Exercise 2.6
Membrane-bound organelles Ready to Go Beyond In eukaryotic cells, a network of fine fibres forms the cytoskeleton, which provides structural support, maintains cell shape, and enables cell movement and internal transport. It is visible only under an electron microscope as a separate entity. The cytoplasm may also store starch (in plant cells), or crystals of calcium oxalate or silica (in some plant cells). These are known as cell inclusions.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Because surface area is what a mitochondrion works with, and many small ones carry far more of it than one giant one of the same volume.
Cellular respiration happens on the inner membrane. That is precisely why the inner membrane is thrown into cristae, finger-like projections that increase the surface area for the chemical reactions and so make more energy available. More membrane means more sites where ATP can be made.
Surface area grows as the square of the size while volume grows as the cube, so anything larger has less surface per unit of volume:
one cube of side \(\displaystyle 2 \): surface \(\displaystyle = 6 \times 2^2 = 24 \), volume \(\displaystyle = 2^3 = 8 \), ratio \(\displaystyle = 24 / 8 = 3 \)
eight cubes of side \(\displaystyle 1 \): surface \(\displaystyle = 8 \times 6 \times 1^2 = 48 \), volume \(\displaystyle = 8 \times 1 = 8 \), ratio \(\displaystyle = 48 / 8 = 6 \)
the same total volume, but twice the working surface once it is split up.
Small mitochondria can also be spread right through the cytoplasm and sit next to whatever needs ATP; one giant mitochondrion would leave the far corners of the cell short.
And a cell can build, repair or dispose of them one at a time as its energy demand changes — losing one of many is not losing everything.
Also in the supplied text — Activity $\displaystyle 2.4$, Table $\displaystyle 2.1$
| Cell structure | Bacterial cell | Plant cell | Animal cell |
| Cell membrane | Present | Present | Present |
| Cell wall | Present | Present | Absent |
| Cytoplasm | Present | Present | Present |
| Well-defined nucleus | Absent | Present | Present |
| Primitive nucleus (nucleoid) | Present | Absent | Absent |
| Membrane-bound organelles | Absent | Present | Present |
Fig. $\displaystyle 2.10$(a), the bacterial cell, is prokaryotic — no well-defined nucleus (a nucleoid instead) and no membrane-bound organelles.
Fig. $\displaystyle 2.10$(b) the plant cell and Fig. $\displaystyle 2.10$(c) the animal cell are eukaryotic — each has a membrane-bound nucleus and several membrane-bound organelles.
