1. Cell membrane transport:
Lecture 6
Prepared by: Dr. Angela T Alleyne Presented by: Dr. Kherie Rowe
BIOC 1015 Introduction to Biochemistry
2. Learning
Outcomes:
After
completing this
lecture students
will be able to
Distinguish
between transport of small and large
molecules
Discuss
the structure and function of ion
transport proteins
Differentiat
e
between active and passive transport in
cells
Differentiat
e
between osmoregulation in plant and
animal cells
Explain
the importance of semi-permeability in
the plasma membrane
Describe
the structure and function of transport
proteins
Discuss the functions of the plasma membrane
3. Fluid mosaic model
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of
various embedded proteins
4.
5. Some membrane lipids
Carbon
No
Name Derived from Formula
12 Laurate Laurus noblis
(plant)
CH3(CH2)10COO-
16 Palmitate Palm oil CH3(CH2)14COO-
18 Stearate Steers or tallow CH3(CH2)16COO-
24 Lignocerate Lignum (oily
woods)
CH3(CH2)22COO-
18 Oleate Oleum oil CH3(CH2)7CH= CH (CH2)7COO-
18 Linoleate Linseed oil CH3(CH2)4 (CH= CHCH2 )2 (CH2)6COO-
20 Arachidonate Arachis
hypogea
(peanut oil)
CH3(CH2)4 (CH= CHCH2 )4 (CH2)2COO-
7. Life at the
Edge
A cell must exchange
materials with its
surroundings, a process
controlled by the plasma
membrane
The plasma membrane
exhibits selective
permeability, allowing
some substances to cross
it more easily than others
9. Cell-Cell Recognition
Cells recognize each other by binding to
surface molecules on the plasma membrane
Membrane carbohydrates are covalently
bonded to lipids or proteins
Glycolipids Glycoproteins
Carbohydrates on the external
side of the plasma membrane
vary among species,
individuals, and even cell
types in an individual
10. Membrane sidedness
• Membranes have distinct inside
and outside faces
• The asymmetrical distribution
of proteins, lipids and
associated carbohydrates in the
plasma membrane is
determined when the
membrane is built by the ER
and Golgi apparatus
13. Membrane permeability
Small hydrophobic (non-polar) molecules, such as hydrocarbons, can dissolve in the lipid
bilayer and pass through the membrane rapidly. Polar molecules, such as sugars, do not
cross the membrane easily.
15. Peripheral proteins: -
Usually associated with
one side of the
membrane. Changes in
pH cause these proteins
to dissociate from the
membrane.
Integral proteins:- span
the entire membrane or
are embedded within the
membrane e.g.
Bacteriorodospin found
in Halobacterium
halobium and consists of
7 alpha helices
Peripheral and integral membrane
proteins
16. Lipid
anchored
proteins
aa side chain linked by an amide
bond or ester to the FA acyl group
many are found in viruses
and eukaryotic cells
In eukaryotes a common
linkage is via
glycosylphosphatidylinositol
( GPI).
Anchored to the membrane by
covalent bond and a lipid anchor
17. Transport
proteins
Are specific for the
substances they move
Allow passage of
hydrophilic substances
across the membrane
Channel proteins have a
hydrophilic channel that
certain molecules or ions
can use as a tunnel
18. Aquaporin
• Transports water, glycerol and
hydrophilic molecules across
membranes
• Consists of 4 sub-units which span the
entire membrane
• 11 aquaporins are found in mammals
• Aquaporin 0, found in large numbers in
eye cells
• Aquaporins transport water in a large
bulk
This Photo by Unknown Author is licensed under CC BY-NC
19. This Photo by Unknown Author is licensed under CC BY
20. Water molecules become attached to
Asparagine (Asn) as they move through
aquaporin
https://sites.google.com/a/zinoproject.com/z008---herb-
zinser-science/biochemistry-war/water-
molecules/theaquaporinbattleofutoyaandtheosmosiswarofosl
o
22. Diffusion
tendency for molecules to
spread out evenly into the
available space
diffusion of a population of
molecules may exhibit a net
movement in one direction
At equilibrium, as many
molecules cross one way as
in the other direction
23. This Photo by Unknown Author is licensed under CC BY-SA-NC
24. Passive transport
• Substances diffuse down
their concentration
gradient
─difference in
concentration of a
substance from one
area to another
• Diffusion of a substance
across a biological
membrane is passive
transport
─It requires no
energy from the cell
25. Facilitated
Diffusion
In facilitated diffusion, transport is
assisted by transport proteins which
speed up the movement of molecules
across the plasma membrane.
