Important Questions Class 11 Biology Chapter 8

CBSE Class 11 Biology Chapter 8 Question Answers: Cell: The Unit of Life

Chapter 8 in the NCERT book Class 11 Biology provides all the essential information about Cell: The Unit of Life. Cells are the basic units of life because they come in discrete and easily recognizable packages. All cells are surrounded by a structure called the cell membrane, like the walls of a house, which serves as a clear boundary between the cell’s internal and external environments. The cells are called the basic unit of life because all living organisms are made up of cells, and all the functions taking place inside the body of organisms are performed by cells. 

Cells are very important because cells provide the right conditions required for the occurrence of metabolic reactions. Cells also provide structure and function for all living things, from microorganisms to humans. Scientists consider them the smallest form of life. Cells house the biological machinery that makes the proteins, chemicals, and signals responsible for everything that happens inside our bodies 

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CBSE Class 11 Biology Chapter 8 Question Answers

Referring to our question and answer bank of Important Questions for Class 11 Biology Chapter 8 helps students upgrade their knowledge of a particular topic. Our subject experts at Extramarks understand this crucial method of imparting education through their own experience and know how to provide a lucid explanation of key concepts which students can easily learn and benefit from . Therefore, we focus on developing content that includes all the necessary questions and solutions related to the topics studied in the chapter. Our question bank of Important Questions for Class 11 Biology Chapter 8 covers full topics from the chapter Cell: The Unit of Life.

Given below are some of the important questions and solutions from Extramarks Important Questions Class 11 Biology Chapter 8.

Q1. Give the fundamental similarities that occur in all cells.

Answer 1-

The fundamental similarities that occur in all cells are:

Hereditary characters are transmitted through nucleic acids containing DNA and RNA.

The basic structure of cell membranes is the same in all cell organelles.

The process and method of aerobic respiration are similar.

The mechanism of synthesis of nucleic acids and the formation of proteins within the cells are also much the same. .

Q2. What are the cytoskeletal structures present in eukaryotic cells?

Answer 2- 

Cytoskeletal Structures help the eukaryotic cells to adopt different types of shapes and to perform directed movement.

Cytoskeletal structures contain three major types of protein filaments which are classified as microfilaments, microtubules, and intermediate filaments.

The microfilaments are 7 nm in diameter and either scattered or organised into the network or parallel arrays within the matrix. They also play a major role in cell motion and help the eukaryotic cells adopt different shapes. Such cellular movements are associated with the microfibres movements of pigment granules, amoeboid movements, and protoplasmic streaming. The microfilaments consist of actin-like proteins.

Q3. What are the major functions of the cell wall?

Answer 3- 

The major functions of the cell wall are as follows:

The cell wall gives a definite shape and helps protect the internal organelles.

The cell wall provides a structural framework and supports the plasma membrane.

It prevents the cell from the process of desiccation.

The cell wall physically counteracts the osmotic pressure produced by the cell organelles.

It also helps transport materials and metabolites in and out of the cell through diffusion.

Q4. List the different functions of the golgi bodies.

Answer 4- 

The functions of Golgi bodies are as follows:

Golgi bodies help in storing, condensing, and packaging important materials.

Many different enzymes are found in Golgi bodies that help during spermatogenesis.

Due to the action of the Golgi bodies, mucilage and gums are secreted in plant cells.

Q5. Describe the structure and functions of the organelles stated below- 

(a) nucleus

(b) mitochondria

(c) plastid

Answer 5 – 

1. a) Nucleus

Electronic studies reveal that the nucleus is an organelle bounded by two membranes forming the nuclear envelope. The outer and inner membranes of the nucleus are separated by a narrow space called perinuclear space.

The outer membrane of the nucleus continues with the organelle endoplasmic reticulum (ER), while the inner membrane surrounds the nucleus organelles. At some points, the nuclear envelope is surrounded by small structures called nuclear pores.

Circular pores surround these pores. The circular pores or circular structures are called annuli. The pores or the annuli unite to form a structure called a pore complex. These pores also help exchange materials between the nucleoplasm and cytoplasm. The nuclear membrane disappears during the cell.-division process. It then reappears during nuclear recognition in the telophase stage of cell division.

The nucleoplasm contains chromatin and nucleolus, where the nucleolus is a rounded structure, and it is not separated from the rest of the nucleoplasm by a single membrane. It is also associated with a specific nucleolar organising region called the NOR of some chromosomes. The nucleolus is also called the “site for ribosomal RNA synthesis”, where the cells which remain engaged in protein synthesis have a larger and more numerous nucleoplasm.

