Transcript Transport Across Cell Membrane - Bioenergetics and Cell Metabolism

Fluid Mosaic Model
1

Membranes are two-dimensional solutions of
oriented lipids and globular proteins

The lipid bi-layer acts as solvent for integral
membrane proteins and a permeability barrier

Membrane lipids: supporting structure
– Phospholipids
– Glycolipids
– Cholesterol
Fluid mosaic model

2
Membrane proteins:
–
Integral (intrinsic) proteins
–
Peripheral (extrinsic) proteins
Membrane fluidity
3

Many membrane processes depend on membrane
fluidity
-transport
-signal transduction

Membrane fluidity is dependent on the properties of the
fatty acid chain

Transition temperature is dependent on the length of the
fatty acid chains and on their degree of unsaturation
Membrane fluidity


Movement of hydrophobic tails
Depends on temperature and lipid composition
How does lipid composition affect fluidity?
4
Lipids and membrane fluidity

5
Interactions between hydrophobic tails decrease
fluidity (movement):
–
Shorter tails have fewer interactions
–
Unsaturated fatty acids are kinked and decrease
interactions
Lipids and membrane fluidity

Cholesterol “buffers”
fluidity:
Prevents interactions
Restricts tail movement
6
Microbial growth at Cold temperatures
Molecular Adaptation to Psychrophily
7

The cytoplasmic membranes of psychrophiles have
a higher content of unsaturated fatty acids.

This helps to maintain a semifluid state of the
membrane at low temperatures
Microbial growth at Cold temperatures
Molecular Adaptation to Psychrophily
8

Lipids of some psychrophiles contain
polyunsaturated fatty acids or other long chained
hydrocarbons with multiple bonds

These fatty acids remain more flexible at lower
temperatures than saturated or monounsaturated
fatty acids
Microbial Growth at High Temperature

Molecular Adaptations to Thermophily
–
Modifications in cytoplasmic membranes to ensure
heat stability
9
 Bacteria
have lipids rich in saturated fatty acids
 Archaea
have lipid monolayer rather than bilayer
Microbial Growth at High Temperature
10

Archaea have lipid
monolayer rather than
bilayer

Lipid monolayers are
quite resistant to
peeling apart

When the lipid layers
peel apart they cause
cell lysis
Membrane asymmetry

11
The inner and outer leaflets of the membrane have
different compositions of lipids and proteins
Membrane asymmetry


12
Sphingomyelin and phophatidyl choline are
located on the outer leaflet
Phosphatdidylserine is located in the inner leaflet
Membrane Rafts

13
Sphingolipids colocalize with cholesterol
in membrane
microdomains called lipid
rafts.
Membrane Rafts

Caveolae are invaginated
lipid raft domains of the
plasma membrane

They have roles in cell
signaling and membrane
internalization
14
Membrane Rafts
15

Caveolin is a protein
associated with the cytosolic
leaflet of the plasma
membrane in caveolae.

Caveolin interacts with
cholesterol and selfassociates as oligomers
that may contribute to
deforming the membrane to
create the unique
morphology of caveolae.
Biomembrane
Cell to cell interactions and adhesions

Integrins are transmembrane
proteins of the plasma
membrane

They act to attach cells to each
other
They carry message between
the extracellular matrix and the
cytoplasm
(extracellular matrix has proteins
such as collagen and
fibronectin)

16
Biomembrane
Cell to cell interactions and adhesions

Integrins regulate many processes
- platelet aggregation at the site of a
wound
- tissue repair
-activity of immune cells
-invasion of tissue by a tumor
Mutation can result in leukocyte
adhesion deficiency. Child dies by
age 2
17
Biomembrane
Cell to cell interactions and adhesions

Other plasma membrane
proteins involved in surface
adhesions:

Cadherins
Immunoglobin-like proteins


Selectins: essential part of
the blood clotting process
18
Biomembrane
Membrane fusion and biological processes




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19
Integral proteins(fusion
proteins) facilitate this
event
Membrane continuity is
maintained
Entry into host cell by
viruses
Fusion of sperm and egg
Release of neurotoxins by
exocitosis
Membrane carbohydrates
20

Membranes play key role in cell-cell recognition

Carbohydrates are usually branched
oligosaccharides with fewer than 15 sugar units
Membrane carbohydrates

21
Oligosaccharides on external of membranes are
different among species, or individuals, or cells
Membrane functions
22

Cell communication and signalling

Cell-cell adhesion and cellular attachment

Cell identity and antigenicity

Conductivity
Membrane functions


Form selectively permeable barriers
Transport phenomena
– Passive diffusion
– Mediated transport
 Facilitated diffusion
– Carrier proteins
– Channel proteins

