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.

H.3.5.6 Plant Growth Regulators and Animal Hormones (E.S)

Plant Regulators

Plant growth regulators [hormones] are chemicals that interact with one another to control a particular development or response.

Plant growth regulators are produced in the meristems and transported through the vascular system of the plant.

Plants only grow at their tips, in small regions of active celldivision called meristems—the tips of shoots, roots and side buds.

IAA (Indoleacetic acid) is an auxin that is made in themeristems of shoots, buds and root, and in the tips ofcoleoptiles.

Plant Regulators

It travels backwards from the tip and causes cells to elongate (get longer) and it regulates the differentiation of the vascular tissue (xylem and phloem).

The coleoptile is the sheath around the leaves and shoot of grass seedlings, such as corn or oats.

A tropism is a plant’s response to a stimulus coming from one direction, e.g. sunlight, gravity.

Phototropism is a growth response of a stem towards light, so that it can receive the maximum amount of light for photosynthesis.

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).

This causes a bending of the stem towards the light.

The extra auxin makes these cells elongate faster than cells on the ‘bright’ side.

Quicker growth here due to more auxin

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].

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.

Animal Hormones

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.

Homeostasis is the maintenance of a constant internal environment and is achieved by ‘feedback mechanisms’.

The thermostat in an oven is an example of how this negative feedback system works.

Control of water, thyroxine & sugar levels are examples of how homeostasis works in humans.

There must be a sensor to detect the change, and an effector to reverse the change.

In negative feedback, the response reduces the strength of the original stimulus—very useful for controlling the release of hormones.

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 tubule and collecting ducts reabsorb water or not.

Control of Water Levels

If you drink a great deal of water

the hypothalamus in the brain detects the diluted blood and turns off ADH 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.

If Water Levels Fall in the Body…..

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 Rise in the Body …..

When the body has excess water, ADH is NOT secreted from the pituitary gland. Less water is reabsorbed and so a largervolume of urine is produced.

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?

The regulation of thyroid hormone secretion depends on a negative feedback loop between the pituitary and the thyroid gland:

Control of Thyroxine Levels

High concentration of thyroxine ‑‑‑ inhibits pituitary ‑‑‑> less TSH ‑‑‑ > less thyroxine --> level drops

TSH = Thyroid Stimulating Hormone

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

THYROXINE control – Feedback

If Sugar Levels Rise in the Body …..

Control of Sugar Levels

If Sugar Levels Fall in the Body …..

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?