2.81 Phototropism

Positive Phototropism:
- Photo = light, Tropism = growth in response to light
- Positive = growth towards the light
- Stem: (example of positive phototropism growth)
- It will grow forwards and upwards when there is light coming from all directions
- The plant bends in the growth towards the lateral light source because the light causes the movement to the opposite side of a compound: Auxin (plant hormone) which causes more growth on the opposite side than the side which has light, therefore, the plant bends.

2.80 Geotropism

- Geo = gravity, tropic = growth response
- An example: seed:
- the embryonic roots grow downwards, this is described as positive geotropism
- the embryonic shoot will grow upwards, this is negative geotropism

- Further experiment: take the same germinating seed and rotating it anti-clockwise to face upwards.
- The shoot grow upwards showing negative geotropism
- The roots will grow downwards showing positive geotropism

2.79 Plants and Stimuli

- Stimuli are changes in the environment (e.g. temperature, light)
- Plants have receptors which detect stimuli and turns into a response
- Responses take the form of growth - this type of response (growth in response to stimulus) is called a Tropism
- Tropisms involving light = Phototropism
- Tropism involving response to gravity = Geotropism
- Connection between receptor and response takes form of plant hormones called Plant Growth Regulators (e.g. Auxin)

2.54 Transpiration

- In order for evaporation to occur in the stomatal pores (water goes from liquid phase to gas phase), heat is required
- As the structure of the leaf absorbs the light energy, which is absorbed from the sun gets, converted to heat
- Water taken up in the root system by osmosis then through the stem and out the petiole.
- Not all of the sunlight is absorbed by the chloroplasts, it is absorbed by other cells which then generate heat.
- Then the gas (water vapour), diffuses through the pores to the outside atmosphere down a steep diffusion gradient

2.53 Uptake of Water

- Root hairs are made of epidermal cells
- Minerals go into the plant by active transport
- Water goes into the root hair by osmosis
- The cell wall expands, therefore, there is more surface area for osmosis to occur
- The xylem transports the minerals and water up the roots and through the whole plant