Introduction to the Light Reactions of Photosynthesis

"Life is woven out of air by light"

• Life on Earth is solar powered.Introduction

PHOTOSYNTHESIS

• Plants and other autotrophs are the producers

of the biosphere.

• On a global scale, photosynthesis is the most important process to the welfare of life on Earth.

Where does photosynthesis occur?

• Green parts of plants

• Leaves are the specialized plant organs where most photosynthesis occurs

• A typical leaf parenchyma cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long.

Parts of the Chloroplast• Thylakoids

– Grana

– Chlorophyll• a - 2-3x, 430nm (violet-

blue), 662nm (orange-red)

• b - 453nm (blue), 642nm (orange)

– Accessory pigments (i.e.carotenoids, xanthophylls)

• Stroma

Photosynthesis

Two steps:

1. Light reactions (=light dependent reactions)

2. Light independent reactions (= Calvin Cycle, Calvin-Benson Cycle, Dark Reactions)

PHOTOSYNTHESIS

6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 + 6H2O

Major Components of Light Reactions

6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +

6H2O

Light

– Where does it come from?

– How does it get into the leaf?

– How does it get into the mesophyll cell?

– Why is it needed?

• When light meets matter, it may be reflected, transmitted, or absorbed.

• The light reactions work with those wavelengths of light that are absorbed.

• In the thylakoids are several pigments that differ in their absorption spectrum.

• Pigments are grouped into two light collecting complexes called Photosynthetic Units (PSI, PSII).

2 parts to each Photosystem

1. Antenna molecules (many)

2. Reaction center molecules (2 chlorophyll a molecules)

Major Components of Light Reactions

6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +

6H2O

Water

– Where does it come from?

– How does it get into the leaf?

– How does it get into the mesophyll cell?

– Why is it needed?

Overview of Light Reactions Part I: Photosystem II

• Light strikes chlorophyll• Reaction center molecules excited to a higher

energy level• Excited electrons captured by electron

acceptor• Hydrogen from water replaces the ‘hole” left

by excited electrons; oxygen released• Electron acceptor passes excited electrons to

another acceptor…down an electron transport chain, ATP formed

Overview of Light Reactions Part 2: Photosystem I

• Light strikes photosystem pigments

• Reaction molecules excited to a higher energy level

• Excited electrons captured by electron acceptor NADP+ [= nicotinamide adenine dinucleotide phosphate]--> NADPH formed

• Excited electrons from reaction center replaced by electrons from PII electron transport chain; no oxygen is released and no water is needed

Bottom Line of Light Reactions

• 6CO2 + 12H2O + light energy --> C6H12O6 + 6O2 +

6H2O

• Energy from sun is captured and converted to chemical form (ATP, NADPH)

• Oxygen is released

Review Questions

• Why is water needed?

Review Questions

• Why is water needed?• Supplies the hydrogens to replace the electrons

“lost” from the chlorophyll

Review Questions

• What energy molecules are formed by the end of the light reactions?

Review Questions

• What energy molecules are formed by the end of the light reactions?– ATP and NADPH

Review Questions

• What is the source of energy for ATP and NADPH?

Review Questions

• What is the source of energy for ATP and NADPH?

–Sunlight

Review Questions

• What wavelengths of light are most important for the light reactions?

Review Questions

• What wavelengths of light are most important for the light reactions?– Violet-blue (400-500nm) and orange-red (600-

700nm)

Review Questions

• Is oxygen needed for the light reactions?

Review Questions

• Is oxygen needed for the light reactions?– No, it is a waste product

Is this all?

• NO!– The light independent reactions follow the light

dependent reactions.– The energy molecules formed in the light

dependent reactions are used, along with the carbon from carbon dioxide, to make glucose.

Why do plants grow better under certain lighting conditions than others?

What factors can affect photosynthesis?

Hydroponic Farming

Pre-Lab Tasks

1. Read the information on photosynthesis and leaf anatomy from your textbook

2. Complete online tutorial on photosynthesis

3. Complete the Planning Form for “Why Do Plants Grow Better Under Certain Lighting Conditions than Others” and turn in by the deadline

Experimental Set Up

Cuvette with buffered spinach solution with DPIP (blue indicator dye) added

Flask filled with H2O and covered with cellophane (blue, red, or clear)

Desk Lamp with 60w or 100w bulb

Other Materials

Light sensor (Vernier Lab Pro)Measures light intensity for visible light in lux

Vernier Lab Pro ColorimeterMeasures the amount of light transmitted through a sample Cellophane film

Allows only certain wavelengths of light to be transmitted through the film

Blue filter = 413nm; 107 luxRed filter = 647 nm; 161 lux

More About DPIP (2,6-dichloropheno-indophonl)

• Replaces NAD– Blue (oxidized) [i.e. NAD]– Colorless (reduced) [i.e. NADPH]

• What will the DPIP allow us to measure?

Other Questions

• What does the desk lamp supply to the experiment?

• What information can the light sensor provide?

• What is the purpose of covering the flask with cellophane?

• Why is the flask filled with water?

• What information will the colorimeter provide?