1. HBC1011 Biochemistry I
Lecture 25 – Lipids and Cell Membranes
Ng Chong Han, PhD
ITAR1010, 06-2523751
chng@mmu.edu.my

2. Overview
• Active and passive molecule transport across a
membrane
• P-type ATPase and ATP-binding cassette (ABC)
transporter/pump
• Ion channels
• Gap junctions & aquaporin
2

3. Introduction
• Lipid bilayer is intrinsically impermeable to ions to
polar molecules, yet certain molecules must be able to
enter for cellular functions.
• Permeability is conferred by membrane proteins, eg
pumps/transporter and channels.
3

4. 4

5. Protein-free lipid bilayers are
impermeable to ions
• The rate of molecule diffusion
depends partly on the size of the
molecule but mostly on its relative
hydrophobicity.
• In general, the smaller the molecule
and the more hydrophobic it is, the
more easily it will diffuse across a
lipid bilayer.
• Small nonpolar molecules, such as O2
and CO2, readily dissolve in lipid
bilayers and therefore diffuse rapidly
across them. Small uncharged polar
molecules, such as water or urea, also
diffuse across a bilayer, albeit much
more slower.
5

6. Transport of molecules across a
membrane may be active/passive
• Many molecules require protein transporters to cross
membranes.
• 2 factors determine whether a molecule will cross a
membrane:
1. Permeability of the molecule in a lipid bilayer -
The molecule must be able to cross a
hydrophobic barrier.
2. Availability of energy source - An energy source
must power the movement
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7. Membrane Transport Proteins:
Carriers and Channels
• Transporters (also called carriers, or permeases) bind the specific
solute to be transported and undergo a series of conformational
changes that facilitate the molecule transport.
• Channels interact with the solute to be transported much more
weakly. They form continuous pores that extend across the lipid
bilayer. When open, these pores allow specific solutes (such as
inorganic ions) to pass through them. Transport through channels
occurs at a much faster rate than transport mediated by
transporters.
7

8. Differences between channels and
transporters
(a) In an ion channel, a transmembrane pore is either open or
closed, depending on the position of the single gate. When it is
open, ions move through at a rate limited only by the maximum
rate of diffusion.
8

9. Differences between channels and
transporters
(b) Transporters have two gates, and both are never open at the same time.
Movement of a substrate through the membrane is therefore limited by the
time needed for one gate to open and close and the second gate to open.
Rates of movement through ion channels can be orders of magnitude
greater than rates through transporters, but channels simply allow the ion
to flow down the electrochemical gradient, whereas active transporters can
move a substrate against its concentration gradient. 9

10. Transporters can function as
uniports, symports, or antiports.
• Transporters that carry a single solute across the membrane are
called uniports. transporters that move multiple solutes are called
coupled transporters. In coupled transport, the solutes can be
transferred either in the same direction, by symports, or in the
opposite direction, by antiports. Uniports, symports, and antiports
can be used for either passive or active transport. Some coupled
transporters, for example, act as pumps, coupling the uphill
transport of one solute to the downhill transport of another. 10

11. Transport of molecules across a
membrane may be active/passive
11
Some small nonpolar molecules such as CO2 can move passively down their
concentration gradient across the lipid bilayer by simple diffusion, without the help
of a transport protein. Most solutes, however, require the assistance of a channel or
transporter. Passive transport, which allows molecules to move down their
concentration gradients, occurs spontaneously; whereas active transport against a
concentration gradient requires an input of energy. Only transporters can carry out
active transport.

12. Ion gradient
12
The electrochemical gradient of a charged solute (an ion) affects its
transport. This gradient combines the membrane potential and the
concentration gradient of the solute. The electrical and chemical gradients
can work additively to increase the driving force on an ion across the
membrane (middle, eg Na+) or can work against each other (right, eg K+).

13. Concentration gradient, membrane potential
and electrochemical gradient
13
• Membrane potential/membrane voltage: the difference in electric
potential between the interior and the exterior of a biological cell.
• Concentration gradient: The movement of solutes through a
membrane from an area of higher concentration to an area of lower
concentration (energy is generated).
Electrochemical
gradient =
concentration
gradient +
membrane
potential

14. Passive transport
• Passive transport is a movement of ions and other molecules
across cell membranes without need of energy input.
• Unlike active transport, it does not require an input of cellular
energy because the substance go down its concentration gradient
across a membrane, eg glucose transporters.
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channel transporter
Passive transport
• Simple diffusion
• Facilitated diffusion

