Photosynthesis

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

Autotrophs u Organisms that can manufacture their own food from inorganic molecules u Self-feeding organisms

PSN General Equation 6 CO H 2 O + light C 6 H 12 O O 2 Requires: Chlorophyll

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

PSN u Has two chemical reactions: 1. Light Dependent Reactions 2. Light Independent Reactions 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 necessary energy for use in PSN.

Too Hot Too Cold Just Right

Action Spectrum u Not all colors are useable to the same degree for PSN. u Red and Blue light - absorbed and used in PSN. 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.

stroma thylakoid sacs

Photosystems u Collection of pigments that serve as a light trap. u Made of chlorophyll and the accessory pigments. u Two photosystems are known: Photosystem I,which uses P 700 ( a type of chlorophyll a energized by the frequency 700 nm), and Photosystem II which uses P 680 ( a type of chlorophyll a energized by the frequency 680 nm)

Noncyclic Photophsphorylation u Uses Photosystem I and Photosystem II. u Splits water, releasing H +, a pair of e -, and O 2. u Produces ATP and NADPH. (e - carrier similar to NADH)

Light Dependent Reactions Photosystem II: Electrons trapped by P 680 are energized by light and accepted by the “primary” electron acceptor

As electrons move down the Electron Transport Chain they lose energy… this energy is used to produce ATP. Some transport proteins, like ferredoxin and cytochrome, include nonprotien parts containing iron. Light Dependent Reactions

Electrons are again energized by light and accepted by a primary acceptor

Light Dependent Reactions Electrons lose energy as they move down the ETC … the energy is used to make NADPH…. The 2 electrons lost to Photo- system II are replaced when water is split…. This is called “Photolysis”.

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

Light Dependent Reaction u Light u Water u ADP + Pi u NADP + u O 2 (released into the environment) u ATP u NADPH RequirementsProducts H 2 O + ADP + P i + NADP + + light ATP + NADPH + O 2 + H +

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

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

Calvin-Benson Cycle…. Light Independent Reactions u How plants actually makes food (carbohydrates). u Don't require light … energy supplied by ATP made during Light Dependent Reactions u Also known as the Calvin cycle or C3 Ps.

Calvin-Benson Cycle…. Light Independent Reactions u Function - to use ATP and NADPH to build food from CO 2 u Location - stroma of the chloroplast.

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

Calvin-Benson Cycle… Light Independent Reactions CO 2 (1C) combines with RuBP (5C) to form 2 PGA (3C) Carboxylation: 6CO RuBP produce 12 PGA

The energy from ATP and the H+ from the NADPH are used to convert 12 PGA to 12 PGAL (G3P)

6 ATP are used to convert 10 PGAL to 6 RuBP… the remaining 2 PGAL are used to make Glucose.

C3 PSN u 6 CO 2 u 18 ATP u 12 NADPH u C 6 H 12 O 6 u 18 ADP + 18 Pi u 12 NADP + RequirementsProducts 6CO ATP + 12 NADPH + + H + 18 ADP + 18 P i 12 NADP Glucose

Cyclic Photophosphorylation u Uses Photosystem I only. u Requires light u Electrons from Photosystem I are used to produce ATP…. the electrons then return to Photosystem 1…. They are not incorporated into NADPH as in Non-Cyclic Photophosphorylation

e-e-

Photorespiration u Rubisco is the most abundant protein on earth. u Occurs when Rubisco accepts O 2 instead of CO 2 as the substrate. u Generates no ATP. u Decreases PSN output by as much as 50%. u Products of Photorespiration are broken down by peroxisomes… found near the chloroplasts.

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

Alternate PSN Methods 1. C4 PSN 2. CAM PSN

C4 PSN u C4 plants high rate of photosynthesis reduces to overall loss of water…. Therefore they are found in dry, hot climates…. This gives them an advantage over C3 plants.

C4 PSN u Uses a different enzyme, PEP, to initially capture CO 2 u Still uses C3 PSN to make sugar, but only does so in the bundle sheath cells…. The bundle cells are near the water source… this makes the C4 plants extremely efficient.

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

C4 PSN u Found in 19 plant families. u Characteristic of hot regions with intense sunlight. 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 PSN 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 PSN 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 Tissues decrease in pH over night, rise in pH during day. u Avoid H 2 O stress by keeping stomata closed during the day. u Generally have slow growth.

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

Importance of Photosynthesis 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.