The structure of an amino acid
pp. 205–210Four groups are attached to the alpha carbon: an amine group, a carboxyl group, a hydrogen atom and an R-group. Only the R-group differs between amino acids.
Guiding questions
- How does the order of amino acids produce such a variety of protein forms and functions?
- How do chemical and physical conditions affect protein molecules?
Slides
Notes
Written for this site from the class slides. The chapter is on pages 205–210 of the course textbook. Image credits are on slide 15.
Condensation forms dipeptides and longer chains
pp. 205–210amino acid + amino acid → dipeptide + water. The -OH from a carboxyl group and an -H from an amine group form water, and a peptide bond links the two. Repeating this adds amino acids to make a polypeptide.
Watch out: a protein contains one or more polypeptides, folded into a working shape.
Dietary requirements
pp. 205–210Essential amino acids cannot be made, and must come from food. Non-essential ones can be made from other amino acids. Vegan diets need attention, to ensure that all the essential amino acids are included.
The limitless variety of peptide chains
pp. 205–210There are 20 amino acids in the genetic code. They can be in any order, and chains can have from a few to thousands of amino acids. A chain of 3 can have 8000 sequences, and one of 6 can have 64 million.
| Protein | Function |
|---|---|
| Haemoglobin | Carries oxygen in red blood cells |
| Immunoglobulin | Antibody that binds to antigens |
| Rhodopsin | Pigment in the retina, absorbs light |
| Collagen | Rope-like strength in skin and tendons |
| Insulin | Hormone that lowers blood glucose |
Effects of pH and temperature
pp. 205–210The shape of a protein is held by weak bonds and interactions between R-groups. Heat makes molecules vibrate, and this breaks those bonds. A change in pH alters charges on R-groups, and this breaks attractions. Either can cause denaturation: the shape changes, usually permanently, and the protein loses its function.
Watch out: denaturation does not break peptide bonds, so the sequence stays the same.
Videos
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Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all five statements, from quick recall to drawings, data and a longer answer. It has 32 marks and takes about 30 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Practical
Denaturing egg white protein
You will need: Egg white, test tubes, water bath at several temperatures, vinegar, sodium hydrogen carbonate solution
- Put equal amounts of egg white into test tubes
- Warm some at different temperatures and add vinegar or alkali to others
- Record the change in appearance
- Explain the changes using bonds in protein structure
Safety: Eye protection. Take care with hot water. Do not taste. Check for egg allergies.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Measurements
Frederick Sanger's sequencing of insulin in the 1950s showed that every protein has a precise order of amino acids.
Theory of knowledge
How can new techniques change the questions we ask?
Before sequencing, some doubted that proteins had a defined order. Consider how methods decide which questions can be answered.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- B1.2.1The general structure shared by amino acidsp. 205
- B1.2.2How condensation joins amino acids into dipeptides and longer chainsp. 207
- B1.2.3The amino acids that must come from the dietp. 208
- B1.2.4Why the possible peptide chains are effectively unlimitedp. 209
- B1.2.5How pH and temperature affect the structure of a proteinp. 210
HL extension
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