Metabolism

Overview: The Energy of Life

Concept 8.1 An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics

The totality of an organism’s chemical reactions is called metabolism.
Metabolism is an emergent property of life that arises from interactions between molecules within the orderly environment of the cell.

The chemistry of life is organized into metabolic pathways.

Metabolic pathways begin with a specific molecule, which is then altered in a series of defined steps to form a specific product.
A specific enzyme catalyzes each step of the pathway.
Catabolic pathways release energy by breaking down complex molecules to simpler compounds.
- A major pathway of catabolism is cellular respiration, in which the sugar glucose is broken down in the presence of oxygen to carbon dioxide and water.
Anabolic pathways consume energy to build complicated molecules from simpler compounds. They are also called biosynthetic pathways.
- The synthesis of protein from amino acids is an example of anabolism.
The energy released by catabolic pathways can be stored and then used to drive anabolic pathways.
Energy is fundamental to all metabolic processes, and therefore an understanding of energy is key to understanding how the living cell works.
- Bioenergetics is the study of how organisms manage their energy resources.

Organisms transform energy.

Energy is the capacity to do work.
- Energy exists in various forms, and cells transform energy from one type into another.
Kinetic energy is the energy associated with the relative motion of objects.
- Objects in motion can perform work by imparting motion to other matter.
- Photons of light can be captured and their energy harnessed to power photosynthesis in green plants.
- Heat or thermal energy is kinetic energy associated with the random movement of atoms or molecules.
Potential energy is the energy that matter possesses because of its location or structure.
- Chemical energy is a form of potential energy stored in molecules because of the arrangement of their atoms.
Energy can be converted from one form to another.
- For example, as a boy climbs stairs to a diving platform, he is releasing chemical energy stored in his cells from the food he ate for lunch.
- The kinetic energy of his muscle movement is converted into potential energy as he climbs higher.
- As he dives, the potential energy is converted back to kinetic energy.
- Kinetic energy is transferred to the water as he enters it.
- Some energy is converted to heat due to friction.

The energy transformations of life are subject to two laws of thermodynamics.

Thermodynamics is the study of energy transformations.
In this field, the term system refers to the matter under study and the surroundings include everything outside the system.
A closed system, approximated by liquid in a thermos, is isolated from its surroundings.
In an open system, energy and matter can be transferred between the system and its surroundings.
Organisms are open systems.
- They absorb energy—light or chemical energy in the form of organic molecules—and release heat and metabolic waste products such as urea or CO2 to their surroundings.
The first law of thermodynamics states that energy can be transferred and transformed, but it cannot be created or destroyed.
- The first law is also known as the principle of conservation of energy.
- Plants do not produce energy; they transform light energy to chemical energy.
During every transfer or transformation of energy, some energy is converted to heat, which is the energy associated with the random movement of atoms and molecules.
A system can use heat to do work only when there is a temperature difference that results in heat flowing from a warmer location to a cooler one.
- If temperature is uniform, as in a living cell, heat can only be used to warm the organism.
Energy transfers and transformations make the universe more disordered due to this loss of usable energy.
Entropy is a quantity used as a measure of disorder or randomness.
- The more random a collection of matter, the greater its entropy.
The second law of thermodynamics states that every energy transfer or transformation increases the entropy of the universe.
- While order can increase locally, there is an unstoppable trend toward randomization of the universe.
- Much of the increased entropy of the universe takes the form of increasing heat, which is the energy of random molecular motion.
In most energy transformations, ordered forms of energy are converted at least partly to heat.
- Automobiles convert only 25% of the energy in gasoline into motion; the rest is lost as heat.
- Living cells unavoidably convert organized forms of energy to heat.
For a process to occur on its own, without outside help in the form of energy input, it must increase the entropy of the universe.
The word spontaneous describes a process that can occur without an input of energy.
- Spontaneous processes need not occur quickly.
- Some spontaneous processes are instantaneous, such as an explosion. Some are very slow, such as the rusting of an old car.
Another way to state the second law of thermodynamics is for a process to occur spontaneously, it must increase the entropy of the universe.
Living systems create ordered structures from less ordered starting materials.
- For example, amino acids are ordered into polypeptide chains.
- The structure of a multicellular body is organized and complex.
However, an organism also takes in organized forms of matter and energy from its surroundings and replaces them with less ordered forms.
- For example, an animal consumes organic molecules as food and catabolizes them to low-energy carbon dioxide and water.
Over evolutionary time, complex organisms have evolved from simpler ones.
- This increase in organization does not violate the second law of thermodynamics.
- The entropy of a particular system, such as an organism, may decrease as long as the total entropy of the universe—the system plus its surroundings—increases.
- Organisms are islands of low entropy in an increasingly random universe.
- The evolution of biological order is perfectly consistent with the laws of thermodynamics.

