The hormone system

  • humans have two control systems: nervous system and endocrine (hormonal) system


  • hormones are secreted by glands into the blood stream via tissue fluid
  • some example of glands are: thyroid, pituitary and adrenal
  • once a hormone has been secreted it diffuses into the blood stream and is carried round all the body to organs
  • however, they only affect target organs which then respond
  • organs have specific receptor molecules in their cells to which the hormone binds
  • these receptors are proteins and are specific to certain types of hormone
  • in this way they form specific hormone-receptor complexes
  • those cells without a receptor ignore the hormone
  • these complexes affect almost all aspects of a cells funtion e.g. metabolism, transport, protein synthesis, cell division
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Comparison of nervous and hormonal systems


  • transmitted by specific neurones
  • effect localised by neurone anatomy
  • fast acting (ms-s)
  • short lived response


  • transmitted by circulatory system
  • effect localised by target cell receptors
  • slow acting (minutes-days)
  • long lived response
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  • means 'standing still' and it refers to the process of keeping the internal body enviroment at a steady state,
  • factors that are controlled include:
  • body tempreture - to keep enzymes working at their optimum and prevent them denaturing
  • blood PH - to keep enzymes working at their optimum
  • blood glucose concentration - to ensure there is enough glucose available for cellular respiration but not to much that water potential is lowered in the blood and cells dehydrated


  • all homestatic mechanisms use negative feedback to maintain a constant value (set point)
  • whenever a change occurs it automatically causes a corrective mechanism, this reverses the original change and brings the system back to normal
  • the bigger the change the bigger the corrective mechanism
  • in a system controlled by negative fedtback the level is never maintained perfectly, it constantly oscillates about the set point, an efficient system minimises these oscillations
  • positive feedback occurs when the change in stimulus stimulates a further change in the same direction (potentially dangerous)
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Thermoregulation - endotherms

  • endotherms include mammals and birds and can generate heat internally and have thermal insulation
  • they maintain a relativly constant body tempreture (37 for humans 42 for birds)
  • they dont keep their whole body tempreture the same: they maintain a constant core tempreture but allow the peripheral tempreture to be colder
  • the advantage is: they can survive in a wide range of eviromental conditons and therefore are able to colonise almost any habitat, they can remain active at night and in cold weather which gives them an advantage over ectothermic prey
  • however: they require large amounts of energy and so must eat more
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Thermoregulation - hypothalamus

  • in humans temperature homeostasis is controlled by two thermoregulatory centres in the hypothalamus:
  • 1) heat loss centre - is activated when core temperature raises above the set point, it sends impulses to several effectors to reduce core temperature
  • 2) heat gain centre - is activated when core temperature falls below the set point, it sends impulses to several effectors to increase core temperature
  • the centres recieve input from two sets of thermoreceptors:
  • 1) receptors in the hypothalamus - these monitor the temp of the blood that passes through the brain (core temperature)
  • 2) receptors in the skin - this monitor pheripheral temperature
  • both peices of information are needed so the body can make appropriate adjustments
  • the thermoregulatory centre is part of the autonomic nervous system, so the various responses are involuntary
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Thermoregulation - responses

LOW TEMPERATURE (controlled by the heat gain centre)

  • muscles is arterioles in the skin contract causing VASOCONSTRICTION, less heat is carried from the core to the surface (extremities can turn blue)
  • no sweat produced
  • skeletal muscles contract and relax repeatedly and involuntarily generating heat by friction
  • erector pili muscles in the skin contract, raising hairs and trapping a warm insulating layer of air close to the skin
  • behaviour - curling up, huddling, finding shelter, putting on more clothes

HIGH TEMPERATURE (controlled by the heat loss centre)

  • muscles is arterioles in the skin relax causing VASODILATIONmore heat is carried from the core to the surface (skin turns red)
  • glands secrete sweat onto the surface of skin where it's evaporated (hairy animals pant instead)
  •  no shivering
  • erector pili muscles in the skin relax, lowering hairs and allowing air to circulate over the skin 
  • behaviour - stretching out, finding shade, swimming, removing clothes
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Thermoregulation - special circumstances

mammals can alter their set point in special circumstaces:

  • FEVER - raise the set point by 2-3 degrees, helps to kill bacteria
  • HIBERNATION - reduce the set point by up to 5 degrees, reduces metabolic rate and conserves their food reserves
  • TORPOR - bats and birds reduce their set point when inactive, they have a high sa:vol ratio so this reduces heat loss

failure of homestasis:

  • HYPOTHERMIA - when heat loss exceeds heat generation due to prolonged exposure to cold tempretures, an example of positive feedback
  • HYPERTHERMIA - heat gain exceeds heat loss, often assiociated with dehydration, this is also an example of positive feedback
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Temperature control - ectotherms

