Class 11 Biology Chapter 8
 Chapter 8 

 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.
Answer. a) Robert Brown discovered the cell.

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

Q3. 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: 
a) ii.
b) iii.
c) i.


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.
Answer: 
c. In prokaryotes, there are no membrane bound organelles.



Q5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Answer:  Mesosome is a special membranous structure which is formed by the extensions of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in formation of cell wall, DNA replication and distribution to daughter cells. they also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.


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?
Answer:  Neutral solutes move across the plasma membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to lower concentration.
No, the polar molecules cannot move across it in the same way.
Polar molecules require a carrier protein of the membrane to facilitate their transport across the membrane.


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.
Answer:  Mitochondria and chloroplasts are the two cell-organelles that are double membrane bound.
The characteristics of these two organelles-
Mitochondrion is a double membrane-bound structure. Its membrane is divided into inner membrane and outer membrane distinctly divided into two aqueous compartments i.e., the inner and outer compartments. The outer membrane is porous and the inner membrane is deeply folded.
The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy for photosynthesis. Chloroplasts are mostly found in the Mesophyll cells of the leaves.
Their functions-
Mitochondria are the sites for cellular respiration. It provides energy in the form of ATP.

 

Chloroplasts involved in the process of photosynthesis.
 



Q8. What are the characteristics of prokaryotic cells?
Answer:  The characteristics of prokaryotic cells:
i. The prokaryotic cells represent by blue-green algae, mycoplasma, bacteria, and PPLQ (Pleuro Pneumonia Like Organisms).
ii. They are generally smaller in size and rapidly multiply.
iii. They have a cell wall surrounding the cell membrane.
iv. No well-defined nucleus is present.
v. They have plasmids.
vi. The presence of mesosome is the characteristic features of prokaryotes.


Q9. Multicellular organisms have division of labour. Explain.
Answer: Multicellular species consist of millions of cells. Many of these cells perform different functions. All the cells specialized for carrying out specific tasks are grouped together as body tissues. Therefore, a common task is performed by a group of cells at a particular position in the body. Similarly, different classes of cells perform different roles, so this is called as division of labour in multicellular organisms.


Q10. Cell is the basic unit of life. Discuss in brief.
Answer:  Cells are the basic units of life that are capable of carrying out all the essential biochemical processes that a normal cell has to do to survive. The essential requirements for all living organisms to live are the same. All living organisms need to breathe, consume food to acquire energy, and rid themselves of metabolic waste. Cells are able to perform all of the body’s metabolic functions.  So, cells are known as fundamental units of life.


Q11. What are nuclear pores? State their function.
Answer: Nuclear pores are the tiny holes found in the nuclear membrane of the nucleus and they are formed by fusion of two nuclear membranes.  They are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.


Q12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their function. Comment.
Answer:  Lysosomes are vesicular membrane-bound structures that hold a variety of enzymes such as proteases, lipases, and amylases. The main aim of lysosomes is to digest worn out cells. They participate in the intracellular digestion of food particles and microbes. They also act like suicidal bags too.
On the other hand, vacuoles act as a storage sacs present in cells. They store the waste products of cells. The food vacuole in unicellular organisms store food particles. It also help in expelling excess water and some wastes from the cell.


Q13. Describe the structure of the following with the help of labelled diagrams.
i. Nucleus
ii. Centrosome

Answer: 
i. Nucleus
Nucleus has nuclear envelop which consists of two parallel membranes with a space between 10 to 50 nm which called the perinuclear space. This space forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane remains continuous with the endoplasmic reticulum and bears ribosomes on it. : Nuclear pores are the tiny holes found in the nuclear membrane of the nucleus and they are formed by fusion of two nuclear membranes.  They are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. The nucleoplasm contains nucleolus and chromatin.
 

ii. Centrosome
Centrosome is an organelle which contains two cylindrical structures called centrioles. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.
 

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.
Answer:  Centromere is a constriction found on the chromosomes where the chromatids are held together.
The chromosomes are classified into four types, based on the position of the centromere-
• The metacentric chromosome has middle centromere forming two equal arms of the chromosome.
• The sub-metacentric chromosome has centromere slightly away from the middle of the chromosome resulting into one shorter arm and one longer arm.
• The acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm.
• The telocentric chromosome has a terminal centromere.






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