Campbell Biology Chapter 2 Review: The Chemical Context of Life

2.1 Elements, compounds and the elements of life

pp. 31–32

On the AP exam? Yes

Topic 1.2 asks which elements build which macromolecules; you won't need body-mass percentages or a list of trace elements.

In the course: Topic 1.2 Elements of Life (notes, videos and more questions)

Key points

Matter is anything that has mass and takes up space. Living things are built from the same matter as rocks and air.
An element is made of just one kind of atom, and no chemical reaction can break it down further. A compound contains more than one element, always in the same proportions, like CO₂ or H₂O.
A compound can act nothing like the elements in it. Hydrogen and oxygen are gases that feed a fire, yet water puts fires out. New properties that appear when parts combine are called emergent properties.
Four elements dominate living matter: carbon, hydrogen, oxygen and nitrogen, which together are about 96% of its mass. Elements like sulfur, phosphorus, calcium and potassium are present in much smaller amounts.
An essential element is one an organism can't grow or reproduce without. A trace element is essential too, but needed only in tiny amounts, like zinc or copper in people.
For the AP exam, know where the main elements go: carbon, hydrogen and oxygen are in all four macromolecule families, nitrogen is in proteins and nucleic acids, phosphorus is in nucleic acids and phospholipids, and sulfur is in some amino acids.
Organisms can't make atoms. They take them in from food, air, water and soil and rearrange them into their own molecules.

Key terms (8)

matter: Anything that has mass and takes up space, from a rock to your own cells.
mass: How much matter something contains. Unlike weight, it doesn't change if gravity changes.
element: A substance made of only one kind of atom, such as carbon or oxygen. No chemical reaction can turn it into anything simpler.
compound: A substance made of two or more kinds of atoms bonded in proportions that never change. Water (H₂O) always has 2 hydrogens for every oxygen.
emergent property: A new property that shows up when parts combine and that none of the parts has alone, like water putting out fires even though hydrogen and oxygen feed them.
essential element: An element an organism can't do without: if it's missing, the organism can't develop, work normally or reproduce.
trace element: An essential element needed only in very small amounts, like zinc or copper in humans. Tiny need doesn't mean optional.
chemical symbol: The one- or two-letter shorthand for an element, such as C for carbon or Na for sodium.

Check yourself: 2.1 Elements, compounds and the elements of life

4 questions on 2.1 Elements, compounds and the elements of life. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of the following is a compound?

Question 2 of 4

Hydrogen gas burns explosively, and oxygen gas makes fires burn hotter. Yet water, made of hydrogen and oxygen, is used to put fires out. Which idea does this best illustrate?

Question 3 of 4

Which element is found in every nucleotide and also in the head of every phospholipid?

Question 4 of 4

Bacteria are grown on a medium in which every nitrogen atom is the heavy isotope ¹⁵N. Which of their macromolecules would become labeled with ¹⁵N?

0 of 4 answered

2.2 Inside the atom: particles, isotopes and electrons

pp. 33–37

On the AP exam? Background

Atomic structure isn't its own CED topic, but valence electrons explain bonding in Topic 1.1, isotope labels appear in experiment questions, and isotope decay dates fossils in Topic 7.6.

In the course: Topic 1.1 Structure of Water and Hydrogen Bonding, Topic 1.2 Elements of Life, Topic 3.4 Photosynthesis, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

An atom is the smallest bit of an element that still acts like that element.
Protons (positive) and neutrons (no charge) are packed into a tiny nucleus, and electrons (negative) move in the space around it. Protons and neutrons each have a mass of about 1 dalton; an electron is about 1,800 times lighter, so it barely adds to an atom's mass.
The atomic number is the number of protons, and it decides which element an atom is. The mass number is protons plus neutrons, so neutrons = mass number − atomic number.
Isotopes are versions of one element with different numbers of neutrons. They react the same way, so scientists swap in a rare isotope as a label to track where atoms go in a cell.
A radioactive isotope has an unstable nucleus that breaks down on its own. Its predictable decay rate lets scientists date fossils, and its radiation makes it easy to detect as a tracer, but that radiation can also damage DNA and other molecules.
Electrons sit at fixed energy levels called shells. An electron moves out to a higher shell only when it takes in an amount of energy that exactly matches the gap between the shells. Chlorophyll relies on this when it absorbs light.
Valence electrons, the ones in the outermost shell, decide how an atom reacts. Atoms with a full outer shell barely react; atoms with unpaired outer electrons react readily.

