3.6.1.1 survival and response

stimulus

change in organisms external or internal environment, eliciting a response

receptor

detects stimuli and transduces energy change into another form

coordinator

formulates responses (CNS in mammals)

effector

produces response

kinesis

increase in random movement to stay longer in favourable conditions, fast + less turns in unfavourable environment, slow + more turns in favourable environment

taxis

purposeful movement in a specific direction, either towards or away from a specific stimulus, requires receptor tissue within organism

tropism

growth in response to a stimulus

positive phototropism

growth towards light

positive geotropism

growth towards gravity

phototropin

molecule which detects light (photoreceptor)

indoleacetic acid (IAA)

hormone produced by cells at tip of plant which stimulates cell elongation

thigmotropism - growth in response to touch

after contact with support, tendril shows less growth in contact region and more growth in no-contact region so can bend over support, touched cells produce auxin and transport it to untouched cells

chemotropism - growth in response to chemicals

enables pollen tube to grow towards ovary to help fertilise gametes

amyloplast

specialised organelle in plants that stores starch

geotropism in roots

amyloplasts settle due to gravity and cause IAA transporter proteins to be inserted into plasma membrane of root cells on underside, moving IAA in direction of pull of gravity, upper side of root has a lower IAA conc so continue to elongate at normal rate, cells on lower side grow slower as IAA inhibits root cell elongation, difference in growth rates causes root to bend towards gravity

phototropism in shoots

phototropin in tip of shoot detects light stimulus, IAA synthesised by shoot cells, diffuses down stem, actively away from light, stimulated phototropin causes more IAA transporter proteins to be inserted in membrane of cells on shady side of shoot so IAA can accumulate, stimulating elongation of cells on the shaded side more than growth of cells in light, causing shoot to grow towards light (pos. phototropism)

acid growth hypothesis

IAA is an acid so H+ ions from IAA are actively pumped by pump into cell wall, acidifying it, H+ ions activate proteins called expansins which increase space between cellulose fibres, cell wall becomes more elastic and cell elongates

reflex

rapid, automatic response to a stimulus, a particular stimulus leads to the same response time and time again

reflex arc

stimulus, receptor (transducer converting energy to electrical impulse), sensory neurone, relay neurone in CNS, motor neurone, muscle/gland, response

nerve

axons from many neuronal cells bundled together

neurone

single nerve cell

afferent nerve

conducts signal which arrives at CNS

efferent nerve

conducts signal which leaves from CNS to muscle

importance of reflex arcs

protection (rapid response with no decision making so brain is free for other functions), survival before language develops e.g. in babies, to maintain posture (small changes generate signals causing immediate response to maintain balance)