Photosynthesis – Early Experiments

Introduction

Photosynthesis is how plants make their food using the energy of sunlight. Plants use the process of photosynthesis to convert light energy into chemical energy. This is how they can grow. This process produces oxygen as a by-product and is thus key to understanding the origins of life on Earth. Scientists have carried out several studies to verify the presence of photosynthesis. In this article, we will take a closer look at the early photosynthesis experiments.

Process of photosynthesis

Photosynthesis is how green plants and some kinds of microorganisms use sunlight to synthesise nutrients from carbon dioxide and water. It is a chemical reaction that converts light energy into chemical energy (through the absorption of electrons). Cornelis van Niel proposed the general equation for photosynthesis first. 

History of photosynthesis experiments

Photosynthesis is one of the most critical processes in nature. It is the procedure through which green plants and some other organisms generate energy by converting carbon dioxide and water into food. These essential characteristics of photosynthesis were found in the mid-19th century. But one of the first experiments was done in the mid-17th century by Jan van Helmont, who concluded that all plant food is derived from water and not from the soil. Later, this experiment was carried forward by many scientists. In 1937, Robert Hill showed the photolysis of water in the light reaction of photosynthesis.

Let us look at some experiments below that were conducted by early scientists to study photosynthesis.

Experiment by Joseph Priestley

-Joseph Priestley was an 18th-century British naturalist who discovered that plants produced oxygen and that this process is the opposite of respiration.

-This discovery was called ‘dephlogisticated air’ because oxygen had not yet been discovered.

-Priestley left his photosynthesising plants in a hermetically sealed container with mice and found that the animals were able to survive without air. 

-Other scientists tried his experiment and also found that it worked.

– In 1770, after a series of studies, Priestley concluded that air is necessary for photosynthesis and plant growth.

– His experiments were criticised by other scientists, but they eventually replicated his work and came to agree with him. 

-Priestley’s work was the basis for future discoveries on photosynthesis and how it works, including the fact that plants take carbon dioxide from the atmosphere to create food energy during photosynthesis.

-In this experiment, Joseph Priestley performed experiments on mice in a bell jar. He lit a candle and watched the flame burn for many hours. 

-He noticed that the flame of the candle became dimmer as time passed and eventually burned out, which he expected. 

-However, he also observed that the mouse died before the candle burned out. This caused him to suspect that something other than air was being consumed by the burning candle. 

-Before the mouse died, Priestley noticed that a plant placed inside the container revived the dying rodent by returning oxygen to its lungs.

– Priestley repeated the experiment several times and eventually concluded that ‘vegetating’ green plants produce oxygen through photosynthesis while burning candles and other substances produce carbon dioxide in their combustion.

-In the first scenario, Joseph Priestley concluded that the air in the bell jar was polluted by the lamp and the presence of the rat.

– However, in the second case, the plant restored the air polluted by the candle and the rat. But it took a few more years to figure out the role of the plants in keeping the rat alive and the candle lit.

Experiment to demonstrate photosynthesis underwater

-Jan Ingenhousz replicated Priestley’s experiments in 1771 and discovered that sunlight is needed for the process.

– He also observed that the gas is created only when the plants are exposed to light.

-This experiment requires a test tube, an aquatic plant (Hydrilla) and a jar. 

-Ingenhousz utilised a similar arrangement to Priestley’s and placed the plant once in the dark and once in the sunlight to conduct his experiment. 

-He demonstrated with an aquatic plant that in bright sunshine, little bubbles formed around the green sections, whereas in the dark, they did not. 

-He later determined that these bubbles were made of oxygen.

-Ingenhousz found that sunlight is essential to the plant process that purifies the air. Only the green parts of the plants release oxygen.

– Although different species undertake photosynthesis in different ways, the process always starts with light energy being received by proteins called reaction centres, which contain green chlorophyll pigments. 

-These proteins are found inside chloroplasts, which are most prevalent in leaf cells in plants. In bacteria, the proteins are found in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable.

-Later scientists did numerous improvised experiments to investigate the key components of photosynthesis. Joseph Priestley (1733–1804) was the first scientist to conduct these tests.

Julius von Sachs in 1854 provided evidence that glucose is produced when plants grow and it is stored as starch. He later showed that the green substance in plants i.e., chlorophyll is located in special bodies (later called chloroplasts) within plant cells.

 T.W. Engelmann (1843-1909), determined the action spectrum of chlorophyll by using green alga, Cladophora. Using a prism he split into its spectral components and then illuminated the alga used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b. By the middle of the nineteenth century, the main features of photosynthesis, that plants use light energy to synthesise carbohydrates from CO2 and water, were known. 

 Cornelius van Niel (1897-1935) a microbiologist made a significant contribution in the  understanding of photosynthesis. His studies on purple and green sulphur bacteria (Photosynthetic bacteria). Demonstrated from a suitable oxidisable compound, it reduces carbon dioxide to carbohydrates.In photosynthetic bacteria H2S is the hydrogen donor which is oxidized to sulphur. They do not release O2 during photosynthesis. In green plants, H2O is the hydrogen donor and its oxidation product is O2. It is, therefore, inferred that the O2 evolved by the green plant comes from H2O and not from carbon dioxide.

Runen, Rassid and Kamen – Nearly a decade later, the view of Van Niel was confirmed by Runen, Rassid and Kamen (1941) using water labelled with radioactive isotope of oxygen evolved as radioactive oxygen (18O). It thus confirmed van Niel’s hypothesis that oxygen evolved in photosynthesis comes from water. This led to the currently accepted general equation of photosynthesis as under.

Conclusion

Trees and other plants use photosynthesis to make their food from simple inorganic compounds such as carbon dioxide (CO2) and water (H2O), in a process called “primary production.” Photosynthesis is one of the essential processes occurring on our planet. It is responsible for converting nearly all land-living matter into energy and oxygen. Scientists established the process of photosynthesis by the mid-nineteenth century. They concluded that light is required for photosynthesis. Plants use carbon dioxide and water to produce glucose (carbohydrate), with water molecules acting as hydrogen donors and oxygen (O2) as a by-product of this biological procedure.