Hello IB Biologists! Welcome to Carbohydrates and Lipids

Welcome to the fascinating world of biomolecules! This chapter, Carbohydrates and Lipids, introduces you to the essential organic compounds that make up all living organisms. Think of these molecules as the fundamental building blocks (like LEGOs) and the primary energy sources (like fuel) that allow your cells to function, grow, and maintain structure.

Since we are focusing on the "Form and function" section of the curriculum, we will specifically explore how the unique chemical structure (the form) of each molecule determines its specific role (the function) inside the cell and organism.

SECTION 1: Carbohydrates – The Energy Currency

Carbohydrates are macromolecules composed of carbon, hydrogen, and oxygen, usually in a ratio of 1:2:1. The basic formula is approximately \(\text{C}_{n}(\text{H}_2\text{O})_{n}\), which is why they are literally called "hydrated carbons."

1.1 Form and Function of Carbohydrates

Carbohydrates serve two main functions in living things:

  • Primary Energy Source: Sugars (like glucose) are quickly broken down to release energy (ATP) via cell respiration.
  • Structural Support: Complex carbohydrates (like cellulose) provide strong, rigid support, especially in plants.

1.2 Monomers and Polymers: The Building Blocks

The basic unit of a carbohydrate is a single sugar molecule called a monosaccharide (mono- means one, saccharide means sugar). When many monosaccharides link up, they form a large chain called a polysaccharide.

Key Monosaccharides (Simple Sugars)

These molecules are sweet and readily soluble in water:

  • Glucose: The most vital monosaccharide; the main source of energy for most living things. It is transported in the blood.
  • Ribose: A five-carbon sugar essential for forming the backbone of RNA and ATP.
  • Galactose: A simple sugar found primarily in milk.
Key Disaccharides (Double Sugars)

These are formed when two monosaccharides are linked together:

  • Maltose (\(\text{Glucose} + \text{Glucose}\)): Used in brewing beer.
  • Lactose (\(\text{Glucose} + \text{Galactose}\)): The sugar found in milk. (Lactose intolerance occurs when the enzyme lactase cannot break this bond).
  • Sucrose (\(\text{Glucose} + \text{Fructose}\)): Common table sugar.

Quick Review: Joining and Breaking Bonds

How do monosaccharides link to form larger molecules? Through chemical reactions:

1. Condensation Reaction (or Dehydration Synthesis):
This is how large molecules are built (synthesis).

Process: Two smaller molecules join together, and a molecule of water is removed (\(\text{H}_2\text{O}\)). This forms a strong covalent bond called a glycosidic bond (in carbohydrates).
Analogy: Imagine two people holding hands only after one drops a water bottle.

2. Hydrolysis Reaction (Hydro- means water, -lysis means splitting):
This is how large molecules are broken down.

Process: A large molecule is split into two smaller molecules by adding a molecule of water. Water breaks the glycosidic bond. This is essential for digestion.
Analogy: A beaver splits a log using the water from the river.

1.3 Key Polysaccharides: Long Chains, Diverse Roles

Polysaccharides are large polymers that function mainly for energy storage or structural support. Their function is directly related to the arrangement (form) of their glucose subunits.

Storage Polysaccharides (For Energy)

  • Starch (Found in Plants):
    • Form: Made up of coiled, helical chains (amylose and amylopectin). This coiled form makes it good for storage.
    • Function: Long-term energy storage in plants (e.g., potatoes, rice).
  • Glycogen (Found in Animals):
    • Form: Highly branched structure. This branching creates many "ends" where glucose can be quickly added or removed.
    • Function: Short-term energy storage in animals, primarily stored in the liver and muscles.

Structural Polysaccharides (For Support)

  • Cellulose (Found in Plant Cell Walls):
    • Form: Made of straight, unbranched chains of glucose molecules. Crucially, the glucose subunits are linked in an arrangement that allows adjacent chains to form strong hydrogen bonds with each other.
    • Function: Provides immense strength and rigidity to plant cell walls, allowing plants to stand upright. Animals cannot easily digest cellulose (it’s known as dietary fiber).
Key Takeaway: Carbohydrates

Carbohydrates are quick energy suppliers and structural providers. Remember: condensation builds, hydrolysis breaks. The branching (glycogen) or straightness (cellulose) of the chain determines its role!

SECTION 2: Lipids – Fats, Oils, and Membranes

Lipids are a diverse group of non-polar, hydrophobic (water-fearing) compounds, including fats, oils, waxes, and steroids. They are generally insoluble in water but soluble in organic solvents.

