cell-membrane transport mechanism

1. CELL- membrane
transport mechanism
and related disorders
Nitin
MSc Medical
Biochemistry
AIIMS Rishikesh

2. Learning objectives
• Cell membrane
• Transport across cell membrane
• Passive transport and active transport
• Diseases related to defective transport mechanism

3. CELL MEMBRANE
 Cell membrane is the structure that separates
the external environment from the internal
environment of the cell
 The cell membrane controls what enters and
exits the cell to maintain an internal balance
called homeostasis
 Provides protection and support for the cell
 Cell membranes are made of a double-layer
sheet called a lipid bilayer

4. iii
 It is a double
layer of
phospholipid
s- bilayer.
 It contains
almost
proteins
called
membranes
proteins
Cell-membrane

5.  Phosphate head
is polar(water
loving)
 Fatty acid tails
non-polar
(water fearing)
 Proteins
embedded in
membranes

6. Proteins
Most cell membranes contain protein , which are embedded in
the lipid bilayer.
 Some of the proteins form channels and pumps that help to
move material across the cell membrane.

7. Studies of red blood cells have provided good examples of both
peripheral and integral proteins associated with the plasma
membrane
Most of these are peripheral membrane proteins that have been
identified as components of the corticol cytoskeleton, which
underlies the plasma membrane and determines the cell shape.
For example , the most abundant peripheral membrane protein
of red blood cells is spectrin, the major cytoskeletal protein of
erythrocytes
Other examples actin,
ankyrin, and band 4.1.

8. Integral proteins: the two major integral proteins of red blood
cells, glycophorin, and band 3,
Glycophorin crosses the membrane with a single membrane-
spanning α helix of 23 amino acids, with its glycosylated amino-
terminal portion exposed on the cell surface.
The band 3 , is the anion transporter responsible for the
passage of bicarbonate (HCO3) and chloride (cl) ions across the
red blood cell membrane

9. Glycocalyx
The glycocalyx is the fuzzy, gel-like, sticky layer made up mainly
of proteins and sugars, it surrounds the outermost cellular
membrane of cells
Carbohydrates portions of glycolipids, glycosylated proteins on
the outer surface of the plasma membrane form a carbohydrate
coat known as the glycocalyx
It protects cell from ionic and mechanical stress and is a barrier
to invading microorganisms

10. TRANSPORT
ACROSS
MEMBRANES

11. Membrane Transport
Passive
SIMPLE diffusion:
Smaller molecules
crossing the
membranes
(gasses- oxygen ,
carbon dioxide)
FACILITATED
Diffusion:
Bigger molecules
(glucose,
potassium)
Crossing the
membrane
through a
protein
channel
Active
Pump Vesicle
Primary
Secondary
Exocytosis Endocytosis
Pinocytosis
Phagocytosis
Receptor
mediated

12. PASSIVE TRANSPORT
Passive transport is the movement of molecules in or out of the
cell without the use of energy
Can only occur if the molecules moving in and out of the cell
are:
Small
Uncharged
Move from an area of high concentration to an area of low
concentration
There are two types of passive transport: simple diffusion and
facilitated diffusion

13. SIMPLE DIFFUSION
Movement of molecules from higher
concentration to lower concentration till equilibrium is reached
doesn’t require energy
Examples are fatty acids, steroids, alcohols, oxygen, carbon
dioxide

14. .
Molecules that diffuse through cell
membrane
1)Oxygen - Non-polar so diffuses
very quickly.
2)Carbon dioxide – polar but very
small so diffuses quickly.
3)Water – polar but also very small
so diffuses very quickly.

15. Facilitated diffusion
Facilitated diffusion is the movement of specific molecules (or
ions) across the plasma membrane, assisted by a carrier protein
These molecules pass through protein channels, these channels
called Facilitated Diffusion
Movement of molecules is still passive just like ordinary
diffusion, the difference is, the molecules go through a protein
channel instead of passing between the phospholipids.
Larger polar molecules such as glucose and amino acids, cannot
cross the membrane via simple diffusion
 Molecules move with the concentration gradient
Does not require energy from the cell

16.  Transmembrane
proteins
recognise a
particular
molecule and
help it to move
across the
membrane.
 The direction it
moves is
dependent on
the
concentration
gradient.

17. Ion channels
 A cell membrane channel that is selectively permeable to certain
ions.
 A protein that acts as a pore in the cell membrane and permits the
selective passage of ions (such as potassium pump, sodium pump,
and calcium ions).
 Ion channels are highly selective in the type of ions transported.

