NCERT Exemplar Class 11 Biology Solutions Chapter 8 Cell The Unit Of Life

Multiple Choice Questions:

Question:1

A common characteristic feature of plant sieve tube cells and most of mammalian erythrocytes
a) Absence of mitochondria
b) Presence of cell wall
c) Presence of haemoglobin
d) Absence of nucleus

Answer:

Ans. The answer is the option (d) Absence of nucleus
Explanation: Sieve tubes are one of the components of phloem, and they do not contain a nucleus. In a similar manner, erythrocytes in mammalian cells also do not contain a nucleus.

Question:2

Select one which is not true for ribosome
a) Made of two subunits
b) Form polysome
c) May attach to mRNA
d) Have no role in protein synthesis

Answer:

Ans. The answer is the option (d) Have no role in protein synthesis

Explanation: Ribosome are known to be the protein factory, and they have an especially important role in the photosynthesis of plants. Other statements are correct.

Question:3

Which one of these is not a eukaryote?
a) Euglena
b) Anabaena
c) Spirogyra
d) Agaricus

Answer:

Ans. The answer is the option (b) Anabaena
Explanation: Anabaena belongs to the Monera kingdom, which has a unicellular prokaryotic arrangement. Others show eukaryotic features.

Question:4

Which of the following stains is not used for staining chromosomes?
a) Basic Fuchsin
b) Safranin
c) Methylene blue
d) Carmine

Answer:

Ans. The answer is the option (b) Safranin
Explanation: Safranin is used as a counterstain for endospore staining and gram staining. It can also be used for staining cartilage and mast cell granule. Other stains are used for staining chromosomes and nuclei.

Question:5

Different cells have different sizes. Arrange the following cells in ascending order of their size. Choose the correct option among the followings: -
a) Mycoplasma
b) Ostrich eggs
c) Human RBC
d) Bacteria
Options:
(a) (i), (iv), (iii) & (ii)
(b) (i), (iii), (iv) & (ii)
(c) (ii), (i), (iii) & (iv)
(d) (iii), (ii), (i) & (iv)

Answer:

Ans. The answer is the option (a) (i), (iv), (iii) & (ii)
Explanation: In ascending order, Mycoplasma is 0.3 micron in length and therefore the smallest. Ostrich egg is the largest living cell and is huge in size. Bacteria are 3-5 micron in length while RBCs are 7 microns in length.

Question:6

Which of the following features is common to prokaryotes and many eukaryotes?
a) Chromatin material present
b) Cell wall present
c) Nuclear membrane present
d) Membrane-bound sub-cellular organelles present

Answer:

Ans. The answer is the option (a) Chromatin material present
Explanation: Option (c) and option (d) are absent in prokaryotes and option (b) is absent in eukaryotes.
So, option (a), chromatin material is common to both prokaryotes and eukaryotes.

Question:7

Who proposed the fluid mosaic model of the plasma membrane?
a) Camillo Golgi
b) Schleiden and Schwann
c) Singer and Nicolson
d) Robert Brown

Answer:

The answer is the option (c), Singer and Nicolson
Explanation: The Golgi Complex was discovered by Camillo Golgi, cell theory was proposed by Schleiden and Schwann, and Robert Brown was the first who attempted to describe the nucleus.

Question:8

Which of the following statements is true for a secretory cell?
a) Golgi apparatus is absent
b) Rough Endoplasmic Reticulum (RER) is easily observed in the cell
c) Only Smooth Endoplasmic Reticulum (SER) is present
d) Secretory granules are formed in the nucleus.

Answer:

The answer is the option (b) Rough Endoplasmic Reticulum (RER) is easily observed in the cell
Explanation: RER is easily observed in the cells that are involved in secretion and protein synthesis. Other options are incorrect as Golgi apparatus is not present in a secretory cell,SER is present with RER in a secretory cell, and no secretory granules are formed in the nucleus

Question:9

What is a tonoplast?
a) Outer membrane of mitochondria
b) Inner membrane of chloroplast
c) Membrane boundary of the vacuole of plant cells
d) Cell membrane of a plant cell

Answer:

The answer is the option (c) Membrane boundary of the vacuole of plant cells
Explanation: In a plant cell, the vacuole is bound by a single membrane boundary called tonoplast.