(b) mitochondrion 

Ultrastructure of the organelle mitochondria: They are commonly spherical, elongated, or rod-like cell organelle, known as the “powerhouse of the cell.” Mitochondria were first discovered by the scientist Hofmeister in 1851 in the cells of a pteridophyte. They were later named ‘mitochondrion’ by Benda in 1897.

Mitochondria is a double membranous cell organelle where the outer membrane is smooth, and the space between the two membranes is called the outer chamber. On the other hand, the space surrounded by the inner membrane is called an inner chamber filled with homogeneous fluid.

The double membrane of the mitochondria is actively associated with aerobic respiration and the release of energy for cellular activity. This biological oxidation of the fats & carbohydrates releases a small amount of energy utilised by mitochondria for ATP synthesis. 

(c) plastid.

Ultrastructure of plastids: They are green-coloured cell organelles in the form of plastids bounded by two membranes with about 300 A in total thickness. Each plastid membrane is about 40 to 60 A’ thick and is separated by a clear space of about 25 – 27 A.

The plastid’s inner membrane is intricately elaborated to form a system of lamellae, and the chloroplasts are divisible into two main parts. Stroma- This is a colourless, ground substance found in the chloroplast’s inner side. 

And the Membrane system is made of closed flattened sacs called thylakoids. These thylakoids are closely packed & appear as piles of coils. . These piles of coil structures are called Grana.

The arrangement of this form can be in the form of simple parallel sacs running lengthwise or in a complex interconnecting network of sacs. These chloroplasts invariably have starch granules, which often accumulate near a special region known as pyrenoid in algae.

Q.6 Explain in brief about the bacterial cell wall.

Answer 6 –

The cell wall of Bacteria is formed of murein, also known as peptidoglycan. It consists of polysaccharides that are cross-linked with short amino acid chains. In Gram-negative bacteria, the covering is formed of lipopolysaccharide and is present around the cell wall. It may provide. Specific adhesion properties to these cells. It determines the shape of the cell

Q7. How is a multicellular organisation more advanced than a unicellular organisation?

Answer 7- 

The multicellular organisation of the human body is more advanced than the unicellular organisation because:

There is a presence of division of labour that occurs among cells which increases efficiency. Also, in a multicellular organism, cells are differentiated, and specialised cells are formed. These specialised cells perform specific functions that increase their survival capacity. Multicellular cells contain a varied type of cells which is more efficient than a single-celled organism. One of the major advantages of multicellular organisms is that the death of a few cells does affect the whole organism.

Q8. Give some examples of functions performed by specialised cells.

Answer 8- 

They contain nerve cells which are specialised to transmit nerve impulses.

They also contain muscle and red blood cells specialised for muscle contraction and oxygen transport.

The cells of the pancreas secrete insulin.

Q9. Justify the statement “Cell theory has shortcomings”. 

Answer 9 –

The shortcomings of cell theory are stated below – 

It is believed that Schleiden and Schwann were not the first scientists to prove that living beings are made up of cells.

Rudolf Virchow was the first scientist to state that “all-new cells arise from the pre-existing cells”, explaining why all cells contain hereditary material that passes from one generation to the next through cell division. It also ensures the transfer of characters with genetic material from parents to their offspring.

Q10. Why does, in a unicellular organism, the efficiency of a cell decrease with an increase in size?

Answer 10- 

With the increase in size, the volume gradually increases. Still, the surface area exposed to the outer environment does not increase correspondingly, limiting the exchange of information and materials through the surface. Through this, the efficiency of the cell as an autonomous unit decreases.

Q11. What is the significance of the plasma membrane?

Answer 11 –

Significance of plasma Membrane:(i)It forms the outer boundary of the cell, thus giving the cell a definite shape (ii) It protects the inner contents of the cell. (iii) It forms a molecular boundary between the cell & its environment.

Q12.Who proposed cell theory? Also, give its postulates.

 Answer 12 –

1. J. Schleiden and Theodore Schwann gave the famous cell theory, which states:

(i) All living things present are made of cells and cell products. 

(ii)The cell is all living organisms’ basic structural and functional units. Also, the cell is the basic unit of the living organism. 

(iii) All metabolic reactions in living beings occur within the cell. The cell theory was later modified by the scientist Rudolf Virchow, who was the first to state that “all-new cells arise from the pre-existing cells”, explaining why all cells contain hereditary material that passes from one generation to another by the process of cell division. It also ensures the transfer of characters with genetic material from parents to their offspring.

Q13.Which cell organelle is known as the powerhouse of a cell and why? 

Answer 13- 

 The double membrane of the mitochondria is actively associated with aerobic respiration and the release of energy for cellular activity. This biological oxidation of the fats & carbohydrates releases a small amount of energy utilised by mitochondria for ATP synthesis. They are commonly spherical, elongated, or rod-like cell organelles known as the “powerhouse of the cell.” Mitochondria were first discovered by the scientist Hofmeister in 1851 in the cells of a pteridophyte. They were later named ‘mitochondrion’ by Benda in 1897.

