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Outer membrane of Cell – Cell membrane / Plasma membrane

 Outer membrane of Cell

[A] Cell membrane / Plasma membrane

This is the outer covering of the animal and plant cells which encloses the mass of cytoplasm and gives separate identity to the cell.

(i) Selectively permeable so allow only selected substances to pass in (water, salt, oxygen) and out (carbon dioxide, waste products) of the cell and to remain in the cell (enzymes, protein).
(ii) It is very thin, elastic and living membrane.
(iii) It possesses fine pores.
(iv) It is made of lipid and protein – a lipid bilayer (2 molecules thick layer) having integrated protein molecules. Therefore, it is also known as ‘number of protein icebergs floating in the sea of lipids’.

(i) Gives boundary to the cytoplasm (shape of the cell) and also provides mechanical support.
(ii) Due to selectively permeable nature, it regulates in (e.g. oxygen, water, salts) and out (CO2, waste materials) of certain solutes and ions and homeostasis.

Now, we will discuss how the movement of substances takes place in and out of a cell. Substances move in and out of cells through certain activities such as osmosis, diffusion, endocytosis and active transportation.

(a) Diffusion

It is the spontaneous movement of molecules from a region of higher concentration to a region of lower concentration.

  • Smell of perfume – If you have a bottle of perfume and take the cap off. The perfume particles will waft out and diffuse into the air where there is a lower concentration of them.
  • Other examples are bread baking, food coloring in water, sugar in tea, ink in water etc.

Factors that affect the rate of diffusion

  1. Concentration difference between the inside and outside of the cell
    The bigger the difference between the concentrations, the diffusion will be faster.
    For example – cellular waste CO2 gas accumulates in high concentration inside the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.
  2. Size of the chemical substance Oxygen molecule (O2) has two atoms. Protein is massive. Oxygen diffuses easily across a cell membrane whereas proteins don’t move at all.
  3. Temperature – At higher temperature, molecules move faster.
  4. Whether the chemical substance is water- soluble or lipid- soluble. The lipid soluble goes through faster because the cell membrane is phospholipids and can easily diffuse through a fatty membrane.


(B) Osmosis 

It is the movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration. Osmosis is simply the diffusion of water. Any time water flows in or out, it is called osmosis. The water flows to wherever there is more solute.

  1. Osmosis is a special type of diffusion that does not require energy.
  2. Osmosis is passive.
  3. Osmosis takes place through a partially permeable (or semipermeable) membrane. It means that the water molecules move from a place of their high concentration to a place of their low concentration through a membrane with pores in it.

(a) Isotonic Solution

  • The cytoplasm of a cell is 80% water and 20% solute. Anything that is not water are called solutes, such as electrolytes. If we surround a cell, for e.g. Red Blood Corpuscles (R.B.C.) with a fluid of same composition, that is 80% water and 20% “other stuff” on the outside of the cell. The “other stuff” may not be the same things as the inside of the cell. In osmosis we only consider the concentration of the water molecules. As the concentration of solutions outside and inside the cell are exactly same, there will be no net movement across the cell membrane. Such solution surrounding the cell is called isotonic (Iso– means same and –tonic means tension).
  • In an isotonic solution, the cell maintains the same size.
    – Water crosses the cell membrane in both directions.
    The amount of water going in the cell = The amount of water going out of the cell. 

(b) Hypotonic Solution

  • In the second condition, R.B.C. cell is surrounded with 100% distilled water or a very dilute solution (a higher water concentration solution). Osmosis will occur because water will flow inside the cell. The cell will swell up. Such a solution is known as a hypotonic solution because it is low in solute.
  • In a hypotonic solution, the cell swells up or turbid.
    – Water crosses the cell membrane in both directions.
    The amount of water going in the cell > The amount of water going out of the cell.

(c) Hypertonic Solution

  • In the third condition, we place the R.B.C. cell in a very concentrated salt solution with 20% water only or in a low water concentration solution. Osmosis will occur because water will flow out of the cell causing it to shrink. This is called crenation. Such a solution is known as a hypertonic solution because it has high concentration of solute.
  • In a hypertonic solution, the cell shrinks.
    – Water crosses the cell membrane in both directions.
    The amount of water going in the cell < The amount of water going out of the cell.



(C) Active Transport

A kind of transport where ions or molecules move against a concentration gradient, which means movement in the direction opposite that of diffusion or movement from an area of lower concentration to an area of higher concentration. Hence, this process will require expenditure of energy and is called active transport.

  • Mnemonic – When you place a cell in a hypotonic solution, it swells up like a hippo. 
  • In a hypotonic solution, the cell swells up until it ruptures and when it pops it is called -lysis. When a red blood cell pops it is called hemolysis. Thus, it would not be a good idea to inject distilled water into the blood.
  • Remember, the water flows wherever there is more solute, like –
    (a) When someone puts salt on a snail, the snail dies because the water goes to the salt and the snail gets dehydrated (its cells crenate due to the hypertonic solution).
    (b) When a de-shelled egg / dry raisins is placed in pure water, it swells up or turbid because the water goes into it.
    (c) When a de-shelled egg / dry raisins is placed in a concentrated solution, it shrinks because the water passes out of the egg.
  • Homeostasis (homeo = unchanging + stasis = standing) – The tendency of an organism or a cell to regulate a condition of equilibrium or stability within its internal environment when dealing with external changes.
    For example, in humans, homeostasis happens when the body regulates body temperature in an effort to maintain an internal temperature around 98.6 degrees Fahrenheit. For this, we sweat to cool off during the hot summer days, and we shiver to produce heat during the cold winter season.



(D) Endocytosis

It is a process of cellular ingestion by which the cell membrane folds inward to bring substances (food and other materials) into the cell from its external environment. This happens due to the flexible nature of cell membrane.

For e.g. Amoeba takes its food through endocytosis.


Read Next –

Outer membrane of Cell – Cell Wall


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