Transport across the plasma membrane

1. CELLULAR
PHYSIOLOGY
SUB-TOPIC
TRANSPORT ACROSS CELL MEMBRANE
By Mr. D.R Siwale (MSc. Human Physiology, BSc Human biology,
BSc.BioSciences, Dip.Ed.)

2. INTRODUCTION
Part 1

3. Structure of the cell
membrane
• Proteins
(55%).
• Phospholipids
(25%).
• Cholesterol
(13%).
• other lipids
(4 %).
• carbohydrate
s (03%)

4. Importance of transport
across cell membrane
• 1. to supply cells with essential materials e.g
O2, nutrients, water, electrolyte etc
• 2. to get rid of metabolic wastes or excess
substance of the cell e.g CO2

5. 2 basic mechanism involved in
transport across cell
Membranes
Transport of substances across the cell
membrane involves the following basic
mechanisms:
• 1. Passive transport mechanism.
• 2. Active transport mechanism.

7. PASSIVE TRANSPORT
Part 2

8. 3 Features of Passive transport
• 1. it occurs down an electrochemical gradient/
chemical or electro gradients.
• 2.does not require metabolic energy.
• 3. it is also known as diffusion or down hill
movement.
• Passive transport is like swimming in the
direction of water flow in a river

9. Two types of diffusion
Diffusion is of two types, namely:
1. simple diffusion: is the movement of
molecules through a cell membrane without
using the channels formed by integral
membrane protein.
2. facilitated diffusion: A type of diffusion which
occurs with the help of the carrier proteins of the
cell membrane
Others: osmosis, filtration and Bulk flow

11. Example of simple diffusion

12. Facilitated diffusion

13. Example of facilitated diffusion

14. Other types of passive transport
• Osmosis
• Bulk flow
• Filtration

15. Facilitated diffusion of amino acids

16. factors affecting Diffusion
1. Temperature
2. Cell membrane thickness
3. Size of molecule
4. Size of ion.
5. Solubility of diffusing substance
6. Concentration or electrical gradient
7. Permeability of the membrane.
8. Charge of the ion.

17. summary

18. ACTIVE TRANSPORT
Part 3

19. Two type
• Primary active transport, also called direct active transport,
directly uses chemical energy (such as from adenosine
triphosphate or ATP in case of cell membrane) to transport all
species of solutes across a membrane against their
concentration gradient.
• In secondary active transport, the movement of the sodium
ions down their gradient is coupled to the uphill transport of
other substances by a shared carrier protein (a cotransporter).
... The carrier protein uses the energy of the sodium gradient
to drive the transport of glucose molecules.

21. Active transport and Facilitated
transport
Active transport mechanism is different from facilitated
Diffusion by two ways:
1. Carrier protein of active transport needs energy,
whereas the carrier protein of facilitated diffusion does
not need energy
2. In active transport, the substances are transported
against the concentration or electrical or electrochemical
gradient. In facilitated diffusion, the substances are
transported along the concentration or electrical or
electrochemical gradient

22. Carrier proteins involved in active
transport are of two
types:1. Uniport 2. Symport or antiport.
• 1. Uniport: Carrier protein that carries only one substance
in a single direction is called uniport. It is also known as
uniport pump.
• 2. Symport or Antiport
• Symport or antiport is the carrier protein that transports
two substances at a time.
• if two different substances in the same direction is called
symport or symport pump.
• If two different substances in opposite directions is called
antiport or antiport pump.

23. ionic and non-ionic substances
transported by active transport
• Substances, which are transported actively,
are in ionic form and non-ionic form.
• Substances in ionic form are sodium,
potassium, calcium, hydrogen, chloride and
iodide.
• Substances in non-ionic form are glucose,
amino acids and urea.

24. Examples of primary active transport in physiologic
systems

25. The Na+-K+ ATPase is an
electrogenic pump.
• 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.
• Therefore, the Na+-K+ pump is said to be electrogenic
because it creates an electrical potential across the
cell membrane

26. Structure of the Na+-K+ pump
• Made up of two protein subunit molecules, an α-
subunit with a molecular weight of 100,000 and a β-
subunit with a molecular weight of 55,000.
• Transport of Na+ and K+ occurs only by α-subunit. The
β-subunit is a glycoprotein the function of which is not
clear.
α-subunit of the Na+-K+ pump has got six sites:
• i. Three receptor sites for sodium ions on the inner
(towards cytoplasm) surface of the protein molecule
• ii. Two receptor sites for potassium ions on the outer
(towards ECF) surface of the protein molecule
• iii. One site for enzyme adenosine triphosphatase
(ATPase), which is near the sites for sodium.

