B.sc. biochemistry sem 1 introduction to biochemistry unit 3.1 hormones

Course: B.Sc. BiochemistrySub: Introduction to biochemistry

Unit 3.1

The term hormone (hormao G = to excite) was first used byWilliam M. Bayliss and his brother-in-law Ernest H.Starling, both of London University College, in 1904, whoshowed that a chemical substance (secretin) from theintestine could stimulate the action of a pancreaticsecretion.

These substances were then called as ‘chemicalmessengers’. Went and Thimann (1937) defined a hormoneas “a substance which, produced in any one part of anorganism, is transferred to another part and thereinfluences a specific physiological process.”

The tissues or organs where they are produced are called aseffectors and those where they exert their influence astargets.

These have low molecular weight and diffuse readily. Asthey are readily oxidized, their effects do not remainpermanent unless these are supplied continuously.

Based on their site on action, the hormones are of twotypes : local and general. The local hormones, obviously,have specific local effects, whence their nomenclature.

These may be exemplified by acetylcholine, secretin,cholecystokinin etc. The general hormones, on the otherhand, are secreted by specific endocrine glands and aretransported in the blood to cause physiologic actions atpoints remote from their place of origin.

A few of the general hormones affect almost all cells of thebody, e.g., growth hormones (GH) and thyroid hormones ;whereas other general hormones, however, affect specifictissues far more than other tissues, e.g.,adrenocorticotropin (a hormone secreted fromadenohyprophysis and stimulating the adrenal cortex) andovarian hormones (affecting the uterine endometrium).

Composed of glands that secrete hormones into the circulatory system.

Hormones are secreted in minute amounts into the interstitial space.

Hormones eventually enter the circulatory system and arrive at specific target tissues.

Characteristics of theEndocrine System

Functions are similar to the nervous system.

Differences;

Amplitude-modulated vs. Frequency-modulated

Response of target tissue to hormones is usually slower and of longer duration than that to neurons.

Characteristics of theEndocrine System

Peptides & Proteins: Most hormones are either peptides or proteins and are usually referred to as peptide hormones.

Amines: Amine hormones are derivatives of the amino acid tyrosine.

Lipids & Steroids: Steroid hormones are produced by the adrenal cortex and the gonads.

Chemical Structureof Hormones

Hormones control the rates of many activities in the body.

The rate at which each hormone is secreted is controlled by a negative feedback mechanism.

Three major patterns of regulation:

Non-hormone substance (e.g. insulin)

Control ofSecretion Rates

1



Three major patterns of regulation:

Non-hormone substance (e.g. insulin)

Stimulation by the nervous system (e.g. epinephrine)


2



Three major patterns of regulation: Non-hormone substance (e.g. insulin)

Stimulation by the nervous system (e.g. epinephrine)

Hormone from another endocrine tissue (e.g. TRH, TSH)


3

Hormones are dissolved in the blood plasma and transported in free form or bound to a protein carrier.

As a result, hormones can be distributed throughout the body relatively quickly.

Hormones diffuse from the capillary to the interstitial space.

Transport and Distributionin the Body

Lipid-soluble hormones diffuse through the walls of all capillaries.

Water-soluble hormones must pass through pores.

Transport and Distributionin the Body

Hormones are only active in the body for a certain time because they are destroyed and eliminated shortly after they are secreted.

Half-life = length of time that it takes to eliminate half of the total amount of hormone that was secreted.

Metabolism and Excretion

Water-soluble hormones have relatively short half-lives because they are rapidly broken down by enzymes.

These hormones normally have concentrations that increase and decrease rapidly in the blood.

They generally regulate activities that have a quick onset and a short duration.


Lipid-soluble hormones are usually bound to protein carriers.

The rate at which these hormones are broken down is greatly reduced.

Therefore, these hormones have longer half-lives.


Hormones are removed from the blood in four ways.

Excretion (kidney, liver)

Metabolism (enzymes)

Active transport (actively transported into cells and secreted again)

Conjugation (attach water-soluble molecules to hormone and then excreted by kidney or liver).


Hormones only interact with cells that have binding sites that are specific for the particular hormone.

Interaction of Hormoneswith Their Target Tissues.

Hormones can be placed into one of two major categories.

Hormones that cannot pass through the plasma membrane.

Hormones that can pass through the plasma membrane.

As a result, hormone receptors need to be located in different locations.

Classes ofHormone Receptors.

Some receptors are located in the membrane of the target tissue.

After a hormone binds to the receptor, the receptor initiates events that lead to a response.

Some receptors alter membrane permeability.

