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Energy and Life Part I
Metabolism, Energy, and Life
The living cell is a chemical industry in miniature, where thousands of chemical reactions occur
within a microscopic space. Small molecules are assembled into polymers. Sugars are broken
down to extract energy. In plants and animals, many cells export chemical products that will be
used in other parts of the organism. These activities and processes are precisely coordinated
and controlled.
The totality of an organism’s chemical reactions is called metabolism. Metabolism is an
emergent property of life that results from the interactions between molecules within the
orderly environment of the cell.
We can think of a cell’s metabolism as an elaborate road map of the thousands of chemical
reactions that occur in that cell. The chemical reactions are arranged in a particular order so
that molecules are changed in a series of steps. Proteins called enzymes speed up chemical
reactions in organisms. Mechanisms are also in place to regulate supply and demand (to
prevent too much or too little of each molecule) that are analogous to the red, yellow, and
green lights that control the flow of road traffic.
The role of metabolism is to manage the chemical and energy resources of a cell. Some
metabolic pathways release energy by breaking down complex molecules into simpler
compounds. The major catabolic pathway (breaking down) in organisms is called cellular
respiration, in which the sugar glucose and other organic fuels are broken down to extract
energy, and carbon dioxide and water are released as waste. The energy that was extracted
can then be used by the cell to do work. Examples of the work cells must perform are anabolic
pathways (building), which consume energy to build complicated molecules from simpler ones.
An example is the building of proteins from the monomers called amino acids.
Organisms Transform Energy
Energy is the capacity to do work – the ability to arrange a collection of matter. The work of life
depends on the ability of cells to transform energy from one type into another. Anything that
moves possesses a form of energy called kinetic energy. Water gushing through a dam turns
turbines; electrons flowing along a wire run household appliances; contraction of leg muscles
pushed bicycle pedals. Light is also a type of kinetic energy that can be harnessed to perform
work, such as powering photosynthesis in green plants. Heat, or thermal energy, is kinetic
energy that results from the random movement of molecules.
Another form of energy is potential energy, energy that matter possesses due to its location or
structure. Water behind a dam, stores energy because it is higher than the stream below the
dam. Chemical energy is a form of potential energy especially important to biologists; it is
stored in molecules as a result of the arrangement of the atoms in those molecules.
How is energy converted from one form to another? Chemical energy can be tapped when
chemical reactions arrange atoms of molecules in such a way that the potential energy stored
in the molecules is converted to kinetic energy. This transformation occurs in the engine of an
automobile when the hydrocarbons of gasoline react explosively with oxygen, releasing energy
that pushed the pistons. Chemical energy also fuels organisms. Cellular respiration is the
name for the series of chemical reactions that release the chemical energy stored in sugar and
other complex molecules and make that energy available for cellular work. The chemical
energy stored in the fuel molecules was actually derived from light energy by plants during
photosynthesis. Organisms are energy transformers.
The Energy Transformations of Life are Subject to Two Laws of Thermodynamics
A closed system, such as a liquid in a thermos bottle, represents matter that is isolated from its
surroundings. In an open system, however, energy and often matter can be transferred
between the system and the surroundings. Organisms are open systems. They absorb energy –
for example, light energy or chemical energy in the form of organic molecules – and they
release heat and waste products, such as carbon dioxide, to the surroundings.
According to the first law of thermodynamics, the energy of the universe is constant. Energy
can be transferred and transformed, but it cannot be created or destroyed. This law is also
known as the principle of conservation of energy. By converting light to chemical energy, a
green plant acts as an energy transformer not an energy producer. A car converts the chemical
energy of gasoline fuels to kinetic energy. But what happens to this energy after it has
performed work in a machine or organism? If energy cannot be destroyed, what prevents
organisms from recycling their energy indefinitely? The second law answers this question.
The second law of thermodynamics states that every energy transfer or transformation
increases the disorder of the universe. Scientists use a quantity called entropy as a measure of
disorder or randomness. In many cases, increased entropy is visible in the physical
disintegration of a system’s organized structure. For example, the gradual decay of an
unmaintained building. Much of the increasing entropy of the universe is actually less obvious
because it takes the form of an increasing amount of heat, which is the energy of random
molecular motion. For example, only about 25% of the energy stored in the chemical energy of
the fuel tank of a car is transformed into motion of the car; the remaining 75% is lost from the
engine as heat. In performing various kinds of work, living cells unavoidably convert organized
forms of energy (fuel molecules) into heat – this can make a room crowded with people
uncomfortably warm for example.
