Principles of Biology

By

Frank H. Osborne, Ph. D.

Energy Capture by Plants

Photosynthesis• Photosynthesis is the use of light energy to synthesize organic materials (glucose) from inorganic raw materials (CO2 and H2O).

Photosynthesis•Photosynthesis has some reactions that require light in order to occur. These are the photochemical reactions also called the light reactions and occur only in the light.• Other reactions do not require light. These are the carbon-fixation reactions. They occur all the time and are sometimes called dark reactions.

Photosynthesis• Photosynthetic reactions involve the transfer of electrons and so they are known as oxidation-reduction reactions.• Photosynthesis requires

lightcarbon dioxide (CO2)chlorophyll

• Using light energy, the chloroplast converts carbon dioxide to glucose.

• The photosynthesis reaction is:

• Water and CO2 are the raw materials.

• Oxygen is the waste product of photosynthesis.

Raw materials

Hydrogen in glucose comes from water

Glucose = product

PhotosynthesisThe Light Reactions (photochemical reactions)• Light is absorbed by chlorophyll and converted to electrical energy.• The electrical energy is converted to chemical energy and stored in molecular form.• Water is used to replace the electrons used in the conversion.• Oxygen is released as the waste product.

Photosynthesis• Light is absorbed by chlorophyll.• Green plants absorb red and blue light. That is why they reflect green light.

• Photosynthetic reactions involve enzymes, cytochromes and other biochemical molecules.

Cyclic Photophosphorylation• Light is absorbed during the photochemical reactions only.

Non-Cyclic Photophosphorylation• The process of non-cyclic photophosphorylation involves three system. These are as follows:

• Photosystem I - responds to red light

• Photosystem II - responds to both red and blue light• Photolysis of water - replaces the electrons used in the photosystems

• Non-cyclic photophosphorylation uses red and blue light.

PhotosynthesisThe Dark Reaction (carbon-fixation reactions)• The carbon-fixation reactions occur all the time whether light is present or not.• The carbon-fixation reactions will not occur without carbon dioxide.• The carbon-fixation reactions use the stored molecular energy to convert the carbon dioxide to glucose.

PhotosynthesisThe Dark Reaction (carbon-fixation reactions)

• The hydrogen from water is incorporated into the glucose sugar molecule.• The carbon-fixation reactions will not occur without carbon dioxide.

The End

Principles of Biology

Energy Capture by Plants