Ch 10 NOTES (part 1): Photosynthesis in Nature

10.1 – Photosynthesis converts light energy to the chemical energy of food.

PHOTOSYNTHESIS: ● PHOTOSYNTHESIS = the capture of light energy & the conversion of this energy to stored chemical energy (in the form of sugar) -AUTOTROPHS: “producers”; capable of producing their own food PHOTOAUTOTROPHS: use light energy to synthesize food (e.g. plants)

● Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes ● These organisms feed not only themselves but also the entire living world… thank you plants! 

Plants Unicellular protist Purple sulfur bacteria Multicellular algae Cyanobacteria 40 µm

● HETEROTROPHS: “consumers”; live on compounds produced by other organisms -animals that eat plants or other animals ● Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen

**some heterotrophs are: -decomposers: live on dead matter

Chloroplasts: The Sites of Photosynthesis in Plants ● Leaves are the major locations of photosynthesis ● Their green color is from CHLOROPHYLL, the green pigment within chloroplasts ● Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast ● Through microscopic pores called STOMATA, CO2 enters the leaf and O2 exits

CHLOROPLASTS: ● organelles in which photosynthesis occurs ● found primarily in leaves (about 500,000 per mm2) ● contain green pigment CHLOROPHYLL (absorbs light energy) ● found mainly in mesophyll (tissue inside leaf) ● a typical mesophyll cell has 30-40 chloroplasts

Leaf cross section Vein Mesophyll Stomata CO2 O2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm

● CO2 enters and O2 leaves through STOMATA ● water is delivered to leaves from the roots via veins

CHLOROPLAST: ● enclosed by 2 membranes ● STROMA = fluid within the chloroplast ● THYLAKOID MEMBRANES are interconnected & contain the thylakoid space, or lumen (are usually in stacked columns called GRANA) ● chlorophyll is stored in the thylakoid membranes

PATHWAYS OF PHOTOSYNTHESIS ● TOTAL equation for photosynthesis: 6CO2 + 12H2O + light  C6H12O6 + 6O2 + 6H2O energy ● NET EQUATION: 6CO2 + 6H2O + light  C6H12O6 + 6O2

The Splitting of Water ● Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

Tracking Atoms Through Photosynthesis: Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2

Photosynthesis as a Redox Process ● Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

(the required energy boost comes from light energy) ● like respiration, photosynthesis is a series of redox reactions…BUT, the electron flow is reversed!…the electrons increase in potential energy as they move from water  sugar… an uphill process! (the required energy boost comes from light energy)

Two Stages of Photosynthesis: 1) the LIGHT REACTIONS   2) the CALVIN CYCLE (a.k.a. the “dark reactions” or light-independent reactions)

The Two Stages of Photosynthesis: A Preview ● Photosynthesis consists of the light reactions (the photo part) and the Calvin cycle (the synthesis part)

LIGHT REACTIONS: ● convert solar energy  chemical energy ● light is absorbed by chlorophyll and drives a transfer of electrons and H from water to an acceptor called: NADP+ ● water is split and O2 is produced as a “waste product” ● NADP+ is reduced to NADPH…ALSO, ATP is generated via phosphorylation of ADP (photophosphorylation!)

CALVIN CYCLE: ● CO2 from air is incorporated into organic molecules by carbon fixation ● NADPH and ATP from the light reactions power the production of sugar ● also called: the “dark” reactions or light-independent reaction

● Light reactions occur in the THYLAKOIDS Locations of the Photosynthesis Reactions: ● Light reactions occur in the THYLAKOIDS ● Calvin cycle occurs in the STROMA

H2O Light LIGHT REACTIONS Chloroplast

H2O Light LIGHT REACTIONS ATP NADPH Chloroplast O2

H2O CO2 Light NADP+ ADP + CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2