RowQ
The Vault
RowQ
The Vault
CBSE Class 9 Biology · 12 questions · 29 marks
Hold a drop of pond water under a microscope and you will see something no naked eye can catch: tiny compartments, each running its own chemistry, each alive. This chapter treats the cell as the smallest unit that can genuinely be called living, and walks through the membrane that guards it, the nucleus that instructs it, and the organelles that keep the work divided. Once you can name what each part does, the differences between a plant cell, an animal cell, and a bacterium stop being a list to memorise and start making sense.
A raisin left in a bowl of plain water for two hours becomes plump and swollen. Which process explains this best?
Answer
Endosmosis of water into the raisin is correct — the dried raisin has a much higher concentration of dissolved substances than plain water, so water moves inward through its semi-permeable membrane and swells it. Exosmosis would occur only in a concentrated sugar or salt solution, active transport moves solutes and not bulk water, and evaporation would shrink rather than plump the raisin.
Which of the following structures would you expect to find in a bacterial cell but never in an animal cell?
Answer
Cell wall is correct — bacteria have a rigid protective wall outside the plasma membrane, whereas animal cells have only a flexible plasma membrane. Ribosomes, a plasma membrane and cytoplasm are present in both bacterial and animal cells.
Which organelle is directly responsible for releasing the energy that a muscle cell uses during exercise?
Answer
Mitochondrion is correct — it carries out aerobic respiration and stores the released energy in ATP molecules that power muscle contraction. The Golgi apparatus packages materials, lysosomes digest waste, and vacuoles mainly store substances.
The large central vacuole of a mature plant cell contributes most directly to which of these?
Answer
Turgidity and rigidity of the cell is correct — the vacuole fills with cell sap and presses outward against the cell wall, keeping soft plant parts firm and upright. Protein synthesis happens on ribosomes, spindle formation involves centrosomes in animal cells, and sunlight is trapped by chloroplasts.
Assertion (A): A plant cell placed in a strong salt solution shrinks away from its cell wall, but the cell does not lose its outer shape. Reason (R): The cellulose cell wall is fully permeable and rigid, so it stays in place even when the plasma membrane pulls inward.
Answer
Both A and R are true and R is the correct explanation of A — in a hypertonic salt solution water leaves the cell by exosmosis and the plasma membrane detaches from the wall, a condition called plasmolysis. Because the cellulose wall is rigid and freely permeable, it neither collapses nor stops the solution from reaching the membrane, so the cell keeps its outline while the living contents shrink.
State three differences between a plant cell and an animal cell.
Answer
First, a plant cell has a rigid cellulose cell wall outside the plasma membrane, while an animal cell has only a plasma membrane and no cell wall. Second, plant cells contain plastids, including chloroplasts that carry out photosynthesis, whereas animal cells have no plastids at all. Third, a mature plant cell has one large central vacuole occupying most of its volume, while an animal cell has either no vacuole or several small, temporary ones. In addition, centrioles are typically present in animal cells but absent in most plant cells.
Lysosomes are described as the suicide bags of the cell. Explain why this name is appropriate.
Answer
Lysosomes are membrane-bound sacs filled with powerful digestive enzymes that normally break down worn-out organelles, food particles and foreign material entering the cell. If the cell is injured, starved or aged, the lysosome membrane may rupture and release these enzymes into the cytoplasm, where they digest the cell's own contents and destroy it. Because the organelle can thus cause the death of the very cell that contains it, it is called a suicide bag.
Distinguish between diffusion and osmosis, giving one example of each in a living cell.
Answer
Diffusion is the net movement of any particle — solid, liquid or gas — from a region of higher concentration to a region of lower concentration, and it does not require a membrane. An example is carbon dioxide produced during respiration diffusing out of a cell into its surroundings. Osmosis is a special case involving only the solvent, water, moving through a semi-permeable membrane from a solution of higher water concentration (dilute) to one of lower water concentration (concentrated). An example is water entering a root hair cell from the surrounding soil water. Both processes are passive and require no energy from the cell.
Why is the plasma membrane called a selectively permeable membrane?
