Chapter 8: Cell – The Unit of Life – Long Answer Type Questions

CBSE Class 11 Biology Long Answer Questions: Cell – The Unit of Life (NCERT-Based)

Course and Examination Framework

Course: CBSE Class 11 Biology

Board: Central Board of Secondary Education

Unit III: Cell Structure and Function

Chapter 8: Cell – The Unit of Life

Question Type: Long Answer Type Questions

Answer Length: 120–150 Words

Syllabus Compliance: 100% NCERT

Exam Relevance: CBSE Class 11 Annual & School Examinations

Section A: Cell Discovery and Cell Theory (Questions 1–5)

Q1. Describe the discovery of the cell and the development of cell theory.

Ans:
The discovery of the cell marked a turning point in biology. In 1665, Robert Hooke observed thin slices of cork under a compound microscope and identified small box-like structures, which he named “cells.” Later, Antonie van Leeuwenhoek observed living cells using improved microscopes. The systematic understanding of cells led to the formulation of cell theory by Matthias Schleiden and Theodor Schwann. They proposed that all living organisms are composed of cells and that the cell is the basic unit of life. Rudolf Virchow later added that all cells arise from pre-existing cells. Together, these ideas established the foundation of modern cell biology.

Q2. Explain the postulates and significance of cell theory.

Ans:
Cell theory consists of three main postulates: all living organisms are composed of cells and cell products; the cell is the basic structural and functional unit of life; and all cells arise from pre-existing cells. This theory is significant because it explains the continuity of life and the cellular basis of growth, reproduction, and heredity. It establishes unity among diverse organisms by showing that all life forms share a common cellular organization. Cell theory also forms the basis for understanding disease processes, development, and evolution, making it fundamental to all branches of biology.

Q3. Why is the cell regarded as the basic unit of life?

Ans:
The cell is regarded as the basic unit of life because it performs all essential biological functions necessary for survival. These include metabolism, respiration, growth, excretion, and reproduction. In unicellular organisms, a single cell carries out all life processes independently. In multicellular organisms, specialized cells work together to perform complex functions. Furthermore, all organisms begin their life as a single cell, which divides and differentiates to form tissues and organs. Thus, the cell forms the structural, functional, and biological foundation of all living organisms.

Q4. Differentiate between unicellular and multicellular organisms.

Ans:
Unicellular organisms are composed of a single cell that performs all life functions independently. Examples include Amoeba and bacteria. These organisms show simplicity in structure but high efficiency at the cellular level. Multicellular organisms consist of many cells that are structurally and functionally specialized. In such organisms, cells are organized into tissues, organs, and organ systems, as seen in plants and animals. Division of labour among cells increases efficiency and complexity. Thus, multicellular organisms exhibit higher levels of organization compared to unicellular organisms.

Q5. Explain the contribution of Antonie van Leeuwenhoek to cell biology.

Ans:
Antonie van Leeuwenhoek significantly advanced cell biology by being the first scientist to observe living cells. Using simple microscopes of high magnification designed by him, he observed bacteria, protozoa, sperm cells, and red blood cells. His observations provided the first evidence that cells are living entities capable of independent existence. This discovery complemented Robert Hooke’s work, which involved dead cork cells, and expanded the understanding of cellular diversity. Leeuwenhoek’s contributions laid the foundation for microbiology and experimental cell biology.

Section B: Prokaryotic and Eukaryotic Cells (Questions 6–10)

Q6. Describe the general characteristics of prokaryotic cells.

Ans:
Prokaryotic cells are simple, small-sized cells lacking a true nucleus and membrane-bound organelles. Their genetic material is present as naked circular DNA in a region called the nucleoid. Prokaryotes possess 70S ribosomes and a cell envelope composed of glycocalyx, cell wall, and plasma membrane. They may also have appendages like flagella and pili for movement and attachment. Prokaryotic cells are commonly found in bacteria and cyanobacteria. Their simple structure allows rapid growth and adaptation to diverse environments.

