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

1

We all reside in a world where non-living things coexist. And, since we are human beings and have the power to speak and understand and make others understand, we always ask ourselves - "what makes a living organism, living? The answer to all such questions is the presence of the basic unit of life in living organisms, i.e. the Cells. All types of living organisms are made from cells, which is present inside their body. Some living creatures consist of just a single cell who are termed unicellular organisms.

While others who are made up of many cells are known as multicellular organisms. So, the "Cells - The Unit of Life" chapter in NCERT Solutions for class 11 Biology is everything about Cells. The sub-topics covered under Cells are as follows:

  • Meaning of a cell
  • What is Cell Theory?
  • A summary of cells
  • Prokaryotic cells
  • Eukaryotic cells

A cell is defined as the functional and structural unit of a living organism, and this chapter discusses every detail about cells, like the meaning & types of cells, composition of cells, functions of cells, etc. Earlier, we have learned that entities possessing a single cell are called unicellular entities, and those composed of multiple cells are termed multicellular organisms. In this chapter, two broad groups of cell classification have been shown, which are eukaryotes and prokaryotes. A cell comprises various components, a few of which are absent in plants, while few are missing in animal cells.

Q1. Which of the following is not correct?

     1.Robert Brown discovered the cell.

     2.Schleiden and Schwann formulated the cell theory

     3.Virchow explained that the cells are formed from pre-existing cells.

     4.A unicellular organism carries out its activities within a single cell.

Answer:

     (a) is not correct. All the others are correct.

  • Robert Brown discovered the nucleus and not the cell. Cells were first discovered by Robert Hook when he was studying the cork cells of plants.Schleiden and Theodore Schwann (1938) proposed the cell theory stating that all organisms are made up of cells.
  • However this theory did not explain the origin of cells and was later modified by Rudolf Virchow (1855). The cell theory states that
  1. All living organisms are composed of cells and products of cells.
  2. All cells arise from pre-existing cells. (This was suggested by Rudolf Virchow).

 

2.Schleiden and Schwann formulated the cell theory

Answer:

  • Correct statement. Metthias Schleiden and Theodore Schwann( 1938) proposed the cell theory which was later modified by Rudolf Virchow(1855). The cell theory states that
  1. All living organisms are composed of cells and products of cells.
  2. All cells arise from pre-existing cells.

 

3.Virchow explained that cells are formed from pre-existing cells.

Answer:

Correct statement. Earlier cell theory as proposed by Schleiden and Schwann stated that all organisms are made up of cells. The cells are the basic structural and functional units of life. However, this theory did not explain the origin of cells. Later on, Virchow suggested that all cells arise from pre-existing cells.

 

Q2. New cells generate from

  (a)bacterial fermentation

  (b)regeneration of old cells

  (c)pre-existing cells

  (d)abiotic material

  Answer:

  (c) New cells generate from pre-existing cells.

  • According to Rudolf Virchow, the new cells arise from pre-existing cells by their division - omnis cellula e cellula.

 

Q3. Match the following

       Column I                        Column II

  1. Cristae                     (i) Flat membranous sacs in stroma
  2. Cisternae                 (ii) Infoldings in mitochondria
  3. Thylakoids              (iii) Disc-shaped sacs in Golgi apparatus

Answer:

  • The correct matching is (a)- (ii), (b)- (iii), (c)- (i)

 

     Column I                          Column II

  1. Cristae                 (i) Infoldings in mitochondria
  2. Cisternae             (ii) Disc-shaped sacs in Golgi apparatus

      c.Thylakoids           (iii) Flat membranous sacs in stroma..

 

Q4. Which of the following is correct:

a.Cells of all living organisms have a nucleus.

b.Both animal and plant cells have well-defined cell walls.

c.In prokaryotes, there are no membrane-bound organelles.

d.Cells are formed de novo from abiotic materials.

Answer:

  • The correct statement is (c).
  • In prokaryotes, there are no membrane-bound organelles.
  • All other statements are incorrect as cells of some organisms can lack a nucleus (e.g. RBC ), animal cells do not possess a cell wall, and cells are formed from pre-existing cells.

 

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

Answer:

  • Mesosome is a complex membranous structure formed by the infoldings of the plasma membrane in prokaryotic cells. The functions performed by mesosome are as follows:
  1. Mesosomes play important roles in cell wall formation, DNA replication, etc.
  2. Mesosomes are folded structures, this quality helps to increase the surface area of the plasma membrane to carry out enzymatic activities.
  3. Mesosome also helps in cellular respiration and secretion.

 

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:

  • Neutral solutes are lipid-soluble and move across the plasma membrane by directly crossing through the lipid bilayer.
  • Their rate of movement across the plasma membrane depends on the concentration gradient and lipid solubility of neutral solutes.
  •  No, polar molecules can not move across the membrane in the same manner as neutral solutes.
  •  The polar molecules require carrier proteins in order to pass through the membrane.
  • The carrier proteins are integral membrane proteins that have an affinity for some specific molecules and during transport, these carriers carry these molecules to the other side of 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 labeled diagrams of both.

