NCERT Solutions: Cell: The Unit of Life

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. 
Ans: (a)
Explanation: Robert Brown did not discover cells; he discovered the nucleus in 1831. The first observations of cells were made by Robert Hooke in 1665 when he described the structure of cork and coined the term "cells". Matthias Schleiden and Theodor Schwann later formulated the cell theory that all living organisms are composed of cells. Rudolf Virchow added that new cells arise from pre-existing cells. A unicellular organism indeed performs all its life activities within a single cell. Therefore option (a) is incorrect.

Q2: New cells generate from
(a) Bacterial fermentation
(b) Regeneration of old cells
(c) Pre-existing cells
(d) Abiotic materials 
Ans: (c)
Explanation: 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

Q3: Match the following:

Ans:

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. 
Ans: (c)
Explanation: Prokaryotic cells lack membrane-bound organelles such as a true nucleus, mitochondria and chloroplasts; thus option (c) is correct. Option (a) is false because prokaryotes do not have a true nucleus (their genetic material is not enclosed by a nuclear membrane). Option (b) is false because only plant cells (and many bacteria) have a rigid cell wall; animal cells do not. Option (d) is incorrect because cells arise from pre-existing cells, not from abiotic materials.

Q5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs. 
Ans: A mesosome is an infolded or convoluted region of the plasma membrane seen in some prokaryotic cells, formed by invagination of the cell membrane.
Its functions are as follows:

• They assist in synthesis of the cell wall and in DNA replication, helping ensure proper segregation of genetic material during cell division.
• They increase the surface area of the plasma membrane, providing more space for membrane-bound enzymes and metabolic reactions.
• They are associated with certain respiratory enzymes and may aid in secretion processes.

Mesosome

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? 
Ans: The plasma membrane is a selectively permeable barrier that regulates movement of substances into and out of the cell.

• Movement of neutral solutes: Small neutral (non-polar) molecules move across the membrane by simple diffusion. This is a passive process in which molecules move from a region of higher concentration to a region of lower concentration until equilibrium is reached.
• Movement of polar molecules and ions: The interior of the phospholipid bilayer is non-polar and restricts free passage of polar molecules and ions. Such substances cross the membrane with the help of specific carrier proteins or channel proteins. Transport may be:
• Passive (facilitated diffusion) - movement down their concentration gradient via proteins without energy expenditure.
• Active - movement against a concentration gradient that requires metabolic energy and involves transport proteins (pumps).

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

Mitochondria

Characteristics of mitochondria:

• Mitochondria are double-membrane-bound organelles with an outer membrane and a highly folded inner membrane forming cristae.
• The inner membrane folds increase surface area for reactions and contain enzymes involved in energy conversion.
• Mitochondria contain their own DNA and ribosomes, making them semi-autonomous.

Characteristics of chloroplasts:

Chloroplasts are also double-membrane-bound. They contain an internal membrane system and two main regions:

(i) Grana - stacks of flattened sacs called thylakoids that contain chlorophyll and are the sites where light-dependent reactions occur. Thylakoids of adjacent grana are connected by stroma lamellae.
(ii) Stroma - a fluid matrix surrounding the grana that contains enzymes for synthesis of carbohydrates and also its own DNA and ribosomes.

Chloroplasts

Functions of mitochondria:

• They are the sites of cellular respiration and convert energy from nutrients into forms usable by the cell (for example, ATP).
• They supply energy necessary for vital cellular activities.
• Because they have their own DNA and ribosomes, they can synthesise some of their own proteins.
• They house enzymes involved in synthesis of certain biomolecules.

Functions of chloroplasts:

• They capture solar energy and convert it into chemical energy by carrying out photosynthesis.
• They contain enzymes for carbohydrate synthesis and may participate in synthesis of some proteins and lipids.

Q8: What are the characteristics of Prokaryotic cells? 
Ans: A prokaryotic cell is a unicellular organism that lacks membrane-bound organelles.

