Page 68 - Understanding NCERT Science 09

damaged, lysosomes may burst and the In plant cells vacuoles are full of cell sap
enzymes digest their own cell. Therefore, and provide turgidity and rigidity to the cell.
lysosomes are also known as the ‘suicide bags’ Many substances of importance in the life of
of a cell. the plant cell are stored in vacuoles. These
include amino acids, sugars, various organic
5.2.5 (iv) MITOCHONDRIA acids and some proteins. In single-celled
organisms like Amoeba, the food vacuole
Mitochondria are known as the powerhouses contains the food items that the Amoeba has
of the cell. Mitochondria have two membrane
consumed. In some unicellular organisms,
coverings. The outer membrane is porous specialised vacuoles also play important roles
while the inner membrane is deeply folded. in expelling excess water and some wastes
These folds increase surface area for ATP- from the cell.
generating chemical reactions. The energy
required for various chemical activities needed
for life is released by mitochondria in the form uestions
of ATP (Adenosine triphopshate) molecules.
ATP is known as the energy currency of the 1. Can you name the two organelles
cell. The body uses energy stored in ATP for Q we have studied that contain
making new chemical compounds and for their own genetic material?
mechanical work. 2. If the organisation of a cell is
Mitochondria are strange organelles in the destroyed due to some physical
sense that they have their own DNA and or chemical influence, what will
ribosomes. Therefore, mitochondria are able happen?
to make some of their own proteins. 3. Why are lysosomes known as

suicide bags?
5.2.5 (V) PLASTIDS
4. Where are proteins synthesised
Plastids are present only in plant cells. There inside the cell?
are two types of plastids – chromoplasts
(coloured plastids) and leucoplasts (white or Each cell thus acquires its structure and
colourless plastids). Chromoplasts containing ability to function because of the organisation
the pigment chlorophyll are known as of its membrane and organelles in specific
chloroplasts. Chloroplasts are important for ways. The cell thus has a basic structural
photosynthesis in plants. Chloroplasts also organisation. This helps the cells to perform
contain various yellow or orange pigments in functions like respiration, obtaining nutrition,
addition to chlorophyll. Leucoplasts are and clearing of waste material, or forming
primarily organelles in which materials such new proteins.
as starch, oils and protein granules are stored. Thus, the cell is the fundamental structural
The internal organisation of the Chloroplast unit of living organisms. It is also the basic
consists of numerous membrane layers functional unit of life.
embedded in a material called the stroma. These Cell Division
are similar to mitochondria in external
structure. Like the mitochondria, plastids also New cells are formed in organisms in order to
grow, to replace old, dead and injured cells,
have their own DNA and ribosomes.
and to form gametes required for
5.2.5 (vi) VACUOLES reproduction. The process by which new cells
are made is called cell division. There are two
Vacuoles are storage sacs for solid or liquid main types of cell division: mitosis
contents. Vacuoles are small sized in animal and meiosis.
cells while plant cells have very large vacuoles. The process of cell division by which most
The central vacuole of some plant cells may of the cells divide for growth is called mitosis.
occupy 50-90% of the cell volume. In this process, each cell called mother cell

THE FUNDAMENTAL UNIT OF LIFE 57

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