27. Cont’d….

28. The protein carrier involved in active
transport have two conformations E1 and
E 2.
• In the E1 state, the ion-binding sites face the
intracellular fluid, and the enzyme has a high
affinity for Na+; (eg in Na+-K+ pump)
• In the E2 state, the ion-binding sites face the
extracellular fluid, the affinity for Na+ is low, and
the affinity for K+ is high.
• Cycling between E1 and E 2 occurs and is powered
by ATP hydrolysis
• (see soft copy levy and bern)

29. Mechanism of action of the Na+-K+
pump
• The Na+,K+-ATPase also cycles between E1 and E2 states in a cycle
powered by ATP hydrolysis.
• In the E1 conformation, the ion binding sites face the cytosol and
have high affinity for Na+ and low affinity for K+.
• In the E2 conformation, the ion binding sites face the extracellular
fluid and have low affinity for Na+ and high affinity for K+.
• As the transporter is driven around the transport cycle from E1 to
E2 and back to E1 again, it picks up Na+ from the cytosol and
releases it to the extracellular fluid. It also picks up K+ from the
extracellular fluid and releases it into the cytosol.
• In each cycle of the pump one molecule of ATP is hydolyzed, three
Na+ ions are ejected from the cytosol, and two K+ ions are taken up
into the cytosol

30. Drugs that inhibit Na+-K+ pump
• Cardiac glycosides (e.g., ouabain and digitalis) are a class
of drugs that inhibits Na+-K+ ATPase.
• Treatment with this class of drugs causes certain
predictable changes in intracellular ionic concentration:
• The intracellular Na+ concentration will increase, and the
intracellular K+ concentration will decrease.
• Cardiac glycosides inhibit the Na+-K+ ATPase by binding to
the E2∼P form near the K+-binding site on the extracellular
side, thereby preventing the conversion of E2∼P back to E1.
• By disrupting the cycle of phosphorylation-
dephosphorylation, these drugs disrupt the entire enzyme
cycle and its transport functions

31. State some drugs that inhibit Na+-K+ pump and
how do they inhibit the pump?
• Cardiac glycosides (e.g., ouabain and digitalis) are a
class of drugs that inhibits Na+-K+ ATPase.
• Treatment with this class of drugs causes certain
predictable changes in intracellular ionic concentration:
• The intracellular Na+ concentration will increase, and
the intracellular K+ concentration will decrease.
• Cardiac glycosides inhibit the Na+-K+ ATPase by binding
to the E2∼P form near the K+-binding site on the
extracellular side, thereby preventing the conversion of
E2∼P back to E1. By disrupting the cycle of
phosphorylation-dephosphorylation, these drugs
disrupt the entire enzyme cycle and its transport
functions

32. Secondary active transport?
• Secondary active transport processes are those in
which the transport of two or more solutes is coupled.
• One of the solutes, usually Na+, moves down its
electrochemical gradient (downhill), and the other
solute moves against its electrochemical gradient
(uphill).
• The downhill movement of Na+ provides energy for the
uphill movement of the other solute. Thus, metabolic
energy, as ATP, is not used directly, but it is supplied
indirectly in the Na+ concentration gradient across the
cell membrane. (The Na+-K+ ATPase, utilizing ATP,
creates and maintains this Na+ gradient.)
• The name secondary active transport, therefore, refers
to the indirect utilization of ATP as an energy source

33. Types of secondary active transport
Secondary active transport is of two types:
• 1. Cotransport
• 2. Counter transport.

34. Cont’d…..

35. Sodium cotransport is the process in which, along with
sodium, another substance is transported by a carrier
protein called symport. Examples…
1. Sodium cotransport of glucose (see diagram
below)
2. Sodium contransport of aminoacids.

36. Sodium counter transport
1. Sodium-calcium counter transport.
2. Sodium-hydrogen counter transport.
3. Other counter transport systems, are
- sodium-magnesium counter transport.
- sodium-potassium counter transport.
- calcium-magnesium counter transport.
- calcium- potassium counter transport.
- chloride bicarbonate counter transport.
- chloride sulfate counter transport.

37. Cont’d……….
• Sodium-calcium counter transport: In this, ions
move in opposite directions with the help of a carrier
protein. This type of transport of sodium and calcium
ions is present in all the cells.
• Sodium-hydrogen counter transport: In this the
hydrogen ions are exchanged for sodium ions and this occurs
in the renal tubular cells. The sodium ions move from tubular
lumen into the tubular cells and the hydrogen ions move from
tubular cell into the lumen.

38. Na+ - Ca2+ counter transport

39. Endocytosis and what form of transport is
it?
• Endocytosis is defined as a transport mechanism by
which the macromolecules enter the cell.
• Macromolecules (substances with larger molecules)
cannot pass through the cell membrane either by
active or by passive transport mechanism. Such
substances are transported into the cell by
endocytosis.
• It is a SPECIAL CATEGORY OF ACTIVE TRANSPORT.

