Download MBBS Physiology Presentations 62 Membrane Transport Lecture Notes

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Elimination from

the cells

electrical

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potentials at the

membrane

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Cell Membrane

? surrounds entire cell and cell organelles

? Fluid in nature ? movement of molecules

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? Phospholipid bilayer ? head ? polar/hydrophilic
tail ? nonpolar/hydrophobic

? Proteins

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Integral ?carrier & channel
Peripheral-receptors & antigen
Peripheral protein

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Intrigal protein

(ankyrin)

Band 3

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cytoskelatal protein

spectrin

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Extrinsic protein

Glycolipid

Head

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ECF

(hydrophilic)

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Glycoprotein

r
l
a

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ye

d bi
L
i

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pi

Tail

(hydrophobic)

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cholesterol

Integral proteins

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Intrinsic protein

ICF
Functions of cell membrane

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? Acts as semi permeable barrier ?(selective)

oMaintains difference in composition of

ICF & ECF & fluid in various organelles

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oProtects cell from toxic substances

oExcretion of waste products

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oTransport of nutrients



? Receives signals from the outside

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?Chemical signals

?Electrical signals

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?Site for attachment to the neighboring cells

Transport across cell membrane

Transport Mechanisms

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Passive

Active

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?Simple diffusion

?Primary active transport

?Facilitated diffusion ?Secondary active transport

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?Filtration

?Endo/Exocytosis

?Osmosis

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?dialysis
Methods of transport

Passive

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Active

Diffusion

Osmosis Filtration

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Dialysis

Simple

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facilitated

?Lipid bilayer

?Protein

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Leaky channels

voltage gated

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channels

Gated channels

Ligand gated

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Simple diffusion -
Movement of molecules from higher
concentration to lower concentration till
equilibrium is reached

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.Diffusion can takes place through:

a) Lipid bilayer

i) Lipid soluble substances-

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O2,Co2,alcohol, steriods etc

ii) Lipid insoluble ? water (through

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spaces bet lipid mol) urea, sugar

(less or no permeability)

iii) Electrolytes ? impermeable

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? charge on fatty acid chain

- Hydrated forms are larger

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.
b) Protein Channels Open/leaky ? Na+ channels,

K+ channels

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Gated ?channels open under specific conditions

Ligand gated Voltage gated
Na+

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Na+,

K+

K+

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Ca++,

Mutation of ionic channels produce channelopathies ?affecting

muscle and brain ? paralysis or convulsions

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Factors affecting rate of diffusion
? Lipid solubility
? Molecular size & wt.

Molecular

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? Temperature
? Thickness of membrane

Membrane related

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? Surface area
? Concentration gradient
? Pressure gradient

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Gradients

? Electrical gradient

Fick's law of diffusion ?

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Imp.

Q

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CPA

MW X

Q = net rate of diffusion

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C = conc. gradient of a substance
P = permeability of membrane to the sub.
A = surface area of a membrane
MW = molecular wt. of sub.
X = thickness or distance

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II. Facilitated diffusion :
- for larger water soluble mols.


- type of passive transport

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- along the conc. Gradient
- carrier mediated transport

Me - re

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chanism

ceptor site on one side

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- Rate of transport ? Vmax

i
t
y

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Vmax

Simple diffusion

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l
oc

Facilitated diffusion

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ve

Conc. gradient

Initially, rate is directly proportional to conc. gradient

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Till it reaches ( l

Vmax

imitation because of no. of

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carrier mols. & rate of conformational change)
Hormonal regulation by changing #of carriers.
- Peculiarities of carrier mediated transport ?
?

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specificity,

? competitive or noncompetitive inhibition ?

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phloridzin for glucose

? saturation,
? blocking of receptor

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V

? max

-Examples ? transport of glucose, amino acids,

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galactose, etc. in the peripheral cells or counter

transport of Ci and HCO3 in renal tubules
III. Osmosis & osmotic pressure?

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when two solutions of different concentrations are

separated by a semi permeable membrane (

impermeable to solute and permeable to water ) water

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mols. diffuse from solution having less conc. To the

sol. having higher conc.

