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Cell Junction

A. Tight junction: membranes fuse tightly, no transport between adjacent cells possible. Eg blood brain barrier, GIT, Kidney & choroid plexus.


: membranes are separated by 15-20 nm, but no transport of substances takes place. Act as anchoring junctions.


Gap Junctions
 : 2-20 nm separation. Made up of Connexons. Contain ion channels responsible for spread of impulses, found in cardiac muscles (in intercalated discs- make Cardiac muscle “one unit” or syncytium), Electrical synapses, Nodal tissue & Single unit (visceral) smooth muscle.



Cytoskeletal system


Cytoskeleton type

Diameter (nm)

Subunit & Structure


A.  Microfilaments


 Made of actin, double helix

Cell junctions

Muscle Contraction

Slow axoplasmic transport

B.  Intermediate filaments


vimentin (mesenchyme)


keratins (epithelial cells)

Cell adhesion

Maintain cell shape

C.   Microtubules


 - and -tubulin

Intracellular transport

Cilia and flagella.

Centriole- mitotic spindle.



D. Molecular motors: Cytoskeletal motors & Polymerization motors

1.  Cytoskeletal motors

a.  Myosin is responsible for muscle contraction

b.  Kinesin moves cargo inside cells away from the nucleus along microtubules

d.  Dynein produces the axonemal beating of cilia and flagella and

e.  also transports cargo along microtubules towards the cell nucleus


2.  Polymerisation motors

a.  Actin polymerization generates forces and can be used for propulsion. ATP is used.

b.  Microtubule polymerization using GTP.

c.   Dynamin is responsible for the separation of clathrin buds from the plasma membrane.


3.  Fast cytoplasmic transport (20-400mm/day): use molecular motors which run on Microtubule filaments

a.   anterograde: This is driven by kinesin

b.   Retrograde: This is driven by Dyenin


4.  Slow cytoplasmic transport (0.2-4mm/day)

This is always anterograde. The cytoskeleton (Microtubules) moves as a whole due to the continual polymerization at the leading end (+ end) and depolymerization at the trailing end (- end).

Concept 1-Body Composition: Values and Measurement


a.  Body Composition (as % of Body weight)










*Body Water as % Body weight
TBW = 60%
ECF = 20%
Plasma = 5%
Interstitial fluid → 15%
ICF = 40%
(Total blood volume = 8% of body weight; since plasma volume is 5% of body weight, blood cell volume = 3% of body weight)


b.  Measurement of the various body fluid compartments

This is done by the principle of volume of distribution. (or dye-dilution method)


= volume

Q = quantity of indicator given

C = concentration of the indicator

e = The amount of indicator which has either been lost or  metabolized


c.  Indicators


Indicator used

Plasma volume

Evans’ blue (T1824)

RBC volume

Tagging RBC with 51Cr, 59Fe, 32P; also antigenic tagging

ECF volume

Inulin, sucrose, Mannitol, Sodium thio sulphate, sodium thiocyanate

Interstitial fluid

Cannot be measured directly; can be calculated as ECF volume – Plasma volume


Cannot be measured directly; can be calculated as Total body water – ECF

Total body water

D2O is most frequent used; also, tritium oxide, aminopyrine


d.  Other points

Water content of lean body mass = 71-72 mL/ 100 gm of tissue (Lean body mass = body mass devoid of fat)

Fat does not hold water.


Total body water:

  i. Somewhat lower in women

 ii. Tends to decrease with age

Concept 2: Expressing Solute Concentrations

Mole (or mol)

It is molecular weight of a substance in grams i.e. it is gram molecular weight.



a.  Calcium → Molecular weight = 40;; therefore, 40 gm = 1 mol of calcium
b.  NaCl → Atomic weight of
Na = 23
CL = 35.5
Therefore, 23+35.5 = 58.5 gm of NaCl = 1 mol of NaCl



1. In S.I. system, mole is the standard used to express amount of any substance
    Dalton: It is a unit of mass; 1 Dalton = 1/12th of the mass of carbon atom – 12
2. Molecular weight is a dimensionless ratio.



Equivalent (Eq.)



a.  1 equivalent of calcium = 40/2 = 20gm

b.  1 equivalent of Sodium = 23/1 = 23gm


1 OSMOLE  = Mol ÷ Number of freely moving particular each molecule liberates in solution        

It expresses concentration of osmotically active particles.



1 mol of NaCl = 2 osmoles because each NaCl molecule given one Na+ and one Cl- particle is solution
1 mol of Na2SO4 = 3 osmoles because each Na2SO4 molecule gives 2 Na+ and 1 SO4 is solution
1 mol of CaCl2 = 3 osmoles, because each molecule of CaCl2 give 3 particles (1 calcium and 2 Cl-) in solution
1 mol of Na2SO4 has 4 equivalents and 3 osmoles




One osmole (or one can say 1 mol of an undissociated substance in an ideal solution ) of any substance has the following properties :
1. it depresses freezing point by 1.860C*
2. it exerts an osmotic pressure of 22.4 atmospheres
3. it has 6 X 1023, molecules (Avagadro’s number)
*This fact can be used to measure the osmolal concentration of a substance.



Difference between osmolrity and osmolality
Osmolarity is the number of osmoles  per litre of solution.

