Chapter 8 Cell : The Unit of Life

Class 11 Biology Chapter 8 Cell : The Unit of Life Exercise Questions and Answers :

1. 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 discovered the cell.

Robert Brown is credited with discovering the cell nucleus, not the discovery of the cell itself.

2. New cells generate from

(a) bacterial fermentation (b) regeneration of old cells

(c) pre-existing cells (d) abiotic materials

Answer : New cells generate from (c) pre-existing cells.

This fundamental concept is known as cell theory and it states that cells are the basic structural and functional units of all living organisms and that new cells arise from the division of pre-existing cells.

3. Match the following

Column I	Column II
(a) Cristae	(i) Flat membranous sacs in stroma
(b) Cisternae	(ii) Infoldings in mitochondria
(c) Thylakoids	(iii) Disc-shaped sacs in Golgi apparatus

Answer : The correct match :

(a) Cristae -------  (ii) Infoldings in mitochondria

(b) Cisternae -------  (iii) Disc-shaped sacs in Golgi apparatus

(c) Thylakoids -------  (i) Flat membranous sacs in stroma

4. 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) In prokaryotes, there are no membrane-bound organelles.

Explanation :

 (a) Cells of all living organisms do not have a nucleus. Prokaryotic cells lack a true nucleus and are found in organisms like bacteria.

(b) Animal cells typically do not have a cell wall, while plant cells have a well-defined cell wall.

(d) Cells are not formed de novo from abiotic materials; they are generated from pre-existing cells, following the principle of cell theory.

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

Answer : A mesosome in a prokaryotic cell is a special membranous structure formed by extensions of the plasma membrane into the cell. These extensions appear in the form of vesicles, tubules, and lamellae.

The function of  Mesosomes are :

 (i) Mesosomes assist in the synthesis of the cell wall components.

 (ii) They play a role in DNA replication and ensuring the distribution of genetic material to daughter cells during cell division.

 (iii) Mesosomes are involved in cellular respiration, particularly in the attachment of respiratory enzymes and the electron transport chain components.

 (iv) They participate in various secretion processes within the cell.

 (v) Mesosomes help to increase the surface area of the plasma membrane, which can enhance the efficiency of various cellular processes.

(vi) They contain enzymes that aid in various metabolic functions within the cell.

Mesosomes are an important and versatile feature of prokaryotic cells, contributing to their structural and functional complexity.

6. 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 :  Neutral solutes move across the plasma membrane through simple diffusion, driven by concentration gradients. This process is effective for non-polar molecules. In contrast, polar molecules face difficulty crossing the lipid bilayer due to its hydrophobic nature. Polar molecules typically require specialized transport proteins like channel proteins (for ions) or carrier proteins to facilitate their movement across the membrane, as they cannot easily pass through the hydrophobic core by simple diffusion.

7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labeled diagrams of both.

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

Characteristics of mitochondria:

(i) Double-membrane structure with an outer membrane and inner membrane.

(ii) The inner membrane forms folds called cristae, which increase the surface area for energy production.

(iii) Mitochondria contain their own circular DNA, similar to prokaryotic DNA.

The functions of mitochondria :

(i) Mitochondria are the main sites for cellular respiration, where they produce ATP (adenosine triphosphate), the cell's primary energy source.

(i) They play a crucial role in breaking down glucose and fatty acids to generate energy.

(ii) Mitochondria are involved in regulating apoptosis, the programmed cell death.

The Characteristics of chloroplasts :

(i) Double-membrane organelles found in plant cells.

(ii) Contain green pigment chlorophyll.

(iii) Chloroplasts consist of two main regions: the grana (stacks of thylakoid membranes) and the stroma (a semi-fluid matrix).

The functions of chloroplasts :

(i) Chloroplasts are essential for photosynthesis, the process by which plants convert light energy into chemical energy (glucose).

(ii) The grana are the sites for the light-dependent reactions of photosynthesis.

(iii) The stroma is where the light-independent (Calvin cycle) reactions occur.

    

8. What are the characteristics of prokaryotic cells?

Answer :  Prokaryotic cells are characterized by their simplicity, lacking a true nucleus and membrane-bound organelles. They contain genetic material in a single, circular chromosome and have a cell wall for structural support and protection. Prokaryotic cells are typically small and often single-celled organisms. They have minimal cytoskeletal structures and ribosomes . Prokaryotes reproduce through binary fission and display a wide range of metabolic diversity, thriving in diverse environments. These features distinguish them from eukaryotic cells, which are more complex and organized, found in plants, animals, fungi, and protists.

9. Multicellular organisms have division of labour. Explain.

Answer :  Multicellular organisms exhibit division of labor, where cells and tissues specialize in distinct functions. This specialization enhances efficiency, with different cells dedicated to tasks like muscle contraction, nerve signal transmission, or nutrient absorption. It optimizes resource allocation, adaptation, and survival. The resulting complexity and diversity enable organisms to thrive in diverse environments. Redundancy ensures resilience, as backup mechanisms maintain crucial functions, contributing to the overall success of multicellular life forms.

