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Overview of AP Biology Units, Chapters, and Big Ideas

Updated: Sep 22, 2023



AP biology is a fascinating class, but it is also one of the densest. In fact, one of the most common complaints from students is that there is too much information and detailed vocabulary to memorize. Knowing the overarching big ideas and keeping them in mind as you learn the detailed topics will really help you understand the material better. Doing this will help you make connections between different processes and structures, which will in turn help you remember all of them without feeling overwhelmed. This approach is far more effective than forcefully trying to memorize everything.

This is an overview of the AP biology units and chapters. The organization of topics here is based on the most popular AP biology textbook, by Campbell and Reece. The book has been used as the AP course textbook for many years. The information is presented in an organized and logical fashion. It is also full of helpful and detailed diagrams. Also, it doesn’t matter much which edition you get, as long as it is relatively recent. Editions 7 through 12 will all work well.

The AP biology curriculum starts with the little things, such as chemical interactions, molecules, and synthesis of key molecules inside organisms. Then, it moves on to larger-scale topics such as genetics, the evolution of populations, diversity of different types of organisms (from plants to fungi to mammals), animal structure, behavior, and development, and finally, ecology.

Here are the units covered in the curriculum:

1. The Chemistry of Life

The properties of water and how they allow life on Earth, diversity of carbon molecules structure (including the functional groups in biochemistry), and function of macromolecules. You will also learn about the building blocks that make up these large, diverse molecules. Concepts and vocabulary to know:

  • Unique properties of water, which are ultimately due to water’s ability to hydrogen bond

  • The various forms that carbon molecules (i.e., organic molecules) can take on and the functional groups

  • The unique properties and functions of the four macromolecules: lipids, proteins, nucleic acids, and carbohydrates. Know the different types of lipids and proteins, and the four stages of protein structure. Know examples of different carbohydrates (e.g., starch, glycogen chitin, cellulose, peptidoglycan), their functions, and where they are found

2. The Cell

Cell structure and organelles, cell membrane structure, enzymes and metabolism, cellular respiration, fermentation, photosynthesis, communication and signaling, and the cell cycle. Some of the key things to know from this unit are:

  • the general structure and function of each organelle, the structural differences between plants vs. animal vs. prokaryotic cells

  • structure and function of the cell membrane, which is composed of a lipid bilayer with various proteins embedded or attached. Also know the fluid mosaic model

  • The different types of transport in and out of cells, especially as they relate to the cell membrane

  • The location, inputs, outputs, and overall function of each within cellular respiration, fermentation, and photosynthesis. Students often find the electron transport chain (ETC) difficult to understand. It is helpful to draw out a diagram and label all the structures and steps in the ETC

  • The three stages of cell signaling, which is another difficult topic for students. Signal transduction, which is the second stage of cell signaling, involves different pathway possibilities and steps, and should be studied carefully

  • The cell cycle, including mitosis. Know the four stages of mitosis: prophase, metaphase, anaphase, and telophase. Also know how malfunctions in the cell cycle can lead to cancer

3. Genetics

Genetics involves some big concepts, as well as some probability calculation problems. You will learn about meiosis, Mendel’s discoveries in genetics, chromosomes and their structure, inheritance, DNA synthesis, protein synthesis, regulating gene expression, viruses, and biotechnology techniques (most of which use genetic engineering). Key terms and ideas to know:

  • Stages of meiosis

  • How Mendel's pea plant experiments laid the foundation of genetics

  • Alleles, genes, genotype, and phenotype

  • Different types of inheritance patterns, including complete and incomplete dominance, codominance, linked genes, and sex-linked genes, polygenic inheritance

  • Drawing and analyzing pedigrees for family history of a condition or other phenotype

  • Drawing Punnett squares and using them to compute genotype and phenotype probabilities for offspring. Test crosses are important to understand, as these come up often on the AP exam

  • Chromosome structure, how linked genes relate back to chromosomes

  • Structure of DNA and RNA

  • Gene expression in prokaryotes and eukaryotes, how operons work

  • The different molecules and enzymes involved in DNA replication and protein synthesis. The textbook has a great diagram of these biosynthesis processes. Also understand the differences between transcription and translation and protein synthesis, and the differences in the process for eukaryotes vs. prokaryotes

  • The structure of viruses, method of infection (lytic vs. lysogenic cycles), types, and specific examples (HIV, influenza). It is important to note that viruses are not considered living organisms

  • Biotechnology techniques and tools: PCR, gel electrophoresis, various genome sequencing techniques, and methods of analyzing gene expression and function

  • Different types of DNA, such as transposons, retrotransposons, and short tandem repeats (STRs). it is important and interesting to note that the vast majority of our genome does not code for any product

Why are birds' beaks so diverse in shape and size?

