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

Q. What would happen if plasma membrane ruptures or breaks down?

Ans. Plasma membrane is the basic external layer of any biological cell and it is a vital component of a biological cell. Plasma membrane acts as a selectively permeable membrane between cell component and the surroundings. It allows only selected materials to move inside and outside of the cell and maintains the interior and exterior environment of the cell. It maintains homeostasis, i.e., constant internal temperature of the cell. Due to the rupture or breakdown of plasma membrane, the exposure of cell components to the external environment would occur and ultimately the death of cell would occur.

Q. Which cell organelle controls most of the activities of the cell?

Ans. Nucleus controls most of activities of the cell. The nucleus is a large, centrally located, spherical cellular component. It is bounded by two nuclear membranes, both forming a nuclear envelope. Nuclear envelope encloses a space between two nuclear membranes and is connected to a system of membranes called the Endoplasmic Reticulum. It encloses nucleolus, nuclear pore and nucleoplasm.


Q. How does Amoeba obtain its food?

Ans. Amoeba obtains its food by the process of phagocytosis. It means ‘cell eating’ and is the common method of feeding among the Protozoa. It is also the way in which white blood cells (leucocytes) engulf cellular debris and uninvited microbes (viruses and bacteria) in the blood. In the phagocytosis, an area of the plasma membrane, coated initially with actin-myosin, forms a pocket and engulfs the solid material (e.g., bacteria, debris). The membrane-enclosed vesicles and phagosomes, then detach from the cell surface (plasma membrane) into the cytoplasm where its contents are digested by lysosomal enzymes.


Q. Make a comparison and write down ways in which plant cells are different from animal cell.


Plant cells Animals Cells
These cells are generally large in size with definite shape. These cells are small in size and irregular.
The cell is surrounded by the rigid cell wall along with the plasma membrane. The cell is enclosed by the flexible thin plasma membrane cell wall is absent.
Plastids are present. Plastids are absent.
The cytoplasm is not so dense. A large vacuole is present inside the cell. Cytoplasm is denser and more granular and almost fills the entire cell.
Centrosomes are absent. Centrosome is present.
Golgi apparatus is simple and has small units called as dictyosomes. Golgi apparatus is highly complex and prominent

Q. A person takes concentrated solution of salt, after sometime, he starts vomiting. What is the phenomenon responsible for such situation? Explain.

Ans. Concentr ated salt solution is a hypertonic solution (water concentration lower as compared to the cell) therefore, it causes exosmosis. The outward movement of water from the cell results in irritation and excessive dehydration.

Q. How are chromatin, chromatid and chromosomes related to each other?

Ans. Inside the nucleoplasm, there is tangled mass of thread-like structures called chromatin. They are formed of an acid called Deoxyribonucleic Acid (DNA) and proteins. When a cell starts to divide, the tangled mass of chromatin condenses into long threads and finally, rod-like bodies called chromosomes. The chromosomes contain stretches of DNA which carry information for protein synthesis. Agene is called the hereditary unit and DNA is called the hereditary material.
A chromat id is one copy of duplicated chromosome which is generally joined to the other copy by a centromere.

Q. State in brief what happens when: 
(i) Dry apricots are left for sometime in pure water and later transferred to sugar solution.
(ii) Ared blood cell is kept in concentrated saline solution.
(iii) The plasma membrane of a cell breaks down.
(iv) Rhoea leaves are boiled in water first and then a drop of sugar syrup is put on it.
(v) Golgi apparatus is removed from the cell.

Ans. (i) The apricots swell due to endosmosis initially and when transferred to sugar solution it shrink again due to exosmosis.
(ii) RBCs shrink due to exosmosis.
(iii) It would lead to scattering of cell organelles and there will be no functioning of the organs.
(iv) There will be no change in cell shape or size because the cells are dead due to boiling.
(v) Function of golgi apparatus is packing, storing and transfer of protein. It would affect the functioning of cell.

Q. How is a prokaryotic cell different from a eukaryotic cell?


Prokaryotic Cell Eukaryotic Cell
Size of a cell is generally smaller (1-10 mm). Size of a cell is generally larger (5-100 mm).
Nucleus is absent (Nuclear region or nucleoid is not surrounded by a nuclear membrane). Nucleus is present (Nuclear material is surrounded by a nuclear membrane).
It contains single chromosome. It contains more than one chromosome.
Nucleolus is absent. Nucleolus is present.
Membrane-bound cell organelles are absent. Membrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, etc., are present.
Cell division takes place by fission or budding (no mitosis). Cell division occurs by mitotic or meiotic cell division.

Q. Draw a well labelled diagram of the animal cell.


well labelled