NCERT Solutions for Class 11 Biology Chapter 8 Cell: The Unit of Life

NCERT
SOLUTIONS
CLASS - 11th

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Class : 11th
Subject : Biology
Chapter : 8
Chapter Name : The Unit of Life

Q1 Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.

Answer. (a) Robert Brown did not discover the cell. The cell was discovered by Robert Hooke.

Q2 New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials

Answer. (c) According to the biogenic theory, new cells can only arise from pre-existing cells. Only
complete cells, in favourable conditions, can give rise to new cells.

Q3 Match the following

Answer.

Q4 Which of the following is correct:
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well de ned cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.

Answer. (c) Membrane-bound organelles are organelles surrounded by a double membrane.
Nucleus, mitochondria, chloroplasts, etc., are examples of such organelles. These cell organelles
are absent from prokaryotes.
(a) Only eukaryotic cells have nuclei. They are absent from prokaryotes.
(b) Cell walls are only present in plant cells. They are absent from all animal cells.
(d) All cells arise from pre-existing cells.

Q5 What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

Answer. Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the
invagination of the plasma membrane. Its functions are as follows:
(1) These extensions help in the synthesis of the cell wall, replication of DNA. They also help in
the equal distribution of chromosomes into the daughter cells.
(2) It also increases the surface area of the plasma membrane to carry out various enzymatic
activities.
(3) It helps in secretion processes as well as in bacterial respiration.

Q6 How do neutral solutes move across the plasma membrane? Can the polar molecules also move
across it in the same way? If not, then how are these transported across the membrane?

Answer. Plasma membrane is the outermost covering of the cell that separates it from the
environment. It regulates the movement of substances into the cell and out from it. It allows the
entry of only some substances and prevents the movement of other materials. Hence, the
membrane is selectively-permeable.
Movement of neutral solutes across the cell membrane — Neutral molecules move across the
plasma membrane by simple passive diffusion. Diffusion is the movement of molecules from a
region of higher concentration to a region of lower concentration. Movement of polar molecules
across the cell membrane — The cell membrane is made up of a phospholipid bilayer and proteins.
The movement of polar molecules across the non-polar lipid bilayer requires carrier-proteins.
Carrier-proteins are integral protein particles having certain af nity for speci c solutes. As a
result, they facilitate the transport of molecules across the membrane.

Q7 Name two cell-organelles that are double membrane bound. What are the characteristics of

these two organelles? State their functions and draw labelled diagrams of both.

Answer. Mitochondria and chloroplasts are the two organelles that are double-membrane-bound.

Characteristics of the mitochondria:-
Mitochondria are double- membrane-bound structures. The membrane of a mitochondrion is
divided into the inner and outer membranes, distinctly divided into two aqueous compartments —
outer and inner compartments. The outer membrane is very porous (containing the organelle),
while the inner membrane is deeply-folded. These folds are known as cristae. Cristae increase the
surface area inside the cell. They are the sites for ATP-generating chemical reactions. The
membrane of a mitochondrion contains speci c enzymes meant for speci c mitochondrial
functions. Hence, the mitochondria are the sites for aerobic respiration. They have their own DNA
and ribosomes. Thus, they are able to make their own proteins. This is why they are considered
Characteristics of chloroplasts Chloroplasts are double-membrane-bound structures.
They are divided into outer and inner membranes, further divided into two distinct regions:
(i) Grana are stacks of attened disks containing chlorophyll molecules. The attened
membranous sacs are called thylakoids. The thylakoids of adjacent grana are connected by
membranous tubules called stroma lamellae.
(ii) Stroma is a homogeneous mixture in which grana are embedded. It contains several enzymes
that are used for the synthesis of carbohydrates and proteins. It also contains its own DNA and
ribosomes.

Functions of the mitochondria:
(i) They are the sites for cellular respiration.
(ii) They provide energy in the form of ATP for all vital activities of living cells.
(iii) They have their own DNA and ribosomes. Hence, they are regarded as semi-autonomous
organelles.
(iv) They have several enzymes, intermediately required for the synthesis of various chemicals
such as fatty acids, steroids, and amino acids.
Functions of chloroplasts:
(i) They trap solar energy and utilise it for manufacturing food for plants. Hence, they are involved
in the process of photosynthesis.
(ii) They contain the enzymes required for the synthesis of carbohydrates and proteins.

Q8 What are the characteristics of prokaryotic cells?

