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In a presentation given at a meeting for science teachers in the late 1960s, physicist Richard Feynman said that energy is “a subtle concept. It is very, very difficult to get right.” Indeed, in his lecture on the conservation of energy he notes that, “in physics today, we have no knowledge of what energy is.” We can’t see or touch the forces that govern the movement of objects and the chemistry of living things, although we can measure them. In this section, we examine this sometimes difficult-to-grasp idea, reviewing the forms that energy can take and discussing the ways cells harness energy to power the processes that keep them alive.
Water behind the Kurobe Dam in Japan is channeled through turbines inside the wall of the dam, movement that generates electricity. Metabolic energy can also be stored and converted from one form to another via metabolic reactions.
Energy and Work
As Franklin Harold describes it, “energy is to biology what money is to economics: the means by which living things purchase useful goods and services.” Practically speaking, energy is the ability to do work. The word work has a special meaning in science. For a cell, work includes all kinds of activities. Some, like crawling or contracting, are mechanical. Others involve carrying or pumping substances across membranes. Cellular work also includes synthesis, the assembly of complex structures or molecules (a protein or nucleic acid, for example) from simpler components (amino acids or nucleotides, for example).
Energy comes in many forms. Some, like electrical energy, thermal (heat) energy, and light energy, are familiar from everyday life. These forms of energy can be readily converted from one to another. That’s because at the level of atoms, all energy is the same—a mixture of stored, or potential energy, and kinetic energy, the energy of motion.
Molecules, too, contain energy. Chemical energy is stored in the bonds that bonds are broken or rearranged, this potential energy can be released and converted to kinetic energy, which the cell can use to carry out work—synthesizing another molecule, pumping ions across a membrane, or driving cell movement.
Cells capture and temporarily store energy in two main ways. First, they use the energy extracted from food molecules to form a chemical bond in a molecule of ATP (adenosine triphosphate). When the bond is later broken, this stored energy is released. The energy stored in ATP can be used to pay for other work done by the cell—in particular, the synthesis of proteins, lipids, nucleic acids, carbohydrates, and other important molecules.
Second, cells store energy in electrochemical gradients, differences in the concentration and charge of ions or molecules on either side of a cell membrane. Cells expend energy to generate these gradients, which represent a form of potential energy similar to that of water held on one side of a dam. When ions are allowed to flow back across the membrane, down their electrochemical gradient, the stored energy is released and can be used to carry out the cell’s work—like capturing the energy of water channeled through a dam to drive turbines and create electricity. In the cell, much of the energy released in metabolism is used to generate ATP.
c07_med_kineticAndPotentialEnergy_011014_fullRes-Apple Devices HD Best Quality_050114.m4vPotential and Kinetic Energy - Potential energy is energy that’s stored—the energy of an object perched on a ledge, for example, or the energy in chemical bonds. Kinetic energy is the energy of motion. Potential and kinetic energy can be converted from one to the other. The potential energy of a ball at the top of a slope turns into kinetic energy as the ball rolls down the hill.
c07_med_energyStorage_021614_fullRes-Apple Devices HD Best Quality_050114.m4vIon Gradients and Energy - When ions are trapped together on one side of a cell membrane, the repulsive forces that push them apart are a form of potential energy; this energy increases with the concentration of ions. When trapped ions are freed, this potential energy is converted to kinetic energy.
Sources of Energy
The energy used by living systems comes from the environment. For most living things on Earth, the ultimate source of energy is the sun. Plants, algae, and photosynthetic prokaryotes capture the energy of sunlight and use it to make sugars and other organic molecules through the process of photosynthesis. Sunlight is made up of a stream of energetic photons. When one of these energy particles strikes a receptive electron in a chlorophyll molecule (a light-capture molecule in plants) the photon disappears as its energy is transferred to the electron. The “energized” electron is more likely to participate in chemical reactions than is an electron in its resting state. It’s that extra spark of electricity that metabolism builds on, as excited electrons hop from one molecule to another, rearranging the bonds in molecules and redistributing their energy.
But not all living things get their energy from the sun. A few organisms, including the bacteria and archaea that thrive in hydrothermal vents on the ocean floor, can extract energy from inorganic molecules—for example, hydrogen sulfide—that bubble up through the steaming vents. These inorganic molecules release energy when they react with other molecules. For vent organisms, those reactions are steps in metabolic processes that channel the released energy into the cell’s work.
c07_lightAndMatter_fullRes_54-Apple ProRes 422 HQ-1.m4vCharging Electrons with Sunlight - The red photon shown is a packet of energy from the sun. When photons are absorbed by chlorophyll molecules, the energy of light is converted into chemical energy in the form of energized electrons. The captured energy can be transferred between electrons and used to drive chemical reactions.
