Chapter 8 Cellular Respiration Variations on a theme

Cell Respiration Breaking stuff to build stuff

Cellular Respiration Goal: To extract usable energy from organic molecules. –For us today: use sugar to make ATP. Start: Glucose End: CO 2 + H 2 O + Energy “oxidize glucose to carbon dioxide” “reduce oxygen to water” –Remember that electrons get accompanied by H + !

(ATP)

Overview Cells slowly break down glucose, releasing its chemical (potential) energy in the process. –The more carefully you pull something apart, the more useful energy you can get out of it. ~40% of glucose’s chemical energy is converted to ATP if all goes well. This can be as low as 2.1% if we don’t have oxygen (more on that later)

Overview Tools: NAD+ and FAD Just like in photosynthesis, we need a way to physically move the high-energy electrons around. –These energy carriers are also considered coenzymes. Both accept high-energy electrons from breakdown products of glucose. These electrons are ultimately used to build up a proton gradient – just like in photosynthesis.

Overview Tools: NAD+ and FAD (flavin AD) NAD+ takes 2 hydrogens away from it’s substrate. 1 of those hydrogens carries an extra electron (H-) and the other has none (H+). –The H- + NAD+ = NADH –The H+ is released into the surroundings. FAD accepts 2 H (and the electrons). –FAD is built from vitamin B2 (riboflavin). That’s why we need vitamin B2.

Overview 4 Phases of cellular respiration: –1. Glycolysis –2. Preparatory Phase –3. Citric Acid Cycle (Kreb’s Cycle) –4. Electron Transport chain Alternative: –5. Fermentation

Overview During glycolysis, glucose is broken down into 2 smaller pieces. (2 ATP are invested to ‘get the ball rolling”) The energy from these pieces are slowly extracted –NADH and ATP are produced. At the end of glycolysis, glucose is broken into 2x pyruvate. We’ve made 4 ATP (but used 2, so net =2) and 2 NADH.

Overview Next: in the presence of O2, pyruvate is pumped into the mitochondria. –Without O2, NADH attacks pyruvate (fermentation) Inside the mitochondria, a huge enzyme attaches the C 3 Pyruvate to a coenzyme (CoA). –CO 2 is released in this process (there goes 2 carbon from the original glucose!) – why 2? –1 NADH per reaction is made here as well.

pyruvate dehydrogenase complex

Overview The 3 rd phase, the Kreb’s cycle, also takes place in the matrix. The energy inside acetyl-CoA is slowly extracted. –High-energy electrons are given to NAD+ and FAD and (along with some protons) NADH and FADH 2 are created. –In addition, 2 ATP are produced. 2 CO 2 per cycle are produced here (there goes the last bit of carbon from our original glucose)

Overview The 4 th phase, the electron transport chain, is where the REAL payoff is. –All the NADH and FADH 2 that has been generated up to this point give up their high-energy electrons to the Electron Transport Chain (ETC). The energy goes “downhill”, pulling in protons from the cytoplasm into the intermembrane space, just like we learned in photosynthesis. This chemiosmotic gradient is then used – just like in photosynthesis light reactions – to generate ATP.

Cytoplasm Outer Membrane Inner Membrane Matrix Cristae Details Inter-membrane Space

Glycolysis First step in extracting energy from glucose. –Also the most ancient means of extracting energy from glucose. Happens in the cytoplasm. Overall goal: Glucose (high energy)  pyruvate (less energy) + chemical energy (in the form of ATP and NADH 2 )

Glucose Glucose-6-Phosphate -1 ATP Investment Return +2 NADH +2 ATP G3P BPG PEP Pyruvate 3PG +2 H 2 0 G3P BPG PEP Pyruvate 3PG -1 ATP

Glucose Glucose-6-Phosphate -1 ATP Investment Return +2 NADH +2 ATP +2 H 2 0 G3P Pyruvate G3P Pyruvate Level of Glycolysis Details required from official syllabus +2 ATP -1 ATP

Notice we just made some ATP. Up to this point, there has only been one way to generate ATP. What is this way? This new way is different – instead of a chemiosmotic gradient generating pressure, we are using an enzyme (which needs a substrate). We call this way of creating ATP “Substrate Level Phosphorylation”. Remember this.

+2 NADH G3P BPG P PP First Substrate-level Phosphorylation Part I PiPiPiPi +2 NAD + (3 carbons, 1 phosphate) (3 carbons, 2 phosphate) Oxidation of high energy G3P (loss of electrons, given to NAD along with a H+) puts the energy to work by creating a phosphate bond (BPG)… +H +

… which can be used to make ATP! BPGPP 3PG P ADPPP ATPPPP (remember there are 2x BGP so 2x ATP are made) (3 carbons, 2 phosphates) (3 carbons, 1 phosphate) First Substrate-level Phosphorylation Part II

3PG PEP P P (oxidation by removing water) +H 2 O (high energy molecule) Second Substrate-level Phosphorylation, Part I

Second Substrate-level Phosphorylation, Part II PEP P pyruvate ADPPP ATPPPP

pyruvate What have we built so far? ATP PPP x2 x4 e- NADH x2 1x GLUCOSE

H20H20 What’s our waste so far? x2 (investment)ATP PPP Total for Glycolysis: 2 net ATP, 2 NADH, and 2 pyruvate molecules

Where can we go from here? Do we have oxygen? Yes How can we extract even more energy? No Kreb’s Cycle Fermentation Prep Phase

Fermentation When oxygen is NOT present, pyruvate is not pumped into the mitochondria. Instead, it is reduced by NADH to alcohol and carbon dioxide, or to lactate (depending on the organism), making a single ATP. pyruvate NADH lactate Ethanol + CO 2 or NAD+ (back to glycolysis) ATP

