Exercise 12.2
Mode of nutrition — autotrophic or heterotrophic.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
This item's stored line is a stray fragment of the §$\displaystyle 12.3.1$ criteria list. The chapter's Pause and Ponder $\displaystyle 2$ (page $\displaystyle 237$) asks: how can a single-celled organism carry out all its life processes when billions of cells are needed for the same functions in us?
Because in a unicellular organism the single cell is the whole body — that one cell performs nutrition, movement, response, waste removal and reproduction by itself.
The division of labour happens inside the cell, among organelles, not among tissues and organs. A true nucleus controls the cell, a contractile vacuole handles excess water, and cilia or flagella move it — Amoeba, Paramecium and Euglena in Fig. $\displaystyle 12.7$ are the chapter's examples.
Small size is what makes this possible. Every part of the cell is close to the surrounding water, so food, gases and wastes pass directly in and out across the cell membrane. No transport system is required.
A large multicellular body cannot work that way — its inner cells are far from the surface. So its cells specialise into tissues and organs, and the body needs transport tissue.
The chapter shows the same logic twice more: a thallus exchanges gases, nutrients and water directly with its surroundings, and a flatworm's flattened body lets gases diffuse without any respiratory organ.
