quick exam recall

• Xylem


  Transports water and minerals
• Phloem


  Transports sugars and organic nutrients
• Direction of flow


  • Xylem: Unidirectional (roots to leaves)
  • Phloem: Bidirectional
• Cell types


  • Xylem: Dead cells (tracheids, vessel elements)
  • Phloem: Living cells (sieve tube elements, companion cells)
• Cell wall composition


  • Xylem: Thick, lignified walls
  • Phloem: Thinner, non-lignified walls
• Location


  • Xylem: Inner part of vascular bundles
  • Phloem: Outer part of vascular bundles
• Transport mechanism


  • Xylem: Passive (transpiration pull, root pressure)
  • Phloem: Active (pressure-flow hypothesis)
• chemoreceptors

  • found in carotid arteries + aorta
  • role: detect changes in blood pH
  • transfer chemical energy ti electrical energy, which is sent as an impulse to the CNS
  • this tells the CNS that more O2 is needed to neutralise the blood pH
• systems controlling heart rate

  nervous system: rapid response, short term

  hormonal system (e.g. adrenaline): slow response, long lasting
• fight/flight response - increasing heart rate

  • sympathetic neuron releases Noradrenaline
  • which stimulates adrenal glands to release adrenaline + noradrenaline
  • increasing force + rate of heart contraction
  • thus more cardiac output

  
• decreasing heart rate:


  parasympathetic signal releases acetylcholine
• homeostasis


  maintaining optimal internal environmental conditions in response to a stimulus
• homeostasis helps maintain:

  • optimal pH
  • optimal blood sugar
  • optimal temp: 37.5°C
• importance of homeostasis

  • ENZYME ACTIVITY: optimal pH and °C needed for optimal enzyme activity
  • CELL SIZE: water potential of blood must be maintained otherwise cells may burst
• what happens if sodium ion channels open


  membrane depolarises
• Saltatory Conduction


  A method of nerve conduction where the action potential jumps from node to node along the axon, allowing for faster transmission of signals.
• Myelin


  A fatty insulation layer surrounding the axon of neurons, produced by oligodendrocytes in the CNS and Schwann cells in the PNS. Insulates, protects, and provides structural support to the neuron.