Cell : The Unit of LifeClass 11 Biology NCERT Solutions
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Q1Exercises
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.
Solution
The correct answer is (a).
Explanation:
Robert Brown discovered the nucleus in 1831. The cell was first observed by Robert Hooke in 1665 (dead cork cells), and the first live cell was seen and described by Antonie Von Leeuwenhoek. Therefore, the statement that Robert Brown discovered the cell is incorrect.
Q2Exercises
New cells generate from
(a)
bacterial fermentation
(b)
regeneration of old cells
(c)
pre-existing cells
(d)
abiotic materials
Solution
The correct answer is (c).
Explanation:
Rudolf Virchow in 1855 modified the cell theory with his statement Omnis cellula-e cellula, which means that all new cells arise from the division of pre-existing cells. This principle is a fundamental tenet of the modern cell theory.
Q3Exercises
Match the following Column I (a) Cristae (b) Cisternae (c) Thylakoids Column II
(i)
Flat membranous sacs in stroma
(ii)
Infoldings in mitochondria
(iii)
Disc-shaped sacs in Golgi apparatus
Solution
The correct match is as follows:
- (a) Cristae - (ii) Infoldings in mitochondria: The inner membrane of the mitochondria forms a number of infoldings called cristae, which increase the surface area for ATP synthesis.
- (b) Cisternae - (iii) Disc-shaped sacs in Golgi apparatus: The Golgi apparatus consists of many flat, disc-shaped sacs or cisternae stacked parallel to each other.
- (c) Thylakoids - (i) Flat membranous sacs in stroma: In the stroma of chloroplasts, there are a number of organised flattened membranous sacs called thylakoids.
Q4Exercises
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.
Solution
The correct answer is (c).
Explanation:
- (a) is incorrect because prokaryotic cells lack a membrane-bound nucleus. Also, some eukaryotic cells, like mature mammalian erythrocytes, lack a nucleus.
- (b) is incorrect because animal cells do not have a cell wall; it is a characteristic feature of plant cells, fungi, and some other organisms.
- (c) is correct. Prokaryotic cells lack membrane-bound organelles like the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus.
- (d) is incorrect. According to the cell theory, all cells arise from pre-existing cells, not spontaneously from non-living (abiotic) matter.
Q5Exercises
What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Solution
A mesosome is a specialized membranous structure found in prokaryotic cells, such as bacteria. It is formed by the infoldings or extensions of the plasma membrane into the cytoplasm. These extensions can be in the form of vesicles, tubules, and lamellae.
Functions of Mesosomes:
Mesosomes perform several important functions in the prokaryotic cell:
- Cell Wall Formation: They help in the synthesis of cell wall materials.
- DNA Replication: They assist in the replication of the bacterial chromosome and its distribution to daughter cells during cell division.
- Respiration and Secretion: They are involved in respiration and secretion processes, as they increase the surface area of the plasma membrane and contain enzymes.
- Increased Surface Area: They increase the surface area of the plasma membrane, which aids in various enzymatic activities.
Q6Exercises
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?
Solution
Neutral solutes move across the plasma membrane by a process called simple diffusion. This is a type of passive transport where molecules move along the concentration gradient, from an area of higher concentration to an area of lower concentration, without any expenditure of energy.
No, polar molecules cannot move across the plasma membrane in the same way as neutral solutes. The plasma membrane has a nonpolar lipid bilayer at its core. This hydrophobic core repels polar molecules and prevents them from passing through freely.
Therefore, polar molecules are transported across the membrane through the following mechanisms:
- Facilitated Diffusion: This is a form of passive transport where polar molecules move across the membrane with the help of carrier proteins embedded in the membrane. These proteins provide a channel or bind to the molecule to facilitate its movement down the concentration gradient. This process does not require energy.
- Active Transport: This process moves molecules against their concentration gradient (from lower to higher concentration). It is an energy-dependent process that requires ATP and involves specific carrier proteins (pumps). An example is the pump.
Q7Exercises
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.
Solution
Two cell-organelles that are double membrane-bound are Mitochondria and Chloroplasts.
1. Mitochondria
Characteristics:
- They are typically sausage-shaped or cylindrical organelles.
- They are enclosed by two membranes: a smooth outer membrane and an inner membrane that is folded into numerous infoldings called cristae.
- The inner compartment is filled with a dense, homogeneous substance called the matrix.
- The matrix contains a single circular DNA molecule, a few RNA molecules, ribosomes (70S type), and enzymes for respiration.
- They are semi-autonomous as they can synthesize some of their own proteins and divide by fission.
Functions:
- They are the sites of aerobic respiration.
- They generate cellular energy in the form of ATP (Adenosine Triphosphate), hence they are called the 'powerhouses' of the cell.
(For the diagram of a mitochondrion, please refer to Figure 8.7 in the textbook).
