Previous year questions: Cell: The Unit of Life

Previous year questions: cell, the unit of life

All ten items are from the official NCERT Exemplar Problems in Biology for Class 11. The exemplar does not print marks against individual questions, so the marks shown here follow the section the question is printed in: one mark for a multiple choice item, two marks for a very short answer, three marks for a short answer and five marks for a long answer. Each heading names the exemplar chapter and the question number inside its section. Work each answer before reading the model answer, because several items test the same vocabulary from different angles.

Q1 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 8, Multiple Choice Questions, Q1. What is common to plant sieve tube cells and most mammalian erythrocytes?

Answer: Both lack a nucleus. Sieve tube cells of vascular plants and the red blood cells of most mammals lose their nucleus as they mature, which is why the absence of a nucleus is the shared feature. Mitochondria, a cell wall and haemoglobin are not common to both: a plant sieve tube cell has a wall while a mammalian red cell does not. A mature mammalian red cell also lacks most organelles, which is why the absence of the nucleus is the only feature that fits both. A sieve tube cell is alive and supported by companion cells, while the red cell circulates for about four months, and both manage without a nucleus.

Q2 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 8, Multiple Choice Questions, Q2. Which statement about ribosomes is not true?

Answer: The false statement is that ribosomes have no role in protein synthesis. Ribosomes are the site of protein synthesis; they are made of two subunits, they attach to mRNA and several of them can form a polysome that translates the mRNA. A ribosome is not bound by a membrane, which is another point the option can test.

Q3 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 8, Multiple Choice Questions, Q3. Which of these is not a eukaryote?

Answer: Anabaena is not a eukaryote; it is a cyanobacterium, a prokaryote. Euglena, Spirogyra and Agaricus are all eukaryotes, and their cells have a membrane-bound nucleus. Anabaena, being a cyanobacterium, has naked DNA and no membrane-bound organelles.

Q4 (NCERT Exemplar, 1 mark)

NCERT Exemplar Class 11 Biology, Chapter 8, Multiple Choice Questions, Q7. Who proposed the fluid mosaic model of the plasma membrane?

Answer: Singer and Nicolson proposed the model in 1972. Benda is linked to mitochondria, Schleiden and Schwann to the cell theory, and Robert Brown to the discovery of the nucleus, so none of them fits. The model describes a quasi-fluid lipid bilayer with proteins scattered through it, and that picture is why the word mosaic is used.

Q5 (NCERT Exemplar, 2 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Very Short Answer Type Questions, Q2. What does the S refer to in 70S and 80S ribosomes?

Answer: The S stands for the Svedberg unit, which measures the sedimentation coefficient of a particle. The value depends on density and size, so a 70S ribosome is not simply the sum of its 50S and 30S subunits. Prokaryotic ribosomes are 70S and eukaryotic cytoplasmic ribosomes are 80S. The S value is a sedimentation measure, so the subunits do not simply add up: 50S plus 30S gives a 70S particle, not an 80S one. That oddity is a good one-line answer to a question that asks why the numbers are not additive.

Q6 (NCERT Exemplar, 2 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Very Short Answer Type Questions, Q3. Name a single-membrane-bound organelle rich in hydrolytic enzymes.

Answer: The lysosome is the single-membrane organelle rich in hydrolytic enzymes. Its lipases, proteases and carbohydrases work best at acidic pH and digest carbohydrates, proteins, lipids and nucleic acids. Lysosomes are formed by packaging in the Golgi apparatus. Their enzymes digest worn-out organelles and material taken in from outside, so the lysosome is the cell's digestive compartment as well as its clean-up system.

Q7 (NCERT Exemplar, 2 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Very Short Answer Type Questions, Q5. What is the function of a polysome?

Answer: A polysome is a chain of ribosomes attached to one mRNA molecule. All the ribosomes together translate the same mRNA, so the cell makes many copies of one protein quickly. The arrangement also protects the mRNA and uses one message efficiently. For a cell that needs a large amount of one protein quickly, the polysome is the fastest route from one gene to many protein copies. A single mRNA can therefore keep several ribosomes busy at once, and the cell does not need to make a new message for every protein molecule.

Q8 (NCERT Exemplar, 3 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Short Answer Type Questions, Q5. Differentiate between rough and smooth endoplasmic reticulum.

Answer: Rough ER carries ribosomes on its surface and is common in cells that make and secrete proteins, so it is the site of protein synthesis and transport. Smooth ER has no ribosomes, is the main site of lipid synthesis, and in animal cells it makes steroid hormones of the lipid kind. Rough ER joins the outer membrane of the nucleus. Both are membrane networks in the cytoplasm, and both belong to the endomembrane system. A useful contrast is the cell type that dominates each: a secretory cell is rich in rough ER, while a cell making steroid hormones is rich in smooth ER. That link between structure and the work of the cell is the point of the three-mark question.

Q9 (NCERT Exemplar, 3 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Short Answer Type Questions, Q7. What are plasmids and what is their role in bacteria?

Answer: Plasmids are small circular DNA molecules that lie outside the genomic DNA in a bacterial cell. They are not enclosed in a membrane and they carry extra genes, which give the bacterium characters not coded by the main chromosome. Antibiotic resistance is the standard example, and plasmids also serve as tools in genetic engineering, where they carry foreign DNA into bacteria. Because a plasmid replicates inside the bacterium, the gene it carries is passed on to the daughter cells, which is what makes it useful in the laboratory.

Q10 (NCERT Exemplar, 5 marks)

NCERT Exemplar Class 11 Biology, Chapter 8, Long Answer Type Questions, Q5. Group the cell organelles into those not bound by a membrane, those bound by one membrane and those bound by two membranes.

Answer: Among the organelles without a membrane are ribosomes, centrioles and the centrosome, and the nucleolus is also a non-membranous structure inside the nucleus. The single-membrane organelles include the lysosome, the vacuole, the Golgi apparatus, the endoplasmic reticulum, the peroxisome and the microbodies. The double-membrane organelles are mitochondria, chloroplasts and the nucleus, because the nuclear envelope is a double membrane. This grouping matters because a double membrane is a sign of an organelle with an evolutionary or functional identity of its own: mitochondria and chloroplasts carry their own DNA and ribosomes, and the nucleus owes its envelope to the two-membrane boundary that separates genetic material from the cytoplasm. A single membrane keeps the organelle separate while allowing exchange, and no membrane at all suits structures whose job is assembly or movement. In an answer, list the groups first and add the reason after each group, because the reasons carry the higher marks. The nucleolus belongs with the non-membranous structures, and the nuclear envelope is best described as a double membrane even though it is continuous with the ER. That single example shows why the classification has to be learned organelle by organelle rather than guessed from size. A final check on the grouping question is to name one function for each member, because a list without functions earns only part of the five marks. The membrane count is the first thing to write, and the reason comes second; both are needed for full credit. Marks in this answer follow the list and the reason, and the list alone is not enough. The grouping answer is complete only when both parts are present.