6. Responding to changes in the environment

• Plant growth factors are chemicals that regulate plant growth response to directional stimuli
• Plant growth factors are produced in plant growing regions, specifically in apical meristems
• Plant growth factors diffuse from cell to cell or through phloem mass transport
• Shoots show positive phototropism because:

  • Indoleacetic acid (IAA) diffuses to the shaded side of the shoot tip
  • As IAA diffuses down the shaded side, it causes active transport of H+ ions into the cell wall
  • Disruption to H-bonds between cellulose molecules & action of expansins make the cell more permeable to water (acid growth hypothesis)
  • Cells on the shaded side elongate faster due to higher turgor pressure
  • Shoot bends towards light
• Roots show positive gravitropism because:

  • Gravity causes IAA to accumulate on the lower side of the root
  • IAA inhibits elongation of root cells
  • Cells on the upper side of the root elongate faster, causing the root tip to bend downwards
• Mammalian hormones and plant growth factors contrast in:

  • Concentration response not always dependent on concentration for mammalian hormones, while plant growth factors show a proportional response to concentration
  • Mammalian hormones bind to complementary proteins in/on target cells, while plant growth factors can affect all cells
  • Mammalian hormones are synthesized in specialized glands, whereas plant growth factors are synthesized in various tissues in growing regions
• • Mammalian hormones are transported through the circulatory system, while plant growth factors diffuse or undergo phloem translocation
  • Mammalian hormones are faster-acting for homeostasis, while plant growth factors are slower-acting for plant growth
• Taxis is directional movement in response to an external stimulus, while kinesis is a non-directional response to the presence and intensity of an external stimulus
• Many organisms respond to temperature and humidity via kinesis rather than taxis due to less directional stimuli and often no clear gradient from one extreme to the other
• To recognize kinesis in an organism's movement:

  • Organism crosses a sharp division between a favorable & unfavorable environment, leading to increased turning
  • If the organism moves a considerable distance into an unfavorable environment, turning slowly decreases, and it begins to move in long, straight lines with sharper turns
• In a simple reflex arc:

  • Receptor detects stimulus
  • Sensory neuron transmits the signal
  • Relay neuron in the CNS coordinates the response
  • Motor neuron carries the response to the effector
• Advantages of a simple reflex:

  • Rapid response to potentially dangerous stimuli since only 3 neurons are involved
  • Instinctive
• Chi squared is a suitable statistical test to determine whether a factor has a significant effect on the movement of an animal in a choice chamber
• Common features of all sensory receptors:

  • Act as energy transducers which establish a generator potential
  • Respond to specific stimuli
• The basic structure of a Pacinian corpuscle includes:

  • Single nerve fiber surrounded by layers of connective tissue separated by viscous gel and contained by a capsule
  • Stretch-mediated Na+ channels on the plasma membrane
  • Capillary runs along the base layer of tissue
• A Pacinian corpuscle responds to pressure by:

  • Deforming the membrane, causing stretch-mediated Na+ ion channels to open
  • If influx of Na+ raises the membrane to threshold potential, a generator potential is produced
  • Action potential moves along the sensory neuron
• The 2 types of photoreceptor cells located in the retina are cone cells and rod cells
• Rod cells are evenly distributed around the periphery but not in the central fovea, while cone cells are mainly in the central fovea with no photoreceptors at the blind spot
• Comparison of rod and cone cells:

  • Rod cells contain rhodopsin pigment, while cone cells have 3 types of iodopsin pigment
  • Rod cells have low visual acuity, while cone cells have high visual acuity
  • Rod cells have monochromatic color sensitivity, while cone cells have trichromatic color sensitivity
  • Rod cells are very light-sensitive, while cone cells are less sensitive and not involved in night vision
• The pathway of light from a photoreceptor to the brain is:

  • Photoreceptor → bipolar neuron → ganglion cell of the optic nerve → brain
• General structure of a motor neuron:

  • Cell body: contains organelles & high proportion of RER
  • Dendrons: branch into dendrites which carry impulses towards cell body
  • Axon: long, unbranched fibre carries nerve impulses away from cell body
• Myogenic is defined as the contraction of the heart being initiated within the muscle itself rather than by nerve impulses
• Additional features of a myelinated motor neuron:

  • Schwann cells: wrap around axon many times
  • Myelin sheath: made from myelin-rich membranes of Schwann cells
  • Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath
• Processes Schwann cells are involved in:

  • Electrical insulation
  • Phagocytosis
  • Nerve regeneration
• The 2 nodes involved in heart contraction are the Sinoatrial node (SAN) within the wall of the right atrium and the Atrioventricular node (AVN) near the lower end of the right atrium in the wall that separates the 2 atria
• How an action potential passes along an unmyelinated neuron:

  1. Stimulus leads to influx of Na+ ions. First section of membrane depolarises.

  2. Local electrical currents cause sodium voltage-gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise.

  3. Sequential wave of depolarisation
• Heartbeats are initiated and coordinated by:

  • SAN initiating a wave of depolarization
  • Wave of depolarization spreading across both atria
  • AVN conveying the wave of depolarization down the septum via the Bundle of His and Purkinje fibers along the ventricles
  • Ventricles contracting from the apex upwards
• Why myelinated axons conduct impulses faster than unmyelinated axons:

  • Saltatory conduction: Impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator. So impulse does not travel along whole axon length
• The formula for cardiac output is: cardiac output (CO) = stroke volume (V) x heart rate (R)
• The autonomic nervous system controls involuntary actions of glands and muscles and has 2 subdivisions: sympathetic & parasympathetic
• Resting potential:

  • Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90 mV, usually about -70 mV in humans)
• How resting potential is established:

  1. Membrane is more permeable to K+ than Na+.

  2. Sodium-potassium pump actively transports 3Na+ out of cell & 2K+ into cell. Establishes electrochemical gradient: cell contents more negative than extracellular environment
• The difference between the sympathetic and parasympathetic nervous system is that the sympathetic system is involved in the 'fight or flight' response, stimulating effectors to speed up activity, while the parasympathetic system is involved in normal resting conditions, inhibiting effectors to slow down activity
• Stages in generating an action potential:

  1. Depolarisation

  2. Repolarisation

  3. Hyperpolarisation

  4. Return to resting potential
• Receptors involved in changing heart rate are:

  • Baroreceptors (detect changes in blood pressure) located in the carotid body
  • Chemoreceptors (detect changes in pH) located in the carotid body & aortic body
• During depolarisation:

  1. Stimulus→facilitated diffusion of Na+ ions into cell down electrochemical gradient.

  2. p.d. across membrane becomes more positive.

  3. If membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open.

  4. Significant influx of Na+ ions reverses p.d. to +40mV
• Responses to changes in blood pressure:

  • Increase in blood pressure leads to more impulses to the cardioinhibitory center, decreasing heart rate
  • Decrease in blood pressure leads to more impulses to the cardioacceleratory center, increasing heart rate and strength of contraction
• During repolarisation:

  1. Voltage-gated Na+ channels close and voltage-gated K+ channels open.

  2. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient.

  3. p.d. across membrane becomes more negative
• Response to an increase in CO2 concentration:

  • Chemoreceptors detect pH decrease and send more impulses to the cardioacceleratory center, increasing heart rate and blood flow to the lungs for gas exchange and ventilation
• During hyperpolarisation:

  1. ‘Overshoot’ when K+ ions diffuse out = p.d. becomes more negative than resting potential.

  2. Refractory period: no stimulus is large enough to raise membrane potential to threshold.

  3. Voltage-gated K+ channels close & sodium-potassium pump re-establishes resting potential