diffusion can operate bidirectionally while
active transport is usually unidirectional.
 Active transport always occur against electrical and
chemical gradient and so it requires energy.

29. Kinetics of Facilitated diffusion Vs
Passive diffusion29
Solute concentration

30. Ping-Pong model explains facilitated
diffusion30
Facilitated diffusion is determined by:
1. Concentration gradient across membrane.
2. Amount of carrier available.
3. Rapidity of solute-carrier interaction.
4. The rapidity of the conformational change for both and loaded and the
unloaded carrier.

31. Active transport
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 Energy is required.
 Energy comes from:
hydrolysis of ATP,
electron
movement, and
light.
 Pump used: Na+-
K+-ATPase

32. Glucose transport involves several
mechanisms32

33. Transport of glucose into myocyte by GLUT4
is regulated by insulin (Facilitated diffusion)
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34. Summary of transport types
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35. 1. Harper’s Biochemistry, 25th. Edition
2. Lehninger’s Biochemistry, 7th. Edition
3. Essential Cell Biology, 3rd. Edition
References
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