Q7. Explain the structure of a typical prokaryotic cell.

Ans:
A typical prokaryotic cell consists of a cell envelope that includes the glycocalyx, cell wall, and plasma membrane. The cytoplasm lacks membrane-bound organelles but contains ribosomes and inclusion bodies. The nucleoid region contains a single circular DNA molecule without histone proteins. Some prokaryotes possess plasmids that carry additional genetic information. Appendages such as flagella help in locomotion, while pili and fimbriae assist in attachment and conjugation. This structural simplicity supports efficient metabolic activity and survival in diverse conditions.

Q8. Describe the major features of eukaryotic cells.

Ans:
Eukaryotic cells are complex and larger than prokaryotic cells. They possess a true nucleus enclosed by a nuclear membrane and contain membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. Their genetic material is organized into linear chromosomes associated with histone proteins. Eukaryotic cells have 80S ribosomes in the cytoplasm. These cells are found in plants, animals, fungi, and protists. The compartmentalization of functions enhances efficiency and regulation of cellular processes.

Q9. Compare prokaryotic and eukaryotic cells.

Ans:
Prokaryotic cells lack a true nucleus and membrane-bound organelles, whereas eukaryotic cells possess both. Prokaryotes contain circular naked DNA, while eukaryotes have linear DNA associated with histones. Ribosomes are 70S in prokaryotes and 80S in eukaryotes. Prokaryotic cells are smaller and simpler, while eukaryotic cells are larger and structurally complex. These differences reflect increasing levels of organization and functional specialization in eukaryotic cells.

Q10. What is the nucleoid? Explain its significance.

Ans:
The nucleoid is an irregular, non-membranous region found in prokaryotic cells that contains the genetic material. It consists of a single circular DNA molecule without histone proteins. The nucleoid controls cellular activities such as metabolism, growth, and reproduction. Its simple organization allows rapid replication and gene expression, enabling prokaryotes to adapt quickly to changing environments. Although it lacks a nuclear membrane, the nucleoid performs functions similar to the nucleus of eukaryotic cells.

Section C: Cell Membrane and Cell Wall (Questions 11–15)

Q11. Describe the structure of the plasma membrane according to the Fluid Mosaic Model.

Ans:
The plasma membrane is described by the Fluid Mosaic Model proposed by Singer and Nicolson. It consists of a phospholipid bilayer in which proteins are embedded. The hydrophilic heads of phospholipids face outward, while hydrophobic tails face inward. Proteins are arranged in a mosaic pattern and can move laterally, giving fluidity to the membrane. Carbohydrates are attached to proteins and lipids, forming glycoproteins and glycolipids. This structure provides flexibility, selective permeability, and efficient transport of substances.

Q12. Explain the functions of the plasma membrane.

Ans:
The plasma membrane acts as a selectively permeable barrier, regulating the movement of substances into and out of the cell. It maintains the internal environment and protects cellular contents. The membrane facilitates transport through diffusion, active transport, endocytosis, and exocytosis. It also plays a role in cell recognition and communication through membrane proteins and receptors. By controlling interactions with the external environment, the plasma membrane ensures cellular integrity and proper functioning.

Q13. Describe the structure and functions of the cell wall.

Ans:
The cell wall is a rigid outer layer present in plant cells, fungi, and bacteria. In plants, it is primarily composed of cellulose. The cell wall provides shape, rigidity, and mechanical support to the cell. It protects against physical injury and prevents osmotic bursting when water enters the cell. The porous nature of the cell wall allows free movement of substances. Thus, the cell wall plays a crucial role in maintaining cell structure and stability.

Q14. Explain diffusion and active transport across the cell membrane.

Ans:
Diffusion is the passive movement of molecules from a region of higher concentration to lower concentration without the use of energy. It helps in the exchange of gases and small molecules. Active transport, on the other hand, involves the movement of substances against the concentration gradient using energy in the form of ATP and carrier proteins. Active transport is essential for maintaining ionic balance and absorbing essential nutrients in cells.

