Introduction

- Hormone is a molecule that is secreted into the extracellular fluid, circulates in the blood or heamolymph and communicates regulatory message throughout the body.

- Endocrine gland is made up of ductless glands that release hormone into the bloodstream.

- Exocrine gland has duct that carry secreted substances onto body surface or into body cavities.

Comparison between endocrine system and nervous system

Endocrine system Feature Nervous systemTransmitted in the form of chemical substance

Nature of information Transmitted in the form of electrical impulse

By the circulatory system Mode of transmission By the nervous systemSlow Speed of transmission RapidLong-lasting Duration of effect Short-livedCarried by blood to targeted cell

Localization of effect Transmitted by nerve fibre to particular destination

Many different Nature of chemical coordination

Only a few types

Graded response Nature of response Follows all-or-none law

There are two types of hormones

1. Water-soluble hormone (peptide and amino-based hormone)- ADH

- Insulin

- FSH

- LH

2. Lipid-soluble hormone (steroid hormone)- Progesterone

- Oestrogen

- Testosterone

Mechanism of action via gene activation

1. Some hormones such as steroid hormones are lipid-soluble so they can enter easily through the phospholipid layer of the cell.

2. In the cytoplasm, the hormone binds to its receptor to form a hormone-receptor complex.

3. The hormone-receptor complex then enters nucleoplasm and binds to a specific section of DNA which functions as a gene.

4. The complex stimulates the gene to transcribe messenger RNA (mRNA).5. The mRNA moves to the cytoplasm and directs the synthesis of proteins that alter

the activity of the cell.

Mechanism of action via cAMP (signal transduction mechanism)

1. Peptide and amino-based hormone serves as the first messenger binds to the receptor (G protein-coupled receptor) in the plasma membrane and forms hormone-receptor complex.

2. The hormone-receptor complex binds to and activates G-protein in the membrane.3. The activated G-protein then binds to the enzyme adenylyl cyclase and activates

it.4. The activated adenylyl cyclase catalyses the conversion of ATP to cAMP.5. cAMP acts as the second messenger which initiates a complex chain reaction.6. cAMP activates the inactive enzyme protein kinase which in turn activates

inactive enzyme phosphorylase kinase.7. Active phosphorylase kinase then activates the inactive enzyme glycogen

phosphorylase that catalyses the breakedown of glycogen to glucose phosphate.8. This results in a cascade where the action of an enzyme in turn activates another

enzymatic reaction producing many product molecules. This brings about an amplified and rapid response to the non-steroid hormone.

Menstrual cycle

1. The cycle begins with the release from the hypothalamus of GnRH2. Which stimulates the anterior pituitary gland to secrete small amounts of FSH and

LH.Follicular phase

3. FSH stimulate follicle growth, aided by LH 4. The cell of growing follicle starts to make oestrogen. The low level of oestrogen

inhibits secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low.

5. When oestrogen secretion by the growing follicle begins to rise steeply,6. The FSH and LH levels increase markedly. Whereas a low level of oestrogen

inhibits the secretion of pituitary gonadotrophins, a high concentration has the opposite effect. It stimulates gonadotrophins secretion by acting on the hypothalamus to increase its output of GnRH. The effect is greater for LH because high concentration of oestrogen increases the GnRH sensitivity to LH-releasing cell in the pituitary gland. In addition, follicle responds more strongly to LH at this stage because more of their cells have receptor for LH. The increase in LH concentration by increased oestrogen secretion from the growing follicle is an example of positive feedback. The result is final maturation of the follicle.Ovulation

7. The maturing follicle, which contains an internal fluid-filled cavity, grows very large, forming a bulge near the surface of the ovary. The follicular phase ends at ovulation, about a day after the LH surge. In response to the peak in LH levels, the follicle and adjacent wall of the ovary rupture, releasing secondary oocyte. Luteal phase

8. LH stimulates the follicular tissue left behind the ovary to transform into the corpus luteum. Under continued stimulation of LH, the corpus luteum secretes oestrogen and progesterone. As the oestrogen and progesterone levels rise, the combination of these hormones exerts a negative feedback to the hypothalamus and pituitary, reducing the secretion of LH and FSH to very low levels. MenstruationNear the end of the luteal phase, low gonadotrophins levels cause the corpus luteum to disintegrate, triggering a sharp decrease in oestrogen and progesterone concentration. The decreasing levels of ovarian steroid hormones liberate the hypothalamus and pituitary gland from the negative-feedback effect of these hormones. The new cycle begins with the secretion of FSH and LH by pituitary gland.

