I remember writing these notes in Y12, I know first hand the amount of time that has to be invested into this subject and how difficult it can be to identify the key information. These notes are time savers, giving you more time to practice questions and past papers. The notes used for all my A-lev...
whichstimuli affect plants
Plants are sensitive to light Lightaffects how muchtheygrow the directionofgrowthand when
they
reproduce
They are also sensitiveto gravity water and temperature In somecases they may besensitiveto
Touch and Chemicals
Dicerentparts of the plantmay react differently to the same stimulus e g rootsand tipo stem
slant responses
Chemicals in plants are called planthormonesthey are analogous to animal hormones Theyare
produced
in one area and have their effect on cells elsewhere
Plants respond to chemical messages in a number of different
ways
Directional growthresponses to specific environmental cues are called tropisms
ow plantsgrow
Growth is a permanentincrease in the size of an organism It is brought about
by cell division and
a
assimilation oe new materials cuithin the cells that result from the divisions followedbycellexpansio
Main areas of plant cell division are called meristerms they are just behind the tipo a rootorsh
toxins
Auxin s are growth stimulants effective in very low
concentrations They are produced in youngshoo
and always move from shoot to root This movement involves an activetransportof calcium ions
In low concentrations theypromote rootgrowth The more auxin transported down the stem the
more root growthwill occur
IE the tipo the stems are removed removing the source of aux ins the stimulation of root
growthis removed so it slows and stops
tow do aux ins work in a plant
i Molecules oe IAA bind
to specificreceptorsites of cell membranes activate the active pump
they
of It ions into cell wall spaces This changes the pit to around s providingoptimumpH for enrym
to break bonds with adjacentcellulosemicrofibrils
This allows microfibrils to slide past each other The cell walls become elastic When the ce
takes water in osmosis it is now flexible so the wall expands As the cell matures IAA
by
is destroyed the pit of the wall rises bonds
between cellulose microcibrils and the wall
becomerigid and stopsgrowing
Light causes auxin to move laterally across the
shoot producing a greater concentration on the
dark side This stimulates growth on the said
side causing the shoot to bend directly to the light
Once the shoot aims directly towards
light the
unilateral stimulus is removed thus the as metric
y
transporto auxin ends
More plant hormones
Gibberellins Aee ect the internodes of the stems stimulating elongation of the growingcells
Theypromote the growth of Evil and are involved in breakingdormancy in seeds
They will stimulate the formation of enzymes inseeds E g stimulate amylasetobreakdown
starch stores and begin to respire
, g
More plant hormones
Cytokinins Growth regulators that promote cell division in the apical meristemas and the
cambium
through interactions with auxins They promote lateral bud development
which overcome apical dominance is leading shoot is removed fordamagea
may
It also works with ether e in theabscission
oe leaves flowers and fruits
and
Synergy antagonism
Mostplant hormones don't work in isolation but in interaction with other substances Th
provides a very eine control over responses Growth regulators may act together synerg
or
against each
other antagonism
Auxins and gibberellins work synergistically Auxins and cytokineins work antagonistically
Apical dominance
Balance between auxins and cytotokinins is crucial The most important interaction is the
maintenance of apical dominance This involves
one lead shoot in a youngplant to grow Easter
than the rest and the growth of the lateral buds is
inhibited
As the first shoot
grows away the inhibition of
the auxin is reduced and the cytokinin becomes
dominant so the lateral buds lower down the plant
start to grow
IE the apical bud is removed auxin inhibition
on the lateral buds is removed and
aytokin in can now have it s full effect lateral budsg
E auxin is artificially added to the out apical stem the antagonistic effect returns and the
lateral Shootgrowth is slowed down
, Control of the heart rate in mammals
Responding to demand
Homoestasis is the maintenance of a state of dynamicequilibrium
Rhythm of the heart is controlledby impulses of the SAN These giveregular coordinated
heartbeats
but they are not enoughto cope with changes in demand
When your body demands more glucose the heart beat and volume of the blood exerted
can be increased The rate of these is known as the cardiac output
Cardiac volume can be increasedbyefficient ventricle contraction
Nervous control of the heart
marathon the cardiac contra centre in me meara
jjjjjj
Oblongata
Chemical stretch and pressurereceptorssend impulses from bloodvessels and the heartchambe
Cardiaccentre sends impulsesthrough sympathetic nerves these release noradrenaline
and stimulate the SAN increasing the frequency of signals from the pacemaker
accelerating
the heart Thesesignals also pass to ventricles and increase the force ofcontract
The cardiac control centre sends signalsthrough parasympathetic nerves releasing
acetylcholine inhibiting the SAN and slowing it down
The role of baroreceptors
Baroreceptors in the sinuses of the carotid arteries in the
neck and aorta are crucial in the feedbackcontrol of the
heart rate during exercise
At rest send steadysignals to the cardiac centre when
they
exercise starts blood vessels dilate in responseto adrenaline
released in anticipation to exercise Bloodpressurecalls
causing
the baroreceptors to almost stopresponding The cardiac
control centre will respondbysendingsignals along the sympathetic nerves to stimulate
heart rate and increase blood pressureagainby vasconstriction
When exercise stops bloodpressureincreases as the pump continues to pumpheavily
this stretches the baroreceptors which send a higherfrequency of nerve impulses to the
cardiac centre which willrespond signallingthroughthe parasympatheticsystemslowing
by
the heart rate and dilating thevessels
Role of chemoreceptors in the aorta
Walls of the aorta and carotid arteries also containchemoreceptors which are sensitive to
carbon dioxide levels
When Cor levels increase the pit is reduced This is detectedbychemoreceptors and signaled
to the cardiac control centre which in turn responds sending impulses down the
by
system accelerating the heart and thussympathetic
increasingblood flow to the lungs where con
is removed
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