1. LAB EVALUATION OF
CELL DISORDERS
2) Transport across cell membranes
Ola H. Elgaddar
MBChB, MSc, MD, CPHQ, LGBSS
Lecturer of Chemical Pathology
Medical Research Institute
Alexandria University
Ola.elgaddar@alexu.edu.eg

2. Sources:
1. Grey's anatomy, 39th ed, 2008
2. Ganong physiology, 23rd ed,2010
3. Robbins pathology, 7th ed, 2005
4. Kumar clinical medicine,6th ed,2005
5. Molecular cell biology, 5th ed, 2003

3. - The cell performs many functions like
support, transport, communication,
recognition and acts as selective barriers.
All these functions are achieved through
movement across cell membranes.
- There are several mechanisms of transport
across cellular membranes. Pathways
include passive and active transport.

5. - Small, non-polar molecules (including O2 and N2) and
small uncharged polar molecules such as CO2 diffuse
across the lipid membranes of cells (Passive simple
diffusion)
-However, the membranes have very limited
permeability to other substances. Instead, they cross
the membranes by endocytosis and exocytosis or by
passage through highly specific transport proteins.
These are transmembrane proteins that are either ion
channels , ATP-powered pumps or carrier proteins.
- The limited permeability applies even to water, with
simple diffusion being supplemented throughout the
body with various water channels (aquaporins).

6. A)Ion channels:
-Channel proteins transport water or
specific types of ions and hydrophilic
small molecules down their
concentration or electric potential
gradients.
- Such protein-assisted transport
sometimes is referred to as facilitated
diffusion.

8. - Channel proteins form a hydrophilic
passageway across the membrane
through which multiple water
molecules or ions move
simultaneously, single file at a very
rapid rate.
- Some ion channels are open much of
the time; these are referred to as non-
gated channels.
- Most ion channels, however, open
only in response to specific chemical
or electrical signals; these are referred
to as gated channels.

11. -Though ion channels shuttle between a closed
state and an open state, many ions can pass
through an open channel without any further
conformational change. For this reason,
channels are characterized by very fast rates of
transport, up to 108 ions per second.
- There are ion channels specific for K+, Na+,
Ca2+, and Cl–, as well as channels that are
nonselective for cations or anions.

13. Ways in which ion channels form pores:
(A) K+ channels are tetramers, with each protein
subunit forming part of the channel.
(B) In ligand-gated cation and anion channels such
as the acetylcholine receptor, five identical or
very similar subunits form the channel.
(C) Cl– channels are dimers, with an intracellular
pore in each subunit.
(D) Aquaporin water channels are tetramers with
an intracellular channel in each subunit.

14. B) Pumps (ATP-powered pumps):
- They are ATPases that use the energy of
ATP hydrolysis to move ions or small
molecules across a membrane against a
chemical concentration gradient or
electric potential or both.
- This process, referred to as active
transport, is an example of a coupled
chemical reaction. In this case, transport
of ions or small molecules “uphill” against
an electrochemical gradient, which
requires energy, is coupled to the
hydrolysis of ATP, which releases energy.

16. Examples of ATPases:
Na, K ATPase: known as the Na, K pump.
H, K ATPases: in the gastric mucosa and the
renal tubules.
Ca2+ATPase: pumps Ca2+ out of cells.

17. Na, K -ATPase:
- It catalyzes the hydrolysis of ATP to ADP and uses
the energy to extrude three Na+ from the cell and
take two K+ into the cell for each molecule of ATP
hydrolyzed (Active transport)
- It is an electrogenic pump in that it moves three
positive charges out of the cell for each two that it
moves in, and it is therefore said to have a coupling
ratio of 3:2
- It is found in all parts of the body. Its activity is
inhibited by ouabain and related digitalis
glycosides used in the treatment of heart failure.

