Unit 4: Respiration & coordination 4.2 Response to changes in the external environment By Mr. Wilson

The retina is the light sensitive part of the eye 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.