topic 6 - biology

• what cells detect an increase in blood glucose levels?

  beta cells
• what cells detect an decrease in blood glucose levels?

  alpha cells
• what do beta cells release after the detection of increase blood glucose levels?

  insulin
• what do alpha cells release after the detection of decrease blood glucose levels?

  glucagon

  & adrenal glands release adrenaline
• what is glycogenesis?

  the process of excess glucose being converted to glycogen when blood glucose is higher than normal.

  • this mainly occurs in the liver
• what is glycogenolysis?

  the hydrolysis of glycogen back into glucose in the liver

  • this occurs when blood glucose levels are lower than normal
• what is gluconeogenesis?

  the process of creating glucose from non-carbohydrate stores in the liver

  • this occurs if all glycogen has been hydrolysed into glucose and your body still needs more glucose
• how does insulin decrease blood glucose?

  • attaching to receptors on the surfaces of target cells, this changes the tertiary structure of the channel proteins resulting in more glucose being absorbed by facilitated diffusion
  • more protein carriers are incorporated into cell membranes so that more glucose is absorbed from the blood into cells
  • activating enzymes involved in the conversion of glucose to glycogen, this results in glycogenesis in the liver
• how does glucagon increase blood glucose?

  • attaching to receptors on the surfaces of target cells(liver cells)
  • when glucagon binds it causes a protein to be activated into adenylate cyclase and to convert ATP in a molecule called cyclic AMP(cAMP). cAMP activates an enzyme protein kinase, that can hydrolyse glycogen into glucose
  • activating enzymes involved in the conversion of glycerol and amino acids into glucose
• role of adrenaline to increase blood glucose

  • adrenaline attaches to receptors on the surface of target cells, this causes a protein(G protein) to be activated and to convert ATP into cAMP
  • cAMP activates an enzyme that can hydrolyse glycogen into glucose
  • this is known as the second messenger model of adrenaline and glucagon action, because the process results in the formation of cAMP, which acts as a second messenger
• type 1 diabetes

  type 1 diabetes is due to the body being unable to produce insulin, it starts in childhood and could be result of an autoimmune disease where the beta cells were attacked

  • after eating, the blood glucose level rises and stays high, this is called hyperglycaemia
  • insulin therapy has to be carefully controlled because too much can produce a dangerous drop in blood glucose levels, this is called hypoglycaemia
  • eating regularly and controlling simple carbohydrate intake(intake of sugars) helps avoid a sudden rise in glucose
  • treatment involves injections of insulin
• type 2 diabetes

  type 2 diabetes is due to receptors on the target cells losing their responsiveness to insulin, it usually develops in adults because of obesity and poor diet.

  • occurs when beta cells don't produce enough insulin or when the body's cells don't respond properly to insulin
  • cells don't respond properly because the insulin receptors on their membranes don't work properly so cells don't take up enough glucose, meaning the blood glucose concentration is higher than normal.
  • it is controlled by regulating intake of carbohydrates, increasing exercise and sometimes insulin injections
• stimulus 


  a detectable change in the environment
• receptors

  cells that detect stimuli

  each receptor responds only to specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which can cause a response

  three key receptors:

  • pacinian corpuscle
  • rods
  • cones
• tropism 


  term given to when plants respond via growth to stimuli
• two types of tropism

  postive - growing towards stimulus

  negative - growing away from stimulus

  plants respond to light and gravity
• tropisms are controlled by specific growth factors and one key example is indoleacetic acid(IAA)
• indoleacetic acid(IAA)

  a type of auxin and can control cell elongation in shoots and inhibit growth of cells in the roots. It is made in the tip or the roots and shoots but can diffuse to other cells
• phototropism


  term given to the tropism where the plant is responding to light

  for shoots - light is needed for the LDR in photosynthesis so plants grow & bend towards light, this is positive phototropism

  for roots - roots do not photosynthesise so do not require light, they must anchor the plant deep in the soil, this is negative phototropism
• positive phototropism 


  in shoots

  1. shoot tip cells produced IAA(indoleacetic acid), causing cell elongation
  2. the IAA diffuses to other cells
  3. if there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there
  4. the cells on the shaded side to elongate more & results in the plant bending towards the light source
• negative phototropism


  in roots

  • high concentration of IAA(indoleacetic acid) inhibits cell elongation causing root cells to elongate more on the lighter side and so root bends away from light
• gravitropism 


  term given to the tropism where the plant is responding to gravity

  shoots - IAA(indoleacetic acid) diffuses from upper to lower side of shoot, plants grows away from gravity, this is negative gravitropism

  roots - IAA(indoleacetic acid) moves to lower side of roots, roots grow down towards gravity, this is positive gravitropism
• negative gravitropism


