Response to stimuli

• Taxes

  • Simple response in which an organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus. When organism moves towards a stimuli it is described as a positive stimulus but when an organism moves away it is a negative taxis
• Kinesis

  • Changes speed of movement and rate it changes direction as organism moves.
  • If an organism moves from an area where they are beneficial stimuli to an area with harmful its kinesis response is to increase rate it changes direction to return back to the favourable conditions quickly
• Response in flower plants

  tropism : when plants response via growth to stimuli, can be positive and negative. Growing towards or away from stimuli, responding to light, gravity and water.

  Controlled by specific growth factors e.g IAA (auxin controlling cell elongation in shoot and inhibit growth of cells in the roots. Made in the tip or roots and shoots but can diffuse to other cells.
• Phototropism

  • Shoots need light for LDR in photosynthesis which is why plants grow and bend towards light, Controlled by IAA this is positive phototropism.
  • If there is unilateral light, IAA will diffuse towards the shaded side of the shoot resulting in higher concentration of IAA there.
  • IAA causes the cells on the shaded side to elongate causing plant to bend towards light source
• Phototropism Roots

  Roots dont require light as they dont photosynthesis, anchor the plant is rtheyre deep in the soil away from light.

  In roots a high concentration of IAA inhibits cell elongation causing root cells to elongate more on the lighter side so root bends away from light.

  Negative phototropism.
• Gravitropism


  Shoots: IAA diffuse from upper to lower side of a shoot, if plant is vertical cause plant cells to elongate and plant grows upwards if plant of side it'll cause the shoot to bend upwards = negative gravitropism.
• Gravitropism roots

  Roots: IAA moves to lower side of roots so that upper side elongates and the root bend down towards gravity and anchors the plant = positive gravitropism
• Pacinian corpuscle

  • Responds to pressure changes, occurring deep in skin mainly in fingers and feet.
  • Consists of a single sensory neuron wrapped with layers of tissue separated by gel. Has special channel proteins in plasma membrane.
• How does pacinian corpuscle uses stretch mediated sodium channels.

  • Plasma membranes contain channel proteins allowing ion transportation and membranes surrounding sensory neurons has stretch mediated sodium channels. They will open and allow Na + to enter sensory neurons only when stretched and deformed ( pressure has to be applied ) so Na+ diffuses in leading to a exceeding the threshold and generating an action potential.
  • No pressure, stretch mediated sodium channels are closed and Na+ channels are too narrow for Na+ to diffuse into SN so resting potential is maintained.
• Rods

  • Cannot distinguish different wavelengths of light and processes images in black and white
  • Detects light of low intensity due to many rod cells connect to one sensory neuron. ( Retinal convergence ) 
• How do rods create a generator potential?

  • pigment of rod cells ( rhodopsin ) break down by light energy. Enough energy from low intensity light to cause the breakdown. Enough pigment has to broken down for the threshold met in bipolar cell ( between sensory neuron and rod cells ) triggers an action potential.
  • Threshold can be reached even at low light as so many rod cells are connected to a single bipolar cell (summation)
• Advantage and disadvantage or Rod cells

  Advantage: still able to see in black and white even in low light intensity so it is a survival mechanism.

  Disadvantage: Retinal convergence means brain cannot distinguish between the separate sources of light. Two light sources close together cannot be seen as separate = low visual acuity
• Cone cells

  • Three types, different types of iodopsin  pigment all absorb different wavelengths of light. Depending on proportion of each cone cell stimulated we perceive colour images.
  • Iodopsin only broken down if there is a high light intensity so action potentials only generated with enough light
• • no retinol convergence and only one cone cell connects to a bipolar cell so no spatial summation occurs and cones only respond to high light intensity which is why we cannot see colour in the dark.
  • Positive: brain can distinguish between separate sources of light giving a high visual acuity.
• Distribution of rod and cone cells

  • Distribution of rods and cone cells in retina is uneven
  • Light is focused by the fovea receiving the highest intensity of light so most conse cells located near fovea as they only respond to high light intensities and rod cells further away as they respond to lower light intensities.
• Control of the heart

  • Cardiac muscle is myogenic = it contract on its own accord but rate of contraction is controlled by wave of electrical activity.
• Key components affecting systole and diastole

  • Sinoatrial node is located in the right atrium known as the pacemaker ( a group of cells release wave of electricity (depolarisation) and when this reaches cardiac muscle, it causes it to contract.)
  • Atrioventricular node located near border of right and left ventricle
  • Bundle of his: run down septum of heart
  • Purkyne fibres : in the walls of the ventricles. Conductive tissues
• Control of heart

  1. SAN releases wave of depolarisation across the atria causing contract. Stimulating atrial systole.
  2. AVN release another wave of depolarization (WOD) when the first reaches it, non-conductive tissue layer prevents wave of depolarisation travelling to ventricles.
  3. WOD can't go downwards so it goes through conductive tissue in septum - bundle of his. It conducts and passes the wave of depolarisation down septum and the purkyne fibres.
  4. The apex and ventricles then contract, short delay after atrial systole
  5. This delay allows enough time for atria to pump all blood into ventricles.
• Autonomic nervous system

  • This controls how fast the SAN releases the wave of depolarisation.
  • Medulla oblongata in the brain controls heart rate. Two parts depending how quickly wave of depolarisation is released. If to make it more rapid to increase heart rate or make it slower to decrease heart rate.
  • A CENTRE LINKED TO SINOATRIAL NODE INCREASES HEART RATE VIA SYMPATHETIC NERVOUS SYSTEM AND ANOTHER DECREASING HEART RATE VIA PARASYMPATHETIC NERVOUS SYSTEM.
• Homeostasis

  • Heart rate changes in response to pH and blood pressure and these stimuli detected by chemoreceptors and pressure receptors in aorta and carotid artery.
• Regulating pH and pressure

  • Pressure: blood pressure too high can cause damage to lining of walls fo arteries causing blood clots leading to stroke etc. if blood pressure too low, may be insufficient supply of oxygenated blood to respiring cells and insufficient removal of waste
  • pH: decreases during times of high respiratory rate due to production of CO2 and lactic acid. Exces acid must be removed before enzymes denature this is done by increasing heart rate so CO2 can diffuse out into the alveoli more rapidly.
• Process of Increased pressure

  1. Stimuli = increasing in pressure
  2. Receptor = pressure receptors in wall of aorta and carotid artery are stretching blood vessel if there is high blood pressure this simultaneously stretches pressure receptors triggering action potential along sensory neuron
  3. Coordination: More electrical impulses sent to medulla oblongata and  more impulses sent via parasympathetic nervous system to decrease this frequency of electrical impulses
  4. Effector: Cardiac muscle and SAN releasing fewer waves of depolarisation reducing heart rate.
• Process of decreased pressure

  1. stimuli - decrease in pressure
  2. Receptor: pressure receptor in wall of aorta and carotid artery not stretched if low blood pressure
  3. coordination : more electrical impulses sent to medulla oblongata and these impulses send via sympathetic nervous system to sinoatrial node tissues increasing waves of depolarisation
  4. Increasing in heart rate,