Cell Respiration AP style…

Background - Equation becomes oxidized Becomes reduced

Background Cell Respiration (CR) occurs in 3 phases: 1.) Glycolysis Glucose  2 pyruvate 2.) Krebs Cycle Pyruvate + Coenzyme A Acetyl Co-A Acetyl Co-A + OAA citric acid 3.) Electron Transport Chain Also called “oxidative phosphorylation” Energy from passing e- and H+ are used to make ATP

Glycolysis Occurs in the cytoplasm of eukaryotic cells Does not require oxygen, therefore is anaerobic respiration.

Glycolysis Glucose is broken down in 2 steps: 1 Glucose  2 PGAL Requires 2 ATP to break glucose in half. (2ATP  2 ADP + 2 Pi) 2 PGAL  2 pyruvate 2 NAD+  2NADH 4 ADP  4 ATP

Glucose summary 1 glucose  2 pyruvate + 2 NADH + 2 ATP ATP produced here is by substrate-level phosphorylation. e- and H+ picked up by NAD+ (making NADH) are carried to the E.T.C.

Energy investment phase Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ Figure 9.8 The energy input and output of glycolysis 2 Pyruvate + 2 H2O Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

Glycolysis Animation Click me… Click me, shape me, anyway you want me…

In-between step… If oxygen is present, pyruvate enters the mitochondrion via a transport protein (active transport). The pyruvate is combined with co-enzyme A to form Acetyl Co-A. In this reaction, 1 CO2 and 1 NADH are produced for each pyruvate that enters.

CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Pyruvate Fig. 9-10 CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle Pyruvate Coenzyme A CO2 Transport protein

Krebs Cycle 2nd step in C.R. Also called the “citric acid cycle”. Occurs in the mitochondrial matrix

Krebs Cycle Acetyl Co-A + OAA  Citric acid (citrate) The citric acid is then broken down through a series of steps to regenerate OAA. During the break down, the following products are made: 3 NADH 1 FADH2 1 ATP 2CO2

Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 Fig. 9-11 Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 Figure 9.11 An overview of the citric acid cycle FADH2 3 NAD+ FAD 3 NADH + 3 H+ ADP + P i ATP

Krebs cycle WAIT…how many pyruvate were produced from glycolysis? Right, 2 of them. So, each pyruvate must enter the mitochondrion separately, bind with Coenzyme A, form Acetyl co-A and enter the Krebs Cycle.

Krebs Cycle Summary For each turn of the krebs cycle you get: 3 NADH 1 FADH2 1 ATP 2 CO2

Overall summary of Krebs after both pyruvates are completely broken down 6 NADH 2 FADH2 2 ATP 4 CO2

Just for fun… Let me show you what actually happens, but is beyond the scope of AP Biology….

Fig. 9-12-1 Acetyl CoA Oxaloacetate Citrate Citric acid cycle CoA—SH 1 Oxaloacetate Citrate Citric acid cycle Figure 9.12 A closer look at the citric acid cycle

Fig. 9-12-2 Acetyl CoA Oxaloacetate Citrate Isocitrate Citric acid CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate Citric acid cycle Figure 9.12 A closer look at the citric acid cycle

Fig. 9-12-3 Acetyl CoA Oxaloacetate Citrate Isocitrate Citric acid CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle

Citric acid cycle Succinyl CoA Fig. 9-12-4 Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 CO2 NAD+ NADH Succinyl CoA + H+

Citric acid cycle Succinyl CoA Fig. 9-12-5 Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 CoA—SH 5 CO2 NAD+ Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

Citric acid cycle Succinyl CoA Fig. 9-12-6 Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 6 4 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

Citric acid cycle Succinyl CoA Fig. 9-12-7 Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle NADH 3 7 + H+ H2O CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 6 4 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

Citric acid cycle Succinyl CoA Fig. 9-12-8 Acetyl CoA CoA—SH NADH +H+ 1 H2O NAD+ 8 Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle NADH 3 7 + H+ H2O CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 6 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP

See…it could be MUCH more detailed. Be happy.

Krebs Cycle Summary Video Click ME! Click me baby, one more time…

Electron Transport Chain Also called “Oxidative Phosphorylation” OP is the process of extracting ATP from NADH and FADH2. These electrons pass along a series of protein complexes, giving up energy as they are passed. The energy given off is used to phosphorylate ADP to ATP.

Each NADH can generally phosphorylate 3 ATP Each FADH2 can generally phosphorylate 2 ATP. Oxygen is the final e- acceptor.

Say whaaaat??? During oxidative phosphorylation, the NADH and FADH2 transfer their electrons to the ETC. The ETC moves the electrons “downhill” to oxygen. No ATP is made directly, but the energy transfer is sliced into small amounts. ATP synthesis occurs via chemiosmosis.

The proteins of the ETC are embedded in the inner membrane of the mitochondria.

Chemiosmosis As the e- are passed down the ETC from protein to protein, they give up some energy. This energy is used to pump H+ out into the mitochondrial intermembrane space.

