Response to change

• What is a stimulus?

  An internal or external change in the environment
• What is a response?

  A change in an organism's behaviour due to stimuli
• Why is a response important?

  To give the organism greater chance of survival and reproduction to pass on genes
• What is a positive response?

  Movement towards a stimulus
• What is a negative response?

  Movement away from a stimulus
• What is taxes?

  When an organism moves in response to a directional stimulus
• What is kinesis?

  When an organism increases the number of turns or changes in direction in response to directional stimuli
• What is tropism?

  The change in plant growth in response to a directional stimulus
• What are auxins?

  Plant growth hormones
• Explain how IAA causes positive phototropism

  -Light hits a plant > produces IAA -Diffuses evenly down the stem

  -Collects on the shaded side of the plant

  -Causes cell elongation

  -Results in plant leaning in the direction of the light source
• Central nervous system


  Brain and spinal chord
• Peripheral nervous system


  Sensory and motor neurones that originate from either the brain or spinal chord throughout the rest of the body
• Voluntary nervous system

  Carries nervous impulses to body muscle and is under voluntary control
• Autonomic nervous system


  Carries nervous impulses to glands and cardiac muscle and is under involuntary contro
• Explain how myogenic contraction occurs

  -Medulla sends electrical impulse

  -SA node stimulates atria

  -Nerve impulse travels down septum to AV node

  -AVN stimulates ventricles

  -Electrical fibres travel down bundle of His > Purkinje fibres, stimulating ventricles
• How does CO2 alter heart rate?

  - High blood acidity detected by chemoreceptors in bloodstream - triggers a sympathetic nervous response by the medulla oblongata

  - Stimulates the SA node > AV node > bundle of His > Perkinje fibres which cause contraction of cardiac muscle, speeding up heart rate
• How does high blood pressure alter heart rate?

  Detected by mechanoreceptors in bloodstream - triggers a parasympathetic nervous response by the medulla oblongata

  Electrical impulse sent to SA node > AV node > bundle of His > Perkinje fibres which causes inhibition, slowing heart rate
• What is a reflex?

  An autonomic response via a pathway of 3 nerves
• Pathway of a reflex arc

  Stimulus > receptors > sensory neurones > relay neuron > motor neuron > effectors > response
• Why are autonomic reflexes important?


  They don't require decision making time and remove the body from harmful stimuli, ensuring survival
• What is a pacinian corpuscle?


  A mechanoreceptor - detects pressure
• How does the pacinian corpuscle work?

  The [Na+] is low at its resting potential with no pressure

  Stimulus of pressure means stretch-mediated protein channels allow an influx of Na+ ions into the membrane, creating a generator potential

  Once a threshold value is crossed, an action potential is created, causing an electrical impulse to travel up the neurone to the CNS
• What two types of photoreceptors are found in the eye?

  Cone cells and rod cells
• How are rod cells specialised?

  - Spatial summation means action potential is reached sooner - easier to stimulate in low light

  - Low light sensitivity means low visual acuity resulting in black and white vision

  - Located throughout the retina, meaning peripheral vision is strong
• How are cone cells specialised?

  - Cones cells come in three types, each adapted for a particular wavelength of light

  - Temporal summation means stimulus is more easily distinguished

  - High sensitivity means high visual acuity, resulting in coloured vision

  - Concentrated in the fovea
• How is an action potential generated?

  - The neurone has a baseline negative internal charge due to sodium exiting at a faster rate than potassium entering.

  - This causes a sudden reversal in charge due to gated ion channels opening (depolarisation),

  -This triggers a chain effect of other sodium channels opening, carrying the potential down the axon
• How do neurotransmitters trigger an action potential?

  - The neurotransmitters diffuse across the synaptic cleft and binding to the neuroreceptors on the post-synaptic knob; causing depolarisation due to the sudden increase in charge
• Describe a cholinergic synapse?

  - An action potential reaches the end of the pre-synaptic neuron, causing the release of calcium ions

  - The calcium ions trigger the production of the neurotransmitter acetylcholine, which are housed in vesicles

  - The vesicles exit the pre-synaptic membrane via exocytosis, allowing acetylcholine to diffuse across the synapse and bind to neuroreceptors

  - Causes depolarisation due to increase in calcium ions

  -Breakdown into acetyl and choline which diffuse back to the pre-synaptic knob
• How does a myelin sheath speed up an action potential?

  The nodes of Ranvier are permeable, whereas Schwann cells are impermeable, so depolarisation triggers over a larger area, which is quicker than triggering each adjacent section of the axon
• How does temperature affect the speed of an axon

  Diffusion of sodium ions into membrane is faster due to more kinetic energy, causing depolarisation faster
• Sliding filament theory

  - At rest, tropomyosin is blocking the actin-myosin binding site, held in place by troponin

  - Calcium binds to the troponin, causing the troponin to uncover the binding site. Cross bridge formation - myosin enters a high energy state

  - Calcium activates ATP hydrolase, breaking down ATP into ADP and Pi - powerstroke

  - ATP binds to the myosin head, breaking the bridge - ATP is hydrolysed, which returns myosin to its high energy state

  - The cycle repeats as long as ATP and Ca2+ is present
• Define homeostasis

  Maintaining a constant internal environment despite external changes
• Name ways that the body controls thermoregulation

  -Metabolism

  -Vasoconstriction/ vasodilation

  -Perspiration to evaporate heat

  -Shivering to generate heat

  -Hair standing up to trap air
• Name factors that must be maintained via homeostasis

  -Body temperature

  -[Blood glucose]

  -[Blood salt]

  -Water potential of blood

  -Blood pressure

  -[CO2]
• Negative feedback

  The reversing of a change in the internal environment
• What are factors that induce negative feedback responses?

  -Change must be detected by receptors

  -Change must be signalled to other cells

  -Response induced to reverse the change
• Negative feedback loop

  Optimum condition

  Change occurs

  Receptor detects change

  Communication system informs effector

  Effector reacts to reverse change

  Return to optimum conditions
• Negative feedback pathway

  Stimulus

  Receptor

  Communication pathway

  Effector

  Response
• Endotherm


  Warm blooded
• Ectotherm


  Cold blooded