–
23
Gated or non-gated channels
Active transport
Transport of Ions and Small
Molecules Across Cell Membrane
24
Membrane transport


All cells require the molecules and ions they
need from ECF (extracellular fluid).
There are two problems to be considered
Relative concentrations
-diffusion
-active transport
2. Lipid bilayers are impermeable to most
essential molecules and ions
1.
25
Membrane transport
26
Solving the Problem
Mechanisms by which cells solve this problem
include:
1.
Active transport
2. Facilitated diffusion
27
Active Transport
Active transport is the pumping of molecules or
ions through a membrane against their
concentration gradient. It requires
28

a transmembrane protein (a complex of
them) called a transporter

Energy. ATP (source)
Active Transport

29
Active transport
enzymes couple net
solute movement
across a membrane
to ATP hydrolysis.
Active Transport

30
An active transport
pump may be a
uniporter, or it may
be an antiporter
that catalyzes ATPdependent
transport of 2
solutes in opposite
directions.
Active Transport
.

31
ATP-dependent ion
pumps are grouped
into classes, based
on transport
mechanism, genetic
& structural
homology.
Active Transport
The energy of ATP may be used directly or
indirectly
There are two types of active transport
 Direct / Primary

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Indirect/Secondary
Active Transport

33
Primary /Direct
– The transport system is an ATPase.
– The energy for transport comes directly
from ATP.
– Some cation transport systems fall into
this category. The NaK-pump is the prime
example.
Active Transport

Secondary/Indirect
–
–
34
The transport system utilizes the Na+
electrochemical gradient as an energy source to
move a solute against its electrochemical
gradient.
Na+ is transported down its electrochemical
gradient in the process. This is also referred to as
a Na-coupled or gradient-coupled transport.
Active Transport
Indirect Active Transport.
Transporters use energy already stored in the
gradient of a directly pumped ion.

35
Membrane Transport
Transporters are of two general classes:
carriers and channels.
These are exemplified by two ionophores (ion
carriers produced by microorganisms):
valinomycin (a carrier)
gramicidin (a channel).
36
Energetics of active transport

37
Active transport
– Metabolic energy expenditure is required.
– Solute moves against a gradient of
electrochemical potential.
Carrier mediated membrane transport

Carriers exhibit saturation kinetics with
respect to solute concentration.

Carriers exhibit stereospecificity.
–
38
Glucose carrier transports D-glucose but not Lglucose.
Carrier mediated membrane transport

Carriers are susceptible to inhibition.

Carrier rates are susceptible to hormonal
control (although channels may be as well).
Influence of insulin on the glucose transporter
Influence of aldosterone on the Na-K transporter
(NaK-pump).
–
39
Kinetics of transport carriers
Carriers exhibit Michaelis-Menten kinetics.
The transport rate mediated by carriers is
faster than in the absence of a catalyst, but
slower than with channels.
A carrier transports only one or few solute
molecules per conformational cycle.
40
Energetics of carrier-mediated
transport
Diffusion
 Passive transport (facilitated diffusion)
–
–
–
–
41
No metabolic energy required.
Solute moves down a gradient of
electrochemical potential in combination with a
carrier.
Km is the same on the two sides of membrane.
Example - glucose transport in most cells.
Carrier proteins
42

Proteins that act as carriers are too large to move
across the membrane.

They are transmembrane proteins, with fixed
topology.

Example: GLUT1 glucose carrier, found in plasma
membranes of various cells, including erythrocytes.

GLUT1 is a large integral protein, predicted via
hydropathy plots to have 12 transmembrane ahelices.
Carrier proteins
conformation
change
conformation
change
Carrier-mediated solute transport

43
Carrier proteins cycle between
conformations in which a solute binding site
is accessible on one side of the membrane
or the other.
Carrier proteins
conformation
change
conformation
change
Carrier-mediated solute transport
44

There may be an intermediate conformation
in which a bound substrate is inaccessible to
either aqueous phase.

With carrier proteins, there is never an open
channel all the way through the membrane
Classes of carrier proteins
45
Classes of carrier proteins
Uniport
Uniport (facilitated
diffusion) carriers
mediate transport of
a single solute.
Examples include
GLUT1 and
valinomycin.
46
Classes of carrier proteins
Uniport
47

These carriers can
undergo the
conformational change
associated with solute
transfer either empty or
with bound substrate.

