Membrane transport biochemistry for nurses

1. Mrs. Namita Batra Guin
Associate Professor
BIOLOGICAL MEMBRANE AND
TRANSPORT

2. MEMBRANE COMPOSITION
● Lipids- 50%: membrane lipids are comprised of both
hydrophobic and hydrophilic regions. (amphipathic molecules).
Polar head and non- polar tail. Fatty acids present are both
saturated and unsaturated.
● Phospholipids: major portion of the cell membrane. Referred
as neutral lipids. Out of the two major classes,
phosphoglycerides are the more common. Simplest
phosphoglyceride is phosphatidic acid. Plasma membrane
consists of both neutral lipids and sphingolipids.
● Proteins- 20%: present as structural component, transport
system, enzymes and receptors. Two types: integral proteins
and peripheral proteins

4. FLUID MOSAIC MODEL OF CELL
MEMBRANE
● Singer and Nicholson (1972)
● Membrane structure in which protein molecules float
about in fluid of phospholipid layer.
● Scattered protein molecules resemble a mosaic.
● Phospholipid bilayer is fluid, so it forms fluid mosaic
structure.
● Two types of proteins: peripheral proteins and integral
proteins.
● Peripheral proteins includes both intrinsic and extrinsic
proteins.

5. FEATURES OF MEMBRANE
● 7nm thick
● Hydrophilic phosphate heads of phospholipids facing
outward.
● Hydrocarbon tail face inwards and creates hydrophobic
interior.
● Some fatty acid tails are saturated while some are
unsaturated. Unsaturated tails are bent and fit together
more loosely. More unsaturated tails leads to more
fluidity of the membrane.

6. FEATURES OF MEMBRANE
● Proteins stay in membrane because they have regions of
hydrophobic amino acids, which interacts with fatty
acid tails to exclude water. Rest of the proteins are
hydrophilic.
● Membrane also contains cholesterol. It is important for
those living in low temperatures. It also increases
flexibility and stability of the membrane. Without it
membrane may break up.

7. FEATURES OF MEMBRANE
● Membrane has fluidity, which is due to presence of
unsaturated fatty acids. Degree of fluidity depends
upon the composition of lipids in a membrane.
● Cholesterol concentration and Ca2+ concentration
affect the membrane fluidity.

8. FUNCTIONS OF MEMBRANE
● Cell membrane may be associated wth several enzymes
e.g. phospholipase A etc. that are used as markers of the
membrane.
● membranes contain specific receptors for external
stimuli, such as hormones.
● Carbohydrate component of glycoproteins in a
membrane act as recognition sites for some of the
substances, such as for antibodies.
● It also acts as a barrier, thereby preventing loss of useful
substances while permitting the entry of nutrients.

9. CELL JUNCTIONS
● Gap juctions: intercellular channels, permits free passage
of ions and small molecules between the cells.
● Helps in intercellular communication. They are composed
of an integral membrane protein called connexin.
● They are gated and their closure is triggered, primarily by
phosphorylation of connexin subunits and abnormally high
calcium ion concentration.
● Desmosomal junctions: cell structure specialised for cell
to cell adhesion. Found in those tissues that subjected to
mechanical stress such as cardiac muscles, epithelial layer of
skin and uterine cavity.

10. CELL JUNCTIONS
● Tight junctions: Specialised sites of contact that block solutes
from diffusing between the cells in an epithelium. Tight junctions
strands are composed of proteins designated occludins and
claudins.
● They act as barrier for the material between two interacting
cells.
● Some tight junctions are permeable to specific ions or solutes.
● They are also involved in signalling pathways that regulate
numerous cellular processes.
● These junctions hold cells together and helps in maintaining the
polarity of the cells.
● Plays important role in maintaining blood-brain barrier,
which prevents substances from passing from blood stream into
the brain. Although cells of the immune system are able to pass
across the endothelium through these junctions.

11. CYTOSKELETON
● A system of protein fibres that pervades the cytoplasm with
analogous functions.
● Functions:
● Provides structural support that determines the shape of the
cell and resists forces that tries to deform it.
● Functions as internal framework that is responsible for
positioning various organelles.
● Provides a network of tracks that directs the movement of
material and organelles within the cell.
● It functions as force- generating apparatus that moves cells
from one place to another like: locomotion of WBC’s,
fibroblasts etc.
● It is responsible for separating the chromosomes during
mitosis and meiosis.