4. Evolution

Evolution is a very large-scale, long-term process, so it’s important to keep the overarching mechanism of evolution in mind for this unit. Remember that evolution happens in populations, not individuals. It occurs over a long period of time as the advantageous genes are passed on and are more likely selected for in the process of natural selection. Other key concepts to know:

  • Descent with modification vs. Lamark’s theory of use and disuse

  • Early conditions and life on Earth

  • Darwin’s discovery with the Galapagos finches, and the origin of species

  • Several possible types of evolution: gradual, punctuated equilibrium, and adaptive radiation

  • Types of natural selection: directional, stabilizing, and disruptive selection

  • Hardy-Weinberg equilibrium, which describes an ideal situation in which no evolution occurs. The Hardy-Weinberg math problems are often challenging for students. It is helpful to have the two Hardy-Weinberg equations memorized and do plenty of practice problems

  • How speciation and changes in gene frequency can change

5. Organism Diversity and Classification

In this unit, you will learn about the different types of organisms and how they are classified. Key things to know:

  • Evolutionary history

  • The Linnaean system of taxonomy, which classifies organisms

  • Phylogenetics, including how to interpret phylogeny trees

  • Diversity of organisms, which includes: bacteria and archaea, protists, plants, fungi, invertebrates, and vertebrates. These can be organized into formal groups such as phyla, or informal groups called clades

6. Plant Form and Function

As you can tell, this unit is all about plants. You will learn about plant structure and growth, vascular plants, angiosperms, and plant responses and hormones. Here are the specific concepts to know:

  • How early plants evolved into more advanced, larger plants that could live on land in dry environments

  • Defining traits of the following groups of plants: mosses (bryophytes), ferns, gymnosperms, and angiosperms. Know which ones came first, and which are the most advanced

  • Vascular tissue, which includes xylem and phloem. Know their structure and function, as well as which types of plants have vascular tissue

  • Monocots and dicots, which are both types of angiosperms

  • The plant hormones that help with phototropism (moving toward the sunlight), cell division and elongation, seed dormancy/germination, senescence, fruit growth, and other important functions

7. Animal Form and Function

This is essentially the physiology unit. It goes over the organ systems in animals. Toward the end of this unit, you will also learn about animal behavior, such as the famous Pavlov experiment. Physiology is a popular unit, but students may feel overwhelmed by all the vocabulary. Taking it one body system at a time, drawing out the structures and organs, and labeling everything really helps. Here are the body systems and key concepts to know:

  • Digestive, circulatory, respiratory, immune, excretory, endocrine, reproductive, musculoskeletal, and nervous systems

  • Stages of early animal development. This is often confusing for students. The diagrams in the textbook help to clarify these processes

  • Basic animal psychology and behavior. This includes imprinting and classical and operant conditioning

Coral reefs are a marine ecosystem with especially high biodiversity

8. Ecology

Ecology is fascinating because it explores how different species interact with each other as well as with their environments. Just like the physiology unit, there is quite a bit of new vocabulary to learn here. The following are the key concepts and terms to know:

  • Biomes. Know the different types of environments based on vegetation and climate. This often shows up on the AP exam, and shows up often on the SAT subject test

  • What ecological niches are

  • Abiotic vs. biotic factors

  • Types of survivorship curves and K versus R-selected species

  • Types of interspecific interactions, such as competition, predation, parasitism, commensalism, and symbiotic relationships

  • Food chains, energy and biomass pyramids, biomagnification

  • Ecological succession, both primary and secondary types. Know some examples of each, as this shows up often on the AP exam


Now that we have an understanding of the course content and organization, let's take a look at the four overarching themes/big ideas in AP biology. These are recurring concepts in biology. Understanding these will help you connect the dots between different topics.

  1. Evolution This theme is connected to many other important topics in biology, such as genetics, organism diversity and classification, and animal development. Evolution is one of the big forces of nature, and it involves changes in specific organism populations over long periods of time. it all starts, though, with baby genetics. genetic variation is what allows for evolution.

  2. Processes and feedback loops Throughout the course, you will come across many different examples of Pathways involved in positive or negative feedback loops. Feedback loops are crucial in organisms because they help to maintain homeostasis. Homeostasis, an important term in biology, is a constant steady state inside an organism. Our bodies processes all serve to either maintain homeostasis or carry out functions that allow for homeostasis. For example, in unit 2, you will learn about cell communication. This describes the way cells receive signals from each other. You will also learn about the endocrine system, which consists of many different hormones traveling throughout our body. These hormones help promote homeostasis, such as maintaining stable blood glucose levels and blood pressure levels.

  3. Systems Systems can be large or small, and they range from ecosystems to organ systems. Feedback loops are also a key component of systems, as they keep everything balanced. You will learn about how these systems are regulated and how they work with other systems.

  4. Genetics Genetics is another recurring theme in biology. It involves hereditary information that is present in all living things, in the form of DNA or RNA. This information is unique to each individual organism and is passed onto offspring. Shifts in the gene pool of a population over time are what drive evolution. That is at the macro level. At the molecular level, biosynthesis processes such as DNA replication and protein synthesis help explain how genes work. Gene expression allows specific genes to be expressed more or less, depending on what the organism needs.

In addition to these big themes and specific chapters, there are also 11 to 12 labs that students typically do at school in their AP Biology class. You can find more information on each of these labs on College Board's website. We will be sharing another article here soon, which will be dedicated to the labs, how they prepare you for the AP exam, and how to do well on them.


Want to test how well you really know all these concepts?

Check out our AP Bio Practice Portal, which is our popular vault of 300+ AP-style MCQ and FRQ problem sets with answers and explanations for every question. Don't waste any more time Googling practice problems or answers - try it out now!



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