MadSci Network: Earth Sciences
Query:

Re: what is photosynthesis.

Date: Fri Oct 23 16:34:14 1998
Posted By: Mark Schneegurt, Faculty, Biological Sciences, University of Notre Dame
Area of science: Earth Sciences
ID: 909098633.Es
Message:

First, let's look at the word - "photosynthesis". "photo" means light or related to light. "synthesis" means making something, usually something complex from something simple. Thus, photosynthesis involves making things using light. Plants, algae and bacteria can use light energy to make their own food through the process of photosynthesis. First the energy in the light is captured by special proteins called light harvesting complexes or antennae. These special proteins contain pigments like chlorophyll (the green stuff in plants) which can capture the energy of photons (the particles that make up light). The light energy is converted into electrical energy (electrons moving through proteins that act like wires). This energy is funnelled into an important protein complex called the reaction center. Here some chemistry takes place and an electron is given a lot of energy and becomes excited. This special electron comes from water. When water is split into hydrogen and oxygen, an electron can be captured and excited by the electrical energy from the light harvesting complexes. The splitting of water is how photosynthesis produces the oxygen we breath. Plants and algae are an important source of oxygen in the atmosphere.

OK. So now we have an excited electron in the reaction center. This electron can be thought of as water at the top of a waterfall. As the electron becomes less excited, it releases some energy. Just as water loses energy as it falls from the top of a waterfall. The little bits of energy released are captured by another set of proteins. These convert the electrical energy of the excited electron into chemical energy. The chemical energy takes the form of high energy chemical bonds in things like ATP and NADPH.

The ATP and NADPH then participate in a set of enzymatic (chemical) reactions called the dark reactions. These reactions can take place without light since the light energy has been captured in ATP and NADPH. The dark reactions, called the Calvin cycle, use the chemical energy of ATP and NADPH to "fix carbon". Here, carbon dioxide from the air is converted into sugars. This is a very important process. Other reactions in the cell can convert these sugars into proteins, nucleic acids like DNA, fats, waxes, and carbohydrates (starches). Thus, all the things that a cell or plant needs can be made from the carbon in the sugars produced by the dark reactions of photosynthesis. Animals like ourselves need to eat plants (or animals that ate plants) in order to survive. We take the complex molecules like sugars and proteins and burn them through respiration, using oxygen and releasing carbon dioxide.

You can see how plants and animals complement eachother. Plants use carbon dioxide and animals produce carbon dioxide. Plants make oxygen and animals use oxygen. Plants are special in that they can make their own food (sugars) from just light, air and water. These thype of creatures are called autotrophs - they make their own food. Animals are heterotrophs, they eat foods made by autotrophs. So, in a nutshell, photosynthesis is the process of capturing light energy and converting it into chemical energy to drive the fixation of carbon dioxide into complex organic molecules.

I hope this helps. If I was too technical, feel free to write me with further questions. If you want more information you may also try this website and the links there:

http://photoscience.la.asu.edu/photosyn/default.html

Cheers,

Dr. Mark.
schneegurt.1@nd.edu


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