Thus they can mediate
net solute transport.
Classes of carrier proteins
Uniport
Valinomycin is a carrier for K+.
Valinomycin reversibly binds a single K+ ion.
48
Classes of carrier proteins
Uniport
Valinomycin is highly
selective for K+
over Na+.
Why???
49
Classes of carrier proteins
Symport
Symport (cotransport)
carriers bind 2
dissimilar solutes
(substrates) & transport
them together across a
membrane.
Transport of the 2 solutes
is obligatorily
coupled.
50
Classes of carrier proteins
Symport
An example is the plasma
membrane glucoseNa+ symport.
A gradient of one
substrate, usually an
ion, may drive uphill
(against the gradient)
transport of a cosubstrate.
51
Classes of carrier proteins
Symport
Trans-epithelial
transport:
In the example shown, 3
carrier proteins
accomplish absorption
of glucose & Na+ in the
small intestine.
glucose Na+
glucose-Na+ symport
apical end
Na+
glucose
ATP
ADP + Pi
basal end
Na+ pump
GLUT2
K+
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intestinal epithelial cell
Classes of carrier proteins
Symport
.
The Na+ pump, at the
basal end of the cell, keeps
[Na+] lower in the cell than
in fluid bathing the apical
surface.
Na+
glucose-Na+ symport
glucose
apical end
Na+
glucose
ATP
ADP + Pi
basal end
Na+ pump
GLUT2
K+
intestinal epithelial cell
53
Classes of carrier proteins
Symport
.
•The Na+ gradient drives
uphill transport of glucose
into the cell at the apical
end, via glucose-Na+
symport. [Glucose] within
the cell is thus higher than
outside.
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Na+
glucose-Na+ symport
glucose
apical end
Na+
glucose
ATP
ADP + Pi
basal end
Na+ pump
GLUT2
K+
intestinal epithelial cell
Classes of carrier proteins
Symport
.
•Glucose flows passively
out of the cell at the basal
end, down its gradient, via
GLUT2 (uniport related to
GLUT1).
Na+
glucose-Na+ symport
glucose
apical end
Na+
glucose
ATP
ADP + Pi
basal end
Na+ pump
GLUT2
K+
intestinal epithelial cell
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Classes of carrier proteins
Antiport
Antiport (exchange
diffusion) carriers
exchange one
solute for another
across a
membrane.
Uniport
A
Symport
A
B
Antiport
A
B
56
Classes of carrier proteins
Antiport
Example: ADP/ATP
exchanger
(adenine nucleotide
translocase) which
catalyzes 1:1
exchange of ADP
for ATP across the
inner mitochondrial
membrane.
57
Uniport
A
Symport
A
B
Antiport
A
B
Classes of carrier proteins
Antiport


58
Usually antiporters
exhibit "ping pong"
kinetics.
One substrate is
transported across
a membrane and
then another is
carried back.
Uniport
A
Symport
A
B
Antiport
A
B
Active Transport





59
ATP dependent ion pumps are grouped into
classes, based on transport mechanisms as
well as genetic and structural homology.
Examples include
P-class pumps
F-class pumps
V-class pumps
Active Transport
There are four types of Direct Active transport
1.
2.
3.
4.
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The Na+/K+ ATPase
The H+/K+ ATPase
The Ca 2+ ATPases
The ABC transporters
P-Type transporters
1.Na+/K+ ATPase
 H+/K+ ATPase
 Ca 2+ ATPase
They use the same basic mechanism:

61

Conformational change in proteins as they are
reversably phosphorylated by ATP

All three pumps can be made to run backwards

If the pumped ions are allowed to diffuse back through the membrane complex,
ATP can be synthesized from ADP and inorganic phosphate
P-Type transporters
The reaction mechanism
for a P-class ion pump
involves transient covalent
modification of the
enzyme.
O
Enzyme-C
OH
ATP
Pi
ADP
H2O
O
Enzyme- C
O
O
P
O-
P-Class Pumps
62
O-
Direct Active Transport
The Na+/K+ ATPase




63
K+ is 20 X higher in cytosol than
extracellular fluid
Na+ in extracellular fliud is 10X greater than
in cytosol
Concentration gradient is maintained by
active transport of both ions
The Na+/K+ ATPase transporter does both
jobs
Direct Active Transport
The Na+/K+ ATPase
The Na+/K+ ATPase transporter uses energy
from the hydrolysis of ATP to
64

Actively transport 3 Na+ ions out of the cell

For each 2 K+ ions pumped into the cell
The Na+/K+ ATPase transporter
–
Na+/K+-ATPase,
in plasma
membranes of
most animal cells,
is an antiport
pump.
Inward
3 Na+
Sodium
Flux
Extracellular
Cytosol
Mg++
ATP
2 K+
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ADP + Pi
Outward
Potassium
Flux
The Na+/K+ ATPase transporter
–
Gradients for Na+
and K+ needed for
action potentials &
synaptic potentials
Inward
3 Na+
Sodium
Flux
Extracellular
Cytosol
Mg++
ATP
2 K+
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ADP + Pi
Outward
Potassium
Flux
Direct Active Transport
The Na+/K+ ATPase Transporter
What does this accomplish
 It helps to establish a net charge across the
plasma membrane
 The accumulation of sodium ions outside of
the cell draws water out of the cell and
enables it to maintain osmotic balance.
Why is this important?
67
Direct Active Transport
The Na+/K+ ATPase Transporter
What does this accomplish
 The gradient of sodium ions is harnessed to
provide the energy to run several types of
indirect pumps
68
The Na+/K+ ATPase transporter
Inhibited by :
– Cardiac
glycosides
– Metabolic
inhibitors
– Heavy Metals
Inward
3 Na+
Sodium
Flux
Extracellular
Cytosol
Mg++
ATP
2 K+
69
ADP + Pi
Outward
Potassium
Flux
Digitalis inhibits the Na+/K+ Pump
70

