cell specialisation

Outline how stem cells develop into specialized cells during development of the embryo. (2)

1. (by a process called) cell differentiation;

2. different genes are expressed/ switched on;

3. different concentrations of signaling molecules/ molecules determine position of cells in embryo;

State the main characteristic of stem cells.

undifferentiated/pluripotent/ability to divide/differentiate into any types of cells/differentiate along different pathways;

Outline the significance of surface area to volume ratio in the limitation of cell size. (4)

a. surface area of the cell affects the rate of material exchange;
b. when the cell increases in size, so does its chemical activity/metabolism;
c. (when the cell increases in size/grows) more substances need to be taken in / more waste products need to be excreted;
d. as the volume of the cell increases, so does the surface area, however not to the same extent
OR
when the cell gets bigger, its surface area to volume ratio gets smaller;
e. substances will not be able enter the cell fast enough/cell volume will not be supplied

OR
metabolic rate will exceed the rate of exchange
OR
when the surface area: volume ratio is higher, the diffusion rate increases;
f. some cells have adaptations to increase their surface area/flatten/microvilli/shape of red blood cells;
g. cells in growth areas tend to divide and remain small
OR
cells divide when maximum size is reached;

Outline two adaptations for water conservation in leaves of desert plants (2)

• (thick) wax layer/cuticle;

• hairs on leaves/rolled leaves;

• sunken stomata/stomata in pits/stomata opening at night/CAM physiology;

• (leaves reduced to) spines/needles/no/few/small leaves/low surface area (to volume ratio) of leaves;

• thick stems/water storage tissue/vertical stems (to avoid most intense sunlight);