TRANSPORT ACROSS CELL MEMBRANE
Two types of transport process occur across the membrane.
1. Non-mediated transport
2. Mediated transport
Non-mediated transport occurs through the simple diffusion process and the driving force for
the transport of a substance through a medium depends on its chemical potential gradient.
Whereas mediated transport requires specific carrier proteins. Thus, the substance diffuses in
the direction that eliminates its concentration gradient; at a rate proportional to the magnitude
of this gradient and also depends on its solubility in the membrane’s non-polar core.
Mediated transport is classified into two categories depending on the thermodynamics of the
system:
1. Passive-mediated transport or facilitated diffusion: In this type of process a specific
molecule flows from high concentration to low concentration.
2. Active transport: In this type of process a specific molecule is transported from low
concentration to high concentration, that is, against its concentration gradient.
Passive mediated transport:
Substances that are too large or polar diffuse across the lipid bilayer on their own through
membrane proteins called carriers, permeases, channels and transporters. Unlike active
transport, this process does not involve chemical energy. So the passive mediated transport is
totally dependent upon the permeability nature of cell membrane, which in turn, is function of
organization and characteristics of membrane lipids and proteins.
Types of passive transport:
1. Diffusion: The process of the net movement of solutes from a region of high
concentration to a region of low concentration is known as diffusion. The differences of
concentration between the two regions are termed as concentration gradient and the diffusion
continues till the gradient has been vanished. Diffusion occurs down the concentration
gradient.
2. Facilitated diffusion: The process of the movement of molecules across the cell
membrane via special transport proteins that are embedded within the cellular membrane is
known as facilitated diffusion or called carrier-mediated diffusion. Many large molecules,
such as glucose, are insoluble in lipids and too large to fit into the porins, therefore, it will
bind with its specific carrier proteins, and the complex will then be bonded to a receptor site
and moved through the cellular membrane.
3. Filtration: Filtration is the process of the movement of water and solute molecules across
the cell membrane due to hydrostatic pressure generated by the system. Depending on the
size of the membrane pores, only solutes of a certain size may pass through it. The membrane
