Photosynthesis
In 1779, Jan Ingenhousz demonstrated that plants require sunlight to revive a mouse trapped under an inverted jar. This experiment revealed the fundamental chemical equation where carbon dioxide and water combine to produce glucose and oxygen. Most photosynthetic organisms store this energy within carbohydrates like sugars, starches, and cellulose. The process converts light energy into chemical bonds necessary for metabolism. Plants absorb red and blue spectra of light while reflecting green wavelengths. This reflection gives most vegetation its characteristic color. Oxygenic photosynthesis releases oxygen as a byproduct when splitting water molecules. Anoxygenic forms exist in some bacteria but do not generate oxygen gas.
Light-dependent reactions occur inside thylakoid membranes within chloroplasts. A single photon absorbed by chlorophyll triggers electron transfer through pheophytin to quinone molecules. This flow generates ATP and NADPH using proton gradients across the membrane. Water photolysis splits two water molecules to yield one oxygen molecule and four hydrogen ions. The Calvin cycle then uses these products to fix atmospheric carbon dioxide into three-carbon sugars. RuBisCO enzymes capture CO2 from the air and attach it to ribulose bisphosphate. Five out of six resulting sugar molecules regenerate the starting compound to keep the cycle running. Only one-sixth exits the cycle to form sucrose or starch for plant growth. C4 plants separate carbon fixation spatially to reduce photorespiration in hot climates. CAM plants perform carbon uptake at night to conserve water during arid conditions.
Fossils dated 3.4 billion years old suggest filamentous photosynthetic organisms existed early in Earth's history. Thylakoid membranes preserved in cherts provide direct evidence dating back 1.75 billion years. Oxygenic photosynthesis became significant around two billion years ago during the Paleoproterozoic era. Cyanobacteria produced excess oxygen that eventually oxygenated Earth's atmosphere. This event enabled the evolution of complex life forms requiring oxygen for respiration. Early systems likely used hydrogen sulfide instead of water as electron donors. Anoxygenic bacteria dominated the euxinic Canfield oceans during the Boring Billion period. Purple Earth hypothesis suggests archaeal photosynthesis using retinal pigments may have preceded cyanobacterial evolution. The biochemical capacity to use water evolved once in a common ancestor of extant cyanobacteria between 2450 and 2320 million years ago.
Cyanobacteria remain the only prokaryotes performing oxygenic photosynthesis today. Chlorobi, Heliobacteria, and Proteobacteria represent anoxygenic lineages using Type I or Type II photosystems. Green sulfur bacteria utilize hydrogen and sulfur as electron donors. Purple nonsulfur bacteria employ various organic molecules for energy transfer. Eukaryotic algae include red algae with phycoerythrin and brown algae containing fucoxanthin. Dinoflagellates and chromerids belong to the superphylum Myzozoa. Euglenids exist within the Excavata clade but differ from other groups. Some marine mollusks like Elysia viridis maintain symbiotic relationships with captured chloroplasts. These slugs survive solely by photosynthesis for months at a time. Symbiosis also explains the origin of chloroplasts through endosymbiotic theory involving early eukaryotic cells engulfing photosynthetic bacteria.
Jan van Helmont measured soil mass changes in the mid-17th century to hypothesize plant growth sources. Joseph Priestley discovered that plants restore air injured by burning candles or mice. Jean Senebier demonstrated green plants consume carbon dioxide under light influence in 1796. Cornelis Van Niel proved hydrogen reduces carbon dioxide in purple sulfur bacteria studies. Robert Emerson identified two distinct light reactions absorbing different wavelengths up to 600 nm and 700 nm. Samuel Ruben and Martin Kamen used radioactive isotopes to confirm oxygen originates from water. Melvin Calvin, Andrew Benson, and James Bassham elucidated the carbon reduction cycle using carbon-14 isotopes. A Nobel Prize in Chemistry was awarded to Melvin Calvin in 1961 for this work. Otto Kandler presented experimental evidence for photophosphorylation in vivo during 1950.
Light intensity affects carbon assimilation rates until reaching a plateau at higher irradiance levels. Temperature influences the rate of carbon fixation when light is abundant but has little effect at low irradiance. Water availability limits photosynthesis through stomatal closure in dry conditions. Carbon dioxide concentration determines how fast sugars are produced before other factors intervene. RuBisCO enzymes bind oxygen instead of carbon dioxide when CO2 levels drop. This process called photorespiration wastes energy without producing sugars. Phosphoglycolate generated by oxygenase activity inhibits photosynthesis at high concentrations. C4 plants like maize achieve leaf photosynthetic rates around 38 to 40 micromoles per square meter per second. These species show no apparent photorespiration compared to C3 types such as cotton or sunflower. Quantum walk phenomena allow excitons to cover wider areas while choosing efficient routes simultaneously.
Common questions
What did Jan Ingenhousz demonstrate about plants and sunlight in 1779?
Jan Ingenhousz demonstrated that plants require sunlight to revive a mouse trapped under an inverted jar. This experiment revealed the fundamental chemical equation where carbon dioxide and water combine to produce glucose and oxygen.
When did oxygenic photosynthesis become significant in Earth's history?
Oxygenic photosynthesis became significant around two billion years ago during the Paleoproterozoic era. Cyanobacteria produced excess oxygen that eventually oxygenated Earth's atmosphere.
How do C4 plants like maize differ from C3 types such as cotton or sunflower regarding photorespiration?
C4 plants like maize achieve leaf photosynthetic rates around 38 to 40 micromoles per square meter per second. These species show no apparent photorespiration compared to C3 types such as cotton or sunflower.
Which scientist won a Nobel Prize in Chemistry for elucidating the carbon reduction cycle in 1961?
Melvin Calvin won a Nobel Prize in Chemistry in 1961 for this work. He, Andrew Benson, and James Bassham elucidated the carbon reduction cycle using carbon-14 isotopes.
What is the biochemical capacity to use water associated with cyanobacteria evolution between 2450 and 2320 million years ago?
The biochemical capacity to use water evolved once in a common ancestor of extant cyanobacteria between 2450 and 2320 million years ago. This event enabled the evolution of complex life forms requiring oxygen for respiration.