Question:10

Which of the following is not true of a eukaryotic cell?
a) Cell wall is made up of peptidoglycans
b) It has 80S type of ribosome present in the cytoplasm
c) Mitochondria contain circular DNA
d) Membrane bound organelles are present

Answer:

The answer is the option (a) Cell wall is made up of peptidoglycans
Explanation: Cellulose makes the cell walls in a eukaryotic cell.

Question:11

Which of the following statements is not true for plasma membrane?
a) It is present in both plant and animal cell
b) Lipid is present as a bilayer in it
c) Proteins are present integrated as well as loosely associated with the lipid bilayer
d) Carbohydrate is never found in it

Answer:

Ans. The answer is the option (d) Carbohydrate is never found in it
Explanation: On carrying out a biochemical investigation on the plasma membrane, it was found that the two biochemical components that is found in the membrane are protein and carbohydrates. Other statements are true.

Question:12

Plastid differs from mitochondria on the basis of one of the following features. Mark the right answer.
a) Presence of two layers of membrane
b) Presence of ribosome
c) Presence of thylakoids
d) Presence of DNA

Answer:

Ans. The answer is the option (c) Presence of thylakoids
Explanation: Thylakoids are the structural elements of chloroplast found in the plastids and not in mitochondria. Other options are common to both plastids and mitochondria.

Question:13

Which of the following is not a function of cytoskeleton in a cell?
a) Intracellular transport
b) Maintenance of cell shape and structure
c) Support of the organelle
d) Cell motility

Answer:

The answer is the option (a) Intracellular transport
Explanation: Cytoskeleton consists of microtubules and microfilaments that are responsible for intercellular movement and not intracellular.

Question:14

The stain used to visualize mitochondria is
a) Fast green
b) Safranin
c) Aceto carmine
d) Janus green

Answer:

The answer is the option (d) Janus green
Explanation: Janus green is a basic vital stain that is used to demonstrate mitochondria as the oxidation-reduction sites in the cell. When the stain reacts with oxygen, it changes its colour to blue.

Very Short Answer Type Questions:

Question:1

What is the significance of vacuole in a plant cell?

Answer:
Vacuole helps in maintaining the balance of cell fluids and the shape of the cell by maintaining its turgidity.

Question:2

What does ‘S’ refer in the 70S & an 80S ribosome?

Answer:

The letter ‘S’ in ribosomes refers to Svedberg’s unit, which is used to measure the sedimentation coefficient. In cells, the higher the sediment coefficient, heavier is the structure.

Question:3

Mention a single membrane bound organelle which is rich in hydrolytic enzymes.

Answer:

Lysosomes are rich in hydrolytic enzymes like hydrolase, proteases, carbohydrase, and lipases.

Question:4

What are gas vacuoles? State their functions?

Answer:

Ans. Gas vacuoles are air vacuoles that are freely permeable to gas. They are present only in prokaryotes. It stores metabolic gases and also helps prokaryotic organisms in regulating buoyancy.

Question:5

What is the function of a polysome?

Answer:

Ans. Polysome is made up of a cluster of ribosomes that are held together by a strand of messenger RNA (mRNA) in rosette or helical group. They facilitate the translation of genetic code and play a vital role in the formation of multiple copies of the same polypeptide.

Question:6

What is the feature of a metacentric chromosome?

Answer:

Ans. In the metacentric chromosome, the centromere lies in the middle of the chromosome, forming two equal arms of a chromosome.

Question:7

What is referred to as satellite chromosome?

Answer:

Chromosomes with secondary constrictions at certain Locations formed by chromatin thread are called satellite chromosomes.

Short Answer Type Questions:

Question:1

Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis.