The inner part of the mitochondrial membrane contains many copies of a protein complex called ATP synthase. This is the enzyme that actually phosphorylates ADP making ATP. Chemiosmosis makes use of a proton gradient which exists between the mitochondrial matrix and the intermembrane space.

Chemiosmosis As the e- are passed down the ETC from protein to protein, they give up some energy. This energy is used to pump H+ out into the mitochondrial intermembrane space.

So what actually happens in the ETC? 1.) e- are passed from protein to protein. 2.) As they pass, the energy is used to pump H+ from the matrix to the intermembrane space. 3.) This creates a proton gradient (sometimes called the “proton motive force”), with more H+ in the intermembrane space and fewer in the matrix.

So what actually happens in the ETC? 4.) Due to the high concentration now in the intermembrane space, the protons will want to move to the area of lower concentration, i.e. back into the matrix. 5.) They move back into the matrix (down the gradient) through a transmembrane protein called ATP synthase.

So what actually happens in the ETC? 6.) As the H+ move through the ATP synthase, the energy of the gradient (the proton motive force) is harnessed in order to phosphorylate ADP  ATP. ***It’s like using the kinetic energy of water moving through a dam’s spillway to turn a turbine to generate electricity.***

Summary Video ETC

ATP Production About 36 to 38 ATPs are produced by the complete oxidation of glucose. There are three main reasons why we cannot put an exact number on this.

ATP Production 1. Phosphorylation and redox reactions are not directly coupled to one another in a 1:1 ratio. One NADH generates a proton motive force that creates about 3 ATPs. FADH2 enters lower in the ETC so it only generates about 2 ATPs.

ATP Production 2. NADH generated from glycolysis can’t get into the mitochondrion. Thus, its electrons are passed via a shuttle system to either NAD+ or FAD+ inside the mitochondrion. If NAD+ is the acceptor, 3 ATP are produced. If FAD+ is the acceptor, 2 ATP are produced.

ATP Production 3. Some of the proton-motive force generated is used to power the uptake of pyruvate from the cytosol and is not used to power ATP production.

ATP Production Thus, if all of the proton-motive force were used, a maximum of 34 ATPs would be produced + the 4 from substrate level phosphorylation giving a total of 38 ATP.

Evolutionary Significance of Glycolysis Ancient prokaryotes likely used glycolysis for energy production before O2 was present in the atmosphere. Oldest prokaryotic fossil is 3.5 byo, O2 began accumulating in the atmosphere 2.7 bya Glycolysis is the most widespread form of energy production indicating it evolved early on. Location in the cytosol indicates it’s very old, older than membrane bound organelles.

Catabolism Much of what we’ve discussed regarding cellular respiration deals with glucose as the “food,” but this isn’t always the case. Often the foods we eat are high in carbohydrates, proteins and fats. Many of the carbs get broken down into glucose and other monosaccharides that can be used by cellular respiration.

Protein Catabolism Proteins are also used as fuel. First, deamination removes an amino group (excreted in urea) and the intermediates are then fed into glycolysis and the TCA cycle.

Fat Catabolism Fats undergo a series of steps producing various intermediates of glycolysis and the TCA cycle which can then be used as fuel. Fats get converted to glycerol and fatty acids. Glycerol gets converted to G-3-P. -oxidation then converts the fatty acids into 2 carbon fragments that enter the TCA cycle as acetyl CoA.

Summary of Fat & Protein Catabolism Fats: Lipids are broken down into glycerol and fatty acids. Both of these components undergo enzymatic reactions that eventually produce acetyl-CoA Proteins: Broken down into amino acids. Some amino acids are converted to acetyl CoA; Other amino acids are converted into other Krebs cycle ingredients, like Oxaloacetate (4C).

Fermentation When there isn’t enough oxygen for aerobic CR to occur, the organism needs a way to regenerate the NAD+ from the NADH since O2 cannot function as a final e- acceptor. Bottom line: without O2, you cannot regenerate NAD+, so respiration shuts down. Fermentation is a way to fix this… So, two methods of fermentation can help with this situation.

Alcoholic fermentation Occurs in yeast and some bacteria. Basic eqn: CO2 Pyruvate ethanol 2NADH 2NAD+ + 2 H+

Lactic Acid Fermentation Occurs in some bacteria, fungi and animal cells, e.g. humans. Pyruvate is changed to lactate. Eqn: Pyruvate lactate 2NADH 2NAD+ + 2 H+

Differences between respiration and fermentation: With aerobic cellular respiration: one glucose  36-38 ATP With fermentation: one glucose  only 2 ATP

Weird Organisms 1.) Obligate Anaerobes: 2.) Facultative Anaerobes Can only perform fermentation Cannot survive in the presence of oxygen A few bacterial species, Clostridium being one. 2.) Facultative Anaerobes Can survive using either respiration or fermentation equally well, depending on whether oxygen is present. Yeast and many bacteria.