Question

Discuss how the movement of substance throughout the cell happened and contribute to its maintenance/ survival. Add a reference

Answer 1

DIFFUSION
Diffusion refers to passive transport of molecules from
areas of higher concentration to areas of lower concentration.
Diffusion through cell membrane is divided into two sub-
types called: simple diffusion and facilitated diffusion.
SIMPLE DIFFUSION
In simple diffusion, transport of atoms or molecules occurs
from one place to another due to their random movement.
Due to constant random movement, the molecules collide
with each other and also strike with the cell membrane. The frequency of collision and the probability of striking to the cell membrane will be higher on the side of the membrane having higher concentration of that particular molecule. In this process there occurs a net flux of the molecules from the areas of high concentration to areas of low concentration. The net movement of the molecules ceases when the concentration of molecules equals, and there occurs a condition of diffusional equilibrium. Quantitatively, the net movement of the molecules across a permeable membrane where only simple diffusion is occurring is expressed by Fick’s law of diffusion, which states that rate of diffusion (J) is directly proportional to the difference in the concentration of the substance in two regions (concentration gradient, i.e. C1− C2) and cross-sectional area (A) and inversely proportional to the distance to be travelled, i.e. thickness of the membrane (T).Thus, J = D A(C1− C2)T , where D is the diffusion coefficient.
The diffusion of molecules across the biological mem-
branes differs depending upon the lipid solubility, water
solubility, type of electrical charge and size of the mole-
cules. Further, selective permeability of thesemipermeable
cell membrane also affects the diffusion of different mole-
cules. How the different molecules diffuse across a cell
membrane are discussed below.
Simple diffusion of lipid soluble substances through
the cell membrane
The rate of diffusion through the lipid bilayer of the cell
membrane is directly proportional to the solubility of a sub-
stance in lipids. Therefore, molecules of substances like oxygen, nitrogen, carbon dioxide, alcohol, steroid hormones and weak organic acids and bases, being lipid soluble, diffuse very rapidly through the lipid bilayer of the cell membrane.
Simple diffusion of water and other lipid insoluble
molecules through the cell membrane
Astonishingly, water and other lipid insoluble substances
can also pass easily through the cell membrane. It has been shown that it is possible due to the presence of the so-called protein channels (made from transmembrane proteins) in the cell membrane.