NCERT Solutions for Class 11 Science Biology Chapter 8 Cell: The Unit Of Life are provided here with simple step-by-step explanations. These solutions for Cell: The Unit Of Life are extremely popular among Class 11 Science students for Biology Cell: The Unit Of Life Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 11 Science Biology Chapter 8 are provided here for you for free. You will also love the ad-free experience on Meritnation’s NCERT Solutions. All NCERT Solutions for class Class 11 Science Biology are prepared by experts and are 100% accurate.
Page No 38:
Question 1:
A common characteristic feature of plant sieve tube cells and most of mammalian erythrocytes is
a. Absence of mitochondria
b. Presence of cell wall
c. Presence of haemoglobin
d. Absence of nucleus
Answer:
Plant sieve tube cells are present in the phloem. It lacks a nucleus and is thus dependent on companion cells for loading and unloading of sugars. Mammalian mature erythrocytes also lack a nucleus.
Hence, the correct answer is option d.
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Question 2:
Select one which is not true for ribosome
a. Made of two sub-units
b. Form polysome
c. May attach to m+ RNA
d. Have no role in protein synthesis
Answer:
Hence, the correct answer is option d.
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Question 3:
Which one of these is not a eukaryote?
a. Euglena
b. Anabaena
c. Spirogyra
d. Agaricus
Answer:
Hence, the correct answer is option b.
Page No 38:
Question 4:
Which of the following stains is not used for staining chromosomes?
a. Basic fuchsin
b. Safranin
c. Methylene green
d. Carmine
Answer:
Basic fuchsin is used to stain chromosomes, cardiac and skeletal muscles. Methylene green stains the nucleus or DNA. Carmine is also used in nuclear staining. However, safranin is used as a counter-stain in Gram staining and endospore staining.
Hence, the correct answer is option (b).
Page No 38:
Question 5:
Different cells have different sizes. Arrange the following cells in an ascending order of their size. Choose the correct option among the followings
i. Mycoplasma
ii. Ostrich eggs
iii. Human RBC
iv. Bacteria
a. i, iv, iii & ii
b. i, iii, iv & ii
c. ii, i, iii & iv
d. iii, ii, i & iv
Answer:
So, the correct ascending order of their size is i>iv>iii>ii.
Hence, the correct answer is option a.
Page No 39:
Question 6:
Which of the following features is common to prokaryotes and many eukaryotes?
a. Chromatin material
b. Cell wall present
c. Nuclear membrane present
d. Membranes bound sub-cellular organelles present
Answer:
Hence, the correct answer is option b.
Page No 39:
Question 7:
Who proposed the fluid mosaic model of plasma membrane?
a. Benda
b. Schleiden and Schwann
c. Singer and Nicolson
d. Robert Brown
Answer:
Hence, the correct answer is option c.
Page No 39:
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 nucleus.
Answer:
Cells that are secretory in nature are abundant in RER and golgi apparatus. Ribosomes present on the endoplasmic reticulum helps to synthesize proteins that are majorly secreted outside of the cell.
Hence, the correct answer is option (b).
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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:
Plant cells usually have large, single vacuoles. Their vacuoles are surrounded by a single membrane called tonoplast.
Hence, the correct answer is option c.
Page No 39:
Question 10:
Which of the following is not true of a eukaryotic cell?
a. Cell wall is made up of peptidoglycans
b. 80S type of ribosomes are present in the cytoplasm
c. Mitochondria contain circular DNA
d. Membrane bound organelles are present
Answer:
Eukaryotic cells are of two types: plant cells and animal cells. Animal cells do not have cell walls. In plant cells the cell wall is made up of cellulose. Bacteria (prokaryotes) have a cell wall made up of peptidoglycans.
Hence, the correct answer is option (a).
Page No 40:
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:
The detailed structure of the plasma membrane was observed with the electron microscope. It reveals that the plasma membrane is majority composed a phospholipid bilayer. Later biochemical investigations showed that the plasma a membrane also contains proteins and carbohydrates in it.
Hence, the correct answer is option (d).
Page No 40:
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:
Hence, the correct answer is option c.
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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:
Some of the functions performed by the cytoskeleton are:
mechanical support,
motility,
maintenance of the shape of the cell etc.
Hence, the correct answer is option a.
Disclaimer: Cytoskeleton also helps in intracellular trafficking of the vesicles. Thus, all of the above mentioned options are functions of the cytoskeleton of a cell.
Page No 40:
Question 14:
The stain used to visualise mitochondria is
a. Fast green
b. Safranin
c. Acetocarmine
d. Janus green
Answer:
Janus green is a special type of strain that becomes coloured when oxidized. It is used to stain the mitochondria where major oxidation and reduction of the cell takes place.
Hence, the correct answer is option d.
Page No 40:
Question 1:
What is the significance of vacuole in a plant cell?