Digitalis is a mixture of cardiotonic steroids

Digitoxigen and ouabain inhibitors

cardiotonic steriods – strong effect on heart

Increases the force of contraction of the heart
Foxglove (Digitalis purpurea)



71
William Withering
conducted studies on
Foxglove
Conducted the first
scientific study on its
effects
“Old woman of
Shropshire”
Digitalis inhibits the Na+/K+ Pump

Inhibit dephosphorylation of the
phosphorylated form of ATPase on the
extracellular face of the membrane

Leads to higher Na+ in the cytosol

Diminished Na+ gradient leads to slower
exclusion of Ca 2+ by Na-Ca exchanger
(antiporter)
72

Increase in intracellular levels of Ca 2+ enhances the ability of the
cardiac muscle to contract
Digitalis inhibits the Na+/K+ Pump
73
Inhibititors of the Na+/K+ Pump
74

Oubain (Samali for arrow poison steriod
derivative of ouabain)

Binds to the form of the Na+K+ ATPase that
is open to the extracellular side

Locks in 2 Na+ and prevents the change in
conformation necessary for transport of ions
Inhibitors of the Na+/K+ Pump





75
Palytoxin (produced by coral found in Hawaii)
Binds to Na+K+ ATPase and locks it in
position so that the ion binding sites are
permanently accessible form both ends
Open channel
Exit of K+ from cells
Toxic
Direct Active Transport
The H+/K+ ATPase Transporter
76

The H+/K+ ATPase transporter plays a part in
maintaining the acidity of the stomach

Mammalian stomach contains a 0.1M solution of
HCl

This stongly acidic medium kills many ingested
pathogens and denatures many ingested proteins
before they can be degraded by proteolytic
enzymes (pectin) that function at acid pH
Direct Active Transport
The H+/K+ ATPase Transporter

HCl is secreted into the stomach by specialized
epithelial cells called parietal cells in the gastric
lining

These cells contain the H+/K+ ATPase in their
apical membrane which faces the stomach lumen

This generates a million fold H+ gradient
pH in stomach lumen is 1.0 whereas pH in the
cytosol is 7.0

77
Direct Active Transport
The H+/K+ ATPase Transporter
78

The H+/K+ ATPase is a P class pump that is
similar in structure and function to the Na+/K+
pump found in the plasma membrane

The numerous mitochondria in the parietal cells
produces enough ATP needed for use by the
H+/K+ pump
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells
79

If parietal cells simply
exported H+ ions in
exchange for K+ ions ,
the loss of protons
would lead to arise in
OH- ions in the cytosol

This would lead to an
increase in cytosolic
pH
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells
80

This is avoided by using Cl/HCO3- antiporters in the
basolateral membrane

This exports the excess OHions in the cytosol into the
blood

This antiporter is activated
at high cytosolic pH
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells
81

In a reaction catalysed
by carbonic anhydrase
the excess cytosolic
OH- combines with CO2
that diffuses in from
the blood

This forms HCO3-
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells

82
Catalysed by the
basolateral anionic
antiporter, this
bicarbonate ion is
exported across the
basolateral membrane
in exchange for Cl- ions
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells

83
The Cl- ions then exit
through Cl- channels in
the apical membrane,
entering the stomach
lumen
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells

84
To preserve the
electronuetrality, each
Cl- ion that moves into
the stomach lumen
through the apical
membrane is
accompanied by a K+
ion that moves outward
through a separate K+
channel
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells
85

In this way the excess
K+ ions pumped inward
by the H+/K+ ATPase
are returned to the
stomach lumen

Thus maintaining the
normal intracellular K+
concentrations
The H+/K+ ATPase Transporter
Acidification of stomach lumen by parietal cells
86

The net result is
secretion of equal
amounts of H+ and Clions (HCl) into the
stomach lumen

While the pH in the
cytosol remains nuetral
and excess HCO3- ions
are transported into the
blood.
Direct Active Transport
The Ca 2+ ATPase Transporter
87

