topic 9

• Homeostasis


  The maintenance of a state of internal dynamic equilibrium
• Homeostasis


  • Ensures that a constant internal environment consisting of factors such as temperature, water potential and pH is maintained, despite changes of external factors
  • Temperature and pH are maintained to ensure optimal enzyme activity and cell membrane integrity
  • Water potential is controlled to avoid negative osmotic effects which can damage the cell
• Negative feedback


  Counteracts any change in internal conditions to restore optimal conditions
• Positive feedback


  Acts in the same direction as the original disturbance, therefore reinforcing the original stimulus
• Positive feedback


  • Dilation of the cervix during child birth
  • Urination
  • Blood clotting
• Hormones


  • Signalling proteins excreted by endocrine glands directly into the bloodstream
  • Only effect target organs and cells which contain complimentary receptors on their plasma membrane, making them very specific
• Hormone action (membrane-bound)


  1. Hormone binds to receptor on target cell membrane

  2. Triggers a series of intermembrane bound reactions

  3. Stimulates the release of a second messenger

  4. Second messenger actives enzymes to alter the metabolism of the cell
• Hormone action (nuclear)


  1. Hormone passes through cell membrane and binds to receptor inside the cell

  2. Form a hormone-receptor complex

  3. Complex passes into the nucleus and acts as a transcription factor to regulate gene expression
• Auxins


  • Growth stimulants
  • Maintain apical dominance and suppress the growth of lateral buds
  • Promote root growth
  • Promote trophic responses to unilateral sunlight (directional growth responses, e.g. phototropism and geotropism)
• Auxin-mediated cell elongation


  1. Auxins cause active transport of H+ into cell walls, lowering the pH

  2. Low pH = flexible walls > cell walls can stretch to accommodate for more water > enabling the expansion and growth of cells
• Auxin distribution in shoot

  • When shoot is illuminated from all sides, auxins are distributed evenly and move down the shoot tip = elongation of cells across the zone of elongation
  • When shoot is illuminated from one side, auxins move towards shaded part of the root, causing elongation of just the shaded side which results in the bending of the shoot towards the light
• Gibberellins


  • Stimulate elongation at cell internodes
  • Stimulate growth of fruit
  • Stimulate germination
  • Stimulate 'bolting' > rapid growth and/or flowering
• Gibberellin-mediated seed germination


  1. Seeds absorb water which activates the embryo

  2. Activated embryo secretes gibberellins which diffuse to aleurone layer

  3. Aleurone layer produces amylase which diffuses into endosperm layer and breaks down starch into glucose
• Cytokinins


  • Promote cell division in apical meristems/later bud development
  • Work synergistically with ethene to promote abscission of leaves
• Interaction of plant hormones

  • Auxins and gibberellins have a synergistic effect (for the same effect)
  • Auxins and cytokinins have an antagonistic effect (for the inverse effect)
• Phytochrome


  A plant pigment that exists as two interconvertible forms: PR (biologically inactive, absorbs red light) and PFR (biologically active, absorbs far red light)
• Phytochrome conversion


  When the phytochrome absorbs one of the two types of light, it is converted to the other form at a rate dependent on light intensity
• Day-neutral plants


  Different flowering triggers
• Etiolated plants


  • Tall and thin
  • Fragile stems with long internodes
  • Small yellowed leaves
  • Little root growth
• Phytochrome action in etiolated plants

  PFR acts as a transcription factor – moves through nuclear pores and binds to nuclear protein > complex activates transcription and controls aspects of growth and development
• Central Nervous System (CNS)


  A specialised concentration of nerve cells that processes incoming information, sends impulses through motor neurones and carries impulses to effectors
• Peripheral Nervous System (PNS)


  Neurones not in the CNS that spread throughout the body
• Components of PNS


  • Autonomic (not under conscious control)
  • Sympathetic
  • Parasympathetic
  • Voluntary (under conscious control)
• Sympathetic and parasympathetic systems

  • Work antagonistically
• Hypothalamus


  • Thermoregulation, osmoregulation, hormone secretions, basic drives
• Cerebellum


  • Smooth movements, balance/posture
• Cerebrum


  • Voluntary behaviour – personality, etc...
• Medulla Oblongata


  • Reflex centres – breathing, heart rate, peristalsis, etc...
• Sensory Neurones


  Transmit impulses from receptors to CNS
• Relay Neurones


  Located within CNS, involved in transmitting electrical impulses from sensory to motor neurones
• Motor Neurones


  Involved in transmitting electrical signals from the CNS to the muscles and glands in the body
• Resting potential


  The difference in voltage across the neurone membrane, with a value of -70mV
• Generation and maintenance of resting potential

  1. Sodium-potassium pump moves sodium ions out of the membrane and potassium ions into the neurone

  2. Creates an electrochemical gradient as the concentration of sodium is higher outside of the cell because the membrane is not permeable to sodium ions

  3. Potassium ions diffuse back out due to presence of potassium ion channels
• Depolarisation


  1. Excitation of a neurone cell triggered by a stimulus causes sodium channels to open, making it more permeable to sodium ions, which subsequently diffuse into the neurone down the electrochemical gradient > making inside less negative

  2. Upon reaching the threshold of -55mV, even more sodium channels open which gives a potential difference of +30mV which is the end of depolarisation and start of repolarisation

  3. Sodium channels closing and potassium ion channels opening allows potassium ions to diffuse out of the neurone down a concentration gradient and eventually restore the resting potential
• Absolute refractory period


  Sodium channels are blocked, and it is impossible for another action potential to be generated
• Relative refractory period


  Sodium channels are not blocked, but potassium channels are open so effectively the threshold is still raised
• Saltatory conduction


  The action potential jumps between gaps of myelin sheath, called Nodes of Ranvier, due to the myelin sheath being impermeable
• Synaptic transmission


  1. Upon arrival of an action potential, the presynaptic membrane depolarises = calcium channels open = calcium ions enter neurone

  2. Presence of calcium ions in neurone causes the fusion of synaptic vesicles filled with a particular neurotransmitter with the presynaptic membrane, releasing the neurotransmitter into the synaptic cleft

  3. Neurotransmitter binds to receptors on the postsynaptic membrane and either stimulates opening of cation channels (causing depolarisation and triggering another action potential) or stimulates opening of anion channels (causing hyperpolarisation and making triggering a new action potential difficult)
• Effects of drugs on the nervous system

  • Nicotine mimics the effects of acetylcholine and triggers the release of dopamine, at high doses nicotine binds to and blocks the acetylcholine receptors
  • Lidocaine blocks voltage-gated sodium ion channels
  • Cobra venom binds to and blocks acetylcholine receptors
• Receptors


  Cells specialised for detection of stimuli