The characteristics of prokaryotic cells are as follows:

• Most prokaryotes are unicellular.
• They are generally small, typically ranging from 0.5 to 5 μm in size.
• They lack a true nucleus; the nuclear region (nucleoid) is not bounded by a nuclear membrane.
• Genetic material is usually a single circular chromosome and is not associated with histones; many also contain small circular plasmids.
• They may have membranous infoldings called mesosomes, which assist in processes like cell wall synthesis and DNA replication.
• Membrane-bound organelles such as mitochondria, plastids and endoplasmic reticulum are absent.
• Many have a three-layered envelope - glycocalyx (outermost), cell wall (middle) and plasma membrane (innermost) - providing protection and shape.
• Examples: bacteria and cyanobacteria (blue-green algae).

Q9: Multicellular organisms have division of labour. Explain. 
Ans: In multicellular organisms, cells specialise for particular tasks. Similar specialised cells group to form tissues. Different tissues combine to make organs, and organs cooperate in organ systems. Because different groups of cells perform different functions at specific places, the organism benefits from division of labour. This leads to greater efficiency and allows complex functions to be carried out simultaneously - for example, muscle cells contract to produce movement while nerve cells conduct signals to coordinate that movement.

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

• Cells are the basic structural units of living organisms: they form tissues, organs and systems that build the body.
• Cells are also the basic functional units: each cell carries out essential life processes such as uptake of nutrients, respiration to release energy, synthesis of biomolecules and removal of wastes.
• All vital processes - growth, reproduction, metabolism and response to the environment - occur at the cellular level, making the cell the fundamental unit of life.

Cell: Basic Unit of life

Q11: What are nuclear pores? State their function.
Ans: The nuclear envelope is composed of two membranes that enclose the nucleus. Where the two membranes fuse, they form small openings called nuclear pores. These pores serve as selective gateways that allow controlled exchange of materials between the nucleus and cytoplasm. They permit movement of RNA and ribosomal subunits out of the nucleus and allow certain proteins and signalling molecules to enter the nucleus from the cytoplasm.

Q12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Ans: Both lysosomes and vacuoles are part of the endomembrane system and are enclosed by membranes, but their primary functions differ:

• Lysosomes: small, membrane-bound organelles containing hydrolytic enzymes. They digest worn-out organelles, macromolecules and invading pathogens, and play a role in intracellular digestion and recycling of cellular components.
• Vacuoles: typically larger compartments used largely for storage. In plant cells a large central vacuole stores water, ions and metabolites and helps maintain turgor pressure; in animal cells vacuoles are smaller and assist in storage, transport and waste disposal.

Q13: Describe the structure of the following with the help of labelled diagrams. 
(i) Nucleus 
(ii) Centrosome
Ans: 
(i) Nucleus
The nucleus controls cellular activities and is usually spherical. It comprises the following parts: 

• Nuclear envelope: a double membrane that separates the nucleus from the cytoplasm. The perinuclear space lies between the two membranes. Nuclear pores in the envelope allow selective exchange of molecules between nucleus and cytoplasm.
• Nucleoplasm (nuclear matrix): a granular, semifluid substance that contains the nucleolus and chromatin.
• Nucleolus: a dense, non-membrane-bound region rich in RNA and proteins where ribosomal RNA is synthesised and ribosome subunits are assembled.
• Chromatin: thread-like complexes of DNA and proteins (histones) that condense to form chromosomes during cell division.

Nucleoplasm

(ii) Centrosome: The centrosome is the main microtubule-organising centre in animal cells. It consists of two cylindrical centrioles arranged at right angles to each other, embedded in a proteinaceous matrix. Each centriole is made up of nine triplets of microtubules arranged in a circular pattern (the cartwheel structure). The centrosome organises the spindle fibres and astral rays during cell division and gives rise to basal bodies that form the core of cilia and flagella.

Centrosome

Q14: 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. 
Ans: The centromere is the constricted region of a chromosome where the two sister chromatids are held together and where spindle fibres attach during cell division. Based on the position of the centromere, chromosomes are classified into four types:

• Metacentric: centromere is approximately in the middle, producing two arms of nearly equal length.
• Sub-metacentric: centromere is off-centre, producing one arm slightly longer than the other.
• Acrocentric: centromere is close to one end, giving one long arm and one very short arm.
• Telocentric: centromere is at the terminal end of the chromosome (one arm essentially absent).