1.1 What Is Science?

Lesson Summary

What Science Is and Is Not Science is an organized way of gathering and analyzing

evidence about the natural world. The goals of science are to provide natural explanations for

events in the natural world and to use those explanations to make useful predictions. Science is

different from other human works in the following ways:

Science deals only with the natural world.

Scientists collect and organize information about the natural world in an orderly way.

Scientists propose explanations that are based on evidence, not belief.

They test those explanations with more evidence.

Scientific Methodology: The Heart of Science Methodology for scientific investigation

involves:

Making an observation. Observation involves the act of noticing and describing events or

processes in a careful, orderly way. Scientists use their observations to make inferences. An

inference is a logical interpretation based on what scientists already know.

Suggesting hypotheses. A hypothesis is a scientific

explanation for a set of observations that can be tested in

ways that support or reject it.

Testing the hypothesis. Testing a hypothesis often involves

designing an experiment. Whenever possible, a hypothesis

should be tested by a controlled experiment—an

experiment in which only one variable (the independent

variable, or manipulated variable) is changed. The variable

that can change in response to the independent variable is

called the dependent variable, or responding variable. The

control group is exposed to the same conditions as the

experimental group except for one independent variable.

Collecting, recording, and analyzing data, or

information gathered during the experiment.

Drawing conclusions based on data.

1.2 Science in Context

Lesson Summary

Exploration and Discovery: Where Ideas Come From Scientific methodology is

closely linked to exploration and discovery. Good scientists share scientific attitudes, or habits of

mind, that lead them to exploration and discovery. New ideas are generated by curiosity, skepticism,

open-mindedness, and creativity.

Ideas for exploration can arise from practical problems.

Discoveries in one field of science can lead to new technologies; the new technologies give rise

to new questions for exploration.

Communicating Results: Reviewing and Sharing Ideas Communication and

sharing of ideas are vital to modern science. Scientists share their findings with the scientific

community by publishing articles that undergo peer review. In peer review, scientific papers are

reviewed by anonymous, independent experts. Publishing peer-reviewed articles scientific journals

allows scientists to

share ideas.

test and evaluate each other’s work.

Once research has been published, it enters the dynamic marketplace of scientific ideas. New ideas fit

into scientific understanding by leading to new hypotheses that must be independently confirmed by

controlled experiments.

Scientific Theories In science, the word theory applies to a well-tested explanation that unifies

a broad range of observations and hypotheses and that enables scientists to make accurate predictions

about new situations.

No theory is considered absolute truth.

Science is always changing; as new evidence is uncovered, a theory may be reviewed or replaced

by a more useful explanation.

Science and Society Using science involves understanding its context in society and its

limitations. Understanding science

helps people make decisions that also involve cultural customs, values, and ethical standards.

can help people predict the consequences of their actions and plan the future.

Scientists strive to be objective, but when science is applied in society, it can be affected by bias,

a point of view that is personal rather than scientific.

1.3 Studying Life

.Lesson Summary Characteristics of Living Things Biology is the study of life. Living things share these

characteristics: They are made of cells and have a universal genetic code; they obtain and use

materials and energy to grow and develop; they reproduce; they respond to signals in their

environment (stimuli) and maintain a stable internal environment; they change over time.

Big Ideas in Biology The study of biology revolves around several interlocking big ideas:

CelCellular basis of life. Living things are made of cells.

Information and heredity. Living things are based on a universal genetic code written in a

molecule called DNA.

Matter and energy. Life requires matter that provides raw material, nutrients, and energy. The

combination of chemical reactions through which an organism builds up or breaks down

materials is called metabolism.

Growth, development, and reproduction. All living things reproduce. In sexual reproduction,

cells from two parents unite to form the first cell of a new organism. In asexual reproduction, a

single organism produces offspring identical to itself. Organisms grow and develop as they

mature.

Homeostasis. Living things maintain a relatively stable internal environment.

Evolution. Taken as a group, living things evolve, linked to a common origin.

Structure and function. Each major group of organisms has evolved structures that make

particular functions possible.

Unity and diversity of life. All living things are fundamentally similar at the molecular level.

