8.7 Exercise Problems

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: (a) Robert Brown did not discover the cell.
The cell was discovered by Robert Hook in the year 1665 by using the microscope. Hooke actually observed the dead cell walls of plant cells (cork) as it appeared under the microscope and remarked that it looked strangely similar to cellula or small rooms which monks inhabited, thus deriving the name.

Q2. New cells generated from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials

Answer: (c) pre-existing cells
According to the theory of biogenesis, new cells can only arise from preexisting cells. Only complete cells, in favourable conditions, can give rise to new cells.

Q3. Match the following

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Column I }} & & {\text { Column II }} \\
\hline \text { a } & \text { Cristae } & \mathbf{i} & \text { Flat membranous sacs in stroma } \\
\hline \text { b } & \text { Cistenae } & \mathbf{ii} & \text { Infolding in matrix } \\
\hline \text { c } & \text { Thylakoids } & \mathbf{iii} & \text { Disc-shaped sacs in Golgi apparatus } \\
\hline
\end{array}
\)

Answer: 

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Column I }} & & {\text { Column II }} \\
\hline \text { a } & \text { Cristae } & \mathbf{ii} & \text { Infolding in matrix } \\
\hline \text { b } & \text { Cistenae } & \mathbf{iii} & \text { Disc-shaped sacs in Golgi apparatus } \\
\hline \text { c } & \text { Thylakoids } & \mathbf{i} & \text { Flat membranous sacs in stroma } \\
\hline
\end{array}
\)

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: (c) Membrane-bound organelles are organelles surrounded by a double membrane. Nucleus, mitochondria, chloroplasts, etc., are examples of such organelles. These cells organelles are absent from prokaryotes.

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

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

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: The plasma membrane is the outermost covering of the cell that separates it from the environment. It regulates the movement of substances into the cell and out of it. It allows the entry of only some substances and prevents the movement of other materials. Hence, the membrane is selectively permeable. Movement of neutral solutes across the cell membrane – Neutral molecules move across the plasma membrane by simple passive diffusion. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Movement of polar molecules across the cell membrane – The cell membrane is made up of a phospholipid bilayer and proteins. The movement of polar molecules across the non-polar lipid bilayer requires carrier proteins. Carrier proteins are integral protein particles having a certain affinity for specific solutes. As a result, they facilitate the transport of molecules across 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 labelled diagrams of both.

Answer: Mitochondria and chloroplasts are the two organelles that are double-membrane-bound. 

Characteristics of the mitochondria
Mitochondria are double-membrane-bound structures. The membrane of a mitochondrion is divided into the inner and outer membranes, distinctly divided into two aqueous compartments – outer and inner compartments. The outer membrane is very porous (containing the organelle), while the inner membrane is deeply folded.
These folds are known as cristae. Cristae increase the surface area inside the cell. They are the sites for ATP-generating chemical reactions. The membrane of a mitochondrion contains specific enzymes meant for specific mitochondrial functions. Hence, the mitochondria are the sites for aerobic respiration. They have their own DNA and ribosomes. Thus, they are able to make their own proteins. This is why they are considered as semi-autonomous organelles Characteristics of chloroplasts Chloroplasts are double-membrane-bound structures. They are divided into outer and inner membranes, further divided into two distinct regions:

• Grana are stacks of flattened discs containing chlorophyll molecules. The flattened membranous sacs are called thylakoids. The thylakoids of adjacent grana are connected by membranous tubules called stroma lamellae.
• Stroma is a homogenous mixture in which grana are embedded. It contains several enzymes that are used for the synthesis of carbohydrates and proteins. It also contains its own DNA and ribosomes.

Functions of the mitochondria:

• They are the sites for cellular respiration.
• They provide energy in the form of ATP for all vital activities of living cells.
• They have their own DNA and ribosomes. Hence, they are regarded as semiautonomous organelles.
• They have several enzymes, intermediately required for the synthesis of various chemicals such as fatty acids, steroids, and amino acids.

Functions of chloroplasts:

• They trap solar energy and utilise it for manufacturing food for plants. Hence, they are involved in the process of photosynthesis.
• They contain the enzymes required for the synthesis of carbohydrates and proteins.

Q8. What are the characteristics of prokaryotic cells?

