Photosynthesis As a Metabolic Process

What is it?

• Process where green plants (and certain photosynthetic bacteria) absorb sunlight and convert it to stored chemical energy

• Energy transformation from radiant to chemical energy (Laws of Thermodynamics apply)

• A knowledge of both chemistry and physics are relevant to an understanding of photosynthesis

Summary Equation

CO2 + H2O (CH2O)n + O2

• State of knowledge in 1900• Energy requiring reaction • Green plants and sunlight

needed• Oxidation/reduction reaction

Global Impact• CO2 and O2 are recycled

by plants• Photosynthesis changed

the atmosphere for aerobes

• Photosynthesis ties up greenhouse gases (CO2) in vegetation

Light absorption is primarily carried out by chlorophyll

Chlorophyll has a characteristicabsorption spectrum

Why are green plants green?

Two types of chlorophyll “a” and “b”

What is the advantage for plants tohaving two types of chlorophyll?

What is the purpose of accessoryPigments?

Photosynthetic pigments are found in the chloroplast in protein-pigment complexes

• Photosynthetic pigments are found in thylakoid membranes

Thylakoids have three types of pigment complexes

• Photosystem I (mostly chl a) - P680 reaction center• Photosystem II (mostly chl a) - P700 reaction center• LHC (Light Harvesting Complex) (~1:1 chl a to b)• Role of LHC

What happens to light filtered through a plant canopy?

• Role of LHC?• Chl a/b ratio

What happens when a photon (of correct energy) is absorbed by chlorophyll?

• Electron returns to ground state (heat emitted)

• Electron returns to ground state (photon emitted as fluorescence)

• Electron transferred to an acceptor molecule (photochemistry)

Photon energy absorbed by electronleads to an unstable “excited state”

How do the photosystems cooperate?

What are the products of the light reactions?

• NADPH

• ATP

• O2

The synthesis of carbohydrates in photosynthesis involves the reduction of CO2

• CO2 (CH2O)n

• Where does the energy come from?• Metabolic process called the Calvin Cycle

Key reaction in the Calvin Cycle

Rubisco – the most abundant protein on earth

Calvin Cycle has two essential tasks

Rubisco has both carboxylase and oxygenase activities

• O=C=O and O=O look similar to enzyme active site

• Oxygenase reaction:

O2 + RUBP 3 PGA + 2 phosphoglycolate (2C)

2(2 phosphoglycolate) 3 PGA + CO2

• Release of CO2 is photorespiration

Impact of photorespiration?•Air: 21% O2 and 0.03% CO2

•CO2 and O2 compete for active site of rubisco•34% of rubisco activity is oxygenase•Alternate CO2 assimilation metabolism: C4 and CAM