2.1 Form and Function of Lipids

The unique non-polar structure of lipids allows them to perform functions that carbohydrates cannot:

  • Long-Term Energy Storage: They store about twice as much energy per gram as carbohydrates (less weight to carry around!).
  • Thermal Insulation: Fat layers under the skin protect and insulate (e.g., blubber in marine mammals).
  • Structural Component: Phospholipids form the essential structure of all cell membranes.
  • Hormones: Steroids (like cholesterol and sex hormones) act as chemical signals.

Did you know? If humans stored all their energy as carbohydrates instead of lipids, we would weigh significantly more because carbohydrates need to be stored alongside water. Lipids are compact and anhydrous (without water).

2.2 Triglycerides: The Storage Lipids

The most common type of fat used for energy storage is the triglyceride (a neutral fat).

Structure of a Triglyceride:

A triglyceride is formed by linking three fatty acid molecules to one glycerol molecule.

  • This reaction occurs via three condensation reactions, releasing three water molecules.
  • The bonds formed are called ester bonds.

2.3 Fatty Acids: Saturated vs. Unsaturated

Fatty acids are long hydrocarbon chains that form the "tails" of many lipids. The structure of these tails is crucial for determining the physical properties (form) of the fat.

Saturated Fatty Acids (The "Straight" Ones)
  • Form: The carbon atoms are linked by single covalent bonds only. The chain is straight and can pack tightly together.
  • Function/Properties: Usually solid at room temperature (e.g., butter, animal fat). Too much can be linked to cardiovascular disease.
Unsaturated Fatty Acids (The "Kinky" Ones)
  • Form: Contains one or more double bonds between carbon atoms. These double bonds introduce a "kink" or bend in the chain, preventing tight packing.
  • Function/Properties: Usually liquid at room temperature (e.g., olive oil). Considered healthier.
HL Extension: Cis and Trans Isomers

The double bonds in unsaturated fatty acids can exist as two different geometrical isomers, which dramatically affect the molecule's shape and function:

  1. Cis-Isomers (Natural Kinks):
    The two hydrogen atoms around the double bond are on the same side. This causes the distinct, natural bend in the chain. These are typically easily metabolized.
  2. Trans-Isomers (Processed, "Straightened" Kinks):
    The two hydrogen atoms are on opposite sides of the double bond. This straightens the chain, allowing it to pack almost like a saturated fat.

    Important: Trans-fats are often artificially created during the hydrogenation of oils and are strongly associated with increased risk of heart disease because their unusual shape makes them harder for the body to break down.

2.4 Phospholipids: Building Cell Barriers

Phospholipids are arguably the most important structural lipid in the body, as they define the boundaries of life.

Structure of a Phospholipid:

It is similar to a triglyceride, but one fatty acid is replaced by a phosphate group. This small change gives it two very different ends:

  • Head: Contains the phosphate group; it is hydrophilic (water-loving).
  • Tails: The two fatty acid chains; they are hydrophobic (water-fearing).

Function: When placed in water, phospholipids spontaneously arrange themselves into a bilayer, with the hydrophilic heads facing the watery environment and the hydrophobic tails tucked safely inside. This forms the basis of the cell membrane. This self-assembling structure is a perfect example of form determining function!

2.5 Assessing Health: Body Mass Index (BMI)

Since lipids are the primary storage molecule, health professionals often use Body Mass Index (BMI) to screen for health issues related to body fat.

The calculation requires two measurements (mass in kg and height in m) and is expressed using the following formula:

\[\text{BMI} = \frac{\text{Mass in kilograms}}{\text{(Height in metres)}^2}\]

Common Mistake to Avoid: Remember that BMI is only a screening tool. A very muscular person might have a high BMI but a low body fat percentage, demonstrating that BMI doesn't distinguish between muscle mass and fat mass.

Key Takeaway: Lipids

Lipids are compact, long-term energy stores and are essential for cellular structure. The crucial difference between saturated and unsaturated fats is the presence of double bonds, which cause kinks and affect density. Phospholipids’ dual nature (hydrophilic head, hydrophobic tails) makes them perfect for forming cell membranes.

Summary Comparison: Carbohydrates vs. Lipids

It is important to be able to compare the energy storage roles of these two key molecules:

Energy Storage Comparison

Carbohydrates (Glycogen)

  • Advantages: Easily and quickly digested, transported, and metabolized. Good for short bursts of high-intensity energy.
  • Disadvantages: Stores less energy per unit mass; requires water for storage (making it heavy).

Lipids (Fats)

  • Advantages: Stores approximately twice as much energy per unit mass; stored without water (more compact). Ideal for long-term, sustained energy demands.
  • Disadvantages: Takes longer to break down and utilize for energy.

Keep practicing those structures and reactions—they are the foundation of biochemistry! Good luck!