AQA Biology unit 5 - responding to the environment

revision notes for section 1 

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Nervous & Hormonal communication

responding to the environment increases animals chances of survival

Nervous system

made up of a complex system of neurons

3 main types:

1) sensory neurone -> transmit impulses from receptors to the CNS

2) relay neurone -> transmits impulses from sesnory neurone to motor neurone

3) motor neurone -> transmits impulses from CNS to effectors

nervous communication is localised, short lived and rapid

Hormonal system

sends information via chemical signals, system made up of glands and hormones

hormones travel around the body via the blood stream therefore they have a widespread response, slower and are longer lasting

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receptors are sepecfic to one kind of stimulus

Pacinian Corpuscle

example of a mechanoreceptor that detect mechanical stimuli e.g. pressure

they contain a naked nerve ending that's wrapped in loads of layers of lamellae (connective tissue)

when stimulated these press down on the nerve ending causing stretch-mediated sodium channels to open

sodium floods in creating a generator potential, when the threshold level is reached an action potential will be produced

The eye

made up of cone and rod cells -> these contain pigments sensitive to light (rhodopsin in rod cells & idopsin in cone cells)

when light hits the pigments it degrades them producing a small change in voltage potential = generator potential

Rod cells have poor visual acuity because they synapse together = retinal convergence Rod cells used when dark, generator potentials add up to reach threshold

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cell membranes are polarised at rest -> outside positive, inside negative, difference in voltage at rest called resting potential (approx -70mv)

action potential

stimulus triggers sodium ion channels to open causing a rapid change in potential difference this is known as a action potential

1) stimulus excites neurone -> sodium channels open and sodium diffuses in, making the inside less negative

2) depolarisation -> if potential difference reaches the threshold approx -50mv more sodium channels open so more sodium diffuses in

3) Repolarisation -> at +30mv sodium channels close and potassium channels open, potassium diffuses in, this makes the inside negative again

4) hyperpolaristaion -> potassium channels are slow to close, too many ions diffuse in making it more negative

5) Resting potential -> sodium-potassium pumps restore the balance by pumping 3 sodium ion out and 2 potassium in

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when an action potential happens, some sodium diffuses sideways causing sodium channels to open = action potential moves

waves move away during the refractory period

refractory Period time where ion channels are recovering and can't bopened

acts as a time delay so action potentials don't overlap

All or nothing

when a threshold value is reached it will always fire, if it isn't reached then it wont fire

bigger stimulus = more impulses

factors affecting speed of transmission

1) myelination ->action potential jumps between nodes of ranvier = quicker, on non-myelinated neurons it passes all the way along

2) axon diameter -> conducted along wider axons -> less resistance

3) temperature -> faster at higher temperatures -> diffuse faster

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Synaptic Transmission

how a nerve impulse is transmitted across a synapse:

  • action potential reached synaptic knob of pre synaptic neurone
  • stimulates calcium ion channels to open
  • calcium diffuses into synaptic knob
  • causes vesicles containing acetylcholine to bind with pre synaptic membrane
  • acetylcholine released into synaptic cleft
  • attaches to receptors of post synaptic neurone
  • causes sodium channels on post synaptic neuron to open
  • sodium ions diffuse in resulting in an action potential
  • acetylcholine broken down by acetylcholinearse and absorbed by pre synaptic neurone

synapses are unidirectional -> acetylcholine only released and reabsorbed by pre synaptic neurone

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Synaptic Transmission


spatial summation = many neurones connect to one neurone

temporal summation = two or more impulses arrive in quick seccession


1) agonists -> mimic neurotransmitter (same shape) means more receptors activated

2) antagonists -> block receptors so neurotransmitters can't bind, fewer activated

3) inhibitory -> stop neurotransmitter from being released, less receptors activated

4) amphetamines -> stimulate release of neurotransmitters so more receptors activated

Neuromuscular Junctions (synapses between neurones and muscles)

same as synaptic transmission with a bit extra:

  • post synaptic membrane contain lot of folds called t-tubules
  • action potential travels down t-tubules
  • causes sarcoplasmic reticulum to release calcium ions
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Muscle Contraction

sliding filament theory

myosin and actin filaments slide over each other to make sacromeres contract


  • A bands stay the same length
  • I band gets shorter
  • H zone gets shorter
  • z lines get closer

Muscle Contraction

  • calcium ions bind to tropnin -> causes tropomyosin to expose binding site
  • myosin head binds to actin filament
  • calcium also activated ATPase which breaks ATP into ADP+Pi to provide energy for muscle contraction
  • energy released from ATP moves myosin head in a rowing action
  • ATP provides energy to break myosin head from actin filament
  • myosin head binds to actin further along -> cycle repeated until contracted
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Muscle Contraction

ATP and Phosphocreatine provide energy for muscle contraction

Phosphocreatine (PCr)

ATP made by adding a phosphate group to ADP

PCr is stored in cells and generates ATP qucikly

PCr runs out very quickly so only used during vigorous exercise and in short bursts

slow and fast twitch fibres

slow -> contract slowly, endurance activities, aerobic respiration, red in colour due to large amounts of myoglobin

fast -> contract quickly, short bursts of speed & power, anaerobic respiration, whitish in colour

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Response in Animals

Control of Heart Rate

SAN node generates electrical impulses to cause cardiac muscles to contract

Animals alter their heart rate in response to stimuli

stimuli detected by pressure receptors (baroreceptors) and chemical receptors (chemoreceptors)

baroreceptors found in aorta & chemoreceptors found in carotid artery

impulses sent to medulla oblongata to increase or deacrease heart rate

medulla send impulses along sympathetic or parasympathetic nervous system

Reflex arcs

involuntary -> not under concious control

rapid -> avoid damage

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Response in Animals

Simple Responses

Taxes = directional response determined by direction of stimulus, can be positive or negative

Kinesis = non directional movement -> more unpleasant the stimulus the more rapid the movement and change of direction

Chemical Mediators

chemical messengers that act locally

similar to hormones -> cells release chemicals that bind to specific receptors on target cells

Histamine = released when body is injured, increases permeability of capaillaries to allow more white blood cells to move out.

Prostaglandins = involved in inflammation, blood clotting and blood pressure regulation

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Response in Plants

plants respond to stimuli to increase their chance of survival, response in plants is called tropism

tropism = growth of plant in response to a directional stimulus

e.g. positive phototropism -> directional growth towards light

negative geotropism -> growth against gravity

Growth factors

growth factors bring about responses, they move around the plant by diffussion and active transport

they can speed up or slow down growth

e.g. Indoleacetic acid (IAA)

- produced in tips and roots of plants

- it controls tropisms by having different amounts in different areas of the plant

- causes uneven growth so that it grows towards the more favourable conditions

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