Responding To Change

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Homeostasis means maintaining the correct conditions in your body so that everything works as it should.

Conditions in your body need to be kept regular so the cells can work properly. For example:

  • Osmoregulation (regulating water content) you need to keep a balance between the water you put in (Drink, food) and the water going out. (Pee, sweat, breath)
  • Thermoregulation (regulating body temp) you need to get rid of exess body heat when you're hot, but keep heat when you're cold.
  • Blood glucose regulation you need to keep the glucose in your blood at a steady level.

A mecanism called negative feedback helps to regulate these things.

  • Changes in environment cause a response that counteracts the changes.
  • This means that the internal environment tends to stay at a norm. (Level where cells work best)
  • This only works within limits, if the environment changes too much it could be impossible to counteract it.
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All enzymes work between cerain temperatures, the enzymes in the human body work best at around 37°.

  • There is a part of the brain called hypothalamus that acts as a thermostat.
  • It holds sensetive receptors for blood temperature in the brain and it gets impulses from receptors in the skin (nerve endings) which provide information about skin temperature.
  • When the hypothalamus senses a change it causes response in the dermis. (Deep layer of skin)

When it detects you're too hot the following happen:

  • Erector muscles relax (making hairs lie flat), Lots of sweat (Holding water and salts), Blood vessels close to the surface dialate (widen) & is called vasodilation.

When it detects you're too cold the folowing happen:

  • Erector muscles contract (making hairs stand up), very little sweat is produced, blood vessles near the surface constrict (get smaller) & is called vasoconstriction.
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Hormones & nerves

There are 2 ways for siganls to be sent from one bit of the body to another, using slow hormones or very speedy nerves.

Hormones are chemical messages sen in the blood.

  • They're chemicals produced in different glands called endocrine.
  • They get released straight into the blood which then carries them to other parts of the body.
  • They travel all over the body but only affect certsin cells in certain places.
  • Those affected cells are called target cells and have the correct receptors to respond to that hormone. An organ containing target cells is called a target organ.
  • Hormones travel at the "speed of blood" and have long lasting affects. 

Hormones and nerves have similar jobs but have differences:

  • Nerves - send very fast messages, act for short periods of time, act on very precise areas & send an electrical message.
  • Hormones - Send slower messages, act for very long periods of time, act in a more general way & send a chemical message.
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Neurones (nerve cells) send infromation as electrical impulses around the body.

  • Neurones have branched endings called dendrons so they can connect with other neurones.
  • The electrical impulses get passed along the axon of the cell.
  • there's a myelin sheath along the axon that acts like an electrical insulator that stops the impulse from getting lost and also speeds up the electricsl impulse.
  • Neurones are long, and this speeds up the impulse but connecting with other neurons slows the impulse down so that one long neuron is faster than lots of short ones joined together.
  • The connection between 2 neurons is called a synapse. This is basically a tiny gap. (The nerve impulse is sent by chemicals called neurotransmitters that diffuse across the gap. The neurotransmitters then set off a new electrical impulse in the next neurone.)
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The Nervous System

The nervous system is what helps you react to the things happening around you.

Sense organs find stimui. A stimulous is a change in your environment that might have to react to. You have to permenantly be on the lookout for what's going on so you can respond if needed. You have 5 sens organs:

  • Eyes (Light receptors)
  • Ears (Sound and "balance" receptors)
  • Nose (Smell receptors - sensitive to chemical stimuli)
  • Tongue (Taste receptors - sensitive to chemical stimuli like bitter or sweet)
  • Skin (Sensitive to pressure and temperature change)

They all have different receptors. Receptors are groups of cells that are sensitive to a stimulous and they change stimulus energy into electrical impulses.

A stimulous can be light, sound, touch, pressure, chemical or change in position or temperature.

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The Nervous System #2

The central nervous system coordinates a response.

  • When a stimulous is found by receptors in a sense organ, the information is sent as electrical impulses along sensory neurones to the CNS. (Central nervous system) 
  • The CNS is made up of the brain and the spinal cord.
  • The CNS coordinates the responses.
  • Then the CNS sends information to a muscle or gland (effector) along a motor neurone and the effector responds appropriately.


  • Effectors - Muscles and glands are called effectors and respond in different ways. Muscles contract to nervous impulses but glands dispense substances.
  • Motor Neurones - lots of short dendrons and one long axon that carry nerve impulses from the CNS to the effectors,
  • Relay Neurones - Lots of short dendrons and axons that carry nerve impulses from sensory neurones to motor neurones.
  • Sensory Neurones - Long dendrons and short axons that carry nerve impulses from the receptors in the CNS.
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Stimuli & Reflexes

Reflexes help to stop and prevent injury.

  • Reflexes are automatic responses to some stimuli and can reduce chances of injury.
  • For instance, when bright light is shined into the eyes, the pupils automatically reduce in size so less light gets into the eye to stop them from being damaged.

