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
Solution:
(a) It was not Robert Brown who discovered the cell; Robert Hooke discovered the cell.
Q2. New Cells generate from:
- Bacterial fermentation
- Regeneration of old cells
- Pre-existing cells
- Abiotic materials
Solution:
(c) According to the biogenic theory, new cells can only emerge from pre-existing cells. Under favorable conditions, only fully developed cells have the capacity to generate new cells.
Q3. Match the following:
- Cristae (i) flat membranous sacs in stroma
- Cisternae (ii) infoldings in mitochondria
- Thylakoids (iii) disc-shaped sacs in Golgi apparatus
Solution:
a. ii
b. iii
c. i
Q4. Which of the following is correct?
- Cells of all living organisms have a nucleus
- Both animal and plant cells have a well defined cell wall
- In prokaryotes, there are no membrane bound organelles
- Cells are formed de novo from abiotic materials
Solution:
(c)
Membrane-bound organelles are those enclosed by a double membrane. Examples of these organelles include the nucleus, mitochondria, and chloroplasts, among others. Prokaryotic cells, on the other hand, lack these particular cellular structures.
Q5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Solution:
The mesosome is a complex membranous structure that arises in prokaryotic cells through the inward folding of the plasma membrane. Its roles include the following:
i. These extensions facilitate cell wall synthesis, DNA replication, and the even distribution of chromosomes into daughter cells.
ii. Additionally, it enhances the surface area of the plasma membrane to support a range of enzymatic activities.
iii. It plays a role in both secretion processes and bacterial respiration.
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?
Solution:
The plasma membrane serves as the cell’s outermost barrier, separating it from the external environment. It controls the passage of substances into and out of the cell, permitting the entry of certain materials while inhibiting the movement of others. Consequently, the membrane is termed selectively permeable.
The transport of neutral solutes across the cell membrane occurs through simple passive diffusion. Diffusion is the process by which molecules move from areas of higher concentration to regions of lower concentration.
The transport of polar molecules across the cell membrane relies on the composition of the membrane, which consists of a phospholipid bilayer and proteins. To traverse the non-polar lipid bilayer, polar molecules necessitate carrier proteins. These integral protein structures exhibit a specific affinity for particular solutes, thereby facilitating the transportation of molecules 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.
Solution:
Mitochondria and chloroplasts are double membrane bound organelles.
Characteristics of Mitochondria
Mitochondria, also known as chondriosome / plastosome / sarcosome is a double-membrane bound structure found in eukaryotic cells. Mitochondrial membrane is divided into the inner and outer membranes. The space between the outer and inner membrane is called intermembrane space. However, the outer membrane is more permeable for biochemicals and the inner membrane is selectively permeable and folded in nature.
The folds on the inner membrane are called cristae. Infoldings are responsible for increasing the surface area inside the cell. Cristae is the site for ATP generating reactions. Mitochondria are also the sites for aerobic respiration due to the presence of specific enzymes on the mitochondrial membrane. Mitochondria also possess their own DNA and ribosomes. Since mitochondria possess ds circular DNA, 70s ribosomes, and binary fission – it can be concluded that the origin of mitochondria is from bacteria i.e., bacterial endosymbionts.
Functions of Mitochondria
- Involved in the oxidation of food stuffs such as protein, carbohydrates and fats.
- Mitochondria is the site for respiration (Krebs Cycle, ETS)
- Involved in the synthesis as well as elongation of fatty acids.
Characteristics of Chloroplast
Chloroplasts are double-membrane bound organelle present only in eukaryotic cells. The matrix contains two regions. The first region is called stroma which is a fluid part of the organelle. The matrix also contains stacks of flattened discs, collectively called as granum and each disc is called thylakoid. Each disc contains chlorophyll molecules. Two thylakoids get connected through stroma lamella
Functions of Chloroplast:
- Involved in photosynthesis (light and dark reaction)
- Storage of starch
Q8. What are the characteristics of prokaryotic cells?
Solution
Prokaryotic cell is the characteristic of unicellular organisms that lack membrane bound organelles. Following are the characteristics of prokaryotic cells:
- Cell wall: non-cellulosic cell wall present.
