Plasma Membrane or Cell Membrane or Plasmalemma

All living cells have a plasma membrane that encloses their contents. It is flexible and made up of organic molecules called lipids, proteins and small fraction of carbohydrates.

The average thickness of plasma membrane is 75-100Å. Plasma membrane is flexible and is made up of a bilayer of lipids (called phospholipids) and proteins. This structure was explained by Singer and Nicolson in the fluid mosaic model of plasma membrane.

Flexibility enables the cell to engulf in food and other materials (endocytosis). e.g. Amoeba. It serves as a selectively permeable barrier to the outside environment. The plasma membrane is permeable to specific molecules, however, it allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules such as oxygen, carbon dioxide are able to pass freely across the membrane by diffusion but the passage of larger molecules, such as amino acids and sugars, is carefully regulated. Carbohydrates are attached to the extra cellular face of the plasma membrane in the form of glycoproteins attached to intrinsic or peripheral proteins.

1.0Plasma Membrane Performs The Following Functions

(i) It protects the internal components of the cell.

(ii) It helps to maintain the shape of the cell.

(iii) It allows materials to enter and leave the cell through the tiny holes called pores.

2.0Transport Across Plasma Membrane

In all living cells, substances may pass across the membrane with or without expenditure of energy. So there are two types of transport:

Active Transport 

Transport of substances across plasma membrane against the concentration gradient with expenditure of energy is called active transport. 

Passive Transport

Transport of substances across plasma membrane along the concentration gradient i.e. from higher concentration to lower concentration without expenditure of energy is called passive transport.

Types of passive transport:

Passive transport is of two types-

 Diffusion

The process of movement of substances (solid, liquid & gas) from the region of its higher concentration to the region of its lower concentration to spread uniformly in the given space is called diffusion.

Diffusion Across Cell Membrane

Metabolic gases (CO₂ and O₂) move out and into the cells through diffusion. Respiration in cell produces carbon dioxide. As the concentration of CO₂ (which is cellular waste) increases inside the cell as compared to the outside, CO₂ diffuses out of the cell into the external medium. Similarly, concentration of oxygen is always higher in the external medium as compared to the cell where it is being consumed in respiration. Therefore, oxygen diffuses from outside to the inside of cell.

Osmosis

Osmosis is the net diffusion of water across a selectively permeable membrane towards a higher solute concentration.

Example of osmosis: 

(i) Absorption of water by plant roots.

(ii) Absorption of water by unicellular fresh water organisms.

Types of osmosis

Osmosis is of two types, endosmosis and exosmosis. Endosmosis is the osmotic entry of water into a cell.  Exosmosis is the osmotic withdrawal of water from a cell or system.

3.0Cell Placed in Solution

Plant and animal cells placed in salt or sugar solution will behave in one of the following ways depending upon the concentration of external solution.

Hypotonic Solution

The external solution is dilute as compared to cell contents. It has more water content while the water content is lower inside the cell. The cell membrane allows passage of water in both directions. Due to difference in concentration of water molecules, there is net flow of water molecules into the cells. The phenomenon is called endosmosis.

Isotonic Solution

The external solution has the same concentration as that of the cell contents. Therefore, water content is equal on both sides. The amount of water going inside the cell is equal to the amount of water coming out of the cell. Therefore, there is no net movement of water. The cell size will remain the same.

Hypertonic Solution

The external solution has more concentration as compared to that of cell. There is less concentration of water in the external solution than inside the cell. Since more water is present inside the cell, more of it will pass out. The phenomenon is called exosmosis.

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