A reflex arc goes through the CNS.

  • Neurones in the reflex arc go through the spinal cord or unconscious part of the brain.
  • When a stimulous is found by receptors, impulses are sent along a sensory neurone to the CNS.
  • In the CNS the sensory neurone send the message to a relay neurone.
  • Relay neurones relay the impulses to a motor neurone.
  • Then the impulses travel along the motor neurone to the effector.
  • The muscle then contracts and because you don't have to think about the response it's quicker.
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Insulin & Glucose

Blood glucose is controled as part of homostatis using insulin and glucagon hormones. Insulin and glucagon controle blood sugar levels.

  • Eating foods that have carbohydrate puts a type of sugar called glucose into the blood from the gut.
  • The normal metabolism of cells takes glucose out of the blood.
  • High intensity exersise removes much more glucose from the blood.
  • To control the level of blood glucose there is a way to add and remove glucose from the blood.

Blood glucose level too high = Insulin is added.

  • Blood with too much insulin, insulin is secreted by the pancreas, too much glucose and insulin, insulin makes liver turn glucose into glycogen, glucose is removed by liver, blood glucose reduced and insulin stopps being secreted.

Blood glucose level too low = Glucagon is added.

  • Blood with too little glucose, glucagon secreted by pancreas, too little glucose and glucagon, glucagon makes liver turn glycogen into glucose, glucose added by liver, blood glucose increased so glucagon stops being secreted.
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Type 1 diabetes is a condition when the pancreas makes little or no insulin. This can cause the blood sugar to rise to a level that culd kill them. This can be controled by:

  • Avoiding foods high in carbohydrates.
  • Injecting insulin into the blood at mealtimes. (This will make the liver remove glucose as it enters the blood from the gut during digestion.)
  • Insulin is normally injected into fatty tissue just uner the skin. (Subcutaneous tissue)
  • The amount of insulin that has to be injected depends on the persons diet and how much exersise they do. 

Type 2 diabetes is a condition when the pancreas doesn't make enough insulin or when a person becomes resistant to insulin and blood suager levels rise.

  • Obese people have increased chances of develpoing type 2 diabetes.
  • Type 2 diabetes can be controled by eating healthy diet, getting regular exersise and sometimes loosing weight. Some people with type 2 diabetes hlso have medication or insulin injections.
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Plant Growth Hormones

Plants respond to stimuli by regulating their growth. A plants growth response is called a tropism. A positive tropism is growing towards a stimulus.
Plant growth hormones (Auxin, gibberelin) allow plants to grow in response to stimuli.

  • Auxin is a plant hormone which controls growth at the ends of shoots and roots.
  • It is made in the ends and diffuses backwards to stimulate cells behind the ends to grow longer. 
  • If the tip of a shoot is taken off no auxin will be ther and the shoot won't grow.Auxin makes the shoot grow but high concentrations also stops growth in the root.
  • Gibberellin is a type of plant hormone that stimulates seed germination, stem growth and flowering.
  • It stimulates the stems of plants to grow by stem elongation to grow tall.
  • Auxin and gibberellin can work together to have a huge impact on plant growth.
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Plant Growth Hormones #2

Shoots are postively phototrophic

  • When a shoot tip is exposed to light, more auxin gather on the side that's in the shade than the side in the light.
  • This side makes the cells grow quicker on the shaded side so the shoot bends towards the light.
  • The response lets plants absorb more light for photosynthesis which provides sugar which provides energy for growth.

Roots are positively gravitropic

  • When a root grows sideways, gravity makes an unequal distribution of auxin in the tip, with more auxin on the lower side.
  • In a root the extra auxin inhibits growth meaning that cells on top grow quicker and roots bend downwards.
  • That response lets plants extend their roots deep into the soil so they're well anchored.The plants can absorb more water and minerals.
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Commercial Use Of Plant Hormones

Plant hormones can be extracted or fake versions can be made.

  • Selective weed killers - most weeds that grow in fields of crops or in a lawn are broad leaved. As opposed to grasses and cereals that have narrow leaves. Selective weed killers have been made from plant growth hormones that only affect broad leaved plants. They completely disrupt the normal growth patterns that soon kill them, while leaving the grass and crops untouched.
  • Growing from cuttings with rooting powder - A cutting is part of a plant that has been taken from it . Usually, if you putt cuttings in soil they won't grow but if rooting power is added then they rapidly produce roots and grow as new plants.
  • Controling the ripening of fruit - This can be controled whilst fruit is still on the plant or during transportation to the shop, meaning that fruit can be picked whilst unripe and ripening hormone is added during the transportatin and will be just right as it reaches the shelves.
  • Producing seedless fruit - Fruit with seeds in the middle only tend to grow on plants pollinated by insects and if the plants don't get pollinated the fruit and seeds won't grow. If growth hormones are put into unpollinated flowers of certain plants the fruit will grow but seeds won't.
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