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Osmotic pressure is the minimum pressure

applied on the solution with high conc. which

prevents osmosis.

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- depends upon total no. of particles of dissolved

solutes rather than type of the particles

Osmols or mOsmols ? expresses conc. of

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osmotically active particles
1 osmol = total no. of particles in gram molecular
wt. of non diffusible substance per kg. of water
Applied -
Isotonic, hypotonic & hypertonic solutions

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Isotonic solution ? fluids having osmolarity

same as that of plasma ( 290 mOsmols ) . Red

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cells suspended in such solution do not shrink

or swell. ( 0.9 % NaCl, 5% glucose )
In Hypotonic soln. RBCs swell and hemolysis

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may occur.
In hypertonic solution RBCs shrink because

water moves out.

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Gibbs ? Donnan Equilibrium

Explains difference in the conc. of

diffusible ions in two compartments separated by

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semi permeable membrane, when one
compartment contains non diffusible ions

Na +

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Na +

Proteins are non

Cl -

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Cl -

diffusible anions in A

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Pr -

Conc. Of Na + is more

in A as compared to B

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A

B
A

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B

Na+ 30

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Na+ 30

conc. Gradient for Cl -

Pr - 30

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Cl -30

Na+ 30

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Na+ 30

Cl - 15

Cl - 15

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More ?vity in A

Pr - 30

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electrical gradient

Na+ 45

Na+ 15

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Conc. gradient

Cl - 15

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Cl - 15

electrical gradient

Pr - 30

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Na+ 40

Na+ 20

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Cl - 10

Cl - 20

Pr - 30

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Explaination ?
1) All the solutions are electrically neutral.
( total no. of anions = total no. of cations )
2) Product of diffusible cations and an

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anions in both the compartment is equal.
( Na+A x Cl-A = Na+B x Cl-B )
Applied ?
In ICF conc. of diffusible K+ is more because of
presence of non diffusible Pr - and PO4 -

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Diffusion potential or Equilibrium potential - E

Potential generated across the cell membrane in
the presence of non diffusible ions in one
compartment.

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Magnitude of potential developed can be
calculated by Nernst equation.

Nernst equation -

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Equilibrium potential or diffusion potential (E)
= + 61 log ------------

Conc. inside

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Conc. outside

EK = - 94 mV
ENa = + 61 mV
ECl = - 90 mV

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Goldmann-Hodgkin's equation =

C

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- 61 log --------------------------------------

Nai.PNa + CKi.PNa + CClo.PClo

CNao.PNa + CKo.PNa + CClo.PCli

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IV. Filtration

Filtration is a process in which fluid along
with solutes passes through a membrane due to
difference in pressures on both sides.

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e.g. Filtration at capillary
Capillary hydrostatic pressure ? 28mm Hg
Interstitial fluid hydrostatic pressure - -2mm Hg
Colloidal osmotic pressure - 25mm Hg
Net Filtration pressure = 28 - (- 2 + 25) = 5 mm Hg

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V. Dialysis ?

separation of larger dissolved particles from

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smaller particles

It is used for elimination of waste products in the

blood in case of renal failure.

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Active transport

? Primary active transport
? Secondary active transport
? Endocytosis

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? Pinocytosis
? Phagocytosis

? Exocytosis

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Peculiarities of active transport

1) Carrier mediated transport

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2) Rapid rate of transport

3) Transport takes place against electrochemical

gradient ( uphill )

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4) Expenditure of energy by transport protein

which incorporates ATPase activity
5) Carrier protein shows specificity, saturation

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competitive inhibition, blocking

6) Substances transported ? Na+ , K+, H+, Cl -, I - ,

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Glucose, Amino acids

I. Primary active transport ?

Examples - Na+ - K+ pump, Ca++ pump

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H+-K+ pump

- Inner surface of carrier mol. has ATPase

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which is activated by attachment of specific

ions and causes hydrolysis of ATP molecule

- Energy released from ATP causes

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conformational change in the carrier which

transports ions to the opposite side.
a) Na+ -K + pump- electrogenic pump

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- Attachment of 2K+ on outer side & 3 Na+ on inner side

Activation of ATPase

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3Na+

Conformational change

Efflux of 3 Na+ & influx of 2K+

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ATP 2K+

aes

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Creates high K+ conc. & - vity inside the cell

Helps in maintaining cell volume

Na-K pump is one of the major energy using

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process in the body & accounts for a large part

of basal metabolism.