19. C) Carrier proteins
(Transporters):
- They bind ions and other molecules
and then change their configuration,
moving the bound molecule from one
side of the cell membrane to the other.
Two types of transporters:
1) Uniporters
2) Co-transporters

21. 1) Uniporters:
transport a
single type of
molecule
down its
concentration
gradient via
facilitated
diffusion.
Ex: GLUT 1

22. 2) Co-transporters:
-Couple the movement of one type of
ion or molecule against its
concentration gradient (the driven
substrate) with the movement of one or
more different ions down its
concentration gradient (the driving
substarte).
(Secondary active transport)
- The direction of transmembrane
movement of the driven substrate can
be either the same as (symport), or
opposite to (antiport), that of the
driving substrate.

23. The luminal
membranes of
mucosal cells in
the small
intestine contain
a symport that
transports
glucose into the
cell only if Na+
binds to the
protein and is
transported into
the cell at the
same time.

24. In the heart,
Na, K ATPase
indirectly
affects Ca2+
transport. An
antiport in the
membranes of
cardiac muscle
cells normally
exchanges
intracellular
Ca2+ for
extracellular
Na+

25. - Like ATP pumps, co-transporters mediate
coupled reactions in which an energetically
unfavorable reaction (i.e., uphill movement of
molecules) is coupled to an energetically
favorable reaction.
- Note, however, that the nature of the energy-
supplying reaction driving active transport by
these two classes of proteins differs. ATP pumps
use energy from hydrolysis of ATP, whereas co-
transporters use the energy stored in an
electrochemical gradient.

26. - ATP-powered pumps and
transporters undergo a cycle of
conformational changes exposing a
binding site (or sites) to one side of
the membrane in one conformation
and to the other side in a second
conformation. Because each cycle
results in movement of only one (or a
few) substrate molecules, these
proteins are characterized by
relatively slow rates of transport
ranging from 10 to 104 ions or
molecules per second.

29. Transport across cell membranes

31. - Secreted proteins, lipid, small molecules and other
cellular products destined for export from the cell
are transported to the plasma membrane in small
vesicles released from the trans face of Golgi.
- This pathway is either constitutive, in which
transport and secretion occur more or less
continuously, or it is regulated by external signals,
as in the control of salivary secretion by autonomic
neural stimulation.

33. -Exocytosis is achieved by fusion of the
secretory vesicular membrane with the plasma
membrane and release of the vesicle contents
into the extracellular domain.
- Excess plasma membrane generated by vesicle
fusion during exocytosis is rapidly removed by
concurrent endocytosis.

34. - The process of endocytosis involves
the internalization of vesicles derived
from the plasma membrane.
- The vesicles may contain: engulfed
fluids and solutes from the
extracellular interstitial fluid
(pinocytosis);larger macromolecules,
often bound to surface receptors
(receptor-mediated endocytosis);
particulate matter, including
microorganisms or cellular debris
(phagocytosis).

36. 1 - Pinocytosis generally involves
small fluid-filled vesicles and is a
marked property of capillary
endothelium, e.g. where vesicles
containing nutrients and oxygen
dissolved in blood plasma are
transported from the vascular
lumen to the endothelial basal
plasma membrane.

37. Interstitial fluid containing dissolved
carbon dioxide is also taken up by
pinocytosis for simultaneous
transportation across the
endothelial cell wall in the opposite
direction, for release into the
bloodstream by exocytosis. This
shuttling of pinocytotic vesicles is
also termed transcytosis.

38. N.B: Transcytosis is a process used by some
cells for the apical-basolateral sorting of certain
membrane proteins. This process of
transcellular transport, which combines
endocytosis and exocytosis, also can be
employed to import an extracellular ligand from
one side of a cell, transport it across the
cytoplasm, and secrete it from the plasma
membrane at the opposite side.

40. 2 - Receptor-mediated endocytosis, also known
as clathrin-dependent endocytosis, is initiated
at specialized regions of the plasma membrane
known as clathrin-coated pits.