  • all animals except mammals and birds rely on external heat sources to warm up and do not have thermal insulation (a common example is reptiles)
  • their body temperature varies with the enviromental temperature
  • they cannot warm up by methods such as shivering because when their temp is low they cannot respire fast enough to make ATP for rapid muscle contraction
  • they instead regulate their temperature by thermoregulatory behavious e.g.
  • 1) basking on rocks until their metabolic rate is fast enough to become active
  • 2) laying down on warm ground to gain heat and raising themselves up when they get to hot
  • 3) the prevent overheating midday they take shelter under rocks and vegetation
  • 4) they adjust the amount of heat they gain by changing their angle to the sun and increasing the SA exposed to it
  • 5) move between land and water
  • 6) shelter in burrows at night to provide insulation and reduce heat loss (also hides them from predators)
  • advantages: use far less energy as they have a much lower metabolic rate, at night their metabolic rate drops further so dont need to eat as much and can survive weeks without eating
  • disadvangtages: at certain times of the day they can only move slowly, this makes them easy prey and poor predators
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Blood glucose sources and fates

  • glucose is used in all animal cells
  • very low concentrations cause cells to die as they have no energy for respiration
  • high concentrations lowers water potential of blood and kills cells by dehydration
  • therefore blood glucose is strictly controlled around 80-100mg 100cm-3
  • the main sources and fates of glucose are:
  • 1) digestion and absorbtion of carbohydrates (especially starch)
  • 2) glucose is mainly used in respiration and can be converted to co2, lactate or amino acids
  • 3) storage glucose can be converted into glycogen in liver and muscle cells (GLYCOGENESIS), when glucose is needed the glycogen can be broken down again (GLYCOGENOLYSIS)
  • 4) excess glucose can be converted to triglycerides in the liver (LIPOGENESIS) then transported to tissue for storage, they can also be used in aerobic respiration but not to make glucose
  • 5) animals dont usually synthesise glucose but when it is scarce proteins and nucleic acid can be used to make it (GLUCONEOGENESIS)
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Control of blood glucose

  • glucose concentration is contolled by the PANCREAS
  • regions of the pancreas called the ISLETS OF LANGERHANS serve as both glucose receptors and endocrine cells
  • there are 2 types of ISLET cell which both have glucose receptors:
  • 1) alpha cells - detect low glucose and respond by secreting GLUCAGON
  • 2) beta cells - detect high glucose and respond by secreting INSULIN
  • the process works as follows:
  • after a meal glucose concentration is HIGH and this is detected by the BETA cells in the PANCREAS
  • these cells secrete INSULIN which converts GLUCOSE to GLYGOGEN in the LIVER (this reduces blood glucose)
  • INSULIN is no longer secreted
  • if glucose levels FALL too far the ALPHA cells of the PANCREAS detect this and release GLUCAGON
  • GLUCAGON causes the liver to break down some of its GLYCOGEN stores to GLUCOSE which diffuses into the blood
  • the pancreas stops secreting GLUCAGON
  • this negative feedback loop occurs all day
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The second messenger model

  • this mechanism is used by hormones GLUCAGON and ADRENILIN (adrenilin does the same as glucagon to provide more energy for the fight or flight response)


  • the hormone (glucagon or adrenilin) is the first messenger, it binds to specific receptors on the membrane of the target cell to form a hormone-receptor complex
  • this complex activates an enzyme inside the membrane, this enzyme converts ATP to cyclic AMP (cAMP)
  • cAMP acts as a second messenger molecule that activates other enzymes that in turn, convert glycogen to glucose


  • insulin works differently: it attatches to receptors on the membrane and in doing so changes the tertiary structure of the glucose transport proteins
  • this causes them to change shape and open allowing more glucose to enter the cells
  • it also activates the enzyme that converts glucose to glycogen and fat
  • the glucose concentration is then lowered by the ways mentioned earlier
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caused by a failure of glucose homestasis - there are two forms of the disease:

  • INSULIN DEPENDENT or TYPE 1 - there is a severe insulin deficiency due to the immune response killing beta cells, usually appears in childhood
  • INSULIN INDEPENDENT or TYPE 2 - insulin is produced but the insulin receptors in the target cells dont work so insulin has no effect, this type often appears in overweight people around their 40's (accounts for 90% of diabetes)
  • in both cases, there high blood glucose conc. after a meal that cant be absorbed, most of the glucose is lost in urine which leads to the following symptoms:
  • 1) high thirst due to osmosis leaving cells (dehydration) and entering the blood which has low water potential
  • 2) excess urine production due to excess water in the blood
  • 3) poor vision due to loss of water from the eye lens
  • 4) tiredness due to loss of glucose in the urine and poor uptake of glucose in the liver and muscle cells
  • 5) muscle wasting due to glucogenesis caused by increased glucagon

there is treatment e.g. insulin injections, careful diet and possibly islets of langerhan transplants

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Control of the oestrous cycle

  • the oestrous cycle is called the mentrual cycle in humans as it occurs once a month
  • it is controlled by 4 hormones, 2 of which are released by the pituitary gland (found at the base of the brain):
  • 1) FOLLICLE STIMULATING HORMONE (FSH) - stimulates the development of follicles in the ovary which contain eggs and stimulates follicles to release oestrogen
  • 2) LUTEINISING HORMONE (LH) - causes ovulation to occur and stimulates the ovary to produce progesterone from the corpus luteum
  • the remaining 2 hormones are produced by the ovaries:
  • 1) OESTROGEN - causes the rebuiliding of the uterus after mentruation and stimulates the puiuitary gland to release LH
  • 2) PROGESTERONE - maintains the lining of the uterus in readiness to recieve fertilised eggs and inhibits the production of FSH from the pituitary gland
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Control of the oestrous cycle cont.