  IAA(indoleacetic acid) diffuses from upper to lower side of shoot

  • if a plant is vertical this causes the plant cells to elongate & the plants grows upwards
  • if a plant is on its side, it will cause the shoot to bend upwards
  • growing away from gravity
• positive gravitropism 


  IAA(indoleacetic acid) moves to lower side of roots so that the upper side elongates and the root bends down towards garvity and anchors the plant in
• reflex 


  a rapid, automatic response to protect you from danger
• reflex arc

  made up of three neurons:

  • sensory neuron
  • relay neuron
  • motor neuron
• simple responses


  keep organisms within the favourable conditions of their environment(light,moisture, chemicals)

  eg taxes and kinesis
• taxes


  an organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus

  when organisms move towards a stimulus this is known as positive taxis and when an organism moves away it is described as negative taxis
• kinesis

  an organism changes the speed of movement and the rate it changes direction

  • if an organism moves from an area where there are beneficial stimuli to an area with harmful stimuli, its kinesis response will be to increase the rate it changes direction to return to the favourable conditions quickly
  • if an organism is surrounded by negative stimuli, the rate of turning decreases to keep it moving in a relatively straight line to increase the chances of it finding a new location with favourable stimuli
• Pacinian corpuscle:


  located deep in skin, mainly in fingers and feet

  the sensory neurone in the Pacinian corpuscle has special channel proteins in its plasma membrane

  • the membranes of the Pacinian corpuscle have stretch-mediated sodium channels
  • these open and allow Na+ to enter the sensory neurone only when they are stretched and deformed
  • when pressure is applied it deforms the neurone plasma membrane, stretches and widens the Na+ channels so Na+ diffuses in which leads to the establishment of a generator potential
• the retina contains two types of photoreceptors:

  • rods
  • cones
• Rods

  rods process images in black and white

  • to create the generator potential, the pigment of rod cells(rhodopsin) must be broken down by light energy
  • they can detect light of very low intensity as many rod cells connect to one sensory neurone(retinal convergence)
  • this means the brain cannot distinguish between the seperate sources of light that stimulated it - low visual acuity
• Cones

  cones process imges in colour

  there are three types that contain different types of iodopsin pigment(red,green+blue) which all absorb different wavelengths of light

  iodopsin is only broken down if there is a high light intensity so action potentials can only be generated with enough light

  one cone cell connects to a bipolar cell, therefore cones can only respond to high light intensity which is why we can't see colour when it is dark

  as each cone is connected to one bipolar cell the brain can distinguish between seperate sources of light detected - cone cells give high visual acuity
• distribution of rods & cones

  the distribution in the retina is uneven

  light is focused by the lens on the fovea, which will receive the highest intensity of light

  • most cone cells are located near the fovea
  • rods cells further away
• control of the heart

  cardiac muscle is myogenic: it contracts on its own accord, but the rate of contraction is controlled by wave of electrical activity

  • the sinoatrial node(SAN) is located in the right atrium and is known as the pacemaker
  • the atrioventricular node(AVN) is located near the border of the right and left ventricle within the atria
  • the bundle of His runs through the septum
  • the purkyne fibres in the walls of the ventricle
• SAN releases a wave of depolarisation across the atria causing it to contract and then AVN releases another wave of depolarisation when the first reaches it.
• A non-conductive layer between the atria & ventricles prevents the wave of depolarisation travelling down to the ventricles.
• The bundle of His conducts the wave of depolarisation down the septum & the Purkyne fibres.
• The apex & then walls of the ventricles contract, there is a short delay before this happens while the AVN transmits the second wave of depolarisation.
• This delay allows enough time for atria to pump all the blood into the ventricles.