Why Do Plants and Animals Break Down Sugar

Metabolism

To some, Justus von Liebig was one of the outstanding chemists of the early 1800's. Born at Darmstadt in 1803, he studied in Bonn and Erlangen, but finding no one interested in chemistry, moved onto Paris and worked in the famous laboratory of Guy-Lussac. His hot temper went everywhere with him, and he often alienated his friends with his pointed remarks and aggressive writing. He thought that fermentation, and the recent work by Pasteur, all wrong. He opposed the theory of catalysis and considered that fermentations were brought about by vibrations spreading from one part of the fermentation to another.

According to Liebig, animals and plants were complimentary; plants synthesized complex materials from simple substances and animals ate this material and broke it down again. Animal metabolism, therefore was a simple, one-way flow of food that began with the intake of proteins and carbohydrates and ended with the excretion of carbon dioxide and urea.

This simple picture of metabolism did not last long. Claude Bernard discovered that the animal liver was capable of synthesizing glycogen, Eduard Buchner discovered 'cell-free' fermentation and the breakdown of sugars to carbon dioxide, and McCollum found that important materials such as amino acids were both made by animals and also required in their diet. Cells in both animals and plants, it seemed, were complex factories in which raw materials were constantly being transformed: sometimes synthesized into complex structures, and sometimes broken down into simpler forms.

Metabolism came to mean the thousands and thousands of inter-related processes of building, maintaining, using and degrading that cells carry out every minute of every day. Not long after Mendel, this murky, chaotic picture began to clarify somewhat, and it was realized that there were pathways of events leading to the synthesis or breakdown of a substance. In these pathways separate component enzymes catalyzed discrete steps in a multi-step set of chemical reactions that transformed one type of molecule into a different type.

Some phenomena, like fermentation, were simply the results of one of these pathways in action. Yeasts take in sugar, and breakdown the sugar molecules in a series of steps and stages, all regulated and controlled by enzymes. Energy is released and stored as ATP, and eventually the remnants of the sugar molecule become alcohol and carbon dioxide. As Buchner showed, if you take the enzymes out of the cell, they still catalyze the same reactions and produce the same result.

We now recognize, however, that there are several large components within the general category of 'metabolism'.

Energy Manipulation Photosynthetic plants have the ability to trap light energy and convert it to chemical energy. Some of this energy is then used to make storage molecules like sugar and starch. Both plant and animal cells have the ability to breakdown the sugar and transferring the stored energy into molecules such as ATP, which become the short term energy currency of the cell.

Catabolism - breaking down a variety of different molecules from old worn out cellulose (the walls around plant cells) to old worn out enzymes (proteins). Once broken down into simpler forms, this material can be either broken down further to carbon dioxide and water and the released energy stored and used, or the materials can be recycled and used again.

Anabolism - building up or synthesizing complex substances, like proteins, from simpler substances such as amino acids. Biosynthesis is the construction of large molecules and other cellular structures that need to take place all the time (to replace worn out structures and macro-molecules), or which are needed for growth and division of cells. In rapidly growing cells, like bacteria, biosynthesis is a huge requirement, and at its maximum (when a bacterial cell can divide every 20 - 30 minutes), such a cell is making over 1,000 new protein molecules a second.

Why Do Plants and Animals Break Down Sugar

Source: http://www.brooklyn.cuny.edu/bc/ahp/BE/BioE/BE.ChemReact.2.07.html

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