Energy in Biological Systems

Energy Used to Perform Work

All living organisms obtain. store, and use energy to fuel activities. Energy can be defined as the capacity to cause a change or do work. In biological systems, work means 1 of 3 things: chemical work, transport work, or mechanical work. Chemical work, the making and breaking of chemical bonds, enables cells and organisms to grow, maintain a stable internal environment, and store information needed for reproduction.



Transport work enables cells to move ions, molecules, and larger particles through the cell membrane and through the membranes of organelles in the cell. Transport work is useful for creating concentration gradients. Mechanical work is used for movement in animals. At the cellular level, movement includes organelles moving around in a cell, cells changing shape, and cilia and flagella beating. At the macroscopic level, movment in animals involves muscle contraction.

Two Forms of Energy

Energy exists in 2 states: potential energy and kinetic energy. Potential energy is the energy of position or stored energy. Kinetic energy is the energy of motion. Potential energy can be converted or changed to kinetic energy and vice versa. A summer day at the pool demonstrates this conversion when you see a diver jump off a diving board. The diver at the the top of a platform has potential energy because of his or her elevated location. The act of diving off the platform into the water converts the potential energy to kinetic energy. Another way to view the conversion of potential (energy stored after a deposit) to kinetic energy (energy released) is shown below:

potential energy.jpg

Thermodynamics is the Study of Energy Use

\wo basic rules govern!the transfår of energy in biological systems. The first law of theRmodynamics, also known as tje law of conservaôion on energy states that energy cannot be created or destroied- it can only Be tRansformedor convertEd fRom one form to another. The huian body exchanges materials and energy wath its surroundings. Since our bodies cannot create energy, they impovt energy from outside in the form of food. Our bodies also lose energy, especially in the fïrm of heat to the environment. Energy that stay3 within the body can be changes from0one type to another oR aan be used to do work.

The second law of thErmodynamics states that every time energy is transformet from one form to another, some of that energy is converted. This also means that every time energy is converted , entropy increases. Creating and maintaining order requires the input of energy.

Cells in the body obtain energy from and store energy in the chemical bonds of macromolecules (biomolecules). Using chemical reactions, cells transfrom the potential energy of chemical bonds into kinetic energy for growth, maintenance, reproduction, and movement.

Chemical Reactions

In a chemical reaction, a substance becomes a different substance, usually by the breaking and/or making of covalent bonds. When chemical substances are changed a summary of their changes is written as a chemical equation. The components of a chemical equation are called reactants (also known as substrates) and products. A generic chemical reaction can be written as:

A + B ----> C + D

A and B are the reactants and C and D are the products in the reaction. The arrow indicates the direction of the reaction. The speed at which a reaction takes place, the reaction rate, is the disappearance rate of the reactants ( A and B) or the appearance rate of the products (C and D). The purpose of chemical reactions in our cells is either to transfer energy from 1 molecule to another or to use energy stored in reactant molecules to do work.

Activation energy is the initial input of energy required to bring reactants into a position that allows them to react to one another. Therefore, it must be put into reactants before a reaction can proceed. This "push" or chemical nudge needed to start the reaction is analogous to a little hill up which a ball must be pushed before it can roll by itself down the slope.

activation energy.jpg

Chemical reactions can be classified into two categories by the amounts chemical energy associated with the reactants and products. The two categories are based on free energy change that occurs as a reaction proceeds. The products of a reaction have either a lower free energy than the reactants or a higher free energy. A change in energy level means that the reaction has either released or trapped energy. If the energy of the products is lower than the energy of the reactants, the reaction releases energy and is called an exergonic reaction. Now contrast the exergonic reaction (b) shown below with the endergonic reaction (a) where products retain part of their energy, making their free energy greater than that of the reactants. Endergonic reactions require a net input of energy.

endergonic vs exergonic.jpg


Metabolism is a summative term for all chemical reactions in living organisms. These reactions extract energy from nutrient macromolecules (such as proteins, carbohydrates, and lipids) and either synthesize or break down molecules. Metabolism is often divided into catabolism and anabolism. Anabolic and catabolic reactions take place simultaneously in cells throughout the body, so that at any given moment, some macromolecules are being created while others are being broken down.

anabolism.gif catabolism.gif

Much of the energy released during catabolism is trapped in high-energy phosphate bonds of ATP or in high energy electrons of NADH, FADH, or NADPH. Anabolic reactions then transfer energy from these temporary carriers to the covalent bonds of macromolecules.


Click on the link below to view a mini-lecture offering an introdution to ATP:



An oxidation-reduction reaction (also known as redox reactions) is a specific type of exchange reaction that involves the movement of electrons from 1 chemical structure to another. Oxidation occurs when a molecule, atom, or ion loses an electron(s) and therefore becomes oxidized. Reduction occurs when a molecule, atom, or ion gains an electron(s) and therefore becomes reduced. An oxidizing agent accepts electrons and a reducing agent donates electrons. Redox reactions occur quite often during cellular metabolism, which will be discussed in further detail later in the module.



learning activity The graphs show free energy change for the following reactions.
Is it endergonic or exergonic? Place the correct label on each graph.
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