NCERT Solutions for Class 9th Science Chapter 2 Pause and Ponder — Cell division
Book page 22 Updated on2026-09-08
Q6.
Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?
Answer
Because energy production happens on membrane surfaces, and many small mitochondria give far more surface area than one big one of the same total volume.
For a sphere of radius r:
surface area A = 4πr² and volume V = (4/3)πr³
so A/V = 3/r — the smaller the radius, the greater the surface per unit volume
Split one sphere of radius R into 8 spheres of the same total volume:
8 × (4/3)πr³ = (4/3)πR³ → 8r³ = R³ → r = R/2
total surface area = 8 × 4π(R/2)² = 8 × 4πR²/4 = 2 × 4πR²
That is twice the surface area of the single large mitochondrion, for exactly the same volume.
Dividing the same volume into smaller pieces multiplies the surface without changing the amount of material.
Three further reasons follow from the same idea:
Delivery. ATP has to reach every corner of the cell. Many mitochondria can be positioned exactly where energy is needed — in a muscle fibre they line up along the contracting fibres; in a sperm they are packed into the tail.
Supply. Oxygen and glucose must diffuse into the mitochondrion. Diffusion is slow over long distances, so a giant organelle would starve at its centre.
Safety and repair. If one small mitochondrion is damaged it can be digested by a lysosome and replaced, with barely any loss. Losing one giant mitochondrion would leave the cell with no energy supply at all.
Why it happens: the same principle explains the shape of a single mitochondrion. Its inner membrane is folded into finger-like projections called cristae, which increase the surface area available for the chemical reactions of cellular respiration without increasing the organelle's size. Nature solves the surface-area problem twice over — by folding inside each mitochondrion, and by having many of them.
Q7.
If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?
Answer
The cut would not heal properly. The skin would be replaced by cells carrying only half the chromosome number, which cannot function as skin — and after one round of meiosis those cells would stop dividing altogether.
If meiosis occurred instead:
skin cell (46) → 4 cells with 23 chromosomes each
half the genes missing → many proteins cannot be made → cells are not skin cells and soon die
Wrong chromosome number. Each daughter cell would get only one of each pair of chromosomes, so a large part of its genetic instructions would be missing. It could not build the keratin, the pigment and the junction proteins that skin needs.
Wrong number of cells, and no continuation. Meiosis produces four cells and then stops — gametes do not divide again. Healing needs division to be repeated until the gap is closed.
Result: the wound would stay open or be filled with weak, abnormal tissue; the skin would lose its barrier function and infection would follow.
Why it happens: the two divisions are built for two different jobs, and the difference is the point of the chapter. Mitosis maintains the genetic information across all body cells, which is exactly what repair needs — the replacement must be an exact copy of what was lost. Meiosis halves it deliberately, because gametes are going to be combined with another gamete at fertilisation. Halving is right in the testes and ovaries and catastrophic anywhere else, which is why the body confines meiosis strictly to reproductive organs.