NCERT solution of class 9 The fundamental unit of life
Page
no.59
Question
1: Who
discovered cells and how?
Answer
1: Cells
were discovered in 1665 by an English Botanist, Robert Hooke. He used a
primitive
microscope to observe cells in a cork slice.
Question
2: Why
is the cell called the structural and functional unit of life?
Answer
2: Cells
constitute various components of plants and animals. A cell is the smallest
unit
of life and is capable of all living functions. Cells are the building blocks
of
life.
This is the reason why cells are referred to as the basic structural and
functional
units of life. All cells vary in their shape, size, and activity they
perform.
In fact, the shape and size of the cell is related to the specific functions
they
perform.
Page
no. 65
Question
1: Can
you name the two organelles we have studied that contain their own genetic
material?
Answer
1: Mitochondria
and plastids are the two organelles that contain their own genetic material.
Both these organelles have their own DNA and ribosomes.
Question
2: If
the organisation of a cell is destroyed due to some physical or chemical
influence, what will happen?
Answer
2: Cell
is the smallest unit of life, which is capable of all living functions. If the
organisation of a cell is destroyed due to some physical or chemical influence,
then the ability of the cell to perform all living functions such as
respiration, nutrition, excretion, etc. would be affected.
Question
3: Why
are lysosomes known as suicide bags?
Answer
3: Lysosomes
are membrane-bound vesicular structures that contain powerful digestive
enzymes. These enzymes are capable of breaking down any foreign food particle
or microbes entering the cell. Sometimes, lysosomes can cause self-destruction
of a cell by releasing these digestive enzymes within the cells. Hence, they
are also known as 'suicidal bags'.
Question
4: Where
are proteins synthesized inside the cell?
Answer
4: Ribosomes
are the site for protein synthesis. Ribosomes are very small structures found
either in a free state, suspended in the cytoplasm, or attached to the surface
of the endoplasmic reticulum. They are composed of ribonucleic acids and
proteins.
Page
no. 61
Question
1: How
do substances like CO2and water move in and out of the cell? Discuss,
Answer
1: The
cell membrane is selectively permeable and regulates the movement of
substances
in and out of the cell.
Movement
of CO2:
CO2
is produced during cellular respiration. Therefore, it is present in high
concentrations
inside the cell. This CO2 must be excreted out of the cell. In the
cell's
external environment, the concentration of CO2 is low as compared to that
inside
the cell. Therefore, according to the principle of diffusion, CO2 moves from
a
region of higher concentration (inside the cell) towards a region of lower
concentration
(outside the cell). Similarly, O2 enters the cell by the process of
diffusion
when the concentration of O2 inside the cell is low as compared to its
surroundings.
Movement
of water:
Water
moves from a region of high concentration to a region of low concentration
through
the plasma membrane. The plasma membrane acts as a semi-permeable
membrane,
and this movement of water is known as osmosis. However, the
movement
of water across the plasma membrane of the cell is affected by the
amount
of substance dissolved in water.
Question
2: Why
is the plasma membrane called a selectively permeable membrane?
Answer
2: The
cell membrane or the plasma membrane is known as a selectively permeable membrane
because it regulates the movement of substances in and out of the cell.This
means that the plasma membrane allows the entry of only some substances
and
prevents the movement of some other materials.
Exercise
Question
1: Make
a comparison to write down ways in which plant cells are different from animal
cells
Question
3: What
would happen if the plasma membrane ruptures or breaks down?
Answer
3: Plasma
membrane is a selectively permeable membrane of the cell that maintains its
homeostasis, i.e., constant internal composition of the cell. If it ruptures or
breaks down the constant internal chemical composition of the cell will be lost
and it will not be able to perform its basic functions. Such a cell with
ruptured plasma membrane is killed.
Question
4: What
would happen to the life of a cell if there is no Golgi apparatus?
Answer
4: The
materials synthesized in the ER are stored, sorted, modified, packaged and
dispatched to various targets inside and outside the cell through the Golgi
apparatus packs products in vesicles, the secretary vesicles. In some cases
complex sugars e.g. cellulose, may be made from simple sugars in Golgi
apparatus. The Golgi apparatus is also involved in the formation of the cells
which will not be possible if Golgi apparatus is not there.
Question 5: Which
organelle is known as the powerhouse of the cell? Why?
Answer 5: Mitochondria are known as the powerhouse of the cell
because they contain enzymes that are needed for stepwise oxidation of food
stuffs (carbohydrate, fats and lipids) present in the cells to CO2 and water.
Oxidation of food releases energy which is used to form high-energy ATP (adenosine triphosphate) molecules.
ATP is known as Energy Currency of the cell and it is used as cellular fuel.
Energy stored in ATP is used to bring about energy requiring activities of the
cell such as photosynthesis, protein synthesis and muscle contraction.
Question 6: Where
do the lipids and proteins constituting the cell membrane get synthesised?
Answer 6:
Rough
Endoplasmic Reticulum (RER) - synthesizes proteins constituting cell membrane.
Smooth
Endoplasmic Reticulum (SER) - synthesizes lipids constituting cell membrane.
Question 7: How
does Amoeba obtain its food?
Answer 7: Amoeba
has flexible cell membrane. It enables amoeba to engulf in food by the process
called endocytosis.
Question 8: What
is osmosis?
Answer 8: The
diffusion of water or solvent through a semi-permeable membrane from a solution
of lower concentration of solutes to a solution of higher concentration of
solutes, to which the membrane is relatively impermeable, is called osmosis.
