Chapter 9: Cellular Respiration, Fermentation, Catabolic Processes

Oxidation-Reduction

Important processes associated with the production of energy and synthesis of substances in biological systems linked biological systems As one molecule is oxidized the other is reduced

O2

In an explosion, oxygen gas is reduced in one step Energy released as heat and light

How redox reactions are involved in energy exchanges

Utilized by cells in catabolism to extract energy from nutritive molecules Involve transfers of hydrogen atoms Each H atom is composed of 1 electron and 1 proton In a simple sense each hydrogen that is gained or lost is also an electron being gained or lost

Anaerobic conditions

Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. In alcohol fermentation, pyruvate is converted to ethanol in two steps. First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO2. Second, acetaldehyde is reduced by NADH to ethanol. Alcohol fermentation by yeast is used in brewing and winemaking.

Chemiosmosis

Using an electron transport chain and energy provided from electrons H+ ions are actively transported (pumped) to create H+ gradient Proton Motive Force used in combination with ATP synthase to generate ATPs Potential energy of gradient/kinetic energy of PMF, provides energy to ATP synthase to produce ATP

dinucleotide

electron carriers, NAD+/NADP/FAD

Anaerobic oxidative phosphorylation

non-oxygen, inorganic final acceptor The production of ATP is a form of chemiosmosis

Redox reactions

release energy when electrons move closer to electronegative atoms. Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to synthesize ATP

reduction

endergonic The addition of electrons to a substance involved in a redox reaction.

Energy payoff phase

(4)ATP is produced by substrate-level phosphorylation and (2)NAD+ is reduced to (2)NADH by electrons released by the oxidation of glucose.

Glycolysis Summary

2 Net ATP produced by SLP 2 NADH produced No Carbon Dioxide 2 pyruvic acid (3c) formed from 2 3 C phosphogylceraldehyde Occurs whether oxygen gas is present or not

So far in cellular respiration:

4 net ATP's produced through substrate-level phosphorylation 2 FADH 10 NADH

ATP

A modified nucleotide with three phosphate groups attached The bonds holding the two terminal phosphates are high energy bonds

Substrate-level phosphorylation

ATP can also be made by transferring phosphate groups from organic molecules to ADP

ATP Reaction

ATP is the key short term energy transport molecule within cells Energy from exergonic reactions can be captured by ATP (ADP + Pi + energy --- ATP) This energy is transported to the site of an endergonic reactions to provide energy Energy from ATP can be released to endergonic reactions (ATP --- ADP + Pi + energy)

Phosphorylation

ATPs are produced through the process of phosphorylation Forms are defined by energy source used to produce ATP

Citric acid cycle

Acetyl CoA is oxidized to two CO2 molecules. During this cycle, more ATP and NADH are produced, as well as FADH2

Citric Acid Cycle Summary

Acetyl Formation: 2 NAD+ reduced to 2 NADH Krebs: Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per acetyl CoA. (x 2) Total: 8 NADH 2 FADH2 6CO2 2 ATP (SLP) Completion of the decomposition of original cycle Electrons from glucose are loaded on NADH/FADH2 (electron carriers)

Acetyl Formation

After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A or acetyl CoA. A carboxyl group is removed as CO2. The remaining two-carbon fragment (acetyl) is oxidized to form acetate. An enzyme transfers the pair of electrons to NAD+ to form NADH. Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA. Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further oxidation.

Breathing supplies oxygen to our cells and removed carbon dioxide

Breathing and cellular respiration are closely related

Mechanisms of Release and Storage

Cells tap energy from electrons transferred from organic fuels to oxygen Glucose gives up energy as it is oxidized

Chemiosmosis

Cells use the energy released by "falling" electrons to pump H+ ions across a membrane The energy of the gradient is harnessed to make ATP by the process of chemiosmosis

Cellular Respiration banks in ATP molecules

Cellular respiration breaks down glucose molecules and banks their energy in ATP The process uses oxygen gas and releases carbon dioxide and water

How does the electron transport chain pump protons?

Certain members of the electron transport chain accept and release H+ along with electrons. At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution. The electron carriers are spatially arranged in the membrane in such a way that protons are accepted from the mitochondrial matrix and deposited in the intermembrane space. The H+ gradient that results is the proton-motive force. The gradient has the capacity to do work.

chemiosmosis

Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work. In mitochondria, the energy for proton gradient formation comes from exergonic redox reactions, and ATP synthesis is the work performed. Chemiosmosis in chloroplasts also generates ATP, but light drives the electron flow down an electron transport chain and H+ gradient formation. Prokaryotes generate H+ gradients across their plasma membrane. They can use this proton-motive force not only to generate ATP, but also to pump nutrients and waste products across the membrane and to rotate their flagella.

