Biology (9610) | Biological molecules

Unit 1: The Diversity of Living Organisms (3.1.1 Biological Molecules)

Welcome to one of the most fundamental and important topics in Biology! Biological molecules are the essential building blocks of life—they are literally what you, I, plants, and bacteria are made of. Understanding their structure is the key to understanding how organisms function, exchange materials, and even how they are classified.

Don't worry if the chemical structures look a little intimidating; we will break them down step-by-step, focusing only on the details the syllabus requires. Let’s get started!

3.1.1.1 Monomers and Polymers: The Building Blocks

Think of biological molecules like giant LEGO structures.

Key Definitions

• Monomers: The small, repeating individual units (like single LEGO bricks).
• Polymers: The large molecules formed when many monomers join together (the completed LEGO castle).

Most large biological molecules (carbohydrates, proteins, and nucleic acids) are polymers, built from smaller monomers.

Making and Breaking Polymers: Condensation and Hydrolysis

The reactions that build and break these large molecules are crucial. You must know these two terms inside out:

1. Condensation Reaction (Joining)

• What it does: Joins two monomers together to form a larger molecule (or polymer).
• How it works: A chemical bond is formed between the monomers, and simultaneously, a molecule of water is eliminated (removed).
• Analogy: It's like welding two things together and steam (water) comes off as a byproduct.

2. Hydrolysis Reaction (Breaking)

• What it does: Breaks the chemical bond holding two molecules (monomers) together.
• How it works: A molecule of water is added, which splits the bond.
• Memory Aid: Hydro- means water, and -lysis means to split or break. So, Hydrolysis is "splitting with water."

3.1.1.2 Carbohydrates: Energy and Structure

Carbohydrates are essential for energy storage and structural support in organisms. Their monomers are monosaccharides (simple sugars).

Monosaccharides (Single Sugars)

The most common monosaccharides you need to know are glucose and fructose.

Glucose Isomers: \(\alpha\)-glucose and \(\beta\)-glucose
Glucose exists as two structural isomers. Isomers have the same chemical formula but different arrangements of atoms.

• The difference lies in the position of the H and OH groups on carbon atom 1.
• For \(\alpha\)-glucose, the OH group on C1 points Down.
• For \(\beta\)-glucose, the OH group on C1 points Up. (This tiny difference is vital for forming huge structural molecules like cellulose!)

(Note: In the exam, you must be able to recognize and potentially draw or interpret how molecules like these join together.)

Disaccharides (Double Sugars)

When two monosaccharides join by condensation, they form a glycosidic bond and create a disaccharide.

• Maltose: Formed from two \(\alpha\)-glucose molecules. (Found in germinating seeds).
• Sucrose: Formed from one \(\alpha\)-glucose molecule and one fructose molecule. (This is common table sugar, transported in plants).

Exam Tip: You may be shown the structures of *unspecified* carbohydrates and asked to demonstrate where condensation would join them, or where hydrolysis would break them. Always look for the OH groups involved in bonding (condensation) or the existing bond that needs water (hydrolysis).

Polysaccharides (Many Sugars)

Many monosaccharides join by condensation to form polysaccharides. These are polymers used for long-term storage or structure.

1. Starch (Storage in Plants)

• Formed by the condensation of thousands of \(\alpha\)-glucose molecules.
• Composed of two components:
  • Amylose: Unbranched chain, coils into a helix (makes it compact for storage).
  • Amylopectin: Branched chain.
• Function: Starch is a great storage molecule because it is large and insoluble, so it doesn't affect the water potential of the plant cell.

2. Cellulose (Structure in Plants)

• Formed by the condensation of thousands of \(\beta\)-glucose molecules.
• The alternating orientation of \(\beta\)-glucose monomers causes the chains to be straight and unbranched.
• These straight chains form strong microfibrils (fibres) held together by hydrogen bonds.
• Function: Provides structural support in plant cell walls, giving the cell strength and rigidity.

Biochemical Tests for Carbohydrates

You must know the practical tests used to identify these molecules:

1. Testing for Reducing and Non-Reducing Sugars (e.g., glucose, maltose, fructose)

• Reagent: Benedict’s reagent (a blue solution).
• Method (Reducing Sugars): Add Benedict’s to the sample and heat in a water bath.
• Positive Result: Colour change from blue through green, yellow, orange, to brick-red precipitate (due to the formation of copper oxide).
• Method (Non-Reducing Sugars, e.g., Sucrose): First, hydrolyse the sample by heating it with dilute acid. Neutralise the acid, then carry out the Benedict’s test.
• Reason: Hydrolysis breaks the non-reducing sugar into its component reducing sugars (\(\alpha\)-glucose and fructose), allowing the test to work.

2. Testing for Starch (Polysaccharide)

• Reagent: Iodine/Potassium Iodide Solution (a brown/orange solution).
• Method: Add a few drops of the iodine solution directly to the sample.
• Positive Result: Colour change from brown/orange to blue-black.

3.1.1.3 Lipids: Fats, Oils, and Membranes

Lipids are diverse, but they are all generally non-polar and insoluble in water. They are vital for energy storage, insulation, and forming cell membranes.