Answer:
Vacuoles are membrane-bound cell organelles found in plant cells. In plant cell, a single, large vacuole is observed. The vacuole mainly acts as a storage organelle that stores water, sap, excretory product and other materials not useful for the cell. It maintains the turgidity of the plant cell.
Page No 40:
Question 2:
What does ‘S’ refer in a 70S & an 80S ribosome?
Answer:
'S' refers to the Svedberg unit in the 70S & an 80S ribosome. Svedberg unit stands for the sedimentation coefficient. It is indirectly a measure of density and size.
Page No 40:
Question 3:
Mention a single membrane bound organelle which is rich in hydrolytic enzymes.
Answer:
The lysosome is a single membrane-bound organelle which is rich in hydrolytic enzymes. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
Page No 40:
Question 4:
What are gas vacuoles? State their functions?
Answer:
Gas vacuoles are present in many cyanobacteria. These vacuoles are permeable to gas and provide buoyancy to the bacteria.
Gas vacuoles majorly help photosynthesizing bacterias to get optimal light by inflating and deflating the vacuoles.
Page No 40:
Question 5:
What is the function of a polysome?
Answer:
When multiple ribosomes attach to an mRNA strand it is called a polysome. A polysome helps in protein synthesis.
Page No 40:
Question 6:
What is the feature of a metacentric chromosome?
Answer:
A metacentric chromosome has centromere in the middle forming two equal arms of the chromosome.
Page No 40:
Question 7:
What is referred to as satellite chromosome?
Answer:
Along with the primary constriction, some chromosome also have non-straining secondary constitution at a constant location. This creates a small fragment called satellite and chromosomes bearing satellite are called satellite chromosomes.
Page No 41:
Question 1:
Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis.
Answer:
The nucleolus is a spherical, membrane-less structure suspended in the nucleoplasm. It is a site of ribosomal RNA (rRNA) synthesis that takes part in ribosome formation. Thus, larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
Page No 41:
Question 2:
Explain the association of carbohydrate to the plasma membrane and its significance.
Answer:
Short chains of carbohydrates may be attached to the protein or lipid of the plasma membrane. It can make up to 1-5% of the total molecular weight of the plasma membrane. The carbohydrates present in the plasma membrane are mainly oligosaccharides and are located on the outer surface of the plasma membrane.
Significance: Two major significance of membrane carbohydrates are-
- They help in cell to cell adhesion.
- They also help in cell recognition. Like oligosaccharide chains present on RBC helps to detect the blood group type.
Page No 41:
Question 3:
Comment on the cartwheel structure of centriole.
Answer:
Two centrioles lie to each other perpendicularly forming a centrosome. The organisation within each centriole is like a cartwheel.
- They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibrils is a triplet.
- The adjacent triplets are also linked.
- The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
Page No 41:
Question 4:
Briefly describe the cell theory.
Answer:
Cell theory was postulated by two scientists. One is German botanist Matthias Schleiden (1838) who examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. Another is British zoologist Theodore Schwann (1839) who proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory. But at that time it was not clear how new cells form. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula) and modified the pre-existing cell-theory. Cell theory as understood today is:
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
Page No 41:
Question 5:
Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
Answer:
| Rough Endoplasmic Reticulum (RER) | Smooth Endoplasmic Reticulum (SER) |
| 1. Ribosomes are attached to the surface. | 1. Ribosomes are not attached to the surface. |
| 2. Takes part in proteins synthesis. | 2. Takes part in lipids or steroids synthesis. |
| 3. It may develop from the nuclear membrane. | 3. It may develop from the RER. |
Page No 41:
Question 6:
Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?
Answer:
⇒ Inside the plasma membrane, the lipid molecular are arranged in a bilayer with the polar heads of the phospholipid facing outside and the non-polar tails facing inside. The non-polar tails create a hydrophobic core.
Page No 41:
Question 7:
What are plasmids? Describe their role in bacteria?
Answer:
Plasmids are small, circular, extra-chromosomal DNA present in some bacteria.
Role of plasmid in bacteria:
1. Some plasmids provide resistance against certain anti-biotics like tetracycline, ampicillin etc.
2. Some plasmids also provide resistance against-harmful toxic metals.
Page No 41:
Question 8:
What are histones? What are their functions?
Answer:
Histones are basic proteins that are found inside the nucleus of the eukaryotes.
As the histone proteins are basic in nature, they have a positive charge. DNA carrying phosphate group is negative in charge. So, DNA is easily coiled around the histone proteins and the histones proteins help in DNA packaging.
Page No 41:
Question 1:
What structural and functional attributes must a cell have to be called a living cell?
Answer:
Cell is the smallest structural and functional unit of every living organism.
- All living cells have a plasma membrane, enclosing the boundary of the cell. This membrane acts as a selectively permeable layer that allows the entry and exit of only certain molecules in the cell. It also separates the internal environment of the cell from the outer surroundings.