Role of hormones during pregnancy

- When fertilized egg is anchored in the endometrium, HCG hormone is produced to replace the role of LH to maintain the secretion of oestrogen and progesterone in the first trimester.

- HCG hormone drops and corpus luteum degenerates. The role of corpus luteum is taken over by placenta in the second trimester.

- Combination of oestrogen and progesteroneMaintain the development of endometrium and thus prevent menstruationInhibit the release of FSHInhibit the release of prolactin

- OestrogenStimulate the development of mammary glands for lactationIncrease the size of myometriumTrigger the formation of receptor to hormone oxytocin in the uterine muscle

- ProgesteroneInhibit contraction of myometriumStimulate the development of mammary glands for lactation

- Towards the end of pregnancy (third trimester), the level of progesterone decrease and trigger onset of parturition.The decrease of progesterone levels causes the contraction of myometrium.Oestrogen increases the sensitivity of myometrium to oxytocin.The contraction of myometrium some more stimulates the posterior pituitary gland to secrete more oxytocin.At the same time, it stimulates the placenta to produce prostaglandins to increase the power of myometrium contraction.

- After the birth, the progesterone and oestrogen concentration decline sharply due to the loss of placenta.

- Prolaction is no longer inhibited and is released to stimulate mammary glands to produce milk.

- Oxytocin stimulates the release of milk from mammary glands.

Lactation

- Lactation is the production of milk for nourishing the young.

- After birth, prolactin is secreted by the anterior pituitary gland to stimulate milk production.

- When baby suckle, the posterior pituitary secretes oxytocin to eject milk from the alveoli into the duct.

- Breastfeeding promotes recovery of the uterus because oxytocin released during breastfeeding stimulates the uterus to contract to normal size.

Plant hormone

Auxin

1. Role in cell elongation- Low concentration of auxin may stimulate cell elongation in the apical meristem

but higher concentration may inhibit cell elongation by inducing production of ethylene, hormone that inhibits cell elongation.

- Acid growth hypothesis – auxin stimulate plasma membrane to pump H ions into cell wall and lowers its pH. Acidification of cell wall activates expansions that break the cross-link between cellulose myofibrils. Lower water potential triggers intake of water and increases the turgor pressure and in turn increase cell wall plasticity which causes cell to elongate.

- Stimulate the sustained growth response (addition of cytoplasm and wall material) for cell elongation.

2. Lateral and adventitious root formation- Auxin involves in branching of root.

- Treating detached leaf or stem with rooting powder containing auxin cause adventitious root to form.

3. As herbicide

- Monocot can rapidly inactivate auxin as herbicide.

- Eudicots cannot and therefore die from hormonal overdose.4. Other effect- Auxin affects secondary growth by increasing cambial activity and influencing

differentiation of cambial initials.- Promote fruit growth.

- Induce fruit development without pollination.

Cytokinins

1. Control of cell division and differentiation- Work with auxin at certain ratio to stimulate cell differentiation.

- Increase cytokinins levels, shoot buds develop.

- Increase auxin levels, roots form.2. Control of apical dominance- Auxin and cytokinins act antagonistically in regularing axillary bud growth.

- Auxin inhibits axillary bud growth.

- Cytokinins counter the action of auxin by signaling axillary bud to begin growing.3. Anti-aging effects- Cytokinins slow down the deterioration of cell by inhibiting protein breakdown,

stimulating RNA and protein synthesis and mobilizing nutrients from surrounding tissues.

Gibberellins

1. Stem elongation- Stimulate stem and leaves growth but have little effect on foots.

- In stem, it stimulates cell elongation and cell division.