15. Passive transport - Simple diffusion
• Some molecules can pass through cell membranes
without interacting with another molecule.
• Dissolve in lipid bilayer: lipophilic molecule, i.e.: steroid
hormones (cholesterol)
• Pass through a membrane down their concentration
gradient: simple diffusion.
• Molecule spontaneously move from a region of higher
concentration to one of lower concentration.
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16. Passive transport - Facilitated diffusion
• Uniporter can be either ion channels or carrier proteins
• Uniporter carrier proteins transport one molecule across a membrane
down its concentration gradient.
• Uniporter channels open in response to a stimulus and allow the free flow
of specific molecules.
• Both rely on passive transport, as they do not directly require cellular
energy to function. 16

17. Active transport: Ion-concentration
gradients
• Active transport occur when a molecule moves against a
concentration gradient with external energy source. The
electrochemical potential measures the combined ability of a
concentration gradient and an uneven distribution of charge to
drive species across a membrane. 17
Orange: Actively
transported
molecules
Red: Energy
source

18. Active transport: Ion-concentration
gradients
• Coupled transporters harness the energy stored in concentration gradients
to couple the uphill transport of one solute across the membrane to the
downhill transport of another.
• ATP-driven pumps couple uphill transport to the hydrolysis of ATP.
• Light- or redox-driven pumps, which are known in bacteria, archaea,
mitochondria, and chloroplasts, couple uphill transport to an input of
energy from light, as with bacteriorhodopsin. 18
Orange: Actively
transported
molecules
Red: Energy
source

19. Primary active transport
• Primary active transport: use energy source from ATP
hydrolysis or light absorption to directly drive the
transport of a solute against its concentration gradient
• Undergo sequential conformational changes to
transport specific small molecules across membranes,
• Energy transducer, convert one form of free energy into
another.
• 2 types of ATP driven pumps:
– P-type ATPases
– ATP-binding cassette (ABC)
transporters
19

20. Primary active transport
P-type pumps are structurally and
functionally related to multipass
transmembrane proteins.
They are called “P-type” because they
phosphorylate themselves during the
pumping cycle.
Responsible for setting up and
maintaining gradients of Na+, K+, H+,
and Ca2+ across cell membranes.
20

21. Na+ pump in animal cells uses ATP to expel
Na+ and bring in K+
• The concentration of K+ is typically 10–30 times higher
inside cells than outside, whereas the reverse is true of Na+.
A P-type Na+-K+ pump/Na+ -K+ ATPase, found in the plasma
membrane of virtually all animal cells maintains these
concentration differences. 21

22. Plasma membrane Na+-K+ pump
• The Na+-K+ pump operates as an ATP-driven antiporter,
actively pumping Na+ out of the cell against its steep
electrochemical gradient and pumping K+ in.
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Functions
Controls cell volume,
Renders neurons &
muscle cells
electrically excitable,
Drives the active
transport of sugars &
amino acids.

23. Primary active transport
• ABC transporters (ATP-
Binding Cassette
transporters) differ
structurally from P-type
ATPases and primarily pump
small molecules across cell
membranes.
23

24. ABC transporter
• ABC transporters harvest the energy released upon ATP
binding and hydrolysis to drive transport of solutes across
the bilayer. The transport is directional toward inside or
toward outside, depending on the particular conformational
change in the solute binding site that is linked to ATP
hydrolysis 24

25. ABC transporter
• The superfamily of ABC transporters is the largest family
of membrane transport proteins and is especially
important clinically.
Example- Proteins that are responsible for cystic fibrosis
(mutation in cystic fibrosis transmembrane conductance protein
– CFTR causes misregulation of ion concentration in the
extracellular fluid, especially in the lung).
Drug resistance in cancer cells
(multidrug resistance (MDR) protein-
pumping out drugs efficiently before
the drugs can exert their effects ).
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26. Secondary active transport
• Secondary active transport: Mediate the transport
of ions and small molecules across the membrane
using electrochemical gradient without direct
coupling of ATP. eg E. coli lactose transporter.
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The free energy released during
the movement of an ion down an
electrochemical gradient is used as
the driving force to pump other
solutes uphill, against their
electrochemical gradient.