By combining the first and second laws of thermodynamics, we can conclude that the quantity
of energy in the universe is constant, but its quality is not. Heat is the lowest grade of energy
because it represents the random movement of molecules. If the temperature is uniform, as in
a living cell, then the only use for heat energy is to warm a body of matter, such as an organism.
(Modified by Linda Lenz from Biology sixth edition, Campbell & Reece)
Life and Energy Part II
Is the fact that life is so orderly (composed of complex organic molecules) in conflict with the
second law of thermodynamics – the unstoppable increase of entropy of the universe? No, the
key is to remember that organisms are open systems and therefore exchange energy and
materials with their surroundings. It is true that organisms create ordered structures from less
organized starting materials. For example, amino acids are ordered in a specific way when
creating protein chains. But organisms also take in organized forms of matter and energy from
their surroundings and release less ordered forms as waste products. For example, an animal
obtains starch, proteins, and other complex molecules from the food that it eats and then
releases carbon dioxide and water – small, simple molecules that store less chemical energy
than the food did. The reduction of chemical potential energy is accounted for by the release
of heat during the breakdown of food molecules. On a large scale, energy flows into an
ecosystem as light and leaves in the form of heat. The entropy (disorder) of organisms may
actually decrease, as long as the entropy of the universe (organism’s surroundings) increases.
Thus, organisms are islands of low entropy in an increasingly random universe.
Metabolic Disequilibrium
Reactions in a closed system eventually reach equilibrium and can do no more work. An
example would be a closed hydroelectric system. As soon as the level of water in the two
chambers reaches an equal level, water no longer will flow past the turbine and generate
electricity. A cell, on the other hand, maintains disequilibrium because it is an open system.
The constant flow of materials into and out of the cell keeps the metabolic pathways from ever
reaching equilibrium. This system can be illustrated by an open hydroelectric system in which a
faucet supplies a constant influx of new water and a drain allows the water in the second
chamber to continually be removed from the system. In this way, the turbine continues to spin
and generate electricity. As long as the cell has a steady supply of glucose or other fuel
molecules and is able to get rid of its waste products, its metabolic pathways never reach
equilibrium and continue to do the work of life.
We see once again how important it is to think of organisms as open systems. Sunlight
provides a daily source of energy for an ecosystem’s plants and other photosynthetic
organisms. Animals and other non-photosynthetic organisms depend on energy transfusions in
the form of organic molecules produced during photosynthesis.
ATP Powers Cellular Work
A cell does three main types of work:
1. Mechanical work: contraction of muscle cells, movement of chromosomes
2. Transport work: pumping of substances across cell membranes
3. Chemical work: building polymers out of monomers
In most cases, the immediate source of cellular energy is provided in the form of a molecule
called ATP. Adenosine Triphosphate (ATP) has a nitrogenous base called adenine bonded to a
ribose sugar, attached to the sugar is a chain of three phosphate groups. The bonds between
the phosphate groups, referred to as ATP’s tail, can be broken by hydrolysis (addition of a water
molecule). The bonds between the phosphate groups are relatively unstable and this is why
their hydrolysis yields energy. These bonds are unstable because all three phosphate groups
are negatively charged. These like charges are crowded together, and their repulsion
contributes to the instability of this region of the ATP molecule. The triphosphate tail of ATP is
the chemical equivalent of a loaded spring.
ATP is able to let the cell perform work when enzymes (proteins that speed up chemical
reactions) transfer a phosphate from ATP to some other molecule. The molecule that has the
attached phosphate is said to be phosphorylated and is now more reactive (less stable) than
the original molecule. Nearly all cellular work depends on ATP’s energizing another molecule
by transferring phosphate groups. For example, ATP powers the movement of muscled as it
transfers phosphate groups to contractile proteins.
An organism at work uses ATP continuously. A working muscle cell consumes 10 million ATP
per second. Luckily, organisms can recycle their ATP – placing the phosphate groups back onto
the original molecule after they have been removed. This is the process that occurs in the
mitochondria, the power-plant, of every living cell. Food molecules such as the sugar glucose
are consumed so that the phosphate groups may be added once again to form recharged ATP
molecules. If ATP could not be recycled, humans would have to consume nearly their body
weight in ATP every day.
(Modified by Linda Lenz from Biology sixth edition, Campbell & Reece)
Life and Energy Part III
The laws of thermodynamics tell us what can and cannot happen under given conditions but
they say nothing about the speed of these processes. A spontaneous reaction may occur
extremely slowly, for example. Table sugar is a disaccharide that will spontaneously hydrolyze
(breakdown) to glucose and fructose. Yet, a solution of table sugar may stay dissolved in sterile
water for years at room temperature without breaking down into glucose and fructose.