Concept 8.2 The free-energy change of a reaction tells us whether the reaction occurs spontaneously

How can we determine which reactions occur spontaneously and which ones require an input of energy?
The concept of free energy provides a useful function for measuring spontaneity of a system.
Free energy is the portion of a system’s energy that is able to perform work when temperature and pressure is uniform throughout the system, as in a living cell.
The free energy (G) in a system is related to the total enthalpy (in biological systems, equivalent to energy) (H) and the entropy (S) by this relationship:
- G = H - TS, where T is temperature in Kelvin units.
- Increases in temperature amplify the entropy term.
- Not all the energy in a system is available for work because the entropy component must be subtracted from the enthalpy component.
- What remains is the free energy that is available for work.
Free energy can be thought of as a measure of the stability of a system.
- Systems that are high in free energy—compressed springs, separated charges, organic polymers—are unstable and tend to move toward a more stable state, one with less free energy.
- Systems that tend to change spontaneously are those that have high enthalpy, low entropy, or both.
In any spontaneous process, the free energy of a system decreases.
We can represent this change in free energy from the start of a process until its finish by:
- ΔG = Gfinal state - Gstarting state
- Or ΔG = ΔH - TΔS
For a process to be spontaneous, the system must either give up enthalpy (decrease in H), give up order (increase in S), or both.
- ΔG must be negative for a process to be spontaneous.
- Every spontaneous process is characterized by a decrease in the free energy of the system.
- Processes that have a positive or zero ΔG are never spontaneous.
The greater the decrease in free energy, the more work a spontaneous process can perform.
Nature runs “downhill.”
A system at equilibrium is at maximum stability.
- In a chemical reaction at equilibrium, the rates of forward and backward reactions are equal, and there is no change in the concentration of products or reactants.
- At equilibrium ΔG = 0, and the system can do no work.
- A process is spontaneous and can perform work only when it is moving toward equilibrium.
- Movements away from equilibrium are nonspontaneous and require the addition of energy from an outside energy source (the surroundings).
Chemical reactions can be classified as either exergonic or endergonic based on free energy.
An exergonic reaction proceeds with a net release of free energy; ΔG is negative.
The magnitude of ΔG for an exergonic reaction is the maximum amount of work the reaction can perform.
The greater the decrease in free energy, the greater the amount of work that can be done.
- For the overall reaction of cellular respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O
- ΔG = -686 kcal/mol
- For each mole (180 g) of glucose broken down by respiration, 686 kcal of energy are made available to do work in the cell.
- The products have 686 kcal less free energy than the reactants.
An endergonic reaction is one that absorbs free energy from its surroundings.
- Endergonic reactions store energy in molecules; ΔG is positive.
- Endergonic reactions are nonspontaneous, and the magnitude of ΔG is the quantity of energy required to drive the reaction.
If cellular respiration releases 686 kcal, then photosynthesis, the reverse reaction, must require an equivalent investment of energy.
- For the conversion of carbon dioxide and water to sugar, ΔG = +686 kcal/mol.
Photosynthesis is strongly endergonic, powered by the absorption of light energy.
Reactions in a closed system eventually reach equilibrium and can do no work.
- A cell that has reached metabolic equilibrium has a ΔG = 0 and is dead!
Metabolic disequilibrium is one of the defining features of life.
Cells maintain disequilibrium because they are open systems. The constant flow of materials into and out of the cell keeps metabolic pathways from ever reaching equilibrium.
- A cell continues to do work throughout its life.
A catabolic process in a cell releases free energy in a series of reactions, not in a single step.
Some reversible reactions of respiration are constantly “pulled” in one direction, as the product of one reaction does not accumulate but becomes the reactant in the next step.
Sunlight provides a daily source of free energy for photosynthetic organisms.
Nonphotosynthetic organisms depend on a transfer of free energy from photosynthetic organisms in the form of organic molecules.

Concept 8.3 ATP powers cellular work by coupling exergonic reactions to endergonic reactions

A cell does three main kinds of work:

Mechanical work, such as the beating of cilia, contraction of muscle cells, and movement of chromosomes during cellular reproduction.
Transport work, the pumping of substances across membranes against the direction of spontaneous movement.
Chemical work, driving endergonic reactions such as the synthesis of polymers from monomers.

Cells manage their energy resources to do this work by energy coupling, the use of an exergonic process to drive an endergonic one.
In most cases, the immediate source of energy to power cellular work is ATP.
ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups.
The bonds between phosphate groups can be broken by hydrolysis.
- Hydrolysis of the end phosphate group forms adenosine diphosphate.
- ATP -> ADP + Pi
- This reaction releases 7.3 kcal of energy per mole of ATP under standard conditions (1 M of each reactant and product, 25°C, pH 7).
- In the cell, ΔG for hydrolysis of ATP is about -13 kcal/mol.
While the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, these are actually fairly weak covalent bonds.
- However, they are unstable, and their hydrolysis yields energy because the products are more stable.
The release of energy during the hydrolysis of ATP comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.
Why does the hydrolysis of ATP yield so much energy?
- Each of the three phosphate groups has a negative charge.
- These three like charges are crowded together, and their mutual repulsion contributes to the instability of this region of the ATP molecule.
In the cell, the energy from the hydrolysis of ATP is directly coupled to endergonic processes by the transfer of the phosphate group to another molecule.
- This recipient molecule is now phosphorylated.
- This molecule is now more reactive (less stable) than the original unphosphorylated molecules.
Mechanical, transport, and chemical work in the cell are nearly always powered by the hydrolysis of ATP.
- In each case, a phosphate group is transferred from ATP to another molecule and the phosphorylated molecule undergoes a change that performs work.
ATP is a renewable resource that can be regenerated by the addition of a phosphate group to ADP.
- The energy to phosphorylate ADP comes from catabolic reactions in the cell.
- A working muscle cell recycles its entire pool of ATP once each minute.
- More than 10 million ATP molecules are consumed and regenerated per second per cell.
Regeneration of ATP is an endergonic process, requiring an investment of energy.
- ΔG = 7.3 kcal/mol.
Catabolic (exergonic) pathways, especially cellular respiration, provide the energy for the exergonic regeneration of ATP.
The chemical potential energy temporarily stored in ATP drives most cellular work.