AP Biology:
Study Guide
AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this
product.
,Key Exam Details
The AP® Biology exam is a 3-hour, end-of-course test comprised of 60 multiple-choice questions,
for which you will have 1 hour and 30 minutes (this counts for 50% of your score) and 6 free-
response questions, for which you will have 1 hour and 30 minutes (this counts for 50% of your
score).
The exam covers the following course content categories:
• Chemistry of Life: 8–11% of test questions
• Cell Structure and Function: 10–13% of test questions
• Cellular Energetics: 12–16% of test questions
• Cell Communication and Cell Cycle: 10–15% of test questions
• Heredity: 8–11% of test questions
• Gene Expression and Regulation: 12–16% of test questions
• Natural Selection: 13–20% of test questions
• Ecology: 10–15% of test questions
This guide will offer an overview of the main tested subjects, along with sample AP multiple-
choice questions that look like the questions you will see on test day.
Chemistry of Life
About 8–11% of the questions on your AP Biology exam will cover the topic Chemistry of Life.
Water and the Elements of Life
Water is made of two hydrogen molecules covalently bonded to an oxygen molecule. The oxygen
atom pulls most of the electrons in the water molecule toward it, giving it a slightly negative
charge and the hydrogen atoms a slightly positive charge. Molecules like water that have distinct
regions of charge based on bond structure are called polar compounds. The charge structure of
water also creates a unique shape, where the hydrogen molecules are concentrated on one side
of the oxygen atom.
The polar nature and shape of water molecules make them ideal for forming hydrogen bonds
between water molecules. Hydrogen bonds are weak bonds that form between a proton in one
molecule and an electronegative atom of another molecule. In the case of water, this is between
the electronegative oxygen of one molecule and the slightly positive hydrogen of another water
molecule. The polar nature of water is important to life for many reasons. For one, it makes water
1
,a solvent to many other molecules. This means that many chemicals that are important to life
are readily dissolved in water and can be distributed throughout an organism due to its
movement.
Water also has properties of cohesion and adhesion. Cohesion occurs when molecules of the
same kind tend to stick together. In water, this is due to hydrogen bond cohesion between water
molecules. Cohesion causes surface tension, which is the tendency of liquid surfaces to shrink to
minimize surface area. This is due to water molecules at the water-air surface interfacing and
forming stronger hydrogen bonds with water molecules below, causing a shrinking of the space
between them. Surface tension causes water droplets to form and allows solid matter to float at
the surface of water.
Adhesion, on the other hand, is the tendency of dissimilar molecules to be attracted to each
other. Adhesive forces can be strong between water and charged molecules and are responsible
for capillary action, which is the movement of liquid through spaces on its own, sometimes in
opposition to gravity. Capillary action is the result of adhesive forces between water and the
surface it is touching, which draws the liquid towards it. Due to cohesive forces, the water also
pulls more water molecules behind it. These properties of water are essential to all life on Earth.
For example, in plants, capillary action is responsible for moving water from the roots up through
the rest of the plant.
Carbon, hydrogen, nitrogen, and oxygen comprise 99% of all living matter. Organic molecules,
which include most molecules with carbon, are the basis of life on Earth.
Carbon has the unique chemical property of being able to form four bonds with other elements,
making it an ideal element to form the backbone of complicated biological molecules. Carbon-
based molecules are able to take on many configurations, as carbon can form single, double, or
triple bonds with other elements. These molecules can take on many shapes: rings, branches, or
long chains. Thus, carbon is the elemental basis of the major biological macromolecules:
carbohydrates, proteins, lipids, and nucleic acids. In addition to carbon, nucleic acids and proteins
rely on nitrogen and phosphorus to build their structure, which we will discuss in more detail
below.
The Makeup and Properties of Macromolecules
Large biological molecules are the building blocks of life. For your AP exam, you should be familiar
with carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, proteins, and nucleic acids
are usually types of molecules called polymers, which are structures made of repeating smaller
units called monomers. The monomers that make DNA are nucleotides, amino acids make
proteins, and sugars make carbohydrates. The monomer units in each of these cases are not
necessarily identical but are of the same kind of molecule. Large polymers are also called
macromolecules. Lipids, on the other hand, are not generally polymers, thus are not always
2
, considered to be macromolecules. In the formation of biological macromolecules, the
composition and order of monomers affect their function.
Macromolecules form through dehydration synthesis of monomers. In dehydration synthesis, a
covalent bond forms between two monomers, releasing water in the process. The reverse
process breaks down polymers into monomers; this is called hydrolysis, meaning the bond is
lysed by water. Synthesis reactions generally use energy, which is then stored within the covalent
bonds of the macromolecule. When hydrolysis occurs, this energy is released for the cell to use.
Proteins comprise the majority of organic molecules in organisms and have huge diversity in
structures and function. Proteins are made of strings of amino acids connected by covalent
bonds. There are 20 types of amino acids in biological organisms, but they all share similar
structural features.
