NCERT Solutions for Class 11 Biology Chapter 8: Cell The Unit of Life

The incorrect statement is (a) Robert Brown discovered the cell.

Explanation: This statement is incorrect. Robert Hooke was the first to observe cells (dead cork cells) in 1665. Robert Brown is credited with the discovery of the nucleus within the cell in 1831.

The correct option is (c) pre-existing cells.

Explanation: This is a core tenet of the modern cell theory, famously stated by Rudolf Virchow as *"Omnis cellula-e cellula"* (all cells arise from pre-existing cells).

The correct match is:

• (a) Cristae — (ii) Infoldings in mitochondria
• (b) Cisternae — (iii) Disc-shaped sacs in Golgi apparatus
• (c) Thylakoids — (i) Flat sacs in stroma

The correct statement is (c) In prokaryotes, there are no membrane-bound organelles.

Explanation:

• (a) is incorrect because prokaryotic cells (like bacteria) lack a nucleus.
• (b) is incorrect because animal cells do not have a cell wall.
• (d) is incorrect because cells only arise from pre-existing cells, not non-living (abiotic) materials.

A mesosome is a characteristic convoluted, infolded structure formed by the extension of the plasma membrane into the cytoplasm of a prokaryotic cell, typically a bacterium.

Functions of Mesosomes:

1. Cell Wall Formation: They help in the synthesis of the cell wall during cell division.
2. DNA Replication & Distribution: They provide a surface for the attachment of the bacterial chromosome (nucleoid) and help in its replication and equal distribution to daughter cells.
3. Respiration: They contain respiratory enzymes and increase the surface area for cellular respiration, analogous to the mitochondria in eukaryotes.
4. Secretion Processes: They help in increasing the surface area of the plasma membrane for secretion.

The movement of substances across the plasma membrane depends on their chemical nature.

• Movement of Neutral Solutes: Neutral solutes are typically nonpolar and lipid-soluble. They can move across the plasma membrane easily by simple diffusion. They move from a region of higher concentration to a region of lower concentration, down the concentration gradient, without any energy expenditure.
• Movement of Polar Molecules: No, polar molecules cannot move across the membrane in the same way. The plasma membrane has a nonpolar, hydrophobic lipid bilayer core, which repels polar (hydrophilic) molecules. Therefore, polar molecules require special mechanisms for transport:
  1. Facilitated Diffusion: They are transported across the membrane with the help of carrier proteins or channel proteins. This process does not require energy as it still follows the concentration gradient.
  2. Active Transport: If polar molecules need to be moved against their concentration gradient (from low to high concentration), it requires energy in the form of ATP. This process is called active transport and is carried out by specific protein pumps.

Two double membrane-bound cell organelles are Mitochondria and Chloroplasts.

1. Mitochondrion (plural: Mitochondria)

• Characteristics: Sausage-shaped or cylindrical; bounded by a smooth outer membrane and an inner membrane folded into cristae; matrix contains its own circular DNA and 70S ribosomes, making it semi-autonomous.
• Function: They are the sites of aerobic respiration and are responsible for producing cellular energy in the form of ATP. Hence, they are called the "powerhouse of the cell".

[Pro-Tip: For exams, you must draw a neat, labelled diagram of a mitochondrion showing the outer membrane, inner membrane, cristae, and matrix.]

2. Chloroplast

• Characteristics: Lens-shaped; bounded by a double membrane; the inner space (stroma) contains flattened sacs called thylakoids arranged in stacks called grana; also semi-autonomous with its own DNA and ribosomes.
• Function: Chloroplasts are the site of photosynthesis. They trap solar energy and use it to synthesize food (glucose) for the plant.

[Pro-Tip: A clear diagram of a chloroplast showing the double membrane, stroma, grana, and thylakoids is essential for scoring full marks.]

Prokaryotic cells are structurally simpler than eukaryotic cells. Their key characteristics are:

• Size: They are generally smaller and multiply more rapidly than eukaryotic cells.
• Nucleus: They lack a well-defined, membrane-bound nucleus. The genetic material is located in a region called the nucleoid.
• Membrane-Bound Organelles: Organelles like mitochondria, chloroplasts, ER, Golgi apparatus, and lysosomes are absent.
• Ribosomes: They have 70S type ribosomes scattered in the cytoplasm.
• Cell Wall: Most prokaryotes have a rigid cell wall made of peptidoglycan (except in Mycoplasma).
• Examples: Bacteria, Blue-Green Algae (Cyanobacteria), Mycoplasma.

"Division of labour" in multicellular organisms means that different cells and tissues are specialized to perform different functions for the good of the whole organism. This specialization allows for greater efficiency and complexity.

• In a unicellular organism, a single cell performs all life functions.
• In contrast, a multicellular organism like a human has specialized cells:
  • Muscle cells for movement.
  • Nerve cells for transmitting signals.
  • Red blood cells for transporting oxygen.
  • Kidney cells for filtration and excretion.

This specialization allows the organism to perform a wide range of complex activities that a single cell could not.

The statement "Cell is the basic unit of life" is a fundamental principle of biology for the following reasons:

1. Structural Unit: All living organisms are composed of one or more cells, which provide the basic structure for the body.
2. Functional Unit: All metabolic activities and physiological processes that define life occur within the cell.
3. Genetic Unit: Each cell contains the complete genetic blueprint (DNA) of the organism, passed on during cell division.
4. Origin of Life: According to the cell theory, all new cells arise from pre-existing cells, underpinning growth, repair, and reproduction.

