b1 edexcel SCIENCE for tomorrowWatch
1) A reflex action is triggered by a change in the environment (stimulus) which is detected by a receptor (light/sound/pressure/temperature). This receptor sends an electrical impulse down the sensory neurone, across a synapse, through a relay neurone and into the central nervous system. The CNS will coordinate a response and this will be sent through the relay neurone, across a synapse and through the motor neurone to the relevant affector. Usually, this is a muscle (the muscle will contract) or a gland (hormones will be secreted). Across a synapse, the electrical impulses are converted to chemical messengers which diffuse across the synapse and become electrical impulses again at the other end.
2) The main plant hormone you need to know about is auxin. It is present on the tips of shoots and roots.
Auxins encourage growth in the shoots and inhibit growth in the roots.
- Shoots will grow toward sunlight. Auxin accumulates on the side that has the least sunlight, causing that side to grow more and making the plant grow in the direction of the sunlight. (phototropism)
- Shoots will grow away from gravity. Auxin accumulates on the bottom of the shoot, thus promoting upward growth. (gravitropism/geotropism)
- Roots grow towards moisture. Auxin accumulates on the side with the most moisture, inhibiting growth on that side, causing the plant to grow towards the moisture. (hydrotropism)
- Roots grow towards gravity. Auxin accumulates on the bottom of the root, inhibiting growth there, thus promoting growth downwards. (gravotropism/geotropsim)
Auxin can be added to clones grown as plant cuttings as it helps them to grow roots. These hormones are in rooting powder.