Question

10. In a given molecule which are the determinants for its transport through a membrane (active/passive/receptor mediated etc.)? Explain in detail. (5 points)

Answer 1

Three primary factors determine whether a molecule will diffuse across a cell membrane are concentration, charge and size. In general molecules diffuse across membranes from areas of high concentration to low concentration.  Cell membranes prevent charged and large molecules from entering the cell, small molecules may be able to slip through the membrane regardless of their charge.

Passive transport : Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes, in this type of transport substances simply move from an area of higher concentration to an area of lower concentration,until they are equal on either side, which does not require the input of energy. Some materials diffuse readily through the membrane, but others require specialized proteins, such as channels and transporters, to carry them into or out of In facilitated transport, hydrophilic molecules bind to a carrier protein, this is a form of passive transport.

Active transport, to move substances against the membrane’s electrochemical gradient, the cell utilizes active transport, which requires energy from ATP. Active transport helps in movement of small-molecular weight material and small molecules, there are two mechanism involved Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP, for eg,  hydrophilic molecules also bind to a carrier protein, but energy is utilized to transport the molecules against their concentration gradient; Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP. Ions are often used in secondary transport systems because they generate an electrochemical gradient.

Bulk transport these mechanisms enable large molecules and even larger objects to cross the plasma membrane.The process of exocytosis expels large molecules from the cell and is used for cell secretion. Receptor-mediated endocytosis allows specific large molecules to be taken in by the cell.

Receptor mediated endocytosis is a form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule.When the receptors bind to their specific target molecule, endocytosis is triggered, and the receptors and their attached molecules are taken into the cell in a vesicle, though most of the time this type of transport is useful some times less friendly particles like flu viruses, diptheria, cholera toxins may gain entry by this route