Photosynthesis

Overview of Photosynthesis PS is the process whereby light energy is converted to chemical energy Photoautotroph: autotroph which uses light energy synthesize energy rich organic compounds Raw materials needed: sunlight, carbon dioxide, water Products: glucose, oxygen gas

Where does Photosynthesis Occur? PS occurs in leaves of plants (stems of cacti) Tremendous surface area for PS

Anatomy of a Leaf Waxy layer on surface of leaf minimizes water loss due to evaporation—called cuticle. Cuticle is produced by the upper epidermis. Pallisade Mesophyll: group of tightly packed cells rich in chloroplasts; where most PS occurs in leaf Spongy Mesophyll

Anatomy of a Leaf Vascular Tissue: xylem conducts water, phloem conducts sugars Stomata: microscopic pores used in gas exchange; stoma=single pore; carbon dioxide enters stomata and oxygen gas exits the stomata; primarily on underside of leaf

Anatomy of a Leaf Guard cells regulate the size of the opening

Chloroplasts Primary site of PS Double membrane Stroma: fluid-filled region where Calvin cycle (carbon fixation) occurs; where ‘dark reactions’ occur Thylakoids: disk-like structures containing chlorophyll---where ‘light reactions’ occur; organized into stacks of disks (like pancakes) called grana

Stages of Photosynthesis 1) light reaction; 2) dark reaction (aka ‘light independent reaction’ Light reactions occur at thylakoids, dark reactions in stroma

Overview of the Process Photon of light excites electrons in chlorophyll a The activated chlorophyll molecule passes the excited electrons down an electron transport chains, producing ATP and the electron carrier NADPH Energy from ATP and NADPH can be used to power carbon fixation

Overview of the Process So…the light reactions transform light energy into ATP and NADPH which can then be used to power carbon fixation in the stroma

The Light Reactions Pigments: light absorbing compounds clustered in units on thylakoids Chlorophyll a: primary pigment of photosynthesis Accessory pigments include all pigments other than chlorophyll a. Accessory pigments include chlorophyll b, carotenoid pigments

The Light Reactions All pigments within a unit can “capture” light, but most cannot excite the electrons The high energy electrons are passed from one pigment molecule to another with energy being lost at each pass. Eventually, the electron reaches the reaction center (primary electron acceptor).

The Light Reactions Two types of reaction centers: Photosystem I (PS I) and PS II. Each has a chlorophyll a molecule that accepts a particular wavelength of light. P680 is the reaction of PSII (max absorption at wavelength of 680 nm) PS700 is reaction center of PSI

The Light Reactions The light energy is used to form ATP and NADPH. The process of forming ATP using light energy is called photophosphoryla- tion (using light, ADP, and phosphates to produce ATP) Two types: Noncyclic Photophosphorylation (most plants) and cyclic photophosphorylation

Noncyclic Photophosphorylation P680 in PSII captures light and passes excited electrons down ETC to produce ATP P700 in PSI captures light and passes excited electrons down an ETC to produce NADPH A molecule of water is split by sunlight, releasing electrons, hydrogen, and free oxygen (photolysis)

Noncyclic Photophosphorylation Hydrogen ions (from splitting of water molecules) are pumped into a thylakoid compartment, creating an electrochemical gradient. As H+ then flow thru ATP synthases into the stroma, the energy is used to produce ATP.

Noncyclic Photophosphorylation Chemiosmosis: electrochemical gradient of H ions used to synthesize ATP

Cyclic Phosphorylation P700 in PSI captures light and passes excited electrons down ETC to produce ATP NO NADPH produced however Water is not split by sunlight

The Dark Reactions Aka ‘light independent reactions’, meaning light is not required for carbon fixation Occur in stroma of chloroplasts Carbon dioxide is ‘fixed’ to produce glucose Primary source of carbon is atmospheric carbon dioxide

The Dark Reactions An inorganic compound (carbon dioxide) is converted into energy rich glucose (energy rich organic compound—a carbohydrate)

The Dark Reactions Carbon fixation is endergonic process Reactions require ATP and NADPH. ATP delivers energy by transferring P groups, NADPH by donating H ions and electrons

The Calvin Cycle Aka ‘Dark Reactions’ Carbon dioxide enters Calvin cycle and combines with RUBP (ribulose bisphosphate—a 5-C compound). The product is an unstable 6-C compound, which splits into two molecules of PGA (3-C; phosphoglycerate). Because most plants form PGA, called C3

The Calvin Cycle PGA is phosphorylated to make PGAL. Some of this is used to make phosphorylated glucose (w/ P stuck on, glucose is primed for making starch or sucrose). The remainder is recycled to make more RUBP.

The C4 Pathway Corn, sugar cane, plants in hot, dry climates A different way to fix carbon dioxide Carbon dioxide combines with PEP (phosphoenolpyru- vate) to form oxaloacetate (a 4-C compound)

The C4 Pathway Oxaloacetate has a carbon dioxide molecule cleaved; in essence, original carbon dioxide used to make oxaloacetate is fixed, along with the carbon dioxide cleaved from oxaloacetate. So… these plants are very efficient at using a small amount of carbon dioxide, fixing it twice.

The C4 Pathway Stomata close on hot, dry days, so the plant gets less CO2 If oxygen builds up in leaf, and there is little carbon dioxide, photorespiration occurs (fixed carbon dioxide is wasted) With C4 pathway, plant can get by with much smaller amount of carbon dioxide since it is fixed twice

The CAM Pathway Cacti, succulents Keep stomata closed during day to conserve water May fix carbon dioxide formed from cell respiration Open stomata only at night, perform carbon fixation at night