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Positive feedback
Positive feedback is less common, which is understandable, as most
changes to steady state pose a threat, and to enhance them would be
most unhelpful. However, there are a few examples:
A baby begins to suckle her mother's nipple and a few drops of milk are
released (the stimulus). This encourages the baby and releases a
hormone in the mother which further stimulates the release of milk (the
response). The hungry baby continues to suckle, stimulating more milk
release until she stops. (Positive feedback, it would not have helped the
baby if suckling decreased milk flow, as in negative feedback!)
A ripening apple releases the volatile plant hormone ethylene (the
stimulus). Ethylene accelerates the ripening of unripe fruit in its
vacinity so nearby fruit also ripens, releasing more ethylene (the
response). All the fruit quickly becomes ripe together. ("One 'bad' apple
has ruined the whole lot." The biological explanation - positive
feedback - for an old saying!)
Ethylene
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Ethylene (or ethene) is a gas at room temperature, the
bp is 104 degrees C
Ethylene is the simplest of a class of hydrocarbons called
Alkenes. Alkenes contain carbon-carbon double bonds
Ethylene is used extensively in industry as a starting
material for larger organic molecules.
The polymerization of ethylene produces polyethylene.
Ethylene is also a plant hormone involved in flower and seed
production, and the ripening of fruits. It stimulates a variety of enzymes
which convert starch and acids of the unripe fruit to sugars, and softens
the fruit by breaking down pectins in the cell walls.
It has a positive feedback effect: the first fruit that begins to ripen
emits ethylene triggering the surrounding fruit to ripen. Overripening
can occur, thus one "bad apple" can really "spoil the whole bunch".
Apples are usually stored in
refrigerated rooms in an atmosphere enriched with CO2. By saturating
the atmosphere with CO2, the action of ethylene is inhibited.
Plant growers often pick fruits before they are ripe. After traveling to
the market they are sprayed with ethylene to ripen quickly for sale.
Fruits tend to travel better when they are not ripe, and this method also
ensures that the fruit doesn't arrive at your supermarket overripe and
mushy.
What are the two types of fruit ripening?
Which type is caused by a hormone? What is the hormone and how
does its production vary as ripening proceeds.
The two types of fruit ripening are climacteric and non-climacteric.
Climacteric ripening is caused by the rapid (exponential) increase in
ethylene. Ethylene increases so rapidly because of two feedback
mechanisms. First, ethylene stimulates its own production in a positive
feedback look, and second, ethylene suppresses auxin production,
which would normally suppress ethylene production. Non-climacteric
ripening is not accompanied by exponential increases of ethylene.
Ethylene production is autocatalytic. That is, exposure to ethylene
stimulates the synthesis of more ethylene. This occurs because the
genes for the biosynthetic enzymes (e.g. ACC SYNTHASE) are ethylene
inducible. The result is a positive feedback loop. Furthermore, the
Never-ripe gene is ethylene inducible, resulting in a positive feedback
loop for ethylene sensitivity as well. Both these factors contribute to
the dramatic burst of ethylene production during ripening.
Fruit softening involves a partial breakdown of cell walls. Several
enzymes are known to be involved in this process. Polygalacturonase
hydrolyzes bonds in pectins. The gene for this enzyme is ethylene
inducible.
Changes in fruit color involve changes in the expression of pigment
biosynthetic genes. The major pigment in tomato is a carotenoid. The
first committed step in carotenoid biosynthesis is catalyzed by
phytoene synthase, and the gene for. his enzyme is induced by
ethylene.
Ethylene is a plant hormone that is responsible for the ripening of fruits
and vegetables in a positive feedback cycle, lending a scientific
explanation for one bad apple ruining the bunch.
Ethylene
19th century fruit farmers of mango and pineapple learned that
something in gas fires near trees resulted in synchronized flowering
and fruit ripening. Greenhouse growers also observed that gas lighting
(carburo) seemed to stimulate premature leaf withering. We now know
that these effects are due to a gaseous hormone known as ethylene,
and it promotes fruit ripening, plant senescence, and fruit and leaf
drop.
Ethylene production is triggered when auxin concentrations pass a
certain critical level. Ethylene then counteracts the effects of auxin.
Ethylene binds to receptors in the plasma membrane of fruit cells, and
triggers the activation of genes which code for enzymes that convert
starches and acids into sugars, thus ripening the fruit, or other
enzymes which soften the fruit.
In apples, bananas, and tomatoes, the release of ethylene results in a
surge of metabolism and ripening called the climateric. This is due to a
positive feedback effect, ethylene production triggers more ethylene
production and more rapid ripening, hence "one bad apple....."
Fruits which exhibit a climateric may be stored for longer periods under
cool conditions with carbon dioxide enriched atmospheres, this inhibits
ethylene. Non-climateric fruits such as cherries, lemons, and oranges
have a steady ethylene concentration and keep better during shipping
and storage, however the fungus Penicillium may grow on the surface
of an orange and promote overripening. Some citrus fruits are picked
green, shipped, then exposed to ethylene to promote ripening just prior
to marketing.
Since high ethylene concentrations inhibit lateral bud development, a
ripe apple stored with potatoes will inhibit sprouting of the potatoes.
Other Chemical Regulators
In addition to the five major plant hormones, other regulatory
substances have recently been discovered.
Molecules called
polyamines (containing many amino groups) inhibit and counteract the
effects of ethylene. Tomatoes have been genetically altered to high
polyamine levels to promote longer shelf life.
A polypeptide called systemin was recently shown to be produced in
some plants. At concentrations as low as 1 in 10 trillion it triggers the
production of proteinase inhibitors that may kill leaf-eating insects.
Oligosaccharides are produced in wounded tissue, and trigger localized
production of proteinase inhibitors, or the production of phytoalexins, a
class of plant antibiotics.
Salicylic acid, from the willow, helps defend plants against pests or
pathogens by activating genes for a systemic acquired resistance
response. Acetylsalicylic acid (aspirin) is a derivative of this compound.