18. 1.) Classification by
the nature
of gating.
2.) Classification by
the types of
ion passing
through those
gates.
3.) Classification by
the Localization of
Proteins in the
Cell.
4.) Other
classifications of ion
channels.
 Voltage-gated
ion channel.
 Ligand-gated
ion channel.
 Other gated
ions channel.
 Chloride
channel.
 Potassium
channel.
 Sodium
channel.
 Calcium
channel.
 Proton
channel.
 Plasma
membrane
 Channel.
 Intracellular
channel.
 This type of
classification
contains many
types of
means
classification
on the basis
of a number
of gates .
Classification of ion channels

19. Classification on the basis of gating:
• Ion channels may be classified by gating means what opens and
closes the channels.
Types:
Voltage-gated ion channel: voltage-gated ion channels open or
depending on the voltage gradient across the plasma membrane.
Ligand-gated ion channel: ligand-gated ion channels open or close
depending on ligands to the channels ( to the extracellular domain of
membrane protein).
Other gated ion channels: these channels open or close in
to light and second messenger. Eg. Light gated ion channel, indirect
signaling.

20. Active transport
Active transport is the movement of molecules across a
membrane from a region of their lower concentration to a
region of their higher concentration---- in the direction against
the concentration gradient.
Requires energy
Involves carrier protein in the membrane
Hydrolysis of ATP releases the energy required for active
transport

21. Two types of active transport
Primary active transport: that uses ATP
Secondary active transport: that uses an electrochemical
gradient

22. Primary active transport:
Primary active transport, also called direct active transport
because directly uses metabolic energy (ATP) to transport
molecules across a membrane.
Na+, K+ , Mg2+ , and Ca2+ ions transport by primary active
transport.
Sodium-potassium pump calcium pump best examples.
Transport of charged particles require ion pumps (channels)

23. Sodium-potassium pump
 Exists in most cell
membranes.
 Actively removes
sodium ions from
the cell while
actively
accumulating
potassium ions
into them from
their
surroundings

24. Sodium potassium pump
• Present in all eukaryotic cells
Functions:
Maintains sodium potassium concentration difference across
the cell membrane.
Maintains volume of the cell.
Causes negative electrical charge inside the cell-electrogenic
pump
Essential for oxygen utilization by the kidneys

26. Calcium pump
Calcium ions are normally maintained at extremely low
concentration in the intracellular cytosol of virtually all cells in
the body, at a concentration about 10,000 times less than that in
the extracellular fluid .
This is achieved mainly by two primary active transport calcium
pumps.
One is the cell membrane and pumps calcium to the outside of
the cell.
The other pumps calcium ions into one or more of the
intracellular vesicular organelles of the cells, such as the
sarcoplasmic reticulum of muscle cells and the mitochondria in
all cells.

27. Hydrogen potassium pump
H+ -K+ ATPase
Gastric glands – parietal cells – hydrochloric acid secretion –
pumps hydrogen ions into the gastric lumen in exchange for
potassium.
Renal tubules- intercalated cells in the late distal tubules and
cortical collecting ducts secretions of hydrogen ions and
reabsorption of potassium ions.

28. Proton pump
H+ ATPase
Present in lysosome and endoplasmic reticulum
Pumps proton from cytosol into these organelles

29. Secondary active transport
 In Secondary active transport, also known as
coupled transport or co-transport.
 In secondary active transport, there is no direct
coupling of ATP.
 But it depends upon the electrochemical potential
difference created by pumping of ions in/out of the
cell.
 Best example : sodium-glucose transporters (SGLT)

30.  Sodium-glucose transporters
co-transporters are a family of
glucose transporter found in
the small intestine.
 Firstly Na+/K+ ATPase pumps
pumps out 3 sodium ions and
bringing in 2 potassium ions.
 This action creates a downhill
sodium ion gradient inside of
the cell
 Sodium-glucose transporters
uses sodium ion gradient
created by Na+/K+ ATPase
pump to transport glucose
across the membrane.
Sodium-glucose transporters

31. Can be classified as symporters, antiporters and uniports
depending on whether the substances move in the same or
opposite directions
I. Symport
II. Antiport
III. Uniport
Membrane transporters

32. Symport:
 In an symport two species of ion or other
solutes are pumped in the same
directions across a membrane.
 The coordinated uptake of glucose and
Na+ is an example of symport , the
transport of two molecules in the same
direction.

33. Antiport/counter transport
In an antiport two species of ion or other solutes are pumped in
opposite directions across a membrane.
Sodium-hydrogen counter transport in the proximal tubule of
the kidneys.
 sodium-calcium exchanger (antiporter) in the cardiac cells.
Which allows three sodium ions pumps into the cell to
transport one calcium ion out to the cell.

34. Applied aspects
Cardiac glycosides-digitalis and oubain-management of heart
failure.
Inhibits Na+ -K+ pump.
Accumulation of Na+ inside the cell and prevention of K+
influx.
Intracellular accumulation of Na+, decreases Na+ gradient from
outside to inside.
Calcium efflux through sodium-calcium exchanger in the
membrane utilizes sodium gradient.
Decreased sodium gradient decreases calcium efflux causing
increase in cytosolic calcium concentration , that promotes
myocardial contractility.

35. Uniport
In contrast, the facilitated diffusion of glucose is an example
of uniport, the transport of only a single molecule.
A uniporter is a integral membrane protein that is involved in
facilitated diffusion .
Integral membrane protein can be either ion channel or carrier
proteins.

