Photosynthesis: Cellular Processes, Light Reactions, and Evolution

Overview and Chemical Processes

• Photosynthesis is the biological process by which light energy is converted into chemical energy, synthesizing organic compounds that fuel life on Earth.
• The process is generally divided into two main stages: the light-dependent reactions and the light-independent reactions (commonly referred to as the Calvin cycle).
• To find more detailed contextual information regarding this biological phenomenon, you can visit the Wikipedia Main Page.

Photosynthetic Membranes and Organelles

• In eukaryotic phototrophs, such as plants and algae, photosynthesis takes place within specialized intracellular organelles called chloroplasts.
• In prokaryotic phototrophs, such as cyanobacteria, the photosynthetic machinery is localized within specialized folds of the plasma membrane called thylakoid membranes.

Light-Dependent Reactions

• The light-dependent reactions capture light using photosynthetic pigments (predominantly chlorophylls and carotenoids) organized into photosystems.
• The flow of electrons through the electron transport chain is commonly illustrated by the Z scheme, which maps the pathway of electrons from water to NADP+.
• Water photolysis occurs at the oxygen-evolving complex of Photosystem II, where water molecules are split to release oxygen gas ($O_2$), protons, and electrons.

Light-Independent Reactions and Carbon Concentrating Mechanisms

• The light-independent reactions use the ATP and NADPH generated during the light stage to fix atmospheric carbon dioxide ($CO_2$) into carbohydrates through the Calvin cycle.
• Some land plants have evolved specialized carbon concentrating mechanisms, such as the C4 and CAM pathways, to minimize water loss and reduce the wasteful process of photorespiration.
• Aquatic phototrophs also utilize distinct carbon-concentrating mechanisms to actively transport inorganic carbon across cellular membranes in water-based environments.

Efficiency, Evolution, and Prokaryotic Lineages

• The thermodynamic efficiency of photosynthesis is highly variable, with typical plants converting only about 1% to 2% of solar energy into chemical energy.
• According to the endosymbiotic theory, eukaryotic chloroplasts originated from an ancient ancestral engulfment of photosynthetic cyanobacteria.
• Various photosynthetic prokaryotic lineages exist today, utilizing diverse pigments like bacteriochlorophylls to perform either oxygenic or anoxygenic photosynthesis.

Experimental History and Affecting Factors

• The experimental history of photosynthesis includes early milestones such as the discovery of oxygen production by Joseph Priestley and the identification of light's role by Jan Ingenhousz.
• Key factors that limit or regulate the rate of photosynthesis include light intensity, the specific wavelengths of light absorbed, environmental temperature, and atmospheric carbon dioxide levels.