Answer: A prokaryotic cell is a unicellular organism lacking membrane-bound organelles.
The characteristics of prokaryotic cells are as follows:

• Most of them are unicellular.
• They are generally small in size. The size of a prokaryotic cell varies from \(0.5-5 \mu \mathrm{m}\).
• The nuclear region of a prokaryotic cell is poorly defined because of the absence of a nuclear membrane. Hence, a prokaryotic cell lacks a true nucleus.
• The genetic materials of prokaryotic cells are naked. They contain single, circular chromosomes. In addition to the genomic DNA, they have a small, circular plasmid DNA.
• They have specialised membranous structures called mesosomes. Mesosomes are formed by the invagination of the cell membrane. 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.
• Membrane-bound cell organelles such as mitochondria, plastids, and endoplasmic reticulum are absent from a prokaryotic cell.
• Most prokaryotic cells contain a three-layered structure – outermost glycocalyx, middle cell wall, and the innermost plasma membrane. This structure acts as a protective unit. Examples of prokaryotic cells include blue-green algae, bacteria, etc.

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

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. This is known as division of labour in multicellular organisms.

Q10. Cell is the basic unit of life. Discuss in brief.

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 functional units of life.

Q11. What are nuclear pores? State their function.

Answer: Nuclear pores are tiny holes present in the nuclear membrane of the nucleus. They are formed by the fusion of two nuclear membranes. These holes allow specific substances to be transferred into a cell and out from it. They allow molecules such as RNA and proteins to move in both directions, between the nucleus and the cytoplasm.

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

Answer: Lysosomes are membrane-bound vesicular structures holding a variety of enzymes such as lipases, proteases, and carbohydrases. 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 system 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.

Q13. Describe the structure of the following with the help of labelled diagrams.
(1) Nucleus
(ii) Centrosome

Answer: (i) Nucleus
The nucleus controls all the cellular activities of the cell. It is spherical in shape. It is composed of the following structures:
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. The nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into a cell 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 for ribosome formation. Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called histones.

(ii) Centrosome
Centrosome consists of two cylindrical structures called centrioles. Centrioles lie perpendicular to each other. Each has a cartwheel-like organisation. A centriole is made up of microtubule triplets that are evenly spaced 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.

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: 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 chromosome
The chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome.

(ii) Sub-metacentric chromosome
The 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.

(iii) Acrocentric chromosome
The 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.

(iv) Telocentric chromosome
The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. What is the significance of vacuole in a plant cell? 

Answer: Vacuole in plant cells help in the storage, waste disposal and cell elongation and protection.

Q2. What does ‘S’ refer in a 70S & an 80S ribosome?

Answer: Svedberg’s Unit or sedimentation coefficient.

Q3. Mention a single membrane bound organelle which is rich in hydrolytic enzymes.

Answer: Lysosome

Q4. What are gas vacuoles? State their functions? 

Answer: Gas vacuoles are aggregates of hollow cylindrical structures called gas vesicles. They are located inside some bacteria. The inflation and deflation of the vesicles provides buoyancy, allowing the bacterium to float at a desired depth in the water.

Q5. What is the function of a polysome?

Answer: Several ribosomes may attach to a single mRNA and form a chain called polyribosome or polysome. The ribosomes of a polysome translate the mRNA into proteins.

Q6. What is the feature of a metacentric chromosome?

Answer: The metacentric chromosome has middle (medial) centromere forming two equal arms of the chromosome. Shape of metacentric chromosome is V-shaped.

Q7. What is referred to as satellite chromosome?

Answer: Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite or trabant. These chromosomes are called sat (satellite) chromosome. Nucleolus is formed by sat chromosome.

SHORT ANSWER TYPE QUESTIONS

Q1. Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis.

Answer: Nucleolus is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

Q2. Explain the association of carbohydrate to the plasma membrane and its significance.

Answer: Carbohydrates forms glycoproteins and glycolipids by glycosylation. Glycoproteins and glycolipids are biochemicals that involved in cell recognition and adhesion.

Q3. Comment on the cartwheel structure of centriole.

Answer: Centrosome is an organelle usually containing two cylindrical structures called centrioles. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub.

Q4. Briefly describe the cell theory.