Answer:

Nucleolus consists of DNA, RNA, and proteins. It is also called the ribosome factory. It receives and stores ribosomal proteins that are produced in the cytoplasm. These ribosomal proteins are the sites for protein synthesis in a cell.

Question:2

Explain the association of carbohydrate to the plasma membrane and its significance.

Answer:

Sugar residues attach with proteins or lipids in the plasma membrane, which can give rise to structures in short chains. They may be involved in cell recognition giving rise to different blood antigens, i.e. AB and O due to difference in the arrangement of carbohydrates on the surface of RNA. Cell surface differences that are related to the characteristic surface because of carbohydrate component also play a vital role in the specificity of cell action with bacteria, drugs, viruses, and hormones.

Question:3

Comment on the cartwheel structure of centriole.

Answer:

Ans. Centrosome consists of two cylindrical structures called centrioles. Both centrioles lie perpendicular to each other in which each appears like a cartwheel.

The centrioles are made up of nine peripheral fibrils of tubulin protein that are evenly spaced. The peripheral fibrils come in triplets, and all the adjacent triplets are linked to each other. The central portion of the proximal region of the centriole contains proteins and is called the hub. The hub is connected to the peripheral triplets by the help of radial spokes that are also proteinaceous.

Question:4

Briefly describe the cell theory.

Answer:

The Cell Theory was first proposed by Schleiden and Schwann in 1838-39. In their theory, they stated that (1) all living beings are products that are formed by one or more cells.(2) cells are the structural and functional units of life. A third statement was added to this theory by the works of Nagelli and Virchow. The cell theory was thus expanded to three statements –

(1) All living beings are products that are formed by one or more cells.
(2) Cells are the structural and functional units of life.
(3) new cells come from pre-existing cells.
In the modern cell theory, other points were also added. Some of them are-
(4) The basic composition is the same in all the cells.
(5) Energy flows occur in the cell.

Only viruses are an exception this Cell Theory as they are not made up of cells but of DNA and RNA materials.

Question:5

Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth

Answer:

Question:6

Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?

Answer:

Chemical composition of the plasma membrane can be summed up like 20-79% lipids, 20-70% proteins, 1-5% carbohydrates and 20% water.

Lipids are one of the major components that form the plasma membrane. The lipid molecules contain polar hydrophilic (water-loving), and non-polar hydrophobic (water resisting) ends. Hydrophilic part forms the head of the molecule and the hydrophobic part forms two tails of the molecule.

The tails are found near the centre of the membrane.

Question:7

What are plasmids? Describe their role in bacteria?

Answer:

The bacterial cell consists of genomic DNA as well as a small circular DNA. This circular DNA is a plasmid which is used to transfer information from one cell to another. The plasmid is used to track bacterial transformation with foreign DNA. It develops unique phenotype characteristics in the bacteria like the resistance to antibiotics. It also enables the bacterial cell to metabolize nutrients. It also facilitates the conjugation of bacteria. In modern experiments, plasmids are used to develop cloning vectors.

Question:8

What are histones? What are their functions?

Answer:

Long Answer Type Questions:

Question:1

What structural and functional attributes must a cell have to be called a living cell?

Answer:
Structural and functional attributes that a cell must have to be called a living cell are:

• Plasma Membrane: A cell should have a plasma membrane because, without it, there will be no container to hold the contents of the cell, and cells will not have any independent existence.

Functional Attributes of Plasma Membrane: The plasma membrane should not be fully permeable or impermeable; rather, it should be selectively permeable to allow the exchange of important materials between the cell and its environment.

• Protoplasm: Protoplasm is the living material in a cell which contains all other important organelles of a cell which is required for life to continue. If protoplasm dries up, the cell becomes dead just like the sclerenchyma cells.

• Endoplasmic Reticulum: Endoplasmic reticulum is the main site of protein and lipid synthesis and secretion. Without the endoplasmic reticulum, cells will not get the required material for its development and will die.

Mitochondria are not mandatory organelles for all living cells like Red Blood Cells do not contain mitochondria; still they are living cells. In the same way, prokaryotes also do not have mitochondria still they are living cells.

