Table of contents

1. Matter and Measurements4h 31m
2. Atoms and the Periodic Table5h 23m
3. Ionic Compounds2h 18m
4. Molecular Compounds2h 18m
5. Classification & Balancing of Chemical Reactions3h 17m
6. Chemical Reactions & Quantities2h 27m
7. Energy, Rate and Equilibrium3h 46m
8. Gases, Liquids and Solids3h 25m
9. Solutions4h 10m
10. Acids and Bases3h 29m
11. Nuclear Chemistry56m
BONUS: Lab Techniques and Procedures1h 38m
BONUS: Mathematical Operations and Functions47m
12. Introduction to Organic Chemistry1h 34m
13. Alkenes, Alkynes, and Aromatic Compounds2h 12m
14. Compounds with Oxygen or Sulfur1h 6m
15. Aldehydes and Ketones1h 1m
16. Carboxylic Acids and Their Derivatives1h 11m
17. Amines39m
18. Amino Acids and Proteins1h 51m
19. Enzymes1h 37m
20. Carbohydrates1h 41m
21. The Generation of Biochemical Energy2h 8m
22. Carbohydrate Metabolism2h 22m
23. Lipids2h 26m
24. Lipid Metabolism1h 45m
25. Protein and Amino Acid Metabolism1h 37m
26. Nucleic Acids and Protein Synthesis2h 54m

18. Amino Acids and Proteins

Amino Acid Classifications

GOB Chemistry

18. Amino Acids and Proteins

Amino Acid Classifications

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Problem 50

Textbook Question

Is phenylalanine hydrophilic or hydrophobic? Explain why.

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Phenylalanine is an amino acid with a side chain that contains a benzyl group (a benzene ring attached to a CH2 group). To determine if it is hydrophilic or hydrophobic, we need to analyze the properties of its side chain.

Hydrophilic molecules or groups are typically polar or charged, meaning they can interact with water through hydrogen bonding or ionic interactions. Hydrophobic molecules or groups are nonpolar and do not interact well with water.

The benzyl group in phenylalanine's side chain is nonpolar because the benzene ring is composed of carbon and hydrogen atoms, which share electrons relatively equally, resulting in no significant polarity.

Since the side chain of phenylalanine is nonpolar, it does not form hydrogen bonds or interact favorably with water. This makes phenylalanine hydrophobic.

In summary, phenylalanine is hydrophobic because its side chain is nonpolar and does not interact well with water.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hydrophilicity and Hydrophobicity

Hydrophilicity refers to the tendency of a molecule to interact favorably with water, often due to the presence of polar or charged groups. In contrast, hydrophobicity describes a molecule's aversion to water, typically associated with nonpolar structures. Understanding these properties is crucial for predicting how substances behave in biological systems, particularly in relation to solubility and interaction with cellular membranes.

Amino Acid Structure

Amino acids, the building blocks of proteins, have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain). The nature of the R group determines the amino acid's properties, including whether it is hydrophilic or hydrophobic. In the case of phenylalanine, its aromatic side chain contributes to its hydrophobic characteristics.

Phenylalanine's Chemical Properties

Phenylalanine is an aromatic amino acid with a nonpolar side chain, which makes it hydrophobic. The presence of the benzyl group in its structure prevents it from forming hydrogen bonds with water, leading to its preference for nonpolar environments. This property is significant in protein folding and function, as hydrophobic interactions play a key role in stabilizing protein structures.

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Related Practice

Textbook Question

Valine is an amino acid with a nonpolar side chain and serine is one with a polar side chain. Draw the two amino acids.

a. Why is the side chain for valine nonpolar, whereas the side chain for serine is polar?

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Textbook Question

Which amino acid is hydrophilic (dissolves in aqueous solutions)? Why?

a. isoleucine

b. phenylalanine

c. aspartic acid

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Textbook Question

Cell membranes are studded with proteins. Some of these proteins, involved in the transport of molecules across the membrane into the cell, span the entire membrane and are called transmembrane proteins. The interior of the cell membrane is hydrophobic and nonpolar, whereas both the extracellular and intracellular fluids are water-based.

a. List three amino acids you would expect to find in the part of a transmembrane protein that lies within the cell membrane.

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Textbook Question

Four of the most abundant amino acids in proteins are leucine, alanine, glycine, and valine. What do these amino acids have in common? Would you expect these amino acids to be found on the interior or on the exterior of the protein?

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Textbook Question

Which of the following amino acids is most likely to be found on the outside of a soluble protein, and which of them is more likely to be found on the inside? Explain each answer. (Hint: Consider the effect of the amino acid side chain in each case and that the protein is folded up into its globular form.)

a. Valine

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Textbook Question

List the amino acids with side chains that are capable of hydrogen bonding. Draw an example of two of these amino acids hydrogen bonding to one another. For each one, draw a hydrogen bond to water in a separate sketch. Refer to Section 8.2 for help with drawing hydrogen bonds.

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Textbook Question

What are some differences between each of the following pairs?

c. polar and nonpolar amino acids

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Textbook Question

How does the polarity of the R group in leucine compare to the R group in serine?

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