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Natural Science - Year II

Unit 58: Calvin and Photosynthesis

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History Weblecture for Unit 58

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History Lecture for Unit 58: Calvin and Photosynthesis

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Discovering How Cells Store Energy

We've spent last fall learning about the structure of the atom and more recently, how atoms join together, either absorbing energy or releasing it, to form molecules by sharing or stealing electrons. Now we look at the interpretation of two basic life phenomena as a simple chemical reaction.

Cellular respiration is the process by which the mitochondria releases of stored energy so that the cell can do work--form other molecules, build proteins, move chemicals through its membrane. Photosynthesis is the process by which the chloroplasts within plant cells change light energy from the sun into chemical energy stored in sugars. Research into how these processes occur is still going on, but the original work was done in the first half of the century by teams of scientists working at American and British universities.

Institutional Science

This new way of doing research is an outgrowth of the work done in the later half of the nineteenth century by teams such as Marie and Pierre Curie. Early teams usually formed as friends agreed to work together on joint projects. Often the members of these teams were at different educational institutions, and sometimes even in different countries. Several trends discouraged this kind of cooperation, though:

The new model of institutional science that developed redirected resources in scientific investigations. Instead of individuals who were free to work on whatever interested them, institutions now hire scientists and treat them as employees who work together for the good of the institution as well as to advance science. The teams write grant proposals asking for funding (usually to pay salaries and buy equipment) to support a particular research goal. The institution (a university, business, or government agency) choses research it thinks will benefit its own purposes, and apportions the funds accordingly. Funding is often directed to research with the most immediate benefits (in the case of industry) or to politically popular causes or national defense (in the case of government research). Since much academic research money also comes into the university from business and government sources, even universities are pressured to chose projects with the most likely practical benefits.

This environment increases competition to be the first to come up with a particular result or prove a popular theory; sometimes this competitive atmosphere leads to premature publication (as in the announcement of successful "Cold Fusion" in the 1980s by University of Utah professors). It puts pressure on research teams to perform and get results even in the face of difficult experiments, so much so that there have been a number of cases where research teams have falsified evidence in order to retain grant allocations.

The search for new products and patents prevents teams studying similar phenomena from sharing their results, particularly in medical research, where pharmaceutical companies spend millions of dollars in hopes of coming up with the first (and only) cure for common diseases. An exception to this problem was the unprecedented cooperation of several Canadian and US teams which shared the results of their individual studies of human DNA to identify the gene defect that causes cystic fibrosis. Without such cooperation, it would have taken three to four extra years for either team to isolate and map the gene. At stake was a commercial method of testing for the defect; both sides agreed to share the profits through a joint patent.

Melvin Calvin became a professor of chemistry at the University of California, Berkeley, after studying at the University of Minnesota (his native state), and at Manchester in Britain. At UC Berkeley, he led a team of researchers in a study of the role of chlorophyll in photosynthesis, and for his explanation, he won a Nobel Prize in 1961. Calvin eventually became the head of the Lawrence Radiation Lab, retiring in 1980.

Notice that we have entered the world of institutional science. All three of the scientists we study in these two units (Calvin in this unit, Krebs and Lippmann in the next) received training at American or British Universities, often studying under the same professors. Their laboratory experiments required equipment which was generally beyond the reach of independent scientists, so they relied on positions as researchers and teachers in the university environment. The universities gained reputations for hiring the best scientists (which attracts the best students), and the scientists gained access to research facilities they could not otherwise afford.

Plants and their Environment

Before Calvin began work on photosynthesis, however, there were a number of crucial observations and discoveries. In the 1600s, Jan van Helmont proved that plants increase in mass more than the amount of soil depleted. He assumed that water was the chief source of matter gained by the plant.

Read about Jan Baptista van Helmont's experiment with air and the willow tree, and work through the questions at the site.

Helmont's work was tested by a contemporary, John Woodward, who was a professor at Cambridge University. Woodward's measurements showed that most of the water drawn up through the plant was actually given off into the air (transpiration), and so could not be the primary source of the plant's nutrition. So if earth was not the source of plant material (van Helmont), and water was not the source of plant material (Woodward), where did the extra weight of the plant come from?

Joseph Priestley, whom we met when discussing the discovery of oxygen, performed a number of experiments with plants and animals in air. One of his most famous experiments involved keeping mice in closed jars, some with plants and some without. Mice in jars without plants died sooner than those kept in jars with plants, leading Priestley to conclude that plants give off a gas that animals breath in.

In the late 1700s, Jan Ingenhousz showed that without light, plants do not perform the process that restores to the air whatever breathing animals (and burning candles) take out. He was able to verify that it was light itself, and not heat energy, that allowed plants to perform this necessary process.

The next clues came from work done by German chemists to discover how carbon is "fixed" or combined to oxygen to form CO2. Around 1865, Julius von Sachs suggested that chlorophyll could catalyze synthetic reactions to make starches out of carbon dioxide, and Theodor Engelmann was able to prove that these reactions occurred in the chloroplast structures found in plant cells, but only when the cells were exposed to particular red and blue hues of light. But the complexity of the process kept chemists and biologists stumped until the discovery of radioactivity. By using radioactive oxygen (O-18) and setting up experiments with neutral oxygen in CO2 and radioactive oxygen in water H2O, then reversing the experiment, Martin Kamen was able to show that the oxygen in CO2 from the atmosphere and in H2 added to plant environments was broken out of these molecules, and recombined into gaseous free oxygen O2 and sugars in the plant (C6H12O6). Calvin used radioactive carbon (C-14) to trace the process of carbon fixation during photosynthesis and discover the repeated sequence of reactions we now call the Calvin cycle.

Read the brief biography of Melvin Calvin at the Nobel Prize Winners site.

  • What are halogens? What interested Calvin about them?
  • What was Calvin trying to learn when he worked out the process of photosynthesis?
  • What other pioneering work did Calvin do?

Study/Discussion Questions:

Further Study/On Your Own