Membrane-BoundHormone Receptors.




Some receptors activate G proteins.





Some receptors activate G proteins.

Some receptors alter intracellular enzyme activity.


Some receptors are located in the target cell.

They are located either in the cytoplasm or in the nucleus of the target cell.

Once the hormone binds to the receptor, the effects of the hormone take place.

IntracellularHormone Receptors.

4

Metabolism and tissue maturation.

Ion regulation.

Water balance.

Immune system regulation.

Heart rate and blood pressure regulation.

Control of blood glucose and other nutrients.

Control of reproductive functions.

Uterine contractions and milk release.

Functions of theEndocrine System

Non-specificity and Cross Effects

Hormones are not specific for the organisms in whichthey are produced and may influence bodily processesin other individuals also.

Adrenalin, for example, also influences protozoansand crustaceans besides man and other vertebrates.

Their cross effects have also been found betweenplants and animals. For example, auxin, a growth-promoting plant hormone, can also stimulate aprotozoan, Euglena viridis.

Likewise, some animal hormones also stimulategrowth in root tips of many plants after decapping.

Differences with Nutrient Materials and Enzymes — Adistinction between hormones and nutrient materialsmay be made, as the former are utilized inextremely minute quantities in comparison to thelatter.

For example, a 5 × 10−10 molar solution of ocytocin(about 1 mg in 2,000 litres of solution)will cause thecontraction of uterine muscles.

They also differ from enzymes, which are essentialto initiate and continue reactions, in that thehormones cannot initiate reactions but can influencethe rate at which they proceed.

Furthermore, the hormones are usually consumed inthe process of growth, whereas the enzymes are not.

The hormones conduct a wide variety of functionsranging from growth, vegetative and sexualdevelopment, cellular oxidation to thermal productionand the metabolism of carbohydrates, proteins andfats.

The various functions performed by hormones may, ingeneral, be discussed under following heads :

1. Regulatory or homeostatic function.

The hormones have regulatory effects on thecomposition of the body fluids, the rate of gaseousexchange and the activity of the vascular system andthe central nervous system (CNS).

There always exists a high degree of precision andconstancy in the composition of the body fluids in anormal individual for the conduction of variousactivities.

Such an environment within the cell has been termedthe millieu intérieur (or internal enivirnoment) in 1857by Claude Bernard, a French physician.

Throughout his research, he had been impressedby the way in which organisms were able to regulatephysiological parameters, such as body temperatureand water content, and maintain them within fairlynarrow ranges.

This concept of self-regulation leading to physiologicalstability was summed up by Bernard in the now classicstatement, ‘La fixité du milieu intérieur est la conditionde la vie libre.’ (The constancy of the internalenvironment is the condition of the free life). In otherwords, it means that for an organism to functionoptimally, its component cells must be surrounded bya medium of closely-regulated composition.

Bernard went on to distinguish between the externalenvironment in which organisms live and the internalenvironment in which the individual cells live (inmammals, this is tissue, or interstitial, fluid).

He realized the importance of conditions in the latterbeing continuously stable.

He concluded that an organism is the sum of itsconstituent cells and the optimum functioning of thewhole depends upon the optimum functioning of itsparts.

For performing various metabolic functions, anorganism should maintain a normal, constant internalstate or homeostasis (homoiosG = same or similar ;stasisG = state or standing), a term coined by anAmerican physiologist, Walter B. Cannon in 1932.

Homeostasis can be defined as the tendency tomaintain uniformity or stability in the internalenvironment of the organism and to maintain thenormal composition of the body fluids.

In other words, homoeostasis is the maintenance of aconstant internal environment in the face of changesin the external environment.

Hormones play an important and decisive role inhomeostatic regulation of internal mileu.

2. Permissive function.

Not only does each endocrine gland affect a number ofprocesses, but these glands also affect the functioningof one another.

Thus certain hormones require the presence (or‘permission’) of another hormone for the expression oftheir activity.

This helps in maintaining a perfect hormonal balance.Derangements of this balance, either clinical orexperimental, lead to a variety of metabolicaberrations.

3. Integrative function. The integrative functionof the hormones is reflected in the fact that theysupport the role of nervous system.

However, the integrative properties of the endocrinesystem are slow and steady whereas those of thenervous system are rapid.

This close tie between the two systems has led to theemergence of a new discipline of science calledneuroendocrinology.

4. Morphogenetic function. The hormones governthe ontogenetic development of an individual fromthe embryonic to the adult state.

Thimann (1948) designated the plant hormones by theterm ‘phytohormones’ in order to distinguish themfrom animal hormones.

