of this is going on, metarhodopsin II is chemically modified byan enzyme called rhodopsin kinase. The modified rhodopsin then binds toa protein known as arrestin, which prevents the rhodopsin from activatingmore transducin. So the cell contains mechanisms to limit the amplifiedsignal started by a single photon.Trans-retinal eventually falls off of rhodopsin and must be reconvertedto 11-cis-retinal and again bound by rhodopsin to get back to the startingpoint for another visual cycle. To accomplish this, trans-retinal is firstchemically modified by an enzyme to trans-retinola form containingtwo more hydrogen atoms. A second enzyme then converts the moleculeto 11-cis-retinol. Finally, a third enzyme removes the previously addedhydrogen atoms to form 11-cis-rennal, a cycle is complete. .And that...BEETLE BOMBSThe bombardier beetle is an insect of unassuming appearance,measuring about one half-inch in length. When it is threatened by anotherbug, however, the beetle has a special method of defending itself, squirtinga boiling-hot solution at the enemy out of an aperture in its hind section.16The heated liquid scalds its target, which then usually makes other plansfor dinner. How is this trick done?It turns out that the bombardier beetle is using chemistry. Prior to battle,specialized structures called secretory lobes make a very concentratedmixture of two chemicals, hydrogen peroxide and hydroquinone.The hydrogen peroxide is the same material as one can buy in adrugstore; hydroquinone is used in photographic development. Themixture is sent into a storage chamber called the collecting vesicle.The collecting vesicle is connected to, but ordinarily sealed off from, asecond compartment called (evocatively) the explosion chamber. Thetwo compartments are kept separate from one another by a duct with asphincter muscle, much like the sphincter muscles upon which humansdepend for continence. Attached to the explosion chamber are a numberof small knobs called ectodermal glands; these secrete enzyme catalystsinto the explosion chamber. When the beetle feels threatened it squeezesmuscles surrounding the storage chamber while simultaneously relaxingthe sphincter muscle. This forces the solution of hydrogen peroxide andhydroquinone to enter the explosion chamber, where it mixes with theenzyme catalysts.Now, chemically, things get very interesting. The hydrogen peroxiderapidly decomposes into ordinary water and oxygen, just as a store-boughtbottle of hydrogen peroxide will decompose over time if left open. Theoxygen reacts with the hydroquinone to yield more water, plus a highlyirritating chemical called quinone. These reactions release a large quantityof heat. The temperature of the solution rises tothe boiling point; in fact, a portion vaporizes into steam. The steamand oxygen gas exert a great deal of pressure on the walk of the explosionchamber. With the sphincter muscle now closed, a channel leading outwardfrom the beetle's body provides the only exit for the boiling mixture.Muscles surrounding the channel allow the steam jet to be directed at thesource of danger. The end result is that the beetle's enemy is scalded by asteaming solution of the toxic chemical quinone.You may wonder why the mixture of hydrogen peroxide and quinonedid not react explosively when they were in the collecting vesicle. Thereason is that many chemical reactions occur quite slowly if there is noeasy way for the molecules to get together on the atomic levelotherwise,this book would burst into flame as