General ---------Biology----------------------------------------------------- ------------------Online--------------------------------------- Ch. 9 Cellular Respiration:  Harvesting Chemical Energy

Respiration involves the exchange of gases, i.e. oxygen and carbon dioxide as food is metabolized for energy.  This gas exchange happens at two places in the biological hierarchy.

• at the organism level (breathing in oxygen; exhaling carbon dioxide)
• at the cellular level (mitochondria utilizing oxygen; giving off carbon dioxide)

Cellular respiration involves those chemical pathways associated with the break down of food and production of ATP energy.  The oxygen and carbon dioxide molecules that move into and out of the cell are simply inputs and outputs of the chemical pathways that break down food molecules.  These chemical pathways use oxygen to enzymatically "burn" food molecules, transferring the chemical energy in the food, into chemical energy in ATP molecules.

Locate the two aspects of "respiration" in the hierarchy table.

Important:  Prior to continuing, look at some questions intended to review concepts and terms from pervious chapters that are critical to understanding cellular respiration.  Click here.

food (chemical energy)	+	O2		CO2	+	H2O	+	energy (heat+ATP)
C6H12O6			enzymes & mitochondria

CO2	+	H2O	+	sunlight		C6H12O6	+	O2	+	H2O
					enzymes & chloroplasts	food (chemical energy)

Write a one sentence comparison of the chemical formula for photosynthesis with that of cellular respiration.  ___________________________________________________________.

Is this process anabolic or catabolic? 
Is this process endergonic or exergonic? 

Photosynthesis involves which of the following? 

a.  breaking down polymers into monomers so they can dissolve and diffuse through the body. 
b.  assembling carbon, hydrogen and oxygen into organic polymers of carbohydrate. 
c.  breaking down organic polymers, releasing carbon dioxide and water, and
storing high energy electrons in the form of ATP. 

Radiant energy from the sun enters the biosphere  plants store this energy  in the form of chemical energy in food (a.k.a calories or covalent bonds). The energy is transferred up the food chain as animals eat plants or other animals, and some energy is lost as heat in every transfer.  Plants and animals uses these energy transfers to grow, reproduce and maintain themselves.  Some of the matter is transferred up the food chain, but ultimately all living matter (atoms and molecules) is recycled (dust to dust, ashes to ashes).  Energy flows, and matter recycles.

The text figure below, from Ch. 9, illustrates these processes in a single plant cell.  At the cellular level ATP production is what actually drives all the growth, reproduction and maintenance seen in the ecosystem and in the biosphere.

"Energy flows through the ecosystem, and matter is recycled."

ATP was compared to a rechargeable battery in previous chapters.  If you want to contract a muscle, ATP  phosphorylates a protein muscle fiber, giving it the energy to change shape or contract.
Phosphorylation is the chemical transfer of a high energy phosphate bond.  The resulting ADP diffuses back to a mitochondrion and gets rephosphorylated back to ATP. This is illustrated in a modified text Figure shown below.

mitochondrion provides energy

ATP (rechargeable battery)

cellular work uses energy

For a more detailed Figure, click here.

• pumping molecules across a membrane
• changing the shape of motor proteins to cause cellular movement
• combining two molecules in an endergonic reaction, as in dehydration synthesis
• breaking apart molecules, as in exergonic hydrolysis
• rearranging chemicals in a molecule
• thinking 
• feeling emotions
• contracting muscles

Chemical energy is the energy stored in covalent bonds, the attraction of a proton for an electron.   To visualize this energy, consider the methane molecule (CH₄) from Ch. 2.  Picture the red protons in the carbon nucleus repelling the protons in the hydrogen nuclei with great force, while simultaneously the nuclei are strongly attracted to the electrons they share; a "love-hate relationship."  Chemical energy is this tremendous tension in the bond.  This energy is analogous to the energy in an archer's bow. 

Oxidation: "the removal of an electron."

If you were to reach in and remove an electron from one of the methane bonds above, the bond would break, and the hydrogen proton would exit at great speed.  At room temperature the proton would exit at around the speed of a 22 caliber bullet. The electron freed from the bond falls in energy level quickly, giving off its energy as a small explosion of heat and light.  The removal of an electron is called oxidation.  If you oxidize enough molecules of methane, you get enough heat to produce a flame.  When you burn butane in your gas grill, this oxidation is the source of the heat and flame. 

Cellular respiration is made up of chemical pathways that oxidize food molecules at body temperature.  Enzymes lower the energy of activation enough that the electrons can be removed without causing an explosion or flame.

Enzymatic oxidation.  For oxidation to happen at body temperature, the electrons must be removed from food and accepted by enzymes that eventually transfer some of the electrons to ATP.  Accepting an electron is called reduction.  Oxidation and reduction reactions occur simultaneously and are called redox reactions. (Electrons act like they are made out of real sticky tape; to get rid of the electron you must stick it to something else that has more attraction for the electron than you do.) The high energy electrons in food are passed from enzyme to enzyme, giving off a little energy at each step.  Click to see enzymatic oxidation..  This energy is used to phosphorylate ADP and make ATP.

In this chapter we will investigate the three chemical pathways that breakdown food, in a series of redox reactions, and produce the energy to drive life: 

• glycolysis
• Krebs Cycle
• oxidative phosphorylation