Compare the processes of fermentation and cellular respiration

Fermentation is a type of catabolic process that leads to partial degredation of sugars in the absence of oxygen A more efficient and widespread catabolic process, cellular aerobic respiration, consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules.

How does the inner mitochondrial membrane generate and maintain the H+ gradient that drives ATP synthesis in the ATP synthase protein complex?

Creating the H+ gradient is the function of the electron transport chain. The ETC is an energy converter that uses the exergonic flow of electrons to pump H+ across the membrane from the mitochondrial matrix to the intermembrane space. The H+ has a tendency to diffuse down its gradient. The ATP synthase molecules are the only place that H+ can diffuse back to the matrix. The exergonic flow of H+ is used by the enzyme to generate ATP. This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis.

ETC and Electron carriers

During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons. Each component of the chain becomes reduced when it accepts electrons from its "uphill" neighbor, which is less electronegative. It then returns to its oxidized form as it passes electrons to its more electronegative "downhill" neighbor. Electrons carried by NADH are transferred to the first molecule in the electron transport chain, a flavoprotein.

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate. (cytoplasm)

During glycolysis, glucose, a six carbon-sugar, is split into two three-carbon sugars. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid.

NADH

Each NADH from the citric acid cycle and the conversion of pyruvate contributes enough energy to the proton-motive force to generate a maximum of 3 ATP. The NADH from glycolysis may also yield 3 ATP.

Electron transport and chemiosmosis

Electrons from NADH and FADH2 move through a series of proteins called the electron transport chain. The potential energy released during these reactions is used to create a proton gradient, which is used to make ATP.

Oxidative phosphorylation

Electrons from organic or inorganic molecules Energy from excited electrons uses to create a H+ gradient Electron transport chain molecules responsible for creating H+ gradient ATP synthase (synthetase) enzyme utilizes energy provided by ion gradient to phosphorylate ADP to ATP

Photophosphorylation

Energy from light excited electrons used to create H+ gradient Electrons derived from pigment molecules Electrons are excited by light energy absorbed by pigment molecules ATP synthase used ATP production form of chemiosmosis

Hydrogen carriers such as NAD+ shuttle electrons in redox reactions

Enzymes remove electrons from glucose molecules and transfer them to a coenzyme

The two processes differ in their mechanism for oxidizing NADH to NAD+.

In fermentation, the electrons of NADH are passed to an organic molecule to regenerate NAD+. In respiration, the electrons of NADH are ultimately passed to O2, generating ATP by oxidative phosphorylation. More ATP is generated from the oxidation of pyruvate in the citric acid cycle. Without oxygen, the energy still stored in pyruvate is only available to cells through anaerobic pathways. Under aerobic respiration, a molecule of glucose yields 38 ATP, but the same molecule of glucose yields only 2 ATP under anaerobic fermentative pathways.

Energy investment phase

In the energy investment phase, the cell invests ATP to provide activation energy by phosphorylating glucose. This requires 2 ATP per glucose. Involves dephosphorylation of ATP into ADP. requires energy input (from ATP)

Electron Carriers Role of NAD+ in cellular respiration

NAD+/NADP+/FAD are molecules that act as electron delivery trucks Dinucleotide molecules that also function as coenzymes Reduced at one location, delivering those electrons to another location, where the carriers are then oxidixed The primary location of the delivery of electrons in the electron transport chain or for hydrogen atoms utilized in chemical reactions

Redox reactions release energy

NADH delivers electron carriers in an electron transport chain that terminates with an FEA (Final Electron Acceptor) As electrons move from carrier to carrier, their energy is released in small quantities

Into to Cellular Respiration

Nearly all cells in our body break don sugars for ATP production Most cells of most organisms harvest energy aerobically, like slow muscle fibers

Glycolysis

One molecule of glucose is broken down into two molecules of pyruvate, two ATP are produced, and NAD+ is reduced to NADH

Substrate Level Phosphorylation

Phosphate group transferred from substrate molecule (phosphorylated compound) directly to ADP during a chemical reaction Occur during the catabolic process of glycolysis and the citric acidc cycle Substrate molecule loses energy as the ADP molecule picks up a phosphate group

Why is our accounting so inexact?

Phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of number of NADH to number of ATP is not a whole number. One NADH results in 10 H+ being transported across the inner mitochondrial membrane. Between 3 and 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP. Therefore, 1 NADH generates enough proton-motive force for synthesis of 2.5 to 3.3 ATP.

Pyruvate processing

Pyruvate is processed to form acetyl CoA, during which another NADH is produced.

Cellular Respiration reactions

Regulating these reactions is the key to the metabolic activity of the cell and to homeostasis.