Therefore, no living entity exists without a cellular organization, making the cell the true basic unit of life.

Nuclear pores are tiny, complex protein-lined channels that perforate the nuclear envelope (the double membrane surrounding the nucleus in a eukaryotic cell).

Function:

The primary function of nuclear pores is to regulate the transport of molecules between the nucleus and the cytoplasm. They act as gatekeepers, allowing selective passage of specific molecules.

• Out of nucleus: RNA molecules (mRNA, tRNA, rRNA) and ribosomal subunits.
• Into nucleus: Proteins needed for DNA replication and transcription, and energy molecules (ATP).

Yes, both lysosomes and vacuoles are part of the endomembrane system, but they have distinct functions. Here's a comparison:

Conclusion: While both are membrane-bound sacs, lysosomes are specialized for enzymatic digestion, whereas vacuoles are primarily involved in storage and structural support.

The "powerhouse of the cell" in a eukaryotic cell is the Mitochondrion.

Structure of a Mitochondrion:

• Membranes: It is a double membrane-bound organelle. The outer membrane is smooth, while the inner membrane is folded into finger-like projections called cristae.
• Matrix: The inner compartment is filled with a dense, gel-like substance called the matrix.
• Semi-autonomous: The matrix contains its own circular DNA molecule, 70S ribosomes, and RNA, making the mitochondrion a semi-autonomous organelle.

Function of a Mitochondrion:

• The primary function of mitochondria is to perform cellular respiration.
• They break down food molecules in the presence of oxygen to produce energy in the form of Adenosine Triphosphate (ATP).
• Because they are the main site of ATP synthesis, mitochondria are aptly called the "powerhouse of the cell".

[Hint: A diagram of a mitochondrion showing all the parts mentioned above is crucial for a complete answer in an exam.]

A centromere is a specialized constricted region on a chromosome where the two sister chromatids are held together. It also serves as the attachment point for spindle fibers during cell division.

The position of the centromere is used to classify chromosomes into four types:

1. Metacentric: Centromere is in the middle, forming two equal arms (V-shaped).
2. Sub-metacentric: Centromere is slightly off-center, forming one short (p) and one long (q) arm (L-shaped).
3. Acrocentric: Centromere is near one end, forming a very short arm and a very long arm (J-shaped).
4. Telocentric: Centromere is at the terminal end, resulting in only one arm (i-shaped). (Not found in humans).

[Instruction for drawing: Draw the four types side-by-side and label the centromere, p arm, and q arm.]

Lysosomes contain potent hydrolytic enzymes. If a cell gets damaged, old, or is under severe stress, the lysosomal membrane can rupture. This releases the enzymes into the cytoplasm, which then digest the entire cell's components, leading to cell death. This process of self-destruction is why they are termed "suicidal bags".

The nucleus is the control center of the eukaryotic cell.

Structure:

• Nuclear Envelope: A double membrane separating the nucleus from the cytoplasm, perforated by nuclear pores.
• Nucleoplasm: The semi-fluid matrix inside the nucleus.
• Chromatin: A tangled, thread-like mass of DNA and proteins (histones). Condenses to form chromosomes during cell division.
• Nucleolus: A dense, non-membranous sphere within the nucleus, which is the site of ribosomal RNA (rRNA) synthesis.

Function:

• Controls all metabolic activities of the cell.
• Stores the cell's genetic material (DNA).
• Plays a central role in cell division, growth, and differentiation.

[Hint: A clear diagram showing the nuclear envelope, nuclear pores, nucleoplasm, chromatin, and nucleolus is essential for this answer.]

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. The organelles included are the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.

The cell wall in a plant cell has three main functions:

1. Shape & Rigidity: It provides a definite shape and structural support to the cell.
2. Protection: It protects the cell from mechanical stress and pathogens.
3. Turgor Pressure: It prevents the cell from bursting due to excessive water intake (osmosis).

The Fluid Mosaic Model, proposed by Singer and Nicolson, describes the plasma membrane as a quasi-fluid structure where proteins are embedded within a phospholipid bilayer.

• Fluid: This refers to the lipid bilayer, where individual phospholipid and protein molecules can move laterally, making the membrane flexible and dynamic.
• Mosaic: This refers to the pattern created by various proteins embedded within or attached to the lipid bilayer, much like tiles in a mosaic artwork.

Plasmids are small, extrachromosomal, circular DNA molecules found in many bacteria, separate from the main bacterial chromosome (nucleoid). They often carry genes for specific traits like antibiotic resistance.

If the two arms of a chromosome are equal in length, it means the centromere is located in the middle. This type of chromosome is called a metacentric chromosome.

1. (a) Identify the type of cell the scientist is observing.
   The cell is a prokaryotic cell (likely a bacterium).
2. (b) What is the function of the 70S ribosomes?
   The function is protein synthesis.
3. (c) The scientist notes infoldings of the plasma membrane. What are these called and what is one function?
   These infoldings are called mesosomes. One key function is to increase the surface area for cellular respiration.

[Hint: Draw two diagrams side-by-side. For the plant cell, show a rigid outer wall, large central vacuole, and chloroplasts. For the animal cell, show an irregular shape and no cell wall.]