Which of the following organic molecules contain both an amine and carboxyl group?
A.
Lipids
B.
Chitin
C.
Cellulose
D.
Proteins
The Correct answer and Explanation is:
The correct answer is D. Proteins.
Explanation:
Proteins are complex organic molecules composed of long chains of amino acids. Each amino acid, the building block of proteins, contains both an amine group and a carboxyl group.
Here’s a closer look at these functional groups:
- Amine Group: This group consists of a nitrogen atom bonded to two hydrogen atoms (—NH₂). It acts as a base, accepting a proton (H⁺) to form an ammonium ion (—NH₃⁺). In amino acids, the amine group is attached to the central carbon atom (alpha carbon) of the amino acid structure.
- Carboxyl Group: This group is characterized by a carbonyl group (a carbon double-bonded to an oxygen) and a hydroxyl group (an —OH group) attached to the same carbon (—COOH). This makes the carboxyl group acidic, as it can donate a proton (H⁺) to form a carboxylate ion (—COO⁻). In amino acids, the carboxyl group is also attached to the alpha carbon.
Amino acids, the monomers of proteins, are defined by their structure, which includes these two functional groups:
- General Structure of an Amino Acid:
- Amine Group: —NH₂
- Carboxyl Group: —COOH
- Hydrogen Atom: —H
- R Group (Side Chain): This varies between different amino acids and determines their specific properties.
Proteins are formed when amino acids link together through peptide bonds, which involve the carboxyl group of one amino acid reacting with the amine group of another. This bonding process results in a long chain of amino acids, which then folds into specific three-dimensional structures necessary for their biological function.
In contrast, the other options listed do not contain both an amine and carboxyl group:
- Lipids are primarily composed of fatty acids and glycerol. They do not have both amine and carboxyl groups; rather, they contain ester linkages.
- Chitin and Cellulose are polysaccharides made of sugar units. Chitin is composed of N-acetylglucosamine units, and cellulose is made of glucose units. Neither contains amine groups; cellulose has hydroxyl groups, and chitin has amino groups in addition to the carboxyl groups. However, neither has both functional groups in a single molecule like proteins.
Thus, proteins are the only class of molecules among the given options that contain both an amine and a carboxyl group within each of their constituent amino acids.