Photosynthesis

Photosynthesis is the process by which plants use light energy to make food from simple chemicals. Photosynthesis is vital to all life on Earth since all food comes from this process, either directly or indirectly. People not only eat green plants and their fruit and grains, but they also eat the animals that feed on the green plants.

The word photosynthesis means "putting together with light," and perfectly describes a process by which a plant converts carbon dioxide and water into food by using light. The miracle of photosynthesis is that it captures light energy and converts it into chemical energy that can be used by organisms. Photosynthesis occurs inside the leaf of a plant at the cellular level. Plant cells contain chloroplasts. These chloroplasts contain a green pigment called chlorophyll. The flat leaf, acting as a solar collector, allows the light to strike the chlorophyll, which is stimulated to absorb it. In the chloroplasts, the light reacts with carbon dioxide (that the plant breathes in through microscopic holes in its leaves called stomata) and with water (that the plant takes in through its roots). During a series of complicated reactions, the water molecules are broken down into hydrogen and oxygen, and the hydrogen combines with carbon dioxide to produce glucose, a simple sugar that is used as a building block for starch and other complex carbohydrates. The excess oxygen is later released through the stomata into the atmosphere. The plants use the glucose as food. What they do not use they convert to starch for storage and to build cells walls.

If all life on Earth depends on photosynthesis, then all life really begins with what makes photosynthesis work—light. Sunlight is the energy that travels from the Sun. It arrives on Earth in waves of different lengths, and those lengths give it its different colors. As a combination of different wavelengths, sunlight is really a mixture of violet, blue, green, yellow, orange, and red light. These wavelengths can be observed by passing sunlight through a prism. Chlorophyll is extremely efficient and absorbs red, orange, and blue light, allowing only green light to pass through. This is what makes a leaf look green. When chlorophyll absorbs the Sun's light, the first part of photosynthesis begins as light energy splits up water molecules (hydrogen and oxygen). This process is called photolysis and produces energy-carrying molecules called adenosine triphosphate (ATP). In the second part of photosynthesis, energy from ATP and other energy carriers remove oxygen from carbon dioxide, allowing the carbon and hydrogen to combine and form glucose.

JAN INGENHOUSZ

Dutch plant physiologist (a person who studies how an organism and its body parts work or function normally) Jan Ingenhousz (1730–1799) discovered photosynthesis and plant respiration. He demonstrated that plants use sunlight and carbon dioxide (atmospheric gas) to make their own food, and that they give off oxygen as a by-product. He was the first to indicate the close connection between animals and plants and to show how much animals depended on green plants.

Jan Ingenhousz was born in Breda, The Netherlands, where he received his basic education. He then attended the University of Louvain and later the University of Leyden. After receiving his medical degree, he worked at a private practice in Breda, but left for England when his father died in 1765. While at work in a hospital there, he became an expert in the new technique of smallpox inoculation. This was a hazardous occupation since he administered a live virus instead of today's weakened vaccine. However, his treatments were so effective that he was called to Vienna, Austria, to inoculate the royal family. He was then appointed court physician and given a lifetime income. This allowed him to pursue his research, and in 1779, after seven years in Vienna, he returned to England where he would remain for the remainder of his life.

It was also that year that he began experiments that led to his discovery of photosynthesis, the process by which plants convert sunlight into food. Ingenhousz found that green plants take in carbon dioxide and give off oxygen, but they do this only in the presence of sunlight. In the dark, he found that the opposite happens, and like animals, they absorb oxygen and give off carbon dioxide. This is called respiration. This was the first recognition that sunlight played a key role in the life of plants. Ingenhousz also proved that only the actual, visible light and not the Sun's heat, was necessary for photosynthesis to work. Others had been experimenting with air at this time, and the work of English chemist, Joseph Priestley (1733–1804), showed that a candle flame burning in a closed container eventually would go out. He also found that small animals placed in a similar space eventually died since all the oxygen was consumed and only carbon dioxide was left. Ingenhousz realized that since plants give off oxygen, which is essential to animal life, and then took in the carbon dioxide that animals breathed out as a waste product, there was a fundamental connection between plants and animals that no one before had realized. To Ingenhousz, photosynthesis meant that animals and plants are totally dependent on one another. We now know that photosynthesis is the key to all life on Earth since it provides food, either directly (for plants) or indirectly (for animals that eat the plants or eat other animals that have eaten plants) for virtually every living thing. For Ingenhousz, plants "purified" the air and "revitalized" it. His 1779 book detailed his plant discoveries and laid the foundation for the continued study of photosynthesis. Ingenhousz also broke new ground in physics and chemistry. For example, he improved phosphorous matches, invented a hydrogen-fueled lighter, and mixed an explosive propellant for firing pistols.

It has taken scientists hundreds of years to understand what happens during photosynthesis. Beginning in the early seventeenth century, the work of Flemish physician Jan Baptist van Helmont (1577–1644), and later the English botanist (a person specializing in the study of plants) Stephen Hales (1677–1761), demonstrated that plants needed air and water to grow. In the eighteenth century, chemists began to identify individual gases, and in 1779, the Dutch physician, Jan Ingenhousz (1730–1799), showed that plants take in carbon dioxide and release oxygen when light shines on them. By the 1880s, the German physiologist (a person specializing the study of the processes of living things), Theodor Wilhelm Engelmann (1843–1909), showed that the light reactions that capture solar energy and convert it into chemical energy occur in chloroplasts. It was not until the twentieth century, however, that scientists began to fully understand the complex biochemistry of photosynthesis.

Photosynthesis is a key part of a cycle that not only maintains life on Earth but keeps Earth's levels of carbon dioxide and oxygen in balance. Plants convert carbon dioxide into food and oxygen, which animals "burn" in a process called respiration (combining food with oxygen to release energy). Respiration is therefore the opposite or reverse of photosynthesis. In respiration, oxygen is used up and carbon dioxide and water are given off (which plants use to start photosynthesis again).

[See alsoCarbon Dioxide; Carbohydrates; Chloroplast; Light; Plant Anatomy; Plant Hormones; Plants ]