Photosynthesis: Introduction

CO2

H2O

light

carbohydrate

O2

6CO2 + 6H2O C6H12O6 + 6O2

Photosynthesis is the initial source of energy for almost all biological activities

Light

Heat

ADPNADP+

NADPHATP

C(H2O)O2

CO2H2O

CO2H2O

C(H2O)O2

ADPNAD+

NADHATP

SynthesisMovementIon pumpsGrowthReproduction

Photosynthesis: plants,photosynthetic bacteria

Respiration: plants,animals, bacteria, etc.

Compare photosynthesis to respiration: Respiration: C6H12O6 + 6O2 6CO2 + 6H2O

Go’ = -2840 kJ/mol Photosynthesis in green plants: 6CO2 + 6H2O C6H12O6 + 6O2

Go’ = +2840 kJ/mol Unfavorable Go’ explains the need for energy input. That is the role of light.

In higher plants, photosynthesis occurs in chloroplasts

The aquatic plant, Elodea You may see dark grana inside some chloroplasts.

Chloroplast ofZea mays

Under an electron microscope, chloroplasts have amore complex structure.

stroma grana lamellae stroma lamellae

The concentration of grana varies among chloroplasts.

Chloroplast ofPisum sativum

Photosynthesis occurs ingreen plants,cyanobacteria,and some other bacteria

Photosynthesis as described earlier: 6CO2 + 6H2O C6H12O6 + 6O2

Oxidation of D in H2D; reduction of C and O in CO2

Compare photosynthesis in plants and bacteria

Plants (divided by 6, H2O added to each side) CO2 + 2 H2O C(H2O) + O2 + H2O

Green sulfur bacteria CO2 + 2 H2S C(H2O) + 2S + H2O

Purple non-sulfur bacteria CO2 + 2 CH3CHOHCH3 C(H2O) + 2 CH3COCH3 + H2O isopropanol acetone CO2 + 2 CH3CHOHCOOH C(H2O) + 2 CH3COCOOH + H2O lactate pyruvate

Generalize: CO2 + 2 H2D C(H2O) + 2 D + H2O D = H/electron donor

How to measure photosynthesis

Starch production

Van Helmont (1577-1644): A willow that grew for 5 years in acontainer gained more than a half centner in weight (ca 60 lbs), whereas the soil lost only 2 ounces.

Illuminated leaves can be boiled in ethanol (to break membranes),then stained for starch with iodine.

6CO2 + 6H2O C6H12O6 + 6O2

How to measure photosynthesis

Starch production

Potato starch visualized by light microscopy…with a light iodine stain under polarized light

6CO2 + 6H2O C6H12O6 + 6O2

How to measure photosynthesis

O2 production

Priestly (1733-1804): Green plants give off oxygen and thus improvethe air.

Engelmann (1843-1909): produced the first action spectrum of photo-synthesis using Spirogyraalgae and aerotactic (oxygen-sensing) bacteria.

6CO2 + 6H2O C6H12O6 + 6O2

How to measure photosynthesis

CO2 uptake

Amoroso et al. Plant Physiol. 116:193 (1998)

6CO2 + 6H2O C6H12O6 + 6O2

How to measure photosynthesis

Dye bleaching (DCPIP reduction; Hill reaction)

(ZZZ = e- transport inhibitor)

blue colorless

Light on

Photosynthesis as described earlier: 6CO2 + 6H2O C6H12O6 + 6O2

Oxidation of D in H2D; reduction of C and O in CO2

Compare photosynthesis in plants and bacteria

Plants (divided by 6, H2O added to each side CO2 + 2 H2O C(H2O) + O2 + H2O

Green sulfur bacteria CO2 + 2 H2S C(H2O) + 2S + H2O

Purple non-sulfur bacteria CO2 + 2 CH3CHOHCH3 C(H2O) + 2 CH3COCH3 + H2O isopropanol acetone CO2 + 2 CH3CHOHCOOH C(H2O) + 2 CH3COCOOH + H2O lactate pyruvate

Generalize: CO2 + 2 H2D C(H2O) + 2 D + H2O D = H/electron donor

Oxidation of D in H2D; reduction of C and O in CO2

These reactions are separable. Dye bleaching, which occurswith O2 production, shows that the oxidation of H2O (or H2S,etc.) does not depend on the reduction of CO2

2 DCPIP (blue) + 2 H2O 2 DCPIPH2 (colorless) + O2

In the bacterial and plant photosynthetic reactions,

CO2 + 2 H2D C(H2O) + 2 D + H2O

The separate reactions are,

2 H2D light 2 D

2 NADP+ 2 NADPH + H+ 3 ADP + 3 Pi 3 ATP

C(H2O) + H2O (dark) CO2

The light reactions produce H. (reductant) and energy.

Summary

The reaction of photosynthesis can be summarized as:6CO2 + 6H2O C6H12O6 + 6O2or better,6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

But the product is starch, not C6H12O6

Photosynthesis can be measured as starch production, O2production, CO2 uptake, or dye reduction

The oxidation of H2O and the reduction of CO2 are separable

If a sample of algae, when illuminated, releases 3 x 10-5

mol O2/min, how rapidly will it incorporate CO2? How muchstarch will be formed per minute? How many photons of lightwill be needed per minute?