However, if we add a small amount of an enzyme (protein), to the solution, then all of the
sucrose may be hydrolyzed within seconds. How does an enzyme do this?
Enzymes Speed Up Chemical Reactions
A catalyst is a chemical agent that changes the rate of a reaction without being used up in the
reaction. An enzyme is a catalytic protein. Chemical reactions occur when molecules interact;
bonds break, atoms rearrange and new bonds form. Heat speeds up chemical reactions
because heat speeds up the movement of molecules, making them more likely to collide and
for bonds to break. However, adding too much heat to organisms causes complex molecules to
decompose. For example, adding too much heat will cause the double helix DNA structure to
unzip (breaking hydrogen bonds between the bases A,G,C, T) and will cause the complex 3-D
structure of proteins to unravel, thus killing cells. Living cells must find a different way to speed
up chemical reactions: catalysts called enzymes.
Enzymes are Substrate Specific
There are thousands of unique enzymes. All enzymes are proteins, having a unique 3-D shape.
Each enzyme recognizes and interacts with a very specific molecule or pair of molecules, this
molecule is called the enzymes substrate. The enzyme connects with its substrate in a lockand-key fashion. The enzyme’s active site is the actual spot in the enzyme where the
interaction occurs. Once the substrate docks with the enzyme’s active site, the 3-D shape of
the enzyme is changed so that it embraces the substrate. This is called an induced fit, and is
like a clasping handshake. The induced fit brings chemical groups of the active site into
positions so that they can produce a chemical reaction with the substrate.
In most cases, the substrate is held in the enzyme’s active site by weak interactions such as
hydrogen bonds or ionic bonds. Once the substrate has been transformed into a product, it is
released and the enzyme is free to take another substrate molecule into its active site. The
entire cycle happens so fast that a single enzyme can act on about a thousand substrate
molecules per second.
How Enzymes Speed Up Chemical Reactions
In reactions that involve two or more reactants, the active site provides a space that brings the
molecules together in the proper orientation for a reaction to occur between them. As the
active site clutches the substrates with an induced fit, the enzyme may stress the substrate
molecules, stressing and bending critical chemical bonds.
Another option is that the active site may provide a microenvironment that is conducive to a
particular type of reaction. For example, the active site might provide a pocket of more acidic
conditions. Yet another mechanism involves a brief covalent bonding between the substrate
and a side chain of an amino acid of the enzyme.
What Influences the Rate of Enzyme Action
The rate at which a given amount of enzyme converts substrate into product is influenced by
the amount of substrate available to the enzyme. The more substrate molecules available, the
more frequently they will bump into the enzymes’ active site. When you add so much
substrate that every enzyme molecule is always engaged with a substrate, the enzyme is said to
be saturated and the only way to increase the rate of reaction is to add more enzymes. Cells
can do this by making more enzyme molecules.
The activity of enzymes is affected by the general environment in which they find themselves,
such as temperature and pH, and particular chemicals. Up to a certain point, increasing
temperature increases the rate of enzyme reaction because as molecular motion speeds up, the
enzyme bumps into its substrate more often. However, above a certain temperature, the rate
of reaction drops off completely because the protein of which the enzyme is composed
unravels (looses its 3-D shape). This process is called denaturing. This is what happens when
egg-white is heated and converts from opaque to white.
The control of enzyme activity can occur on several levels. First, enzymes are proteins, and the
code for producing any specific protein is found in the genes of an organism’s DNA. Genes can
be turned on and off, thus affecting how much enzyme is produced in the first place. However,
once the enzyme has already been made, its activity can still be regulated. There are molecules
that can act as activators, turning enzymes on; and other molecules can function as inhibitors,
turning enzymes off. In this way, a cell can be very efficient with its chemical reactions, so it is
not producing substances when it does not need to. The cell follows rules of supply/demand.
Emergent Properties Is Manifest in the Chemistry of Life
We have seen that the unusual behavior of water, so essential for life on Earth, results from the
interactions of the water molecules themselves an ordered arrangement of hydrogen and
oxygen atoms. We reduced the great complexity and diversity of organic compounds to the
chemical characteristics of carbon, but we also saw that the unique properties of organic
compounds are related to the specific structural arrangements of carbon skeletons. We
learned that small organic molecules are often assembled from giant molecules, but we also
discovered that a macromolecule does not behave like a simple composite of monomers. For
example, the unique form and function of a protein are consequences of a hierarchy of primary,
secondary, tertiary, and quaternary structures. And now we have learned that metabolism, the
orderly chemistry that characterizes life, is a concerted interplay of thousands of different kinds
of molecules in an organized cell.
(Modified by Linda Lenz from Biology sixth edition, Campbell & Reece)