Fermentation The net is just 1 more ATP (2 total from 1 glucose molecule) We’ve only recovered about 2.1% of the total energy of glucose. What could possibly be the benefit of Fermenation? pyruvate NADH lactate Ethanol + CO 2 or NAD+ (back to glycolysis) ATP

Prep Reaction So how we increase efficiency, and make more ATP? When O 2 is present, pyruvate gets pumped into the matrix, then oxidized (it loses electrons… and hydrogens) Pyruvate (C 3 ) + NAD + + CoA  Acetyl-CoA + NADH + H + CO 2

Another diagram of the prep reaction Figure 6.10 CoA Pyruvate Acetic acid Coenzyme A Acetyl-CoA (acetyl-coenzyme A) CO 2

What next? In photosynthesis, we saw photosystems hand down high-energy electrons to build up a H+ concentration gradient The ETC in mitochondria work the same way, except it gets high-energy electrons from NADH and FADH 2.

What next? Acetyl-CoA is fed into the Kreb’s cycle. Acetyl-CoA is attached to a 4-carbon molecule (oxaloacetate – where did you see this before?) From here, the energy is slowly released from this high- energy molecule. For each “crank” of the cycle (2 cranks per 1 glucose!) –2 CO 2 are given off (these are the last remaining carbons from the original glucose!) –3 NADH are made. –1 FADH 2 is made. –1 ATP is made. Remember there are TWO cranks per original glucose molecule! –Totals: 4 CO 2, 6 NADH, 2 FADH 2, 2 ATP total.

Overall, the Krebs cycle extracts the energy of sugar by breaking the acetic acid molecules (from pyruvate) all the way down to CO 2. –The cycle uses some of this energy to make ATP –The cycle also forms NADH and FADH 2

A series of oxidation / reduction reactions “hand off” the high energy electrons down a chain, until at the very bottom, oxygen (O 2 ) recieves them (along with some hydrogen ions) Therefore, oxygen is reduced to H 2 0. Electron Transport Chain!

A series of protein complexes stuck into the cristae. NADH hands its high-energy electron to the first ‘link’ in the chain. The H it was attached to (now H+) is pumped into the intermembrane space. The high-energy electron is continually passed down the line, until much of its energy is spent. Oxygen is waiting a the bottom to accept them (along with an H+) WATER IS FORMED. –You can think of these 2 electrons as coming from the original glucose molecule. Electron Transport Chain!

For every 1 NADH inside the mitochondria, 3 ATP molecules can be formed. For every 1 FADH 2, only 2 ATP molecules can be formed. From 1 glucose molecule (x2 pyruvate) –2 NADH from the prep reaction –6 NADH and 2 FADH 2 from Kreb’s Cycle –8 total NADH * 3 ATP = 24 ATP –2 total FADH 2 * 2 ATP = 4 ATP Total of 28 ATP so far (from oxidative phosphorylation) Energy Accounting

There were 2 NADH molecules made during glycolysis… out in the cytoplasm! But we can only make ATP from this INSIDE the mitochondria. Therefore, these molecules pass their energy – through a poorly understood mechanism – into the mitochondria Sometimes, this energy can be directly handed to the ETC. In this case, they produce 3 ATP. However, most cells pass these off to FAD (to make FADH 2 ) – which means only 2 ATP can be made. With direct handing off to the ETC, we can therefore make a maximum of 6 additional ATPs. Otherwise, if the cell uses FAD, we can only make 4 additional ATPs. Sub totals from oxidative phosphorylation = = 34 What about the NADH from glycolysis?

The ETC, by using the energy from NADH and FADH2, make 32 or 34 total ATP. This is called Oxidative Phosphorylation, because O2 grabs the cooled off electrons (along with a pair of protons) and becomes water. Do you remember how we made ATP during glycolysis and the Kreb’s Cycle? How much did we make from a single glucose molecule? 2 net ATP were made from glycolysis. 2 ATPs were made during the Kreb’s Cycle. Overall Totals = = 38 Now let’s account for the other ATP that was made

 Gs Another way to look at it: Glucose  H 2 O + CO 2  G = -686 kcal ATP  ADP + P i  G = -7.3 kcal We only managed to store about 270 kcal of the 686 kcal of free energy stored in glucose. Overall, fermentation (2 ATP or 14.6 kcal) is 2.1% efficient, while aerobic respiration is 39% efficient. 36 ATP = kcal 38 ATP = kcal

The metabolic pool Other molecules besides glucose can be used as energy Fat: glycerol + fatty acids –Glycerol can be converted to pyruvate –Fatty acids can be broken down to form acetyl-CoA –An 18 carbon chain can make 108 ATP! Amino acids: –Deamination (removes NH 3, which enters the urea cycle) –From here, they can go through Glycolysis Be converted to acetyl-CoA Go directly into the citric acid cycle

Anabolism The oxidized ‘breakdown’ components from cellular respiration can be used as building blocks. (these traveled downhill, giving up their energy to make ATP, right?) we can’t make some amino acids. –“essential amino acids”, we get these from our diet –The ones we can make are “non-essential” amino acids. Plants can make all of them.

Input Acetic acid ADP 3 NAD  FAD Krebs Cycle Output 2 CO

Cytosol Mitochondrion High-energy electrons carried by NADH High-energy electrons carried mainly by NADH Glycolysis Glucose 2 Pyruvic acid Krebs Cycle Electron Transport

Electron Transport Chain Glycolysis Cellular Respiration glucose pyruvate Kreb’s Cycle NADH NAD+ ATP Acetyl CoA ATP NADH Fermentation -O2 (+O2) Lactic acid CO2 H2O