Plants occupy a fundamental position in the food web, since they serve as the primary source (producers) of organic compounds for animals and humans. Plants utilize their leaves to produce sugars in a process that involves light and which is called photosynthesis. Simplified (Fig. 1), water (H2O, the source of H and O) and carbon dioxide (CO2, the source of C) are transformed into sucrose (a sugar = C6H12O6) and oxygen (= O2 to the atmosphere) in a process in which light energy is consumed:
 |
A. Flow of photosynthetic components in the leaf, B. Chlorofyll molecuule (thanks to Dennis Vriezema), C. Absorption curves of chlorofyll a, b and caronetoiuml;ds, D. Chloroplast and details of thylakoids (Transmission electron microscopy), E. Very simplified representation of the photosynthesis reactions in a mesophyll cell
1 Cell wall, 2 Cytoplasm, 3 Vacuole, 4 Chloroplast envelope (2 membranes), 5 Tonoplast, 6 Plasma membrane, 7 Grana, 8 Stroma thylakoids, 9 Starch grains, 10 Stroma |
Photosynthesis starts with a so-called light reaction. Light energy from the sun is captured by pigment molecules (chlorophyll a, chlorophyll b en carotenes; Fig B and C) that are located in the internal membranes (thylakoids) of chloroplasts (Fig. D and E). This energy is consumed to produce reducing power (H+ ions = protons) and molecular oxygen (O2) from water molecules (H2O). The reducing power (the protons) is used to generate ATP (adenosine triphosphate) and NADPH (reduced nicotine amide adenine dinucleotide phosphate; fig F).
Then, in the so-called dark reaction (a process that does not require light,but does not require darkness neither) ATP and NADPH are invested to include CO2 into organic molecules.
The CO2 fixation and net production of carbohydrates (sugar-like) molecules occurs in the stroma (the fluid part of the chloroplasts) in a complex cascade of reactions named the Calvin cycle (Fig. F). Most angiosperms and gymnosperms are so-called C3 plants, because the first product of CO2 incorporation is a three-carbon-compound: 3-phosphoglycerate.
In C4 plants, like corn and sugarcane, the Calvin cycle is preceded by fixation of CO2 into a four-carbon-molecule (oxaloacetate) under control of an enzyme (phosphoenolpyruvate [=PEP] carboxylase) with a high affinity for CO2. The efficient C4 mechanism is correlated to a unique leaf anatomy: Kranz anatomy (Kranz = wreath: see micrograph), in which veins are surrounded by a CO2 fixing bundle sheath, which in turn stays in direct contact with the mesophyll cells where the Calvin cycle occurs.
Go to the Bioplek for beautiful animations and explanations on photosynthesis (secondary school level, in Dutch but very visual)
In autumn most leaves loose their green color and become yellowish or reddish. How does this come? (see answer) 
