The movement of molecules through a membrane by filtration depends upon the presence of

The movement of molecules through a membrane by filtration depends upon the presence of:

A. Osmotic pressure
B. Hydrostatic pressure
C. Atmospheric pressure
D. None of the above

The correct answer and explanation is :

Correct Answer: B. Hydrostatic pressure

Explanation:

Filtration is a passive transport process where molecules move through a membrane due to a pressure gradient. This process is driven by hydrostatic pressure, which is the force exerted by a fluid against a membrane.

In biological systems, hydrostatic pressure is primarily generated by the pumping action of the heart. For example, in capillaries, blood pressure forces water and small solutes through the walls while preventing the movement of larger molecules like proteins. This is how filtration occurs in the kidneys during urine formation, where blood pressure forces water, ions, and small molecules from the bloodstream into the kidney tubules, forming the initial filtrate.

Now, let’s analyze why the other options are incorrect:

1. Osmotic Pressure (Option A):
   Osmotic pressure is the force exerted by a solution to pull water across a semi-permeable membrane due to solute concentration differences. It is critical in osmosis but does not drive filtration. Instead, osmotic pressure works against hydrostatic pressure in systems like capillary exchange.
2. Atmospheric Pressure (Option C):
   Atmospheric pressure refers to the force exerted by air molecules on a surface. While it plays a role in respiration and gas exchange, it does not significantly influence the movement of molecules through biological membranes by filtration.
3. None of the Above (Option D):
   This option is incorrect because hydrostatic pressure is the primary force responsible for filtration.

Conclusion:

Filtration depends on hydrostatic pressure, which pushes fluids and small molecules through a membrane while larger molecules remain behind. This process is vital in physiological functions like kidney filtration and capillary exchange, where blood pressure drives the movement of molecules across membranes.