Answer:

  • The two double membrane-bound organelles are mitochondria and chloroplast.

Characteristics of Mitochondria-

  1. Mitochondria are cylindrical-shaped cell organelles that can be stained by Janus green and observed under the microscope. MItochondria consists of 60-70% proteins, 25- 35 % lipids, 5-7% RNA, DNA etc.
  2. Mitochondria are encircled by two membranes i.e. an outer membrane and an inner membrane. These two membranes are separated by peri-mitochondrial space. The outer membrane is smooth and porous in nature. Porins are present in the outer membrane.
  3. The inner membrane of mitochondria is invaginated into folds that are called cristae. Electron transport chains and peroxisomes are found in the inner membrane.
  4. Inside the mitochondria, a semi-fluid matrix is present which consists of protein particles, ribosomes, RNA, DNA, and enzymes that are used in the Krebs cycle, amino acid synthesis, etc.
  5. Mitochondria are semi-autonomous organelles as they possess their own DNA and protein synthetic machinery. The DNA of mitochondria is naked and circular.

Functions of mitochondria:

  1.  Mitochondria are the sites of cellular respiration.
  2. Mitochondria provide important intermediates for the synthesis of several biochemicals like chlorophyll. cytochromes, pyrimidines, steroids, etc.
  3. Synthesis of many amino acids and fatty acids occurs in the mitochondria.
  4. Mitochondria may store and release calcium when required
  5. Mitochondria are involved in providing maternal inheritance.

 

Characteristics of chloroplast:

  1. The chloroplast is a green-colored plastid that possesses photosynthetic pigments like chlorophyll, carotenoids, etc, and takes part in photosynthesis.
  2. Chloroplasts can have various shapes i.e. they can be plate-like, ribbon-like, cup-shaped, etc.
  3. Chloroplast consists of 50-60% protein, 25-30% lipids, 5-10% of chlorophyll, 1-2% carotenoids, up to 0.5% DNA and 2-3% RNA.
  4. The structure of chloroplast mainly consists of envelope, matrix, and thylakoids.
  5. The envelope of the chloroplast is made up of two smooth membranes separated by an intermembrane space of 100-200 angstrom width. The outer membrane is more permeable than the inner membrane.
  6. The ground substance of a chloroplast is known as matrix or stroma. It is a semi-fluid colloidal complex made up of 50% soluble proteins and RNA, DNA, ribosomes, enzymes, etc. Throughout the stroma, membrane-lined flattened sacs are present.

These structures take part in photosynthesis. Thylakoids stack one over the other to form grana. 40-60 grana may occur in 1 chloroplast. Each granum may have 2-100 thylakoids. In the thylakoids membrane, photosystems are present.

Functions of chloroplast:

  1. Chloroplasts are the site of photosynthesis.
  2. Chloroplasts contain the enzymes for the synthesis of carbohydrates and proteins.
  3. They temporarily or permanently store starch.

 

Q8. What are the characteristics of prokaryotic cells?

Answer:

Characteristics of prokaryotic cells:

  • Prokaryotic cells are those cells that do not have a well-defined nucleus. The most important characteristics of prokaryotic cells are as follows:
  1. The nucleus of prokaryotic cells is not well defined i.e. it does not possess a nuclear envelope. The DNA of these cells lies freely in the cytoplasm. The DNA is naked and coiled. Extrachromosomal DNA Is also found in these cells.
  2. Prokaryotic cells are devoid of membrane-bound cell organelles.
  3. The cell wall is present in prokaryotic cells of bacteria and cyanobacteria. However, it is absent in mycoplasma.
  4. Instead of chloroplasts, chromatophores are present in prokaryotes to carry out photosynthesis.
  5. Ribosomes of prokaryotic cells are of the 70S type.

 

Q9. Multicellular organisms have the division of labor. Explain.

Answer:

  • Division of labor refers to the differentiation of certain components or parts to perform specific functions to cause an increase in the efficiency and survival rate of that organism.
  •  Multicellular organisms are composed of millions of cells. All these cells are different from each other with respect to their structure and functions.
  •  These cells together are responsible for the survival of the organism.
  • The various necessary functions are executed via the division of labor. In simple words, we can say that in multicellular organisms the functions essential for the survival of the organism are divided and allotted to different tissues and tissue systems.
  •  All these issues and their systems work in a coordinated manner. For example, every cell of a multicellular organism cannot obtain food from outside. The organism requires a system for obtaining food, its digestion, and distribution.
  • Therefore, a digestive system and system of transport are also required. So, there are tissues responsible for digestion and transportation.
  • Similarly, tissues are there for excretion, respiration, etc.
  •  Similarly, certain cells of the body take over the function of reproduction. Others take part in the repair and replacement of worn-out or injured portions.
  • Hence, multicellular organisms show the division of labor.

 

Q10. The cell is the basic unit of life. Discuss in brief.

Answer:

  • A living organism is composed of various organ and organ systems. These organ systems are made up of tissues that are composed of cells.
  •  A cell is the smallest unit of life that can survive on its own and perform all the essential functions necessary for its survival.
  • It not only supports the structure of an organism but also performs various functions. Due to this reason, the cell is regarded as the basic structural and functional unit of life.

