---

Introduction to Photosynthesis

Photosynthesis is arguably the most important biological activity on the planet. Without photosynthesis, life as we know it would be impossible. It is the process by which energy in the form of visible light is converted into the chemical energy needed by all living things. Because of photosynthesis, the energy needs of living cells and organisms are met ultimately by the ONLY source of usable energy available: the radiant energy of the sun.

Photosynthesis can be dissected into two distinct, but coupled sequences of reactions. In the light reaction, light energy is converted into chemical energy, in the form of ATP and reduced NADP. NADP is a small molecule that carries energetic electrons. In the dark reaction (the Calvin cycle), the ATP and reduced NADP act to chemically reduce CO₂ to high energy carbohydrates, a process generally referred to as CO₂ fixation. These carbohydrates are then either stored, or used to synthesize other molecular building blocks needed by all living things to support their metabolism, growth, and reproduction.

Chemically of course, when an atom of molecule gains electrons and becomes reduced, some other atom or molecule must be lose electrons and be thus oxidized. In photosynthesis, the electrons that reduce CO₂ are ultimately removed from H₂O. As a consequence, water molecules fall apart and O₂ is released to the atmosphere. This oxygen will be eventually be used in aerobic (oxygen-requiring) respiratory activities, as living organisms, "harvest" (via respiration) the energy stored in carbohydrates, fats, etc. and use it to perform the biological "work" of growth and reproduction. Interestingly enough, the O₂ so essential to aerobic organisms such as ourselves is a waste product of photosynthesis.

The equation below summarizes the overall photosynthetic reaction:

where:

n = the number of CO₂ molecules "fixed"

light = photosynthetically active visible radiation

chloroplast = the organelle within which photosynthesis occurs in eukaryotic cells.

By thinking about this equation, it is clear that we could study photosynthesis by measuring the light-driven production of O₂ and/or the removal of CO₂ from the air. In this laboratory module, that is precisely what we are going to do. In the three exercises that follow, we will study various aspects of the photosynthetic process, paying particular attention to the environmental parameters that control the rate, or speed, of the reaction, as well as characterizing the molecules directly involved in the absorption of light.

To download and print a worksheet for this exercise, select Resources from the navigation bar to the left.

To begin, select Action Spectrum from the links in the navigation bar on the left.