Mitochondria is a double membranous cell organelle where the outer membrane is smooth, and the space between the two membranes is called the outer chamber. On the other hand, the space surrounded by the inner membrane is called an inner chamber filled with homogeneous fluid.

Q14. What are the main functions of a cell wall?

Answer 14-

The main functions of a cell wall are:

(i)It provides structural support to the shape of the cell.

 (ii)It protects the inner contents of cells

 (iii)It also protects the delicate plasma membrane present in it. 

(iv)It allows the transportation of various substances to and fro from the cell.

 (v) It prevents the cell contents from drying up.

Q15. Why is the plasma membrane described as a” protein iceberg in the sea of lipids”? 

Answer 15- 

The plasma membrane was first described by Singer & Nicolson as a fluid mosaic model type. The lipids and proteins were arranged in a mosaic fashion. The matrix is a highly viscous fluid consisting of two layers of phospholipid molecules with two types of globular proteins. The two types of globular proteins are- 

1. i) peripheral or extrinsic proteins 
2. ii) intrinsic or integral proteins. 

The proteins present superficially with the membrane are enzymatic and can move across the matrix & help in the active & passive transport of ions through the membrane.

Q16.Explain the fluid mosaic model given by Singer and Nicolson of the plasma membrane.

Answer 16 –

 The plasma membrane was first described by Singer & Nicolson as a fluid mosaic model type. According to this theory, each phospholipid layer is bimolecular, where their hydrophilic ends are pointed towards the top & bottom, respectively. The lipids and proteins were arranged in a mosaic fashion. The matrix is a highly viscous fluid consisting of two layers of phospholipid molecules with two types of globular proteins. The two types of globular proteins are- 

1. i) peripheral or extrinsic proteins 
2. ii) intrinsic or integral proteins. The integral proteins are tightly bonded in place by strong hydrophobic or hydrophilic interactions and are very difficult to remove from the plasma membranes. Two peripheral proteins are arranged superficially on either side of the membrane and selectively permeable. Thus, this model explains that the cell membrane is quasi-fluid & is made up of “protein icebergs in the sea of lipids’ ‘. 

Q17.Describe in brief the eukaryotic chloroplast. 

Answer 17- 

Chloroplasts are bounded by two membranes, about 3000 A0 in total thickness. Each membrane is 40-60 thick. The eukaryotic chloroplast’s inner membrane is intricately elaborated to form a system of stroma lamellae. Internally these chloroplasts are divisible into two parts.

(a) Stroma- This is a colourless, ground substance found in the chloroplast’s inner side. 

(b)Membrane system is made of closed flattened sacs called thylakoids. These thylakoids are closely packed & appear as piles of coils. . These piles of coil structures are called Grana. The arrangement of this form can be in the form of simple parallel sacs running lengthwise or in a complex interconnecting network of sacs. These chloroplasts invariably have some starch granules, which often accumulate near a special region known as pyrenoid in algae

Q18.Mention three similarities and three differences between mitochondria & chloroplasts.

 Answer 18- 

Similarities between mitochondria & chloroplast (i)Mitochondria & chloroplasts are semiautonomous organelles and possess their DNA RNA ribosomes. (ii)They both develop and originate in the same way, formed by the division of pre-existing organelle (iii) They contain circular DNA.

 Differences between mitochondria & chloroplast (i)Mitochondria occurs in all eukaryotic cells, while chloroplast is present only in plant cells. (ii) Pigments are also absent in mitochondria but are always present in chloroplast. (iii)The inner membranes of mitochondria are folded into cristae, whereas cristae is absent in chloroplast.

Q19.What do you mean by tonoplast?

1. The outer membrane of organelle mitochondria 
2. The inner membrane of a eukaryotic chloroplast
3. Membrane boundary of the vacuole present in plant cells
4. The cell membrane of the plant cell

Answer 19: Option (c) is the answer.

Explanation-  Tonoplast is an organelle in the vacuole of plant cells.

Q20. Which of the following is not true for a eukaryotic cell?

1. Polypeptides and peptidoglycans form the cell wall. 
2. 80S types of ribosomes are present in the eukaryotic cell. 
3. Mitochondria contain circular DNA.
4. Membrane-bound organelles are present

Answer: Option (a) is the answer.

Explanation – The cell wall of a eukaryotic cell is made of cellulose.

Q21. Why are Lysosomes called the suicidal bag of the cell?