Interdependence in nature. All forms of life on Earth are connected into a biosphere—a living

planet.

Science as a way of knowing. Science is not a list of facts but “a way of knowing.”

Fields of Biology Biology includes many overlapping fields that use different tools to

study life. These include biotechnology, global ecology, and molecular biology.

Performing Biological Investigations Most scientists use the metric system as a way to

share quantitative data. They are trained in safe laboratory procedures. To remain safe when you are

doing investigations, the most important rule is to follow your teacher’s instructions.

Chapter 1 Vocabulary Review For Questions 1–8, complete each statement by writing the correct word.

1. The act of noticing and describing events or processes in a careful, orderly way is called .

2. The information gathered during an experiment is called .

3. A(n) is a logical interpretation based on what scientists already know.

4. A(n) is a scientific explanation for a set of observations that can be tested in

ways that support or reject it.

5. A(n) is a well-tested explanation that unifies a broad range of observations and

hypotheses.

6. In reproduction, the new organism has a single parent.

7. A(n) is a signal to which an organism responds.

8. is an organized way of gathering and analyzing evidence about the natural

world.

For Questions 9–17, write the letter of the definition that best matches each term on the line

provided.

Term

9. biology

10. bias

11. homeostasis

12. metabolism

13. DNA

14. control group

15. independent variable

16. dependent variable

17. biosphere

Definition

A. in an experiment, the group exposed to the same

conditions as the experimental group except for one

independent variable

B. the study of life

C. living things maintaining a relatively stable internal

environment

D. a molecule containing the universal genetic code

E. a point of view that is personal rather than scientific

F. a living planet

G. the combination of chemical reactions through which an

organism builds up or breaks down materials

H. in an experiment, the variable that is manipulated

I. in an experiment, the responding variable

2.1 The Nature of Matter

Lesson Summary

Atoms The atom is the basic unit of matter,

made up of three subatomic particles.

Protons have a positive charge and neutrons carry

no charge. Strong forces bind protons and

neutrons together in the nucleus.

An electron is a negatively charged particle

that has only about 1/1840 the mass of a

proton. Electrons constantly move around the

space surrounding the atom’s nucleus.

Because an atom has the same number of protons and electrons, if it is electrically neutral.

Elements A chemical element is a pure substance that consists entirely of one type of atom.

Chemical Compounds A chemical compound is a substance formed by the chemical

combination of two or more elements in definite proportions. The physical and chemical properties

of a compound are usually very different from those of the elements from which it is formed.

Scientists use formulas to show the ratio of elements that make up a compound.

Chemical Bonds The atoms in compounds are held together by chemical bonds. Electrons that

are available to form bonds are called valence electrons.

An ionic bond (left) forms when one

or more electrons are transferred from

one atom to another, forming ions. An

atom that loses electrons becomes

positively charged. An atom that gains

electrons becomes negatively charged.

A covalent bond (right) forms when electrons are shared rather

than transferred. The structure formed by atoms joined by

covalent bonds is called a molecule. The molecule is the smallest

unit of most compounds.

2.2 Properties of Water

Lesson Summary

The Water Molecule Water molecules (H2O) are polar

because of an uneven distribution of electrons, creating a slight

negative (–) charge in the oxygen atom and a slight positive (+)

charge in each hydrogen atom. The attraction between a

hydrogen atom of one water molecule and the oxygen atom of

another water molecule is called a hydrogen bond.

Cohesion is an attraction between molecules of the same

substance. It causes water molecules to be drawn together,

producing surface tension

Adhesion is an attraction between molecules of different substances. It causes capillary action, an

effect that causes water to rise in a narrow tube against the force of gravity.

Solutions and Suspensions A mixture is a material composed of two or more elements or

compounds that are physically mixed together but not chemically combined. A solution is a mixture

in which all the components are evenly spread out: the substance dissolved is the solute; the

substance that causes the dissolving is the solvent. Mixtures of water and undissolved materials are

suspensions.

Acids, Bases, and pH A water molecule (H2O) can split apart to form a hydrogen ion (H+) and

a hydroxide ion (OH–).

The pH scale measures the concentration of hydrogen ions in a solution. The scale ranges

from 0 to 14. Pure water has a pH of 7.

