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.
Ans: (a)
Sol: Robert Brown discovered the cell is false because cells were first discovered by Robert Hooke. Mathias Schleiden, Theodar Schwann, and Rudolf Virchow formulated the cell theory, according to which all living organisms are composed of cells. Much of the work of Rudolf Virchow was in proving that cells arise from preexisting cells. Organisms consisting of a single cell carry out all functions of life by means of that single cell. So, the correct answer is '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
Ans: (c)
Sol: According to the biogenic theory, new cells can only arise from pre-existing cells. Only complete cells, in favourable conditions, can give rise to new cells.
Q3: Match the following:
Ans:
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.
Ans: (c)
Sol: Membrane-bound organelles are organelles surrounded by a double membrane. Nucleus, mitochondria, chloroplasts, etc., are examples of such organelles.
These cell organelles are absent from prokaryotes.
(a) Only eukaryotic cells have nuclei. They are absent from prokaryotes.
(b) Cell walls are only present in plant cells. They are absent from all animal cells.
(d) All cells arise from pre-existing cells.
Q5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Ans: Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane.
Its functions are as follows:
Mesosome
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?
Ans: The plasma membrane is the outermost covering of the cell that separates it from the environment. It regulates the movement of substances into the cell and out of it. It allows the entry of only some substances and prevents the movement of other materials. Hence, the membrane is selectively permeable.
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.
Ans: Mitochondria and chloroplasts are the two organelles that are double membrane-bound.
Mitochondria
Characteristics of Mitochondria:
Characteristics of Chloroplasts:
Chloroplasts are double-membrane-bound structures.
They are divided into outer and inner membranes, further divided into two distinct regions:
(i) Grana are stacks of flattened discs containing chlorophyll molecules. The flattened membranous sacs are called thylakoids. The thylakoids of adjacent grana are connected by membranous tubules called stroma lamellae.
(ii) Stroma is a homogenous mixture in which grana are embedded. It contains several enzymes that are used for the synthesis of carbohydrates and proteins. It also contains its own DNA and ribosomes.
Chloroplasts
Functions of the mitochondria:
Functions of chloroplasts:
Q8: What are the characteristics of Prokaryotic cells?
Ans: Prokaryotic cell is a unicellular organism lacking membrane-bound organelles.
The characteristics of prokaryotic cells are as follows:
Q9: Multicellular organisms have division of labour. Explain.
Ans: Multicellular organisms are made up of millions and trillions of cells. All these cells perform specific functions. All the cells specialised for performing similar functions are grouped together as tissues in the body. Hence, a particular function is carried out by a group of cells at a definite place in the body. Similarly, different functions are carried out by different groups of cells in an organism. This is known as division of labour in multicellular organisms.
Q10: Cell is the basic unit of life. Discuss in brief.
Ans:
Q11: What are nuclear pores? State their function.
Ans: Nuclear envelope bounds the nucleus from outside and separates it from cytoplasm. It consists of two membranes, with outer membrane continuous with endoplasmic reticulum. The nuclear envelope is interrupted by minute nuclear pores, at a number of places, which are produced by the fusion of its two membranes. These nuclear pores 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 functions. Comment.
Ans: The endomembrane is an intercellular system responsible for the flow of materials from one part to another through vesicles. Some of its components are vacuoles, plasma membrane, endoplasmic reticulum, lysosomes and Golgi apparatus. Lysosomes are membrane-bound organelles that release lytic enzymes to digest worn-out cells; hence they are known as suicidal bags. On the other hand, vacuoles help cells to maintain their shape. Vacuoles also store food, water and waste products.
Q13: Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus
(ii) Centrosome
Ans:
(i) Nucleus
Nucleus controls all the cellular activities of the cell. It is spherical in shape. It is composed of the following structures:
(ii) Centrosome: Centrosome consists of two cylindrical structures called centrioles. Centrioles lie perpendicular to each other. Each has a cartwheel-like organisation.
A centriole is made up of microtubule triplets that are evenly spaced in a ring. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub is connected to the triplets via radial spokes. These centrioles help in organising the spindle fibres and astral rays during cell division. They form the basal body 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.
Ans: Centromere is a constriction present on the chromosomes where the chromatids are held together. Chromosomes are divided into four types based on the position of the centromere.
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1. What is the basic structural and functional unit of all living organisms? |
2. What are the main types of cells? |
3. What are the components of a eukaryotic cell? |
4. How do plant cells differ from animal cells? |
5. What is the role of the cell membrane? |
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