The Ca 2+ ATPase is located in the plasma
membrane of all eukaryotic cells

1 ATP is used to pump 1 Ca 2+ out of the cell

20,000 fold conc gradient between Ca 2+ in
the cytosol and that in the extracellular fluid
(ECF)
The Ca 2+ ATPase Transporter

88
Ca 2+ -ATPase pump, in endoplasmic reticulum (ER)
& plasma membranes catalyze transport of Ca 2+
away from the cytosol, either into the ER lumen or
out of the cell.
There is some evidence that H+ may be transported in
the opposite direction.
– Ca 2+ -ATPase pumps keep cytosolic Ca 2+ low
(10-7M vs. 10-3 M in plasma), allowing Ca 2+ to
serve as a signal.
Direct Active Transport
The Ca 2+ ATPase Transporter
89

Resting skeletal muscle there is a higher
conc of Ca 2+ ions in the endoplasmic
reticulum than the cytosol

Activation of muscle fibre allows Ca 2+ to
pass into the cytosol, triggering contraction
Direct Active Transport
The Ca 2+ ATPase Transporter

After contraction the Ca 2+ is pumped back
into the sarcoplasmic reticulum
This is done by another Ca 2+ ATPase pump
Uses energy from each molecule of ATP to
pump 2 Ca 2+ ions
 The Ca 2+ pump is called SERCA

90
The Ca 2+ ATPase Transporter

The catalytic cycle begins with the enzyme in its
unphosphorylated state with 2 calcium ions bound

In the E1 conformation the enzyme can bind ATP.
Conformational change occurs and the Ca2+ ions are trapped
inside
91
The Ca 2+ ATPase Transporter

The phosphoryl group is then transferred from ATP
to aspartate

Upon ADP release the enzyme changes its overall
conformation (E2-P). This process is called eversion
92
The Ca 2+ ATPase Transporter

In the E2-P conformation the calcium binding sites
become disrupted and the calcium ions are released
to the side of the membrane opposite to which they
entered

E2-P is then hydrolysed releasing the inorganic
phosphate
93
The Ca 2+ ATPase Transporter

With the release of the phosphate the stabilization of
the E2 form is lost and the enzyme everts back to the
E1 conformation

The binding of two calcium ions from the cytosolic
side completes the cycle
94
SERCA:Sarco Endo(plasmic)
Reticulum Ca 2+ ATPase
95
SERCA:Sarco Endo(plasmic)
Reticulum Ca 2+ ATPase
96
Direct Active Transport
ABC Transporters
ABC (ATP-Binding-Cassette) transporters are
transmembrane protein that
 Expose a ligand-binding domain at one
surface and a
 ATP-binding domain at the other surface
 The ligand binding domain is restricted to a
single type of molecule
97
Direct Active Transport
ABC Transporters

98
The ATP bound to its domain provides the
energy to pump the ligand across the
membrane
ABC Transporters
Mechanism
99

The catalytic cycle begins
with the transporter being
free of both ATP and
substrate

The transporter can
interconvert between closed
and open forms

Substrate enters the central
cavity of the open form of
the transporter from inside
the cell
ABC Transporters
Mechanism
10
0

Substrate binding results in
a conformational change in
the ATP binding cassette
that increases their affinity
for ATP

ATP binds to the ATPbinding cassettes, changing
their conformations so that
the two domains interact
strongly with each other
ABC Transporters
Mechanism
10
1

The strong interaction
between the ATP-binding
cassettes induces a change
in the relation between the
two domains releasing the
substrate to the outside of
the cell

The hydrolysis of ATP and
the release of ADP and
inorganic phosphate resets
the transporter for another
cycle
ABC Transporters (Mechanism)
10
2
Direct Active Transport
ABC Transporters
10
3
The human genome contains 48 genes for
ABC transporters.
 CFTR- the cystic fibrosis transmembrane
conductance regulator
 TAP-the transporter associated with antigen
processing
 ABC transporters that pump
chemotherapeutic drugs out of cancer cells
Physiological effects of defects in
ABC Transporters
Genetic diseases such as:
 Cystic fibrosis
 Tangier disease
 Retinal degeneration
 Anemia
 Liver failure
10
4
Effects of ABC Transporters
ABC transporters can confer antibiotic
resistance in pathogenic microbes such as:
 Pseudomonas aeruginosa
 Staphylococcus aureus
 Candida albicans
 Neisseria gonorrhoeae
10
5
Transepithelial transport
10
6

Absorption of nutrients from the intestinal lumen
occurs by:

import of molecules on the luminal side of the
intestinal epithelial cells

And their export on the blood-facing (serosal) side, a
two stage process called transcellular transport

An intestinal epithelial cell is polarized
Transepithelial transport
Movement of glucose and amino acids across
Eipthelia