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

Answer : The cell is the fundamental unit of life. All living organisms are composed of cells or cell aggregates, and they exhibit variations in terms of size, shape, and functions. Cells are categorized as eukaryotic or prokaryotic based on the presence of a membrane-bound nucleus and other organelles. Eukaryotic cells typically consist of a cell membrane, nucleus, and cytoplasm, with plant cells also having a cell wall. These cells house various organelles like the endomembrane system, which includes the endoplasmic reticulum, Golgi complex, lysosomes, and vacuoles, each serving distinct functions. Additionally, the nucleus plays a crucial role in controlling cellular activities and heredity. In sum, the cell is the essential structural and functional unit of life, with diverse organelles orchestrating various biological processes.

11. What are nuclear pores? State their function.

Answer : Nuclear pores are large, complex protein structures found in the nuclear envelope, which surrounds the nucleus of eukaryotic cells. These pores serve a vital function in the regulation of molecular transport between the nucleus and the cytoplasm.

Their main functions are:

(i) Regulate the passage of molecules between the nucleus and the cytoplasm.

 (ii) Control the movement of RNA, proteins, and ions.

(iii) Ensure selective permeability, allowing specific molecules to pass.

(iv) Facilitate the transport of mRNA for protein synthesis.

(v) Maintain the separation of genetic material in the nucleus from the cytoplasm.

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

Answer :   Lysosomes and vacuoles, while both part of the endomembrane system, differ significantly in their functions and roles within the cell.

The function of lysosomes:

(i) Lysosomes are single-membrane organelles containing enzymes used for the digestion of various macromolecules, such as proteins, nucleic acids, and lipids.

(ii) Their primary function is intracellular digestion, breaking down waste materials, cellular debris, and foreign invaders like bacteria or damaged organelles.

(iii) Lysosomes are often referred to as the "garbage disposals" of the cell.

The function of vacuoles :

(i) Vacuoles are also membrane-bound organelles, but they serve a more diverse range of functions depending on the type of cell and organism.

(ii) In plant cells, central vacuoles primarily store water, nutrients, and waste products, contributing to turgor pressure and supporting the plant's structure.

(iii) In some protists and animal cells, vacuoles can store materials, transport substances, and even play a role in regulating cell volume.

(iv) Contractile vacuoles, found in certain single-celled organisms, help maintain water balance by expelling excess water.

13. Describe the structure of the following with the help of labelled diagrams.

(i) Nucleus (ii) Centrosome

Answer :  Nucleus: The nucleus is a membrane-bound organelle that houses an organism's genetic information.

Its structure can be described as follows:

(i) Nuclear Envelope: The nucleus is enclosed by a double membrane called the nuclear envelope. It consists of an inner and an outer membrane with small openings called nuclear pores that allow the exchange of molecules between the nucleus and the cytoplasm.

(ii) Nucleoplasm: The interior of the nucleus is filled with a semi-fluid substance called nucleoplasm, which contains the genetic material, including DNA.

(iii) Chromatin: Within the nucleoplasm, you find chromatin, which is a complex of DNA and proteins. It appears as a network of thin threads. Chromatin contains the genes that carry the instructions for the cell's activities.

(iv) Nucleolus: Inside the nucleus, you can also find a small, round structure called the nucleolus. It's involved in the production of ribosomes, which are essential for protein synthesis.

Centrosome: The centrosome is an organelle involved in organizing microtubules during cell division.

 

Its structure can be described as follows:

(i) Centrosome Matrix: The centrosome consists of a central region called the centrosome matrix.

(ii) Centrioles: Within the centrosome matrix, there are two small cylindrical structures called centrioles. These centrioles are usually arranged at right angles to each other.

(iii) Microtubules: The centrioles act as the organizing centers for microtubules. Microtubules are a component of the cytoskeleton, and during cell division, they form the spindle apparatus, which helps segregate chromosomes.

(iv) Pericentriolar Material: Surrounding the centrioles is an area called the pericentriolar material, which contains proteins and other molecules that assist in microtubule organization.

14. 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 .

Answer : A centromere is a specialized region of a chromosome, a constriction point where two sister chromatids are tightly joined together. It plays a critical role in the structure and function of chromosomes during cell division. The centromere is essential for proper chromosome segregation during cell division, as it serves as the attachment point for spindle fibers, which are responsible for pulling the chromatids apart into the daughter cells.

The position of the centromere on a chromosome is a key feature used in the classification of chromosomes. The four tupes of chromosomes are :

(a) Metacentric Chromosomes : In metacentric chromosomes, the centromere is positioned at or very near the center of the chromosome. This results in two arms of roughly equal length.

(b) Sub-metacentric Chromosomes : Sub-metacentric chromosomes have the centromere slightly off-center, causing one arm to be shorter than the other. These chromosomes have an appearance somewhat between metacentric and acrocentric.

(c) Acrocentric Chromosomes : In acrocentric chromosomes, the centromere is located very close to one end, leading to one very short arm and one very long arm.

(d) Telocentric Chromosomes : Telocentric chromosomes have the centromere at or near one end of the chromosome, resulting in a single-arm structure. These chromosomes appear linear, with the centromere located at one end.

  Types of chromosomes based on the position of centromere