The nucleus is also not a mandatory requirement for all living cells. Prokaryotic cells do not have a nucleus.

Question:2

Briefly give the contributions of the following scientists in formulating the cell theory.
a) Rudolf Virchow
b) Schleiden and Schwann

Answer:
The Cell Theory was first proposed by Schleiden and Schwann in 1838-39. In their theory, they stated that
(1) All living beings are products that are formed by one or more cells.
(2) Cells are the structural and functional units of life.
A third statement was added to this theory by the works of Rudolf Virchow. The cell theory was thus expanded to three statements –

(1) All living beings are products that are formed by one or more cells.
(2) Cells are the structural and functional units of life.
(3) New cells come from pre-existing cells. The last statement was the contribution of Virchow who through his experimentation, proved that cells arise from the pre-existing cell and not through crystallization as was proposed by Schleiden and Schwann earlier.

All the three scientists helped in paving the way to formulate the modern cell theory. Main points of the modern version of cell theory are-
(1) All living beings are made from one or more cells.
(2) Cells are formed from the pre-existing cell.
(3) Cell is the functional and structural unit of life.
(4) Energy flows within cells.
(5) All cells have the same basic composition.

Question:3

Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.

Answer:

Yes, extra genomic DNA is present in prokaryotes as well as in eukaryotes.
Extra Genomic DNA in Prokaryotes: In prokaryotes, circular DNA is the plasmid which is used to transfer information from one cell to another. The plasmid is used to track bacterial transformation with foreign DNA. It develops unique phenotype characteristics in the bacteria like the resistance to antibiotics. It also enables the bacterial cell to metabolize nutrients. It also facilitates the conjugation of bacteria. In modern experiments, plasmids are used to develop cloning vectors.

Extra Genomic DNA in Eukaryotes: Extra genomic DNA is present in mitochondria and plastids of eukaryotes. Mitochondrial DNA makes up almost 20000-25000 genes of the total genes in the human body. It has instructions for enzymes secretion and action. Plastids are found in plants that help in its growth by trapping radiation and storing food and other important nutrients for the plant. The DNA in the plastids facilitate the synthesis of protein.Since DNA is present in mitochondria and plastids, they have self-replication abilities and hence, they are semi-autonomous organelles.

Question:4

Structure and function are co-relatable in living organisms. Can you justify this by taking the plasma membrane as an example?

Answer:

The statement is true.
A cell should have a plasma membrane because without it, there will be no container to hold the contents of the cell, and cells will not have any independent existence.The plasma membrane should not be fully permeable or impermeable; rather, it should be selectively permeable to allow the exchange of important materials between the cell and its environment. The lipid bilayer that makes the plasma membrane contains protein molecules which act as a channel for transportation of substances in the cell or out of the cell. The fluid layer of lipid allows sure that only those molecules are passed through the plasma membrane that can travel easily through osmosis.

Question:5

Eukaryotic cells have organelles which may
a) Not be bound by a membrane
b) Bound by a single membrane
c) Bound by a double membrane
d) Group the various subcellular organelles into these three categories.

Answer:
(a) Ribosome is the organelles which are not bound by a membrane. They occur freely in the cytoplasm. These can also be found attached to the surface of endoplasmic reticulum, giving it a rough appearance.Ribosomes are also found in the matrix of mitochondria and stroma of plastids.

(b) Lysosomes are the cell organelles that are bound by a single membrane. Lysosomes arethe tiny sac-like structure that is found in animal cells but not in prokaryotes. Lysosomes help in destroying and preventing harmful substances and foreign materials from entering the cells. Most of the organelles of the cells are bound by double membranes.

(c) Mitochondria, plastids, nucleus, endoplasmic reticulum are some examples of double-membrane cell organelles. These organelles serve important functions in the cell-like synthesis of proteins, storing materials, providing energy to the cells.

Question:6

The genomic content of the nucleus is constant for a given species whereas the extrachromosomal DNA is found to be variable among the members of a population. Explain.