40. Three types of endocytosis
• 1. Pinocytosis: “Cell drinking”; movement of
extracellular fluid into a cell by infolding of
plasma membrane to form a vesicle e.g Solutes in
extracellular fluid.
• 2. Phagocytosis: Phagocytosis is the process by which
particles larger than the macromolecules are engulfed into the cells.
It is also called cell eating.
• 3. Receptor-mediated endocytosis.
Receptor-mediated endocytosis is the transport of macromolecules
with the help of a receptor protein Surface of cell membrane has some
pits which contain a receptor protein called clathrin. Together with a
receptor protein (clathrin), each pit is called receptor-coated pit or
Clathrin/AP2 coated pits and vessicle). These receptor-coated pits are
involved in the receptor mediated endocytosis

41. What steps are involved in phagocytosis?
• Material to be phagocytosed, makes contact
with the cell membrane.
• The cell membrane invaginates the material
• The invagination is pinched off forming a
membrane enclosed vacuole. The vacuole
contains the engulfed material

42. What role dose clathrin-mediated endocytosis play?
1. Internalisation of many receptors and ligands
bound to them;
• Example:
- Nerve growth factor (NGF)
- Low density lipoproteins (LDL).
- Transferin (iron-binding proteins).
- Other glycoproteins
2. Plays a role in synaptic function e.g uptake of
neurotransmitters into presynaptic neuron.

43. Give a general view of steps involved in clathrin-mediated
endocytosis
1. The macromolecule attaches
to the clathrin/Ap2 coated
pits forming receptor- ligand
complex forming a vesicle.
2. At the neck of a vesicle,
Dynamin (GTP binding
protein) helps in pinching off
the vessicle.
3. Vesicle fuse and damps its
content in the early
endosome.
4. The early endosome can
become late endosome and
fuse with lysosomes. Also a
vesicle can bud off from the
early endosome and return to
cell membrane.
5. In the lysosomes, contents
are digested by the lysosomal
proteases

44. Describe the mechanism of clathrin- mediated endocytosis
of lipoproteins?
Step1 : Cell-surface LDL
receptors bind to an apoB
protein embedded in the
phospholipid outer layer of
LDL particles. Interaction
between the NPXY sorting
signal in the cytosolic tail
of the LDL receptor and
the AP2 complex
incorporates the receptor-
ligand complex into
forming endocytic vesicles.

45. Step 2 and 3
Step2 : Clathrin-coated pits
(or buds) containing
receptor-LDL complexes are
pinched off by the same
dynamin-mediated
mechanism used to form
clathrin/AP1 vesicles on the
trans-Golgi network .
Step 3: After the vesicle coat is
shed, the uncoated endocytic
vesicle (early endosome) fuses
with the late endosome. The
acidic pH in this compartment
causes a conformational
change in the LDL receptor
that leads to release of the
bound LDL particle.

46. Step 4 and 5
Step4 : The late endosome
fuses with the lysosome, and
the proteins and lipids of the
free LDL particle are broken
down to their constituent
parts by enzymes in the
lysosome.
Step5 : The LDL receptor
recycles to the cell surface
where at the neutral pH of the
exterior medium the receptor
undergoes a conformational
change so that it can bind
another LDL particle.

47. Describe the mechanism of receptor- mediated endoctytosis?

48. Cont’d..
• Receptor-mediated endocytosis is the transport of macromolecules with the help of a
receptor protein.
• Surface of cell membrane has some pits which contain a receptor protein called clathrin.
Together with a receptor protein (clathrin), each pit is called receptor-coated pit.
•
• These receptor-coated pits are involved in the receptor mediated endocytosis .
• Mechanism of receptor-mediated endocytosis
• i. Receptor-mediated endocytosis is induced by substances like ligands.
• ii. Ligand molecules approach the cell and bind to receptors in the coated pits and form
ligand-receptorcomplex.
• iii. Ligand-receptor complex gets aggregated in the coated pits. Then, the pit is detached from
cell membrane and becomes the coated vesicle. This coated vesicle forms the endosome.
• iv. Endosome travels into the interior of the cell. Primary lysosome in the cytoplasm fuses
with endosome and forms secondary lysosome.
• v. Now, the hydrolytic enzymes present in secondary lysosome are activated resulting in
release of ligands into the cytoplasm.
• vi. Receptor may move to a new pit of the cell membrane

49. What is the meaning of exocytosis?
• Exocytosis is the process by which the
substances are expelled from the cell. In this
process, the substances are extruded from cell
without passing through the cell membrane.
This is the reverse of endocytosis.

