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

NCERT Solutions for class 11 Chapter 8 Cell The Unit of Life: The chapter on Cell: The Unit of Life is important for students approaching the home exams. This article introduces NCERT solutions designed to help students explain the concepts of further learning and how to write to get good grades on exams. The solutions are presented in very simple language for ease of understanding.

NCERT CBSE Chapter 8 Cell The Unit of Life of Class 11 explains that all living organisms are made up of smaller units, called cells. Cells provide structure and function for all living things. A cell has three main parts, i.e. cell membrane, nucleus and cytoplasm. When the cells become damaged, our body makes new cells and replaces the damaged cells. This process is called cell division. Small organ-like structures present inside a cell are known as cell organelles. Revise the basic concepts of Cell: The Unit of Life for quick revision and class notes.

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:

The correct answer is (a). The cell was discovered by Robert Hooke. Robert Brown discovered the nucleus.

Q2: New Cells Generate from

• (a) Bacterial Fermentation
• (b) Regeneration of Old Cells
• (c) Pre-existing Cells
• (d) Abiotic Materials

Answer:

The correct answer is (c) pre-existing cells. This statement was given by Rudolf Virchow.

Q3: Match the Following

Column I	Column II
(a) Cristae	(i) Flat membranous sacs in stroma
(b) Cisternae	(ii) Infoldings in mitochondria
(c) Thylakoids	(iii) Disc-shaped sacs in Golgi apparatus

Answer:

Column I	Column II
(a) Cristae	(ii) Infoldings in mitochondria
(b) Cisternae	(iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids	(i) 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 a well-defined cell wall.
• (c) In prokaryotes, there are no membrane-bound organelles.
• (d) Cells are formed de novo from abiotic materials

Answer:

The correct answer is (c) in prokaryotes, there are no membrane-bound organelles.

Q5: What is a Mesosome in a Prokaryotic Cell? Mention the Functions that it Performs.

Answer:

Mesosome is the sub-organelle formed by the infolding of the plasma membrane of prokaryotes, e.g. bacteria. This is analogous to cristae, which are finger-like projections, present in the mitochondrion of eukaryotes. The functions of mesosomes are stated below:

1. It increases the surface area of the cell.
2. It aids in DNA replication and distribution to the daughter cells. 
3. It helps in cellular respiration.
4. It supports respiration and secretion processes in a cell
5. It enhances the surface area of the plasma membrane and enzyme concentration.
6. It helps in cell wall formation.

Q6: How do Neutral Solutes move across the Plasma Membrane? Can the Polar Molecules also move across it in the same way? If not, how are these Transported Across the Membrane?

Answer:

Neutral solutes move across the membrane by simple diffusion, the movement of molecules from an area of higher concentration to an area of lower concentration. Polar molecules cannot cross the membrane in the same way. For their transport, carrier proteins or transport proteins or membrane channel proteins are needed because their size and charge make them difficult to penetrate. In addition, polar molecules are hydrophilic in nature; they are water-loving and like the exterior part of the membrane. So, therefore they are excluded from the interior of the membrane. Facilitated diffusion allows polar and charged molecules, i.e. carbohydrates, amino acids, nucleosides, and ions to move across the plasma 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 cell organelles which are double membrane-bound.

Characteristics of mitochondria:

• They are the double-membraned and rod-shaped structures which are present in both plant and animal cells.
• They have an outer membrane, an inner membrane and a gel-like substance, known as the matrix.
• Their inner membrane has some infoldings, known as cristae.

Structure of a mitochondrion

Functions of mitochondria:

• They produce ATP molecules. ATP is known as the energy currency of the cell, it is like a battery which derives energy. Therefore, mitochondria are known as the powerhouse of the cell.
• They help in apoptosis i.e. programmed cell death (facilitates cell division also)
• They help in the storage of calcium ions.
• They help in thermoregulation i.e. heat production in living beings like mammals.
• They help in the detoxification of ammonia in the liver cells. 

Characteristics of the chloroplast:

1. They are oval-shaped and possess an outer membrane and an inner membrane. The gap present between these membranes is known as intermembrane space.
2. They are a type of plastid which is involved in the synthesis and storage of food for the plants. Therefore, they are known as the kitchen house of the cell.
3. The internal membrane is highly folded and has thylakoids (closed disc-like structures).
4. The thylakoids are arranged like some stacks known as grana (plural) /granum (singular). Many grana are linked by stromal lamellae.

