Learning Objectives

In this section, you will explore the following questions:

What are metabolic pathways?
What are the differences between anabolic and catabolic pathways?
How do chemical reactions play a role in energy transfer?

Connection for AP® Courses

Connection for AP^® Courses

All living systems, from simple cells to complex ecosystems, require free energy to conduct cell processes such as growth and reproduction.

Organisms have evolved various strategies to capture, store, transform, and transfer free energy. A cell’s metabolism refers to the chemical reactions that occur within it. Some metabolic reactions involve the breaking down of complex molecules into simpler ones with a release of energy (catabolism), whereas other metabolic reactions require energy to build complex molecules (anabolism). A central example of these pathways is the synthesis and breakdown of glucose.

The content presented in this section supports the Learning Objectives outlined in Big Idea 1 and Big Idea 2 of the AP^® Biology Curriculum Framework listed below. The AP^® Learning Objectives merge Essential Knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP^® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP^® exam questions.

Big Idea 1	The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.B	Organisms are linked by lines of descent from common ancestry.
Essential Knowledge	1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.
Science Practice	3.1 The student can pose scientific questions.
Learning Objective	1.14 The student is able to pose scientific questions that correctly identify essential properties of shared, core life processes that provide insight into the history of life on Earth.
Essential Knowledge	1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.
Science Practice	7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective	1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms.
Essential Knowledge	1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.
Science Practice	6.1 The student can justify claims with evidence.
Learning Objective	1.16 The student is able to justify the scientific claim that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today.
Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A	Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge	2.A.1 All living systems require a constant input of free energy.
Science Practice	6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective	2.1 The student is able to explain how biological systems use free energy based on empirical data that all organisms require constant energy input to maintain organization, to grow and to reproduce.

The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP^® exam. These questions address the following standards:

[APLO 2.1]
[APLO 2.3]
[APLO 4.3]
[APLO 4.15]
[APLO 4.17]
[APLO 2.21]

In addition, content from this chapter is addressed in the AP^® Biology Laboratory Manual in the following lab(s):

7 Enzymatic Activity

Scientists use the term bioenergetics to discuss the concept of energy flow (Figure 6.2) through living systems, such as cells. Cellular processes, such as the building and breaking down of complex molecules, occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish what has been used, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. All of the chemical reactions that take place inside cells, including those that use energy and those that release energy, are the cell’s metabolism.

This diagram shows energy from the sun being transferred to producers, such as plants, as well as releasing heat. The producers in turn transfer the energy to consumers and decomposers, which release heat. Animals also transfer energy to decomposers.

Figure 6.2 Most life forms on earth get their energy from the sun. Plants use photosynthesis to capture sunlight, and herbivores eat those plants to obtain energy. Carnivores eat the herbivores, and decomposers digest plant and animal matter.

Metabolism of Carbohydrates

The metabolism of sugar—a simple carbohydrate—is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. The breakdown of glucose, a simple sugar, is described by the equation

C_{6} H_{12} O_{6} + {6O}_{2} \to 6 {CO}_{2} + {6H}_{2} O + energy

Carbohydrates that are consumed have their origins in photosynthesizing organisms, like plants (Figure 6.3). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO₂) into sugar molecules, like glucose (C₆H₁₂O₆). Because this process involves synthesizing a larger, energy-storing molecule, it requires an input of energy to proceed. The synthesis of glucose is described by this equation—notice that it is the reverse of the previous equation

{6CO}_{2} + {6H}_{2} O + energy \to C_{6} H_{12} O_{6} + {6O}_{2}

During the chemical reactions of photosynthesis, energy is provided in the form of a very high-energy molecule called ATP, or adenosine triphosphate, which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. The sugar (glucose) is stored as starch or glycogen. Energy-storing polymers like these are broken down into glucose to supply molecules of ATP.

