Topic 9 - Control Systems Flashcards
Homeostasis
The maintenance of a constant internal environment
e.g. temp, water, pH
Negative feedback
Counteracts any change in internal conditions, restored to optimum
Positive feeback
Acts in same direction as original disturbance therefore reinforcing original stimulus
e.g. blood clotting
What are hormones and where do they come from?
They are signalling proteins secreted by endocrine glands directly into the bloodstream (specific)
2 modes of hormone action
1) Hormones (e.g. adrenaline) bind to a receptor on target cell membrane. Triggers intracellular membrane-bound reactions and stimulates release of second messanger (e.g. cAMP). This activates enzymes to alter metabolism of cell.
2) Hormones (e.g. oestrogen) pass through cell membrane and bind to a receptor inside the cell. They form a hormone-receptor complex, passes into nucleus & acts as transcription factor to regulate gene expression.
What do auxins do?
- Growth stimulants ~(e.g. IAA)
- Maintain apical dominance & suppress the growth of alteral buds
- Promote root growth
- Promote trophic responses to unilateral light (directional growth responses e.g. phototropism, geotropism)
- Functions; rooting powder, weed killers
How do auxins work?
- Cause cell elongation via active transport of hydrogen ions into cell walls
- This lowers the pH of the walls
- This makes the walls mroe flexible to stretch to accommodate more water (enabling expansion and growth of cells)
How does light influence auxins?
- When shoot is illuminated from all sides, auxins evenly distributed & move down shoot tip causing elongation of cells in zone of elongation
- When shoot is only illuminated from one side, auxins move to shaded part of shoot causing elongation of shaded side only (bends towards the light
What do gibberellins do?
- Stimulate elongation at cell internodes
- Stimulate growth of fruit
- Stimulate germination
- Stimulate ‘bolting’ (rapid growth/flowering)
How do gibberellins work?
- Seed absorbs water (activates the embryo)
- Activated embryo secretes gibberellins
- Gibberellins diffuse to aleurone layer
- Aleurone layer produces amylase
- Amylase diffuses to endosperm layer and breaks down starch into glucose
What do cytokinins do?
- Promote cell division in apical meristems/lateral bud development
- Work synergistically with ethene to promote abcission of leaves
What is phytochrome
Plant pigment (blue-green) that exists as 2 interconvertible forms;
- Pr = biologically inactive form, absorbs red light (like sunlight)
- Pfr = biologically active form, absorbs far red light
When do the 2 forms of phytochrome interchange?
When phytochrome absorbs one of the two respective types of light, it is converted to the other form (or in darkness, converted to Pr) at a rate dependent on light intensity
What happens in long/short day plants and neutral plants?
(in terms of Pfr)
- Long-day plants, Pfr stimulates flowering
- Short-day plants, Pfr inhibits flowering
- Day-neutral plants have different flowering triggers
What are etiolated plants?
(& characteristics)
Plants grown in the dark (where phytochrome is in Pr);
- Tall and thin
- Fragile stems with long internodes
- Small yellowed leaves
- Little root growth
What happens when an etiolated plants breaks through soil?
- Pfr acts as a transcription factor
- Moves through nuclear pores and binds to nuclear protein
- The complex activates transcription and controls aspects of growth and development
Central nervous system
Specialised concentration of nerve cells that processes incoming information, sends impulses through motor neurons and carries impulses to effectors
Consists of brain and spinal cord
Peripheral nervous system and consists of what 2 parts ?
