This cheat sheet summarizes the major themes and concepts used across IB Biology. It helps students connect details from cells, genetics, evolution, ecology, and physiology into a smaller set of repeatable ideas. Students need these themes because IB questions often test explanation, comparison, data interpretation, and links between different biological scales.
The reference is designed as a clean printable guide for quick review before lessons, labs, and exams.
Unity and diversity explain how all organisms share core cellular and molecular features while showing variation through evolution. Form and function show that biological structures are shaped by the jobs they perform, from enzymes to organs. Interaction and interdependence focus on systems, feedback, energy flow, nutrient cycling, and relationships among organisms.
Important quantitative tools include magnification = image size / actual size, surface area to volume ratio = surface area / volume, and percentage change = ((new value - original value) / original value) x 100.
Key Facts
- Magnification is calculated as magnification = image size / actual size, with image size and actual size in the same units.
- Surface area to volume ratio is calculated as SA:V = surface area / volume, and smaller cells usually have a higher SA:V for faster exchange.
- Percentage change is calculated as percentage change = ((new value - original value) / original value) x 100.
- Unity in biology means organisms share features such as DNA, cell membranes, ribosomes, metabolic pathways, and a common genetic code.
- Diversity in biology comes from mutation, recombination, natural selection, genetic drift, isolation, and adaptation over generations.
- Form follows function because the shape, size, structure, and chemical properties of biological parts affect what they can do.
- Interactions in biological systems include enzyme-substrate binding, cell signaling, predator-prey relationships, competition, and symbiosis.
- Interdependence means organisms and systems rely on one another through food webs, nutrient cycles, energy flow, feedback loops, and shared environments.
Vocabulary
- Unity
- Unity is the shared set of biological features found across living organisms, such as DNA, cells, and basic metabolic processes.
- Diversity
- Diversity is the variation among organisms, populations, species, genes, structures, behaviors, and ecological roles.
- Form and function
- Form and function is the idea that a biological structure's shape and organization are linked to the job it performs.
- Interaction
- An interaction is an effect that one molecule, cell, organism, or system has on another.
- Interdependence
- Interdependence is the reliance of organisms or biological systems on one another for resources, regulation, survival, or reproduction.
- Emergent property
- An emergent property is a new function or behavior that appears when simpler parts work together as a system.
Common Mistakes to Avoid
- Confusing unity with uniformity is wrong because unity means shared core features, not that all organisms are the same.
- Treating form and function as separate ideas is wrong because IB Biology often expects students to explain how structure supports a specific biological role.
- Ignoring units in magnification calculations is wrong because image size and actual size must be converted to the same unit before using magnification = image size / actual size.
- Assuming larger cells are always more efficient is wrong because increasing size usually lowers surface area to volume ratio and can slow exchange.
- Describing food chains without energy loss is wrong because energy transfer between trophic levels is inefficient and much energy is lost as heat through respiration.
Practice Questions
- 1 A cell image is 40 mm wide on a micrograph, and the actual cell is 20 micrometers wide. What is the magnification?
- 2 A cubical cell has sides of 2 micrometers. Calculate its surface area, volume, and surface area to volume ratio.
- 3 A population increases from 240 individuals to 300 individuals in one year. Calculate the percentage change.
- 4 Explain how one example from human physiology shows the theme of form and function and one example from ecology shows interdependence.