Photosynthesis Chapter 8

Light & Color  Primary colors of light: red, green, blue  White light = equal “amounts” of red, green, and blue  Pigments absorb certain colors of light and reflect others  Primary colors of pigment: yellow, cyan, magenta  Black = equal amounts of yellow, cyan, and magenta  White = no pigment  Sun produces white light  Objects appear the color of the wavelength of light they reflect

Plant Pigments  Plants contain a number of pigments, yielding a variety of colors and shades  Chlorophyll a is the primary pigment involved in photosynthesis  Blue-green in color  Accessory pigments = all other pigments

The Chloroplast

Chlorophyll and Photosystems  Chlorophyll is embedded in the thylakoid membrane  Photosystem = collection of pigments in thylakoid membrane  Photosystems capture energy from the sun to carry out reactions of photosynthesis

Overview of Photosynthesis  Overall equation: 12 H 2 O + 6 CO 2  6 H 2 O + 6 O 2 + C 6 H 12 O 6  Photosynthesis is a two-stage process:  The Light Reactions  Hydrolyze water to obtain electrons and produce ATP and NADPH, oxygen is a waste product  The Calvin Cycle  Products of the light reactions power this stage to create sugar from carbon dioxide light energy

The Light Reactions 1. Water is hydrolyzed by chlorophyll a when excited by light – Photosystem II  Electrons are liberated  Two split waters = one oxygen molecule 2. Electrons pass through a series of proteins in the thylakoid membrane producing ATP  Electron Transport Chain 3. Electrons used to reduce NADP+ into NADPH – Photosystem I  NADPH is an electron carrier that delivers electrons to the Calvin Cycle

The Light Reactions

The Calvin Cycle 1. Carbon dioxide is fixed to RuBP (ribulose biphosphate), a 5-carbon molecule  Catalyzed by RuBisCO 2. Newly created 6-carbon molecule with two phosphates splits apart  3-carbon molecules created 3. ATP hydrolyzed to phosphorylate the 3-carbon molecule 4. NADPH is oxidized to NADP+ when it reduces the 3- carbon molecule  End product is a 3-carbon sugar = Glyceraldehyde 3-phosphate (G3P)

The Calvin Cycle

Photosynthesis Animation (view animation file on public server)

Arid Environments & Carbon Fixation  Plants close stomata on hot, dry days to avoid dehydration  In leaf, CO 2 level falls, O 2 level rises  Rubisco attaches oxygen to RuBP = photorespiration  C 4 and CAM plants use alternative methods for carbon fixation

C 4 and CAM Plants