Photosynthesis: Evidence and Early Experiments

Why can plants build food from light?

Most animals obtain organic molecules by eating other organisms. Green plants instead use energy from sunlight to construct organic molecules from simpler raw materials, principally carbon dioxide and water. This is photosynthesis: a set of interconnected physical and chemical processes rather than a single instantaneous reaction. Photosynthetic production of carbohydrates supplies organic carbon to plants themselves and, directly or indirectly, to many other organisms. Oxygenic photosynthesis also releases molecular oxygen, which helps maintain the oxygen-rich atmosphere used by aerobic life.

The light provides energy, not atoms of carbon. Carbon in newly formed sugars comes from CO₂, while water supplies electrons and hydrogen equivalents through the light-driven reactions. In a simplified balanced overall description, 6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6H₂O + 6O₂. Cancelling water on both sides gives the familiar net equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, but the longer form helps keep water's role in oxygen evolution visible. A glucose molecule is a representative carbohydrate product; living plants also produce sucrose, starch and numerous other molecules.

The evidence for light and chlorophyll

A variegated leaf contains green and non-green regions. After an appropriate period in light, removing stored starch beforehand and then testing for newly formed starch, the illuminated green regions develop a positive iodine result while non-green regions generally do not. A similar comparison uses a leaf with part of its surface covered by an opaque screen. When the cover is removed and the leaf is tested, only the previously illuminated region is expected to produce starch under otherwise suitable conditions. The controls matter because a dark region may already contain starch from earlier photosynthesis unless the plant is first destarched.

A separate experiment tests the role of CO₂. In a sealed enclosure around part of a leaf, potassium hydroxide absorbs atmospheric carbon dioxide; the other part remains exposed to ordinary air and light. After the usual destarching and starch-test procedure, the enclosed CO₂-depleted portion has little or no newly formed starch, while the control region gives a positive result. The manipulation changes CO₂ availability, not just light. Reliable conclusions require comparable leaf tissue, sufficient illumination, time and control of leaks.

Historical experiments established the interpretation

Joseph Priestley placed candles, animals and green plants in enclosed air and observed that the presence of a plant could alter conditions that otherwise prevented burning or animal survival. He inferred that plants restored something removed by burning and respiration. We can now explain the result in terms of oxygen release in light, although the original experiment did not establish every modern component of gas exchange. In particular, an enclosed system is affected by light, the quantity of plants, respiration and time.

Jan Ingenhousz introduced a decisive comparison between sunlight and darkness. In water plants exposed to bright light, bubbles appeared mainly around green parts; the bubbles were subsequently identified as oxygen. The dark treatment did not produce the same oxygen evolution. These observations showed that plant oxygen release is associated with light and green tissue. Julius von Sachs later linked green cell bodies, now recognised as chloroplasts, with production of carbohydrate and its accumulation as starch. Together, these studies moved understanding from air 'restoration' to chemical synthesis in illuminated green cells.

Why the oxygen must come from water

Cornelius van Niel compared green plants with photosynthetic bacteria that use hydrogen donors other than water. Some sulphur bacteria, for instance, use H₂S and produce elemental sulphur or related oxidised sulphur products instead of O₂. A generalised reaction is CO₂ + 2H₂A + light energy → [CH₂O] + H₂O + 2A. The specific oxidised product depends on the hydrogen donor. This comparison suggested that the released oxygen of green plants originates from oxidation of water, not from the oxygen atoms of carbon dioxide.

Isotope-labelling experiments provided stronger independent evidence by tracing oxygen atoms. Water enriched with an oxygen isotope yields oxygen gas containing that label, whereas labelling CO₂ alone does not produce the corresponding primary label in evolved O₂. Thus water splitting is associated with oxygen evolution. The inference matters because it separates two pathways that are easily confused: carbon from CO₂ is assimilated into organic compounds, while O₂ released into the atmosphere comes from water. Oxygenic photosynthesis therefore involves both carbon reduction and water oxidation.

From historical inference to a modern mechanism

Understanding progressed by identifying what each experiment actually measured. A starch test indicates accumulation of carbohydrate but does not, by itself, reveal where its electrons originated. Bell-jar observations reveal changes in air composition but are influenced by respiration as well as photosynthesis. Ingenhousz made sunlight a controlled variable. Van Niel's comparisons and isotope tracers addressed the source of released oxygen. Modern mechanistic knowledge combines all of these findings with chloroplast structures and electron transport.

Quick Quiz

Model Answers

1. Light drives photochemical energy conversion; CO₂ supplies carbon fixed into carbohydrate; water supplies electrons through PS II-linked oxidation and is the source of evolved O₂.

2. Destarch a healthy potted plant, cover part of a green leaf with opaque paper, expose it to light, then test covered and uncovered regions for starch. Positive starch in the exposed part supports the role of light while the covered region serves as a matched control.

3. KOH absorbs CO₂ in one enclosed leaf region; an equivalent illuminated control region remains supplied with CO₂. After destarching and treatment, differing starch results indicate that carbon dioxide availability limits starch formation.

4. Priestley observed restoration of air by plants; Ingenhousz established the role of sunlight and green parts in oxygen evolution; Sachs linked green chloroplast-containing tissue to starch; van Niel inferred that evolved oxygen originates from the hydrogen donor water.

5. It compresses multiple photochemical and carbon-fixation reactions occurring in different chloroplast compartments, using intermediates and energy-carrying molecules rather than one direct mixing event.