chapter 9 phototropism

• Phytochromes is a blue-green pigment molecule which exist as 2 inconvertible forms which is PR phytochrome red (p660) that absorb red light and PFR phytochrome far red (P730) which absorb far red light .
• In short-day plant , PFR must be low while in long-day plant, PFR must be high.
• Phototropism is the response of plant to the relative lengths of daylight and darkness.
• exapmle of long-day plant are spinach, lettuce,clover.

  example of short-day plant are strawberry ,chrysanthemum, tobacco
• A short-day plant will only flower when the dark period is longer than the critical night length.

  A long-day plant will only flower when the dark period is shorter than the critical night length.

  When there is a subsequent red flash light interrupting the dark period,PR will absorb the red light and instantly converted to PFR. The concentration of PFR will increase and long-day plant is able to flower while the short-day plant cannot flower (inhibit )
• How to maintain resting potential

  1. Sodium-potassium pump pumps 3 Na+ out of the axoplasm and 2 K+ into the axoplasm

  2. This is an active transport and needs ATP

  3.creating a concentration gradient of Na+ ion and K+ ion across the axolemma
• facilitated diffusion in maintaining resting potential

  • More non-gated K+ channels than non-gated Na+ channels
  • Permeability of axolemma towards K+ ion is higher
  • Higher concentration of K+ ion in the axon
  • More K+ ion will diffuse out from the axon
  • Lesser Na+ ion will diffuse in to the axon through passive transport
  • Axon will remain negative
• Large anions

  • Carry negative charge such as protein and nucleic acid in the axoplasm
  • Cannot diffuse out from the cell
• The voltage-gated ion channels are all closed
• how to maintain resting potential ?

  1)sodium-potassium pump

  2)more non-gated potassium ion channel and lesser non-gated sodium ion channel

  3)large anion (protein and nucleic acid )

  4)voltage- gated ion channel all closed
• -resting potential is the potential difference between the internal and the external surface of the axon membrane when it is not transmitting nerve impulses.

  -the potential difference are -70mV and is more negative in the axoplasm

  -the axon is polarised
• Action potential

  -is a nerve impulse

  -an electrical excitation that travels down along the axon all the way till it reach the synaptic knob at the axon terminal

  -will change the potential difference of the axolemma from -70mV to +40mV

  -cause by voltage-gated sodium ion channel on the axolemma

  -Event of generation of action potential : depolarisation
• How action potential is generated

  1. Stimulus

  2. Voltage-gated sodium ion channel opens

  3. Sodium ion diffuses from interstitial fluid to axoplasm

  4. Axoplasm becomes more positive

  5. Potential difference reaches threshold level (-50mV)

  6. All voltage-gated sodium ion channels open

  7. Depolarisation occurs

  8. Voltage-gated potassium ion channel remains closed

  9. Sodium-potassium pump stops working

  10. Influx of sodium ion increases potential difference

  11. Potential difference reaches +40mV

  12. Action potential is generated
• What is action potential ?

  -A short term of change in the potential difference on the axolemma membrane of a neurone in response to stimulation which then spreads rapidly from one end to the other.
• Propagation of nerve impulse along axon

  • The action potential does not travel along the axon, it is repeatedly generated along the axon
  • self-propagated
• Function of myelin sheath

  • Electrical insulator to prevent the movement of ions such as Na+ and K+
  • to speed up the transmission of nerve impulse
• Transmission along the axon (non-myelinated axon)

  1. Continuous conduction

  2. Stimulus triggers voltage-gated sodium channel

  3. Sodium ion influx into axoplasm

  4. Depolarisation occurs

  5. First action potential generated

  6. Depolarisation spreads to adjacent region

  7. New action potential generated at adjacent region

  8. Axolemma repolarising as potassium ions diffuse out

  9. Previous region undergoes hyperpolarisation

  10. Previous region enters absolute refractory period
• Continuous conduction


  Propagation from one patch of axolemma to another through local current in one direction
• Action potential

  Generated when the axoplasm is more positive and the potential difference is +40v
• The stimulus will trigger the voltage-gated sodium channel to open
• This causes more sodium ion influx into the axoplasm
• Depolarisation happens and the first action potential is generated
• The depolarisation of the first action potential spreads to the adjacent region of the axoplasm
• New action potential is generated at the adjacent region
• At the previous region, the axolemma is repolarising as potassium ions diffuse out
• The previous region also undergoes hyperpolarisation
• The previous region will also undergo an absolute refractory period
• During the absolute refractory period, the previous region cannot generate a new action potential for a certain period of time
• The action potential moves

  In one direction
• Characteristic of nerve impulses

  -refractory period

  -all-or-none law

  -speed of transmission
• The characteristic of nerve impulses refractory period

  -refractory period (a period immediately after generation of action potential )

  -During the absolute refractory period, a new action potential cannot be generated

  -During the relative refractory period, a new action potential can be generated if the stimulus is more intense than the normal one.
• -The nerve impulse also follow all-or-none law

  -which the amplitude is always +40mv regardless of the intensity of the stimulus

  -The frequency of nerve impulse will only determine the stimulus ,the stronger the stimulus,the higher the frequency of the nerve impulses generate per miliesecond. 
• -The speed of the transmission of nerve impulse can be increase by presence of myelin sheath along the axon on myelinated axon and also increasing the diameter of axon of a non-myelinated axon.
• Synapse


  A small gap between the axon terminal of presynaptic neurone and the cell body of a postsynaptic neurone
• Structure of synapse

  • Synaptic cleft
  • Thin space containing interstitial fluid
  • Presynaptic membrane
  • Postsynaptic membrane
  • Synaptic knob
  • Membrane-bound synaptic vesicle
  • Mitochondria
  • Golgi apparatus
• Synaptic cleft

  A thin space containing interstitial fluid between the presynaptic membrane and the postsynaptic membrane
• Synaptic knob


  An enlarged bulbous structure at the terminal end of the axon
• Presynaptic membrane

  • Contains voltage gated calcium channel
• Postsynaptic membrane

  • Contains ligand-gated sodium channel
• Synaptic vesicle


  Contains neurotransmitters like acetylcholine and noradrenaline