Osmolality is the number of osmoles per kg of solvent. Osmolality is not affected by changes in volume of solution or by temperature. 


This is the osmolality of a solution with respect to plasma osmolality



0.9% NaCl is isotonic ; 5% glucose is isotonic initially; later is become hyptomic
Plasma osmolality : 290 mosm
Approximate formula for finding the plasma osmolality:
Plasma osmolality = Na+ concentration (in mEq/l) x 2 +  glucose(mg/dL)/18 +  BUN (mg/dL)/2.8
Out of the 290 mosm,

1.  Na+ and its associated ions (Cl- / HCO3-) =      270 mosm
2.  Urea           =      5 mosm
3.  Glucose =      5 mosm



Osmolal Gap:

  1. The most accurate way of finding out the osmolality is by freezing point depression (see above)
  2. The approximate formula is as given above
  3. If there is a difference between the two calculations, it is called osmolal gap. Osmolol gap indicates the presence of a foreign substance.


ECF Volume

Body Osmolarity

ICF Volume

Loss of isotonic fluid Hemorrhage Diarrhea Vomiting

no change

no change

Loss of hypotonic fluid Dehydration Diabetes insipidus

Gain of isotonic fluid

no change

no change

Gain of hypotonic fluid

Gain of hypertonic fluid e.g. mannitol

Concept 3: Transport Across Cell Membranes

1. Active: Primary and secondary

2. Passive

3. Exocytosis / Endocytosis (This is an active process)

  1. Active Transport

Definition: Energy is used. Against gradient


a. Primary active transport:  Energy is derived directly by hydrolysis of ATP by the transporter itself.


Note: All transporters ending with “ATPase” are primary active


Exmaple: Na+ K+ ATPase pump


b. Secondary active transport :  Energy is derived indirectly                         


Example: Sodium – linked glucose transport (i.e. SGLT) in kidney & GIT


The secondary active transport can be a


Note: All transport mechanisms which are linked to Na+ entry or K+ exit are secondary active.eg. Na+-I- symport

i. Counter transport
or antiport (exchanger) where two substance are transported in the opposite direction


ii. Co- transport (symport) where two substances are transported in the same direction

Na+ K+ ATPase pump:
It is a universal pump responsible for maximum energy consumption in basal state. Coupling ratio of 3:2 i.e. 3 Na+ out and 2 K+ inside the cell. Blocked by digitalis & Ouabain. The Na+/K+-ATPase helps to generate resting membrane potential(5-10%), active transport (primary as well as secondary active) and regulate cellular volume(by pumping out Na+ & therefore water-failure will lead to cellular swelling)

  1. Passive transport – (No energy required as along gradient)
This can be:
a. Simple diffusion    
b. Facilitated diffusion
c.  Nonionic diffusion
a. Simple diffusion
i. No carrier molecule involved
ii. No Tm (No transport maximum i..e not saturable)
iii. Follows Fick’s law of diffusion
iv. Fock's law of diffusion     

Fick’s Law of diffusion
        J =   - DA C
J = Net rate of diffusion
D = Diffusion coefficient
A = Area
(Delta C = Concentration difference on the 2 sides of the membrane)
(Delta x = thickness of the membrane)
(The negative sign indicates the direction of diffusion )
(For diffusion from higher to lower concentration, the sign is negative)
The time required for diffusion is directly proportional to the square of the diffusion distance
Example of simple diffusion : O2/CO2 exchange in alveoli

b. Facilitated disffusion
  1. No energy is required
  2. A carrier molecule is involved to which the substance binds, therefore, it is also called passive carrier – mediated transport
  3. Has a Tm (it is saturable)
  4. It can be competitively and noncompetitively inhibited
  5. It follows the enzyme – substrate kinetics of Michaelis - Menten
Example: Glucose transport by glucose transporters (GLUT)

c.  Non – ionic diffusion
  1. In case of weak acids or bases, where the acid / base can cross the membrane in the non – ionized form but cannot cross the membrane in the ionized form
Ammonia transport in GIT / Kidney. The mucosal toxicity of non – steroidal anti inflammatory drugs can be explained on this basis.

Diffusion of a solvent into a higher concentration of solute to which the membrane is impermeable.
  1. Osmotic pressure for an ideal solution is given by

                                                P= nRT
                          Where, P      =      osmotic pressure
                                     N      =      number of particles
                                     R      =      Gas constant
                                             =      volume
                                     T      =      Absolute temperature
  • Osmotic pressure depends on the number rather than the type of particles in solution. Such properties which depend on the number the rather than the type of particles in called fundamental colligative property.
Example: Other examples of fundamental colligative property are:
o   Vapour pressure lowering
o   Freezing point depression
o   Boiling point elevation
  1. Filtration: This is the movement of fluid across capillaries, this is governed by Starling’s forces. The starling’s forces are
  • Hydrostatic pressure (a ‘push’ force) and
  • Oncotic pressure (a ‘pull’ force)
  • Bulk flow or solvent drag

This refers to the movement of particles / solutes along with movement of water

Vesicular transport: Endocytosis (eg phagocytosis) & Exocytosis. Membrane area increases in exocytosis and decreases in endocytosis. All require Ca2+. Proteins involved are Clathrin (Receptor mediated endocytosis), Dynamine, Caveolin etc.

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