An acid is any compound that forms H+ ions in solution. Acidic solutions have pH values below

7. A base is a compound that forms OH– ions in solution. Basic, or alkaline, solutions have pH

values above 7.

Buffers are weak acids or bases that can react with strong acids or bases to prevent sudden

changes in pH.

2.3 Carbon Compounds

Lesson Summary

The Chemistry of Carbon Organic chemistry is the study of compounds with bonds

between carbon atoms. Carbon atoms have four

valence electrons, allowing them to form strong

covalent bonds with many other elements, including

hydrogen, oxygen, phosphorus, sulfur, and nitrogen.

Living organisms are made up of molecules made of

carbon and these other elements.

One carbon atom can bond to another to form chains

and rings.

Carbon can form millions of different large and

complex structures.

Macromolecules Many of the carbon molecules in living things are so large they are called

macromolecules. Macromolecules form by polymerization, in which smaller units called monomers

join together to form polymers. Biochemists sort the macromolecules in living things into groups

based on their chemical composition.

Carbohydrates (starches and sugars) are composed of carbon, hydrogen, and oxygen.

Carbohydrates are polymers of monosaccharides. Carbohydrates are the main energy source

for living things.

Complex carbohydrates:

o Plants use cellulose for structure and starch for energ storage.

o Animals use glycogen for energy storage.

Simple Sugars:

o Glucose is one example of a monosaccharide and is C6H12O6.

Lipids (fats, oils, and waxes) are made mostly of carbon and hydrogen atoms. Lipids can be used

to store energy and form parts of biological membranes and waterproof coverings. Steroids

manufactured by the body are lipids as well.

Nucleic acids contain hydrogen, oxygen, nitrogen, carbon, and phosphorus. They are polymers

of nucleotides. A nucleotide has three parts: a 5-

carbon sugar, a phosphate (–PO4) group, and a

nitrogenous base. Nucleic acids store and

transmit hereditary (genetic) information.

There are two kinds of nucleic acids: DNA

(deoxyribonucleic acid) and RNA (ribonucleic

acid).

Proteins are made up of nitrogen, carbon,

hydrogen, and oxygen. Proteins are polymers of amino acids. An amino acid molecule has an

amino group (–NH2) on one end and a carboxyl group (–COOH) on the other end. Proteins

control the rate of reactions, regulate cell processes, form cellular structures, carry substances

into or out of cells, and help fight disease.

More than 20 different amino acids are found in nature. Any amino acid can bond with

any other.

Covalent bonds called peptide bonds link amino acids together to form a polypeptide.

Amino acids are assembled into polypeptide chains according to instructions coded in

DNA.

2.4 Chemical Reactions and Enzymes

Lesson Summary

Chemical Reactions Everything that happens in an organism is based on chemical reactions. A

chemical reaction is a process that changes one set of chemicals into another set of chemicals.

The elements or compounds that enter into the reaction are the reactants.

The elements or compounds produced by the reaction are the products.

Chemical reactions involve changes in the chemical bonds that join atoms in compounds.

Energy in Reactions Some chemical reactions release energy; others absorb energy.

Chemical reactions that release energy often occur on their own.

Chemical reactions that absorb energy require a source of energy. The energy needed to get a

reaction started is called the activation energy.

Enzymes An enzyme is a protein that acts as biological catalyst. A catalyst is a substance that

speeds up the rate of a chemical reaction. Catalysts work by lowering a reaction’s activation energy.

In an enzyme-catalyzed reaction, the reactants are known as substrates. Substrates bind to a part

of an enzyme called the active site and remain bound to the enzyme until the reaction is complete,

when the products are released.

Temperature, pH, and regulatory molecules can affect the

activity of enzymes.

Every enzyme has an optimum pH and optimum

temperature where the rate of reaction is the

highest.

If temperature or pH stray too far from the

optimum, the enzyme may denature, or change

shape and permanently lose all function.

Chapter Vocabulary Review

Crossword Puzzle Use the clues below to fill in the spaces in the puzzle with the correct

words.