12. CYTOSKELETON
● Flagella, cilia and microtubules are examples
● Made up of three kinds of protein filaments:
● Intermediate filaments: provides tensile strength to the
cell. keratin of hair and nails, neurofilaments of nervous tissues.
● Actin filaments: (microfilaments): Fine thread like protein
fibres. Composed of contractile protein i.e. actin. Carries
out muscle contractions and cellular movement.
● Microtubules: composed of subunits of protein tubulin.
Determines shape of the cell. Provides tracks for cell
organelles to move on. Forms spindle fibres for separating
chromosomes during mitosis. Also used for locomotion (cilia
and flaggela)

13. MEMBRANE TRANSPORT
● Ability of the membrane to allow passage of gases,
liquid and dissolved substances through it.
● Passage of substances across bio-membrane occurs by
three modes:
● Passive transport
● Active transport
● Bulk transport

14. Passive Transport
● No energy required
● Move due to gradient
● differences in concentration, pressure, charge
● Move to equalize gradient
● High moves toward low

15. Types of Passive Transport
1. Diffusion
2. Osmosis
3. Facilitated diffusion
4. Filteration

16. Diffusion
● Molecules move to equalize concentration

17. Diffusion
● Higher concentration region to lower concentration region.
● Diffusion pressure: during movement of diffusing
particles, collision usually occurs and pressure is generated,
which is called diffusion pressure.
● Pressure is directly proportional to the concentration of
molecules and temperature.

18. ● Factors affecting rate of diffusion:
● Temperature: increases with increase in temperature
● Concentration of medium: decreases with increased
concentration.
● Size and mass of diffusing particles: smaller
particles diffuses more quickly.
● Solubility of solutes: More solubility faster diffusion.
● Density of diffusing particles: rate of diffusion is
inversely proportional.

19. ● Diffusion of a substance may occur through
Channels or pores in the cell
● Channel: they are pore forming membrane proteins
whose function includes establishing a resting membrane
potential, shaping action potentials and other electrical
signals by gating the flow of ions across the cell
membrane.
●

20. IMPORTNACE OF DIFFUSION
● Helps in uniform distribution.
● Helps in respiratory gas exchange
● Transpiration in plants occur by diffusion.
● Helps in cell to cell movement of water in plants.

21. DIALYSIS
● Diffusion of small dissolved molecules from one
solution through semi-permeable membrane into
another fluid.
● In renal insufficiency patients, dialysis employs process
of diffusion across semi-permeable membrane to
remove unwanted, toxic substances from blood while
adding desirable components.

22. Osmosis
● Fluid flows from lower solute concentration
● Transport of water or any solvent molecules from a region
of lower conc. of solute to a region of higher conc. of
solute through a semi-permeable membrane.
● Involves movement of water
● Into cell
● Out of cell

23. Solution Differences & Cells
● solvent + solute = solution
● Hypotonic
● Solutes in cell more than outside
● Outside solvent will flow into cell
● Isotonic
● Solutes equal inside & out of cell
● Hypertonic
● Solutes greater outside cell
● Fluid will flow out of cell

24. 0.9% NaCl and 5% glucose solution is isotonic for
RBCs.

25. TYPES OF OSMOSIS
● Endosmosis: entry of water into cell when
surrounded by hypotonic solution. E.g. swelling up of
raisins when soaked in water.
● Exosmosis: exit of water from the cells when
surrounded by hypertonic solution. E.g. surface of
sliced cucumber when salted exudes water due to
exosmosis.

26. DIFFERENCE
DIFFUSION OSMOSIS
● Movement of substances
from higher concentration
to lower concentration.
● Occurs in solids, liquids
and gases.
● Does not require semi-
permeable membrane.
● Little effect of other
substances
● Movement of solvent from
solution of lower solute
concentration to region of
higher solute concentration
● Occurs only in liquids.
● Requires semi-permeable
membrane.
● Depends upon number of
solute particles

27. IMPORTANCE
● Helps in absorption of water from soil through roots.
● Distribution of water from one cell to another.
● Helps in maintaining turgidity of the cells.

28. Facilitated Diffusion
● Differentially permeable membrane
● Refers to the transport of solutes through membrane
transport proteins without expenditure of metabolic
energy.
● Example: glucose transporters (GLUT), ATP/ADP
translocase (exchanges anions between cytosol and
mitochondrial matrix)

29. Process of Facilitated Transport
● Protein binds with molecule
● Shape of protein changes
● Molecule moves across membrane

30. FILTERATION
● Diffusion under pressure across membrane having
minute pores.
● Ultra-filteration in uriniferous tubules of kidneys is the
example.
● High blood pressure in glomerular capillaries causes
high filtration pressure.
● Pressure exceeds that of osmotic pressure of blood
colloids and so squeezes water and many dissolved
substance from blood to Bowman’s capsule through fine
pores.
● Filtered fluid is glomerular filtrate containing sodium,
potassium, glucose, amino acids, urea and water.

