Lectures 12-14 Photosynthesis
“Life’s clever. It figures out how to suck electrons out of everything we eat and keep them under control” Kenneth Nealson “You eat sugars that have excess electrons, and you breathe in oxygen that willingly takes them.”
• Living is work (with energy!). • To perform their many tasks, cells require transfusions of energy from outside sources. • Mostly, powered by the Sun Introduction
• Autotrophs: produce organic molecules from CO 2 or other inorganic materials. – Producers. • Heterotrophs: live on organic compounds produced by other organisms. – Consumers. Two types of life
• Ultimate sources of organic compounds for all non-autotrophic organisms. • Photo autotrophs use light as the energy source. • Plants, algae, some protists and prokaryotes. • Chemo autotrophs harvest energy from oxidizing inorganic substances, e.g. sulfur, ammonia. • Only in limited bacteria. Autotrophs
Purple sulfur bacteria Cyanobacteria Protist Euglena Kelp Higher plants Diatom Algae
• Occurs mostly in the chloroplasts • Nourishes almost all of the living world directly or indirectly • Synthesizes 160 billion metric tons of carbohydrate per year • Renewable resources – connection to biofuels Life on Earth is Solar Powered via Photosynthesis
Key Concepts • Photosynthesis consists of two linked sets of reactions: the light-dependent reactions, and the light- independent reactions of the Calvin cycle. In eukaryotic cells, both processes take place in chloroplasts. • The light-dependent reactions transform the energy in sunlight to chemical energy in the form of electrons with high potential energy. These electrons are used to produce ATP and the electron carrier NADPH. • Then the reactions of the Calvin cycle use the ATP and NADPH to reduce carbon dioxide to carbohydrate.
An Overview of Photosynthesis • Photosynthesis is the process of using sunlight to produce carbohydrate. This process requires sunlight, carbon dioxide, and water, and produces oxygen as a byproduct. The overall reaction is as follows: CO 2 + 2 H 2 O + light energy (CH 2 O) n + H 2 O + O 2 where (CH 2 O) n stands for carbohydrate. • Usually, glucose (C 6 H 12 O 6 ) is the carbohydrate made: 6 CO 2 + 12 H 2 O + light energy C 6 H 12 O 6 + 6 H 2 O + 6 O 2
Photosynthesis: Two Linked Sets of Reactions • Photosynthesis consists of two sets of reactions: the light- dependent reactions produce O 2 from H 2 O, and then the light-independent reactions of the Calvin cycle produce sugar from CO 2 . • These two sets of reactions are linked by electrons. Electrons are released in the light-dependent reactions when water is split to form oxygen gas. These electrons are transferred to the electron carrier NADP + , forming NADPH.
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- Fall '12