NCERT Solutions For Class 9 Science Chapter 5
The Fundamental Unit of Life

Osmosis can be defined as the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane.

Amoeba acquires its food by the process of endocytosis. Amoeba captures food from its surroundings through its plasma membrane. Being flexible in nature, plasma membrane enables the cell to engulf food particles from the surrounding medium by extending itself into the pseudopodia.  
 

Lipids and proteins constituting the cell membrane are synthesised in the endoplasmic reticulum. Lipids are synthesised in smooth endoplasmic reticulum. Ribosomes present on the rough endoplasmic reticulum serve as the site for protein synthesis.

Mitochondria are known as the powerhouse of the cell. During cellular respiration, mitochondria produce energy in the form of ATP that is required to perform various activities in a cell. ATP (Adenosine triphosphate) is known as the energy currency of the cell. Due to this, mitochondria are known as the powerhouse of the cell.  

In the absence of the Golgi apparatus, following activities will not take place in a cell:

1. Functions like storage, modification and packaging of products synthesised in the endoplasmic reticulum will not take place in a cell as all these functions are performed by the Golgi apparatus.
2. Formation of complex sugars from simple sugars will be hampered in the absence of Golgi apparatus. 
3. Cleaning of the cell will be disturbed due to the absence of Golgi apparatus as it is involved in the formation of lysosomes that acts as a waste disposal system of a cell.

Rupturing or breakdown of plasma membrane would not allow the cell to exchange substances from its surrounding through diffusion or osmosis. The selectively permeable nature of the plasma membrane will be disturbed as the cytoplasm of the cell would come in direct contact of the surrounding medium and the act of selection would stop. In case of severe rupturing of plasma membrane, the contents of the cell will leak out and would eventually cause the death of the cell.

The type of cell division which is required for the growth and repair of the body is mitosis. Mitosis maintains the same chromosome number in parent and daughter cells. Thus the daughter cells retain the characters of the parent cell. This results in the growth of tissues and the overall development of the organism. With the help of mitosis, tissues are repaired as the worn out cells are replaced.

For the formation of gametes, meiotic division is required. It is also called reductional division. In this type of cell division, the chromosome number is reduced to half. Thus, this cell division occurs in diploid reproductive cells to form haploid cells that are gametes (sperm and egg).

(i) Water gathers in the hallowed portions of set-up B and C due to osmosis. In the cup of the potato B, sugar is kept and in the potato C, salt is kept. Inside the cups of the potatoes, the concentration of water is low as compared to the cells making the hallowed portion. Hence, water from its high concentration (from the potato cell) moves to the low concentration (out of the cell and in the hallowed portion) through the plasma membrane of the cells. Due to the movement of water from the potato cells to the hallowed portion by osmosis, water gathers in the hallowed portions of the potato cup.

(ii) In this experiment, potato A acts as control set-up. A control set-up is run in an experiment to study the differences that take place in or on the object or group being experiment on. It also increases the reliability of the final result by comparing the observations of the experimental set-up and control set-up.

(iii) Water does not gather in the hallowed portions of potato A and D because potato A is kept empty and is a control set-up in the experiment. Water does not enter the potato D as osmosis did not take place in it because the potato used here was boiled.  Boiling denatured the proteins present in the plasma membrane of the cells of the potato and thus, disrupted the cell membrane. A selectively permeable membrane is required for osmosis to occur which is disrupted in this case. Therefore, osmosis does not occur. Hence, water does not enter the potato D.