NCERT Solutions for Class 9th Science Chapter 2 Ready to Go Beyond — Do cells grow and reproduce forever? (2026-27)

Q1.

How do cells monitor their growth to maintain a balance?

Answer

By balancing the making of new cells against the deliberate, controlled death of old ones. The main mechanism is Programmed Cell Death (PCD).

PCD is a genetically regulated and organised process of selective cell destruction — the cell dismantles itself neatly on instruction, instead of bursting and damaging its neighbours.
It is essential for three things: normal development, cellular quality control, and immune function.
Contact inhibition works alongside it: in many animal cells, division stops when a cell comes into contact with its neighbours, so a tissue stops growing once it is complete.
Every cell also has a definite life span. Mature red blood cells, for example, survive about 120 days and are then replaced.

rate of cell division = rate of cell death → tissue size stays constant
division > death → tumour
division < death → tissue wastes away

Why it happens: the striking example in the book is the human hand. As an embryo develops, PCD removes the cells between the developing digits, and that is how separate fingers are carved out of a paddle-shaped bud — without it we would have webbed hands. So cell death here is not damage at all; it is a sculptor's tool. The same process removes cells that have picked up faulty DNA before they can become cancerous, which is why PCD counts as quality control. Problems arise at both extremes: if cells do not die when they should, tumours form; if they die too early, tissues break down.

Q2.

Explore different ways by which cells maintain themselves.

Answer

A cell stays healthy by running several housekeeping systems at once. Here is what to explore, with the part of the chapter each connects to.

Way of maintaining itself	How it works	Organelle or process involved
Controlling what enters and leaves	Selectively permeable membrane; proteins act as gatekeepers; water balance by osmosis	Cell membrane
Clearing waste and worn-out parts	Enzymes digest unwanted proteins, carbohydrates, fats and damaged organelles; products are recycled into the cytoplasm	Lysosomes
Replacing damaged molecules	New proteins are synthesised and then modified, sorted and packed for delivery	Ribosomes, RER, Golgi apparatus
Supplying energy on demand	Glucose is broken down in cellular respiration and the energy stored as ATP	Mitochondria
Holding shape and moving materials inside	A network of fine fibres gives structural support and enables internal transport	Cytoskeleton
Storing and isolating	Water, minerals, sugars and wastes are locked in cell sap; turgor is maintained	Vacuole
Replacing whole cells	Damaged or dead cells are replaced by identical new ones	Mitosis
Removing cells that are no longer wanted	Selective, genetically controlled self-destruction	Programmed Cell Death

Why it happens: read down the table and one idea keeps returning — a cell survives by constant renewal, not by being permanent. Almost every molecule in a cell is broken down and rebuilt over time; what persists is the pattern, held in the DNA, and the compartments that keep each process separate. Maintenance is therefore inseparable from the organelle structure the chapter has just described: without lysosomes there is no clearance, without mitochondria there is no energy to rebuild, and without a membrane there is no inside worth maintaining.

Try This: take one organelle from Fig. 2.10 and trace what would fail if it stopped working for a day. Doing this for the lysosome, the mitochondrion and the Golgi apparatus in turn will show you how tightly the systems depend on each other.