31. ACTIVE TRANSPORT
● Energy dependent transport of molecules or ions across a
semi-permeable membrane against a concentration
gradient.
● Transport occurs through the carrier molecules, carrier
particles, carrier proteins.
● There is a special carrier molecule for each solute particle.
● Carrier has a binding site where solute particle combines
to form carrier solute complex.
● Carrier undergoes confrontational change which
transports solute to the other side of the membrane.
● E.g. sodium-potassium pump.

32. Active Transport
● Molecular movement
● Requires energy (against gradient)

33. Sodium- Potassium pump
● Active transport system.
● All animal cells have it in the plasma membrane.
● The pump transfers Na+ and K+ ions against their
concentration gradient.
● A resting cell membrane is electropositive on outer side
and electronegative on inner side.
● ECF has more Na+ while ICF has more K+ .
● At rest membrane is more permeable to K+ and Na+.

34. ● When membrane is stimulated, it becomes depolarised due
to rapid diffusion of Na+ from ECF into cytoplasm, thus
making the membrane more electronegative on outside and
electropositive inside.
● After maximum depolarization, cell membrane becomes
more permeable to K+ and then restoring the membrane
electro-positivity on outside and electronegativity on inside.
It is called repolarization.
● Original ionic distribution is maintained by the Na+/K+
pump.
● It pumps 3Na+ ions out and 2K+ ions in for each molecule
of ATP.

35. MECHANISM
● Na+-K+ pumps maintain low concentration of Na+ in the
cytosol by following steps:
● 3Na+ in the cytosol bind to pump station.
● Binding of Na+ triggers hydrolysis of ATP into ADP. The
phosphate thus released binds with pump protein leading to
change in shape of pump proteins.
● It leads to expelling of 3Na+ into ECF.
● Change in shape of pump proteins favours binding of 2K+ in
ECF to pump proteins.
● Binding triggers release of phosphate group from pump
protein, changing the conformation of protein.
● Pump protein reverts back to its original shape releasing K+
ions into cytosol.
● And pump is again ready to bind 3Na+ and cycle repeats.

36. SODIUM – POTASSIUM PUMP
● Works by cyclical process flip-flopping between two
conformation
● Flip-flop governed by phosphorylation
● 3 Na+ bind on cytoplasmic surface
● Binding changes conformation activating ATPase
● ATPase phosphorylates the protein, hydrolysing ATP
● Causes further conformational change
● Moves 3Na+ to extracellular side
● Released because phosphorylation also reduces affinity of protein for Na+
(conformational change)
● 2 K+ bind on extracellular side (conformational change)
● Dephosphorylation occurs (conformational change)
● Restores original conformation
● Protein flips back to cytoplasmic side taking 2K+ with it
● 2K+ released

37. FUNCTIONS
● Maintains a positive potential on the outer membrane
relatively electro-negative potential inside.
● Pump maintains the water balance of the living cells.
● Helps urine formation.
● Takes part in excretion of salts as in marine animals.

38. APPLICATION
● Na+-K+ ATPase in the cardiomyocyte membrane moves
Na+ out of cell in exchange for K+. Low cytosolic Na+ in
turn maintains Na+gradient for functioning of Na+-Ca2+
exchanger so that low cytosolic Ca2+ levels are maintained.
● In failing heart, aim of treatment is to make myocardium
contract with greater force, which is achieved by giving a
drug- digitalis. the drug inhibits the Na+-K+ ATPase, and
abolishes Na+ gradient. because of which Ca2+ cannot
exit the cell and attains sufficiently high intracellular level.
This mediates excitation- contraction coupling and
generates greater contractile force.
● As more blood is pumped, symptoms of heart failure are
alleviated.

39. BULK TRANSPORT
● Transport of larger particles- proteins, nucleic acid,
liquid droplets or may be solid particles, across plasma
membrane.
● Inward transport is called endocytosis
● Outward transport – exocytosis
● It occurs by two methods:
● Pinocytosis (cell drinking)
● Phagocytosis (cell eating)

40. Endocytosis
● Movement of large material
● Particles
● Organisms
● Large molecules
● Movement is into cells
● Types of endocytosis
● bulk-phase (nonspecific)
● receptor-mediated (specific)

41. Process of Endocytosis
● Plasma membrane surrounds material
● Edges of membrane meet
● Membranes fuse to form vesicle

42. Forms of Endocytosis
● Phagocytosis – cell eating
● Pinocytosis – cell drinking

43. Exocytosis
● Reverse of endocytosis
● Cell discharges material

44. Exocytosis
● Vesicle moves to cell surface
● Membrane of vesicle fuses
● Materials expelled

45. IMPORTANCE
● Larger molecules which cannot be transported
normally through cell membrane are transported
through bulk transport.
● Acts in primary defence of the body.
● Worn out cells and organelles can be thrown out of cell
through this process.

46. THAT’S ALL