Cell membrane structure and transport across cell membrane

1. Ashok V. Katta

2. The plasma membrane, lipid bilayer
A flexible yet sturdy barrier that
surrounds and contains the cytoplasm of
a cell.
Is described by the fluid mosaic model.

3.  The molecular arrangement of the pl. membrane
resembles an ever-moving sea of fluid lipids that
contains a mosaic of many different proteins.
 Some proteins float freely like icebergs in the
lipid sea, whereas others are anchored at specific
locations like boats at a dock.
 In simple words, Pl. membrane is phospholipids
bilayer containing embedded proteins and
cholesterol. Carbohydrates are present on the
surface as glycoprotein or glycolipids.
 Cell membrane is selective permeable.

4. Integral (transmembrane)
proteins
Cholesterol
Glycolipid:
Carbohydrate
Lipid
Glycoprotein:
Carbohydrate
Protein
Extracellular
fluid
Channel protein
Lipid
bilayer
Cytosol

5. The basic structural framework is the lipid bilayer.
Mead up of 3 macromolecules………
 Lipids
 Proteins
 Carbohydrate.

6. 75% - phospholipids
20% - cholesterol
5% - glycolipids.
Mostly lipids are Amphipathic having both polar &
nonpolar parts.
Acts as permeability barriers.
Essential for the maintenance of fluidity of membranes.
Membrane lipids:-

7. Head
Tail
“Head” – Polar part – phosphate group
“Tail” – Non polar part – long chain fatty
acids
These includes……
phosphptidylcholine,
phosphptidylethanolamine,
phosphptidylserine,
phosphptidylinositol,
plasmalogens &
sphingomyelin.
Phospholipids
Phosphate gr.
fatty acid

8.  Cholesterol molecules are weakly
amphipathic & are interspersed
among the other lipids in both
layers of the membrane.
 Glycolipids present only in the
membrane layer that faces the
extracellular fluid.

9. Membrane Proteins
Two types of proteins are present in membrane
 Integral proteins
 peripheral proteins
Integral proteins extend through the lipid
bilayer are firmly embedded in it.
Most integral proteins are transmembrane
proteins.
Peripheral proteins are not embedded in the
membrane.
They associate more loosely with the polar
heads of membrane lipids at the inner or outer
surface of the membrane.
Many membrane proteins are glycoproteins.

10. Membrane Proteins

11.  Ion channels
Carriers (transporters)
Receptors
Enzymes
Cell identity markers (glycoproteins &
glycolipids)
Membrane Proteins (functions)

12. Membrane Carbohydrate
Carbohydrate are covalently bound to
lipids to form glycolipids
And to the protein to form glycoproteins.
These are mostly
 Glucose
 Galactose
 Mannose
 N-acetyl glucosaine
 N-acetylgalactosamine

13.  Transport of materials across the plasma
membrane is essential to the life of a cell.
It also essential to maintain equilibrium of
cell
 Certain substances must move into the cell
to support metabolic reactions.
 Other substances which produced by the
cell for export or as cellular waste
products must move out of the cell.
Transport
across Cell membrane

14.  Passive transport.
 Simple diffusion
 Facilitated diffusion
Active transport.
 Primary Active Transport
 Secondary Active Transport
Bulk Transport
 Exocytosis.
 Endocytosis.
Transport across Cell membrane

15. Passive transport.
 Simple diffusion :-
moment of particles from the area of higher
conc. To an area of lower conc. (i.e down the
conc. gradient).
It does not require energy and membrane
transport proteins.
Example –
Transport of gases,
Transport of ions and
Transport lipid soluble
molecules

16. Facilitated diffusion-
 Diffusion of molecules down the conc. gradient
which require carrier protein.
 It does not require energy.
 It more rapid than simple diffusion.
 It works as ping-pong mechanism.
 Example-
 Absorption of fructose,
 Glucose transport.

17. Uniport, antiport.
 Based on direction of moment of molecules,
facilitate diffusion can occur by
Uniport- transport of single type of molecule in
one direction. E.g. absorption of fructose.
Antiport - Transport of molecule in opposite
direction. E.g. chloride-bicarbonate exchanger.
Uniport Antiport

18. Osmosis.
 The diffusion of water through a
semipermeable membrane.
 Movement of water molecules occur from an
area of lower solute concentration to an area of
higher solute concentration.
 Clinical Significance-
 Decreased formation of urine in hypovolemic
conditions.
 Edema due to hypoalbuminemia.
 Tonicity and its effects on red blood cells
(RBCs).

19. Two types
Primary Active Transport.
Secondary Active Transport.
Active Transport.

20. Primary Active Transport.
Transport against conc. gradient in
which energy is used directly.
It is carrier mediated.
It requires energy in the form of ATP &
used directly.
Example –
 sodium-potassium pump,
 calcium pump.

21. Sodium-Potassium Pump
(Na/K ATPase)
 Also called Sodium pump.
In this pump, 3 sodium ions moves out of
the cells and 2 potassium ions moves
inside the cell.
With consumption of 1 ATP molecule.
This is maintain low intracellular Na+
&
high intracellular K+.

22. Sodium-potassium pump

23. Transport against conc. gradient in
which energy is used indirectly.
The transport of two or three molecules
are coupled.
This transport is coupled with Na-K
ATPase, that requires the ATP.
It occurs by symport and antiport.
Secondary Active Transport.

24. Secondary Active Transport.

25. Symport (co-transport)
Molecule move in same direction.
Example-
 Sodium-glucose symport
 Sodium-amino acid symport.
Disorders- cystinuria & Hartnup disease
(mutations in sod-amino acid symport)

26. Antiport (counter transport)
Molecule move in opposite directions.
Example-
 Sodium-hydrogen exchanger (Renal tubule)
 Calcium-hydrogen exchanger (cardiac
muscle)

27.  Also called transport by vesicle formation.
 It involves formation of membrane bound
vesicles.
 It involves transport of macromolecule.
Bulk transport.

28. Expulsing out of molecules from the cell.
Fate of molecule released by exocytosis
may be……..
 Peripheral proteins
 Part of extracellular matrix
 Release to extracellular medium.
Exocytosis.

29. Engulfing large molecules by the cell.
Two type of endocytosis.
 Phagocytosis-
 large substances such as bacteria are
taken within cell. (macrophages)
 Pinocytosis-
 fluid or fluid contents are taken within
the cell.
Endocytosis.

30. Endocytosis.

31. Quick Review
Define the Cell.
Structural and Functional Unit of Life
Which organelle is the control center of the cell?
Nucleus
Which organelle separates the cell from external
environment?
Cell membrane
Which organelles Detoxifies drugs & harmful substances?
Smooth endoplasmic reticulum.
Which organelle helps in Modification, sorting, packaging,
& transportion proteins?
Golgi Complex.
Niemann pick disease is caused by faulty functioning of
which organelles?
Lysosomes.

32. “THANK U”
Contact no. – 07418831766
E mail – ashokktt@gmail.com
Ashok Katta
Dept. of Biochemistry,
Dhanalakshmi Srinivasan Medical College,
Perambalur