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Responses to a changing environment
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Be able to define homeostasis as the maintenance of a stable internal
environment in an organism
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o Basically if something in the changes beyond a certain limit, then the
change is detected and the body automatically responds to balance
things up again eg
 If you blood sugar falls too low (levels controlled by insulin),
more glucose is produced from glycogen,
 if your temperature gets too high, heat-temperature receptors
send a message to the central nervous system-brain which
automatically triggers the bodies cooling mechanism
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2 You must have an understanding of the homeostatic mechanisms of:
o a) thermoregulation and the effect of temperature on enzymes
 Thermoregulation is the maintaining of a steady body
temperature (eg for us ~37
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 If you get too cold you need to retain heat and reduce heat loss
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 You do want to be dehydrated if at all possible
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2
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4 HT only: Be able to explain how thermoregulation takes place, with
reference to:
o a) vasoconstriction
o b) vasodilation
o c) negative feedback

Homeostasis - Extra Notes for sections 2
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4
The internal conditions that are controlled in the body include:

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The water content of the body – water leaves the body via the lungs when
we breathe out and via the skin when we sweat to cool us down, and excess
water is lost via the kidneys in the urine
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o Any loss needs to be replaced, more so in the summer when we sweat
more than in the winter
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o For example, the kidney controls the sodium ion (Na +) concentration
from digested food containing salt
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The body temperature is controlled by the brain to maintain the temperature
at which enzymes work best (~37oC)
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o This 'thermostat' is sensitive to the blood temperature of the brain and
via nerve impulse signals from temperature receptors in the skin
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 This automatic temperature change response is an example of
negative feedback
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Your sweating is
reduced to a minimum since heat is absorbed and therefore lost
in the process of evaporation
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 If you are too hot the erector muscles relax allowing the hairs to
lie flat on the skin, no longer trapping insulating air
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The blood vessels near the skin surface widen (to
dilate - process of dilation, vasodilation) which allows more
blood to flow and hence transfer more heat to the surroundings
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 A very high temperatures make you feel extremely
uncomfortable as your body struggles to cope with the
situation and you suffer from 'heat exhaustion' and then
heatstroke - which can be fatal
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If body respiration can't replace the heat loss, then your
body gradually cools, it begins to malfunction and
eventually ceases to function at all
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When sugary or carbohydrate foods are digested the blood sugar
levels rise as the sugar is absorbed from the gut into the bloodstream
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If you are not doing much physical work your blood sugar level will tend
to rise
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It can be dangerous if your blood sugar levels become too high or too
low, so your blood sugar level is regulated by the hormone insulin,
which enables your body to have a regular supply of sugar for a secure
supply of energy
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5 Know that hormones are produced in endocrine glands and are
transported by the blood to their target organs
o Know and understand that many process within the body are
coordinated and controlled by chemical substances called hormones
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o Hormones, being directly released into the blood, are quite rapidly
carried to all parts of the body BUT only affect the function of particular
cells
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o Note on comparing nerve and hormone functions
 Hormones effectively act as 'chemical messages' to trigger
particular biochemical reactions and their effect is more general
and relatively long-lasting compared to eg the nervous impulses
and responses of reflex arc
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2
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o When sugary or carbohydrate foods are digested the blood sugar
levels rise as the sugar is absorbed from the gut into the bloodstream
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o If you are not doing much physical work your blood sugar level will tend
to rise
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o It can be dangerous if your blood sugar levels become too high or too
low, so your blood sugar level is regulated by the hormone insulin,

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which enables your body to have a regular supply of sugar for a secure
supply of energy
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o These changes increase the blood flow to the muscles and so increase
the supply of sugar and oxygen for energy from respiration and also
increase the rate of removal of carbon dioxide - the waste product
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o Glycogen is produced and stored and released for conversion to
glucose on a supply and demand basis
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o The more you physically exercise, the greater the glucose demand, if
this exceeds what is available in the blood stream, then the glycogen
reserves are called upon to fill the energy gap
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7 HT only: Be able to explain how blood glucose levels are regulated by
glucagon causing the conversion of glycogen to glucose
o If your glucose level in the blood is too high the 1st hormone insulin is
secreted by the pancreas
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 Therefore, the conversion of glucose to glycogen, reduces the
glucose concentration in the blood
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 Insulin was once extracted from the pancreas of a pig or
cow, but human insulin is now made genetic engineering
and doesn't give the side effects experienced from
patients using animal insulin
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 Glucagon makes the liver convert glycogen into glucose
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 When the glucose concentration reaches an appropriate level,
secretion of glucagon stops and so does the conversion of
glycogen to glucose stabilising the glucose level
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8 Know that Type 1 diabetes is caused by a lack of insulin, the hormone
which controls the level of glucose in the blood
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o This causes the blood glucose level to rise to potentially lethal levels
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9 Be able to explain how Type 1 diabetes can be controlled, including the
roles of diet and injection of insulin usually into the subcutaneous fat
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 (i) Avoid too much sugary foods and carbohydrates in the diet,
sugars in particular, will cause a rapid rise in glucose levels
which is difficult to remove without the presence of sufficient
insulin
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 This is an inconvenient, but very effective way, of keeping
the blood sugar level in check
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 Diabetics can have a pancreas transplant which, if
successful, can theoretically avoid the need for insulin,
but there is always the danger tissue rejection and costly
immunosuppressive drugs must be taken (with the added
complication of serious side-effects)
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10 Be able to explain how, in Type 1 diabetes, the level of physical activity
and diet affect the amount of insulin required
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o A healthy balanced diet, regular eating and regular exercise will both
help to keep a diabetic in good health and minimise the amount of
insulin needed
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11 Know that Type 2 diabetes is caused by a person becoming resistant to
insulin
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2
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o Type 2 diabetes can be controlled by eating a healthy balanced diet,
regular eating, regular exercise and losing weight if necessary
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2
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o Obese people (BMI > 30) do run the risk of developing type 2 diabetes
and if their BMI is over 30, then action should be taken
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14 Be able to explain how plant growth substances (hormones) bring about:
o a) positive phototropism in shoots
o b) positive gravitropism (geotropism) in roots
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16 You are expected to have investigated, and therefore have some
knowledge of tropic responses e
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g
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2
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18 HT only: You should have an understanding of the uses of plant
hormones, including:

