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. 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.

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.

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

• 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.

Homework

•Read Chapter 10, 11

•Chapter 10 – Mon. Oct. 29

•Rs Lab – Tuesday – Oct. 30

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 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 Dark Reaction.

• Alternate Ps forms.

• Ps:Rs ratios.

U-Tube Links

• Light Reaction:

• http://www.youtube.com/watch?v=hj_W

KgnL6MI