Across

1. element or compound that enters into a

chemical reaction

4. process that changes one set of chemicals into

another

7. positively charged subatomic particle

8. substance formed by the chemical

combination of elements

11. positively or negatively charged atom

12. carbon compound that stores and transmits

genetic information

14. the center of an atom

16. bond formed when electrons are shared

between atoms

17. macromolecule formed when monomers join

together

Down

2. negatively charged subatomic particle

3. compound that forms hydroxide ions in

solution

5. bond formed when one or more electrons are

transferred from one atom to another

6. monomer of nucleic acid

9. monomer of protein

10. compound that forms hydrogen ions in

solution

13. atom of an element that differs in the number

of neutrons compared with other atoms of the

same element

15. basic unit of matter

7.1 Life Is Cellular

Lesson Summary

The Discovery of the Cell The invention of the microscope in the 1600s enabled researchers to see

cells for the first time.

Robert Hooke named the empty chambers he observed in cork “cells.”

Anton van Leeuwenhoek was the first to observe living microorganisms.

Cells are the basic units of life.

Discoveries by German scientists Schleiden, Schwann, and Virchow led to the development of the cell

theory, which states:

All living things are made of cells.

Cells are the basic units of structure and function in living things.

New cells are produced from existing cells.

Exploring the Cell Scientists use light microscopes and electron microscopes to explore the structure of

cells.

Compound light microscopes have lenses that focus light. They magnify objects by up to 1000 times.

Chemical stains and fluorescent dyes make cell structures easier to see.

Electron microscopes use beams of electrons focused by magnetic fields. They offer much higher

resolution than light microscopes. There are two main types of electron microscopes—transmission and

scanning. Scientists use computers to add color to electron micrographs, which are photos of objects seen

through a microscope.

Prokaryotes and Eukaryotes Cells come in an amazing variety of shapes and sizes, but all cells

contain DNA. Also, all cells are surrounded by a thin flexible barrier called a cell membrane. There are two

basic categories of cells based on whether they contain a nucleus. The nucleus (plural: nuclei) is a large

membrane-enclosed structure that contains DNA.

Eukaryotes are cells that enclose their DNA in nuclei.

Prokaryotes are cells that do not enclose their DNA in nuclei.

7.2 Cell Structure

Lesson Summary

Cell Organization Eukaryotic cells contain a nucleus and many specialized structures.

Cytoplasm is the fluid portion of a cell.

Organelles are structures that have specialized functions in eukaryotic cells.

The nucleus contains DNA and controls the activity of a cell.

Organelles That Store, Clean Up, and Support These structures include:

vacuoles: membrane-enclosed saclike structures that store water, salts, and organic molecules

lysosomes: small organelles filled with enzymes that break down large molecules and organelles that are

no longer useful

the cytoskeleton: a network of protein filaments; it helps the cell maintain its shape and is involved in

movement

centrioles: organelles made from tubulins; they help organize cell division in animal cells

Organelles That Build Proteins Three kinds of organelles work with the nucleus to make and

distribute proteins:

ribosomes: small particles of RNA and protein found throughout the cytoplasm in all cells; they produce

proteins by following coded instructions from DNA

the endoplasmic reticulum (ER): an internal membrane system where lipid components of the cell

membrane are assembled, along with proteins and other materials

the Golgi apparatus: an organelle that appears as a stack of flattened membranes; it modifies, sorts, and

packages proteins and other materials from the ER for storage in the cell or release outside the cell

Organelles That Capture and Release Energy Two types of organelles act as power plants of

the cells. Both types are surrounded by two membranes.

Chloroplasts capture the energy from sunlight and convert it into food that contains chemical energy in a

process called photosynthesis. Cells of plants and some other organisms contain chloroplasts, which

contain chlorophyll.

Mitochondria are found in nearly all eukaryotic cells; they convert the chemical energy stored in food to

a usable form.

Cellular Boundaries All cells are surrounded by a cell membrane. Many cells also have a cell wall.

Both cell membranes and cell walls separate cells from the environment and provide support.

Cell walls support, shape, and protect the cell. Most prokaryotes and many eukaryotes have them.

Animals do not have cell walls. Cell walls lie outside the cell membrane. Most cell walls allow materials

to pass through them.