2. Chloroplasts
Characteristics:
- These are large, lens-shaped organelles found in plant cells and euglenoides.
- They are also enclosed by a double membrane: an outer and an inner membrane. The inner membrane is relatively less permeable.
- The space enclosed by the inner membrane is called the stroma.
- Within the stroma, there are flattened membranous sacs called thylakoids, which are arranged in stacks called grana.
- The stroma contains small, double-stranded circular DNA molecules, ribosomes (70S type), and enzymes required for carbohydrate synthesis.
- Thylakoids contain chlorophyll pigments essential for photosynthesis.
Functions:
- They are the site of photosynthesis, where light energy is trapped and converted into chemical energy (food).
(For the diagram of a chloroplast, please refer to Figure 8.8 in the textbook).
Q8Exercises
What are the characteristics of prokaryotic cells?
Solution
Prokaryotic cells are structurally simpler than eukaryotic cells. Their main characteristics are as follows:
- Organisms: They are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
- Size: They are generally smaller (1-10 ) and multiply more rapidly than eukaryotic cells.
- Nucleus: They lack a well-defined, membrane-bound nucleus. The genetic material is a single circular DNA molecule, which lies naked in the cytoplasm in a region called the nucleoid.
- Organelles: They lack membrane-bound organelles such as mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.
- Ribosomes: They have 70S ribosomes (composed of 50S and 30S subunits) that are associated with the plasma membrane.
- Cell Wall: Most prokaryotes have a cell wall surrounding the plasma membrane, which provides structural support. It is absent in mycoplasma.
- Cell Membrane Structures: They possess unique structures like mesosomes (infoldings of the plasma membrane) which are involved in respiration, DNA replication, and cell wall formation.
- Plasmids: Many bacteria contain small, circular, extrachromosomal DNA molecules called plasmids, which may confer characteristics like antibiotic resistance.
Q9Exercises
Multicellular organisms have division of labour. Explain.
Solution
Division of labour is a fundamental characteristic of multicellular organisms. It refers to the specialization of different cells, tissues, organs, and organ systems to perform specific functions for the body as a whole. This specialization allows the organism to function efficiently and carry out complex life processes.
In a simple unicellular organism, a single cell must perform all essential functions like respiration, digestion, excretion, and reproduction. In contrast, multicellular organisms are composed of millions of cells. These cells differentiate and group together to form tissues (e.g., muscle tissue, nervous tissue), which in turn form organs (e.g., heart, brain), and organs work together in organ systems (e.g., circulatory system, nervous system).
For example:
- Muscle cells are specialized for contraction to bring about movement.
- Nerve cells are specialized for transmitting signals to coordinate body activities.
- Red blood cells are specialized for transporting oxygen.
- Cells of the stomach lining are specialized for secreting digestive enzymes.
This distribution of tasks among different parts of the body ensures that all life processes are carried out with greater efficiency and coordination. No single cell or tissue is burdened with all functions, allowing the organism to grow larger, become more complex, and adapt to a wider range of environments.
Q10Exercises
Cell is the basic unit of life. Discuss in brief.
Solution
The statement "Cell is the basic unit of life" is a cornerstone of biology, supported by the cell theory. This can be understood in two main aspects:
-
Structural Unit: All living organisms, from the simplest bacteria to the most complex plants and animals, are composed of one or more cells. The cell is the smallest entity that can be considered a building block of an organism's body. Just as bricks are the structural units of a building, cells are the structural units of all life forms.
-
Functional Unit: A single cell is capable of performing all the essential functions of life, such as metabolism, growth, respiration, excretion, and reproduction. Unicellular organisms like Amoeba and bacteria exist independently as a single cell, demonstrating that a cell is the smallest unit capable of independent existence. In multicellular organisms, the life of the organism is the sum total of the activities and interactions of its constituent cells. Any function that an organism performs is ultimately carried out at the cellular level.
Therefore, because a cell is the fundamental structural and functional entity of all living organisms, and anything less than a complete cell cannot ensure independent living, it is rightly called the basic unit of life.
Q11Exercises
What are nuclear pores? State their function.
Solution
Nuclear pores are minute perforations or channels found in the nuclear envelope, which is the double membrane surrounding the nucleus in eukaryotic cells. These pores are formed by the fusion of the two membranes (the inner and outer nuclear membranes) of the envelope.
Function:
The primary function of nuclear pores is to regulate the transport of molecules between the nucleus and the cytoplasm. They act as selective gateways, controlling the passage of substances. This includes:
- Export from Nucleus: The movement of RNA molecules (mRNA, tRNA, rRNA) and ribosomal subunits from the nucleus to the cytoplasm, where protein synthesis occurs.
- Import into Nucleus: The movement of molecules from the cytoplasm into the nucleus, such as proteins (like DNA polymerase and histones), carbohydrates, signaling molecules, and lipids that are required for nuclear activities like DNA replication and transcription.