10
7
In the first stage of this
process, a 2 Na+/1glucose symport
located in the
microvillar membrane
imports glucose
against its
concentration gradient
from the intestinal
lumen
Transepithelial transport
Movement of glucose and amino acids across
Eipthelia

10
8
This symporter couples
the energetically
unfavourable
movement of 1 glucose
molecule to the
favourable inward
transport of 2Na+
Transepithelial transport
Movement of glucose and amino acids across
Eipthelia

10
9
In the steady state all the
Na+ ions transported from
the intestinal lumen into
the cell during the
Na+/glucose symport
are pumped out across
the basolateral membrane
Transepithelial transport
Movement of glucose and amino acids across
Epithelia
11
0

Thus the low intracellular
Na+ concentration is
maintained

This is accomplished by
the Na+/K+ ATPase
which is found exclusively
in the basolateral
membrane of the small
intestine
Transepithelial transport
Movement of glucose across Eipthelia
11
1

The coordinated operation
of these two transport
proteins allows uphill
movement of glucose
from the intestine into
the cell

The first stage is
ultimately powered by
ATP hydrolysis by the
Na+/K+ ATPase
Transepithelial transport
Movement of glucose across Eipthelia

11
2

In the second stage,
glucose concentrated
inside the intestinal cells
by symporters are
exported down their
concentrated gradients
into the blood via uniport
proteins in the
basolateral membrane.
In the case of glucose this
is mediated by GLUT2
Transepithelial transport
Movement of glucose across epithelia

11
3
The net result of this twostage process is
movement of Na+,
glucose, and amino acids
from the intestinal lumen
across the intestinal
epithelium into the
extracellular medium that
surrounds the basolateral
surface of the intestinal
epithelial cells
Transepithelial transport
Movement of glucose across Eipthelia
11
4

Tight junctions between
the epithelial cells prevent
these molecule from
diffusing back into the
intestinal lumen

Eventually they move into
the blood
Transepithelial transport
Movement of glucose across Epithelia

11
5
The increased osmotic pressure created by
transcellular transport of salt, glucose and amino
acids across the intestinal epithelium draws water
from the intestinal lumen into the extracellular
medium that surrounds the basolateral surface.
Transepithelial transport
Movement of glucose across Epithelia
11
6

Simple rehydration therapy depends on the osmotic
gradient created by absorption of glucose and Na+

Treatment of chlolera and other intestinal
problems (gastroenteritis). Giving patients a
solution of sugar and salt helps in rehydration.

Similar sugar/ salt solutions are the basis of popular
sport drinks used by athletes (gatorade). This is
used to get sugar as well as water into the body
quickly and efficiently
Faciltated Diffusion of Ions




11
7
Facilitated diffusion takes place through
transmembrane proteins
Water filled channel through which the ions
can pass down its concentration gradient
Transmembrane channels that permit
facilitated diffusion can be opened or
closed.
They are said to be gated
Facilitated Diffusion of molecules
Small hydrophilic molecules such as sugars
can pass through cell membranes by
facilitated diffusion
Examples:
Maltoporin: allow disaccharide and maltose
and few related molecules to diffuse into the
cell of E.coli
11
8
Facilitated Diffusion of molecules
Examples:
 Transmembrane proteins of red blood cells
permit diffusion of glucose from blood into
the cell
NB. In facilitated diffusion the channels are
selective. The structure of the protein only
admits certain types of molecules
11
9
Ion Channels

Ionophores: compounds that shuttle ions
across membrane
Valinomycin disrupts the (K+) channel
Monensin disrupts (Na+) channel
12
0
Ion channels
12
1

Bacterial K+ channel

Neuronal Na+ channel

Nicotinic Acetylcholine receptor ion
channel
Ion channels
Differences between Ion channels and
transporters
1. The rate of flux through channels are much
greater
2. Ion channels are not saturable
3. They are gated –opened or closed in
response to some cellular event
12
2
Ion channels
Types of Gated Channels




12
3
Ligand gated
Mechanically- gated
Voltage- gated
Light -gated
Voltage gated ion channel
In cells such as neurons and muscle cells
some channel open and close in response to
changes in the charge (measured in volts)
across the plasma membrane
12
4
Voltage gated ion channel
Example:
 Impulse passes down neuron
 Reduction in voltage opens Na+ channels in
adjacent portion of the membrane
 This allows the influx of Na+ into the neuron
and hence the continuation of the nerve
impulse
12
5
Ion Channels
K+ Ion channel



12
6
The K+ ion channel is a tetramer of identical
subunits
The four subunits come together to form a
pore that runs through the centre of the
structure
The pore starts with a diameter of 10 A and
constricts to a smaller cavity (8 A)
Ion Channels
K+ Ion channel
12
7