Krebs cycle is cyclical

The Krebs cycle is a series of reactions in which enzymes strip away electrons and hydrogen ions from each acetyl group. Completes the oxidation of organic fuel, generating NADH and FADH2

Krebs Cycle/Citric Acid Cycle

The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate. The next seven steps decompose the citrate back to oxaloacetate. It is the regeneration of oxaloacetate that makes this process a cycle. Three CO2 molecules are released, including the one released during the conversion of pyruvate to acetyl CoA. The cycle generates one ATP per turn by substrate-level phosphorylation. Most of the chemical energy is transferred to NAD+ and FAD during the redox reactions. The reduced coenzymes NADH and FADH2 then transfer high-energy electrons to the electron transport chain. Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per acetyl CoA. (x 2)

Reduction

The addition of electron or hydrogen atoms to a molecule Often a molecule that is reduced gains energy

Cellular respiration

The aerobic harvesting of energy from sugar is called cellular respiration Cellular respiration yields carbon dioxide, water, and ATP

How does the mitochondrion couple electron transport and energy release to ATP synthesis?

The answer is a mechanism called chemiosmosis. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and Pi. ATP uses the energy of an existing proton gradient to power ATP synthesis. The proton gradient develops between the intermembrane space and the matrix The proton gradient is produced by the movement of electrons along the electron transport chain. The chain is an energy converter that uses the exergonic flow of electrons to pump H+ from the matrix into the intermembrane space.

The Citric Acid Cycle (mitochondrial matrix)

The citric acid cycle completes the energy-yielding oxidation of organic molecules. More than three-quarters of the original energy in glucose is still present in the two molecules of pyruvate. the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide.

Electronegativity

The more electronegative the atom, the more energy is required to take an electron away from it. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one A redox reaction that relocates electrons closer to oxygen, such as the burning of methane, releases chemical energy that can do work.

How are electrons extracted from food and stored by NADH finally transferred to oxygen? cellular respiration uses an electron transport chain to break the fall of electrons to O2 into several steps.

The electron transport chain consists of several molecules (primarily proteins) built into the inner membrane of a mitochondrion. Electrons released from food are shuttled by NADH to the "top" higher-energy end of the chain. At the "bottom" lower-energy end, oxygen captures the electrons along with H+ to form water. Electron transfer from NADH to oxygen is an exergonic reaction with a free energy change of −53 kcal/mol. Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen, the most electronegative receptor. In summary, during cellular respiration, most electrons travel the following "downhill" route: food ( NADH ( electron transport chain ( oxygen.

ETC II

The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier. The prosthetic group of each cytochrome is a heme group with an iron atom that accepts and donates electrons. The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution to form water. for every two electron carriers (four electrons), one O2 molecule is reduced to two molecules of water.

Human Body

The human body uses energy from ATP for all its activities ATP powers almost all cell and body activities

Hydrogen atoms

The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide).

ATP Synthase

The protons pass back to the matrix through a channel in ATP synthase, using the exergonic flow of H+ to drive the phosphorylation of ADP. Thus, the energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis. From studying the structure of ATP synthase, scientists have learned how the flow of H+ through this large enzyme powers ATP generation. ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides

Oxidation

The removal of electrons/hydrogen atoms from a molecule Often an energy producing reaction

ATP Hydrolysis

When the bond joining a phosphate group to the rest of an ATP molecule is broken by hydrolysis, the reaction supplies energy for cellular work

Fermentation enables some cells to produce ATP without the help of oxygen.

Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation ceases. However, fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen. In glycolysis, glucose is oxidized to two pyruvate molecules with NAD+ as the oxidizing agent.

How does NAD+ trap electrons from glucose?

dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), oxidizing it. The enzyme passes two electrons and one proton to NAD+. The other proton is released as H+ to the surrounding solution. By receiving two electrons and only one proton, NAD+ has its charge neutralized when it is reduced to NADH. the electrons carried by NADH have lost very little of their potential energy in this process. Each NADH molecule formed during respiration represents stored energy. This energy is tapped to synthesize ATP as electrons "fall" from NADH to oxygen.

Chemiosmosis/Electron Transport Chain (cristate/inner mitochondrial membrane)

couples electron transport to ATP synthesis. These reduced coenzymes link glycolysis and the citric acid cycle to oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis. The electron transport chain is a collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion. The folding of the cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion.

oxidation

exergonic The loss of electrons from a substance involved in a redox reaction.

Y, the electron recipient

is the oxidizing agent and oxidizes X

X the Electron donor

is the reducing agent and reduces Y

Aerobic Oxidative Phosphorylation

oxygen is the final electron acceptor

Eukaryotic and Prokaryotic Organisms

prokaryotic organisms (PRO) and eukaryotic organisms (EU) have fermentative and respiratory pathways. EU seem to be limited to aerobic resp and anaerobic fermentative pathways, PRO show a wide range of forms of aerobic resp., anaerobic resp., and fermentative pathways.