• The ability to obtain energy from various sources and to use it to do biological work is a fundamental property, called metabolism, of all living organisms; it must have been acquired very early in the evolution of life.

Energy

• In physical science, energy can be defined as the capacity to produce change. It can also be defined as the capacity to do work and to produce heat.

Types of Energy

• All forms of energy can be classified as either kinetic or potential.

o Kinetic energy (KE), or the “energy of motion”, is the energy an object has because it is moving.

o Potential energy (PE), or the “energy of position”, is the energy an object has due to its position in a force field, such as gravitational, electrostatic, or magnetic. It can be thought of as “stored energy.”

• Thermal energy is the energy a substance contains due to the constant random motion of all the particles (atoms, ions, or molecules) in it. It is therefore a form of kinetic energy.

o The temperature of an object is a measure of the average kinetic energy of its atoms, ions, or molecules.

o Thermal energy cannot do work.

Thermal Energy

• Chemical energy is the potential energy stored in a chemical bond.

o It is due to the positions of electrons in the electrostatic force field created by two or more atomic nuclei.

o Living organisms routinely use chemical bonds for storage and transfer of (potential) energy.

Chemical Energy

• Other major forms of energy in the universe include:

o Light Energy - kinetic energy in the form of electromagnetic radiation. o Mechanical Energy - just another name for kinetic and potential

energy. o Gravitational Energy - the energy of an object due to its position in a

gravitational field. An example is a ball on top of a hill. o Nuclear Energy - the potential energy stored in an atomic nucleus that

keeps it together. o Electrical Energy - energy associated with the movement of electrons.

Energy Can Be Transferred

• Energy transfers take place when energy stays in the same form but is passed between different objects.

o Consider a row of falling dominoes. The kinetic energy of the first domino is transferred to the kinetic energy of the next domino and so on. The energy remains kinetic as it is passed on to different objects.

Heat

• Heat is the thermal energy being transferred between two objects or systems due to a difference in their temperatures.

o So it is correct to say that something contains thermal energy, but not that it “contains” heat, since heat is thermal energy that is being transferred from one thing to another.

o Heat can do work, but not in living things due to homeostasis.

Energy Can Change Forms

• Energy transformations (or conversions) occur when energy changes from one form into another.

o For example, an incandescent light bulb converts electricity into light (5%) and thermal energy (95%).

o Another example of an energy transformation occurs when a ball rolls down a hill. As it does so, it pushes aside air molecules in its path, converting the gravitational potential energy it had at the top of the hill into additional kinetic energy in the air molecules. The molecules in the ground in its path also gain a little kinetic energy. So, the surroundings warm up ever so slightly.

Energy’s Graveyard: Thermal Energy

• Although energy can be converted (changed) from one form to another, it is always conserved: it can neither be created nor destroyed—the total amount of energy in the universe is constant. This statement is called the first law of thermodynamics*.

• In every energy conversion, some energy is converted to thermal energy that is dispersed to the surroundings, and thus cannot be used for work. This statement is called the second law of thermodynamics.

*Thermodynamics is the study of how energy moves and changes form.

• Most of the energy that fuels life on Earth comes from the sun.

o One-billionth of the Sun's total energy output actually reaches the Earth and of this energy, less than one percent is captured by plants and converted into the chemical energy in organic compounds. The rest of the sun’s energy output becomes spread out into deep space.

o In turn, animals eat the plants and break down the organic compounds to obtain energy.

• Energy conversion occurs between levels in a food chain.

o As energy— originally captured from sunlight—passes from one level to the next, a substantial amount is dispersed back to the surroundings, ultimately as thermal energy (heat). Only about 10% of it is stored in the next level. A population at the end of a food chain receives the least amount of energy. This limits the number of levels there can be in a food chain.

• Eventually, all of the original solar energy ends up as relatively low-temperature thermal energy spread out in the surroundings and eventually in deep space. This one-way flow of energy out of living organisms requires a one-way flow of energy into them to stay alive.