Both the openings to
the outside and central
cavity of the pore are
filled with water

K+ can fit into the pore
without losing its shell
of bound water
molecule
Ion Channels
K+ Ion channel
12
8

Approximately twothirds of the way
through the channel the
pore constricts

The K+ ions will have to
shed its bound water
molecules
Ion Channels
K+ Ion channel
12
9

For K+ to relinquish
their water molecule
other polar interactions
must replace those with
water

The restricted part of
the core is built from
residues contributed by
two transmembrane
alpha helices
Ion Channels
K+ Ion channel

13
0
A five amino acid
stretch within this
region functions as the
selectivity filter that that
determines the
preference of K+ ions
over other ions
Ion Channels
K+ Ion channel
13
1

K+ ion channels are
100-fold more
permeable to K+ than
to Na+

The key point is that the
free energy cost of
dehydrating Na+ ions
are considerable.
Ion Channels
K+ Ion channel

13
2
The channel repays the
cost of dehydrating K+
by providing
compensating
interactions with the
carbonyl oxygen atoms
lining the selectivity
filter
Ion Channels
K+ Ion channel
13
3

These oxygen atoms
are positioned so that
they do not interact
favourably with Na+
because it is too small

The Na+ ions are
rejected because they
must stay hydrated to
pass through the
channel
Ion Channels
K+ Ion channel
13
4

Four K+ binding sites
crucial for rapid flow are
present in the
constricted area of the
K+ channel

The ion can move
within the 4 sites of the
selectivity channel
because they have
similar ion affinities
K+ binding sites on the
selectivity pore of K+ channel
Ion Channels
K+ Ion channel

13
5
As each subsequent K+
moves into the
selectivity filter, its
positive charge will
repel the K+ ion at the
nearest site
K+ binding sites on the
selectivity pore of K+ channel
Ion Channels
K+ Ion channel

13
6
This causes it to shift to
a site further up the
channel and in turn
push upward any K+
ion already bound to a
site further up
K+ binding sites on the
selectivity pore of K+ channel
Ion Channels
K+ Ion channel

13
7
Insert photo
K+ binding sites on the
selectivity pore of K+ channel
Ion Channels
Na+ Ion channel (Voltage gated)
13
8

Typically more selective for Na+ over other
monovalent or divalent ions

The Na+ channel’s selection of Na+ over K+
ions depends on ionic radius

It is sufficiently restricted so that small ions
like Na+ and Li+ can pass through but larger
ions are restricted
Ion Channels
Na+ Ion channel (Voltage gated)

Na+ are activated by a reduction in the
transmembrane potential

-60 mV resting
+30 mV activated

13
9
Ion Channels
Na+ Ion channel (Voltage gated)

14
0
Insert photos
Ion Channels
Na+ Ion channel (Voltage gated)
14
1

The depolarization induced by the opening of
Na+ channels causes voltage gated K+
channels to open, and the resulting efflux of
K+ repolarises the membrane locally

Changes in transmembrane potential
produce subtle conformational changes in
channel protein
This is explained by the ball and chain model

Na+ Ion channel (Voltage gated)
14
2
Ball and chain model for channel inactivation
Na+ Ion channel (Voltage gated)
14
3

The first 20 residues of the K+ channel forms a
cytoplasmic unit ( the ball)

This is attached to a flexible segment of the
polypeptide (the chain)
Na+ Ion channel (Voltage gated)
14
4

When the channel is closed the ball rotates freely in
aqueous soln

When the channel is open the ball quickly finds a
complementary site in the open pore and occludes
to it
Na+ Ion channel (Voltage gated)
14
5

The channel opens for only a brief interval before it
undergoes inactivation by occlusion

Shortening the chain speeds in activation
Lengthening the chain slows inactivation

Physiological consequences of
defective ion channels
Defect in Na+ channel results in :
14
6

Hyperkalemic (periodic) paralysis

Paramyotonia congenita (muscles are stiff)
Physiological consequences of
defective ion channels
14
7

Many naturally occuring toxins act on ion channels

Tetrodotoxin (produced by the puffer fish) and
saxitoxin (produced by marine dinoflagellates
Gonyaulax, which causes red tides)

act by binding to the voltage gated Na+ channels of
neurons and prevent normal action potentials
Physiological consequences of
defective ion channels
14
8

Eating shellfish that have been fed on
Gonyaulax can be fatal

Shellfish concentrate the saxitoxin in their
muscles which becomes highly poisonous to
organisms higher up on the food chain
Physiological consequences of
defective ion channels

The venom of the Black mamba snake contains
Dendrotoxin which interferes with the voltage gated
K+ channels.

