A2 Survival and Response: Plant responses

Need to know:

Organisms increase their chance of survival by responding to changes in their environment.

Tropisms are responses to directional stimuli that can maintain a mobile organism in a favourable environment.

A tropism is a plant response in which the direction of growth is determined by the direction of the stimulus.
It may be positive or negative, e.g. the shoot of a germinating seed is positively phototropic (grows towards light), but negatively geotropic/gravitropic (grows in the opposite direction to gravity).

e.g.
stimulus                growth response
light                      phototropism
gravity                  geotropism
touch                    thigmotropism

The communications system of flowering plants involves:
sensor (receptor) --> plant growth substances --> effector
                               (or plant growth regulators)

N.B. Use 'plant growth substances (or regulators)' NOT 'hormones'.

There are five types of plant growth substance.

• Auxins stimulate cell elongation in stems, produce apical dominance (leading bud grows at the expense of others) and stimulate the growth of adventitious roots (grow directly from stem) in stem cuttings. The first auxin to be discovered was indoleacetic acid (IAA), which is involved in phototropism.
• Gibberellins promote stem elongation and stimulate seed germination.
• Cytokinins promote cell division and delay senescence (aging, leaves/fruit falling off).
• Abscisic acid (ABA) inhibits seed germination, thereby promoting seed dormancy, closes stomata in conditions of water stress and helps plant adapt to cold conditions (nature's way of controlling when the seed starts to grow).
• Ethene ripens fruit and is involved in responses to stress.

1. Auxins synthesised in cells of shoot apical meristems.
2. Diffuse away from tip to zone of elongation.
3. Bind to specific receptor sites on cell surface membranes of small, newly formed cells.
4. Activates some molecules in cell surface membranes to pump H+ ions by active transport from cytoplasm to primary cell walls, decreasing the pH (more acidic).
5. Lower pH means optimum conditions for enzymes, break bonds between adjacent cellulose microfibrils. Microfibrils can then slide freely past each other - keeps walls flexible.
6. Cells absorb water (osmosis) and flexible cell walls allow cell expansion as extra water exerts higher hydrostatic pressure.
7. As cells mature, become situated further from tipdue to formation of new cells. Eventually auxins in receptor sites destroyed by enzymes and pH of cell walls rises. Bonds form between increased number of cellulose microfibrils and cells walls become more rigid, cannot expand anymore.

Don't need to know in as much detail, but useful to get better understanding.

Bioassay:

- technique in which concentration of a particular substance is determined by measuring the extent of its biological effect.

- The results of such experiments could be used to compare the relative concentrations of auxins in different agar blocks placed halfway across a broken stem (a higher concentration of auxins will result in a larger angle of curvature away from the direction the auxins are entering the stem).