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.
Understanding IB Biology Themes and Concepts Reference
A useful way to use biological themes is to move between levels of organization. A change in one protein can alter a cell process, affect a tissue, change an organism's survival, then influence a population. For example, a mutation may change the shape of a membrane transport protein.
This can change water or ion balance in cells. The effect may appear later as reduced growth or lower reproductive success.
Strong IB explanations trace these links clearly instead of naming several facts without showing a connection. In data questions, identify the biological level being measured before drawing conclusions.
Shared features across life are evidence for long evolutionary history, not proof that organisms are identical. DNA uses the same basic information system in nearly all organisms, which allows scientists to compare gene sequences and estimate relatedness. Differences in sequences can reveal patterns of descent.
However, similarity does not always mean a recent common ancestor. Similar environments can select for similar features in unrelated groups.
Wings in insects and birds both support flight, but their underlying structures developed differently. Students should distinguish homologous structures, which arise from common ancestry, from analogous structures, which have similar functions because of similar selection pressures.
Form and function can be tested rather than simply claimed. A red blood cell has a flattened shape that gives it a large area for gas exchange. A root hair cell has a long extension that increases contact with soil water.
In both cases, the structure improves movement of substances across a membrane. Surface area to volume ratio explains an important limit on cell size. As a cell becomes larger, its volume increases faster than its surface area.
Its needs for oxygen and nutrients may rise faster than its membrane can supply them. Cells can respond by dividing, changing shape, or using internal transport systems.
When calculating ratios, keep units consistent and show each stage of working. A ratio alone is not enough unless it is linked to exchange rate and cell demand.
Biological interactions often involve feedback. Negative feedback reduces a change and helps maintain stable internal conditions. Body temperature control is one example, where responses counteract a temperature shift.
Positive feedback increases a change for a limited time, such as blood clotting. In ecosystems, predator and prey numbers can affect each other over time, though real food webs are more complex than a single pair of species. Human actions can disturb these relationships through habitat loss, pollution, overharvesting, or introduced species.
In practical work, percentage change helps compare results that began at different values. State what increased or decreased, use the original value as the reference, and consider uncertainty before claiming that a pattern is meaningful.