NCERT Solutions Class 11 Biology Chapter 8

Q.1 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.

Ans-

(a) Incorrect

[Note: Robert Brown did not discover the cell. It was Robert Hooke first discovered the cell in 1665.]

Q.2 New cells generate from

(a) bacterial fermentation

(b) regeneration of old cells

(c) pre-existing cells

(d) abiotic materials

Ans-

(c) pre-existing cells

[Note: The cell theory given by Rudolf Virchow in 1855 states that cells divide and are formed from pre-existing cells only.]

Q.3 Match the following

Ans-

Q.4 Which of the following is a correct statement:

(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.

Ans-

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

Q.5 What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

Ans-

Mesosomes are special membranous structures found in prokaryotic cells and are formed by the extension of the plasma membrane into the cell. They are infoldings of the bacterial cell membrane. These extensions are in the form of vesicles, tubules and lamellae.

Functions: They are involved in various cellular processes like:

1. Cell wall formation during cell division
2. DNA replication and its separation in daughter cells
3. Respiration (oxidative phosphorylation)
4. Secretion by enhancing the surface area
5. They are rich in enzymes

Q.6 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?

Ans-

The plasma membrane is composed of lipids that are arranged in a bilayer with their polar head towards outside and the hydrophobic tails towards the inside. Embedded in this lipid bilayer are proteins and carbohydrates. One of the most important functions of the plasma membrane is the transport of molecules across it.

Neutral solutes move across the plasma membrane by the process of simple diffusion along the concentration gradient i.e. from higher concentration to the lower concentration. This is called passive transport and no energy is required for this process to occur. The respiratory gases, oxygen and carbon dioxide diffuse into and out of the cell.

The polar molecules cannot move across the plasma membrane in the same way since the lipid bilayer is nonpolar in nature. Transport of such molecules requires membrane proteins that facilitate transport across the membrane. This is called as facilitated transport. This is mediated by:

a) carrier proteins that facilitate the movement by combining with the molecule to be transported and delivering it to the other side of the membrane after undergoing a conformational change

b) channel proteins that provide a simple passage across the membrane for the molecules to pass through.

Q.7 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.

Ans-

The two double membrane-bound organelles are mitochondria and chloroplasts.

Characteristics of mitochondria:

• Mitochondria are between 0.5-1 µm in diameter and ~7 µm in length, although the size and shape can vary.
• Their number varies depending upon the physiological activity of the cells.
• They are bound by a double membrane with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments. The inner membrane is folded to form structures called cristae, which project into the matrix.
• The matrix is filled with a gel-like fluid. It contains enzymes that break down carbohydrate-derived products.
• ATP production occurs at the cristae. The outer membrane forms the continuous limiting boundary of the organelle. The two membranes have their own specific enzymes.
• Mitochondria contain their own DNA, a few RNA molecules and ribosomes. They can also produce few of their own proteins.
• They reproduce or divide themselves.

Functions of mitochondria: Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence are called ‘powerhouse of the cell’.

Figure: Structure of Mitochondria

Characteristics of chloroplasts:

• Chloroplasts are lens-shaped, oval, spherical, discoid or even ribbon-like organelles and are about 4-6 µm in diameter and 1-5 µm in length.
• Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll of green leaves.
• They are mostly found in the mesophyll cells of the leaves.
• They are bounded by a double membrane. Of the two membranes, the inner membrane is less permeable.
• The membranes of the thylakoids enclose a space called a lumen.
• Inside the chloroplasts are found numerous membranes which are arranged into flattened sacs called thylakoids.
• The thylakoids are piled up like stacks of coins and each stack is called a granum. The flat membranous connections or tubules connecting the thylakoids of various grana are called lamellae. These membranous structures are located in the stroma.
• Chloroplasts belong to a group of plant organelles known as plastids. Chloroplasts contain chlorophyll and carotenoid pigments.
• Chloroplasts have their own small, double-stranded circular DNA and ribosomes (70S). They can produce their own proteins.
• They can also divide to form more chloroplasts. Thus, they resemble photosynthetic prokaryotic organisms.