A Link in a Long Chain - The American pika is a small relative of rabbits that lives in high mountain areas. The plants it is feeding on trapped energy from the sun. That stored energy will fuel the pika. And that energy in pikas fuels predators of the pika—eagles, hawks, coyotes, bobcats, foxes, and weasels—in a food chain that is powered from bottom to top by energy that originated in the sun.
Organisms that produce their own nutrients—either by harvesting energy from sunlight or by manipulating inorganic materials, like the deep-sea bacteria—are called autotrophs. These “producers” are autonomous: they don’t need other living things to provide them with food. All other organisms are heterotrophs. These “consumers” get energy by feeding on other organisms or on their biological products. The flow of energy—from humble autotroph to hungry heterotroph—constitutes a food chain. Because most heterotrophs eat plants—or other organisms that feed on plants—most food chains are powered primarily by photons from the sun.
Endergonic versus Exergonic - If a reaction is exergonic, meaning it can occur without the input of energy, the opposite reaction is exergonic—energy must be added to the system for it to occur. The release of this arrow would be exergonic, with a release of the enormous energy stored in the taut bow. The reverse reaction is endergonic. Without an input of energy from the archer, loading and drawing the bow, the arrow will not jump back onto the bow and stretch the bow string.
The Flow of Chemical Energy
How much work can any chemical reaction carry out? It depends on the nature of the reactants and products, and on conditions in the cell—in particular, the concentrations of the molecules involved in the reaction.
A chemical reaction is a process that transforms one or more molecules, called reactants, into molecules with different chemical identities, or products. When a molecule undergoes a chemical reaction, chemical bonds are broken and re-formed to generate one or more products. If the products of a reaction are more stable than the reactants, the reaction releases energy. A reaction that releases energy is called exergonic. The breakdown of food molecules by cells via cellular respiration is exergonic—the process releases energy.
Other reactions absorb more energy than they release. These energy-consuming reactions are called endergonic. The conversion of carbon dioxide (CO₂) into sugar is an endergonic process: it doesn’t happen without an input of energy. In photosynthesis, the energy to produce sugars is absorbed from sunlight.
Chemical Equilibrium
The amount of energy released during an exergonic reaction depends on the structure of the reactant—how electrons are distributed in the molecule and how stable the arrangement is—and on how far the reaction is from equilibrium. Remember that all chemical systems have a tendency to move toward equilibrium. In a chemical reaction, equilibrium is the point at which there’s an equal likelihood that the reactants will combine to make a product or that the product will break down to re-form the reactants.
A reaction doesn’t halt completely at equilibrium. Instead, the forward reaction, in which the reactants combine to form products, takes place at the same rate as the reverse reaction, in which the products break down to yield the reactants. The reactants and products are not necessarily present at the same concentration at equilibrium, but the ratio of one to the other will remain the same. In a chemical system at equilibrium, no work gets done.
Consider the analogy of water in a U-shaped tube. When the water is at the same level in each arm of the tube, the system is at equilibrium:
Now, suppose you quickly add water to the right arm of the tube:
Because the system has a tendency to restore itself to equilibrium, water is going to rush over to the left arm of the tube. That movement of water is a form of kinetic energy, which could be readily captured and put to work if there were, say, a tiny turbine in the bottom of the tube.
The same rules apply to chemical reactions carried out by enzymes. A chemical system can be driven away from equilibrium by increasing the concentration of the reactants or products. If we add more of the reactants to the system, the reaction would run forward until reactants and products are in equilibrium again. And if we add more of the product, the reaction would run backward until equilibrium is restored “equilibrium by increasing the concentration of the reactants or products. If we add more of the reactants to the system, the reaction would run forward until reactants and products are in equilibrium again. And if we add more of the product, the reaction would run backward until equilibrium is restored:
This chemical flow, like the sloshing of water in the U-shaped tube, releases energy that can be captured for later use. Many reactions that are carried out by enzymes operate near equilibrium, and run forward or backward depending on whether there’s an increase in the concentration of reactants or products. (We will talk more about enzymes later.)