Answer:

The genomic content of the nucleus is constant for a given species which imparts specific characters to the species. Each species has its own unique set of characters which is entirely different from any other species. DNA is the carrier of traits and genetic information from one generation to another. The chromosomes that carry the DNA are fixed for particular organisms. A human cell contains 23 pairs of chromosomes. Had this unique set of chromosomes not existed, humans would not have got their special characteristics which make them human or cats would not look like cats or dogs would not give birth to puppies.

In case of extra genomic DNA, the number can vary because these are not responsible for the transfer of genetic traits from one generation to another, but these are only responsible for replication of a cell organelle in which they are present like the DNA present in the mitochondria and plastids.

Therefore, the genomic content of the nucleus is always constant for a given species, but the extra genomic content can vary among the members of the same species.

Question:7

Justify the statement, "Mitochondria are powerhouses of the cell.?

Answer:

Ans.

Mitochondria are cylindrical structures that are bound in double membranes- an outer membrane and an inner membrane. The outer membrane forms fold like structures called cristae. The cristae are related to the increase in surface area. Mitochondria serves as the main site for aerobic respiration.

They produce cellular energy in the form of Adenosine Triphosphate (ATP). The respiration occurs in two steps- glycolysis and Kreb’s Cycle. Glycolysis occurs in the cytoplasm, whereas Kreb’s cycle occurs in the mitochondria. Energy is not only produced, but it is also stored in the mitochondria for carrying out vital cell functions. Hence, mitochondria are also called the powerhouses of the cell.

Question:8

Is there a species-specific or region-specific type of plastids? How does one distinguish one from the other?

Answer:

Plastids are species-specific and not region-specific and are found in all plants and euglenoids. Different type of plastid consists of different pigment; thus, imparting different colours at the parts where they are found. On the basis of the colour they impart, plastids can be divided into three types:

Leucoplasts: These are colourless plastids that impart no colour to the parts of the plantwhere they are located. A plant with only leucoplasts is pale white in appearance. The primary function of this plastid is to store foods for the plant. There are three types of leucoplasts- Amyloplaststs, elaioplasts, and aleuroplast. Amyloplast helps in storing starch;elaioplast helps in storing fats, and aleuroplast helps in storing proteins.

Chromoplasts: These are not colourless but essentially non-green plastids. They impart colours other than green to plants. Various colours other than green found in petals and fruits are because of chromoplasts.

Chloroplasts: Chloroplasts consist of a green pigment called chlorophyll which imparts the green colour to the plant and also helps in trapping sunlight which is an essential function for photosynthesis.

Question:9

Write the functions of the following
a) Centromere
b) Cell wall
c) Smooth ER
d) Golgi Apparatus
e) Centrioles

Answer:
(a) It is important for facilitating proper chromosome segregation.

(b) The cell wall gives structure to the cell and protects it from injuries. It also helps with lining all the contents of the cell together.

(c) Smooth ER helps in the synthesizing of protein, lipids and carbohydrates. It also helps in drug detoxification and regulating calcium concentration.

(d) It is a primary site for the formation of glycolipids and glycoprotein. It also plays a vital role during cell division by formation of cell plates.

(e) Centrioles help in the cell division of animals by forming asters. It also forms the microtubules and sperm tail.

Question:10

Are the different types of plastids interchangeable? If yes, give examples where they are getting converted from one type to another.

Answer:

Yes, different types of plastids are interchangeable under different stages of development of the plant or due to some external cause involved.
There are three types of plastids: chromoplasts, chloroplasts and leucoplasts. Chloroplasts are plastids that contain a green pigment called chlorophyll and carotenoid pigments. In some situations, generally in mature leaves, the carotenoid increases in proportion to the chlorophyll.
The green colour decreases due to the loss of chlorophyll. This loss converts the chloroplast to chromoplast giving the leaves dark brown and reddish appearance. Similarly, flowers and fruits get their colour due to the conversion of chloroplast into chromoplast.