The document discusses various modes of transport across the cell membrane, including passive transport mechanisms like simple diffusion and facilitated diffusion, active transport processes like primary active transport and secondary active transport, and vesicular transport mechanisms like endocytosis, exocytosis, and transcytosis. Transport across the cell membrane is essential for cellular functions and is mediated by integral membrane proteins like ion channels and carrier proteins.
2. OBJECTIVES
To get a better understanding of :
Importance of cell membrane
Passive transport
Active transport
Vesicular transport
Other transport processes
3. INTRODUCTION TO CELL
MEMBRANE
Cell membrane/plasma membrane is a thin, elastic, pliable
structure of 7.5-10 nm in diameter.
It is composed almost entirely of proteins & lipids.
It separates the contents of the cell from the external
environment and controls the exchange of substances
between the ECF & ICF.
4. STRUCTURE OF CELL MEMBRANE
The basic structure of CM is a lipid bilayer – two layers of
phospholipids.
1972, Singer and Nicolson proposed the fluid mosaic
model.
5. ARRANGEMENT OF LIPID BILAYER
Two layers of phospholipids.
Head end/polar end – phosphate moiety – hydrophilic –
directed outwards facing the aqueous phase (ECF&ICF).
Tail end/non polar end – two chains of fatty acid –
hydrophobic – arranged towards the center of the CM.
The lipid bilayer is a semi permeable
membrane
6. ARRANGEMENT OF PROTEINS
Protein molecules float about in the lipid bilayer.
Peripheral proteins & integral proteins (channel proteins,
carrier proteins, pumps, receptor proteins, enzyme
proteins).
7. ARRANGEMENT OF
CARBOHYDRATES
The carbohydrates are attached either to the proteins -
Glycoproteins or to the lipids - Glycolipids.
Throughout the surface , the carbohydrate molecules
form a thin loose covering called Glycocalyx.
8. TRANSPORT
The physiological activities of a cell depend upon
substances like nutrients, oxygen and water, which must
be transported into the cell and at the same time,
metabolic waste must be transported out of the cell.
The integral proteins that are required for transport are:
Ion channels
Water channels (Aquaporins)
9. ION CHANNELS
Ion channels are pore-forming membrane proteins that
allow ions to pass through the channel pore.
Most of them are gated-i.e., they open or close
spontaneously or in response to certain stimuli.
Types of ion channels:
Voltage gated
Ligand gated
Mechanical gated
11. CLINICAL SIGNIFICANCE
CHANNELOPATHIES
Channelopathies are a heterogeneous group of disorders
resulting from the dysfunction of ion channels. Mutations
in genes encoding the ion channels are the common cause.
Examples include long QT syndrome, Brugada syndrome,
cystic fibrosis, thyrotoxic hypokalemic periodic paralysis
etc.,
12. WATER CHANNELS
Aquaporins also called as water channels are
channel proteins that form pores in the cell
membrane, mainly facilitating transport of
water between cells.
• Mutations in the aquaporin-2 gene cause
hereditary nephrogenic diabetes
insipidus in humans
CLINICAL
SIGNIFICANCE
14. PASSIVE TRASPORT
It refers to the mechanism of transport of substances
along the gradient without expenditure of energy.
Types: Diffusion & osmosis.
DIFFUSION
It refers to passive transport of molecules from areas of
higher concentration to areas of lower concentration
Types: Simple diffusion & facilitated diffusion.
15. DIFFUSION
FACTORS AFFECTING NET RATE OF DIFFUSION
Fick’s law of diffusion
J=DA (C1-C2)
T
D-Diffusion coefficient
A-Surface area
C1&C2-Concentration on either sides
T-Thickness of the membrane
16. DIFFUSION
FACTORS THAT INFLUENCE DIFFUSION
RATES
EFFECT
Thickness of the membrane ↑ the thickness → ↓ the
rate of diffusion
Size of the molecule ↓ the size → ↑ the
diffusion
Temperature ↑ the Temp → ↑ the motion
of particles → ↑ the rate
of diffusion
Membrane surface area Larger the area, faster is
the diffusion
18. SIMPLE DIFFUSION
In simple diffusion, the kinetic movement of molecules or
ions occur through a membrane opening or through
intermolecular spaces without any interaction with carrier
proteins in the membrane.
SD occurs by two pathways :
1.through the interstices of the lipid bilayer &
2.through watery channels
19. FACILITATED DIFFUSION
It is also known as carrier mediated diffusion because a substance
transported in this manner diffuses through the membrane using a specific
carrier protein.
A conformational change occurs in the carrier protein after the molecule to
be transported is bound at the receptor site. The repetitive spontaneous
configurational changes allow the diffusion of the molecule.
20. FACILITATED DIFFUSION
In FD, the rate of diffusion ↑ with ↑ in concentration
gradient to reach a limit beyond which a further increase
in diffusion can’t occur. This is called saturation point.
All the carrier proteins will be occupied and the system
operates at maximum capacity.
21. OSMOSIS
Osmosis refers to diffusion of water/any solvent molecules through a
semi permeable membrane from a solution of lower concentration of
solutes to a solution of higher concentration of solutes.
22. OSMOTIC PRESSURE
The minimum pressure which when applied on the side of higher
solute conc. that prevents osmosis is osmotic pressure.
23. OSMOLE
It is the unit used in place of grams to express the conc.
In terms of osmotically active particles in a given soln.
