What a functional group is. Some of the key types of functional groups found in biological molecules.
Hydrocarbons, made up entirely of carbon and hydrogen atoms, make wonderful combustion fuels (such fuels include propane, butane, and the bulk of commercial gasoline). But can you build a living organism out of only combustion fuels? Probably not. Most large biological molecules in fact contain many types of atoms beyond just carbon and hydrogen. These additional atoms allow for functionality not typically seen in hydrocarbons.
Large biological molecules are generally composed of a carbon skeleton (made up of carbon and hydrogen atoms) and some other atoms, including oxygen, nitrogen, or sulfur. Often, these additional atoms appear in the context of functional groups. Functional groups are chemical motifs, or patterns of atoms, that display consistent “function” (properties and reactivity) regardless of the exact molecule they are found in. Biological molecules can contain many different types and combinations of functional groups, and a biomolecule’s particular set of groups will affect many of its properties, including its structure, solubility, and reactivity.
A few of the most important functional groups in biological molecules are shown in the table below. Functional groups can be classified as hydrophobic or hydrophilic based on their charge and polarity characteristics. The only hydrophobic group below is the methyl (CH) group, which is nonpolar. The remaining six functional groups in the table all have varying degrees of hydrophilic character.
One example of a strongly hydrophilic group is the carboxyl group (COOH), which can act as an acid and lose a proton to form a negatively-charged carboxylate ion (COO). Carboxyl groups are commonly found in amino acids, fatty acids, and other biomolecules. An example of a less hydrophilic group is the carbonyl group (C=O), an uncharged but polar (contains partial positive and partial negative charges) functional group. Carbonyls are found in many different biological molecules, including proteins, peptides, and carbohydrates.
Common functional groups in biology
|Carboxyl||Charged (forms R-COO) at the pH of most biological systems. Since carboxyl groups can release H into solution, they are considered acidic.|
|Amino||Charged (forms R-NH) at the pH of most biological systems. Since amino groups can remove H from solution, they are considered basic.|
|Phosphate||Charged (forms R-OPO) at the pH of most biological systems. Since phosphate groups can release H into solution, they are considered acidic.|
Table modified from: OpenStax Biology.
In the table above, the letter R is used to represent the rest of the molecule that a functional group is attached to. For instance, R might be an ethyl (CHCH) group, in which case the first entry in the table would correspond to ethanol. But R could also represent the bulk of a much larger molecule, such as a protein. The letter R is used throughout biology and chemistry to simplify chemical structures and highlight the most important parts (often the functional groups!) of a molecule.
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- In the table, what is the difference between a functional group being polar and being charged?(10 votes)
- A charged group is either positive or negative (gains or loses an electron) and a polar group contains atoms that have a difference in electronegativity.(2 votes)
- Are aldehydes and ketones (carbonyls) significantly soluble in water like alcohols and carboxylic acids? If not, why not?(4 votes)
- The short molecules are soluble in polar substances like water because they are relatively polar. The longer the carbon chain is however, the greater the non-polar tail is, and the less soluble aldehydes and ketones become.(7 votes)
- How can you tell if a functional group is acidic or basic just by looking at the functional group?(5 votes)
- how alkanes are less reactive. why?(3 votes)
- Alkanes are less reactive than alkenes.
This is because alkenes have double bonds (C=C).
It is easy to break just one of these bonds in the double bond and make a reaction happen, but breaking the C-C single bond in an alkane is difficult(4 votes)
- please how comes the phosphate group has 5 bonds.whereas phosphorus is a group 5 element and it "needs" just 3 electrons to obey the octate rule?(2 votes)
- There are compounds where phosphorous forms three bonds (this is associated with an oxidation state of +3 or -3), but this element seems to prefer an oxidation state of +5.
The formation of these "extra" bonds is due to hypercoordination (aka. hypervalency, but that may not be a good term to use) and is commonly seen for elements in the 3rd period and beyond.
This is quite complicated to explain, but the current consensus is that the phosphorous in phosphate doesn't actually end up with more than 8 "valence" electrons due to electron withdrawing effects by the oxygens.
(Note: You will find explanations for this that invoke hybridization with d-orbitals, but this has been shown to not be true.)
The wikipedia article on hypercoordination seems like a reasonable place to learn more:
You might also find this section of wikipedia article on phosphorus helpful:
Does that help?(5 votes)
- In the 5th paragraph, there is discussion about carboxyl groups and carboxylate and, although these have been discussed in previous videos, I noticed that I did not remember which was which. The question: Is there a method to the naming of these that might help me remember?(3 votes)
- If something has '-yl' suffix it is an alkyl group.
carboxylate is salt or ester of a carboxyl group (meaning that H is removed).
So you first try to memorize alkyl groups.(0 votes)
- Is all groups hydrophilic except for the methyl and amino groups?(1 vote)
- Amino groups are polar so they are also hydrophilic.
Alkyl groups (e.g., methyl, ethyl, propyl, butyl, etc) in general are hydrophobic.(3 votes)
- Are the groups that don't specify if they are charged or uncharged just neither?(2 votes)
- Some of them are polar, which means that the charge is not absolute, but that the charge is partial.(1 vote)