Answer 21- 

The cell organelles are bound by a single membrane and are classified as Lysosomes.  They are tiny sac-like structures that are found in animal cells. They are not present in prokaryotes. They help destroy and prevent harmful substances and foreign materials from entering the cells; hence, they are also called suicidal bags of the cell. 

Q22. Describe the contributions of the following scientists in formulating the cell theory briefly.

1. Robert Virchow
2. Schleiden and Schwann

Answer 22 –

Rudolf Virchow described the modern theory of cells, which states that new cells are formed from pre-existing cells. He also modified the hypothesis of Schleiden and Schwann to give the famous cell theory.

1. All living organisms are composed of cells and products of cells
2. All cells arise from pre-existing cells

In 1839, a British Biologist Theodore Schwann studied a different type of animal cell and said that cell had a thin layer later known as the plasma membrane. He studied plant cells and concluded that the presence of a cell wall is a unique characteristic of plant cells.

Q23.Write the functions of the following terms-  

1. Centromere
2. Cell wall 
3. Smooth ER 

d.Golgi Apparatus 

e.Centrioles 

Answer:

1. Centromere – Centromere holds the two chromatids or sister chromatids together in a chromosome.
2. Cell wall- The cell wall is present in the plant cells, which gives shape and protection to the cell from mechanical damage.
3. Smooth ER – Smooth Endoplasmic reticulum is the major site for lipid synthesis, and in animal cells, the lipid-like steroidal hormones are synthesised by Smooth ER.
4. Golgi apparatus – Golgi apparatus is an important site for forming glycoproteins and glycolipids.
5. Centrioles – The basal body of cilia or flagella and spindle fibres that give rise to spindle apparatus.

Q24. Are all the different types of plastids interchangeable? 

Answer 24 – 

Yes, all different types of plastids are interchangeable in their form.

There are three major plastids: Chloroplasts, which are green in colour, and Chromoplasts, which are red, yellow, or orange and Leucoplasts which are colourless. Depending upon the different circumstances, these plastids are interchangeable.

For example: Due to replacing the chloroplast with the chromoplasts, the colour of green tomatoes will change from red to blue.

Q25. Which of the following statements is true for a secretary cell?

1. a) Golgi apparatus is absent in the cell
2. b) Rough Endoplasmic Reticulum or the (RER) is easily observed in a cell.
3. c) Only Smooth Endoplasmic Reticulum (SER) is present in the endoplasmic reticulum.
4. d) Secretory granules are easily formed in the nucleus.

Answer – A correct answer is an option (b)

 Explanation – Rough Endoplasmic Reticulum (RER) is easily observed in the cell involved in secretion and protein synthesis. 

Q26. What structural and functional attributes must a cell have to be considered a living cell?

Answer 26- 

The structural and functional attributes that a cell must contain to be called a living cell are as follows- 

Plasma Membrane: A cell must have a plasma membrane because, in the absence of it, there will be no membrane to hold the contents of the cell, and thus the cells will not have any independent existence in the body.

Functional Attributes of the Plasma Membrane: The plasma membrane should never be fully or impermeable; rather, it should always be selectively permeable to allow the exchange of important materials through diffusion between the cell and its external environment.

Protoplasm: Protoplasm contains the living material in a cell. They also contain all other important cell organelles required for life to continue. If the protoplasm dries up, the cell dies like the sclerenchyma cells.

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

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

The nucleus is also not mandatory for all living cells, just like Prokaryotic cells, which also do not have a nucleus.

Q27. Eukaryotic cells contain organelles which may

1. a) Not be bound by a membrane
2. b) Bound by a single membrane
3. c) Bound by a double membrane

Answer 27-

(a) Organelles not bound by a membrane are classified as Ribosomes. They occur freely in the cytoplasm and can also be found attached to the rough surface of the endoplasmic reticulum. They are also found in the matrix of mitochondria and the stroma of plastids.

(b) The cell organelles bound by a single membrane are classified as Lysosomes. Lysosomes are tiny sac-like structures found in animal cells and are not present in prokaryotes. They help destroy and prevent harmful substances and foreign materials from entering the cells; hence, they are also called a suicidal bag of the cell. 

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

Q 28. Why is  cell the basic unit of life?

Answer 28. While a human body is made up of trillions of cells, there are also organisms formed with a single cell, e.g., amoeba. All cells divide and form new cells and possess several organelles to carry out various life functions. The cells of an organism carry the same genetic material and, thus, the same hereditary information as an individual. Hence, a cell is called the basic unit of life.

Q 29. Explain why the Mitochondria are known as the “powerhouse of the cell”?

Answer 29. The Mitochondria carry out anaerobic respiration; thus, they generate ATP for cell functioning. Hence, they are called the “powerhouse of the cell”.

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