Structure of a chloroplast

Functions of Chloroplasts:

1. They produce energy by the process of photosynthesis which results in the production of sugars for the plants. (light energy is converted into chemical energy).
2. Chlorophyll pigment present in the chloroplasts is responsible for giving green colour to the plants.
3. They are responsible for the biosynthesis of some compounds, i.e. amino acids, plant hormones, nucleotides, vitamins, fats, etc.

Q8: What are the Characteristics of Prokaryotic Cells?

Answer:

The characteristics of prokaryotic cells are:

1. They lack a true membrane-bound nucleus. The nuclear content i.e. nucleoid is present in the cytoplasm, and lacks a nuclear membrane around it.
2. They do not possess membrane-bound organelles like mitochondria, chloroplasts, ER, golgi bodies, etc.
3. They have some infoldings of the plasma membrane, known as mesosomes which help in respiration.
4. They are divided into two main domains, i.e. bacteria and archaea.

Q9: Multicellular Organisms have a Division of Labour. Explain.

Answer:

Division of labour depicts some specialised functions of the cell organelles which combine and perform some functions. Multicellular organisms have a division of labour as they are made up of many different types of cells in their body and when they are aggregated or grouped together as a tissue which is specialized to perform some specific functions.

Q10: The Cell is the Basic Unit of Life. Discuss in brief.

Answer:

The cell is known as the basic unit of life because a cell can perform all the necessary functions independently which are essential to survive. All the functions that occur inside our body are performed by the cells. They provide some specific conditions which are required for some metabolic reactions to take place.

Q11: What are Nuclear Pores? State their function.

Answer:

The nuclear pores are the areas where the inner and outer membranes of the nucleus fuse to form a pore or protein-lined channel. They aid in the transportation of various molecules between the nucleus and the cytoplasm. Some examples of such molecules are ribonucleoproteins, lipids, signalling molecules, carbohydrates, etc.

Q12: Both Lysosomes and Vacuoles are Endomembrane Structures, yet they Differ in Terms of their Functions. Comment.

Answer:

Lysosomes are membrane-bound structures which have a number of enzymes like lipases and proteases. The function of lysosomes is to digest worn-out cells by hydrolytic enzymes. They are involved in the intracellular digestion of foreign substances. They act as suicidal bags as they are involved in the self-digestion of cells.

Vacuoles are the storage bag-like structures present in both plant and animal cells. In plant cells, they are prompt and large to store food materials. They also store waste products in the cells. In unicellular organisms like amoeba, the food vacuole contains food particles. They also function in expelling excess water and other wastes from the cell.

Q13: Describe the Structure of the following with the help of Labelled Diagrams.

• (i) Nucleus
• (ii) Centrosome

Answer:

Nucleus

Structure of a nucleus

The nucleus is a sphere-shaped organelle present in eukaryotic cells. The structure of the nucleus includes a nuclear membrane, chromosomes, nucleoplasm and nucleolus. A nuclear membrane is selectively permeable i.e. it allows only certain things to pass across the membrane. It is perforated by some tiny gaps known as nuclear pores. The nuclear envelope encloses a ground substance, known as the nucleoplasm. It is responsible for making essential enzymes to aid in the function of genetic material. The nucleus contains a condensed network of fine fibres called chromatin which is made up of DNA and nuclear proteins. Inside the nucleus, the nucleolus is present for making ribosomes.

Centrosome

Structure of a centrosome

A centrosome is a cellular structure which is composed of two microtubule rings, centrioles. It is involved in the process of cell division. Before cell division, the centrosome divides. Each divided centrosome are moved to the opposite ends of the cell. They are present in the animal cells only.

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 is the constricted region of a chromosome which helps in cell division (mitosis and meiosis). The spindle is the structure which pulls the chromatids to the opposite poles of the cell during cell division. Each chromosome has only one centromere. Centromeres are also involved in the sister chromatid adhesion and separation, movement of chromosomes, microtubule attachment, mitotic checkpoint control, etc. On the basis of the location of the centromere, chromosomes are classified into four types: metacentric, submetacentric, acrocentric, and telocentric.