NCERT Solutions for Class 9th Science Chapter 3 From one cell to an organism: Totipotency — Think as a Scientist

Book page 42 Updated on2026-09-08

Q1.

What do you conclude about the characteristics of phloem cells of carrot?

Answer

That a mature carrot phloem cell is totipotent — although it has already differentiated into a food-conducting cell, it still carries the complete set of instructions to build an entire plant, and it can be made to use them again.

Steward's 1958 experiment shows this in three steps:

Mature phloem cell → dedifferentiation (regains the ability to divide)
→ divides into an undifferentiated mass of unspecialised cells
→ redifferentiation into root, shoot and finally a complete carrot plant

So the characteristics are: the cell is living (a dead xylem cell could never do this), it has retained its full nucleus and complete genetic information, and its differentiated state is reversible given the right nutrients, hormones and conditions.

Why this was so surprising: until then, differentiation looked like a one-way street — once a cell became a phloem cell, that was the end of the story. Steward showed the street runs both ways in plants. A single carrot cell can do what a zygote does: divide and differentiate into an entire organism.

Q2.

In which of the three combinations would you obtain the highest and lowest biomass? What could be the possible reason(s) for this observation?

Answer

Highest biomass: combination 2 — light present, air present, liquid medium + nutrients (20% increase).
Lowest biomass: combinations 1 and 3, where the fresh weight actually reduced below the starting weight.

#	Light	Air	Medium	Change in fresh weight	What was missing
1	✓	✗	Solid + nutrients	reduced	No air — no oxygen for respiration; a solid medium also limits contact with nutrients
2	✓	✓	Liquid + nutrients	20% increased	Nothing — all three factors present
3	✗	✓	Liquid + nutrients	reduced	No light

Now put a number on the growth. The cultures were started from 2-mg fragments (Fig. 3.19):

Increase in fresh weight = initial weight × (percentage increase ÷ 100)
= 2 mg × (20 ÷ 100) = 0.4 mg
Final fresh weight = 2 mg + 0.4 mg = 2.4 mg

For combinations 1 and 3 the change is negative, so the final weight is less than 2 mg.

Reasons. Only combination 2 gives the cells everything at once:

Air supplies oxygen for aerobic respiration. Without it (combination 1) the cells cannot release enough energy to divide, and they consume their own stored food — so fresh weight falls.
Light is needed for photosynthesis and also as a signal for growth and greening. Without it (combination 3) the cells keep respiring but make nothing, so again the stored material is used up.
A liquid medium surrounds every cell with nutrients and lets the culture be stirred, which is how single cells shear off into suspension in the first place. A solid medium touches only one face of the tissue.

Why the weight goes down and not merely stops rising: a living cell must respire whether or not it is growing. If it cannot take in or make food, it oxidises its own reserves, and the fresh weight of the culture drops below where it started.

Q3.

Will you get the same results if you culture animal cells instead of carrot cells?

Answer

No. Animal cells can be grown in culture, but a differentiated animal cell will not regenerate a whole animal the way a carrot phloem cell regenerates a whole plant.

	Carrot (plant) cells	Animal cells
Totipotency of a mature cell	Retained — can dedifferentiate and redifferentiate	Lost in almost all body cells once they have differentiated
What grows in culture	An unorganised mass of cells that then forms root, shoot and a whole plant	Usually only a sheet of the same cell type — a tissue, not an organism
Cell wall	Present; helps the mass hold together and take shape	Absent; cells depend on attachment and on signals from neighbours
Cells that can still form many types	Practically any living parenchyma or phloem cell	Only special stem cells, such as those in bone marrow

Why the difference: an animal's body plan is laid down early in development and its cells then commit permanently to one fate; they also depend on precise positions, blood supply and signals from surrounding tissues that a dish cannot reproduce. A plant, being fixed in one place, must be able to repair and replace any part it loses — so it keeps that flexibility in ordinary body cells all its life.

Did you know? Animal stem cells are the exception that proves the rule. Bone marrow stem cells can still divide and make new blood cells, which is why they are transplanted from a healthy donor to patients with leukaemia or thalassaemia.

Q4.

Think and mention any two commercial applications of the study above.

Answer

Two clear ones:

Micropropagation — mass production of identical, disease-free planting material. A few grams of tissue from one superior plant can be cultured to raise thousands of plantlets in a small laboratory space, at any time of year, all genetically identical to the parent. This is how banana, sugarcane, potato and orchid planting material is produced commercially.
Industrial production of plant chemicals. Cultured cells can be grown in large vessels to make valuable phytochemicals — medicines, pigments, flavours — without growing, harvesting and destroying whole plants, and without depending on the season.

Two more worth knowing:

Crop improvement through genetic engineering. Scientists studied how the bacterium Agrobacterium tumefaciens, which causes crown gall disease, transfers its genetic material into plant cells. That knowledge, combined with tissue culture, now lets useful genes be introduced into crops to produce improved and disease-resistant varieties.
Haploid plants from anther culture, the line of work of Sipra Guha Mukherjee and S. C. Maheshwari, which speeds up the breeding of new varieties.

Why totipotency is the key to all of them: every one of these applications depends on the same single fact — that a mature plant cell can be persuaded to become a whole plant again. Without that, a cultured cell would only ever give you more cells of the same kind.

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