  • mentrual cycle begins when uterus lining is shed along with some blood (period)
  • pituitary gland releases FSH which STIMULATES FOLLICLES to grow and mature, each follicle contains an egg
  • follicles release small amounts of OESTROGEN which causes the uterus lining to BUILD again, this INHIBITS FSH and LH production (negative feedback)
  • as follicles grow MORE OESTROGEN is released until it reached a CRITICAL POINT where it STIMULATES the release of FSH and LH from the pituitary gland (positive feedback)
  • there is a SURGE in FSH and LH, the surge in LH causes one of the follicles to RELEASE its egg (ovulation)
  • after ovulation LH STIMULATES the empty follicle to develop into a CORPUS LUTEUM which RELEASES PROGESTERONE
  • PROGESTERONE maintians the thick lining of the uterus and INHIBITS the release of FSH and LH (negative feedback)
  • if the egg is NOT fertilised the CORPUS LUTEUM degenerates and NO LONGER produces PROGESTERONE
  • less progesterone results in the uterus lining breaking down (mentruation) and FSH production is NO LONGER inhibited
  • FSH production RESUMES and the cycle is REPEATED
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Plant responses

  • plants dont have nervous systems or endocrine systems but they can sense and respond to stimuli
  • e.g. plants can grow towards light or their roots can grow towards water and they can flower at certain times of the year
  • these responses are often DIRECTIONAL growth responses called TROPISMS
  • they can be positive (growing towards a stimulus) or negative (growing away from a stimulus)
  • phototropism - in reponse to light - young shoots (positive) and roots (negative)
  • geotropism - in response to gravity - roots (positive)
  • hydrotropism - in response to water - roots (positive)
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Darwin's experiments


  • method - directed light at one side of the plant shoot
  • response - shoot bends towards light
  • explanation - the shoot is positivly phototropic, bending occurs at the tip


  • method - shoot tip removed and light directed at one side of the shoot
  • response - no response
  • explanation - the tip must either detect the stimulus or produce the messenger (or both) as it's removal prevents any response


  • method - lightproof cover placed on the shoot tip and light directed at one side of the shoot
  • response - no response
  • explanation - the light stimulus must be DETECTED by the tip
  • his hypothesis that the stimulus is detected by the tip of the shoot was proven right
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Boysen-Jensens experiments


  • method - thin, impermeable barrier of mica inserted on illuminated side
  • response - movement of chemical down shaded side, bends towards light
  • explanation - mica prevents chemical passing down illuminated side, causes bending on the side it passes down


  • method - mica inserted on shaded side
  • response - no response, movement of chemical down shaded side is prevented by mica


  • method - tip removed, agar block interted and tip replaced on top
  • response - movement of chemical down shaded side, bends towards light
  • explanation - agar allows chemicals to pass through but not electrical impulses the bending that occurs must be due to a chemical passing from the tip
  • his hypothesis that the 'messenger' is a chemical was proven
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Paals experiment

  • wanted to investigate how the chemical messenger worked


  • method - tips removed and then replaced but displaced to one side, kept in total darkness
  • response - shoots bend towards side where no tip is present
  • explanation - chemical only moves down the side that is in contact with the tip, this side grows more rapidly, causing bending away from that side
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Brigg's experiments

  • the chemical was later found to be IAA (an auxin) the next question was how did light cause uneven distributions of IAA
  • method - one shoot in darkness, one with light from one side, IAA collected from both and compared
  • response - shoot in darkness had no response, shoot in light bent towards it, total IAA approximately the same
  • method - thin glass plate inserted that seperates both sides of the shoots, IAA collected from both sides
  • response - no bending, IAA approximately the same both sides of the glass
  • method - glass plate placed so that IAA can be transferred at the tip, IAA is collected from both sides
  • response - shoot bends towards light, 30% IAA on illuminated side, 70% on shaded side
  • glass plate used as it prevets chemicals but not light passing through
  • shows that IAA is transported from the light side to the shaded side as both sides have equal amounts at the beginning
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  • if a seed is placed on its side the roots and shoots will begin growing horizontally but soon the shoot will turn upwards and the root downwards
  • since the seed is underground this is an example of a geotropism not a phototropism
  • in both, gravity makes starch grains sink to the lower side of cells, this causes auxin to accumalate at the lower side and causes cell elongation which makes the shoot grow upwards
  • the root bends downwards because they have a different response to auxin, it inhibits cell elongation in roots
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