Photosynthesis Summary

In photosynthesis, light energy is converted to chemical energy and carbon is fixed into organic compounds.

Light energy is captured by the living world by means of pigments. The pigments involved in photosynthesis in eukaryotes include the chlorophyll and carotenoids. Light absorbed by the pigments boosts their electrons to higher energy levels. Because of the way the pigments are packed into membranes, they are able to transfer this energy to reactive molecules, probably chlorophyll-a packed in a particular way.

Photosynthesis takes place within cellular organelles known as chloroplasts. These organelles are surrounded by two membranes. Contained within the membranes of the chloroplast are a solution of organic compounds and ions known as the stroma and a complex internal membrane system consisting of fused pairs of membranes that form sacs called thylakoids. The pigments and other molecules responsible for capturing light are located in and on these membranes.

Photosynthesis takes place in two stages.

• The light reaction, in which light energy is captured by chlorophyll and converted to the chemical energy of ATP and NADPH.

• The dark reaction, in which carbon atoms are reduced and carbohydrates formed.

In the currently accepted model of the light reactions in photosynthesis, light energy strikes antennae pigments of photosystem-II. Electrons are boosted uphill from the reactive chlorophyll a molecule P₆₈₀ to an electron acceptor. As the electrons are removed, they are replaced by electrons from water molecules, with the simultaneous production of free oxygen. The electrons then pass downhill to photosystem-I along an electron transport chain, in the course of which ATP is generated. Light energy absorbed in antennae pigments of photosystem-I and passed to chlorophyll P₇₀₀ results in the boosting of electrons to another electron acceptor. The electrons involved from P₇₀₀ are replaced by the electrons from photosystem-II. The electrons are ultimately accepted by the electron carrier NADP. The energy yield from the light reactions is contained in molecules of NADPH and ATP.

In the dark reactions, which take place in the stroma, NADPH and ATP produced in the light reactions are used to reduce carbon dioxide to organic carbon. This is accomplished by means of the calvin cycle. In the calvin cycle, a molecule of CO₂ is combined with the starting material, a five-carbon sugar called ribulose biphosphate. At each turn of the cycle, one carbon atom enters the cycle. Three turns of the cycle produce a three-carbon molecule, glyceraldehyde phosphate. Two molecules of PGAL (six turns of cycle) can combine to form a glucose molecule. At each turn of the cycle RUBP is regenerated. The PGAL can also be used as a starting material for the synthesis of other organic compounds by the cell.

In C₄ plants, CO₂ is initially accepted by a compound known as PEP to yield the 4 c oxaloacetic acid. The oxalaoacetic acid is then oxidised, and CO₂ is transferred to the RUBP of the calvin cycle. Under conditions of drought C₄ plants are more efficient than C₃ plants.

Insectivorous plants are autotrophs whereas parasites are heterotrophs.