# Theme B: Form and Function

> IB Biology SL · IB SL Biology 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-sl-u2-overview/
> Weight: 23-27% of overall IB SL Biology exam

This unit explores the core biological principle that structure determines function, spanning from biological macromolecules at the molecular level to organ systems in human physiology. It makes up roughly a quarter of your overall IB SL Biology exam score.

**Prerequisites:** [Introduction to cells](https://www.owlsprep.com/study/ib-biology-sl-u1-introduction-to-cells/); [Basic biological molecules](https://www.owlsprep.com/study/ib-biology-sl-u1-biological-molecules-intro/)

## Learning objectives

- Relate the molecular and structural form of biological systems to their specific functional roles
- Explain how the structure of macromolecules and mammalian organ systems supports their core functions
- Apply the core form-function relationship to solve exam questions about enzymes and human homeostasis
- Identify and draw key structural features of macromolecules and organ tissues per IB SL requirements

## Unit at a Glance

This unit follows a logical progression from the smallest scale (molecular form and function) to the larger scale of whole human organ systems. The core theme uniting all sub-topics is that every biological structure is shaped to carry out its specific role, and changes to structure directly alter function.

You will first build a foundation in the structure of the four key biological macromolecules, then move to applied function in enzymes and metabolism, before covering major human organ system physiology in the second half of the unit.

This unit includes the following core sub-topics:
- [Carbohydrates and lipids](https://www.owlsprep.com/study/ib-biology-sl-u2-carbohydrates-and-lipids/) — Explore carbohydrate and lipid structure and their functional roles in cells and organisms.
- [Proteins](https://www.owlsprep.com/study/ib-biology-sl-u2-proteins/) — Cover amino acid structure and protein folding, linking 3D shape to protein function.
- [Nucleic acid structure](https://www.owlsprep.com/study/ib-biology-sl-u2-nucleic-acid-structure/) — Learn DNA and RNA structure and how their form supports genetic information storage.
- [Enzymes and metabolism](https://www.owlsprep.com/study/ib-biology-sl-u2-enzymes-and-metabolism/) — Explain how enzyme structure enables substrate specificity and regulation of metabolism.
- [Gas exchange and transport in humans](https://www.owlsprep.com/study/ib-biology-sl-u2-gas-exchange-and-transport-in/) — Examine how lung and circulatory system structure supports gas exchange and oxygen transport.
- [Digestion and absorption in humans](https://www.owlsprep.com/study/ib-biology-sl-u2-digestion-and-absorption-in-humans/) — Explore how the structure of each digestive organ supports its role in breaking down and absorbing nutrients.
- [Muscle and motility](https://www.owlsprep.com/study/ib-biology-sl-u2-muscle-and-motility/) — Understand how the structure of skeletal muscle and bone enables movement in humans.
- [Hormones, homeostasis and reproduction](https://www.owlsprep.com/study/ib-biology-sl-u2-hormones-homeostasis-and-reproduction/) — Explain how hormone structure and organ function maintain homeostasis and support human reproduction.

## Common pitfalls

- **Wrong:** Reversing the core relationship, claiming function determines structure
  - Why it fails: IB exams explicitly test the principle that structure enables function, shaped by evolution
  - Correct: Always link observed structural features to the function they allow when answering questions
- **Wrong:** Mixing up levels of protein folding, especially secondary vs tertiary structure
  - Why it fails: This is a common multiple choice and short answer question topic on IB SL exams
  - Correct: Remember: primary = amino acid sequence, secondary = local H-bond folding, tertiary = full 3D shape of one polypeptide
- **Wrong:** Describing alveoli structure without linking it to gas exchange function
  - Why it fails: Examiners require explicit form-function connections to award full marks for this question
  - Correct: Always connect large surface area, thin epithelium and dense capillary supply to faster diffusion

## Cheatsheet

| Concept | Key Unit-wide Takeaway |
| --- | --- |
| Core Biological Principle | Structure always determines function |
| DNA vs RNA Structure | DNA: double-stranded, deoxyribose, ATCG; RNA: single-stranded, ribose, AUCG |
| Protein Folding Levels | Primary → Secondary → Tertiary → Quaternary, each level builds final functional shape |
| Saturated vs Unsaturated Lipids | Saturated: no double bonds, solid at room temp; Unsaturated: 1+ double bonds, liquid at room temp |
| Enzyme Activity Factors | Rate is affected by temperature, pH, substrate concentration, and enzyme concentration |
| Alveoli Adaptations | Large surface area, 1-cell thick walls, dense capillaries = efficient gas exchange |
| Homeostasis | Negative feedback reverses deviations from a set point to maintain stable internal conditions |
| Core Digestive Rule | Each macromolecule class is broken down by a specific enzyme matched to its structure |

## What's next

Start this unit by working through the first sub-topic on carbohydrates and lipids to build your foundational understanding of molecular form and function. Follow the sub-topics in order to progress smoothly from the molecular scale up to whole human organ system physiology. After completing all sub-topics in this unit, move on to the first sub-topic of the next unit.

- [Carbohydrates and lipids](https://www.owlsprep.com/study/ib-biology-sl-u2-carbohydrates-and-lipids/)
- [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/)

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