Chapter 7.1 Sunlight as an Energy Source AP Biology Fall 2010

Properties of Light Wavelength: horizontal distance between the crests of every two successive waves of radiant energy Light travels in waves, but has particle qualities Photons: consists of fixed amount of energy, when absorbed measured as discrete packet

Properties of Light Electromagnetic spectrum: range of all wavelengths of radiant energy, from shortest (gamma rays) to longest (radio waves)

Properties of Light Organisms use only a small range of wavelengths for photosynthesis, vision, and other processes Most of wavelengths are the ones we see as visible light, a small part of the electromagnet spectrum from the sun

Properties of Light Light energy is packaged as photons, which vary in energy as a function of wavelength ◦Shortest are gamma rays with highest energy; longest are radio waves with lowest energy Only a small range ( nm) of wavelengths is used for photosynthesis Visible light of all wavelengths combined appears white ◦Prism or water – bends light, colours

From Sunlight to Photosynthesis The light reflected from each pigment gives the pigment its colour ◦Example: red pigment absorbs all colours of visible light except red light

From Sunlight to Photosynthesis There is more than one kind of chlorophyll Chlorophyll a: is the most common pigment used in photosynthesis by plants, photosynthetic protists, and cyanobacteria Carotenoid pigments: absorb blue- violet and blue-green but reflect yellow, orange, and red

From Sunlight to Photosynthesis Xanthophylls: reflect yellow, brown, purple, or blue light Anthocyanins: reflect red and purple light in fruits and flowers Phycobilins: reflect red or blue-green light and are accesory pigments found in red algae and cyanobacteria

From Sunlight to Photosynthesis The chlorophylls in green leaves mask the accessory pigments until autumn when the chlorophyll content declines A pigment absorbs light of specific wavelengths by acting as an antenna for receiving photon energy Photosynthesis begins when photosynthetic pigments absorb a photon of light

Key Terms Pigments: class of molecules that absorb photons in particular wavelengths only Chlorophyll a: major pigment in all but one group of photoautotrophs, absorbs violet and red light; reflects green and yellow light Chlorophyll b: reflects green and blue light

How Do Pigments Absorb Light? Pigment molecule has at least one array of atoms in which single covalent bonds alternate with double covalent bonds

How Do Pigments Absorb Light? Electrons of these atoms share one orbital that spans the entire array That array lets the pigment act like an antenna for receiving photon energy Each pigment absorbs light of specific wavelengths- corresponds to photon energy Photon absorbed by pigment if it has enough energy to boost an electron to a higher energy level

How Do Pigments Absorb Light? Excited electron returns to lower energy level almost immediately and emits energy as heat or as photon Will reach a chlorophyll that can give up excited electrons and start photosynthetic reactions

Review 1. List the different photosynthetic pigments 2. What wavelength of light is used in photosynthesis? 3. When is a photon absorbed by a pigment?

Answers 1. Photosynthetic pigments: chlorophyll a, chlorophyll b, carotenoids, xanthophylls, anthocyanins, phycobilins and 750nm wavelengths of light used 3. A photon is absorbed by a pigment if it has exactly enough energy to boost an electron of the pigment’s antenna to a higher energy level