# Carbohydrates and lipids

> IB Biology SL · Theme B: Form and Function
> Source: https://www.owlsprep.com/study/ib-biology-sl-u2-carbohydrates-and-lipids/

This sub-topic explores the structure, properties and key biological roles of two major classes of biomolecules: carbohydrates and lipids. You will learn to link molecular form to core functions in living organisms.

**Prerequisites:** [Introduction to biomolecules](https://www.owlsprep.com/study/ib-biology-sl-u1-introduction-to-biomolecules/)

## Learning objectives

- Distinguish between the structure and function of carbohydrates and lipids
- Classify carbohydrates into monosaccharides, disaccharides and polysaccharides
- Compare structural and functional differences between saturated and unsaturated fatty acids
- Compare the energy storage properties of carbohydrates and lipids
- Link molecular structure of carbohydrates and lipids to their biological roles

## Structure and classification of carbohydrates

**Carbohydrate** — Organic compounds composed of carbon, hydrogen and oxygen, typically with a hydrogen:oxygen ratio of 2:1, used primarily for energy and structural support.

Carbohydrates are classified based on the number of monomer units they contain. Three main classes are assessed in IB Biology SL:

- **Monosaccharides**: Single monomer units, e.g. glucose, ribose, fructose, galactose
- **Disaccharides**: Two monomers joined by a condensation reaction to form a glycosidic bond, e.g. sucrose (glucose + fructose), lactose (glucose + galactose), maltose (two glucose)
- **Polysaccharides**: Long chains of monosaccharides, may be branched or unbranched, e.g. starch, glycogen, cellulose

**Worked example:** Identify the carbohydrate class of cellulose, sucrose, and ribose, then state one function of each.

1. Recall that carbohydrate classification is based on the number of monosaccharide units in the molecule.
2. 1. Ribose is a 5-carbon sugar with one monomer unit, so it is a **monosaccharide**. Its core function is as a component of RNA nucleotides.
3. 2. Sucrose is made of two joined monosaccharides, so it is a **disaccharide**. Its function is sugar transport in vascular plants.
4. 3. Cellulose is a long unbranched chain of glucose monomers, so it is a **polysaccharide**. Its function is structural support in plant cell walls.

> **Exam tip:** Always specify the number of monomers when asked to classify carbohydrates in short answer questions.

## Functions of polysaccharides

Different polysaccharides have distinct 3D structures that lead to very different functional roles, even when all are built from glucose monomers.

| Polysaccharide | Organism | Structure | Core Function |
| --- | --- | --- | --- |
| Amylose (starch) | Plants | Unbranched α-glucose chain | Energy storage |
| Amylopectin (starch) | Plants | Branched α-glucose chain | Energy storage |
| Glycogen | Animals | Highly branched α-glucose chain | Energy storage |
| Cellulose | Plants | Unbranched β-glucose chain | Structural support |

**Worked example:** Explain why cellulose provides structural support in plants but starch does not, even though both are made of glucose monomers.

1. The difference in function arises from the use of different glucose isomers, leading to different overall molecular structure.
2. Cellulose is built from β-glucose monomers linked by 1-4 glycosidic bonds. This arrangement creates straight, unbranched chains that form hydrogen bonds with adjacent chains to create strong, rigid microfibrils.
3. Starch is built from α-glucose monomers, which forms coiled, branched structures ideal for compact energy storage, but not strong enough for structural support.
4. The small difference in monomer structure leads to dramatically different properties suited to their distinct biological roles.

## Structure and classification of lipids

**Lipid** — A diverse group of hydrophobic organic molecules that are insoluble in water but soluble in non-polar solvents. Common types include triglycerides, phospholipids and steroids.

The most common lipids tested in IB SL are triglycerides and phospholipids, both formed from glycerol and fatty acids. Fatty acids are classified as saturated or unsaturated based on the presence of carbon-carbon double bonds:

- **Saturated fatty acids**: No double bonds between carbon atoms, all carbons are fully bonded to hydrogen. Solid at room temperature (e.g. animal fats)
- **Unsaturated fatty acids**: Have one or more double bonds between carbons. Liquid at room temperature (e.g. plant oils)
- Monounsaturated = one double bond; polyunsaturated = multiple double bonds

**Worked example:** Distinguish between the structure of a triglyceride and a phospholipid.

1. Both molecules share a glycerol backbone, where fatty acids are bonded via ester bonds formed by condensation reactions.
2. A triglyceride has three fatty acid chains bonded to the glycerol backbone, with no phosphate group.
3. A phospholipid has only two fatty acid chains bonded to glycerol. The third position is occupied by a polar phosphate group.
4. This gives phospholipids both hydrophobic (fatty acid) and hydrophilic (phosphate) regions, making them uniquely suited to form the bilayer of cell membranes.

