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Homeostasis maintains some conditions inside the body at a constant level. Negative feedback mechanisms respond to a change in a condition to help bring the condition back to the normal level.


  • Osmoregulation controls how much water is lost in urine, and so controls the amount of water in the body.
  • Too much water in the body= lots of watery urine.
  • Too little water in the body= small amount of concentrated urine.


  • Thermoregulation keeps core body temperature steady at around 37°C. This is important because enzymes in the main organs are the most active at this temperature.
  • Body temperature rises= Vasodilation, moresweat released, sebaceous glands produce oil- helps spread sweat over skin.
  • Body temperature falls= Vasoconstriction, less sweat, raised body hairs by erector muscles.
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The nervous system includes:

  • the central nervous system- brain and spinal cord.
  • the sense organs, such as the eyes and ears, which contain receptors that can detect a change in the environment (called a stimulus) and produce an electrical impulse.
  • the nerves that join the central nervous system to the sense organs and effector organs- these are made up of bundles of nerve cells, or neurones.

Types of neurones

There are three main types of neurones:

  • Sensory neurones- carry impulses to the central nervous system.
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Responding to stimuli

  • Myelin sheath- Fatty layers surrounding the axon. Helps to insulate nuerone from surrounding tissue, especially other nuerones and also allows impulses to be carried faster.
  • Nuerotransmitters- chemical substances that transmit impulses across the gaps in synapses. All impulses are slowed down slightly by synapses.
  • reflex arc- 
    • The neurones in the reflex arc go through the spinal cord or through an unconscious part of the brain (you don’t have to think).
    • When a stimulus (e.g. pin) is detected by the receptors, impulses are sent along a sensory neurone to the CNS.
    • In the CNS the sensory neurone passes on the message to a relay neurone.
    • Relay neurones relay the impulses to a motor neurone.
    • The impulses then travel along the motor neurone to the effector (e.g. muscle)
    • The muscle then contracts and moves the hand away from the pin.
    • As you don’t have to think about this response, it’s quicker than a normal response.
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  • Hormones - chemical messengers that travel in the blood
    • the parts of the body respond to their presence
  • Endocrine glands - produce and release hormones
  • An organ that responds to a certain hormone is a target organ
  • Most carbohydrates in food are digested into glucose
  • Concentration of glucose in your blood increases after a meal
    • pancreas releases insulin hormone when its a certain concentration
  • Insulin takes glucose out of the blook and converts in to glycogen
    • which is a store of glucose in the liver
  • Glucagon is a hormone released when glucose levels drop
    • the hormone causes liver cells to turn glycogen back into glucose
    • then its released back into the blood to increase levels
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  • Diabetes - a disease whereby blood glucose levels aren't controlled
    • Low blood glucose concentrations - unconsiciousness
    • High blood glucose concentrations - tiredness, organ damage e.g. eyes
  • Type 1 diabetes - pancreas does not produce insulin
    • body cannot bring down blood glucose levels when they get to high
    • between 5% and 10% of diabetics have it
    • most will inject insulin everyday
  • Must be injected into the fat layer beneath the skin - subcutaneous fat layer
    • easily absorbs insulin
    • spreads to blood vessels to be carried in the blood
  • Diabetics must balance the lowering and increasing factors
    • more exercise = less insulin
    • more insulin needed some days than others
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Diabetes Continued

  • Type 2 diabetes - when cells become 'resistant'/respond less to well to insulin being produced
  • Contributors to aquiring it are:
    • high-fat diets
    • lack of exercise
    • obesity
    • old age
  • Often controlled by changing diet and increasing amounts of exercise
  • Obesity - if a person's Body Mass Index (BMI) is over 30
  • BMI - estimate of how healthy your mass is for your height
  • weight in kilograms
  • height in metres squared
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Plant Hormones

  • Tropism - responding to a stimulus by growing towards of away from it
  • Positive tropsism - growing towards a stimulus e.g. plant shoots
  • Negative tropism - growing away from a stimulus e.g. plan roots
  • Phototropism - a tropisim caused by light
  • Geotropism - a tropism towards the direction of gravity (positive gravitropism)
  • Photosythesis - positively phototrophic, to get light, to make food
  • Plant growth substances = plant hormones
  • Auxins - cause positive phototropism and positive gravitropism
  • Gibberellins - cause starch in a seed to be converted into sugars for growth energy
    • also stimulate flower/fruit production in some species
  • Germination - when roots and a shoot start to grow
    • gibberellins are released after this process
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Plant Hormones continued

  • Auxins produced in the tip of a shoot
    • cause elongation of cells
  • Shoot grows towards light when
    • light coming from one direction
    • auxins move to shaded side of shoot
    • cells on shaded side elongate more
  • Auxins produced in root tips - opposite effect
    • causes cells to stop elongating
    • causes geotropism
  • Anchors the plant in place
  • and to reach moisture underground
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Uses Of Plant Hormones

  • Artificial auxin 
  • used as a seletive weedkiller
    • plants with broad leaves grow out of control and die e.g. dandilions, daisies
    • narrow leaved plants are unaffected e.g. grass, wheat
    • farmers can kill weeds without killing cereal crop
  • used in rooting powders
    • plant cuttings are dipped in rooting powders to increase root development
    • large numbers of plants can be produced quicker than growing seeds
  • Seedless fruits
    • flowers sprayed with hormones that cause fruits but not seeds to develop
    • naturally seedless but small fruits are sprayed with gibberellins to increase size
  • Fruit ripening
    • fruit trees sprayed with hormones to stop fruit falling and being damaged
    • also to speed up ripening to be picked all together
    • unripe fruit so it reaches shops in a 'just ripened' condition
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