CELL, Std-9

Q. How do substances move across                 plasma membrane?

:- The movement of substances in cell take place through the following processes:

      1. Diffusion

      2. Osmosis

🌞 Diffusion

:- The process of movement of substance (solid, liquid and gas)from the region of its higher concentration to the region of its lower concentration so as to a spread uniformly in the given space is called diffusion.

:- Metabolic gases like CO2 and O2 move out and into the cell through diffusion.

🌞Osmosis

:- The diffusion of water or solvent across semipermeable membrane from a region of its higher concentration to the region of its lower concentration is called osmosis.

E.g. 1. Absorption of water by plant roots.

2. Absorption of water by unicellular freshwater organisms.

🌞Types of Osmosis

1. Endosmosis

:- The osmotic entry of water into a cell or 

    system is called endosmosis. 

2. Exosmosis

:- the osmotic withdrawal of water from a cell or system is called exosmosis. 

Q. What will happen when a cell placed into different types of solution?

:- When 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. 

i. Hypotonic Solution

:- The external solution is dilute as compared to the 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 direction.

:- Due to difference in concentration of water molecules come there is net flow of water molecules into the cell.

:- In this case endosmosis take place.

:- As a result cell gets swell up. 

ii. Isotonic Solution

:- The external solution has the same concentration as that of the cell contents.

:- Therefore, there is no net movement of water. In this case the cell size will remain the same. 

iii. Hypertonic Solution

:- The external solution has more osmotic concentration as compared to that of cell.

:- There is less concentration of water in the outside solution than inside the cell. Since more water is present inside the cell, more of it will pass out.

:- In this case exosmosis take place.

:- And as a result cell gets shrink.