## Carbohydrates vs lipids for energy storage

Both carbohydrates and lipids are used for energy storage, but their different properties make them suited for different storage roles:

| Property | Carbohydrates | Lipids |
| --- | --- | --- |
| Energy per gram | ~17 kJ g⁻¹ | ~37 kJ g⁻¹ |
| Water solubility | Soluble (polar) | Insoluble (non-polar) |
| Digestion speed | Easily digested, fast energy release | Slow digestion, slow energy release |
| Storage mass | More mass for same energy | Half the mass for same energy |

**Worked example:** Explain why lipids are more suitable for long-term energy storage in humans than carbohydrates.

1. Lipids release more than twice as much energy per gram than carbohydrates. This means lipids add less than half the body mass for the same amount of stored energy, which is advantageous for mobility.
2. Lipids are insoluble in water, so they do not change the solute concentration of cells, avoiding osmotic water uptake that would occur with soluble carbohydrates.
3. Energy is released more slowly from lipids than carbohydrates, making them ideal for long-term storage rather than immediate energy needs.

> **Exam tip:** This comparison is one of the most common short answer questions for this topic, so memorize the key differences.

## Common pitfalls

- **Wrong:** Confusing α-glucose and β-glucose linkages in starch and cellulose
  - Why it fails: Both molecules are made of glucose, so students assume their linkages are identical
  - Correct: Remember starch and glycogen use α-glucose linkages, while cellulose uses β-glucose linkages
- **Wrong:** Claiming lipids do not contain any oxygen atoms
  - Why it fails: Lipids have a lower oxygen proportion than carbohydrates, leading to the misconception they have no oxygen
  - Correct: Recognize lipids do contain oxygen in the glycerol backbone and fatty acid carboxyl groups
- **Wrong:** Stating all lipids function in energy storage
  - Why it fails: Triglycerides are the most commonly discussed energy-storing lipid, leading to overgeneralization
  - Correct: Remember phospholipids form cell membranes and steroids act as hormones, so lipids have diverse roles
- **Wrong:** Mixing up condensation and hydrolysis reactions for bond formation
  - Why it fails: Students often confuse which reaction forms vs breaks glycosidic/ester bonds
  - Correct: Condensation forms bonds and releases water; hydrolysis breaks bonds and adds water
- **Wrong:** Claiming saturated fatty acids are liquid at room temperature
  - Why it fails: Students mix up the properties of saturated and unsaturated fatty acids
  - Correct: Saturated fatty acids have no double bonds, pack tightly, and are solid at room temperature; unsaturated are liquid

## Cheatsheet

| Category | Key examples | Core function |
| --- | --- | --- |
| Monosaccharides | Glucose, ribose, fructose | Energy / nucleotide component |
| Disaccharides | Sucrose, lactose, maltose | Sugar transport / breakdown |
| Polysaccharides | Starch, glycogen | Short/medium energy storage |
| Polysaccharides | Cellulose | Plant cell wall structural support |
| Triglycerides | Animal fats, plant oils | Long-term energy storage / insulation |
| Phospholipids | Membrane phospholipids | Cell membrane bilayer structure |

## What's next

Understanding the structure and function of carbohydrates and lipids is foundational for almost all subsequent topics in IB Biology SL. The amphipathic nature of phospholipids is the core basis for cell membrane structure and fluidity, which you will explore next. Polysaccharide recognition on cell surfaces is key to understanding cell signaling and immune responses. Energy stored in both carbohydrates and lipids is also released during cellular respiration to produce ATP, which powers all cellular processes in living organisms.

- [Proteins](https://www.owlsprep.com/study/ib-biology-sl-u2-proteins/)
- [Nucleic acid structure](https://www.owlsprep.com/study/ib-biology-sl-u2-nucleic-acid-structure/)
- [Gas exchange and transport in humans](https://www.owlsprep.com/study/ib-biology-sl-u2-gas-exchange-and-transport-in/)

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