Question 1

Which of the following is not correct?

Accepted Answer

Robert Brown discovered the cell - is a false statement.The first person to see a cell was Robert Hook. He used a very primitive microscope and observed empty blocks like structure in the cork cells which was named as cell.

Question 2

New cells are generated from:

Accepted Answer

New cells generate from the pre-existing cells.The cell theory is a widely accepted explanation of the relationship between cells and living things. The cell theory states :• All living things or organisms are made of cells.• New cells are created by old cells dividing into two.• Cells are the basic building units of life.In 1858, Rudolf Virchow concluded that all cells come from pre-existing cells, thus completing the classical cell theory.

Question 3

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

Accepted Answer

Cristae : The inner membrane of mitochondria folds inward into a finger like projection known as cristae. It increases the surface area for absorption.Cisternae : A space containing fluid, such as those occurring between the membranes of flattened sacs of the Golgi apparatus and the endoplasmic reticulum, also between the two membranes of the nuclear envelope.Thylakoids : Thalakoids are disc shaped structure which are arranged in stacks known as grana. These contain the photosynthetic pigments and are site for light reaction during photosynthesis. Hence, the correct answer is as follows : 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

Question 4

Which of the following is correct?

Accepted Answer

(a) Only eukaryotic cells have nuclei. They are absent in prokaryotes.(b) Cell walls are only present in plant cells. They are absent in all animal cells.(c) Membrane-bound organelles are organelles surrounded by a double or a single membrane like nucleus, mitochondria, chloroplasts, lysosomes, ER, Golgi bodies etc., are examples of such organelles. These cell organelles are absent in prokaryotes.(d) All cells arise from pre-existing cells.

Question 5

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

Accepted Answer

Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane. Its functions are as follows :(1) These extensions help in the synthesis of the cell wall and replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells.(2) It also increases the surface area of the plasma membrane to carry out various enzymatic activities.(3) It helps in secretion processes as well as in bacterial respiration.

Question 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?

Accepted Answer

Neutral solutes move across the membrane by the process of simple diffusion along the concentration gradient i.e., from higher concentration to the lower concentration. Polar molecules cannot pass through the nonpolar lipid bilayer, they require carrier proteip of the membrane to facilitate their transport across the membrane. In facilitated diffusion, molecules are transported along concentration gradient by help of ion channels and permeases and it does not involve energy expenditure (passive transport).

Question 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.

Accepted Answer

There are many cell organelles which are membrane-bound, The two organelles which have double membrane are the mitochondria and choloroplast. MitochondriaAre sausage or cylindrical-shaped organelles found in cytoplasmAre the sites of aerobic respiration (complete breakdown of glucose to release energy)Number and size varies.Has 2 membranes; outer membrane and inner membraneInner membrane forms a number of infoldings called the cristae Cristae contains many enzymes required for forming ATP (storage form of energy inside a cell)Fluid-filled space inside the mitochondria is called the matrixMatrix contains a circular DNA, few RNA molecules and ribosomes (70S).Are also called the powerhouses of the cell (because they generate energy required for the cell and store it in the form of ATP) Are semi-autonomous (They have their own DNA and replicate on their own and can also synthesize most of its proteins) ChloroplastThey are mainly present in the in leaf epidermal cells.They are cylindrical-shaped organellesThey are the sites of photosynthesisThey have 2 membranes; outer and inner membraneThe fluid-filled internal space is called the stromaThe stroma contains thylakoids which contain the pigments chlorophyll and carotenoids.The stroma also contains a circular DNA, few RNA molecules and ribosomes (70S).

Question 8

What are the characteristics of prokaryotic cells?

Accepted Answer

Prokaryotic cells are those cell that does not possess membrane-bounded nucleus e.g., bacteria, cyanobacteria. The characteristics of prokaryotic cells are as follows:1. They are small in size 0.1 mm to 10 mm.2. They do not possess membrane-bound organelles.3. They have single circular DNA as genetic material and plasmid.4. They possess mesosomes for respiration.5. Some are autotrophic and some are saprotrophic.

Question 9

Multicellular organisms have division of labour. Explain.

Accepted Answer

Multicellular organisms are made up of millions and trillions of cells. All these cells perform specific functions. All the cells specialised for performing similar functions are grouped together as tissues in the body. Hence, a particular function is carried out by a group of cells at a definite place in the body. Similarly, different functions are carried out by different groups of cells in an organism and this is known as the division of labour in multicellular organisms.

Question 10

Cell is the basic unit of life. Discuss it in brief.

Accepted Answer

Cells are the basic units of life capable of doing all the required biochemical processes that a normal cell has to do in order to live. The basic needs for the survival of all living organisms are the same. All living organisms need to respire, digest food for obtaining energy, and get rid of metabolic wastes. Cells are capable of performing all the metabolic functions of the body. Hence, cells are called as the functional units of life.

Question 11

What are nuclear pores? State their function.

Accepted Answer

Nuclear envelope bounds the nucleus from outside and separates it from cytoplasm. It consists of two membranes, with outer membrane continuous with endoplasmic reticulum. The nuclear envelope is interrupted by minute nuclear pores, at a number of places, which are produced by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.

Question 12

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

Accepted Answer

Lysosomes are membrane-bound vesicular structures holding a variety of enzymes such as lipases, proteases and amylases. The purpose of lysosomes is to digest worn out cells. They are involved in the intracellular digestion of foreign food particles and microbes. Sometimes, they also act as suicidal bags. They are involved in the self digestion of cells. They are a kind of waste disposal systems of a cell. On the other hand, vacuoles are storage sacs found in cells. They might store the waste products of cells. In unicellular organisms, the food vacuole contains the consumed food particles. It also plays a role in expelling excess water and some wastes from the cell.

Question 13

Describe the structure of the following with the help of labelled diagrams.(i) Nucleus(ii) Centrosome

Accepted Answer

(i) NucleusThe nucleus is a double wall membrane structure. It regulates the cellular activities . It constitutes the following parts-Nuclear membrane : It is a double membrane separating the contents of the nucleus from the cytoplasm. The narrow space between the two membranes is called the perinuclear space. Nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into a nucleus and out from it.Nucleoplasm/Nuclear matrix : It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin. The nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules and is the site of ribosome formation. Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called as histones.(ii) CentrosomeCentrosome has two cylindrical structures called centrioles. Each has a cartwheel-like organisation. A centriole is made up of microtubule triplets in a ring. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub is connected to the triplets via radial spokes. These centrioles help in organising the spindle fibres and astral rays during cell division. They form the basal body of cilia and flagella as well.

Question 14

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

Accepted Answer

The centromere is a constriction present on the chromosomes where the chromatids are held together.Chromosomes are divided into four types based on the position of the centromere.(i) Metacentric chromosomeThe chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome. During anaphase, they appear V-shaped.(ii) Sub-metacentric chromosomeThe chromosome in which the centromere is slightly away from the middle region is known as a sub-metacentric chromosome. In this, one arm is slightly longer than the other. During anaphase, they appear L-shaped.(iii) Acrocentric chromosomeThe chromosome in which the centromere is located close to one of the terminal ends is known as an acrocentric chromosome. In this, one arm is extremely long and the other is extremely short. During anaphase, they appear J-shaped.(iv) Telocentric chromosomeThe chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome. During anaphase, they appear I-shaped.

Chapter 8 : Cell: The Unit of Life