 

Q11. What are nuclear pores? State their function.

Answer:

  • Nuclear pores are the areas in the nuclear membrane where two membranes fuse with each other. The nuclear pores allow the transfer of only some specific substances across the nuclear membrane. For example, they allow RNA and proteins to move in and move out of the membrane.

 

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

Answer:

  • Both lysosome and vacuole are endomembrane structures possessing  only a single- membrane.
  • The membrane of vacuole is called tonoplast.
  •  Despite being endomembrane structures, the function of lysosome and vacuole differs considerably.
  • The lysosome consists of hydrolytic enzymes that hydrolyze various substances except cellulose. Lysosomes help to digest worn contents and cellular debris of the cell. Due to this reason, lysosomes are also called suicidal bags.
  • On the other hand, vacuoles are non-cytoplasmic sacs that store substances. For example, sap vacuole stores sap with organic and inorganic substances which helps to  maintains the osmotic pressure anturgidity of the cell. Similarly, food vacuoles store food, and gas vacuoles store metabolic gases.

 

Q13. Describe the structure of the following with the help of labeled diagrams.

(i) Nucleus

Answer:

  • The nucleus is the centrally placed, spherical structure found in the cell. It is the primary organelle that controls all the activities of a cell. The structure of the nucleus comprises a nuclear membrane enclosing nucleoplasm or nuclear matrix.
  1. Nucleoplasm / Nuclear matrix- It refers to the homogeneous, granular fluid present inside the nucleus. Nucleoplasm consists of nucleolus and chromatin material.
  2.  The chromatin material consists of DNA complexed with proteins in the form of a long thread.
  3.  It is mainly involved in the transmission of characteristics from one generation to another.
  4. The nucleolus, on the other hand, is the spherical structure lacking a membrane. It acts as the site of ribosome formation. Nucleolus mainly consists of RNA and protein.

 


https://d2pduerm2meudp.cloudfront.net/media/uploads/2019/06/29/nucleus-structure-2.jpg

 

 

Q13. Describe the structure of the following with the help of labeled diagrams.

(ii) Centrosome

Answer:

  1. The structure of the centrosome is made up of two cylindrical structures called centrioles. These centrioles show a cartwheel-like organization and both lie perpendicular to each other.
  2.  A centriole is made up of microtubule triplets that are evenly spaced in a ring-like structure.
  3. The adjacent triplets are linked together.
  4. A proteinaceous hub is present in the central part of a centriole.
  5. The hub is attached to triplets by means of radial spokes.
  6. Centrioles help in the organization of spindle fibers. It also forms the basal body of cilia and flagella.

 

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

Answer:

  • A centromere refers to a non- stainable area that attaches two chromatids of a chromosome. On the basis of the position of the centromere, chromosomes can be of the following types:
  1. Acrocentric chromosomes- Centromere is present in sub-terminal regions, chromatids are arranged in J-shaped
  2. Metacentric chromosome- Centromere is present in the center and the chromosome appears V-shaped.
  3. Sub-metacentric chromosome- The position of the centromere is sub-median. It appears L-shaped
  4. Telocentric chromosome- Centromere is terminally placed.
     

 

 

Also See    
NCERT Solutions for Class 11 Biology Chapter 1 - The Living World NCERT Solutions for Class 11 Biology Chapter 2 - Biological Classification NCERT Solutions for Class 11 Biology Chapter 3 - Plant Kingdom
NCERT Solutions for Class 11 Biology Chapter 4 - Animal Kingdom NCERT Solutions for Class 11 Biology Chapter 5 - Morphology of Flowering Plants NCERT Solutions for Class 11 Biology Chapter 6 - Anatomy of Flowering Plants
NCERT Solutions for Class 11 Biology Chapter 7 - Structural Organization in Animals NCERT Solutions for Class 11 Biology Chapter 9 - Biomolecules NCERT Solutions for Class 11 Biology Chapter 10 - Cell Cycle and Division
NCERT Solutions for Class 11 Biology Chapter 11 - Transport in Plants NCERT Solutions for Class 11 Biology Chapter 12 - Mineral Nutrition NCERT Solutions for Class 11 Biology Chapter 13 - Photosynthesis in Higher Plants
NCERT Solutions for Class 11 Biology Chapter 14 - Respiration in Plants NCERT Solutions for Class 11 Biology Chapter 15 - Plant Growth and Development NCERT Solutions for Class 11 Biology Chapter 16 - Digestion and Absorption
NCERT Solutions for Class 11 Biology Chapter 17 - Breathing and Exchange of Gases NCERT Solutions for Class 11 Biology Chapter 18 - Body Fluids and Circulation NCERT Solutions for Class 11 Biology Chapter 19 - Excretory Products and their Elimination
NCERT Solutions for Class 11 Biology Chapter 20 - Locomotion and Movement NCERT Solutions for Class 11 Biology Chapter 21 - Neural Control and Coordination NCERT Solutions for Class 11 Biology Chapter 22 - Chemical Coordination and Integration

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