Objectives – What you will need to know from this section

1. H.3.5.6 Plant Growth Regulators and Animal Hormones (E.S) Objectives – What you will need to know from this section • Explain the mechanism of plant response to any one external stimulus. • Describe the feedback mechanism of any one animal hormonal system.

2. Plant Regulators

3. Plant growth regulators [hormones] are chemicals that interact with one another to control a particular development or response. • Plants only grow at their tips, in small regions of active celldivision called meristems—the tips of shoots, roots and sidebuds. • Plant growth regulators are produced in the meristems and transported through the vascular system of the plant.

4. The coleoptile is the sheath around the leaves and shoot of grass seedlings, such as corn or oats. Plant Regulators • IAA (Indoleacetic acid) is an auxin that is made in themeristems of shoots, buds and root, and in the tips ofcoleoptiles. • It travels backwards from the tip and causes cells to elongate (get longer) and it regulates the differentiation of the vasculartissue (xylem and phloem).

5. A tropism is a plant’s response to a stimulus coming from onedirection, e.g. sunlight, gravity. • Phototropism is a growth response of a stem towards light, sothat it can receive the maximum amount of light forphotosynthesis.

6. Quicker growth here due to more auxin • If a plant receives light from one side only, the light causes theauxin travelling down from the meristem to be redirected awayfrom the light and move down the ‘dark’ side of the stem (orcoleoptile). • The extra auxin makes these cells elongate faster than cells onthe ‘bright’ side. • This causes a bending of the stem towards the light.

10. APICAL DOMINANCE • Auxins are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem. • This photograph shows side buds sprouting when the main stem is cut off [pruned].

11. LEARNING CHECK • What is a meristem? • What is IAA and give its function? • Explain phototropism in terms of light and IAA action. • What is apical dominance. • Explain apical dominance in terms of auxin.

12. Animal Hormones

13. Homeostasis is the maintenance of a constant internal environment and is achieved by ‘feedbackmechanisms’. • Homeostasis is necessary if an organism is to be independent of its surroundings, and if its metabolism is to function efficiently. • Most homeostatic mechanisms work by negative feedback, i.e. if there is a change away from the normal optimum value, action is automatically taken to reverse this change.

14. The thermostat in an oven is an example of how this negative feedback system works. • In negative feedback, the response reduces the strength of the original stimulus—very useful for controlling the release of hormones. • There must be a sensor to detect the change, and an effector to reverse the change. • Control of water, thyroxine & sugarlevelsare examples of how homeostasis works in humans.

15. Control of Water Levels • The kidney regulates the amount of water in the body byvarying the amount of urine produced. • This is known as osmoregulation, and it is an example ofhomeostasis. • ADH [Anti-diuretic hormone] controls whether the distal tubuleand collecting ducts reabsorb water or not.

16. If you drink a great deal of water • the hypothalamus in the brain detects the diluted blood and turnsoffADH production. • Less water is reabsorbed, so more water is allowed to escape to the bladder, • and a larger volume of dilute urine is produced. When the body is low on water • ADH is secreted from the pituitary gland. • More water is reabsorbed • and only a small volume of urine is produced.

17. When the body is low on water, ADH is secreted from thepituitary gland. More water is reabsorbed and only a smallvolume of urine is produced. If Water Levels Fall in the Body…..

18. If Water Levels Rise in the Body ….. • When the body has excess water, ADH is NOT secreted from thepituitary gland. Less water is reabsorbed and so a largervolume of urine is produced.

20. LEARNING CHECK • What is homeostasis? • What is osmoregulation? • What is ADH? • Where is it made and where does it act? • What happens in the kidney if we take in a lot of fluid? • What happens in the kidney if we are short of water in the body?

21. Control of Thyroxine Levels • The regulation of thyroid hormone secretion depends on a negative feedback loop between the pituitaryand the thyroid gland:

22. High concentration of thyroxine ‑‑‑ inhibits pituitary ‑‑‑> less TSH ‑‑‑ > less thyroxine --> level drops TSH = Thyroid Stimulating Hormone

23. Low concentration of thyroxine ‑‑‑> stimulates pituitary ‑‑‑> more TSH ---> more thyroxine --> level rises

24. THYROXINE control – Feedback

25. Control of Sugar Levels If Sugar Levels Rise in the Body …..

26. If Sugar Levels Fall in the Body …..

28. LEARNING CHECK • What is meant by feedback? • What is negative feedback? • Can you find out what positive feedback is and give an example from everyday life? • What’s the difference between TSH and Thyroxine? • What is the difference between insulin and glucagon?