Tobocurarine (active componet of curare) and two
other toxins from snake venom Cobrotoxin &
Bungarotoxin Blocks AcH receptors or prevent
opening of its ion channel.
By blocking signals from nerve to muscles these
toxins can cause paralysis and possibly death

14
9
Ligand gated channels
External ligands
Bind to a site on the extracellular side of the
channel

Examples:
 Acetylcholine (ACh).
 Gamma amino butyric acid (GABA)
15
0
Binding of GABA at certain synapses ( designated
GABAA) in the CNS admits Cl- ions into the cell and
inhibits the creation of a nerve impulse
Ligand gated channels
Internal ligands
Internal ligands bind to a site on the channel
protein exposed to the cytosol
Examples
 Cyclic AMP (cAMP) and cyclic GMP (cGMP)

15
1

cGMP (3`,5`-cyclic monophosphate) cAMP (cyclic adenosine
phosphate)

ATP is needed to open the channel that
allows chloride (Cl-) and bicarbonate (HNO3-)
ions out of the cell
The acetylcholine receptor
Ligand gated channel






15
2

Acetylcholine released by motor nuerons
diffuses to the plasma membrane
Binds to AcH receptor
Conformational change in receptor
Causes the ion channel to open
Ca 2+ ,Na+ and K+ passes through with ease
Depolarization of plasma membrane
(contraction)
Movement is unsaturable
The acetylcholine receptor
Ligand gated channel
15
3

Signaling in the nervous system is
accomplished by a network of neurons, that
carry an electrical impulse (action potential)
from one end of the cell through an
elongated cytoplasmic extension (the axon).

The electrical signal triggers the release of
neurotransmitter molecules at a synapse
carrying signal to the next cell in the circuit
The acetylcholine receptor
Ligand gated channel





15
4
Three types of voltage-gated ion channels
are essential to this signaling mechanism
These are
the voltage gated Na+ channels
The voltage gated K+ channels
The voltage gated Ca2+ channels
The acetylcholine receptor
Ligand gated channel
Along the length of the
axon are voltage gated
Na+ channels that are
closed when the
membrane is at rest (60mV)
 They open briefly when
the membrane is
depolorised locally in
responce to acteylcholine
(or other nuerotransmitters)

15
5
The acetylcholine receptor
Ligand gated channel

15
6
The depolorization
caused by the opening of
the Na+ channels causes
the voltage gated K+
channels to open and the
resulting efflux of K+ ions
repolarizes the
membrane locally
The acetylcholine receptor
Ligand gated channel

15
7
A brief pulse of
depolarization traverses
the axon as local
depolarization triggers the
brief opening of Na+
channels then K+
channels
The acetylcholine receptor
Ligand gated channel
15
8

After each opening of the
Na+ channel a brief
refractory period follows,
during which the channel
cannot open again.

This results in a
unidirectional wave of
depolorization from the
nerve cell body to the end
of the axon
The acetylcholine receptor
Ligand gated channel



15
9
At the distal tip of the
axon are voltage gated
Ca2+ channels.
They open and the Ca2+
enters from the
extracellular space
The rise in cytoplasmic
Ca2+ triggers the release
if acetylcholine into the
synaptic cleft
The acetylcholine receptor
Ligand gated channel
16
0

Acetylcholine diffuses to
the postsynaptic cleft
where it binds to
acetylcholine receptors
and triggers
depolorization

The message is passed
to the next cell in the
circuit
The acetylcholine receptor
Ligand gated channel
16
1
Role of voltage gated and ligand gated ion
channels in neural transmission
16
2

Initially the plasma membrane of the presynaptic neuron is
polarized (inside negative) through the action of the
electrogenic Na+ K+ ATPase, which pumps 3 Na+ out for every
2 K+ pumped into the neuron

A stimulus to this neuron causes an action potential to move
along the axon ( white arrow) away from the cell body. The
opening of one voltage gated Na+ channel allows Na+ entry,
and the resulting local depolorization causes the adjacent Na+
channel to open and so on. The directionality of movement of
the action potential is ensured by the brief refractory period that
follows the opening of each voltage gated Na+ channel
Role of voltage gated and ligand gated ion
channels in neural transmission
16
3

When the wave of depolarisation reaches the axon tip, voltagegated Ca 2+ channels open, allowing Ca 2+ entry into the
presynaptic neuron

The resulting increase in internal Ca 2+ triggers exocytic release
of the neurotransmitter acetylcholine into the synaptic cleft

Acetylcholine binds to a receptor on the postsynaptic neuron,
causing its ligand gated ion channel to open
Role of voltage gated and ligand gated ion
channels in neural transmission

16
4
Extracellular Na+ and Ca 2+ enter through this channel
depolarizing the postsynaptic cell. The electrical signal has now
passed to the cell body of the post synaptic neuron and will
move along the axon to a third neuron by the same sequence
of events