3
Study Guide
AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this
product.
,Key Exam Details
The AP® Biology exam is a 3-hour, end-of-course test comprised of 60 multiple-choice questions,
for which you will have 1 hour and 30 minutes (this counts for 50% of your score) and 6 free-
response questions, for which you will have 1 hour and 30 minutes (this counts for 50% of your
score).
The exam covers the following course content categories:
• Chemistry of Life: 8–11% of test questions
• Cell Structure and Function: 10–13% of test questions
• Cellular Energetics: 12–16% of test questions
• Cell Communication and Cell Cycle: 10–15% of test questions
• Heredity: 8–11% of test questions
• Gene Expression and Regulation: 12–16% of test questions
• Natural Selection: 13–20% of test questions
• Ecology: 10–15% of test questions
This guide will offer an overview of the main tested subjects, along with sample AP multiple-
choice questions that look like the questions you will see on test day.
Chemistry of Life
About 8–11% of the questions on your AP Biology exam will cover the topic Chemistry of Life.
Water and the Elements of Life
Water is made of two hydrogen molecules covalently bonded to an oxygen molecule. The oxygen
atom pulls most of the electrons in the water molecule toward it, giving it a slightly negative
charge and the hydrogen atoms a slightly positive charge. Molecules like water that have distinct
regions of charge based on bond structure are called polar compounds. The charge structure of
water also creates a unique shape, where the hydrogen molecules are concentrated on one side
of the oxygen atom.
The polar nature and shape of water molecules make them ideal for forming hydrogen bonds
between water molecules. Hydrogen bonds are weak bonds that form between a proton in one
molecule and an electronegative atom of another molecule. In the case of water, this is between
the electronegative oxygen of one molecule and the slightly positive hydrogen of another water
molecule. The polar nature of water is important to life for many reasons. For one, it makes water
1
,a solvent to many other molecules. This means that many chemicals that are important to life
are readily dissolved in water and can be distributed throughout an organism due to its
movement.
Water also has properties of cohesion and adhesion. Cohesion occurs when molecules of the
same kind tend to stick together. In water, this is due to hydrogen bond cohesion between water
molecules. Cohesion causes surface tension, which is the tendency of liquid surfaces to shrink to
minimize surface area. This is due to water molecules at the water-air surface interfacing and
forming stronger hydrogen bonds with water molecules below, causing a shrinking of the space
between them. Surface tension causes water droplets to form and allows solid matter to float at
the surface of water.
Adhesion, on the other hand, is the tendency of dissimilar molecules to be attracted to each
other. Adhesive forces can be strong between water and charged molecules and are responsible
for capillary action, which is the movement of liquid through spaces on its own, sometimes in
opposition to gravity. Capillary action is the result of adhesive forces between water and the
surface it is touching, which draws the liquid towards it. Due to cohesive forces, the water also
pulls more water molecules behind it. These properties of water are essential to all life on Earth.
For example, in plants, capillary action is responsible for moving water from the roots up through
the rest of the plant.
Carbon, hydrogen, nitrogen, and oxygen comprise 99% of all living matter. Organic molecules,
which include most molecules with carbon, are the basis of life on Earth.
Carbon has the unique chemical property of being able to form four bonds with other elements,
making it an ideal element to form the backbone of complicated biological molecules. Carbon-
based molecules are able to take on many configurations, as carbon can form single, double, or
triple bonds with other elements. These molecules can take on many shapes: rings, branches, or
long chains. Thus, carbon is the elemental basis of the major biological macromolecules:
carbohydrates, proteins, lipids, and nucleic acids. In addition to carbon, nucleic acids and proteins
rely on nitrogen and phosphorus to build their structure, which we will discuss in more detail
below.
The Makeup and Properties of Macromolecules
Large biological molecules are the building blocks of life. For your AP exam, you should be familiar
with carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, proteins, and nucleic acids
are usually types of molecules called polymers, which are structures made of repeating smaller
units called monomers. The monomers that make DNA are nucleotides, amino acids make
proteins, and sugars make carbohydrates. The monomer units in each of these cases are not
necessarily identical but are of the same kind of molecule. Large polymers are also called
macromolecules. Lipids, on the other hand, are not generally polymers, thus are not always
2
, considered to be macromolecules. In the formation of biological macromolecules, the
composition and order of monomers affect their function.
Macromolecules form through dehydration synthesis of monomers. In dehydration synthesis, a
covalent bond forms between two monomers, releasing water in the process. The reverse
process breaks down polymers into monomers; this is called hydrolysis, meaning the bond is
lysed by water. Synthesis reactions generally use energy, which is then stored within the covalent
bonds of the macromolecule. When hydrolysis occurs, this energy is released for the cell to use.
Proteins comprise the majority of organic molecules in organisms and have huge diversity in
structures and function. Proteins are made of strings of amino acids connected by covalent
bonds. There are 20 types of amino acids in biological organisms, but they all share similar
structural features.
3