- Plasma membrane present
- 70s ribosome present
- The inner infolding of plasma membrane is called mesosome. It is the site for DNA replication and possesses respiratory enzymes.
- Outgrowth of plasma membrane – pili and fimbriae. They are meant for attachment.
- True vacuole absent
- Double stranded circular DNA present
- Flagella made up of flagellin protein for locomotion are present.
- Cytoskeleton absent
- Examples of prokaryotes include BGA, bacteria, etc.
Q9. Multicellular organisms have division of labour. Explain.
Solution:
Multicellular organisms consist of countless cells, each with distinct roles. Cells with similar functions aggregate to form tissues within the organism. Consequently, specific functions are executed by clusters of cells situated in precise locations throughout the body. In a multicellular organism, various groups of cells perform diverse tasks, illustrating the concept of division of labor within these organisms.
Q10. Cell is the basic unit of life. Discuss in brief.
Solution
According to cell theory, all living organisms are composed of cells and their products. In other words, cell represents the structural unit of all living organisms. All the activities occurring inside an organism is the sum total of all activities occurring inside the cell. On this basis, we can say that cell is the functional unit of life.
Q11. What are nuclear pores? State their function.
Solution:
Nuclear pores are minuscule apertures found within the nuclear membrane of the nucleus, originating from the fusion of the two nuclear membranes. These openings facilitate the selective exchange of substances, enabling the bidirectional movement of molecules like RNA and proteins between the nucleus and the cytoplasm.
Q12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Solution
Lysosomes are enclosed, membrane-bound structures containing an assortment of enzymes, such as lipases, proteases, and carbohydrases. Their primary function is the degradation of aged or damaged cells, and they play a vital role in intracellular digestion of foreign particles and microorganisms. At times, lysosomes can even act as self-destructive entities, participating in the autophagy of cells. Essentially, they serve as a cellular waste disposal system.
In contrast, vacuoles are sac-like organelles within cells primarily utilized for storage purposes. They may store cellular waste products, and in unicellular organisms, the food vacuole is responsible for containing ingested food particles. Additionally, vacuoles assist in expelling excess water and certain cellular waste materials from the cell.
Q13. Describe the structure of the following with the help of labelled diagrams.
- Nucleus
- Centrosome
Solution:
Nucleus: The nucleus governs all cellular activities within the cell and takes on a spherical shape, comprising the following components:
Nuclear Membrane:
The nuclear membrane is a double-layered barrier that separates the nucleus’s contents from the cytoplasm. This membrane consists of two layers with a narrow space between them, known as the perinuclear space. Within the nuclear membrane are minuscule openings called nuclear pores, which facilitate the selective movement of specific substances into and out of the cell.
Nucleoplasm
Nucleoplasm, also known as the nuclear matrix, constitutes a uniform granular fluid found within the nucleus. It encompasses both the nucleolus and chromatin. The nucleolus is a spherical structure devoid of a membrane, abundant in proteins and RNA molecules, serving as the site for ribosome assembly. Chromatin, on the other hand, is a complex network of thread-like structures containing DNA and foundational proteins referred to as histones.
Centrosome
Centrosome is an organelle containing two cylindrical structures, called centrioles. The teo centrioles lie perpendicular to each other.
A centriole consists of microtubule triplets arranged in a circular pattern with uniform spacing between them. These adjacent triplets are interconnected, and at the center of a centriole, there is a proteinaceous hub. This hub connects to the triplets through radial spokes. Centrioles play a crucial role in orchestrating the formation of spindle fibers and astral rays during cell division. Additionally, they serve as the basal body for the development of cilia and flagella.
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.
Solution
The centromere is a narrow region found on chromosomes responsible for holding the chromatids together.
On the basis of the position of centromere, chromosome can be divided into 4 types:
Metacentric chromosome: the centromere is present in the middle.
Sub-metacentric chromosome: the centromere is present near the centre.
Acrocentric chromosome: the centromere is present near the end
Telocentric chromosome: the centromer is present at the end
CELL THE UNIT OF LIFE CLASS 11 | PROKARYOTIC CELL | NEET 204 SANJEEVANI BATCH | BOTANY BY TARUN SIR