Functions of chloroplasts:

• The enzymes required for photosynthesis are located in chloroplasts. The thylakoids contain the green pigment chlorophyll that captures the solar energy required for photosynthesis.
• The stroma contains all the required enzymes for the synthesis of carbohydrates and proteins.

Figure: Structure of chloroplast

Q.8 What are the characteristics of prokaryotic cells?

Ans-

Characteristics of Prokaryotes:

• Prokaryotes are a group of organisms that do not have a nucleus and membrane-bound cell organelles.
• All the prokaryotic cells have a cell wall surrounding the cell membrane.
• The cytoplasm is the fluid matrix that fills the cell.
• There is no well-defined nucleus. The genetic material, which can be in the form of a single chromosome or a circular DNA is naked and is not surrounded by any nuclear membrane.

• Many prokaryotes especially, bacteria have a small circular DNA, known as plasmids, other than the genomic DNA. The plasmid confers certain unique phenotype characters so these bacteria like antibiotic resistance.
• No membrane-bound organelles are found in prokaryotic cells.
• Mesosomes, special membranous structures are found in prokaryotic cells that are formed by the extension of the plasma membrane into the cell. They are infoldings of the bacterial cell membrane.
• The cell envelope of most prokaryotic cells is very complex – it consists of a tightly bound three-layered structure that acts as a protective covering with each layer performing a distinct function. The outermost glycocalyx is followed by the cell wall and then the plasma membrane.
• Prokaryotes can be motile or non-motile. The motility is provided by flagella.
• Pili and Fimbriae are also surface structures that do not help in motility but help in attachment of bacteria to some surface.
• Bacteria, blue-green algae, mycoplasma etc. are examples of prokaryotes.
• They are generally smaller and multiply rapidly. They vary greatly in shape (bacillus, coccus, vibrio or spirillum) and size.

Q.9 Multicellular organisms have division of labour. Explain.

Ans-

Multicellular organisms are composed of a large number of cells with larger complexity involved in terms of structure and function. These cells vary greatly in size, shape and activities. Some of the examples of various kinds of cells based on the function are: red blood cells are round and biconcave to increase the surface area and nerve cells are long cells as they are required to carry signals over long distances etc. Each kind of cell combines to give rise to tissues, many different tissues organize themselves to form an organ and ultimately the organ system which carries out specific metabolic activities such as respiration, digestion, circulation, excretion etc. Each organ system consists of different organs each of which is assigned specific roles. This shows the division of labour which is needed in a complex body such as that of multicellular organisms. Each cell, tissue, organ and organ system carries out its role depending upon the kind of cell it is made up of.

In simple organisms, like the unicellular Amoeba, which consists of only a single cell, all the functions are carried out within a single cell.

Q.10 Cell is the basic unit of life. Discuss in brief.

Ans-

Cell theory which forms the basis for this statement states that:

• All living things or organisms are made of cells and their products.
• New cells are created by old cells through division.
• Cells are the basic building blocks of life.

All the cells are alive and carry out respiration, reproduction (by the process of mitosis or meiosis) and growth. Cells arise from preexisting cells and become specialized for distinct functions such as; contraction, conduction, secretion, absorption, and protection. All cells have a few things in common e.g. cell membrane, DNA, cytoplasm, and ribosomes which carry out numerous functions required for the activity of the cell. A cell is capable of carrying out all the fundamental activities required to live and thus, is called as a basic unit of life.

Q.11 What are nuclear pores? State their function.

Ans-

Nuclear pores are large protein-lined channels with a complex structure that regulate the transportation of large molecules between the nucleus and the cytoplasm through the nuclear envelope. The nuclear membrane is impermeable to large molecules and thus, safeguards the DNA. In spite of this barrier, nuclear pores allow communication between the nucleus and the cytoplasm.