27. Secondary active transport -Symporter
• A glucose–Na+ symport protein uses the electrochemical Na+
gradient (produce energy, high to low) to drive the active import
of glucose (low to high). Both molecules are transported in the
same direction.
• Example: the glucose symporter SGLT1, which co-transports one
glucose molecule into the cell for every two sodium ions it imports
into the cell. 27

28. Secondary active transport - Antiporter
• Two species of molecules are
pumped in opposite directions
across a membrane.
• One of these species move from
from high to low concentration ,
producing energy to drive the
transport of the other solute
from a low concentration to a
high one.
• An example is the sodium-
calcium exchanger or antiporter,
which allows three sodium ions
into the cell to transport one
calcium out.
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29. Specific channels can rapidly transport
ions across membranes
• For transport efficiency, ion channels have an advantage over
transporters, in that they can pass up to 100 million ions through
one open channel each second—a rate 105 times greater than the
fastest rate of transport mediated by any known transporter.
• However, channels cannot be coupled to an energy source to
perform active transport, so the transport they mediate is always
passive.
• Thus, the function of ion channels is to allow specific inorganic
ions—primarily Na+, K+, Ca2+, or Cl–—to diffuse rapidly down their
electrochemical gradients across the lipid bilayer.
• Examples: Gap Junction and Aquaporin
29

30. Ion channels are ion-selective and gated
Properties: Ion selectivity and Gated channel
• Ion selectivity - Selectivity filter permits some inorganic ions to
pass, but not others. This suggests that their pores must be narrow
enough in places to force permeating ions to shed most of their
associated water molecules so that only ions of appropriate size
and charge can pass. Thus, as the ion concentration increases, the
flux of the ion through a channel increases proportionally but then
levels off (saturates) at a maximum rate.
30

31. Ion channels are ion-selective and gated
• Ion channels are not continuously open. Instead, they are gated,
which allows them to open briefly and then close again.
• Moreover, with prolonged stimulation, most ion channels go into a
closed “desensitized,” or “inactivated,” state, in which they are
refractory (resistant) to further opening until the stimulus has been
removed. In most cases, the gate opens in response to a specific
stimulus.
31
Eg. Olfactory fatigue is
the temporary, normal
inability to distinguish a
particular odor after a
prolonged exposure to
that airborne compound
due to desensitization of
a cation channel.

32. Different types of gated ion channels
respond to different types of stimuli
Depending on the type of channel, the probability of gate opening is
controlled by (a) a change in the voltage difference across the
membrane, (B) the binding of a chemical ligand to the extracellular
face of a channel, (c) ligand binding to the intracellular face of a
channel, or (D) mechanical stress. 32

33. Potassium (K+) channel
• Many ion channels have a common structural framework. In
regards to K+ channels, hydrated potassium ions must transiently
lose their coordinated water molecules as they move to the
narrowest part of the channel, termed the selectivity filter.
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In the selectivity filter,
peptide carbonyl groups
coordinate the ions. Rapid
ion flow through the
selectivity filter is facilitated
by ion-ion repulsion, with
one ion pushing the next ion
through the channel.

34. Potassium (K+) channel
• Many channels spontaneously inactivate after having
been open for a short period of time.
• Some ion channels are voltage gated: changes in the
membrane potential induce conformational changes
that open these channels.
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Voltage-gated
channels work in
concert to generate
action potentials.

35. Aquaporins are permeable to water
but impermeable to ions
• In addition to the direct diffusion of water across the
lipid bilayer, some prokaryotic and eukaryotic cells have
water channels, or aquaporins, embedded in their
plasma membrane to allow water to move more rapidly.
35
Aquaporins are
particularly abundant in
animal cells that must
transport water at high
rates, such as the
epithelial cells of the
kidney.

36. Aquaporins are permeable to water
but impermeable to ions
• Aquaporins do not transport protons.
• To avoid disrupting ion gradients across membranes,
they have to allow the rapid passage of water
molecules while completely blocking the passage of
ions.
36

37. Gap junction
• Cell junction with adjacent cells that are connected through
protein channels. These channels connect the cytoplasm of
each cell and allow molecules, ions, and electrical signals to
pass between them.
• Small hydrophilic molecules/ions can pass through.
• Inorganic ions & most metabolites (sugars, a.a, nucleotides)
can flow between the interiors of cell joined by gap junctions.
• Proteins, nucleic acids and polysaccharides are too large to
traverse these channels.
37

38. Gap junction
Functions:
– allows for direct cell-to-cell communication without having to
go through the extracellular fluid surrounding the cells, eg.
cells in some excitable tissues: heart muscle
• Essential for the nourishment of cells that are distant from
blood vessels: lens & bone.
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Communicating
channels are
important in
development and
differentiation.

39. Summary
1. The transport of molecules across a membrane may
be active or passive.
2. Two families of membrane proteins use ATP
hydrolysis to pump ions across membranes.
3. Secondary transporters use one concentration
gradient to power the formation of another.
4. Specific channels can rapidly transport ions across
membranes.
5. Gap junctions allow ions and small molecules to
flow between communicating cells.
6. Aquaporin increase the permeability of some
membranes to water.
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