Channel proteins provide corridors
that allow a specific molecule or ion
to cross the membrane
Carrier proteins undergo a subtle
change in shape that translocates the
solute-binding site across the
membrane
26. This Photo by Unknown Author is licensed under CC BY-SA-NC
28. Active
Transport
moves substances against
their concentration gradient
requires energy, usually in the
form of ATP
is performed by specific
proteins embedded in the
membranes e.g.. The sodium-
potassium pump
29. Active Transport
• Primary active transport
• Uses a direct energy
source e.g. ATP or
light
• Secondary active
transport
• Uses ion
concentration
gradients
• Uses ATPases
• Maintain ion
concentration across
membranes
• Examples: Na+-K+
ATPase, Ca2+ ATPase
30. Membrane Potential
• Membrane potential is the voltage
difference across a membrane
• Two combined forces, collectively called
the electrochemical gradient, drive the
diffusion of ions across a membrane:
• A chemical force (the ion’s
concentration gradient)
• An electrical force (the effect of the
membrane potential on the ion’s
movement)
This Photo by Unknown Author is licensed under CC BY-SA
31. The proton
pump
• An electrogenic pump is a transport protein
that generates the voltage across a
membrane
• The main electrogenic pump of plants, fungi,
and bacteria is a proton pump
32.
33.
34. Calcium pump
• In muscle cells the
calcium muscle pump is
the main protein in
skeletal muscles
• In skeletal muscle Ca 2+
ion are found in the
sarcoplasmic reticulum
(SR)
• Ca is released into the
lumen of the SR causing
muscles to relax
• Regulation of Ca 2+ pump
in the plasma membrane
is controlled by the
protein calmodulin
Moves calcium into the SR( modified ER)
35. Osmosis
Diffusion of water across a
selectively permeable membrane
Direction of osmosis is determined
only by a difference in total solute
concentration
Water diffuses across a membrane
from the region of lower solute
concentration to the region of
higher solute concentration
This Photo by Unknown Author is
37. Water Balance of Cells
• Tonicity - the ability of a solution
to cause a cell to gain or lose
water
• Isotonic solution: solute
concentration is the same as that
inside the cell; no net water
movement across the plasma
membrane
• Hypertonic solution: solute
concentration is greater than that
inside the cell; cell loses water
• Hypotonic solution: solute
concentration is less than that
inside the cell; cell gains water
This Photo by Unknown Author is licensed under CC BY-SA
38. Osmosis in cells without cell
walls
• Animals and other organisms
without rigid cell walls have
osmotic problems in hypertonic
or hypotonic environments
• Such organisms must have
adaptations for osmoregulation,
the control of water balance
• The protist Paramecium, which is
hypertonic to its pond water
environment, has a contractile
vacuole that acts as a pump
39. Osmoregulation of plant
cells
• Cell walls help maintain water
balance
• A plant cell in a hypotonic
solution swells until the wall
opposes uptake; the cell is now
turgid (firm)
• If a plant cell and its
surroundings are isotonic,
there is no net movement of
water into the cell; the cell
becomes flaccid (limp), and the
plant may wilt
• In a hypertonic environment,
plant cells lose water;
eventually, the membrane pulls
away from the wall, a usually
lethal effect called plasmolysis
40. Co-transport
Co transport occurs when
active transport of a solute
indirectly drives transport
of another solute
Plants commonly use the
gradient of hydrogen ions
generated by proton pumps
to drive active transport of
nutrients into the cell
42. Bulk transport
• Small molecules and
water enter or leave the
cell through the lipid
bilayer or by transport
proteins
• Large molecules, such as
polysaccharides and
proteins, cross the
membrane via vesicles
during exocytosis or
endocytosis
43. Endo- and Exocytosis
Transport vesicles,
fuse with
membrane and
release their
contents
secretory cells use
exocytosis for
export
Exo-
cytosis cell takes in
macromolecules by
forming vesicles
from the plasma
membrane
Phagocytosis,
Pinocytosis and
Receptor-mediated
endocytosis
Endo-
cytosis
44.
45. Examples of transported molecules
Diffusion
• O2, CO2, hormones, drugs
Facilitated
diffusion
• Glucose, amino acids, ions and water
Co-transport
• Glucose, amino acids, ions, sucrose
Active
transport
• Ions, small hydrophilic molecules, lipids
46. References
Lodish, H., Berk, A., Kaiser, C., Scott, M.,
Bretscher, A., Ploegh H. and Mstudaira,
P. Molecular cell Biology 6th edition
(2008) WH Freeman and Co. NY. USA
Campbell, N. and Reece, J. Biology 7th
ed (2005) A tour of the cell by Chris
Romero . Pearson education Inc.,
Benjamin Cummings, USA-
Nelson, D. L and Cox, M. M (2007)
Lehninger: Principles of Biochemistry.
WH Freeman and Co. NY. USA