36. Vesicular transport
When molecules are too large to move through a channel
protein or by using a carrier protein, vesicles are used to move
the “bulk” molecule.
Requires metabolic energy but is independent of concentration
gradient
Divided in two parts:
Endocytosis- movement of substance into the cell
Exocytosis- movement of materials out of the cell

37. Endocytosis
 Occurs when the plasma membrane envelops food
particles and liquids into a vesicle to take into the cell.
 three types:
 Phagocytosis- (cell-eating) when material taken into
the cell is a bacterium or fragment of organic matter.
 Example: engulfment of bacteria by WBCs
,macrophages, monocytes, neutrophils

38. Pinocytosis ( cell-drinking) –
 this process of endocytosis is for any liquids that
are entering the cell
 Example: uptaking of enzymes and hormones from the
extracellular fluid, microvilli in the gut use this process
to absorb nutrients from food.

39. Receptor-mediated endocytosis-
 specific molecules are taken in after they bind to a
receptor
 The receptors are recycled for later use
 Example : apoE protein in lipid metabolism

40. Exocytosis
 Occurs when material is discharged from the cell.
 Vesicles in the cytoplasm fuse with the cell membrane and
release their contents to the exterior of the cell
 Used in animals to secrete hormones, neurotransmitters,
digestive enzymes. etc

41. Difference between facilitated diffusion and
active transport
In both instances , transport depends on carrier proteins
that penetrate through the cell membrane , as is true for
facilitated diffusion.
However, in active transport, the carrier protein functions
differently from the carrier in facilitated diffusion
because it is capable of imparting energy to the
transported substances to move it against the
electrochemical gradient.

42. Activator and inhibitor of Na+ -K+ pump
Activation of Na+ -K+ pump:
thyroxine , insulin , aldosterone
Inhibition of Na+ -K+ pump:
dopamine , digitalis , hypoxia , hypothermia

43. Enzymatic markers of different membranes
(isolation of organelles)

44. Diseases due to defective transport
mechanism
 Hartnup’s disease - caused by a mutation ( six mutations in
SLC6A19 ) of the gene that controls the processes of amino acid
absorption and reabsorption.
Defect: intestinal and renal tubular reabsorption defect of the
neutral amino acids ( alanine, valine, threonine leucine,
tryptophan deficiency)
This leads to nicotinic acid and serotonin deficiency
Clinical findings: photodermatitis, cerebellar ataxia; often
asymptomatic.
Diagnosis- high levels of neutral amino acids in urine and low
levels of neutral amino acids in plasma.

45.  symptoms
• Senstivity to light
• Anxiety
• Rapid mood swings
• Hallucinations

46. Cystinuria
It is an autosomal recessive disorder
Caused by mutations in the SLC3A1 and SLC7A9, these
mutations result in the abnormal transport of cystine in the
kidney and this leads to the symptoms of cystinuria
Defect – renal tubular reabsorption defect of the diabasic
amino acids ( lysine, arginine, ornithine and cystine )
• Clinical findings – nephrolithiasis ( cystine crystallizes above
1250µmol/l at ph 7.5 )
• Diagnosis – positive nitroprusside test in urine, increased levels
of acids Lysine, arginine, ornithine, and cystine in the urine,
plasma levels are generally normal.

47. Cystic fibrosis
• Defect - Mutation in CFTR(CF transmembrane conductance
regulator (CFTR) gene
• In cystic fibrosis , point mutation in the CFTR gene results in
defective chloride transport , so water moves out from lungs
and pancreas .
• The alteration in chloride transport is associated with production
production of abnormally thick secretions in glandular tissues.
The lung bronchioles and pancreatic ducts are primarily
affected.
• This will lead to infection and progressive damage and death at
a young age .

48.  symptoms
• Repeated lung infection
• A persistent cough that produces thick mucus (sputum)
• Wheezing
• Breathlessness
Diagnosis
• Sweat Chloride Test
• Immuno-reactive Trypsinogen (IRT) : New-born Screening

49. Wilson’s disease
• Defect – Wilson disease is caused by mutations in ATP7B gene
present on chromosome 13 which controls for excreting excess
copper into bile and out of the body.
• the protein transporter is located in the trans-golgi network of
the liver and brain.
• The major route of copper excretion (95%) is through the liver,
this excess copper first accumulates in the liver and then spills
into the blood and then to other organ systems.
• The excess copper leads to the generation of free radicals that
causes oxidation of vital proteins and lipids.

50.  symptoms
• Jaundice
• Dark urine or a light stool colour
• Anxiety and depression
• Muscle stiffness

51. • Vitamin D resistant rickets
Defect – reabsorption of phosphate from renal tubules
• Congenital long QT syndrome
defect - defective potassium channel causing delay in
repolarization of heart

52. Summary
Cell membrane is made up mainly of phospholipids and protein
molecules. This is the barrier that surrounds of the cell surface.
Phospholipids have a hydrophilic head and a hydrophobic tail.
Proteins act as transport proteins to act as channels for
substances to move into or out of the cell. Some act as
membrane enzymes and some have important roles in the
membranes of organelles.
Passive transport and active transport regulate materials
entering and exiting cells.

53. Thank you !