Photosynthesis: Turning Light Into Life

11/1/2016

Lets face it, we'd like to think we're the greatest living thing on the Planet, but we know we're not. We may be able to smelt metals, generate electricity, and even split atoms; but those are nothing compared to a plant's ability to use light to manufacture the complex chemicals that keeps this planet teeming with life.

If you still scoff at this notion, then prepare to be humbled.

"We may be nothing more than a complex set of chemical reactions. Solar energy raises the electrons of atoms to high levels of energy in photosynthesis, and all other metabolic reactions allow the energy to flow to background levels."
Stanley A. Rice, 'Life of Earth: Portrait of a Beautiful, Middle-aged Stressed-out World', 2011

Photosynthesis is the process by which plants and other things make food. It is a chemical process that uses sunlight to turn carbon dioxide into sugars the cell can use as energy. As well as plants, many kinds of algae, protists and bacteria use it to get food. Photosynthesis is very important for life on Earth. Most plants either directly or indirectly depend on it. The exception are certain organisms that directly get their energy from chemical reactions; these organisms are called chemoautotrophs.

Photosynthesis can happen in different ways, but there are some parts that are common: 6 CO2(gas) + 6 H2O + photons → C6H12O6(aqueous) + 6 O2(gas)

Although this important process has existed since the beginning of time, everyone was totally oblivious of its existence, and it wasn’t discovered until the 1800s. There isn’t merely one scientist who made this discovery, as several different scientists over a period of more than 200 years contributed to the discovery of this important natural phenomenon.

History

Photosynthesis was partially discovered in the 1600s by Jan Baptista van Helmont, a Belgian chemist, physiologist and physician. Helmont performed a 5-year experiment involving a willow tree which he planted in a pot with soil and placed in a controlled environment. The willow tree was carefully and precisely watered over the 5-year period. At the end of his experiment Helmont concluded that the growth of the tree was the result of the nutrients it had received from the water and not the soil. Helmont’s conclusion was inaccurate but his experiment proved that water contributes to the growth of plants.

Joseph Priestley is another scientist who contributed to the discovery of photosynthesis. He was born in 1733 and later became a chemist, minister, natural philosopher, educator and political theorist. His experiments included placing a lit candle inside a closed jar. The flame quickly went out and Priestley concluded that the air inside the jar had been “injured”. He conducted similar experiments with mice and concluded that the mice had also “injured” the air.

Priestley later discovered that plants could be used to restore air that was “injured” by the candle and the mice. In 1774, the results of Priestley’s experiments were published in “Experiments and Observations of Different Kinds of Air, Volume I.” Although Priestley did not know it at the time, his experiments proved that air contains oxygen.

Jan Ingenhousz is yet another scientist who contributed to the discovery of photosynthesis. He was a Dutch chemist, biologist and physiologist who performed important experiments in the late 1770s that proved that plants produce oxygen. Ingenhousz placed submerged plants in sunlight and then in the shade. He noticed that small bubbles were produced by the plants when they were in the sunlight. When they were transferred to the shade bubbles were no longer produced by these plants. Ingenhousz later concluded that plants use light to produce oxygen.

In 1796, Jean Senebier, a Swiss botanist, pastor and naturalist demonstrated that plants absorb carbon dioxide and release oxygen with the help of sunlight. In the early 1800s Nicolas-Théodore de Saussure demonstrated that while plants need carbon dioxide, the increased mass of growing plants is not the result of carbon dioxide alone but also the uptake of water.

In the 1840s Julius Robert Mayer, a German physician and physicist, stated that energy can be neither created nor destroyed. This is known as the first law of thermodynamics. He proposed that plants convert light energy into chemical energy. It wasn’t until 20 years later, from 1862-64, Julius von Sachs investigated how starch is produced under the influence of light and in relation to chlorophyll.

Reactions

Photosynthesis has two main sets of reactions. Light-dependent reactions need light to work; and light-independent reactions, which do not need light to work.

In the light-dependent reaction, light energy from the sun is used to split water (photolysis) which has been sucked in by plants by transpiration. Water, when broken, makes oxygen, hydrogen, and electrons. These electrons move through structures in chloroplasts and by chemiosmosis, make ATP.

The hydrogen is converted to NADPH which is then used in the light-independent reactions. Oxygen diffuses out of the plant as a waste product of photosynthesis. This all happens in the grana thylakoid of chloroplasts.

Colin Flannery was the first to propose the idea that photosynthesis needs light, in 1779. He recognized it was sunlight falling on plants that was required, although Joseph Priestly had noted the production of oxygen without the association with light in 1772. Cornelius Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce CO2. Then in 1939 Robin Hill showed that isolated chloroplasts would make oxygen, but not fix CO2 showing the light and dark reactions occurred in different places.

Light-independent reactions take place in plant chloroplasts. In this process sugars are made from carbon dioxide. The process, known as the Calvin cycle, uses products of the light-dependent reactions (ATP and NADPH) and various enzymes. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Sugars made in the light-independent reactions are then moved around the plant by translocation.

The Calvin cycle is named after Melvin Calvin, who won a Nobel Prize in Chemistry for finding it in 1961. Calvin and his colleagues did the work at the University of California, Berkeley. Using the radioactive carbon-14 isotope as a tracer, Calvin, Andrew Benson and their team mapped the complete route that carbon travels through a plant during photosynthesis. They traced the carbon-14 from its absorption as atmospheric carbon dioxide to its conversion into carbohydrates and other organic compounds. The single-celled algae Chlorella was used to trace the carbon-14.

Factors Affecting Photosynthesis

There are three main factors affecting photosynthesis: Light intensity, carbon dioxide concentration, temperature.

If there is little light shining on a plant, the light-dependent reactions will not work efficiently. This means that photolysis will not happen quickly, and therefore little NADPH and ATP will be made. This shortage of NADPH and ATP will lead to the light-independent reactions not working as NADPH and ATP are needed for the light-independent reactions to work.

Carbon dioxide is used in the light-independent reactions. It combines with NADPH and ATP and various other chemicals (such as Ribulose Biphosphate) to form glucose. Therefore, if there is not enough carbon dioxide, then there will be a buildup of NADPH and ATP and not enough glucose will be formed.

There are many enzymes working in photosynthetic reactions – such as the enzyme in photolysis. These enzymes will not work as well, or stop working at all at high or low temperatures and therefore, so will the light-dependent and light-independent reactions.

Ponder this

A plant is more than just sugars, how and where do these plants get the materials that build themselves?

Starch is a carbohydrate, which is simply long strings of sugar molecules. Why do some plants store sugars (e.g. apples, oranges, grapes), while others store starch (e.g. potatoes, rice, wheat)? Some even produce and store fats, such as avocados, how is that possible, and for what purpose?

Discuss

Why don't animals (including humans) have their own chloroplasts? What are the advantages and disadvantages of such? Is there a physical, chemical or biological requirement for animals not to produce their own food? Would animals function as they are with it?