Functions:

• Allows small molecules and ions to pass freely, or diffuse, in and out of the nucleus.
• Nuclear pores allow necessary proteins with specific sequence tags (nuclear localization signals) to enter the nucleus from the cytoplasm.
• RNA transcribed in the nucleus and proteins that are destined to enter the cytoplasm have nuclear export sequences and are thus released in the cytoplasm through the nuclear pores.

Q.12 Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

Ans-

The endomembrane is an intercellular system is responsible for the flow of materials from one to another part through vesicles but they are specialised to perform the different functions. Both vacuole and lysosome are its components. Lysosomes are membrane-bound organelles that release hydrolytic digestive enzymes to digest food and also breakdown aged and worn-out cells. This is the reason lysosomes are known as suicidal bags. On the other hand, vacuoles help cells to maintain the shape of the cell. Vacuoles also store food, water and waste products. Vacuoles help in the excretion and osmoregulation in Amoeba.

Q.13 Describe the structure of the following with the help of labelled diagrams.

(i) Nucleus (ii) Centrosome

Ans-

(i) Structure of Nucleus: The nucleus is a prominent structure in the eukaryotic cell that can be seen with a light microscope after straining. It acts as a control centre of the cell overseeing the metabolic functioning of the cell as well as characteristics of the cell. The nucleus is composed of the nuclear matrix and nuclear envelope.

Nuclear matrix (nucleoplasm): Nuclear matrix is semifluid in nature and contains the chromatin and the nucleolus. Chromatin is a threadlike material that undergoes coiling or condensation into rod-like structures (chromosomes) just before the cell divides. Chromatin consists of DNA with proteins and some RNA.

One or more spherical bodies called nucleoli are also present in the nuclear matrix. There is no membranous separation between the nucleoli and the rest of the nuclear matrix. This is the site of active ribosomal RNA (rRNA) synthesis and looks darker than the rest of the chromatin under an electron microscope.

Nuclear envelope: The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelope. These parallel membranes have space between them which is called perinuclear space. It acts as a barrier between the cytoplasm and the nucleus however nuclear pores with a very complex structure allow passage of large molecules (like RNA and proteins). The outer membrane is usually continuous with the endoplasmic reticulum and also bears ribosomes on it.

Diagram of the nucleus:

(ii) Structure of Centrosome: Centrosome is an organelle that usually contains two cylindrical structures called centrioles and serves as the microtubule-organizing centre of the cell. Centrioles are short cylinders with a 9+0 pattern of microtubule triplets which means that each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein. Each fibril, in turn, is a triplet. The central part of the centriole is also made of proteins and is connected with peripheral triplet tubules by radial spokes (also made of proteins). Amorphous pericentriolar materials surround the two centrioles. The two centrioles of a centrosome are perpendicular to each other.

Function: The centrioles form the basal body of cilia and flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells. Before an animal cell divides, the centrioles replicate and the members of each pair are also at right angles to each other.

Diagram of Centrosome:

Q.14 What is a centromere? How does the position of centromere forms the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.

Ans-

Centromere: It is the specialized constricted region of the chromosome where the two sister chromatids remain joined together after replication. There is a disc-shaped structure on the sides of centromere called kinetochore to which spindle fibres attach during cell division.

Classification of chromosomes based on the position of centromeres: Chromosomes are classified into four different groups based on the position of centromeres:

• Metacentric chromosome: The centromere is present in the middle of the chromosome resulting in two equal arms of the chromosome.
• Sub-metacentric chromosome: The centromere is present slightly away from the middle position of the chromosome resulting in a longer and a shorter arm of the chromosome.
• Acrocentric chromosome: The centromere is present close to the end of the chromosome resulting in one extremely short and one very long arm.
• Telocentric chromosome: The centromere is present at one end of the chromosome.