Page 69 - Understanding NCERT Science 09

Fig. 5.7: Mitosis Fig. 5.8: Meiosis

divides to form two identical daughter cells They divide by a different process called
(Fig. 5.7). The daughter cells have the same meiosis which involves two consecutive
number of chromosomes as mother cell. It divisions. When a cell divides by meiosis it
helps in growth and repair of tissues produces four new cells instead of just two (Fig.
in organisms. 5.8). The new cells only have half the number
Specific cells of reproductive organs or tissues of chromosomes than that of the mother cells.
in animals and plants divide to form gametes, Can you think as to why the chromosome
which after fertilisation give rise to offspring. number has reduced to half in daughter cells?

What

you have

learnt

• The fundamental organisational unit of life is the cell.
• Cells are enclosed by a plasma membrane composed of lipids
and proteins.
• The cell membrane is an active part of the cell. It regulates
the movement of materials between the ordered interior of
the cell and the outer environment.
• In plant cells, a cell wall composed mainly of cellulose is
located outside the cell membrane.
• The presence of the cell wall enables the cells of plants,
fungi and bacteria to exist in hypotonic media without
bursting.
• The nucleus in eukaryotes is separated from the cytoplasm
by double-layered membrane and it directs the life processes
of the cell.
• The ER functions both as a passageway for intracellular
transport and as a manufacturing surface.
• The Golgi apparatus consists of stacks of membrane-bound
vesicles that function in the storage, modification and
packaging of substances manufactured in the cell.
• Most plant cells have large membranous organelles called
plastids, which are of two types—chromoplasts and
leucoplasts.

58 SCIENCE

Rationalised 2023-24