Two-way traffic is common in metabolism, depending on the state of the cell and whether, for example, it’s storing or consuming a product.
Inside cells, most chemical reactions are fed by a steady stream of matter and energy that flows through the system: nutrients are ingested and broken down, and their products are whisked away to be reused or discarded. Overall, this constant throughput of matter and energy maintains biological systems a safe distance from equilibrium. Without these efforts to keep equilibrium at bay, the cell would stop functioning: structures would crumble; gradients would dissipate; and all the hard-won biological order would disappear. In other words, life would cease to exist.
ATP: Energy on the Move
Cells work hard to keep certain chemical reactions from reaching equilibrium, because disequilibrium serves as a source of potential energy. Consider, for example, the molecule ATP. As mentioned earlier, cells use ATP, or adenosine triphosphate, as a form of energy currency. The molecule is made up of the nucleotide adenosine coupled to three phosphate groups (the prefix tri- means “three”). When the outermost, or terminal, phosphate group breaks away from ATP, energy is released. The products of this exergonic reaction are ADP (adenosine diphosphate) and a released phosphate molecule.
The amount of energy released in this reaction is related to the concentrations of ATP and ADP. Inside cells, the ratio of ATP to ADP is kept far from equilibrium: the concentration of ATP in cells is 5 times higher than the concentration of ADP. Although that might not sound like much, if the concentrations of ATP and its breakdown products (ADP + phosphate) were allowed to come to equilibrium, the concentration of ATP would be lower than that of ADP—by 10,000 times. So the balance of ATP and ADP in cells is a long, long way from equilibrium, pushed strongly in the direction of more ATP (like water poised far up one side of a U-shaped tube). When a molecule of ATP is converted to ADP, it provides the cell with a substantial dose of energy.
The ADP and phosphate produced when ATP is consumed are constantly recycled to form new ATP, which is synthesized by cells at the same rate it’s used. And cells use a lot of ATP. In the course of a day, the human body burns through nearly its weight in ATP. But it does so by constantly recycling its store of ADP and phosphate in the body, which is equal to about 300 grams, or a bit more than 10 ounces. Each molecule of ATP is recycled hundreds of times a day.
ATP Cycle - In cells, the terminal phosphate group of ATP breaks away to produce ADP and P. This reaction releases energy that is captured and used to drive other reactions. ADP is then recycled to regenerate ATP, a reaction that requires an input of energy. The bond that breaks when ATP releases its terminal phosphate group is often called a “high-energy bond,” but it’s actually a fairly normal chemical bond. What’s unusual about this system is the amount of energy the cell expends to form a vast surplus of ATP.
The Energy in ATP - The reason ATP reacts to form ADP is related to its chemical structure. ATP is “eager” to get rid of the outermost phosphate group because all three phosphate groups are negatively charged: these “like” charges repel one another, creating a pressure that drives the terminal phosphate group away. When this phosphate group breaks loose, the pressure is relieved and energy is released.
Gradients: The Other Energy Stockpile
Chemical reactions are not the only systems that cells maintain in disequilibrium. The cell expends energy to pump ions across cell membranes. This movement of ions generates an electrochemical gradient that serves as a form of potential energy, like water stored behind a dam. In a hydroelectric power plant, the energy released when dammed-up water is allowed to flow back downhill is used to power a turbine that generates electricity. A similar process drives the production of ATP in cells. In living systems, the energy derived from the breakdown of foods is used to pump H⁺ ions, or protons, across a membrane. When these protons are allowed to flow down their electrochemical gradient, toward equilibrium, they pass through a molecular turbine embedded in the membrane. That turbine, called ATP synthase, taps the energy of the proton flow to attach a phosphate to ADP, forming ATP.
Electrochemical gradients are a hallmark of life. In addition to the proton gradients powered by nutrients, which are present in all cells, photosynthetic cells generate proton gradients powered by sunlight. Although these systems have different components, they share a common evolutionary ancestor. We examine them both in detail in the next chapter. Here, it’s enough to understand that the cell uses such systems to capture energy and, ultimately, convert that energy to ATP. What do cells do with all this energy? They use it to drive the interconnected web of chemical reactions that make up cellular metabolism.
ATP Synthase - A difference in the concentration of H⁺ across a membrane is a form of stored energy. In living cells, protein complexes pump H⁺ across membranes to create such a difference. H⁺ is channeled back across the membrane through ATP synthase, releasing energy that is coupled to the formation of ATP.
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