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Out of the 90 or so naturally occurring elements, these are the major plant nutrients:


Bonds depend on valency - how many links an atom can make:

Covalent bonds involve shared electrons and are most stable. Most bonds in most compounds are covalent

Ionic bonds involve loss and gain of electrons and can be broken, especially in solution:

Hydrogen bonds depend on the attraction of the hydrogen nucleus (+) for electronegative atoms
(especially oxygen). These are individually weak but a lot of them can add up to make a strong association
- this is part of the reason that cellulose is so strong.

Organic molecules

In chemistry "organic" means containing carbon, usually in combination with hydrogen.

Thus methane is the simplest organic compound.

If carbons are strung together in chains we have a straight-chain or aliphatic compound - as in fats and waxes

If carbons are in some kind of ring we have a cyclic compound, and if we have a six-membered ring with three double bonds we have an aromatic compound - as in lignin and many other plant secondary metabolites

Hexane also happens to be an example of a saturated compound because it has single bonds between carbons and the rest of the bonds are "saturated" with hydrogens (like in those bad animal fats)

Benzene is an unsaturated compound because it contains double bonds and less hydrogens than would be required to saturate the carbons (like in those good plant fats)

Organic molecules are often shown as stick diagrams:

Every bend or end of a stick represents a carbon and as many hydrogens as necessary to fill the unused carbon bonds. A large part of the imortance of plants in our world is that they can take a single carbon molecule (CO2) and make carbon chains in photosynthesis:

The functions of organic compounds in plants usually depend on additional atoms, often in particular structures or functional groups:

Carboxyl and phosphate groups can ionize to a negatively charged ion (anion). Thus they are acidic groups that can form salts with metals or other cations:

Amino groups tend to acquire hydrogen ions so that they ionize to a positively charged cation. They are basic and can form salts with anions:

Organic acids and bases are weak (relatively undissociated) by comparison with mineral acids and bases. That is why we can eat acetic acid (vinegar) on our salad, but not hydrochloric acid. Carboxylic acids and amines can combine ionically to form salts or covalently to form amides:

Organic acids and alcohols can also combine to form esters (R1 COOR2). These are often important in fruit flavors. For example butyl acetate is present in bananas and iso-amyl acetate in some pears.


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Copyright © Michael Knee
The Ohio State University
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