Solar energy is required to synthesize a molecule of glucose during the reactions of photosynthesis. In photosynthesis, light energy from the sun is initially transformed into chemical energy that is temporally stored in the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate). The stored energy in ATP and NADPH is then used later in photosynthesis to build one molecule of glucose from six molecules of CO₂. This process is analogous to eating breakfast in the morning to acquire energy for your body that can be used later in the day. Under ideal conditions, energy from 18 molecules of ATP is required to synthesize one molecule of glucose during the reactions of photosynthesis. Glucose molecules can also be combined with and converted into other types of sugars. When sugars are consumed, molecules of glucose eventually make their way into each living cell of the organism. Inside the cell, each sugar molecule is broken down through a complex series of chemical reactions. The goal of these reactions is to harvest the energy stored inside the sugar molecules. The harvested energy is used to make high-energy ATP molecules, which can be used to perform work, powering many chemical reactions in the cell. The amount of energy needed to make one molecule of glucose from six molecules of carbon dioxide is 18 molecules of ATP and 12 molecules of NADPH—each one of which is energetically equivalent to three molecules of ATP—or a total of 54 ATP molecule equivalents required for the synthesis of one molecule of glucose. This process is a fundamental and efficient way for cells to generate the molecular energy that they require.

The photo on the left shows acorns growing on an oak tree. The photo on the right shows a squirrel eating.

Figure 6.3 Plants, like this oak tree, use energy from sunlight to make sugar and other organic molecules. Both plants and animals, like this squirrel, use cellular respiration to derive energy from the organic molecules originally produced by plants.

Metabolic Pathways

The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways. A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products. In the case of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic (building) and catabolic (breaking down) pathways, respectively. Consequently, metabolism is composed of building (anabolism) and degradation (catabolism).

Evolution Connection

Evolution of Metabolic Pathways

At the base of the evolutionary tree is the prokaryotic ancestor. This ancestor gave rise to archaebacteria, eubacteria, and Protista, which in turn gave rise to plants, fungi, and animals.

Figure 6.4 This tree shows the evolution of the various branches of life. The vertical dimension is time. Early life forms, in blue, used anaerobic metabolism to obtain energy from their surroundings.

There is more to the complexity of metabolism than understanding the metabolic pathways alone. Metabolic complexity varies from organism to organism. Photosynthesis is the primary pathway in which photosynthetic organisms like plants—the majority of global synthesis is done by planktonic algae—harvest the sun’s energy and convert it into carbohydrates. The by-product of photosynthesis is oxygen, required by some cells to carry out cellular respiration. During cellular respiration, oxygen aids in the catabolic breakdown of carbon compounds, like carbohydrates. Among the products of this catabolism are CO₂ and ATP. In addition, some eukaryotes perform catabolic processes without oxygen (fermentation); that is, they perform or use anaerobic metabolism.

Organisms probably evolved anaerobic metabolism to survive—living organisms came into existence about 3.8 billion years ago, when the atmosphere lacked oxygen. Despite the differences between organisms and the complexity of metabolism, researchers have found that all branches of life share some of the same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor (Figure 6.4). Evidence indicates that over time, the pathways diverged, adding specialized enzymes to allow organisms to better adapt to their environment, thus increasing their chance to survive. However, the underlying principle remains that all organisms must harvest energy from their environment and convert it to ATP to carry out cellular functions.

The early atmosphere lacked oxygen. Why do you think this is the case?

Oxygen is a byproduct of photosynthesis, so there was very little oxygen in the atmosphere until photosynthetic organisms evolved.
Oxygen is a byproduct of anaerobic respiration, so there was very little oxygen in the atmosphere until anaerobic organisms evolved.
Oxygen is a byproduct of fermentation, so there was very little oxygen in the atmosphere until fermentative organisms evolved.

Anabolic and Catabolic Pathways

Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO₂ is one example. Other examples are the synthesis of large proteins from amino acid building blocks, and the synthesis of new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH (Figure 6.5).

ATP is an important molecule for cells to have a sufficient supply of at all times. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve the degradation, or breakdown, of complex molecules into simpler ones. Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 6.5).

It is important to know that the chemical reactions of metabolic pathways don’t take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Anabolic and catabolic pathways are shown. In the anabolic pathway (top), four small molecules have energy added to them to make one large molecule. In the catabolic pathway (bottom), one large molecule is broken down into two components: four small molecules plus energy.

Figure 6.5 Anabolic pathways are those that require energy to synthesize larger molecules. Catabolic pathways are those that generate energy by breaking down larger molecules. Both types of pathways are required for maintaining the cell’s energy balance.

Science Practice Connection for AP® Courses

Think About It

Describe two different cellular functions in different organisms that require energy that parallel human energy-requiring functions, such as physical exercise.

6.1 Energy and Metabolism

6.1 Energy and Metabolism

Learning Objectives