4.3 Proteins
Proteins are complex organic macromolecules that serve as vital components of all living cells. Composed primarily of carbon, hydrogen, oxygen, and nitrogen, proteins are built from basic units called amino acids. Through chemical reactions such as condensation and hydrolysis, amino acids assemble into dipeptides and polypeptides, or break down when needed. Proteins play diverse roles in the cell, ranging from building and repairing tissues to forming essential enzymes, hormones, antibodies, and structural components.
Learning Objectives
- State the elements that make up proteins.
- Identify the monomers of proteins and describe how amino acids form dipeptides and polypeptides through condensation.
- Describe the breakdown of dipeptides into amino acids through hydrolysis.
- State and explain the importance of proteins in a cell and the human body.
Condensation and Hydrolysis of Dipeptides
Two amino acids join by a peptide bond with the removal of water, and hydrolysis breaks the peptide bond by adding water.
Condensation
Two amino acid molecules link together by a peptide bond to form a dipeptide. One water molecule is removed.
Hydrolysis
A dipeptide is broken down into two amino acids by the addition of water.
Short Explanation
What are proteins?
Proteins are biological macromolecules made up of carbon, hydrogen, oxygen, nitrogen, and frequently sulphur and phosphorus. The fundamental monomer of a protein is an amino acid. There are approximately 20 types of natural amino acids, and the sequence in which they are arranged determines the specific type and function of the resulting protein.
Elemental composition
Proteins are complex compounds composed of carbon, hydrogen, oxygen, and nitrogen elements. Most proteins also contain sulphur and phosphorus. Common dietary sources rich in proteins include fish, meat, milk, beans, and eggs.
Polymers and monomers
All proteins are composed of one or more polymers known as polypeptides. Each polypeptide is formed from monomers or small units known as amino acids. A polypeptide chain can consist of fifty to thousands of amino acid molecules.
Formation of proteins
When two amino acids combine, a condensation reaction takes place, releasing a water molecule and creating a peptide bond to form a dipeptide. Repeated condensation reactions add more amino acids, producing a long polypeptide chain of 50 to thousands of units.
Breakdown of proteins
During digestion or cellular breakdown, a hydrolysis reaction uses water to cleave the peptide bonds, splitting dipeptides and polypeptides back into individual amino acids. These amino acids are recycled to construct new body proteins or broken down to yield energy.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the monomer or basic unit of a protein?
What type of bond is formed when two amino acid molecules link together during a condensation reaction?
Name two structural proteins found in the human body mentioned in the textbook.
Common Mistakes
- Forgetting that proteins contain nitrogen in addition to carbon, hydrogen, and oxygen, and that many also contain sulphur and phosphorus.
- Confusing condensation and hydrolysis: assuming water is added during condensation and removed during hydrolysis, rather than the reverse.
- Assuming a dipeptide consists of many amino acids instead of specifically two amino acid molecules joined together.
Concept Misunderstandings
Because there are only about 20 naturally occurring amino acids, only a few kinds of proteins can exist.
Varying the number and sequence of these 20 amino acids allows for an almost unlimited variety of polypeptides and proteins.
Proteins only serve as structural materials for bodybuilding.
Proteins also serve as enzymes, hormones, antibodies, haemoglobin, and an alternative energy source.
Summary
- Proteins consist of the elements carbon, hydrogen, oxygen, nitrogen, and often sulphur and phosphorus.
- The basic building blocks (monomers) of proteins are amino acids.
- Two amino acids link via a condensation reaction, forming a peptide bond and releasing one water molecule to create a dipeptide.
- Hydrolysis uses water to break down a dipeptide into its constituent amino acids.
- A polypeptide chain consists of 50 to thousands of amino acids; proteins contain one or more polypeptides.
- Proteins are necessary for building new cells, repairing tissues, synthesizing enzymes, hormones, antibodies, and haemoglobin, providing structural integrity (keratin, collagen, myosin), and providing energy.
Short Activity
Objective: check your understanding of proteins, their monomers, reactions and functions.
A. Objective Quiz
Which element is present in all proteins in addition to carbon, hydrogen, and oxygen?
What process breaks down a dipeptide into two amino acid molecules by adding a water molecule?
B. Fill in the Blanks
Two amino acid molecules are joined together by a bond during a condensation reaction.
Collagen is an important structural protein found in the of the human body.
C. Matching / Drag and Drop
Match each protein or cellular component with its correct description or location in the human body. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Protein or Term to Its Location or Function
Write the correct protein or term next to each description.
| Location or Function | Protein / Term |
|---|---|
| 1. Structural protein found in the skin | |
| 2. Structural protein found in bones | |
| 3. Protein found in muscle tissues | |
| 4. Monomer that builds polypeptides |
Answer choices: Keratin • Collagen • Myosin • Amino acid