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Regulators of Na-K pump ?
- Incraesed amount of cellular Na conc.
- Thyroid hormones increase pump activity by more # of Na-K

ATPase mol

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- Aldosterone also increases # of pumps
- DOPamine inhibits pump
- Insulin increases pump activity
- Oubain or Digitalis inhibits ATPase (used when weakness of

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cardiac muscle ?maintains Ca conc. In ICF of cardiac muscle
- Ca++ pump ?
present in the membrane of ER,

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mitochondria and cell membrane
- involves uniport carrier
- helps to maintain low Ca++conc. in ICF

II. Secondary active transport

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Active transport depending upon conc.
gradient of Na+ from ECF to ICF created by
utilization of energy
_ carrier does not have ATPase activity

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Substance is transported along with Na+
(Na increases affinity of carrier for gl.)
Na+ is transported only when glucose mol. is
attached
Examples ? a) Reabsorption of glucose & amino

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acids in PCT & Intestinal mucosa ? Co-transport
mechanism

b) H+ secretion by tubular epithelium

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? counter transport mechanism

c)In heart Na-K ATPase indirectly affects Ca transport. ?antiport

in the membrane of cardiac muscle exchanges intracellular Ca

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for extracellular Na

basal

lumen

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Na+
K+
Glucose

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? Na + ? K + pump on basal side

? Electrochemical gradient for Na + on luminal side

? Carrier mediated transport (SGLT-1)of Na+ along

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with glucose ( or amino acid ) through the apical

membrane

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? Transport of glucose by facilitated diffusion

( GLUT-2 ) through basal side
Types of transporters

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Uniport

Synport

Antiport

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Extracellular material to be tackled by

lysosomes is brought into the cell by

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endocytosis

3 types

pinocytosis

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phagocytosis

Specialised

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All cells

Receptor

cells

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mediated

endocytosis

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Requires ATPase, Ca, microfilaments
Pinocytosis

ECF

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ECF

dynamin

Membrane deforming

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coat protein

Endocytic

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vesicle

B. Receptor mediated endocytosis ? highly

selective process to import imp. specific large

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molecules. Requires energy & Ca++.

e.g. endocytosis of low density

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lipoproteins

Coated pit Clathrin, actin, e.g. endocytosis of viruses such as

myosin

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hepatitis, AIDS viruses & excess iron


C. Phagocytosis

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? Internalization of large multimolecular

particles, bacteria, dead tissues by

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specialized cells e.g. certain types of

w.b.c.s ( Professional phagocytes)

? The material makes contact with the

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cell membrane which then invaginates.

bacterium

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Pseudopodia

internalization

Phagoso-

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some

Fusion

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Residual

body

absorption

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digestion

Phagocytosis
Passive transport Active transport

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? No expenditure of

? Expenditure of energy

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energy molecules

mol. ( ATP )

? Takes place along

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? Can take place against

conc., electrical, &

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conc. Gradient

pressure gradient

? Carrier may or may

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? Carrier is always

not be required

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required

? Rate is proportional to

? Rate is proportional to

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conc. difference

availability of carrier

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& energy. (Vmax)

Simple Diffusion Facilitated Diffusion

? Passive transport

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? Passive transport

? For small molecules

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? For large molecules

? No carrier required

? Carrier mediated

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? Rate of transport is

? Initially rate is

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directly proportional to

proportional to conc.

conc. gradient

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gradient till Vmax

? Examples ?

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( saturation of carriers)

Lipid soluble ?

? Examples ?

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O2, CO2, alcohol glucose, amino acids
Lipid insoluble ?

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urea, Na+, K+