41. Clathrin is a protein which plays a major role in
the formation of coated vesicles. It forms a
triskelion shape (a figure with three bent legs),
which is composed of three clathrin heavy
chains and three light chains. The legs are joined
at a central trimerization domain (txd). When
the triskelion interact they form a polyhedral
lattice which surrounds the vesicle.

44. - Adaptors represent a diverse group of proteins
recognizing different classes of cargo receptor.
-The best characterized are a family of closely
related proteins called the adaptor proteins
(APs) comprising AP1, AP2, AP3 and AP4.
- Each of these four classes are localized to
different intracellular compartments and vary in
their receptor specificity

45. -Clathrin assembles onto the adaptors to form
the outer layer of the coat.
-It acts to stabilise the curvature introduced into
the growing pit whilst increasing its deformation
until the entire region invaginates to form a
closed vesicle.

49. 3 - Phagocytosis:
- Phagocytosis is a property of many cell types, but
is most efficient in cells specialized for this activity.
-The professional phagocytes of the body belong to
the monocyte lineage of haemopoietic cells, in
particular the tissue macrophages.
-Other effective phagocytes are neutrophil
granulocytes and most dendritic cells, which are
also of haemopoietic origin.

50. -Phagocytosis is a triggered process, initiated when
a phagocytic cell binds to a particle or organism,
often through a process of molecular recognition.
-Typically, a pathogenic microorganism may first be
coated by antibodies, which are bound in turn by
receptors for the Fc portion of the antibody
molecule expressed by macrophages and
neutrophils.
-This triggers the production of large pseudopodia,
which engulf the organism and internalize it.

52. -The process appears to depend on actin-
myosin-based cellular motility and, unlike
receptor-mediated endocytosis, it is energy
dependent.
- Inside the cell, the phagosome fuses with
lysosomes, which degrade its contents.

54. -Lysosomes are membrane-bound intracellular
organelles that contain a variety of hydrolytic
enzymes, including acid phosphatase,
glucuronidase, sulfatase, ribonu-clease, and
collagenase.
- These enzymes are synthesized in the rough
endoplasmic reticulum and then packaged into
vesicles in the Golgi apparatus.

56. - Lysosomes
(primary) fuse
with membrane-
bound vacuoles
that contain
material to be
digested,
forming
secondary
lysosomes or
phagolysosomes

57. Lysosomes are involved in the breakdown of phagocytosed
or endocytosed materials in one of two ways:
heterophagy or autophagy.

58. Heterophagy is the process of lysosomal
digestion of materials ingested from the
extracellular environment.
- Extracellular materials are taken up by cells
through the general process of endocytosis
(Pinocytosis or phagocytosis).

59. -Heterophagy is most common in the
"professional" phagocytes, such as neutrophils
and macrophages, although it may also occur in
other cell types.
- Examples of heterophagocytosis include the
uptake and digestion of bacteria by neutrophils

61. Autophagy refers to lysosomal
digestion of the cell's own
components.
- In this process, intracellular
organelles and portions of cytosol
are first sequestered from the
cytoplasm in an autophagic vacuole
formed from ribosome-free regions
of the rough endoplasmic reticulum.
- The vacuole fuses with lysosomes
or Golgi elements to form an
autophagolysosome.

62. Autophagy is a common phenomenon involved in the removal of
damaged organelles during cell injury and the cellular remodeling
of differentiation, and it is particularly pronounced in cells
undergoing atrophy induced by nutrient deprivation or hormonal
involution.

63. -The enzymes in lysosomes are capable of
degrading most proteins and carbohydrates, but
some lipids remain undigested.
- Lysosomes with undigested debris may persist
within cell as residual bodies or may be
extruded.

64. Lipofuscin pigment granules
represent undigested material
derived from intracellular lipid
peroxidation.
- Certain indigestible pigments, such
as carbon particles inhaled from the
atmosphere or inoculated pigment
in tattoos, can persist in
phagolysosomes of macrophages
for decades.

66. THANK YOU