Neurons not in the CNS that spread throughout the body
2 parts;
- Autonomic (not under conscious control)
- Voluntary (conscious control)
Autonomic nervous system
- Sympathetic - ganglia close to CNS, neurotransmitter is noradrenaline, coordinates fight/flight response
- Parasympathetic - ganglia far from CNS, neurotransmitter is acetlcholine, coordinates rest/digest response
(Work antagonistically)
Hypothalamus function
- Thermoregulation
- Osmoregulation
- Hormone secretions
- Basic drives
Cerebellum function
- Smooth movements
- Balance/posture
Cerebrum function
- Voluntary behaviour
- Personality
Medulla oblongata function
- Reflex centres
- breathing
- heart rate
- peristalisis
Types of neuron
- Sensory
- Relay
- Motor
Sensory neuron function
Transmits impulses from receptors to the CNS
Relay neuron function
Located in the CNS and are involved in transmitting the electrical impulses from sensory to motor neurons
Motor neurons function
Involved in transmitting electrical signals from CNS to muscles and glands in the body
Resting potential
When the inside of the axon is negatively charged compared to outside of the axon (axon is polarised)
Typically resting potential = -70mV
How resting potential happens
1) Na ions are actively transported OUT of the axon by the Na-K pump
2) K ions are actively transported INTO the axon by the Na-K pump
3) The active transport of Na ions is greater than that of the K ions, 3 Na move out OUT for every 2 K ions that move in
4) Na will naturally diffuse back into the axon & K out of the axon
5) However, most Na channels are closed (less leaky) and most K channels are open (more leaky)
6) This means the electrochemical gradient is maintained
Action potential
- An explosion of electrical activity
- Occurs when a neurone sends info from it’s cell body down it’s axon
- Also known as A SPIKE or FIRING of a neurone
Depolarisation
(action potential)
Movement of sodium ions into the neurone reduces the potential difference across the membrane
When stimulus received by a receptor/nerve endings, its energy causes a temporary reversal of the charge on the axon membrane e.g. inside becomes less negative/more positive
Repolarisation
(action potential)
Movement of potassium ions out of the neurone reverses the depolarisation
Hyperpolarisation
(action potential)
Potassium channels stay open a bit too long
Gradually ion concentrations go back to resting levels
Events at the synapse
1) Action potential depolarises the presynaptic neuron
2) Calcium channels open and calcium diffuses in
3) Synaptic vesicles move to and fuse with the presynaptic membrane
4) The neurotransmitter is released into synaptic gap
5) Neurtotransmitter moves across cleft by diffusion
6) Neurotransmitter binds to specific protein receptors on the sodium channel on the post synpatic membrane
7) Sodium channels open and sodium diffuses in
8) This causes a change in the potential difference of the membrane and an excitatory post-synaptic potential (EPSP) to be set up
Inhibitory post-synpatic potential (IPSP)
- Kind of synpatic potential that makes a postsynaptic neurone less likely to generate an action potential
- Here, different ion channels open in the membrane allowing inward movement of negative ions
- This makes post-synpatic cells more negative than normal resting potential
- This means an action potential is less likely to occur
How are neurotransmitters broken down and what happens after?
- Broken down by hydrolic enzymes
- They then move back across the cleft, back into synaptic knob and are recycled
2 main types of synapses, where they are found and what neurotransmitter they release
1) Adrenergic , sympathetic NS, noradrenaline & adrenaline
2) Chlinergic, parasympathetic NS, acetylcholine
Salactory conduction
- Process where action potentials are transmitted from one node of ranvier to next myelinated nerve
- Speeds up transmission as ionic movements associated with action potential occur less frequency
How does depolarisation occur
1) The excitation of a neuron cell triggered by a stimulus causes the sodium channels to open, making it more permeable to sodium ions which diffuse into the neuron down the electrochemical gradient, making the inside less negative
2) When its reached the threshold, more sodium channels open, eventually giving a potential difference which is the end if the depolarisation and start of repolarisation
Generally how do drugs increase the response of the synpase
- Increases amount of neurotransmitter synthesised
- Increases release of neurotransmotter from vesicles at presynaptic membrane
- Binds to post-synaptic receptors and activates them/ increases effect of the normal neurotransmitter
- Prevents degradation of neurotransmitter by enzymes or prevents reuptake into presynaptic knob
Generally how can drugs decrease the synaptic response
- Block synthesis of neurotransmitters
- Causes neurotransmitters to leak from vesicles and be destroyed by enzymes
- Prevents release of neurotransmitters from vesicles
- Blocks receptors and prevents neurotransmitter binding
What affect does nicotine have on the synapse
- Mimics effect of acetylcholine
- Triggers action potential in post-synaptic neuron but then the receptor remains unresponsive for some time
- At low doses = nicotine has stimulating effects
- At high doses = can be lethal
- Triggers release of dopamine which can be associated with pleasure sensations
What affect does lidocaine have on the synapse
- Used as local anaesthetic by dentists
- Blocks voltage-gated Na channels preventing action potential in sensory neurons
- Used to prevent some heart arrhythmias as it blocks Na channels raises depolarisation threshold so prevents early or extra action potentials in the pacemaker
What affect does cobra venom have on the synpase
- Toxic and often lethal
- Binds reversibly to acetylcholine receptors and so prevents transmission of impulses across synapses
- Means muscles are not stimulated to contract so person becomes paralysed
- Low doses = can be used to relax trachea muscles in asthma patients to save lives
- High doses = stop breathing altogether and cause death
