Phototropism

Light source from window

Direction the plant is growing - positive phototropism

- The plant grows in the direction of the (lateral) light source

- Auxin in the pink side causes more growth

2.81 phototropism

Phototropism - growth responses to light

Positive phototropism - towards light

Stem tip:

uniform light coming in from all directions - grows upwards

lateral light source (from one side) - stem bends towards the light

Auxin (plant hormone) causes more growth in the opposite side (side without light) - bending the plant

2.80 geotropism

Geotropism - growth responses to gravity

seed growing

- embryonic roots growing downwards = positive geotropism

- embryonic shoot growing upwards = negative geotropism

if the seed is rotated, the shoot still grows upwards (neg. geotropism)

the root still grows downwards (pos. geotropism)

2.79 plants and stimuli

Stimuli - changes in the environment

e.g. temperature, light, gravity changes

Receptor - can detect stimuli

therefore making a response

Response

- growth

i.e. tropism - growth in response to stimulus

phototropism - tropism that involves light

geotropism - tropism in response to gravity

Connection between receptor and response - plant hormones (or plant growth regulators - e.g. Auxin)

2.54 Transpiration

Transpiration: evaporation of water from the surface of a plant

H2O (l) ---> H2O (g)

Heat + sunlight (light energy converts to heat as structure of leaf absorbs light)

evaporation through stomatal pores

Water transport = root -> stem -> leaf (by xylem) -> (gas) stomatal pores

Chloroplasts not absorbing all the light - some absorbed by other cell structures

water ---> (xylem) ----> mesophyll layer (spongy)

above stomatal pore: phase change - liquid -> gas (evaporation)

Gas diffuses through pore (steep diffusion gradient)

- humidity

Summary

1. sunlight warms leaf

2. liquid -> gas from heat

3. diffusion

2.53 uptake of water in the roots

- branching pattern in root hair cell

   - increase surface area for absorption of water

   - roots branch out, looks for water source

- hairs emerging from end of root - root hairs

   - epidermal cells

   - cell wall extended (root hair structure) - increases surface area for abs H2O

- root hair structure on epidermal cells

   - surface area concept

   - mechanism - active transport of minerals (movement against conc. gradient, involving energy)

- plant actively concentrates minerals inside root, encourages plant to take in water by osmosis (from dilute (soil water) to concentrated (created by building up minerals in cells))

- water moves across cortex of root to xylem by osmosis