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Section P: homeostasis & Sensitivity (plants (Auxin (image (auxin…
Section P: homeostasis & Sensitivity
key words
homeostasis = the maintenance of a constant internal environment to allow chemical reactions and allow tissues & cells to function properly.
organisms are able to respond to changes in their environment
examples of homeostasis to maintain constant:
temperature due to its effect on enzymes
body water content to maintain osmotic balance in body
body glucose levels
coordinated response:
Stimulus → receptor → effector → response
stimulus = change in environment
receptor = detects the stimulus
effector = produces the response - muscle/gland
response = reaction/movement
stimuli
external
sound, light
internal
rise or drop in core body temperature, rise or drop in blood glucose level
plants
plants respond to stimuli from their environment
they usually respond slower than animals due to altered patterns of growth
plant growth is mostly due to elongation
the growth responses too a stimulus in the plants are called
tropisms
Geotropism
this is a response to gravity
roots are positively geotropic - they grow with gravity
shoots are negatively geotropic - they grow away from gravity
beneficial for the root to be positively geotropic because down is where they are more likely to find water, minerals and stability
Auxin
auxins are plant hormones
made in the tips of the roots and shoots
they diffuse away from the site of production (dissolved in the aqueous cytoplasm)
stimulates the elongation of shoot cells
inhibits the elongation of root cells
auxin accumilation
cells elongate more due to the low concentration of auxin
cells do not elongate so much as auxin is inhibiting it
phototropism
this is a response to light
shoots are positively phototropic which means they grow towards the light
roots are negatively phototropic (or are unaffected by light)
Responses can be controlled by nervous or hormonal communication
Nervous system
form of communication: electrical impulse
how communication carried: in neurons
speed of response: fast
how long responses last: short-lived
hormonal system
form of communication: chemicals
how communication carried: blood - (plasma)
speed of response: slow
how long responses last: long-term effect
the eye
which organ can you get conjunctivitis in? = the eye
eye is an organ because it has lots of tissues
light is a stimulus
stimulus → receptor → sensory neuron → CNS → effector → response
receptor = the retina
A = Iris (colour)
B = pupil (pore)
C = lens
D = retina (has the receptors which are sensitive to light)
E = optic nerve
the iris reflex
the eye needs to be able to control the amount of light entering it
in dim conditions, more light is allowed to enter so that a clear image can be formed on the retina
in bright conditions less light is allowed to enter so that the retina is not damaged
this adjustment is done by two sets of muscles in the iris:
its circular muscles contract to close up the pupil, making the pupil smaller - while its radial muscles contract to open up the pupil, making the pupil larger
