A Historical Perspective of Photosynthesis Is this always the way photosynthesis has been understood?

Aristotle – 350 BC “Green plants obtain their nourishment from the soil. Leaves are mere ‘shading devices’ that provide a level of comfort for the roots.”

Jan Baptista van Helmont (1643) He performed the classic ‘willow tree experiment’ He studied plant growth by weighing a small tree, some soil, and a pot. Then he planted the small tree in the dirt (in the pot). He watered the tree regularly  after several years, he removed the tree and reweighed the tree, soil, and pot. What did he discover? 1.The weight of the pot did not change. 2.The weight of the soil decreased. (very small change - lost several ounces from 200 lbs) 3.The weight of the tree increased. (very large change – gained 165 pounds) What did he conclude? Plants only need water to grow. (We now know that plants also need CO 2 – in 1600, no one had any idea of what air consisted of, let alone any way to measure the weight of gases…

Joseph Priestly (1771) He performed the classic ‘bell jar’ experiments. He observed that when lit candles are placed in a closed container, they soon go out… But, if a living plant was put in the container, the candle would stay lit longer… He was the first person to show that plants produce oxygen gas. For this discovery, the King of England awarded him a medal as he exclaimed – “We are assured that no vegetable grows in vain…but cleanses and purifies our atmosphere!” He confirmed his findings by showing that a mouse will live considerable longer in a bell jar with a plant than without a plant…(and the plant will live longer as well…)

Joseph Priestly Experiments

Jan Ingenhousz (1779) He was a trained doctor from Holland. He finds that aquatic plants produce oxygen bubbles in the light but not in the dark. He concludes that plants need sunlight to produce oxygen.

T.W. Engelmann ( ) German Botanist – Built and developed the ideas of Ingenhousz in more depth. In 1883, he used an alga that has long spiral chloroplasts. (Spirogyra) He illuminated the alga with light shown through a prism  He placed oxygen loving bacteria on the alga. He proposed that oxygen production would be the greatest in regions where light of a given color was absorbed the most. He therefore expected to see the most bacteria in those regions. The bacteria were most concentrated where red light fell, and also where blue light fell. He concluded that plants absorb the light they need (red and blue) and reflect the rest (green)

Wavelengths of light that are used by plants

Melvin Calvin (1948) Traces the chemical pathway that carbon follows to form glucose. (Calvin Cycle)

Rudolph Marcus (1992) Wins the Nobel Prize in Chemistry for describing the process by which electrons are transferred from one molecule to another in the electron transport chain.

So, what do we know today photosynthesis to be?... Light, water, and carbon dioxide are used to make sugar (usually glucose). As by- products, oxygen gas and a small bit of water are also produced… CO 2 + H 2 O + light energy  CH 2 O + O 2 + H 2 O Photosynthesis is when light energy is changed to chemical energy which is stored in glucose molecules…(remember the whole usable vs. unusable relationship?) unusable usable [light glucose ATP]