Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
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Assertion: Rudolf Virchow modified the hypothesis of cell theory given by Schleiden and Schwann.
Reason: Cell theory says that all cells arise from preexisting cells.
b
Assertion: Cells vary greatly in their shape.
Reason: The shape of cell does not depend on the function they perform.
(c) Cells vary greatly in their shape. They may be disc-like, polygonal, columnar, cuboid, thread like, or even irregular. The shape of the cell may vary with the function they perform e.g. xylem vessels are long as they have to form long water conducting channels.
Assertion: Pili are nonmotile appendages of bacteria.
Reason: Pili take part in conjugation.
(b) Pili and fimbriae are non-motile appendages that are outgrowths of membrane. Pili are present in \(\mathrm{F}^{+}\)bacteria and are used to transport \(\mathrm{F}\) factor (fertility factor) from \(\mathrm{F}^{+}\)bacteria to \(\mathrm{F}^{-}\)bacteria during conjugation.
Assertion: The fimbriae are elongated tubular structures made of a special protein.
Reason: The pili occur in Gram negative bacteria.
(d) Pili and fimbriae are surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein (pilin). Pilidevelop in response to \(\mathrm{F}^{+}\) or fertility factor in Gram negative bacteria. The fimbriae are small bristle-like fibres sprouting out of the cell. Fimbriae are involved in attaching bacteria to solid surfaces or host tissues.
Assertion: The cells that have membrane bound organelles are called eukaryotic.
Reason: The cells that lack membrane bound organelles are called prokaryotic.
b
Assertion: Peripheral proteins are partially or totally buried in the membrane.
Reason: The internal proteins are called intrinsic or integral proteins.
(d) Membrane proteins are classified on the basis of ease of extraction as integral or peripheral. Peripheral proteins lie on the surface of membrane while the integral or internal proteins are partially or totally buried in the membrane.
Assertion: The quasifluid nature of lipid enables lateral movement of proteins within the overall bilayer.
Reason: This ability to move within the membrane is called fluidity and is important for cell growth.
(b) The structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model. According to this, the quasifluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity. The fluid nature of the membrane is important for functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.
Assertion: The endomembrane system includes endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
Reason: Mitochondria, chloroplast and peroxisomes are not the part of endomembrane system because their functions are not coordinated with the same.
(b) Endomembrane system is a group of some membrane organelles which function in close coordination with one another, viz., endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. Functions of other organelles are not coordinated. They are not part of endomembrane system, e.g., chloroplasts, mitochondria, peroxisomes and glyoxysomes, etc.
Assertion: The endoplasmic reticulum which lacks ribosomes is called smooth endoplasmic reticulum (SER).
Reason: SER is mainly involved in protein synthesis.
(c) RER is involved in protein synthesis.
Assertion: The Golgi apparatus mainly performs the function of packaging materials.
Reason: Materials to be packed in the form of vesicles from the ER fuse with trans face of the Golgi apparatus.
(c) The Golgi apparatus principally performs the function of packaging materials, to be delivered either to the intracellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the Golgi apparatus and move towards the maturing face. That is why, the Golgi apparatus remains in close association with the endoplasmic reticulum.
Assertion: Lysosomes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
Reason: Lysosomes are rich in hydrolytic enzymes like lipases, proteases and carbohydrases.
(a) Lysosomes are small vesicles which are bounded by a single membrane and contain hydrolytic enzymes. The important enzymes are acid phosphatases, sulphatases, proteases, peptidases, nucleases, lipases and carbohydrases.
Assertion: Mitochondria are called ‘power houses’ of the cell.
Reason: Mitochondria produce cellular energy in the form of ATP.
a
Assertion: The content of inner compartment of mitochondria is called matrix.
Reason: The outer membrane forms a number of infoldings called cristae.
(c) The inner membrane of mitochondria forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area.
Assertion: Ribosomes are non-membrane bound organelles found in the prokaryotic cells only.
Reason: Ribosomes are present in the cytoplasm and some organelles.
(d) Ribosomes are non-membrane bound organelles found in all cells – both eukaryotic as well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER.
Assertion: The arrangement of axonemal microtubules in cilia or flagella is called \(9+2\) array.
Reason: The axoneme usually has nine pairs or doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules.
(a) The study of a cilium or the flagellum shows that they are covered with plasma membrane. Their core is called axoneme, it possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
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