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a) selective weedkillers
b) rooting powder
c) seedless fruit
d) fruit ripening

Notes for sections 2
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18 about plant growth hormones eg auxins
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a) Know and understand that plants are sensitive to, and respond to, light,
moisture and gravity:
o their shoots grow towards light and against the force of gravity,
o their roots grow towards moisture and in the direction of the force of
gravity
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o Auxin is a plant hormone that controls the growth of the tips of shoots
and roots
o Auxin acts by enabling the plant to respond to, and
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the tips of shoots to grow towards light, the effect is called
phototropism,
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the tips of roots to seek moisture in the soil
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 Auxin is produced in the tips and moves back by diffusion to
stimulate cell growth - cell enlargement-elongation
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 Auxin can promote growth in shoots but a high concentration of
auxin can inhibit growth in the root to ensure it grows in the right
direction
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c) The responses of plant roots and shoots to light, gravity and moisture are
the result of unequal distribution of hormones like auxin, causing unequal
growth rates and changes in growth direction
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 This stimulates growth to elongate the cells on the
shaded side so the shoot bends towards the light
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 Therefore the lower side cells are stimulated to grow
faster causing the shoot to grow upwards
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 Roots growing towards moisture - positive hydrotropism
(positively hydrotropic)
 If a root is exposed to an uneven distribution of moisture
ie one side of the root is more moist than the other, more
auxin concentrates on the side with the most moisture
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d) Plant growth hormones are used in agriculture and horticulture as weed
killers and as rooting hormones and is a very important use of plant hormones
like auxin
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 Desired crops of grasses and cereals are narrow leafed plants
but many weeds have broad leaves
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o Some plant cuttings won't always readily grow in soil or compost, but
by adding a rooting powder to the compost containing a plant hormone
like auxin, the growth of roots and subsequent shoots are greatly
encouraged
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o Plant hormones can be used to control the ripening of fruit or produce
seedless fruit
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 You can therefore pick fruit before it is ripe and still quite
firm - which means its less easily damaged in transport
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 By applying growth hormones to the unpollinated flowers
of some fruit plants, the fruits grow BUT not the seeds!
Sometimes the plant hormones are applied after
pollination, but still prevent the seeds developing
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Gibberellin is another plant growth hormone used to stimulate seed
germination, stem growth (taller) and flowering
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19 to 2
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o Any change in your surroundings eg temperature, visual, sound etc
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The stimulus might be chemical, light, pain, position,
pressure, sound, temperature, touch etc
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o In the receptor cells the stimulus input is converted into an electrical
signal - an electrical impulse which is sent to the brain
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b) Cells called receptors can detect stimuli (changes in the environment
outside the organism)
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Sound receptors in the ears that are sensitive to sound
vibrations in the air
 There are also balance receptors in the ears that are
sensitive to changes in position and enable us to keep
our balance
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) or
anything else in contact with the tongue - good or bad!
 The receptors in the nose are also sensitive to chemicals and
enable us to smell all sorts of different things which may be a
pleasant or unpleasant experience
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c) Light receptor cells, like most animal cells, have a nucleus,
cytoplasm and cell membrane
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o
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reflex actions are automatic and rapid,
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e) You should know and understand the role of receptors, sensory neurones,
motor neurones, relay neurones, synapses, myelin sheath, neurotransmitters
and effectors in simple reflex actions
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 Neurone cells have lots of branched endings called dendrons
which connect with lots of other neurones
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o Sensory neurones - the nerve cells that transmit the electrical impulse
signal from the receptors in the sense organs to the spinal chord and
brain (central nervous system)
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o Synapse - a connection between two neurones eg the junction
between a sensory neurone and a relay neurone, it enables the
electrical impulse signal to reach the spinal cord and brain (ie the
central nervous system)
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o Neurotransmitter - chemicals produced that transmit the electrical
signal across a synapse gap between one neurone cell and another
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Motor neurones - the nerve cells that transmit the electrical signals
through the central nervous system from the brain via the spinal cord to
the effector cells of the muscles or glands
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Reflex actions are automatic responses to stimuli detected by the
receptors in the organs of the body
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 If the intensity of light impacting on your eye is too great, your
pupil automatically gets smaller to allow less light
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 If something hot touches your skin, on feeling pain you
immediately try to recoil from the heat source eg on burning your
hand, the muscles rapidly contract to take your hand away
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 At a nerve junction (synapse) between a sensory neurone and
a relay neurone in the central nervous system, a chemical is
released that causes an impulse to be transmitted by a relay
neurone
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 The effector is either a muscle or a gland, a muscle responds by
contracting or a gland responds by releasing (secreting)
chemical substances eg the central nervous systems decides
what is to be done depending on what stimulus is received, so
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 Glands may secrete a particular hormone in response to
a particular stimulus eg adrenalin in a 'flight response'
from a dangerous situation
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 Summary of the reflex arc sequence via the central nervous
system:
 stimulus ==> receptor cell ==> sensory neurone
==synapse==> relay neurone in central nervous

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system =synapse==> motor neurone ==> effector ==>
response
The reflex arc action is fast, no thinking involved, just a
rapid automatic response on the part of your body!



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