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Unit 4: Respiration &
coordination
4.2 Response to changes in the
external environment
By Mr. Wilson
The mammalian eye
• The retina is the
light sensitive part
of the eye
• It contains rods
and cones,
photoreceptor
cells that convert
light rays into
nervous impulses.
Rods & cones
Rod cells
• Sensitive to low light intensity.
• Many rods connect with a single
bipolar neurone (one with two
extensions from the cell body.
• Give low visual acuity
• Concentrated in the retinal periphery
& evenly spread throughout the rest.
Cone cells
• High light intensity colour receptors.
• Mainly in fovea (yellow spot)
• Tightly packed together for high
definition.
• Each cone has a single connection to
the optic nerve.
Structure of the retina
The human retina
Photoreception
• The outer segments of rods & cones
are composed of stacked membrane
layers containing pigments.
• Rods contain rhodopsin & cones
contain iodopsin.
• Rhodopsin is made from the protein
opsin & light absorbing retinine, which
is derived from vitamin A.
Photoreception
• When light strikes a molecule of rhodopsin
(visual purple) it splits into Opsin & free
retinine. This depolarises the membrane
of the cell and an impulse passes to the
brain.
• This process is called ‘bleaching’
• Rhodopsin must be resynthesised from
opsin & retinine before the rod can be
stimulated again.
Photoreception
• Iodopsin in cone cells is less easily broken
down (it requires more energy than rods)
and takes longer to be resynthesised –
cones are therefore not very sensitive in
dim light.
• The trichromatic theory of colour vision
suggests there are 3 types of iodopsin
sensitive to red, green & blue wavelengths
of light. If they were in different cones
this would explain how we see many colours.
Photoreception
• In bright light the eye is said to be light
adapted – most of the rhodopsin in the
rods is bleached.
• If you are then exposed to dim light the
eye needs time to adjust as the rods
resynthesise rhodopsin.
• Once the rods have done this and regained
their response the retina is said to be dark
adapted.
Investigate….
• Can you explain why rods give low
visual acuity at a cellular level?
• How could you explain red-green
colour blindness using the
trichromatic theory of colour vision?
Photoreception in plants
• In some plants flowering is in response to a
period of illumination (photoperiod).
• These short day plants & long day plants
respond to light using a light sensitive
pigment called phytochrome.
• Phytochrome exists in two forms – 660
(Pr), which absorbs red light and 730 (Pfr),
which absorbs far red light.
Photoreception in plants
• When each pigment absorbs its wavelength
of light it is converted to the other form.
• Sunlight contains much more red than far
red so in daylight a lot of 660 is converted
to 730. At night this is converted back as
it is unstable.
• Plants measure day length by the amounts
of each type of phytochrome detected
over time.
Photoreception in plants
• Flowering in plants is initiated by
the hormone ‘florigen’.
• The release of this hormone is
linked to different
concentrations of the two
different types of phytochrome.
Photoreception in plants
• A long day plant is a plant that requires less
than a certain number of hours of night in each
24 hour period to induce flowering. Long day
plants typically flower in the late spring or early
summer.
• A short day plant is a plant that cannot flower
under the long days of summer. Short day plants
typically flower in the Autumn of the year.
These plants require a certain number of hours
of darkness in each 24 hour period (a short day
length) before floral development can begin.