The Need for Energy

When a car runs low on fuel, you put more gas in it. The car needs energy to move, and that energy is locked within the molecules of the gasoline in the car’s tank. Molecules are atoms connected by chemical bonds.

Once you turn the ignition, a process begins. A process is a series of actions that leads to a result. In this process, parts of the car’s engine begin to work, and oxygen is mixed with gasoline. The spark plugs ignite the gas. This causes explosions that provide the force to move the car. As you drive the car, waste products-mostly carbon dioxide and water-leave the car through the tailpipe. About 25 percent of the energy locked within the molecules of gasoline is used to move the car. The rest is lost as heat. That’s why your car heats up as you drive it.

Gasoline comes from fossil fuels. What do you know about fossil fuels? They are made from the remains of once-living plants and animals. When these organisms died, all of the energy stored within them was buried. Millions of years later, we unearth these materials to capture the energy stored in their fossilized remains. We call that energy “fossil fuel.”

Your cells are not completely unlike a car engine. Your fuel is the food you eat. This food, either directly or indirectly, got energy from the Sun. The Sun is the source of all energy on Earth, past and present.

Eating food is like putting gasoline in a car. Your body works to release the energy stored in food molecules, and just as in a car’s engine, your body needs oxygen to do the job effectively.

Opposite Processes

All cells need energy to survive, meaning they need food and oxygen. When plants perform photosynthesis, they create and store food molecules for their own use. The food may be stored in the leaves, stems, ,or roots, and it is in the form of organic compounds called carbohydrates, or sugars. These sugars are the fuel-the source of energy for a plant cell. Like all substances, the sugars stored in a plant’s parts are made of molecules. The chemical bonds that hold these molecules together store the energy produced during photosynthesis. To release the energy, the bonds must be broken. This process of breaking the bonds of food molecules to release energy is called cellular respiration. Cellular respiration is an aerobic reaction, meaning that oxygen must be present for it to occur.

In photosynthesis, carbon dioxide and water combine in the presence of sunlight to make sugar and oxygen. The process looks like this:

carbon dioxide + water + sunlight → sugar + oxygen

In cellular respiration, sugar and oxygen produce high-energy molecules, as well as waste heat, carbon dioxide, and water. The process looks like this:

sugar + oxygen → energy + heat + carbon dioxide + water

Therefore, photosynthesis is considered the opposite process of cellular respiration. Essentially, that’s because photosynthesis removes carbon dioxide from the atmosphere, and cellular respiration puts it back in.



Plant and animal cells, as well as cells in microscopic organisms called protists, are eukaryotic (you-CARE-ee-ah-tik). A eukaryotic cell has a nucleus, or structure containing its hereditary information. Outside the nucleus and within the cell’s membrane is the cytoplasm, a fluid substance that contains organelles, or “little organs.” Organelles each have jobs to do, and the organelles essential to the process of cellular respiration are the mitochondria.

In illustrations and microscopic slides, mitochondria appear to have the same cylindrical shape. However, time-lapse videography shows that they change their shapes, move about in the cell, and divide in two.

A single mitochondrion has two outer membranes. The outer membrane is smooth, unlike the inner membrane, which is folded. The folds are called cristae (KRISS-tee). The cristae form a boundary around the cell’s matrix, a fluid substance that contains enzymes, or proteins. Enzymes increase the speed of the chemical reactions that occur in the mitochondria during cellular respiration.

The purpose of cellular respiration is to capture the energy held in the chemical bonds of sugar molecules. The energy is ultimately stored in molecules of ATP, or adenosine triphosphate. Because the production of ATP is completed in the mitochondria, the structures are often called the “powerhouses” of a cell.

Some cells have thousands of mitochondria. Others have far fewer. The number depends on how much energy a cell needs. Cells that require a lot of energy, such as muscle cells, have many mitochondria.


Inside the Cell

In your body’s cells, the process of cellular respiration begins with a molecule of sugar called glucose. The molecule enters a cell through the cell’s membrane. There, the glucose molecule undergoes glycolysis. The prefix glyc- or glyco- “means carbohydrate and sugar” and the root lysis means “to disintegrate or dissolve.” The word glycolysis then, can mean “to split, break, or disintegrate carbohydrates or glucose.” All together, there are ten steps in glycolysis, and each step involves an enzyme.

