Cellular Respiration Part I: Introduction to Energy Harvesting You may want to have a periodic table handy for the oxidation-reduction part of this lesson.

Big Idea 2 Energy 2.A.1 1. All living systems require constant input of energy.

Curriculum Framework f. Cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that harvest free energy from simple carbohydrates.

It sound simple but…… Why do you have to eat? What does it have to do with entropy? Why do you have to breathe? Can you give an AP Biology level answer to these questions? Ask students to discuss these questions with a partner or in small group. (It is most likely their responses will be fairly general and lacking in detail.)

What do you know about this molecule? Can you name this molecule? What characteristics can you observe? List them in the margin of your notes. Is this molecule more like a protein or DNA? ANS: This molecule is ATP, an important energy molecule.

ATP The cell’s energy currency Energy is released when a phosphate is removed from the molecule. Respiration fuels the addition of P to ADP Unstable, short-term energy storage All cells need energy. Energy is the ability to do work. The most common type of energy molecule that a cell uses is ATP or adenosine triphosphate. ATP contains the pentose sugar, ribose. ATP also contains the nitrogenous base, adenine. This is the same base found in DNA and RNA.

High Energy Phosphate Bonds - The energy “stored” in the ATP molecule is a consequence of its electron configuration. The three phosphate groups have a total of 4 negative charges (as shown in the diagram) confined in a small area. These charges repel each other, so this makes the potential energy of the molecule very high. This repulsion also weakens the bonds between the phosphate groups. Emphasize to students that ”high energy” bonds are “weak” bonds are unstable—they often think about it the other way around! The bonds can be broken releasing up to 7.3 kcal/mole. ATP contains high energy phosphate bonds that provide energy for cellular work….So, what kind of work do cells spend their time doing? Ribose

The work powered by ATP can be categorized into three basic types: transport, mechanical and chemical. All three types of work require energy and ATP is the cell’s most likely energy provider.

Photosynthesis in chloroplasts Cellular respiration in mitochondria Light energy ECOSYSTEM Photosynthesis in chloroplasts  O2 Organic molecules CO2  H2O Cellular respiration in mitochondria In this unit, we will explore the details of how cells convert energy from food into the energy temporarily stored in the phosphate bonds of ATP. Remember, the energy isn’t being created. The energy from the sun was captured by autotrophs and stored in organic molecules. To be useful for cellular work, the energy has to be transferred to the more easily utilized chemical form as found ATP. Figure 9.2 Energy flow and chemical recycling in ecosystems. ATP powers most cellular work ATP Heat energy

Sequence These From Greatest To Least Chemical Energy Lets consider these 5 different forms of stored energy. Which one contains the most energy? Which one contains the least? Arrange them in order from most to least energy. Write your order in the margin of your notes.

Greatest to Least How does your list compare? As we explore the processes of cellular respiration, we will be able to track the flow of energy from our food to ATP.

The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized (its oxidation number increases) In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) Before we get into the details, lets revisit the principle of coupled oxidation and reduction reactions fondly referred to as “redox” reactions. Start by asking students if they know the “oxidation number” for a hydrogen in H2 (zero—all elements have oxidation numbers of zero whether diatomic or not) vs. the hydrogen atom in water (+1). What’s the oxidation number of the oxygen in water? (−2) . You may want to write “H2O” as a molecule to jog their chemistry memory! If they can explain why we need 2 H’s and only 1 oxygen to make a neutral water molecule, then you off to a good start even if they didn’t know the oxidation numbers to start with! Oxidation-reduction reactions are important in the processes of energy transfer in many cellular processes including respiration. Start with reduction, it will make more sense to their “math brain”. If an element gains an electron which has a −1 charge, it’s oxidation number is reduced by one algebraically. The converse is also true, if a substance is oxidized, it loses an electron, thus its oxidation number increases by one algebraically. Remember that a substance that loses electrons is oxidized. A substance that is gaining electrons is reduced. There are many mnemonics used by chemistry teachers to help students remember which is which…OIL RIG translates into “oxidation is loss & reduction is gain” of electrons (of course)!

Methane (reducing agent) Oxygen (oxidizing agent) Example Reactants Products becomes oxidized Energy becomes reduced Some redox reactions do not transfer electrons but rather change the electron distributions in covalent bonds. This is common in combustion reactions. An example is the reaction between methane and O2. The electron donor is called the reducing agent (since it is itself oxidized, think “OIL”—oxidation is loss of electron) & the electron receptor is called the oxidizing agent (since it is itself reduced) (Note these “agent” terms will be leaving the AP Chemistry exam in 2014—the “agent” causes the action. Also know that students do not need to KNOW any oxidation numbers, they just need to recognize the process involves a loss and gain of electrons.) Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water

Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced becomes oxidized becomes reduced The best way to understand is to do the simple math! (Again not necessary, but helpful none the less.) Prior knowledge: Elements in their standard states have oxidation numbers of zero. H is +1 and O is −2 with rare exception within compounds. The sum of the oxidation numbers in a compound must equal zero. (The oxidation numbers in a polyatomic ion such as PO43- must total to the ion’s charge. What’s the oxidation number of C in glucose? ZERO; (H is always 1 and O is always -2 in a compound). A compound is always neutral so, 6C +12 +(−12) must equal zero, and the only way that can happen is if the six carbons are ZERO! (or a combination that totals zero) So, what’s the oxidation number of C in CO2? +4 since O is always −2 in a compound AND there are two of them! Was carbon oxidized or reduced in this reaction? OXIDIZED since C in glucose (zero) “becomes” C in carbon dioxide (+4). (oxidized—oxidation number becomes more positive due to the loss of electrons). What’s the oxidation number of O in the O2 molecule? ZERO—all elements are zero! What’s the oxidation number of O in H2O? Minus 2 since O is always −2 in a compound or because H is always +1 in a compound and 2 of them are present. Was the oxygen oxidized or reduced in this reaction? REDUCED from zero to −2.

In cellular respiration, glucose and other organic molecules are broken down in a series of steps. Electrons from organic compounds are usually first transferred to NAD+, a coenzyme. As an electron acceptor, NAD+ functions as an oxidizing agent (thus is reduced) during cellular respiration Each NADH formed (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP.

Respiration Cellular respiration includes both aerobic and anaerobic respiration but is often refers to aerobic respiration Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat energy) Understanding Redox reactions, Electron carriers and the role of ATP are central to understanding cellular respiration. Be sure that you have a firm grasp of these concepts as we move into the events of cellular respiration.

Big Energy Events of Respiration Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) Glycolysis, Citric Acid cycle and oxidative phosphorylation are the three big energy events of respiration. In our next session, we will look more closely at the events of glycolysis. Historical note (for those “But, why?” inquiring students): The citric acid cycle — also known as the tricarboxylic acid cycle (TCA cycle) since the name of this metabolic pathway is derived from citric acid (a type of tricarboxylic acid) that is first consumed and then regenerated by this sequence of reactions to complete the cycle. In addition, the cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces carbon dioxide. The components and reactions of the citric acid cycle were established in the 1930s by seminal work from the Nobel laureates Albert Szent-Györgyi and Hans Adolf Krebs. (note it’s Krebs NOT Kreb’s—no matter how many times you see it mistyped!)

So…..Why do we have to eat? Answer the question using your AP Biology level understanding of energy transfer. Be ready to share your answer. Press for students to verbalize that eating provides the organic compounds that store energy that was captured by autotrophs. Eating brings the energy into our bodies where it can be converted to the usable form, ATP.

Created by: Debra Richards Coordinator of Secondary Science Programs Bryan ISD Bryan, TX