Physiology sheet (lecture 12)
made by: Farah Badwan
Transport of substances
There are three methods of protein- mediated transport:
1- Facilitated diffusion
2- Primary active transport
3- Secondary active transport
The difference between simple diffusion and carrier mediated transport :
Simple diffusion: a passive process (without energy)
substances move either directly through the cell membrane or through a
channel protein
Carrier- mediated transport (characteristic of carrier-mediated transport)
1) Specificity : each protein specifically transports a certain substance
2) Saturation ‫ التشبع‬: the carrier protein has a maximal capacity of
transport of a substance. when the protein reaches the maximal
capacity , it can not transport any further substances
3) Competition: if a carrier protein transports a substance primarily and
there is another substance that has a similar structure, the two substances
will compete on the protein
Example: * a carrier protein transports Glucose primarily, but if there is
Galactose (which has the similar structure of glucose) especially in a high
concentration, it will compete with glucose on the protein
* In treatment: some receptors associate with a certain chemical
substance and give a certain response in the body. If we don’t want this
response, we give a similar structure substance at a higher concentration;
this substance will associate with all receptors and prevent the effect of
already produced substance in the body.
 Facilitated diffusion :
1- a type of diffusion (a passive process)
2- occurs down an electrochemical gradient (downhill) from a high
concentration area to a low concentration area
3- exhibits specificity, saturation and competition (because it is a
facilitated process>> requires carrier proteins to transport substances)
Blue line: a substance pass through the membrane by simple
diffusion (the relation is proportional)
When the concentration increase the rate of diffusion increase
There is no limitation
Red line: a substance that transported by facilitated diffusion.
When the carrier protein saturated, the movement leveling off
because the carrier protein can not transport any further substances
Example of facilitated diffusion: Glucose transport in muscle, adipose cells
and fat tissues (the carriers for facilitated diffusion of glucose require
insulin): - downhill
- carrier–mediated
- inhibited by sugars such as galactose (competition)
* Urinal tubule: capillaries filtrate fluids into tubule (each capillary filtrate
small molecular substance such as; glucose at the same concentration as that
in plasma)
Under normal conditions glucose is filtered and passes through urinal tubule,
but there is no glucose appears in urine because all the filtered glucose is
reabsorbed.
Reabsorption in urinal tubule is a facilitated diffusion process.
Under normal conditions the filtered glucose is lower than the tubular
maximal capacity of Reabsorption of glucose.
In Diabetes mellitus: blood glucose is high and the filtered glucose is high.
Urinal tubules absorb the maximal amount of glucose and any amount that
above the maximal capacity of Reabsorption will appear in urine.
Factors affecting the net rate of diffusion:
1. Concentration difference
2. Surface area
3. Permeability of the membrane to substance
4. Molecular weight of substance
5. Distance of diffusion (thickness of the membrane)
The rate of diffusion is inversely proportional to the square of distance
of diffusion
Example: respiratory system (there is a thin membrane between
inspired air and pulmonary blood, diffusion and gases exchange occur
through this membrane. In certain respiratory diseases, the thickness
of this membrane increase; so the oxygen level in blood decrease)
 Active Transport of substances through membranes
Requires energy because molecules or ions move uphill:
against a concentration gradient (from a low concentration area to a high
concentration area) or against an electrical gradient (from less negative
charge area to more negative charge area)
* The difference in concentration of any substance between cell membrane
and ECF is related to active transport
Active transport is divided into:
1. primary active transport (the energy is used directly to move a certain
substance such as Na+)
2. Secondary active transport (the energy is used to produce a difference
in concentration of a substance)
Na+ - K+ pump: a primary active transport
Na+ move outside the cell and K+ move inside the cell (it pumps more
Na+ outside the cell than K+ into the cell, so Na+ present at a higher
concentration outside the cell). The pump use the energy primarily to
produce a difference in concentration of Na+ between outside and inside
Na+ tries to move inward according to difference in concentration
through a carrier protein (some carrier proteins can not work unless two
or more substances associate with that protein). This protein can not
allow the inward movement of Na+ unless it’s associated with outward
movement of Ca+2: secondary active transport
The pump is responsible for maintaining the difference in charges
between inside and outside the cell, therefore it is electrogenic.
Na+ - K+ pump control cell volume (if the activity of the pump
decreases, the Na+ concentration will increase inside the cell and the
volume will increase)
*The Inhibition of the pump because of the decreasing of oxygen or
nutrition >> Na+ accumulate inside the cell >> fluid increase inside the
cell >> IC Edema result
Specific inhibitors of Na+ - K+ pump are cardiac drugs (used in heart
failure because the muscle’s contraction is weak): Na+ gradient decrease
so more Ca+2 remain inside the cell >> the muscle contraction increase
Another example of primary active transport: Ca+2 pump
* Na+ - Ca+ exchange (secondary active transport)
* Ca+2 pump (primary active transport)
Ca+2 pump maintain a great difference in concentration inside and
outside the cell. There are mainly two pumps:
1) in the cell membrane (it pumps Ca+2 outside the cell)
2) in the sacroplasmic reticulum (SR) of muscle cells (after stimulation
of muscle (muscle receives nerve impulse), the terminal nerve releases
neurotransmitter on the cell membrane >> change in polarity occur >>
inward movement of Ca+2 and releasing of stored Ca+2 from SR >>
free Ca+2 in cytoplasm increase >> activate the contractile protein >>
contraction of the muscle >>> after that relaxation of the muscle occur
(Ca+2 pump in the cell membrane and in SR return Ca+2 back toward
normal level) )
Example three: secretion of HCl acid (transport of H+):
1- By stomach at a very high concentration (Glands in stomach (pits) are
lined by different types of cells such as; parietal cells which secrete
HCl acid into the lumen of the stomach against a very high
concentration gradient because the acidity in stomach is very high
more than acidity in blood)
2- By urinal tubules (secrete the excessive amount of H+ ions in the
body against a high concentration gradient) >> primary active
secretion of H+ ions.
Good luck