Chemical Energy Concentration of Substances

• Different substances contain different amounts of chemical energy because:

o Different substances contain different numbers of chemical bonds; and o Different chemical bonds contain different amounts of potential energy,

depending on which elements are taking part in the bond. For example, the amount of chemical energy in a single covalent bond between an oxygen atom and a hydrogen atom (O—H) differs from the amount in a double covalent bond between two oxygen atoms (O=O).

A Word about Chemical Equations

• An equation shows what happens in a specific chemical reaction.

o The arrow is read as “produces.” o A number (coefficient) before a

chemical formula represents the amount of that chemical present.

o A chemical equation must have an equal number of atoms of each element on each side of the arrow.

Chemical Reactions

• In chemical reactions, energy is transferred between molecules and some is transformed.

o In spontaneous reactions, the reactants have more energy than the products, so the reaction releases energy. At least some of the energy released is thermal energy.

o In nonspontaneous reactions, the reactants have less energy than the products, so the reaction requires energy to occur. The energy input is stored in the chemical bonds of the products.

Coupled Chemical Reactions

• A nonspontaneous chemical reaction can occur in living organisms only if it is coupled with a spontaneous chemical reaction. In other words, an energy-requiring reaction can occur only in tandem with an energy-releasing reaction.

o Most energy-requiring chemical reactions in cells are coupled with the breakdown of ATP, which releases energy.

• Reactant molecules in both spontaneous and nonspontaneous reactions must collide with enough kinetic energy and in the proper orientation to break the necessary chemical bonds so that new bonds can be formed.

o If molecules are moving too slowly with little kinetic energy, or collide with an improper orientation, they will not react and will just simply bounce off each other.

What determines the speed of chemical reactions?

o Successful collisions briefly produce a transition state in which old bonds are weakened and new bonds begin to form. This state has more chemical (potential) energy than either the reactants or the products.

Activation Energy (EA)

• The extra energy required to break chemical bonds in the reactants is called the energy of activation, or activation energy (EA).

o A spontaneous chemical reaction requires an activation energy.

o A nonspontaneous chemical reaction also requires an activation energy.

• The requirement for activation energy keeps most of the molecules in your body from suddenly turning into carbon dioxide, water, and nitrogen gas! There is not enough thermal energy in the molecules within living organisms to enable them to react with the oxygen in the air.

Life Requires Change

• Nevertheless, certain chemical reactions must occur rapidly or life cannot exist.

Enzymes

• An enzyme is a molecule that enables a chemical reaction to occur (or occur faster) in cells by providing a different "path" to the products that doesn’t need as much activation energy as is needed for reaction by collision.

o The enzyme acts as a catalyst for the chemical reaction. o Nearly all enzymes are made of protein. o Since an enzyme, like a catalyst, never chemically reacts with the

reactant(s), the reactant(s) is called the substrate of the enzyme.

o Enzymes make it easier for spontaneous reactions to occur.

o They also make it easier for nonspontaneous reactions to occur.

The Enzyme/Substrate Complex

• An enzyme/substrate complex forms when the substrate(s) "binds" with the enzyme’s active site(s). Because the shape of the substrate and active site must match, each enzyme is specific for a particular chemical reaction.

o After the reaction, the products are released and the enzyme can then bind to another substrate.

• Every enzyme has a pH and temperature range in which it functions best. Temperatures and pH values outside the optimal ranges may cause the enzyme to lose its three-dimensional shape and thus its function.

o For example, an enzyme that functions in the human stomach, where the pH is very low, will not function in the small intestine, where the pH is much higher.

o The optimal temperature for enzymes in heat-tolerant (thermophilic) bacteria is much higher than for enzymes in humans.

Metabolic Pathways

• A metabolic pathway is a series of enzyme-catalyzed chemical reactions in which a product of each reaction is a substrate for the next reaction.

o The similarity of the basic metabolic pathways and components between even vastly different species is striking and is evidence for their common descent from an ancient ancestor that had these pathways and components.

Substrate A

Substrate B Substrate C D Enzyme 1 Enzyme 2 Enzyme 3

Starting Chemical End Product