Photosynthesis is the conversion of solar energy into chemical energy stored in sugars (biomolecules). It harvests a small portion of the sun’s energy to build biomolecules (anabolic building of biochemicals), especially sugar (glucose).
Photosynthesis is the reason most organisms on Earth exist, directly or indirectly, because it fuels the food chain. Exception mentioned: a hydrothermal-vent ecosystem near the ocean floor uses a different system based on chemical energy and heat, touching on next semester.
The most prevalent photosynthetic organisms on Earth are oceanic photosynthetic organisms (bacteria), even though we often focus on plants.
Plants’ photosynthetic “flaps” are leaves, whose main purpose is to capture solar energy. Leaves are flattened to maximize light capture.
A leaf cross-section includes: epidermis on the top and bottom, stomata (open to exchange CO₂ and O₂), and internal mesophyll layers (palisade mesophyll, spongy mesophyll) where the green photosynthetic cells reside.
Chloroplasts are the organelles that perform photosynthesis. Outer and inner membranes surround a membrane-rich interior; the inner membrane forms stacked membranes (thylakoids) arranged as grana, with the surrounding fluid called the stroma. This is analogous in purpose (keeping components close) to mitochondrial membranes.
The primary photosynthetic organelle (chloroplast) shares conceptual similarities with mitochondria (electron transport, energy extraction), but the enzymes and electron transport chains differ.
Photosynthesis and respiration are complementary processes on opposite ends of the same energy coin: respiration breaks down sugars to produce energy and consumes O₂ while releasing CO₂; photosynthesis uses CO₂ and water to build sugars and releases O₂. The two are linked mechanistically (redox processes) but use different enzymes and electron transport chains.
The most important chemical process is the redox interplay: CO₂ is reduced and water is oxidized in photosynthesis; this requires energy input (endergonic). The reverse in respiration is oxidation of nutrients and production of CO₂.