Unit Overview

Theme B: Form and Function

IB Biology SL· 5 min read 📊 23-27% of overall IB SL Biology exam

1. 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

Explore carbohydrate and lipid structure and their functional roles in cells and organisms.

★★⏱ 8 min

Proteins

Cover amino acid structure and protein folding, linking 3D shape to protein function.

★★★⏱ 7 min

Nucleic acid structure

Learn DNA and RNA structure and how their form supports genetic information storage.

★★⏱ 6 min

Enzymes and metabolism

Explain how enzyme structure enables substrate specificity and regulation of metabolism.

★★★⏱ 10 min

Gas exchange and transport in humans

Examine how lung and circulatory system structure supports gas exchange and oxygen transport.

★★★⏱ 9 min

Digestion and absorption in humans

Explore how the structure of each digestive organ supports its role in breaking down and absorbing nutrients.

★★★⏱ 8 min

Muscle and motility

Understand how the structure of skeletal muscle and bone enables movement in humans.

★★★★⏱ 7 min

Hormones, homeostasis and reproduction

Explain how hormone structure and organ function maintain homeostasis and support human reproduction.

★★★★⏱ 10 min

2. Common Pitfalls

Wrong move:

Reversing the core relationship, claiming function determines structure

Why:

IB exams explicitly test the principle that structure enables function, shaped by evolution

Correct move:

Always link observed structural features to the function they allow when answering questions

Wrong move:

Mixing up levels of protein folding, especially secondary vs tertiary structure

Why:

This is a common multiple choice and short answer question topic on IB SL exams

Correct move:

Remember: primary = amino acid sequence, secondary = local H-bond folding, tertiary = full 3D shape of one polypeptide

Wrong move:

Describing alveoli structure without linking it to gas exchange function

Why:

Examiners require explicit form-function connections to award full marks for this question

Correct move:

Always connect large surface area, thin epithelium and dense capillary supply to faster diffusion

3. Quick Reference 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.