Editing Retina (section)

====Energy requirements====
The energy requirements of the retina are even greater than that of the brain.<ref name="Viegas Neuhauss" />  This is due to the additional energy needed to continuously renew the photoreceptor outer segments, of which 10% are shed daily.<ref name="Viegas Neuhauss" /> Energy demands are greatest during dark adaptation when its sensitivity is most enhanced.<ref name="Kaynezhad Tachtsidis Sivaprasad Jeffery">{{cite journal | last1=Kaynezhad | first1=Pardis | last2=Tachtsidis | first2=Ilias | last3=Sivaprasad | first3=Sobha | last4=Jeffery | first4=Glen | title=Watching the human retina breath in real time and the slowing of mitochondrial respiration with age | journal=Scientific Reports | volume=13 | issue=1 | date=2023 | issn=2045-2322 | doi=10.1038/s41598-023-32897-7 | page=6445| pmid=37081065 | pmc=10119193 | bibcode=2023NatSR..13.6445K }}</ref> The choroid supplies about 75% of these nutrients to the retina and the retinal vasculature only 25%.<ref name="Kolb">{{cite journal|last1=Kolb|first1=Helga|title=Simple Anatomy of the Retina|url=http://webvision.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/|website=Webvision|year=1995|pmid=21413391|access-date=1 January 2018}}</ref>

When light strikes 11-cis-retinal (in the disks in the rods and cones), 11-cis-retinal changes to all-trans-retinal which then triggers changes in the opsins. Now, the outer segments do not regenerate the retinal back into the cis- form once it has been changed by light. Instead the retinal is pumped out to the surrounding RPE where it is regenerated and transported back into the outer segments of the photoreceptors. This recycling function of the RPE protects the photoreceptors against photo-oxidative damage<ref>{{Cite web|title=LIGHT-INDUCED DAMAGE to the RETINA|url=http://photobiology.info/Rozanowska.html|access-date=2023-02-23|website=photobiology.info}}</ref><ref>{{cite web |title=Diagrammatic representation of disc shedding and phagosome retrieval into the pigment epithelial cell |url=http://webvision.med.utah.edu/imageswv/photphag.jpeg |url-status=live |archive-url=https://web.archive.org/web/20120921011139/http://webvision.med.utah.edu/imageswv/photphag.jpeg |archive-date=21 September 2012 |access-date=22 April 2022}}</ref> and allows the photoreceptor cells to have decades-long useful lives.