Chapter 10 Photosynthesis

Photosynthesis u Process by which plants use light energy to make food (not energy).

Ps General Equation 6 CO H 2 O ---> C 6 H 12 O O 2 Requires: Chlorophyll Light

Early Question (Just listen) u Does the Oxygen in sugar come from the CO 2 or from the H 2 O ? u Model: CO H 2 O ----> CH 2 O + O 2

Proof u Used 18 O as a tracer. CO H 2 O ----> CH 2 O + O 2 Both experiments confirm that water is split. u O 2 is a waste product of Ps that altered life on earth.

Ps u Has two chemical reactions: 1. Light Reaction 2. Dark Reaction Names are from “light” as a requirement, not where or when they occur.

Light u A form of electromagnetic radiation. u Visible light has the right energy for use in Ps.

Too Hot Too Cold Just Right

Action Spectrum u Not all colors are useable to the same degree for Ps. u Red and Blue light - absorbed and used in Ps. u Green light - reflected or transmitted.

Photosynthesis Pigments 1. Chlorophylls 2. Accessory Pigments

Chlorophylls u Has CHON and Mg. u Several types possible. u Molecule has a lipophilic tail- can go through membranes u Contains Mg in a reaction center.

Accessory Pigments u Absorb light energy and transfer it to chlorophyll. u Ex: Carotene (orange). Xanthophyll (yellow)

Fall Leaf Colors u Chlorophyll breaks down. u N and Mg salvaged and moved into the stem for next year. u Accessory pigments remain behind, giving the various fall leaf colors.

Chloroplast Structure (Annotate your diagram) u Double outer membrane. u Inner membrane folded and stacked into grana. (Each disk is a thylakoid) u Stroma - liquid that surrounds the thylakoid membranes.

Photosystems u Collection of pigments that serve as a light trap. u Made of chlorophyll and the accessory pigments. u Two types known: PSI, PSII

Cyclic Photophosphorylation u Uses PSI only. u Produces ATP. u Requires light.

Linear or Noncyclic Photophosphorylation u Uses PSI and PSII. u Splits water, releasing H +, a pair of e -, and O 2. u Produces ATP and NADPH. (e - carrier similar to NADH)

Light Reaction u Same thing as Noncyclic Photophosphorylation. u Location - grana of the chloroplast. u Function - to split water and produce ATP and NADPH.

Light Reaction u Light u Water u ADP u NADP + uO2uO2 u ATP u NADPH RequirementsProducts

Chemiosmosis Model u Light energy is used to pump H + across a membrane. u When the H + diffuses back, ATP is generated. u The chloroplast produces ATP in the same manner as the mitochondria in Rs.

Chemiosmosis u H + are pumped into the thylakoid space. u ATP and NADPH are made when the H + diffuse into the stroma.

Comment u There can be a 3 pH unit difference between the thylakoid space and the stroma.

Dark Reactions u How plants actually make food (carbohydrates). u Don't require light directly to run. u Also known as the Calvin cycle or C3 Ps.

Dark Reactions u Function - to use ATP and NADPH (from light rxn) to build food from CO 2 u Location - stroma of the chloroplast.

Rubisco u Ribulose BisPhosphate Carboxylase. u Enzyme that adds CO 2 to an acceptor molecule. u Most important enzyme on earth.

C3 Ps u 6 CO 2 u 18 ATP u 12 NADPH u C 6 H 12 O 6 u 18 ADP u 12 NADP + RequirementsProducts

END of PART 1!!

Photorespiration u When Rubisco accepts O 2 instead of CO 2 as the substrate. u Generates no ATP. u Decreases Ps output by as much as 50%.

Photorespiration u Happens when conditions are too dry that stomata close u May reflect a time when O 2 was less plentiful and CO 2 was more common.

Stomata u Holes in the plant leaf that allow for gas exchange u Close them to prevent water loss

Alternate Ps Methods 1. C4 Ps 2. CAM Ps

C4 Ps u Uses a different enzyme to initially capture CO 2 u Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells.

PEP Carboxylase u Enzyme used for CO 2 capture in C4 Ps. u Can use CO 2 down to 0 ppm. u Prevents photorespiration.

C4 Ps u Found in 19 plant families. u Characteristic of hot regions with intense sunlight, but plenty of water u Examples - sugarcane, Bermuda grass, crab grass

C3 Ps vs C4 Ps Photorespiration Shade to full sun High water use Cool temperatures Slow to moderate growth rates Cool season crops No Photorespiration Full sun only Moderate water use Warm temperatures Very fast growth rates Warm season crops

CAM Ps u Crassulacean Acid Metabolism u Found in plants from arid conditions where water stress is a problem. u Examples - cacti, succulents, pineapples, many orchids.

CAM Ps u Open stomata at night to take in CO 2. u The CO 2 is stored as a C4 acid. u During the day, the acid is broken down and CO 2 is fixed into sugar.

CAM plants u Avoid H 2 O stress by keeping stomates closed during the day. u Generally have slow growth.

Ps to Rs ratios u Predict what will happen u Ps > Rs u Ps = Rs u Ps < Rs

Ps:Rs Ratios u Reflect a plant’s balance in making food and using food. 1. Ps > Rs, energy available for growth and reproduction. 2. Ps = Rs, no growth, but don’t die either. 3. Ps < Rs, death by starvation

Comments - Ps:Rs u Rs happens 24 hours a day. u Ps only in light. u If Ps < Rs, best solution is to increase the amount of light.

Factors That Affect Ps 1. Light - quantity and quality. 2. Temperature - too hot or too cold. 3. CO 2 - often limits C3 plants. 4. Minerals - especially N P K and Mg.

Importances of Ps 1. Food - either directly or indirectly comes from plants. 2. Oxygen in the air. 3. CO 2 balance. 4. Plant products. 5. Life on Earth.

Summary u Know the main Ps equation. u Know Light Reaction. u Know Dark Reaction. u Alternate Ps forms. u Ps:Rs ratios.