NCERT Class 9 Science Chapter 5 – The Fundamental Unit of Life

In 1665, Robert Hooke discovered cells while examining a thin slice of cork through a self-designed microscope. He observed that the cork resembled the structure of a honeycomb consisting of numerous tiny compartments. The minuscule boxes are referred to as cells.

Cells form the structure of an entity. A group of cells form a tissue, further an organ and ultimately an organ system. They perform fundamental functions and life processes such as respiration, digestion, excretion etc., in both unicellular and multicellular entities. They perform all the activities independently. Hence, cells are referred to as structural and fundamental units of life.

CO2 moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO2 in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO2 to diffuse from a region of higher (within the cell) to a lower concentration.

H2O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.

The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.

The two organelles which have their own genetic material are

i)  Mitochondria

ii) Plastids

In the event of any damage to cells and when the revival of cells is not possible, Lysosomes burst, and enzymes digest such cells. This is why lysosomes are often referred to as ‘suicide bags’.

When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

If the plasma membrane ruptures or breaks down, then molecules of some substances will freely move in and out of the cells. As the plasma membrane acts as a mechanical barrier, the exchange of material from its surroundings through osmosis or diffusion in a cell won’t take place. Consequently, the cell would die due to the disappearance of the protoplasmic material.

The Golgi apparatus consists of stacks of membrane-bound vesicles whose functions are as follows:

• Storage of substances

• Packaging of substances

• Manufacture of substances

Without the Golgi apparatus, the cells will be disabled from packing and dispatching materials that were produced by the cells. The Golgi apparatus is also involved in the formation of cells. Hence, in the absence of the Golgi apparatus, cells will not be produced.

Mitochondria are known as the powerhouse of the cell. It is because it releases the energy required for different activities of life. Mitochondria releases energy in the form of ATP (Adenosine triphosphate) molecules, essential for numerous chemical activities of life. Hence, ATP is often referred to as the ‘energy currency of the cell’.

Lipids and proteins are synthesised in the ER (Endoplasmic Reticulum).

Through the process of endocytosis, an Amoeba obtains its food. As its cell membrane is flexible enough, food particles are engulfed, forming a food vacuole girdling it, which is assisted by the pseudopodia. Amoeba secretes digestive enzymes to bring about digestion of the engulfed particle once the food is trapped.

The process of movement of a water molecule from a region of higher concentration to a region of lower concentration through a semipermeable membrane is known as osmosis.

There are two ways in which a cell divides:

• Mitosis

• Meiosis

Mitosis is the type of cell division that is involved in the growth and repair of the body, whereas meiosis is a type of cell division which results in the formation of gametes.

The following table depicts the differences between plant cells and animal cells.

The following are the differences between prokaryotic and eukaryotic cells.

(i) Water accumulates in the hollowed portions of B and C as a difference in the water concentration

is observed. Thereby, endosmosis occurs as the cells act as a semipermeable membrane.

(ii) Potato A is essential in this experiment as it is significant to compare different scenarios seen in

potato cups B, C and D. Potato A in this experiment clearly shows that the potato cavity on its

own cannot bring about water movement.

(iii) Cup in A does not show any change in the water flow concentration for osmosis to occur, which

requires concentration to be higher than the other. Cells in cup D are dead; thus, there is no

existence of a semipermeable membrane for water flow. Consequently, osmosis does not occur.