1 osmole = 1 g molecular wt of undissolved solute
180 g of glucose = I g molecular wt of glucose = 1 osmole
NaCl dissolves into Na and Cl
58.5 g of NaCl = 2 osmoles
1/1000 osmole = 1 milli Osmole
25. TONICITY
Ability of a solution to affect fluid volume &
pressure within a cell.
Isotonic : Osmolality similar to plasma. 0.9 %
NaCl
Hypertonic : Osmolality ↑ than that of plama
Hypotonic: Osmolality ↓ than that of plasma
27. CLINICAL SIGNIFICANCE
Total plasma osmolality ↑ in:
• Severe dehydration, severe Diabetes, ↑levels of urea in pts with
Renal Disease.
• Hyper osmolarity → water flow out of the brain → Hyperosmolar
coma.
↓ Plasma osmolality in:
• Administration of excessive 5% glucose- swelling of body tissues
28. ACTIVE TRANSPORT
It refers to the mechanism of transport of
substances against the chemical/electrical
gradient using energy mainly in the form of ATP.
Ionic substances : Na+ K+ Ca2+ Cl-
Non ionic substances : glucose, amino acids, urea.
Types : Primary and Secondary
30. PRIMARY ACTIVE TRANSPORT
The energy is derived directly from the breakdown of
ATP or some other high energy phosphate compound.
Some of the important pumps involved are
Sodium potassium pump
Calcium pump
Potassium Hydrogen pump
31. SODIUM POTASSIUM PUMP
Present in all the cells of the body.
Active transport of Na ions outwards thro’ the CM and K
ions inwards simultaneously
STRUCTURE
Carrier protein – α and β subunits. α subunit is mainly
concerned.
Binding sites:
3 intracellular – one each for binding Na+ (3Na+),
ATP, phosphorylation site
2 extracellular – one each for binding K+ (2K+) and
33. FUNCTIONS
Controlling cell volume:
This is the most important function of the Na+ K+
pump without which most of the cells of the body will swell
up until they burst.
Electrogenic activity:
Na+K+ pump act as electrogenic pump since it
produces a net movement of positive charge out of the cell
creating electrical potential across the CM. This is a basic
requirement in nerves & muscles to transmit the signals.
34. Na+-K+ PUMP
Activity of Na+-K+ pump is :
↑ by diacyl glycerol, thyroid hormone,
aldosterone, insulin & G actin.
inhibited by low T, O2 lack, dopamine,
ouabain, glycosides (digitalis).
35. CALCIUM PUMP
Two pumps :
one in the CM which extrudes Ca2+ out of the cell and the other
actively transports cytoplasmic Ca into cell organelles
36. SECONDARY ACTIVE TRANSPORT
In secondary active transport, the energy is derived
secondarily from the energy which has been stored in the
form of ionic conc. differences between the two sides of
the membrane.
At many areas of the body, transport of some other
substance is coupled with the active transport of Na+
Two types: Sodium co transport & Sodium counter
transport
37. SODIUM CO TRANSPORT
Carrier protein act as symport. Glucose, amino acids, chloride and
iodine are transported by this method.
Sodium co transport of Glucose
38. SODIUM COUNTER TRANSPORT
The carrier protein act as antiport. Eg., Sodium-Calcium counter
transport, Sodium-Hydrogen counter transport.
40. VESICULAR TRANSPORT
Involved in the transport of macromolecules such
as large protein molecules.
Three mechanisms:
Exocytosis
Endocytosis
Transcytosis
41. ENDOCYTOSIS
It is the process in which the substance is transported into the cell
by infolding of the CM around the substance and internalizing it.
Three types
Pinocytosis(cell drinking)
Phagocytosis(cell eating)
Receptor mediated endocytosis
Two pathways
Constitutive
Clathrin mediated
45. CLATHRIN MEDIATED
STRUCTURE
Clathrin is a protein that plays a major role in the
formation of coated vesicles.
Clathrin was first isolated and named by Barbara Pearse
in 1976
It is triskelion shaped and composed of three clathrin
heavy chains and three light chains.
47. CAVEOLAR ENDOCYTOSIS
Clathrin independent endocytosis involving flask shaped invaginations called
caveolae.
Caveolae have been implicated in cell signalling, Lipid regulation.
48. EXOCYTOSIS
Reverse of endocytosis
Two pathways
Constitutive-proteins from golgi initially enter secretory granules.
Prohormones to mature hormones before exocytosis
Non constitutive-No processing or storage
MECHANISM
Substance to
be extruded
Vesicle/granule Fusion with CM
Contents
released to the
exterior
50. TRANSCYTOSIS
Vesicular transport within the cell. common in epithelial cells
Macromolecules are captured in vesicles on one side of the cell, drawn across
the cell and ejected on the other side
51. OTHER TRANSPORT PROCESSES
Transport across epithelia – naturally assemble in the form of sheets
of tightly linked cells resting on basal membrane. They form tight
junctions.
Transport through cell proper
Ultrafiltration : occurs at the barrier between the blood and the
filtrate in the glomerular capsule of kidneys.
The high hydrostatic pressure forces small molecules like water,
glucose, amino acids, urea through the filter, from the blood in the
glomerular capsule across the basement membrane of the Bowman’s
capsule and into the renal tubule.
52. SUMMARY
The cell membrane plays an active role in transport of
substances,
Lipid soluble substances pass directly through the cell
membrane.
Water and other small ions pass through channel proteins.
Large molecules pass through carrier proteins by active
transport.
ATP is the major source of energy in active transport.
Bulk transport of substances occur by vesicular transport.