AN INTRODUCTION TO METABOLISM Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Metabolism, Energy, and Life

Living is work. To perform tasks, cells require energy from outside sources. In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.1

Energy is the capacity to do work - to move matter against opposing forces. Energy can be converted from one form to another. Potential energy is the energy that matter possesses because of its location or structure. Chemical energy is a form of potential energy in molecules because of the arrangement of atoms. Organisms transform energy Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cellular respiration unleashes energy stored in sugar and other complex molecules. This energy is available for cellular work. The chemical energy stored on these organic molecules was derived from light energy (primarily) by plants during photosynthesis. A central property of living organisms is the ability to transform energy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Reflect and Review Where is energy stored in a molecule? Can energy be lost or is it constantly recycled? What is the ultimate source of energy in our ecosystem?

ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics. Analogy: It is the chemical equivalent of a loaded spring. The close packing of three negatively-charged phosphate groups is an unstable, energy-storing arrangement. Loss of the end phosphate group “relaxes” the “spring”. Cells recycle the ATP they use for work Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A cell does three main kinds of work (kinetic energy): Mechanical work, movement Transport work, pumping substances across membranes Chemical work, synthesis of polymers from monomers. In most cases, the source of energy that powers cellular work is ATP. ATP powers cellular work by coupling exergonic reactions to endergonic reactions Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 6.8a

The bonds between phosphate groups can be broken by hydrolysis. Hydrolysis of the end phosphate group forms adenosine diphosphate [ATP -> ADP + P i ]. The phosphate bonds are weak because each of the three phosphate groups has a negative charge Their repulsion contributes to the instability of this region of the ATP molecule. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 6.8b

The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (P i ). ATP is a renewable resource that is continually regenerated by adding a phosphate group to ADP. An animal cell regenerates ATP from ADP and P i by the breaking down of organic molecules. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 6.10

The transfer of the terminal phosphate group from ATP to another molecule is phosphorylation. This changes the shape of the receiving molecule, performing work (transport, mechanical, or chemical). When the phosphate group is added to the molecule, it becomes more reactive. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.2

Reflect and Review What are the 3 things that make up a molecule of ATP? Based on that information, what type of organic molecule is ATP? Where is the energy stored in a molecule of ATP? What does it mean for a molecule to be phosphorylated?

Catabolic pathways (break down) relocate the electrons stored in food molecules, releasing energy that is used to synthesize ATP. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation- reduction reactions, or redox reactions. The loss of electrons is called oxidation. The addition of electrons is called reduction. Redox reactions require both a donor and acceptor. Redox reactions release energy Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 9.5 Unlike the explosive release of heat energy that would occur when H 2 and O 2 combine, cellular respiration and photosynthesis use an electron transport chain to break the fall of electrons to O 2 into several steps.

Molecules, such as carbohydrates, lipids, and proteins, store high energy electrons in their bonds. Initially, the electrons gain this energy in photosynthesis. These electrons are required to power formation of ATP. Electron carriers such as NAD+ (cellular respiration)and NADP+ (photosynthesis) are able to bind high energy electrons and preserve their energy NADH and NADPH shuttle electrons from food to where they are needed next. Without these electron carriers, the electrons would quickly lose their energy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Reflect and Review (and Preview) Why is an electron transport chain important? What might happen without it? Write out what happens in photosynthesis, generally. Write out the general equation for photosynthesis. How does photosynthesis relate to energy?

Suggested book work Chapter 6 Review questions 2, 3, 5 Self-quiz questions 1, 2, 3, 4, 5