Q15. What is endocytosis? Explain its types.

Ans:
Endocytosis is the process by which a cell engulfs materials from its external environment by forming vesicles from the plasma membrane. It occurs mainly in animal cells. There are two main types: phagocytosis, in which solid particles are engulfed, and pinocytosis, where liquids are taken into the cell. Endocytosis enables cells to intake large molecules and particles that cannot pass through the membrane directly.

Section D: Cell Organelles (Questions 16–25)

Q16. Describe the structure and functions of the nucleus.

Ans:
The nucleus is a membrane-bound organelle surrounded by a double-layered nuclear envelope with pores. It contains nucleoplasm, nucleolus, and chromatin. Chromatin carries genetic information, while the nucleolus synthesizes ribosomal RNA. The nucleus controls cellular activities by regulating gene expression and protein synthesis. It also plays a vital role in cell division and heredity, making it the control centre of the cell.

Q17. Explain the structure and functions of the endoplasmic reticulum.

Ans:
Endoplasmic reticulum is a network of membranous tubules and cisternae. Rough ER has ribosomes attached and is involved in protein synthesis. Smooth ER lacks ribosomes and participates in lipid synthesis, detoxification, and calcium storage. The ER provides an internal transport system and increases the surface area for biochemical reactions, contributing to efficient cellular metabolism.

Q18. Describe the Golgi apparatus and its functions.

Ans:
The Golgi apparatus consists of stacks of flattened membranous sacs called cisternae. It has a cis face for receiving materials and a trans face for dispatching them. The Golgi modifies proteins and lipids, packages them into vesicles, and transports them to their destinations. It also plays a role in cell surface renewal and formation of lysosomes.

Q19. Explain the structure and functions of mitochondria.

Ans:
Mitochondria are double-membraned organelles with an outer smooth membrane and an inner folded membrane forming cristae. The inner space is called the matrix. Mitochondria are the sites of aerobic respiration and ATP synthesis. They contain their own DNA and ribosomes, allowing partial autonomy. Due to their role in energy production, mitochondria are known as the powerhouse of the cell.

Q20. Describe plastids and their types.

Ans:
Plastids are double-membraned organelles found in plant cells. They are involved in photosynthesis, storage, and pigmentation. Chloroplasts contain chlorophyll and carry out photosynthesis. Chromoplasts provide colour to fruits and flowers, while leucoplasts store food materials such as starch, fats, and proteins. Plastids play a vital role in plant metabolism and energy conversion.

Q21. Explain the structure and function of lysosomes.

Ans:
Lysosomes are membrane-bound organelles containing hydrolytic enzymes. They digest worn-out organelles, foreign particles, and cellular debris. Lysosomes help in intracellular digestion and recycling of materials. During cell damage, they may release enzymes causing autolysis, hence called suicidal bags of the cell.

Q22. Describe ribosomes and their functions.

Ans:
Ribosomes are non-membranous organelles composed of ribosomal RNA and proteins. They occur freely in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are the sites of protein synthesis. Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S. They play a crucial role in translating genetic information into functional proteins.

Q23. Explain the cytoskeleton and its functions.

Ans:
The cytoskeleton is a network of protein filaments including microtubules, microfilaments, and intermediate filaments. It maintains cell shape, provides mechanical support, and facilitates cell movement. The cytoskeleton also plays a role in intracellular transport and cell division, contributing to cellular organization and stability.

Q24. Describe centrosome and centrioles.

Ans:
The centrosome is found in animal cells and contains a pair of centrioles arranged perpendicular to each other. Centrioles are cylindrical structures made of microtubules. They play a vital role in spindle formation during cell division and help in the formation of cilia and flagella.

Q25. Explain the structure and functions of vacuoles.

Ans:
Vacuoles are membrane-bound sacs filled with cell sap. In plant cells, a large central vacuole maintains turgidity, stores nutrients, and regulates osmotic balance. In animal cells, vacuoles are smaller and involved in storage and transport. Vacuoles help maintain internal pressure and contribute to cell growth.