In a similar way, two amino acids can be linked to form a dipeptide (Figure 2.5): amino acid + amino acid → dipeptide + water When more than two amino acids are joined in this way, a polypeptide is formed. Polypeptide chains form protein molecules. amino acid
amino acid
R
H N H
O
C
R
H
C
N OH
H
one molecule of water is released
dipeptide O
C
H
H
C
N OH
H
H
R
O
C
C
R
H
condensation reaction
N
C
H
H
O C OH
peptide bond
H2O
Figure 2.5 Two amino acids combine to form a dipeptide.
In another condensation reaction, glycerol links to fatty acids to produce triglyceride lipid molecules (Figure 2.6): glycerol + 3 fatty acids → triglyceride lipid + water glycerol fatty acid H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
H H
C
OH
condensation reaction
one molecule of water is released for each fatty acid that combines with glycerol
HO
C
H
HO
C
H
HO
C
H
H2O
H Each of the –OH groups in glycerol is able to combine with the –COOH group of a fatty acid. ester bond O C
H
O
C
H
O
C
H
O
C
H
O C O C
triglyceride lipid molecule
H
Figure 2.6 How a triglyceride lipid is formed from glycerol and three fatty acids in a condensation reaction.
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