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Biological Rhythms
A biological rhythm or cycle is present when a phenomenon repeatedly goes
through a peak and trough over a specific period of time.
Circadian rhythms are biological processes that follow a 24 hour cycle.
· They include biochemical, (e.g. hormone production) N.B: In humans
circadian rhythms are
physiological (e.g. body temperature), and behavioural (e.g. coordinated by a
sleeping and eating) cycles whose period is approximately 24 `central biological clock'
hours. which is located in the
suprachiasmatic nuclei
· Circadian rhythms are of controlled internally but can be (SCN), a cluster of
influenced by environmental factors (such as ambient light about 10 thousand
nerve cells in the
intensity) hypothalamus.
Circadian Rhythms in animals
Sleep-wake cycle: The central biological clock (SCN)
detects light intensity to control secretion of the
hormone melatonin. As it gets dark the pineal gland
starts to secrete melatonin, which makes a person
sleepy. When levels of melatonin drop in the
morning it causes us to wake up.
Circadian Rhythms in Plants
Flower opening: Plants can respond to light intensity by opening and closing their
flowers at different times of the day. They only need to be open when the pollinators
are active.
Stomata opening: Stomata respond to light intensity by opening during the day when
photosynthesis occurs to let CO2 and O2 in and out, and closing at night to reduce
water loss.…read more

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Body temperature is
another example of a
circadian rhythm.
Jet lag: Jet lag is an example of a
phenomenon that happens when the
body clock becomes out of sync with
environmental cues. When a person
travels across time zones their body
clock circadian rhythms are still
synchronized with the time of day at
home but the information they are
receiving matches the time of day in the
destination giving rise to the
phenomenon of jet lag. It can take a
while for a persons body clock to
readjust and re-synchronise with the
destination time.…read more

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Photoperiodicity in Plants
Photoperiodism: Responses to changing day lengths
Plants are photoperiodic: they respond to changes in day length.
The day length gradually increases from midwinter to midsummer (in the
northern hemisphere between December and June) and then gradually
decreases again.
Some plants only germinate, grow or flower given a certain amount or
intensity of light and dark. Photoperiodic responses allow them to
germinate, grow or reproduce at the time of year that best suits them.
Seeds of some plants only germinate when days are very long to
make sure they only germinate in the middle of summer when the
temperatures are warm.
Some buds use increasing day length to tell them that it is far enough
from winter for them to grow to make sure they do not get killed by
Some plant species may respond to lengthening day lengths by
growing faster. Other plant species may stop growing in autumn.
Long day plants (e.g. spinach) only flower when the day is at least 14
hours to make sure they flower near to midsummer.
Short day plants flower only when days are less than 14 hours to
make sure they flower in early spring/late autumn allowing them to
flower when the right insects are about to pollinate their flowers.…read more

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Plant Defenses
Plants produce a variety of poisonous chemicals (secondary metabolites) that
are in order to defend themselves against being eaten by insect pests or larger
herbivores as well as against pathogens (disease causing organisms.)
Lupins produce toxic chemicals called alkaloids (class of nitrogenous
compounds) in their leaves as a defensive mechanism against
herbivores. Despite the fact that it takes energy to these produce
poisonous chemicals in the leaves, the high energy cost is outweighed
by the protection their chlorophyll containing leaves, the site of
Some varieties of potato produce chemicals against the potato blight
pathogen, a fungus-like organism that acts by destroying the leaves of the
plant, killing it. The potato blight fungus infecting potato crops in Ireland
caused a famine in 1845-6 killing >1million people.
Pests, weeds and pathogens can dramatically reduce crop yields.
· Fruit flies feeding of fruit can ruin entire crops
· Weeds growing near plants compete for nutrients in the soil ­ hindering the growth of
the plant getting the fewer nutrients and therefore lowering crop yield.
· If a plant becomes infected with a pathogen the pathogen takes energy from the host
plant or energy used to replace dead plant tissue ­ as a result there is less energy to
produce useful products reducing yields. Heavy infestation can mean that a whole field
produces nothing at all.
Increases the economic cost of producing food as money must be spent on pesticides or
fungicides and resistant crops. Low crop yields also drive up price for consumers. Since we
depend on plants so much for a food source the impacts can be devastating (especially
famine in developing countries)…read more

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Many chemicals can be extracted from plants to be used as drugs to treat
human diseases or to relieve symptoms. A lot of current medicines were
discovered by studying plants used in traditional cures.
1. Aspirin: used to treat symptoms of disease
such as to lower pain and fever. Aspirin was
developed from a chemical found in the leaves
and bark of the willow tree.
2. Quinine: comes from a South American
cinchona tree, used to be the main
treatment against malaria.
3. Taxol anti-cancer drug, comes from
bark of Pacific yew tree.…read more

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Growth of Bacteria
Prokaryotes reproduce very rapidly by the
process of binary fission (the equal division
of nuclear material and cytoplasm resulting
in 2 new organisms.)
Population growth b binary fission is
exponential: as time increases the rate of
growth of population increases (until there
are no more resources for extra growth
Louis Pasteur showed that microbes cause decay and disease ­ this
disproved the inaccurate theory of spontaneous generation (that
microbes appeared from nowhere) as a result of decays rather than
being the cause of disease. Pasteur argued that there are microbes
in the air causing disease.
Pasteur heated the broth in two flasks then let them open. One of
the flasks was a `swan neck' flask and the bacteria collected in the
loop ­ the broth was free of microorganisms, however when the
broth was placed in the normal flask which was left it was full of
microorganism. Pasteur's experiment showed that it was the
microbes (not the air) causing the broth to decay.
Pasteur then proposed the idea of keeping microorganisms away from people
and foods to prevent the spread of disease and preserve food. This method of
keeping things sterile and reducing contamination by microbes is aseptic
technique. Pasteur invented the process of pasteurization of milk ­ heating it
up to ~70 degrees and cooling it to kill most harmful germs/bacteria.…read more

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