 Dear students, you know that plasma membrane allows substances to pass through it. Plasma membrane ki iss property ko, ham membrane permeability kehte. However, plasma membrane is not permeable to every substance. The structural features of plasma membrane confer it with the property of selective permeability. Due to selective permeability ki vaja se, plasma membrane koch substances ko to aloh karte hai aur koch substances ko pass karna aloh nahi karte. Due to this property, plasma membrane regulates the movement of substances across it. This property is fundamental to the functioning of living cells and the maintenance of an intracellular physiological environment which is required for carrying out various enzymatically controlled metabolic reactions. Membrane permeability varies for various substances. Ye variation doh factors per depend karte hai ek type of the molecules aur dosri size of the molecules par. Karte ke lehaz se, ham discuss karenge ki kis type ke molecules ko plasma membrane easily aloh karte hai aur kis type ke molecules ko plasma membrane dis-aloh karte hai. So first of all, we shall discuss the lipid soluble non-polar substances. You know that plasma membrane has a continuous lipid bilayer. Due to this lipid bilayer, the lipid soluble substances can passively diffuse through the membrane easily by dissolving in the lipid molecules of bilayer. However, the rate of penetration depends on their lipid solubility and the size of these molecules. The more soluble substances will penetrate at more greater rate and similarly, smaller size substances will move comparatively rapidly. In comparison to the lipid soluble and non-polar substances, the polar substances and charged particles that is ions have difficulty in passing through because of the hydrophobic interior of the lipid bilayer that is highly impermeable to water soluble substances and charged substances. Similarly, the large size macromolecules, for example proteins, cannot pass through the membrane because of their size. However, you can understand that such non-diffusible substances are necessary for normal cell functions. So plasma membrane has evolved many structural as well as other mechanical systems which include the active and passive mechanisms and bulk transport mechanisms to allow such substances to pass through the membrane. Dear students, Membrane permeability ka concentration gradient ke saath bohot vaze taluk hai. Ab hum, concentration gradient aur membrane permeability ka mathematical relationship study karenke. The permeability of membrane means that the rate at which it penetrates the membrane passively. If a substance is diffusible and a continuous concentration gradient exists across the membrane, then the permeability of the membrane that is the amount of substance crossing through the membrane per unit time represented as delta QS over delta T is equal to the product of permeability constant of the substance and its concentration gradient present on the two sides of the membrane so that we can read this equation as delta QS over delta T is equal to P into concentration gradient C1 minus C2. This equation is applicable to simple diffusion only. Ye equation actively transport its substances aur electrolytes par applied nahi hoti. Kyunke electrolytes aqueous medium mein charred particles mein dissociate hojate hain aur charred particles simple diffusion se membrane ko cross nahi kar sakte. Dear students, The membrane permeability to a substance is not a fixed or constant entity. It can be altered. Certain substances for example hormones and neurotransmitters can alter the permeability of the membrane to certain substances. Hormones do so by reacting with the receptor sites and influencing the channel size and carrier mechanisms. For example, antidiuretic hormone can increase water permeability of renal collecting duct in mammals up to 10 times. Similarly neurotransmitters in nerve and muscle cells act on integral membrane proteins so that their permeability for ions increases.