- Inside the plasma membrane, there is a thick jelly-like substance, called cytoplasm. The cytoplasm acts as a site where all biochemical reactions essential for a cell to survive to take place. In the case of the eukaryotic cells, membrane-enclosed cell-organelles like mitochondria, golgi body, endoplasmic reticulum, are suspended here.
- Almost all living cell also contains a nucleus. This is the control centre of the cell that regulates all cellular functions.
Page No 41:
Question 2:
Briefly give the contributions of the following scientists in formulating the cell theory:
a. Robert Virchow
b. Schielden and Schwann
Answer:
a. Schleiden and Schwann together formulated the cell theory. But at that time it was not clear how new cells are formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula) and modified the pre-existing cell theory.
b. Cell theory was postulated by two scientists. One is German botanist Matthias Schleiden (1838) who examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. Another is British zoologist Theodore Schwann (1839) who proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.
Cell theory as understood today is:
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.
Page No 41:
Question 3:
Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.
Answer:
Page No 41:
Question 4:
Structure and function are correlatable in living organisms. Can you justify this by taking plasma membrane as an example?
Answer:
Living organisms are either unicellular or multicellular. Cell shape varies according to the function if performs. for example, RBCS have a round and biconcave shape for carrying more amount of hemoglobin and easy transport through the capillaries. NBCS are amoeboid to perform phagocytosis and diapedesis.
The plasma membrane has a quasi-fluid lipid bilayer in which protein molecular can show lateral movement. This fluid nature of the cell is important for growth secretion, endo cytosis, cell division etc.
Page No 41:
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
Group the various sub-cellular organelles into these three categories.
Answer:
a. not be bound by a membrane - ribosome, centrosome, cytoskeleton.
b. bound by a single membrane - vacuole, endoplasmic recticulum, golgi apparaties, lysosome, microbodies.
c. bound by a double membrane - nucleus, mitochondria, plastid.
Page No 41:
Question 6:
The genomic content of the nucleus is constant for a given species where as the extra chromosomal DNA is found to be variable among the members of a population. Explain.
Answer:
Page No 42:
Question 7:
Justify the statement, “Mitochondria are power houses of the cell”
Answer:
The mitochondria is a double membrane bound structure. The inner membrane forms a number of foldings called the cristae towards the matrix. The cristae are embeded with small tennis racket shaped structures called oxysomes or F0 - F1 particles. Mitochondria take part in aerobic respiration and oxysome help in the formation of ATP (adeno sine triphosphate). ATP acts as cellular energy currency and used in performing various cellular activities. As mitochondria produce cellular energy in the form of ATP, hence they are called 'power houses' of the cell.
Page No 42:
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 or region-specific. They are found in all plant cells and in euglenoids. Different plastids bear different pigments or no pigment at all based on the type of pigment present plastids are classified into three groups: chloroplasts, chromoplasts and leucoplasts.
Chloroplasts: These are the green coloured plastids that contain the pigment chlorophyll. These plastids take part in photosynthesis.
Chromoplast: They contain soluble carotenoid pigments like carotene and xanthophyll. The presence of these pigments imparts yellow, orange or red colour in plants. For example chilly, tomato etc.
Leucoplasts: They are colourless plastids. Their major function is storage like amyloplasts stare carbohydrates, elaioplasts stare oils and fats whereas, leucoplast stare proteins. For example potato, castor etc.
Page No 42:
Question 9:
Write the functions of the following
a. Centromere
b. Cell wall
c. Smooth ER
d. Golgi Apparatus
e. Centrioles
Answer:
(b) Cell wall - It is a rigid, dead layer found outside the plant, bacterial and fungal cell. The cell wall provides shape to the cell as well as proteins from the external environment.
(c) Smooth ER - The smooth ER lacks ribosome on it and a major site of lipid synthesis. It also synthesizes steroidal hormones.
(d) Golgi apparatus - It majorly performs the function of packaging and delivering materials either intra-cellular targets or outsides of the cell.
(e) Centrioles - Two centrioles lie to each other perpendicularly and form the centrosome. The centrioles form the basal body of cilia or flagella. They also form spindle apparatus that take part in cellular division.
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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 inter-changeable based on the requirement of the plant. There are three types of plastids are found in the plant cells :
Chloroplasts (green colored, photosynthesizing plastids), chromoplasts (colorful plastids), and leucoplasts (storage plastids)
Some examples of such interchange are -
(i) Potato stem tubers are a type of leucoplast. On exposure to the sunlight, some parts of it change into the chloroplast and appear green.
(ii) Ovary of capsicum contains leucoplasts. When the ovary changes into fruit, the leucoplast converts into the chloroplast. After ripening of the fruit, the chloroplast converts into chromoplast.
(iii) When tomato and chili are rows. They appear green due to the presence of chloroplast. On ripening the chloroplasts change into chromoplasts.
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