- Act in concert with auxin to promote cell elongation.2. Fruit growth- Auxin and gibberellins must be present in order for fruit to develop.3. Germination- Gibberellins signal the seed to break dormancy and germinate.

Abscisic acid

1. Seed dormancy- Inhibits seed dormancy during seed maturation and induce the production of

certain proteins that helps the seed withstand dehydration.2. Drought tolerance

- Accumulate in leaves and causes stomata to close rapidly.

- Cause potassium channel of the guard cell to open, leading to a massive loss of calcium ion and is followed by osmotic pressure.

Ethylene

1. The triple response to mechanical stress- Enable the young shoot to avoid the obstacle when growing up from the soil.

- Triple response = slowing of stem elongation, thickening of the stem, curvature that cause the stem to grow horizontally.

- Whenever there is obstacle, ethylene is produced to inhibit vertical elongation.

- Production of ethylene decreases when no obstacle is detected, vertical elongation is resumed.

2. Senescence- A burst of ethylene is almost always associated with the programmed destruction

of cells, organs or the whole plant during autumn.3. Leaf abscission- A change in the ratio of ethylene to auxin controls abscission.

- Aging leaf produces less auxin, rendering the cells of abscission layer more sensitive to ethylene.

4. Fruit ripening- A burst in ethylene production in fruit triggers the ripening process.

- Stimulate the enzymatic breakdown of cell wall components to soften the fruit and conversion of starch to sugar to make fruit sweet.

- The production of new scents and colours helps advertise ripeness to animals.

Phytochrome

1. Plants have a light-detecting system known as phytochrome which plays a role in plants development, seed germination as well as flowering.

1. A phytochrome has two identical subunits, each consisting of a polypeptide component covalently bonded to a non polypeptide chromophore, the light-absorbing part of the subunit.

2. The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomer forms, depending on the colour of light.

3. In its Pr isomer form, a phytochrome absorbs red light maximally, whereas in its Pfr isomer form, it absorbs far-red light.

4. Pfr triggers mamny of a plant’s development responses to light.5. Conversion to Pfr is faster than the conversion to Pr. therefore the ratio of Pfr to

Pr increase in sunlight.

Photoperiodism

1. Photoperiod is the relative lengths of daylight and darkness that a plant is exposed in a 24-hour cycle.

2. Photoperiodism is the physiological response to photoperiod, such as flowering.

Short-day plant

- Plants that require a light period shorter than a critical length to flower.

- Example: chrysanthemums, poinsettias and some soybean.

Long-day plant

- Plants that require light period longer than a certain number of hours to flower

- Example: spinach, radishes, lettuce, irises and many cereal varieties.

Day- neutral plant

- Plants that are not affected by photoperiod and flower regardless of day length.

1. The photoreceptor of photoperiodism is phytochrome. The balance between two isomers of chromophore, controls the flowering in short-day and long-day plant.

2. Pfr stimulates flowering of long-day plants but inhibits flowering in short-day plants.

3. Florigen, a chemical messenger that transmits information from leaves to flower buds, is secreted in response to relative amounts of two forms of phytochrome.

4. There are two inactive forms of florigen: one of which is converted to florigen in the absence of Pfr, the other in the presence of Pfr.

5. In long-day plants, the inactive hormone is activated by the presence of Pfr while in short-day plants, the inactive hormone is activated by the absence of Pfr.

1. When the dark period is shorter than a critical dark period, the long-day plants will flower but short-day plants will not flower.

2. When the dark period is longer than a critical dark period, the long-day plants will not flower but short-day plants will flower.

3. A flash of red light interrupting the dark period shortens the dark period. Hence, there is flowering in long-day plants but not short-day plants.

4. A subsequent flash red light followed by far-red light cancels the effect of red light. Hence, there is flowering in short-day plants but not long-day plants.

5. If a red light flash followed by a flash of far-red light, and then red light flash again, there is flowering long-day plants but not short-day plants.

6. Therefore, it can be concluded that the flowering of long-day plants or short-day plants is determined by the last flash of light which favours the either plants.