1. Triglycerides (Fats and Oils)

Triglycerides are the main form of energy storage in both plants (oils) and animals (fats).

• Structure: Formed by the condensation reaction between one molecule of glycerol and three molecules of fatty acids.
• Bond: The resulting chemical bond formed is an ester bond.
• Fatty Acid Formula: Fatty acids have the general formula RCOOH, where R is the long hydrocarbon chain.

Saturated vs. Unsaturated Fatty Acids

• Saturated: The R-group hydrocarbon chain has no double bonds between carbon atoms. These molecules pack tightly together, making them solid at room temperature (e.g., animal fats).
• Unsaturated: The R-group hydrocarbon chain contains one or more double bonds. These bonds cause kinks in the chain, preventing tight packing, making them liquid at room temperature (oils).

Did you know? Saturated fats often lead to higher cholesterol levels because their straight chains are easier to pack into artery walls, illustrating the structure-property link!

2. Phospholipids (Membrane Structure)

Phospholipids are structural lipids, crucial for forming the cell membrane (the plasma membrane).

• Structure: Similar to a triglyceride, but one fatty acid is substituted by a phosphate-containing group.
• Properties and Function: This substitution creates a unique structure:
  • The phosphate head is hydrophilic (water-loving).
  • The fatty acid tails are hydrophobic (water-hating).
• In water, phospholipids spontaneously arrange into a bilayer, forming the basic structure of the cell membrane, separating the cell's contents from the environment.

Biochemical Test for Lipids

The Emulsion Test

• Method: Mix the sample with ethanol (or another organic solvent). Pour this solution into a test tube containing cold water.
• Positive Result: If lipids are present, they will form a white, cloudy suspension known as an emulsion.

3.1.1.4 Proteins: Function and Complexity

Proteins are the workhorses of the cell, carrying out almost every function, from catalysis (enzymes) to transport and structural support.

Amino Acids (The Monomers)

The monomers of proteins are amino acids. There are twenty common amino acids found in all organisms.

General Structure: Every amino acid has four main components attached to a central carbon atom (the \(\alpha\)-carbon):

• An Amine group (\(H_2N\))
• A Carboxyl group (\(COOH\))
• A single Hydrogen atom (\(H\))
• A variable R-group (this is what makes each of the 20 amino acids different and determines their specific properties).

Amino acids join together through condensation to form a peptide bond.

• Two amino acids joining form a dipeptide.
• Many amino acids joining form a polypeptide.

Don't worry if this seems tricky at first—just remember that the R-group is the 'personality' of the amino acid.

Protein Structure and Function

The function of a protein is entirely dependent on its highly specific 3D shape, which is determined by the sequence of its amino acids. We describe this shape in four levels:

1. Primary Structure

• This is simply the sequence of amino acids in the polypeptide chain.
• Determined by the genetic code (DNA).

2. Secondary Structure

• The chain begins to fold or coil due to hydrogen bonds forming between atoms in the polypeptide backbone (not the R-groups).
• Common shapes include the \(\alpha\)-helix (spiral) and the \(\beta\)-pleated sheet (zig-zag fold).

3. Tertiary Structure

• This is the final, complex 3D shape of a single polypeptide chain.
• It is held together by various bonds and interactions between the R-groups (side chains) of the amino acids:
  • Hydrogen bonds (weak, but numerous)
  • Ionic bonds
  • Hydrophobic interactions
  • Disulfide bridges (very strong covalent bonds between cysteine R-groups).
• This specific tertiary structure dictates the protein's function (e.g., the shape of an enzyme's active site).

4. Quaternary Structure

• This structure exists only in proteins made up of two or more polypeptide chains that interact and bind together.
• Example: Haemoglobin (which has four polypeptide chains) and some complex enzymes.

The Importance of Bonding: Hydrogen bonds play roles in both secondary and tertiary structures, while strong disulfide bridges are critical for stabilizing the tertiary structure, especially in proteins exposed to harsh environments.

Protein Function and Structure Relationship: You must be able to relate the properties of specific proteins (like enzymes or membrane proteins) to the unique structures listed above. For instance, the function of an enzyme relies totally on its tertiary structure creating a specific active site shape that is complementary to its substrate.

Biochemical Test for Proteins

The Biuret Test

• Reagent: Potassium hydroxide solution followed by copper sulfate solution (Biuret reagent).
• Method: Add Biuret reagent to the sample and mix.
• Positive Result: Colour change from blue to purple/lilac (indicating the presence of peptide bonds).

Summary of Biochemical Tests

A quick table to help you memorize the three crucial tests:

Molecule Tested	Reagent Used	Positive Result
Reducing Sugars	Benedict’s Reagent + Heat	Brick-Red Precipitate
Starch	Iodine/Potassium Iodide Solution	Blue-Black
Proteins	Biuret Reagent (KOH + CuSO4)	Purple/Lilac Colour
Lipids	Ethanol + Cold Water	White Emulsion/Cloudy Suspension

Remember, mastering these molecules—how they are built, what they look like, and how to test for them—is foundational for understanding everything else in A-Level Biology! Keep practicing those condensation and hydrolysis diagrams!