Answer. Prokaryotic cell is a unicellular organism lacking membrane-bound organelles.
The characteristics of prokaryotic cells are as follows:
(i) Most of them are unicellular.
(ii) They are generally small in size. The size of a prokaryotic cell varies from 0.5 — 5
(iii) The nuclear region of a prokaryotic cell is poorly-de ned because of the absence of a nuclear
membrane. Hence, a prokaryotic cell lacks a true nucleus.
(iv) The genetic materials of prokaryotic cells are naked. They contain single, circular
chromosomes. In addition to the genomic DNA, they have a small, circular plasmid DNA.
(v) They have specialised membranous structures called mesosomes. Mesosomes are formed by
the invagination of the cell membrane. These extensions help in the synthesis of the cell wall,
replication of DNA. They also help in the equal distribution of chromosomes into the daughter
cells.
(vi) Membrane-bound cell organelles such as mitochondria, plastids, and endoplasmic reticulum
are absent from a prokaryotic cell.
(vii) Most prokaryotic cells contain a three-layered structure — outermost glycocalyx, middle cell
wall, and the innermost plasma membrane. This structure acts as a protective unit. Examples of
prokaryotic cells include blue green algae, bacteria, etc.

Q9 Multicellular organisms have division of labour. Explain?

Answer. Multicellular organisms are made up of millions and trillions of cells. All these cells
perform speci c functions. All the cells specialised for performing similar functions are grouped
together as tissues in the body. Hence, a particular function is carried out by a group of cells at a
de nite place in the body. Similarly, different functions are carried out by different groups of cells
in an organism. This is known as division of labour in multicellular organisms.

Q10 Cell is the basic unit of life. Discuss in brief.

Answer. Cells are the basic units of life capable of doing all the required biochemical processes
that a normal cell has to do in order to live. The basic needs for the survival of all living organisms
are the same. All living organisms need to respire, digest food for obtaining energy, and get rid of
metabolic wastes.
Cells are capable of performing all the metabolic functions of the body. Hence, cells are called the
functional units of life.

Q11 What are nuclear pores? State their function.

Answer. Nuclear pores are tiny holes present in the nuclear membrane of the nucleus. They are
formed by the fusion of two nuclear membranes.
These holes allow speci c substances to be transferred into a cell and out from it. They allow
molecules such as RNA and proteins to move in both directions, between the nucleus and the
cytoplasm.

Q12 Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their
functions. Comment.

Answer. Lysosomes are membrane-bound vesicular structures holding a variety of enzymes such
as lipases, proteases, and carbohydrases. The purpose of lysosomes is to digest worn out cells.
They are involved in the intracellular digestion of foreign food particles and microbes. Sometimes,
they also act as suicidal bags. They are involved in the self digestion of cells. They are a kind of
waste disposal systems of a cell. On the other hand, vacuoles are storage sacs found in cells.
They might store the waste products of cells. In unicellular organisms, the food vacuole contains
the consumed food particles. It also plays a role in expelling excess water and some wastes from
the cell.

Q13 Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus
(ii) Centrosome

Answer. (i) Nucleus
Nucleus controls all the cellular activities of the cell. It is spherical in shape. It is composed of the
following structures:
Nuclear membrane: It is a double membrane separating the contents of the nucleus from the
cytoplasm. The narrow space between the two membranes is called the perinuclear space. Nuclear
membrane has tiny holes called nuclear pores. These holes allow speci c substances to be
transferred into a cell and out from it.
Nucleoplasm/Nuclear matrix: It is a homogeneous granular uid present inside the nucleus. It
contains the nucleolus and chromatin. Nucleolus is a spherical structure that is not bound by any
membrane. It is rich in protein and RNA molecules, and is the site for ribosome formation.
Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic

(ii) Centrosome
Centrosome consists of two cylindrical structures called centrioles. Centrioles lie perpendicular to
each other. Each has a cartwheel-like organisation.
A centriole is made up of microtubule triplets that are evenly spaced in a ring. The adjacent
triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub
is connected to the triplets via radial spokes. These centrioles help in organising the spindle bres
and astral rays during cell division. They form the basal body of Cilia and agella.

Q14 What is a centromere? How does the position of centromere form the basis of classi cation of
chromosomes. Support your answer with a diagram showing the position of centromere on
different types of chromosomes.

Answer. Centromere is a constriction present on the chromosomes where the chromatids are held
together. Chromosomes are divided into four types based on the position of the centromere.
(i) Metacentric chromosome
The chromosomes in which the centromere is present in the middle and divides the chromosome
into two equal arms is known as a metacentric chromosome.

(ii) Sub-metacentric chromosome
The chromosome in which the centromere is slightly away from the middle region is known as a
sub-metacentric chromosome. In this, one arm is slightly longer than the other.

(iii) Acrocentric chromosome
The chromosome in which the centromere is located close to one of the terminal ends is known as

an acrocentric chromosome. In this, one arm is extremely long and the other is extremely short.

(iv) Telocentric chromosome
The chromosome in which the centromere is located at one of the terminal ends is known as a
telocentric chromosome.