Chapter 10 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

Early 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

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. Calvin Cycle

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.

Plants without chlorophyll

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.

Linear or 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

• Light

• Water

• ADP + Pi

• NADP+

• O2

• ATP

• NADPH

Requirements Products

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.

Calvin Cycle

• How plants actually makes food (carbohydrates).

• Don't require light directly to run.

• Also known as C3 Ps.

Calvin Cycle

• 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.

• Most abundant enzyme on earth.

Dark Reactions/Calvin Cycle

• 6 CO2

• 18 ATP

• 12 NADPH

• C6H12O6

• 18 ADP + 18 Pi

• 12 NADP+

Requirements Products

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 survives.

3. Ps < Rs, death by starvation

Comments - Ps:Rs

• Rs happens 24 hours a day.

• Ps only in light.

• Plants overwinter on stored food made 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 Calvin Cycle.

• Alternate Ps forms.

• Ps:Rs ratios.

Practice FRQ Question (long version)

• 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 reactions and Calvin cycle.

U-Tube Links

• Light Reaction:

• http://www.youtube.com/watch?v=hj_WKgnL6MI