Answer
The plasma membrane does not allow every substance to pass through it freely; it permits some molecules, such as oxygen, carbon dioxide and water, to move across easily while restricting or actively regulating the entry and exit of others, such as certain ions and large molecules. This selective control keeps the internal composition of the cell stable and prevents useful substances from leaking out, which is why it is described as selectively permeable rather than simply permeable.
Describe the structure and functions of the nucleus, and explain what happens to a cell if its nucleus is removed.
Answer
The nucleus is usually the largest organelle in a eukaryotic cell and is bounded by a double-layered nuclear membrane that carries small pores. These pores allow controlled exchange of materials such as RNA and proteins between the nucleus and the cytoplasm. Inside lies a jelly-like nucleoplasm containing one or more nucleoli, which manufacture ribosome components, and a network of thread-like chromatin made of DNA and protein. At the time of cell division, this chromatin condenses into distinct rod-shaped chromosomes. The DNA in these chromosomes is organised into functional segments called genes, which carry the instructions for building proteins and hence determine the characters an organism inherits. The nucleus therefore performs two central roles: it stores and transmits hereditary information from one generation of cells to the next, and it controls all the metabolic activities of the cell by regulating which proteins are made. If the nucleus is removed, the cell loses this control centre. It can no longer produce new enzymes and proteins, cannot repair itself, cannot divide, and its ordinary chemical activities gradually stop. Such an enucleated cell survives for only a short time before it dies. This is also why mature mammalian red blood cells, which lose their nucleus, have a limited lifespan of about 120 days.
Explain how a eukaryotic cell manufactures, modifies and exports a protein, naming the organelles involved at each stage. Why is this arrangement described as division of labour within the cell?
Answer
The instructions for making a protein are stored as a gene in the DNA inside the nucleus. A copy of this instruction leaves the nucleus through a nuclear pore and reaches the ribosomes, the small granular structures that actually assemble amino acids into a protein chain. Ribosomes attached to the rough endoplasmic reticulum feed the newly made protein into the channels of the ER, which acts as an internal transport network carrying it through the cytoplasm. The smooth endoplasmic reticulum, which lacks ribosomes, handles the manufacture of lipids instead. From the ER the protein is delivered to the Golgi apparatus, a stack of flattened membranous sacs. Here it is chemically modified, sorted and packed into small vesicles. Some vesicles store the product, some become lysosomes containing digestive enzymes, and others travel to the plasma membrane and release their contents outside the cell. Throughout this process, mitochondria supply the ATP energy needed, and the plasma membrane governs the final release. This arrangement is called division of labour because each organelle is specialised for one part of the job — the nucleus instructs, ribosomes build, the ER transports, the Golgi packages and dispatches, and mitochondria power the work. Just as a factory works faster when different departments handle different tasks, the cell carries out complex chemical work efficiently by keeping incompatible reactions in separate membrane-bound compartments.
Ritika sets up a simple experiment for her school science fair. She scoops out the centre of a peeled potato to make a small cup, fills the cavity with concentrated sugar solution, marks the starting level, and stands the potato cup in a dish of plain water. After three hours she finds that the level of sugar solution inside the cavity has clearly risen, while a second, boiled potato cup set up in exactly the same way shows no change at all. Answer the following: (a) Name the process responsible for the rise in level in the raw potato cup. (b) Explain the direction of water movement in terms of concentration. (c) Why did the boiled potato cup show no change? (d) State one everyday example of the same process in plants.
Answer
(a) The process responsible is osmosis — the movement of water through the semi-permeable living cell membranes of the potato tissue. (b) The plain water in the dish is a dilute medium with a high concentration of water molecules, while the sugar solution in the cavity is concentrated and has a low concentration of water molecules. Water therefore moves from the dish, through the living potato cells, and into the cavity, raising the level of the sugar solution. (c) Boiling kills the potato cells and destroys the selectively permeable nature of their plasma membranes. Once the membranes are no longer selectively permeable, no controlled osmotic movement of water can occur, so the level in the boiled potato cup stays unchanged. This also proves that osmosis needs a living, semi-permeable membrane. (d) An everyday example is the absorption of water from the soil by root hair cells of a plant, where soil water is more dilute than the cell sap inside the root hair, so water enters by osmosis.
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