Glycolysis happens in the cytosol, inside the cell’s cytoplasm. Energy is necessary for the process to begin. That energy comes from two molecules of ATP. It may seem odd to use energy to make energy, but as you will see, the investment provides a huge payoff.

A molecule of glucose has six carbon atoms in its structure. This molecule splits to make two three-carbon sugars. Chemical changes act upon the new molecules, ultimately changing them into two molecules of a compound called pyruvate. Glycolysis is complete. The entire process generates four molecules of ATP. Two of these molecules can be said to replace the molecules that were used to start the process, leaving a net gain of two molecules of ATP.

By now, half of the steps that occur during cellular respiration are finished, but more than 75 percent of the energy belonging to the glucose molecule remains locked inside the two molecules of pyruvate. Further chemical changes must unlock that energy, and if oxygen is present, those changes occur inside the mitochondria.

Inside the mitochondria, the pyruvate molecules undergo further chemical changes that include the movement of electrons. High-energy electrons shuttle through the cristae, where an enzyme uses the energy in those electrons to build molecules of ATP. By the conclusion of the cellular respiration process, the mitochondria will generate almost 38 molecules of ATP. Given that the process began with an investment of two molecules of ATP, the generation of so many energy-storing molecules is something of an energy bonanza.

The process can be described in simple terms as:

sugar + oxygen → energy + heat + carbon dioxide + water

The waste products carbon dioxide and water leave your body when you breathe out, or exhale.



All living things, even the smallest single-celled organisms, eat food for the same purpose. They eat to provide nutrients and energy for their cells. However, not all organisms depend on the same kinds of food molecules for cellular respiration. For example, some bacteria naturally feed on chemical substances that are environmentally harmful to other living things, including humans. Within their cells, these microorganisms get the energy and nutrients they need for growth and survival from breaking down pollutants, sue~ as fuels or industrial chemicals. In this process of cellular respiration, bacteria give off carbon dioxide and water as waste products, just as your body’s cells do. However, the natural process is slow. That’s when bio-remediation becomes necessary.

Bio-remediation is a branch of biotechnology. It is the application of biological processes to solve problems within industry, medicine, and the environment. The prefix bio- means “living,” and the base word re-mediate means “to correct or solve a problem.” So bio-remediation refers to the use of microorganisms, or chemicals produced by those microorganisms, to solve a problem.

Scientists use natural and developed cells that feed on pollution. In the environment, these custom-made cells, which include microorganisms, plants, and some fungi, get the energy they need and benefit the environment at the same time.

One of the earliest examples of bio-remediation occurred in 1992. Almost two decades earlier, huge quantities of jet fuel from a military base had seeped through the soil and entered the community’s water supply. Scientists studying the soil and water had found bacteria that digested molecules of fuel and gave off carbon dioxide and water as waste, just as your body’s cells do in the process of cellular respiration. However, the bacteria couldn’t consume fast enough to solve the environmental disaster. So, scientists added nutrients to the soil, which increased the bacterial population and digestion rate. Within a year, 75 percent of the pollutant was gone from the groundwater supply. Nearer the sites where nutrients were put into the groundwater, there were no signs of the pollutant at all.

Groundwater forms when precipitation such as rain and snow soaks into the ground. Water travels downward, through soil, sand, gravel, and rock, until the ground is saturated, meaning it cannot hold any more water. The top of this saturated area is called the water table. In the United States, people depend on this groundwater for freshwater.

As the jet fuel example shows, more than precipitation drains into groundwater. So do fertilizers, pesticides, liquid waste from landfills, and even human sewage. Naturally occurring bacteria living in the groundwater will reduce the pollution into less toxic compounds. This process is most effective in areas of groundwater where bacteria are plentiful and where pollution levels are low. The process is speed up through bio-remediation. This involves stimulating the natural bacteria by injecting nutrients and possibly carbon compounds needed by the bacteria into the groundwater. The bacteria consumes the harmful pollutants at a much faster rate.

Bio-remediation is only one of several methods used to remove harmful materials from water. One of its advantages is cost. Bio-remediation can be up to 90 percent less expensive than other water-cleaning technologies. However, there is no perfect solution to cleaning up groundwater pollution. Bringing water back to drinking water standards is very difficult. Scientists are continuing to research bio-remediation to improve its performance. The protection of groundwater sources is vitally important to every living organism.