He defined a phytohormone as “an organic compoundproduced naturally in higher plants, controlling growthor other physiological functions at a site remote fromits place of production and active in minute amounts.”

This definition includes a variety of compounds,besides those responsible for growth curvatures inorgans like Avena coleoptile.

For example, it embraces the hormones which induceflowering, wound-healing and also those vitaminswhich act as growth factors.

A definition of plant hormones with still wider scopehas been given by Johannes van Overbeek (1950).

According to him, the plant hormones are defined as“organic compounds which regulate plant physiologicalprocess— regardless of whether these compounds arenaturally occurring and/or synthetic ; stimulatingand/or inhibitory ; local activators or substances whichact at a distance from the place where they are formed.”

The migratory nature of hormones has beenspecifically emphasized by Meirion Thomas (1956)who stated that “all hormones are migratorycorrelating substances or correlators which play anessential part in the integration of plant behaviour.”

Three types of plant hormones are usually recognized.These are auxins, gibberellins and cytokinins.

These were discovered in the early decades of thetwentieth century, in 1930’s and in 1960's respectively.

Naturally, the knowledge accumulated on auxins andgibberellins is far greater than that gathered forcytokinins.

Plant Hormones

Hormone = Gr. “to excite”

1) active in small amounts

2) produced in one part of plant & transported to another for action

3) action is specific for that site

Each has a Multiplicity of Effects

Depending on site of action

Developmental stage of plant

Concentration of hormone

Auxins stimulate growth but too much inhibits

growth

functions:

1) root initiation, stem elongation

2) retard abscission (loss) of leaves & fruits (Figure 39.10)

3) stimulates cell differentiation

4) apical dominance (Figure 39.6)

Apical dominance5

6

Gibberellins

discovered due to a fungus Giberella which causes Japanese “foolish” rice seedling disease

Figure 39.7

induces flowering

stimulates growth by increasing cell size & numbers (Figure 39.8)

“Foolish seedling disease” in rice

17

39.11 Effect of Gibberellin

7

Dwarf pea plant treated with gibberellin

8

Cytokinins

1) induces cell division (cytokinesis)

2) affects root growth & differentiation

3) stimulates germination

4) delays senescence (aging); the progression of irreversible change that eventually leads to death

Abscisic Acid

1) stimulates abscission

2)converts vegetative buds (active) to dormant buds

3) inhibits growth

Ethylene

1) promotes fruit ripening

2) stimulates production of cellulase

Photochromes

Plant pigment

Important in processes where light is a critical factor, such as, flowering in long day short night plants.

References/Sources IMAGES:1.https://lh6.ggpht.com/gTIke2ZsoCOUYyV8cwUVWfKdjDCy3iqeMDQ_GQW329w1XLLaOV

StAATDkP1k6BAQ5iePHA=s1142.https://lh4.ggpht.com/gJ0IjG4cZ9BYisYLbtVO08xGbpJAZP_H7TfQrSxgpaSC3piBVfMz_5Bx

U1E45x8HicV85Q=s1133.https://lh4.ggpht.com/VfoukrG3Fb6VL_JYE7cseWLTiBTA6O2DXsywQAL08yHxEuXd4H2_u

Mhjhxnncav5vqrnFQ=s1144. https://lh6.ggpht.com/5Gr-nyJeWp0SjL7DSL-fmkj7RIS_sCqOE-

yDF4TgEj5_nKEQ5Bu8v1T1NVpx3EbO6f5VWQ=s1145. https://lh4.ggpht.com/o6NkH3-jWx8yEv8B5mIftQzeX6dGVjjOF-

Vnrllh3qFUmHpBeka6sye4HMZ4R8jifu0eZA=s1286.https://lh4.ggpht.com/Rier6R4QYECfTcN9QL0OHTnyCUqxpYPhJbNlg0JfC5d9qSiQwtv5Ri

XNjhZOBDgEniBqyQ=s1177.https://lh5.ggpht.com/0LCYvItpsuRa7NL4vsi8SNJelbT8on5jFdMWikguZT69tx21lwHLm5bx

T8XtKhSIYqFgSw=s1288.https://lh5.ggpht.com/xoSPnP_TTIuy3kMfS51xVXgLzWcepZe0noDxvgOIthmdOIyuqXAGiU

gUDdYI4gt-uStI=s86

Books/ Web resources Fundamentals of biochemistry by JL Jain and Jain www.shmoop.com › Biology › Plant Biology › In Depth

Education

B.sc. biochemistry sem 1 introduction to biochemistry unit 3.1 hormones