Q12Exercises
Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Solution
Yes, both lysosomes and vacuoles are single-membrane-bound organelles and are part of the endomembrane system. However, they have distinct structures and perform different functions.
Lysosomes:
- Structure: They are small, vesicular structures formed by the process of packaging in the Golgi apparatus.
- Content: They are filled with hydrolytic enzymes such as lipases, proteases, and carbohydrases.
- Function: Their primary function is intracellular digestion. They digest foreign materials that enter the cell (like bacteria), worn-out cell organelles, and macromolecules. They are often called 'suicidal bags' because they can release their enzymes to digest the entire cell if it is damaged.
Vacuoles:
- Structure: They are membrane-bound sacs, with the membrane being called the tonoplast. In plant cells, they are very large, often occupying up to 90% of the cell volume. In animal cells, they are small and temporary.
- Content: They contain water, sap, excretory products, and other materials not immediately useful for the cell.
- Function: Their functions vary:
- In plant cells, they maintain turgor pressure, store nutrients and waste products.
- In Amoeba, the contractile vacuole is crucial for osmoregulation (excreting excess water).
- In protists, food vacuoles are formed to engulf and digest food particles.
In summary, while both are part of the endomembrane system, lysosomes are specialized for enzymatic digestion, whereas vacuoles are primarily involved in storage, excretion, and maintaining turgor pressure.
Q13Exercises
Describe the structure of the following with the help of labelled diagrams.
(i)
Nucleus
(ii)
Centrosome
Solution
(i) Nucleus
The nucleus is a large, double membrane-bound organelle that contains the cell's genetic material. It is the control center of the eukaryotic cell.
Structure:
- Nuclear Envelope: The nucleus is enclosed by a double membrane called the nuclear envelope. It consists of two parallel membranes, an outer and an inner membrane, separated by a space called the perinuclear space. The outer membrane is often continuous with the endoplasmic reticulum and may have ribosomes on its surface.
- Nuclear Pores: The nuclear envelope is perforated by numerous nuclear pores, which regulate the passage of materials between the nucleus and the cytoplasm.
- Nucleoplasm: The interior of the nucleus is filled with a semi-fluid matrix called the nucleoplasm or nuclear matrix.
- Chromatin: Suspended within the nucleoplasm is the chromatin, an indistinct network of nucleoprotein fibres. Chromatin consists of DNA and associated proteins called histones. During cell division, chromatin condenses to form distinct chromosomes.
- Nucleolus: The nucleoplasm contains one or more spherical, non-membranous bodies called nucleoli (singular: nucleolus). The nucleolus is the site of active ribosomal RNA (rRNA) synthesis.
(For a labelled diagram of the Nucleus, please refer to Figure 8.11 in the textbook).
(ii) Centrosome
The centrosome is an organelle typically found in animal cells, located near the nucleus. It plays a crucial role in cell division.
Structure:
- Centrioles: A centrosome usually contains two cylindrical structures called centrioles, which are arranged perpendicular to each other.
- Pericentriolar Material: The centrioles are surrounded by an amorphous mass of protein called the pericentriolar material.
- Cartwheel Organisation: Each centriole has an organisation resembling a cartwheel. It is made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of these fibrils is a triplet (i.e., composed of three microtubules). The adjacent triplets are linked together.
- Hub and Spokes: The central part of the centriole is a proteinaceous hub. This hub is connected to the peripheral triplets by radial spokes made of protein.
Functions: Centrioles form the basal bodies of cilia and flagella, and they organize the formation of spindle fibres that separate chromosomes during cell division in animal cells.
Q14Exercises
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.
Solution
A centromere is the primary constriction present in a chromosome. It is the region where the two sister chromatids of a replicated chromosome are held together. Disc-shaped structures called kinetochores are present on the sides of the centromere, which serve as attachment points for spindle fibres during cell division.
The position of the centromere is a key characteristic used to classify chromosomes into different types. Based on its location, a chromosome is divided into two sections or 'arms'. The classification is as follows:
-
Metacentric Chromosome: The centromere is located in the middle of the chromosome, dividing it into two equal arms. This results in a V-shaped chromosome during anaphase.
-
Sub-metacentric Chromosome: The centromere is situated slightly away from the center of the chromosome. This results in one arm being shorter and the other being longer. This gives the chromosome an L-shape during anaphase.
-
Acrocentric Chromosome: The centromere is located very close to one end of the chromosome. This results in one extremely short arm and one very long arm. This gives the chromosome a J-shape during anaphase.
-
Telocentric Chromosome: The centromere is located at the terminal end of the chromosome. This results in the chromosome having only one arm. This gives the chromosome an I-shape during anaphase.
(For a diagram showing these types of chromosomes, please refer to Figure 8.13 in the textbook).