Answer: Schleiden and Schwann together formulated the cell theory (1838-39). This theory, however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understood today is
(i) All living organisms are composed of cells and products of cells.
(ii) All cells arise from pre-existing cells.

Q5. Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).

Answer: The ER often shows ribosomes attached to their outer surface. The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER). RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus. The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

Q6. Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?

Answer: The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane. These studies showed that the cell membrane is composed of lipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part. This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides. Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids.

Q7. What are plasmids? Describe their role in bacteria? 

Answer: In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. This plasmid DNA is used to monitor bacterial transformation with foreign DNA.

Q8. What are histones? What are their functions? 

Answer: In eukaryotes there is a set of positively charged, basic proteins called histones. Histones are rich in the basic amino acid residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called as histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix.

LONG ANSWER TYPE QUESTIONS

Q1. What structural and functional attributes must a cell have to be called a living cell?

Answer: All cells have an outer membrane called the cell membrane. Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA. Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’.
Besides the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria microbodies and vacuoles. The prokaryotic cells lack such membrane bound organelles. Ribosomes are non-membrane bound organelles found in all cells—both eukaryotic as well as prokaryotic.

Q2. Briefly give the contributions of the following scientists in formulating
the cell theory
a. Robert Virchow
b. Schielden and Schwann

Answer: In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, Theodore Schwann (1839), a British Zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells. On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e c’ellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understood today is :
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.

Q3. Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.

Answer: Yes, extra genomic DNA present in prokaryotes and eukaryotes. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria (prokaryotes) have small circular DNA outside the genomic DNA. These smaller extra genomic DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. This plasmid DNA is used to monitor bacterial transformation with foreign DNA. In eukaryotes, extra genomic DNA is present in two organelles- mitochondria and plastids. 

Q4. Structure and function are correlatable in living organisms. Can you justify this by taking plasma membrane as an example?

Answer: The shape of the cell may vary with the function they perform. For example, RBCs are round and biconcave to pass through capillaries and carry more \(O_2\). WBCs are amoeboid to do phagocytosis and diapedesis.
The quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity. The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.

Q5. Eukaryotic cells have organelles which may
a. not be bound by a membrane
b. bound by a single membrane
c. bound by a double membrane
Group the various sub-cellular organelles into these three categories.

Answer: a. Non-membrane bound cell organelles – Ribosome, Centrosome (Centriole), Nucleolus, Cytoskeletal structures.
b. Single membrane bound cell organelles – ER, GB, Lysosome, Vacuoles, Microbodies (Glyoxysomes and Peroxisomes), Thylakoid.
c. Double membrane bound cell organelles – Plastid, Mitochondria and Nucleus.

Q6. The genomic content of the nucleus is constant for a given species where as the extra chromosomal DNA is found to be variable among the members of a population. Explain.

Answer: The genomic content of the nucleus is constant for a given species whereas the extra chromosomal DNA is found to be variable among the members of a population. For humans (Homo sapiens) the genomic content of the nucleus is constant, i.e. 46 chromosomes. But extra chromosomal DNA is found to be variable among the members of the population like different humans have different amount of extra chromosomal DNA in their mitochondria.

Q7. Justify the statement, “Mitochondria are power houses of the cell” 

Answer: Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e. the outer compartment and the inner compartment. The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function. Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.

Q8. Is there a species specific or region specific type of plastids? How does one distinguish one from the other?

Answer: Yes, plastids are species specific or region specific. Plastids are found in all plant cells and in euglenoids. These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts. The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis. In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour. The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.

Q9. Write the functions of the following
a. Centromere
b. Cell wall
c. Smooth ER
d. Golgi Apparatus
e. Centrioles

Answer: a. Centromere: Every chromosome essentially has a primary constriction or the centromere. Two sister chromatids are joined together at the centromere.
b. Cell wall: Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.
c. Smooth ER: The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.
d. Golgi Apparatus: Golgi apparatus is the important site of formation of glycoproteins and glycolipids.
e. Centrioles: The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Q10. Are the different types of plastids interchangeable? If yes, give examples where they are getting converted from one type to another.

Answer: Yes, different types of plastids are interchangeable.
Conversion of green tomatoes (or chilli) into red form is due to formation of chromoplasts from chloroplasts. Chromoplasts also formed from leucoplasts by development of some pigments (like carotenes in carrot).