A cell membrane consists of a lipid bilayer, a strong but flexible barrier between the cell and its

surroundings. The cell membrane regulates what enters and leaves the cell and also protects and supports

the cell. Most biological membranes are selectively permeable, allowing some substances, but not

others, to pass across them.

7.3 Cell Transport

Lesson Summary

Passive Transport The movement of materials across the cell membrane without using cellular

energy is called passive transport.

Diffusion is the

process by which

particles move from

an area of high

concentration to an

area of lower

concentration.

Facilitated

diffusion is the

process by which

molecules that

cannot directly diffuse across the membrane pass through special protein channels.

Osmosis is the facilitated diffusion of water through a selectively permeable membrane.

Aquaporins are water channel proteins that allow water to pass through cell membranes.

Two adjacent solutions are

isotonic if they have the

same concentrations of

solute.

Hypertonic solutions have

a higher concentration of

solute compared to another

solution.

Hypotonic solutions have

a lower concentration of

solute compared to another

solution.

Osmotic pressure is the force

caused by the net movement of

water by osmosis.

Active Transport The movement of materials against a concentration difference is called active

transport. Active transport requires energy.

Transport proteins that act like pumps use energy to move small molecules and ions across cell

membranes.

The bulk transport of large molecules and clumps of materials into and out of cells occurs by

movements of the cell membrane, which require energy.

7.4 Homeostasis and Cells

Lesson Summary

The Cell as an Organism Sometimes a single cell is an organism. Single-celled organisms must be

able to carry out all the functions necessary for life.

Unicellular organisms maintain homeostasis, relatively constant internal conditions, by growing,

responding to the environment, transforming energy, and reproducing.

Unicellular organisms include both prokaryotes and eukaryotes.

Unicellular organisms play many important roles in their environments.

Multicellular Life Cells of multicellular organisms are interdependent and specialized.

The cells of multicellular organisms become specialized for particular tasks and communicate with one

another to maintain homeostasis.

Specialized cells in multicellular organisms are organized into groups.

A tissue is a

group of similar cells

that performs a

particular function.

An organ is a

group of tissues

working together to

perform an essential

task.

An organ system is a group of organs that work together to perform a specific function.

The cells of multicellular organisms communicate with one another by means of chemical signals that

are passed from one cell to another.

Certain cells form connections, or cellular junctions, to neighboring cells. Some of these junctions hold

cells together firmly.

Other cells allow small molecules carrying chemical signals to pass directly from one cell to the next.

To respond to a chemical signal, a cell must have a receptor to which the signaling molecule can bind.

Chapter Vocabulary Review For Questions 1–4, write True if the statement is true. If the statement is false, change the underlined

word or words to make the statement true.

1. All cells are surrounded by a cell wall.

2. The flexible nature of a cell membrane results from its channel proteins.

3. Selectively permeable membranes allow only certain materials to pass through them.

4. Centrioles are found in animal cells.

For Questions 5–11, match the organelle with its description.

Organelle

5. Ribosomes

6. Endoplasmic reticulum

7. Golgi apparatus

8. Lysosomes

9. Vacuoles

10. Chloroplasts

11. Mitochondria

Description

A. Convert energy from sunlight into chemical energy that is stored

in food

B. Stack of membranes that modifies, sorts, and packages proteins

and other materials for storage or release

C. Convert chemical energy stored in food into a form that can be

easily used by the cell

D. An internal membrane system where lipid components of cell

membranes are made

E. Saclike structures that store materials

F. Small particles of RNA and protein on which proteins are

assembled using instructions from DNA

G. Filled with enzymes used to break down carbohydrates into

smaller molecules

8.1 Energy and Life Lesson Summary

Chemical Energy and ATP Energy is the ability to do work. Organisms need energy to stay alive.

Adenosine triphosphate (ATP) is a chemical compound

cells use to store and release energy.

An ATP molecule consists of adenine, the sugar

ribose, and three phosphate groups.

Cells store energy by adding a phosphate group to

adenosine diphosphate (ADP) molecules.

Cells release energy from ATP molecules by subtracting a phosphate group.

Energy provided by ATP is used in active transport, to contract muscles, to make proteins, and in many

other ways.

