Higher Tier: amino acids join to form polypeptides
| English | 中文 | Pinyin |
|---|---|---|
| amino group/əˈmiːnəʊ ɡruːp/ | 氨基 | ān jī |
| peptide link | 肽键 | tài jiàn |
What would explain this observation?
- Higher Tier: Glycine H₂N–CH₂–COOH has two different functional groups in one molecule. Its amino end and carboxylic-acid end let it join with other amino acids while forming water.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- An amino acid has an amino group 氨基 and a carboxylic-acid group. In the stated glycine example, the amino group is H₂N– and the acid group is –COOH, with CH₂ between them. Condensation links the acid-derived carbonyl carbon of one molecule to the amino nitrogen of another. The C(=O)–NH connection is a peptide link 肽键. Repeated joining forms a polypeptide, represented for glycine by the repeating interior –NH–CH₂–C(=O)–.
- amino group: The nitrogen-containing –NH₂ functional group in the stated amino-acid structure; peptide link: The C(=O)–NH link formed between the stated amino-acid residues by condensation.
Which link joins amino-acid residues in the stated polypeptide?
Each joining event loses OH from one carboxylic-acid group and H from one amino group as H₂O. A chain can extend because functional groups remain at its ends. Different amino acids can combine in the same chain to make proteins; their order and folding affect the resulting protein. Glycine-only repetition is a simple model, not a claim that every natural protein contains one repeating amino-acid type. Detailed zwitterion, stereochemistry and protein-structure classifications are not required here.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Each joining event loses OH from one carboxylic-acid group and H from one amino group as H₂O. A chain can extend because functional groups remain at its ends. Different amino acids can combine in the same chain to make proteins; their order and folding affect the resulting protein. Glycine-only repetition is a simple model, not a claim that every natural protein contains one repeating amino-acid type. Detailed zwitterion, stereochemistry and protein-structure classifications are not required here.
- Use labelled amino-acid cards with distinguishable amino and acid ends. Form a short open chain, mark each peptide link, retain the carbonyl double bonds and account for each water molecule. Then build a chain using several different amino-acid labels to represent sequence variation. Keep chemistry evidence separate from a dietary claim: identifying a peptide bond does not establish the biological role, nutritional value or safety of an unknown sample.
Which two habits make the investigation or model in this case more defensible?
Use labelled amino-acid cards with distinguishable amino and acid ends. Form a short open chain, mark each peptide link, retain the carbonyl double bonds and account for each water molecule. Then build a chain using several different amino-acid labels to represent sequence variation. Keep chemistry evidence separate from a dietary claim: identifying a peptide bond does not establish the biological role, nutritional value or safety of an unknown sample.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: four glycine molecules form a finite open chain with three links. They release three water molecules. Using supplied relative masses glycine=75 and water=18, the chain relative mass is 4×75−3×18=246. This calculation includes its remaining terminal groups, unlike multiplying an ideal interior repeat mass alone. It illustrates conservation of matter; the smaller mass is in water, not destroyed atoms.
Five glycine molecules form an open chain with four links. Given relative masses 75 and water 18, calculate chain relative mass. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Five glycine molecules form an open chain with four links. Given relative masses 75 and water 18, calculate chain relative mass.
The result is 303 . Known: four glycine molecules form a finite open chain with three links. They release three water molecules. Using supplied relative masses glycine=75 and water=18, the chain relative mass is 4×75−3×18=246. This calculation includes its remaining terminal groups, unlike multiplying an ideal interior repeat mass alone. It illustrates conservation of matter; the smaller mass is in water, not destroyed atoms.
Check the conclusion and its limits
- A peptide link C(=O)–NH differs from an ester link C(=O)–O. Do not remove the carbonyl oxygen or lose an entire amino group. The one-water-per-link finite-chain calculation excludes cyclic or branched arrangements. Amino-acid condensation chemistry is HT only, while recognising amino acids as protein monomers in 4.7.3.4 applies on both tiers.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Forming a peptide link removes every nitrogen atom from the amino acids. This claim is false: A peptide link C(=O)–NH differs from an ester link C(=O)–O. Do not remove the carbonyl oxygen or lose an entire amino group. The one-water-per-link finite-chain calculation excludes cyclic or branched arrangements. Amino-acid condensation chemistry is HT only, while recognising amino acids as protein monomers in 4.7.3.4 applies on both tiers.
Higher Tier: amino acids join to form polypeptides: Each joining event loses OH from one carboxylic-acid group and H from one amino group as H₂O. A chain can extend because functional groups remain at its ends. Different amino acids can combine in the same chain to make proteins; their order and folding affect the resulting protein. Glycine-only repetition is a simple model, not a claim that every natural protein contains one repeating amino-acid type. Detailed zwitterion, stereochemistry and protein-structure classifications are not required here.
Forming a peptide link removes every nitrogen atom from the amino acids.
A peptide link C(=O)–NH differs from an ester link C(=O)–O. Do not remove the carbonyl oxygen or lose an entire amino group. The one-water-per-link finite-chain calculation excludes cyclic or branched arrangements. Amino-acid condensation chemistry is HT only, while recognising amino acids as protein monomers in 4.7.3.4 applies on both tiers.
The nitrogen-containing –NH₂ functional group in the stated amino-acid structure: write the technical term.
amino group means The nitrogen-containing –NH₂ functional group in the stated amino-acid structure.