Topic 9

• Homeostasis


  The maintenance of a state of internal dynamic equilibrium
• Homeostasis


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


  Counteracts any change in internal conditions
• Positive feedback


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


  • Dilation of the cervix during childbirth
  • Blood clotting
  • Urination
• Hormones


  Signalling proteins secreted by endocrine glands directly into the bloodstream
• Hormones


  • Only affect target organs and cells which contain complementary receptors on their plasma membrane, thus making them very specific
• Hormone action (membrane-bound)


  1. Hormone binds to receptor on target cell membrane

  2. Triggers a series of intracellular membrane-bound reactions

  3. Stimulates the release of a second messenger

  4. Second messenger activates 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 e.g. IAA
• Auxins


  • Maintain apical dominance and suppress the growth of lateral buds
  • Promote root growth
  • Promote trophic responses to unilateral light (directional growth responses e.g. phototropism, geotropism)
  • Functions include rooting powder and weed killers
• Auxin-induced cell elongation


  1. Auxins cause active transport of hydrogen ions into cell walls, lowering the pH

  2. Low pH makes cell walls flexible, enabling 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, causing elongation of cells across the zone of elongation
  • When shoot is only illuminated from one side, auxins move towards the shaded part of the shoot, causing elongation of the shaded side only, resulting in bending of the shoot towards the light
• Gibberellins


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


  1. Seed absorbs water, activating the embryo

  2. Activated embryo secretes gibberellins

  3. Gibberellins diffuse to the aleurone layer

  4. Aleurone layer produces amylase

  5. Amylase diffuses to the endosperm layer and breaks down starch into glucose
• Cytokinins


  Promote cell division in apical meristems/lateral bud development, work synergistically with ethene to promote abscission of leaves
• Interactions between plant hormones

  Can be synergistic (for the same effect e.g. auxins and gibberellins) or antagonistic (for inverse effects e.g. auxins and cytokinins on apical dominance)
• 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 and flowering


  • In long-day plants, Pfr stimulates flowering
  • In short-day plants, Pfr inhibits flowering
  • Day-neutral plants have different flowering triggers
• Etiolated plants


  • Tall and thin, fragile stems with long internodes, small yellowed leaves, little root growth
• Phytochrome and etiolation


  • In the dark, all phytochrome is in the form Pr
  • When the stem breaks through the soil, Pfr acts as a transcription factor, moving through nuclear pores and binding to a nuclear protein, activating transcription and controlling aspects of growth and development
• Central Nervous System


  A specialised concentration of nerve cells that processes incoming information, sends impulses through motor neurons and carries impulses to effectors
• Components of the Central Nervous System

  • Brain
  • Spinal Cord
• Peripheral Nervous System


  Neurons not in the CNS that spread throughout the body
• Components of the Peripheral Nervous System

  • Autonomic (not under conscious control)
  • Voluntary (under conscious control)
• Sympathetic nervous system


  Ganglia close to CNS, neurotransmitter is noradrenaline, coordinates the fight/flight response
• Parasympathetic nervous system


  Ganglia far from CNS, neurotransmitter is acetylcholine, coordinates the rest/digest response
• Sympathetic and parasympathetic nervous 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.
• Types of neurons


  • Sensory
  • Motor
  • Relay
• Motor neurons


  Involved in transmitting electrical signals from the central nervous system to muscles and glands in the body
• Sensory neurons


  Transmit impulses from receptors to the central nervous system
• Relay neurons


  Located within the central nervous system, involved in transmitting the electrical impulses from sensory neurons to motor neurons
• Resting potential


  The difference in the voltage across the neuron membrane, with a value of -70mV
• Neurons


  • Differ by the position of the cell body within the neuron
• Nerve cells are polarised in their resting state
• Resting potential


  Difference in the voltage across the neuron membrane, with a value of -70mV