A cell joins 25 amino acids end to end into one chain with no branches…
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A cell joins amino acids end to end into one chain with no branches. Take the average mass of an amino acid as daltons, and the mass of a water molecule as daltons. (a) How many peptide bonds does the chain have? (b) How many molecules of water are given off? (c) What is the mass of the chain? (d) Which two groups join at each bond?
Need a reminder? The key ideaShow
Joining two monomers removes a molecule of water. Peptide bond: amino acids, between the carboxyl group of one and the amino group of the next. Glycosidic bond: monosaccharides. Ester bond: glycerol and fatty acids; a triglyceride has three. Phosphodiester bond: nucleotides, a phosphate that makes two ester bonds, linking the 3' carbon and the 5' carbon of neighbouring sugars. A chain of n monomers has n minus 1 bonds. Hydrolysis breaks a bond by adding water, as in digestion.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Bonds. A chain of amino acids has peptide bonds.
Step 2. Water. One molecule of water is given off for each bond, so molecules.
Step 3. Mass. The amino acids add up to daltons. The water given off is daltons. The chain has a mass of daltons.
Step 4. The groups. At each bond, the carboxyl group of one amino acid joins the amino group of the next.
Answer: (a) bonds; (b) molecules of water; (c) daltons; (d) the carboxyl group and the amino group.
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