Key terms (15)

atom: The tiniest piece of an element that still behaves like that element.
proton: A positively charged particle in the nucleus. The number of protons tells you which element an atom is.
neutron: An uncharged particle in the nucleus with about the same mass as a proton.
electron: A tiny negatively charged particle that moves around the nucleus. Electrons are what atoms share or trade when they bond.
atomic nucleus: The small, dense center of an atom, holding its protons and neutrons.
dalton: The unit for the mass of atoms and molecules. A proton or neutron is about 1 dalton. Another name for it is the atomic mass unit (amu).
atomic number: The number of protons in an atom. No two elements share an atomic number, and in a neutral atom it also equals the number of electrons.
mass number: The number of protons plus neutrons in an atom, written as a superscript before the symbol, as in ¹⁴C.
atomic mass: An atom's mass in daltons, roughly equal to its mass number.
isotope: One version of an element. All versions share a proton count but differ in how many neutrons they have.
radioactive isotope: An isotope whose nucleus is unstable and decays with no outside push, releasing radiation. The decay can turn the atom into a different element.
potential energy: Stored energy (energy being the ability to make change happen) that matter has because of where it is or how it's arranged. Electrons farther from the nucleus have more.
electron shell: One of the fixed energy levels an atom's electrons can occupy. The farther a shell sits from the nucleus, the more energy its electrons have.
valence electron: An electron in an atom's outermost shell, which is called the valence shell. These electrons decide how the atom bonds and reacts.
orbital: The 3-D region around the nucleus where you're most likely to find a given electron. Two electrons is the most any one orbital can take.

Check yourself: 2.2 Inside the atom: particles, isotopes and electrons

4 questions on 2.2 Inside the atom: particles, isotopes and electrons. Pick an answer to see if you got it, and why.

Question 1 of 4

Four neutral atoms contain these particles. Atom P: 6 protons, 6 neutrons, 6 electrons. Atom Q: 6 protons, 8 neutrons, 6 electrons. Atom R: 7 protons, 7 neutrons, 7 electrons. Atom S: 5 protons, 6 neutrons, 5 electrons. Which two atoms are isotopes of the same element?

Question 2 of 4

Nitrogen has an atomic number of 7. How many electrons are in the valence (outermost) shell of a neutral nitrogen atom?

Question 3 of 4

Why can a radioactive isotope of carbon be used to follow carbon atoms through a cell's metabolism?

Question 4 of 4

To find where the O₂ released in photosynthesis comes from, researchers gave one group of algae water made with the heavy oxygen isotope ¹⁸O plus ordinary CO₂. A second group got ordinary water plus CO₂ made with ¹⁸O. Only the first group released O₂ containing ¹⁸O. Which conclusion is best supported?

0 of 4 answered

2.3 Chemical bonds and molecular shape

pp. 38–42

On the AP exam? Yes

Polar covalent bonds and hydrogen bonds are core to Topic 1.1. Ionic bonds and van der Waals details are mostly background, and shape-based binding comes back with proteins and cell signaling.

In the course: Topic 1.1 Structure of Water and Hydrogen Bonding, Topic 1.6 Nucleic Acids, Topic 1.7 Proteins, Topic 4.2 Introduction to Signal Transduction (notes, videos and more questions)

Key points

Atoms bond by sharing or transferring valence electrons, which lets them fill their outer shells.
A covalent bond is a pair of shared electrons. One shared pair makes a single bond and two pairs make a double bond. Carbon usually forms 4 bonds, nitrogen 3, oxygen 2 and hydrogen 1.
Electronegativity is how hard an atom pulls on shared electrons. Equal pulling gives a nonpolar covalent bond, like C–H. Unequal pulling gives a polar covalent bond, like O–H, with a slightly negative end (δ−) and a slightly positive end (δ+).
If one atom pulls an electron away completely, both atoms become ions: a positive cation and a negative anion. The attraction between opposite ions is an ionic bond. It's strong in a dry crystal but much weaker once water surrounds the ions.
A hydrogen bond is a weak attraction between a δ+ hydrogen (already bonded to O or N) and a δ− oxygen or nitrogen nearby. Van der Waals interactions are brief, weak attractions between any atoms that get very close.
Because weak bonds form and break easily, they suit brief contacts, such as a signal molecule landing on its receptor. Many weak bonds acting together are strong enough to hold a protein in its folded shape and keep DNA's two strands paired.
A molecule's shape decides what it can bind. Water's bent shape is part of why the molecule is polar, and in cells a signal molecule fits its receptor much as a key fits a lock. A drug shaped like the natural signal can copy it or block it.

Key terms (15)

chemical bond: An attraction that holds two atoms close together, formed when they share or transfer electrons.
covalent bond: A strong bond in which two atoms share a pair of electrons from their outer shells. Covalent bonds hold the atoms of a molecule together.
molecule: A group of atoms joined by covalent bonds, such as O₂ or H₂O.
double bond: A covalent bond in which two atoms share two pairs of electrons, as in O₂ (O=O).
valence: How many covalent bonds an atom usually forms: 1 for hydrogen, 2 for oxygen, 3 for nitrogen and 4 for carbon.
electronegativity: An atom's pull on the electrons it shares in a covalent bond. Oxygen and nitrogen pull hard; carbon and hydrogen pull with similar strength.
nonpolar covalent bond: A covalent bond where the electrons are shared about equally, so neither atom gets a partial charge.
polar covalent bond: A covalent bond where one atom pulls the shared electrons harder, giving it a partial negative charge and its partner a partial positive charge.
ion: A charged particle: an atom, or a group of bonded atoms, that has more or fewer electrons than protons.
cation: A positively charged ion, formed when an atom loses electrons, like Na⁺ or Ca²⁺.
anion: A negatively charged ion, formed when an atom gains electrons, like Cl⁻.
ionic bond: The attraction between a cation and an anion. It's strong in a dry crystal but weak when the ions are in water.
salt: A compound whose ions are held by ionic bonds, like table salt (NaCl). Its formula gives a ratio of ions, not a molecule.
hydrogen bond: A weak attraction between a partly positive hydrogen and a partly negative oxygen or nitrogen on another molecule or another part of the same one.
van der Waals interactions: Very weak, short-lived attractions between atoms that are extremely close, caused by electrons briefly bunching up on one side.