50. Example of exocytosis of insulin

51. Exocytosis adds total amount of membrane, why then
doesn’t the cell enlarge?
• Indeed exocytosis adds amount of membrane
to the cell, however endocytosis also removes
the membrane of the cell.
• Therefore exocytosis-endocytosis coupling
maintains the surface area of the cell at its
normal size.

52. 5 macromolecules that are transported by endocytosis
into the cell
• Receptor-mediated endocytosis play an important role in the
transport of several types of macromolecules into the cells, viz.
• i. Hormones: Growth hormone, thyroid stimulating hormone,
luteinizing hormone, prolactin, insulin, glucagon, calcitonin and
catecholamines
• ii. Lipids: Cholesterol and low-density lipoproteins (LDL)
• iii. Growth factors (GF): Nerve GF, epidermal GF, platelet-derived
GF, interferon
• iv. Toxins and bacteria: Cholera toxin, diphtheria toxin,
pseudomonas toxin, recin and concanavalin A
• v. Viruses: Rous sarcoma virus, semliki forest virus, vesicular
stomatitis virus and adenovirus
• vi. Transport proteins: Transferrin and transcobalamine
• vii. Antibodies: IgE, polymeric IgG and maternal IgG.

53. Some receptor- coated pits are coated with Caveolin,
instead of clathrin. What is the function of caveolin?
• Caveolin-coated pits are concerned with the
transport of vitamins into the cell.

54. Role of Calcium in exocytosis
• Calcium ions play an important role during the
release of some secretory substances such as
neurotransmitters.
• The calcium ions enter the cell and cause
exocytosis. However, the exact mechanism of
exocytosis is not clear

55. Non-constitutive and constitutive pathway.
Non-constitutive pathway:
In the nonconstitutive pathway,
proteins from the Golgi apparatus
initially enter secretory granules,
where processing of prohormones to
the mature hormones occurs before
exocytosis
Constitutive pathway: constitutive
pathway, involves the prompt
transport of proteins to the cell
membrane in vesicles, with little or no
processing or storage.

56. What is transcytosis? What is the mechanism
• Transcytosis is a transport mechanism in which an extracellular
macromolecule enters through one side of a cell, migrates across
cytoplasm of the cell and exits through the other side. Example: Example
of this type of transport is the movement of proteins from capillary blood
into interstitial fluid across the endothelial cells of the capillary. Many
pathogens like human immune deficiency virus (HIV) are also transported
by this mechanism.
• Mechanism of Transcytosis
• Cell encloses the extracellular substance by invagination of the cell
membrane to form a vesicle. Vesicle then moves across the cell and
thrown out through opposite cell membrane by means of exocytosis.
• Transcytosis involves the receptor-coated pits as in receptor-mediated
endocytosis. Receptor protein coating the pits in this process is caveolin
and not clathrin.
• Transcytosis is also called, vesicle trafficking or cytopempsis.
• Transcytosis plays an important role in selectively transporting the
substances between two environments across the cells without any
distinct change in the composition of these environments.

57. Pathological conditions associated with Na+/K+ pump
• Abnormalities in the number or function of
Na+-K+ pump are associated with several
pathological conditions. Important examples
are:
• 1. Reduction in either the number or
concentration of Na+-K+ pump in myocardium
is associated with cardiac failure
• 2. Excess reabsorption of sodium in renal
tubules is associated with hypertension.

58. Mention any 3 ion channel disease or channelopathies?
• 1. Sodium Channel Diseases
• Dysfunction of sodium channels leads to muscle spasm and Liddle’s
syndrome (dysfunction of sodium channels in kidney resulting in
increased osmotic pressure in the blood and hypertension).
• 2. Potassium Channel Diseases
• Potassium channel dysfunction causes disorders of heart, inherited
deafness and epileptic seizures in newborn.
• 3. Chloride Channel Diseases
• Dysfunction of chloride channels results in formation of renal
stones and cystic fibrosis.
• Cystic fibrosis is a generalized disorder affecting the functions of
many organs such as lungs (due to excessive mucus), exocrine
glands like pancreas, biliary system and immune system.

59. All vesicles involved in transport have protein coats. Mention any
four (4) of such proteins?
• AP1 (assembly protein 1); found on vesicles
that transport proteins from Golgi to
lysosomes.
• AP2 (assembly protein 2): found on
endocytotic vesicle that transport to
endosomes.
• COPI 1: vessicle transportin from ER to Golgi
• COPI 2: same as COPI 1

60. RECAP
1. Enumerate the various subcategories of
passive and active transport mechanism?
2. Describe the transport mechanism involved
in the absorption of Glucose from the
intestinal epithelial cells
3. Show that exocytosis of insulin requires
facilitated transport of glucose, blockage of
K+ efflux and influx of Ca++ into the
pancreatic β-cells

61. • Primary
• Secondary
• Endocytosis and exocytosis

62. • …….END…..