1. Unit 3 Science Investigation Skills
Topic D: Enzymes in action
Lesson 1

2. Learning objectives To become familiar with assessment command words
To gain awareness of the method of assessment for Unit 3
To describe peptide linkage of amino acids
To research the different levels of protein structure and the types of bonds involved
To relate protein structure to function

3. Command words
There are many command words that are used to tell you what you need to do in the assessment tasks.
You need to have a firm understanding of the specific meanings of the command words used in assessments.
The command words required are defined in the specification and you should make sure you know what they are asking you to do.

4. Example command word: ‘compare’
The definition of ‘compare’ in the specification is as follows:
You need to look for the similarities and differences of two (or more) things.
The answer must include at least one similarity and one difference.
This should not require the drawing of a conclusion.
The answer must relate to both (or all) things mentioned in the question.
Practice task:
Compare sheep and rabbits, following the above definition.
You could ask a friend to check you have included all the requirements for a good comparison, and have left out points not required.

5. How will you be assessed on this unit?
Unit 3 is an externally assessed unit.
The assessment consists of two parts:
Part A:
You will carry out an investigation following a task brief provided by Pearson. You will have two weeks to do this.
You will need to plan, carry out and record a scientific investigation to create primary data (results/observations) for use in Part B.
You will not be assessed on practical competence.
Part B:
This takes place in May or June, two weeks after beginning part A.
Part B is a written task worth 60 marks.
You will have 1½ hours to complete a task booklet in which you will process, analyse and evaluate your primary data and secondary data.
Sample assessment materials will be available for you to practise your skills.

6. What are the functions of these substances in the body?
keratin
enzymes
haemoglobin
hormones
collagen
antibodies

7. What do all of these substances have in common?
They all consist of protein.
keratin
enzymes
haemoglobin
(some)
hormones
collagen
antibodies

8. Proteins consist of long chains of amino acids (polypeptides).
Figure 5.21 p. 78 Biology 3rd Edition Campbell Benjamin/Cummings ISBN

9. Structure of an amino acid The central carbon atom of an amino acid is bonded to a hydrogen atom and three other groups.
Figure after © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2011 by Steven M. Carr

10. Build a model of an amino acid using a hydrogen atom for the R group as an example.
Figure after © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2011 by Steven M. Carr

11. Amino acids are joined together by peptide bonds which form between the carboxyl group of one amino acid and the amino group of another. A molecule of water is lost, so this is a condensation reaction.
Image taken from : © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2011 by Steven M. Carr

12. How long a polypeptide chain can your class make?
The amino acid glycine has an R group consisting of a hydrogen atom. Add this to your model to form a glycine molecule.
Working with a partner, link two glycines together by removing H2O and form a peptide bond between them.
Image taken from : © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2011 by Steven M. Carr
How long a polypeptide chain can your class make?

13. Different R groups assign different properties to the amino acids
Different R groups assign different properties to the amino acids. There are four main types of amino acids with properties determined by their R groups:
Polar/hydrophilic
Non-polar/hydrophobic
Acidic/negatively charged R groups
Basic/positively charged R groups

14. Polar R groups are hydrophilic and non-polar R groups are hydrophobic.
Test your understanding
In a soluble blood protein, which type of amino acid would you expect to find on the external surface of the protein molecule (exposed to the water-containing plasma)?
Which type of amino acid would you expect to find embedded within the internal part of the molecule?

15. Some R groups have a negative charge (acidic, eg
Some R groups have a negative charge (acidic, eg. COO-) and some have a positive charge (basic, e.g. NH3+).
From the five amino acids shown below, can you choose two that might be attracted to each other and pulled together in a protein structure?

16. Different types of bond form between different types of amino acid R groups within a protein.

17. Research activity
Protein structure is described at three levels: primary, secondary and tertiary.
Use books and Internet sources to find out about these levels of structure and identify:
the structural features involved at each level
the types of R groups, attractions and bonds involved in holding these structural features together.
Extension questions
Some proteins have a further level of structure. What is this called and what does it involve? Can you identify any examples of proteins with this level of structure?

18. The different amino acid R groups:
cause a chain of amino acids (polypeptide chain) to fold into a particular shape
assign properties that enable the protein to carry out its function in a living organism.
Protein shapes come in two main forms: fibrous and globular.
Fibrous
Globular

19. Fibrous proteins are insoluble in water and have strong, elongated structures.
Globular proteins are soluble in water and have more rounded structures.
Can you sort these proteins into the two groups?
elastin
fibrous
globular