Chapter 10 Photosynthesis

Photosynthesis

Process by which plants use light energy to make food (not energy).

Ps General Equation

6 CO2 + 6 H2O ---> C6H12O6 + 6 O2

Requires:

Chlorophyll

Light

Early Question(Just listen)

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

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.

Photosynthesis Pigments

1. Chlorophylls

2. Accessory Pigments

Chlorophylls Has CHON and Mg. Several types possible. Molecule has a lipophilic tail-

can go through 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(Annotate your diagram)

Double outer membrane. Inner membrane folded and

stacked into grana. (Each disk is a thylakoid)

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 Photophosphorylation

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

Same thing as Noncyclic Photophosphorylation.

Location - grana of the chloroplast.

Function - to split water and produce ATP and NADPH.

Light Reaction

Light Water ADP NADP+

O2

ATP NADPH

Requirements Products

Chemiosmosis Model Light energy is used to pump

H+ across a membrane. When the H+ diffuses back,

ATP is generated. The chloroplast produces ATP

in the same manner as the mitochondria in Rs.

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 make food

(carbohydrates). Don't require light directly to

run. Also known as the

Calvin cycle or C3 Ps.

Dark Reactions

Function - to use ATP and NADPH (from light rxn) 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 12 NADP+

Requirements Products

END of PART 1!!

Photorespiration

When Rubisco accepts O2

instead of CO2 as the substrate.

Generates no ATP. Decreases Ps output by as

much as 50%.

Photorespiration

Happens when conditions are too dry that stomata close

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

Stomata

Holes in the plant leaf that allow for gas exchange

Close them to prevent water loss

Alternate Ps Methods

1. C4 Ps

2. CAM Ps

C4 Ps Uses a different enzyme to

initially capture CO2

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, but plenty of water

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

Avoid H2O stress by keeping stomates closed during the day.

Generally have slow growth.

Ps to Rs ratios

Predict what will happen Ps > Rs Ps = Rs Ps < Rs

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. 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 N P K 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.