Photosynthesis

Photosynthesis Process by which plants use light energy to make food. A reduction process that makes complex organic molecules from simple molecules.

Ps General Equation 6 CO2 + 6 H2O ---> C6H12O6 + 6 O2 Requires: Chlorophyll Light

Question Does the Oxygen in sugar come from the CO2 or from the H2O ? Model: CO2 + 2 H2O ----> CH2O + O2

Proof Used 18O as a tracer. CO2 + 2 H2O ----> CH2O + O2 Both experiments confirm that water is split. O2 is a waste product of Ps that altered life on earth.

Ps: a redox process Hydrogens are added to Carbons. Water is a source for the Hydrogens. Complex covalent bonds are made.

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

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

Too Hot Too Cold Just Right

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

Comment In oceans, red light is lost or filtered out early because it has lower energy. Only blue light which has higher energy can reach the lower depths.

Result Many deep ocean fish are bright red in color. Why? They can’t be seen because there is no red light to reflect their color.

Photosynthesis Pigments 1. Chlorophylls 2. Accessory Pigments

Chlorophylls Has CHON and Mg. Several types possible. Molecule has a lipophilic tail that allows it to dissolve into membranes. Contains Mg in a reaction center.

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

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

Chloroplast Structure Double outer membrane. Inner membrane folded and stacked into grana. Stroma - liquid that surrounds the thylakoid membranes.

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

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

Noncyclic Photophsphorylation Uses PSI and PSII. Splits water, releasing H+, a pair of e-, and O2. Produces ATP and NADPH. (e- carrier similar to NADH)

Light Reaction

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

Light Reaction Requirements Products Light Water ADP + Pi NADP+ O2 ATP NADPH

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

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

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

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

Dark Reaction Function - to use ATP and NADPH to build food from CO2 Location - stroma of the chloroplast.

Rubisco Ribulose BisPhosphate Carboxylase. Enzyme that adds CO2 to an acceptor molecule. Most important enzyme on earth.

C3 Ps Requirements Products 6 CO2 18 ATP 12 NADPH C6H12O6 18 ADP + 18 Pi 12 NADP+

Photorespiration When Rubisco accepts O2 instead of CO2 as the substrate. Generates no ATP. Decreases Ps output by as much as 50%.

Photorespiration May reflect a time when O2 was less plentiful and CO2 was more common.

Alternate Ps Methods 1. C4 Ps 2. CAM Ps

C4 Ps Uses a different enzyme to initially capture CO2 Separates CO2 capture from carbon fixation into sugar. Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells.

PEP Carboxylase Enzyme used for CO2 capture in C4 Ps. Can use CO2 down to 0 ppm. Prevents photorespiration.

C4 Ps Found in 19 plant families. Characteristic of hot regions with intense sunlight. 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 Crassulacean Acid Metabolism Found in plants from arid conditions where water stress is a problem. Examples - cacti, succulents, pineapples, many orchids.

CAM Ps Open stomata at night to take in CO2. The CO2 is stored as a C4 acid. During the day, the acid is broken down and CO2 is fixed into sugar.

CAM plants Tissues decrease in pH over night, rise in pH during day. Avoid H2O stress by keeping stomates closed during the day. Generally have slow growth.

Ps:Rs Ratios 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 Rs happens 24 hours a day. Ps only in light. Plants overwinter on stored food when Ps > Rs. 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. CO2 - often limits C3 plants. 4. Minerals - especially NPK and Mg.

Importances of Ps 1. Food - either directly or indirectly comes from plants. 2. Oxygen in the air. 3. CO2 balance. 4. Plant products. 5. Life on Earth.

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