Check yourself: 2.3 Chemical bonds and molecular shape

4 questions on 2.3 Chemical bonds and molecular shape. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of these covalent bonds is the most polar?

Question 2 of 4

A calcium ion, Ca²⁺, which helps trigger muscle contraction, has 20 protons. How many electrons does it have, and what kind of ion is it?

Question 3 of 4

In a laboratory, heating a DNA double helix to about 95 °C separates its two strands, but each strand stays in one piece. Which explanation fits this result?

Question 4 of 4

A protein chain folds back on itself so that two stretches of its backbone lie side by side. Which pair of atoms is most likely to form a hydrogen bond holding the two stretches together?

0 of 4 answered

2.4 Chemical reactions and equilibrium

pp. 42–43

On the AP exam? Background

Chemical equilibrium isn't tested in AP Biology, but you'll use balanced equations and reactants versus products for photosynthesis and respiration in Unit 3.

In the course: Topic 3.3 Cellular Energy, Topic 3.4 Photosynthesis, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

A chemical reaction breaks some bonds and makes new ones, turning the starting substances (reactants) into new ones (products).
Atoms are never created or destroyed in a reaction, only rearranged. A balanced equation has the same number of each kind of atom on both sides, and the coefficients count molecules.
Photosynthesis, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, uses light energy to rearrange carbon dioxide and water into sugar and oxygen. Cellular respiration runs roughly the other way and releases energy. Each is really many smaller steps.
Reactions can, in principle, run both ways. A double arrow (⇌) marks a reversible reaction.
Concentration matters: the more molecules of a substance there are, the more often they meet partners and react. So adding reactant pushes the forward reaction, and a buildup of product pushes the reverse one.
At chemical equilibrium, the forward and reverse reactions run at the same rate, so the amounts stop changing. The reactions haven't stopped, and the amounts usually aren't equal; they settle at a steady ratio.
Cells rarely let a reaction sit at equilibrium. The next step in a pathway uses up each product, which keeps the earlier reaction running forward.

Key terms (9)

chemical reaction: A process in which bonds between atoms break and new ones form, so the starting substances become different substances.
reactant: A starting substance in a chemical reaction, written to the left of the arrow.
product: A substance made by a chemical reaction, written to the right of the arrow.
coefficient: The number in front of a formula in an equation, telling you how many molecules take part, like the 6 in 6O₂.
conservation of matter: The rule that a reaction can't create or destroy atoms, only rearrange them, so every atom in the reactants shows up in the products.
reversible reaction: A reaction that can run forward or backward, shown with a double arrow (⇌).
chemical equilibrium: The point where a reversible reaction goes forward exactly as fast as it goes backward, so the amounts of reactants and products stop changing.
photosynthesis: The process that uses light energy to build sugar from carbon dioxide and water, releasing oxygen.
cellular respiration: The set of reactions cells use to break down sugar and capture its energy as ATP. When oxygen is used, the leftovers are CO₂ and water.

Check yourself: 2.4 Chemical reactions and equilibrium

4 questions on 2.4 Chemical reactions and equilibrium. Pick an answer to see if you got it, and why.

Question 1 of 4

Photosynthesis can be summed up as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. How many molecules of CO₂ must a plant take in to make 3 molecules of glucose?

Question 2 of 4

A reversible reaction, A ⇌ B, was started with only A in a sealed container. Concentrations measured (invented data, in mM): 0 min, A = 10.0 and B = 0.0; 1 min, A = 4.0 and B = 6.0; 2 min, A = 2.8 and B = 7.2; 3 min, A = 2.6 and B = 7.4; 4 min, A = 2.5 and B = 7.5; 5 min, A = 2.5 and B = 7.5; 6 min, A = 2.5 and B = 7.5. Which statement is best supported by the data?

Question 3 of 4

In the same reaction (A ⇌ B), the amounts held steady at A = 2.5 mM and B = 7.5 mM. Suppose the experiment is repeated at the same temperature but starts with 20.0 mM of A and no B. Which result is most likely once the amounts stop changing?

Question 4 of 4

Cellular respiration can be summed up as C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. A student counts the oxygen atoms on each side to check that the equation is balanced. Which statement is correct?

0 of 4 answered