Cells contain only a small amount of ATP at any one time. They regenerate it from ADP as they need it,

using energy stored in food.

Heterotrophs and Autotrophs The energy to make ATP from ADP comes from food. Organisms

get food in one of two ways.

Heterotrophs get food by consuming (eating) other organisms.

Autotrophs use the energy in sunlight to make their own food.

Photosynthesis is the process that uses light energy to produce food molecules.

8.2 Photosynthesis: An Overview

Lesson Summary

Chlorophyll and Chloroplasts In eukaryotes, photosynthesis occurs in organelles called

chloroplasts. Chloroplasts house light-absorbing chemicals.

Light is a form of energy. Sunlight is a mixture of all the different colors of visible light.

Light-absorbing molecules

called pigments capture the

sun’s energy.

Chlorophyll is the principal

pigment in photosynthetic

organisms. Chlorophyll

absorbs blue-violet and red

light but reflects green light.

Chloroplasts have a

complex internal structure that

includes:

thylakoids: saclike

photosynthetic

membranes that contain

chlorophyll and other

pigments and are

arranged in stacks called

grana.

stroma: the fluid portion outside of the thylakoids.

High-Energy Electrons The energy in light raises some of the electrons in chlorophyll to higher

energy levels. These high-energy electrons are used in photosynthesis.

Electron carriers are used to transport the electrons from chlorophyll to other molecules during

photosynthesis.

NADP+ is a compound that can accept and hold 2 high-energy electrons and 1 hydrogen ion. This

process converts NADP+ into NADPH.

An Overview of Photosynthesis Usually summarized by a simple chemical reaction,

photosynthesis is a complex process that involves two interdependent sets of reactions.

The light-dependent reactions require light, light-absorbing pigments, and water to form NADPH, ATP,

and oxygen.

The light-independent reactions do not use light energy. They use carbon dioxide from the atmosphere,

NADPH, and ATP to make energy-rich carbon compounds.

8.3 The Process of Photosynthesis

Lesson Summary

The Light-Dependent Reactions: Generating ATP and NADPH

Photosynthesis begins with these reactions, which occur in thylakoid membranes.

Photosystems are clusters of proteins and chlorophyll in thylakoid membranes.

High-energy electrons form when pigments in photosystem II absorb light. The electrons pass through

electron transport chains, a series of electron carrier proteins.

The movement of electrons through an electron transport chain causes a thylakoid to fill up with

hydrogen ions and generates ATP and NADPH.

ATP synthase is a membrane protein through which excess hydrogen ions escape a thylakoid in a

process that makes ATP.

The Light-Independent Reactions: Producing Sugars They occur in the stroma of

thylakoids and are commonly called the Calvin cycle.

Six carbon dioxide molecules from the atmosphere enter the Calvin cycle and combine with 5-carbon

compounds already present. They produce twelve 3-carbon molecules.

Two 3-carbon molecules are removed from the cycle. They are used by the plant to build sugars, lipids,

amino acids, and other compounds.

The remaining ten 3-carbon molecules are converted back to 5-carbon molecules and begin a new cycle.

Chapter Vocabulary Review Crossword Puzzle Complete the puzzle by entering the term that matches the description.

Across

4. energy carrier cells use to transport high-energy electrons

6. cluster of pigments and proteins that absorbs light

7. a saclike photosynthetic membrane found in chloroplasts

8. energy carrier made as a result of photosystem II

9. process of using the sun’s energy to make food

10. man who worked out the light-independent reactions

Down

1. liquid part of the inside of a

chloroplast

2. chemical that absorbs light for

photosynthesis

3. light-absorbing chemical

5. organism that makes its own food

For Questions 11–16, complete each statement by writing the correct word or words.

11. The light- reactions occur in thylakoid membranes.

12. Carbon dioxide is used to make sugars in the light- reactions.

13. The light-independent reactions are also called the .

14. spins to provide the energy for adding a phosphate group to ADP.

15. Electron move high-energy electrons between photosystems.

16. An animal that obtains food by eating other organisms is called a(n) .

9.1 Cellular Respiration: An Overview

Lesson Summary

Chemical Energy and Food Chemical energy is stored in food molecules.

