Chapter 10 Photosynthesis Photosynthesis u Process by which

Chapter 10 Photosynthesis

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

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

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

Proof u. Used 18 O as a tracer. CO 2 + 2 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: a redox process u. Hydrogens are added to Carbons. u. Water is a source for the Hydrogens. u. Complex covalent bonds are made.

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.

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

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

Photosynthesis Pigments 1. Chlorophylls 2. Accessory Pigments

Chlorophylls u. Has CHON and Mg. u. Several types possible. u. Molecule has a lipophilic tail that allows it to dissolve into 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 u. Double outer membrane. u. Inner membrane folded and stacked into grana. 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.

Noncyclic Photophsphorylation 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

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

Light Reaction Requirements u. Light u. Water u. ADP + Pi u. NADP+ Products u. O 2 u. ATP u. NADPH

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

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

u. Let’s see the light reaction in motion u. Go to here uhttp: //www. science. smith. ed u/departments/Biology/Bio 23 1/ u. And click on “ light reaction”

Dark Reactions u. How plants actually makes food (carbohydrates). u. Don't require light directly to run. u. Also known as the Calvin cycle or C 3 Ps.

Dark Reaction u. Function - to use ATP and NADPH to build food from CO 2 u. Location - stroma of the chloroplast.

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

C 3 Ps Requirements u 6 CO 2 u 18 ATP u 12 NADPH Products u. C 6 H 12 O 6 u 18 ADP + 18 Pi u 12 NADP+

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. May reflect a time when O 2 was less plentiful and CO 2 was more common.

u. Let’s see the dark reaction, Calvin Cycle, in motion u. Go to here uhttp: //www. science. smith. edu/de partments/Biology/Bio 231/ u. And click on “Carbon Fixation in Photosynthesis”

Alternate Ps Methods 1. C 4 Ps 2. CAM Ps

C 4 Ps u. Uses a different enzyme to initially capture CO 2 u. Separates CO 2 capture from carbon fixation into sugar. u. Still uses C 3 Ps to make sugar, but only does so in the bundle sheath cells.

PEP Carboxylase u. Enzyme used for CO 2 capture in C 4 Ps. u. Can use CO 2 down to 0 ppm. u. Prevents photorespiration.

C 4 Ps u. Found in 19 plant families. u. Characteristic of hot regions with intense sunlight. u. Examples - sugarcane, Bermuda grass, crab grass

C 3 Ps vs Photorespiration Shade to full sun High water use Cool temperatures Slow to moderate growth rates Cool season crops C 4 Ps 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 C 4 acid. u. During the day, the acid is broken down and CO 2 is fixed into sugar.

CAM plants u. Tissues decrease in p. H over night, rise in p. H during day. u. Avoid H 2 O stress by keeping stomates closed during the day. u. Generally have slow growth.

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. Plants overwinter on stored food made when Ps > Rs. 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 C 3 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. 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.

Practice Question u. Explain how the molecular reactions of photosynthesis transform light energy into chemical bond energy. Include in your discussion the relationship between chloroplast structure and light and dark reactions.