Energy is released when chemical bonds in food molecules are broken.

Energy is measured in a unit called a calorie, the amount of energy needed to raise the temperature of 1

gram of water 1 degree Celsius.

Fats store more energy per gram than do carbohydrates and proteins.

Overview of Cellular Respiration Cellular respiration is the

process that releases energy from food in the presence of oxygen.

Cellular respiration captures the energy from food in three main stages:

glycolysis

the Krebs cycle

the electron transport chain

Glycolysis does not require oxygen. The Krebs cycle and electron transport

chain both require oxygen.

Aerobic pathways are processes that require oxygen.

Anaerobic pathways are processes that occur without oxygen.

Comparing Photosynthesis and Cellular Respiration The energy in photosynthesis and cellular

respiration flows in opposite directions. Their equations are

the reverse of each other.

Photosynthesis removes carbon dioxide from the

atmosphere, and cellular respiration puts it back.

Photosynthesis releases oxygen into the atmosphere,

and cellular respiration uses oxygen to release energy

from food.

9.2 The Process of Cellular Respiration

Lesson Summary

Glycolysis The word glycolysis literally means “sugar-breaking.” The end result is 2 molecules

of a 3-carbon molecule called pyruvic acid.

2 ATP molecules are used at the start of glycolysis to get the process started.

High-energy electrons are passed to the electron carrier NAD+, forming two molecules of NADH.

4 ATP are synthesized during glycolysis for a net gain of 2 ATP.

The Krebs Cycle The second stage of cellular respiration is the Krebs cycle, which operates only when

oxygen is available. The Krebs cycle is a series of energy-extracting reactions.

Pyruvic acid produced by glycolysis enters mitochondria. In the innermost compartment of a

mitochondrion, or the matrix, pyruvic acid molecules are broken down into carbon dioxide and acetyl-

CoA molecules.

Acetyl-CoA combines with a 4-carbon compound, producing a 6-carbon molecule—citric acid. Energy

released by the breaking and rearranging of carbon bonds is captured in ATP, NADH, and FADH2.

The Krebs cycle produces four types of products:

high-energy electron carriers (NADH and FADH2)

carbon dioxide

2 ATP molecules (per glucose molecule)

the 4-carbon molecule needed to start the cycle again

Electron Transport and ATP Synthesis The electron transport chain uses the high-energy

electrons from glycolysis and the Krebs cycle to convert ADP into ATP.

The electron carriers produced during glycolysis and the Krebs cycle bring high-energy electrons to the

electron transport chain. Oxygen is the final electron acceptor.

The passing of electrons through the electron transport chain causes H+ ions to build up in the

intermembrane space, making it positively charged relative to the matrix.

The charge difference across the membrane forces H+

ions through channels in enzymes known as ATP

synthases. As the ATP synthases spin, a phosphate group is added to ADP, generating ATP.

The Totals Together, glycolysis, the Krebs cycle, and the electron transport chain generate about 36

molecules of ATP per molecule of glucose.

Chapter Vocabulary Review For Questions 1–7, match the term with its definition.

Term

1. anaerobic

2. glycolysis

3. Krebs cycle

4. calorie

5. matrix

6. aerobic

7. fermentation

Definition

A. Innermost compartment of a mitochondrion

B. Process that forms either lactic acid or ethyl alcohol when no oxygen is

present

C. Stage of cellular respiration that starts with pyruvic acid and produces

carbon dioxide

D. Process in which glucose is broken down into two molecules of pyruvic

acid

E. “In air”

F. “Without air”

G. Amount of energy needed to raise the temperature of 1 gram of water

1°C

For Questions 8–10, write the letter of the correct answer on the line at the left.

8. Which is the process that releases energy by breaking down food molecules in the presence of

oxygen?

A. cellular respiration C. glycolysis

B. electron transport D. photosynthesis

9. Which is the electron carrier that accepts electrons during glycolysis?

A. ADP C. NAD+

B. ATP D. NADP+

10. When comparing cellular respiration and photosynthesis, these two processes are best described

as

A. energy-releasing processes. C. opposite processes.

B. energy-storing processes. D. similar processes.