Photosynthesis and Cellular Respiration2.06 Photosynthesishttp://safeshare.tv/w/WJjalpEJUz2.05 Cellular Respirationhttp://safeshare.tv/w/yVgtsDvjXw

In A NutshellTo a plant, sunbathing is life. Literally. In fact, plants have evolved all sorts of ways to maximize their exposure to the sun while at the same time preventing loss of critically needed water. Plants, as well as some algae and bacteria, perform photosynthesis, a process that involves the capture and use of the Sun’s energy to create biological compounds. Photosynthetic organisms generate these compounds using carbon dioxide (CO2) and water (H2O), and the products they release are oxygen (O2) and carbohydrates as byproducts.

Plants provide us with the oxygen we breathe. Now that you know this tidbit of info, you may have a little better understanding of why environmentalists freak out over the rapid destruction of rainforests, wetlands, and natural habitat. The lower the number of plants on the planet, the less O2 produced for important things like, say…breathing.

Photosynthesis can be divided into two processes: the first requires light, and the second does not.

In the first phase of photosynthesis, cellular protein pigments calledchloroplasts are excited by light that propels them into high-energy states. The chloroplasts then transfer this energy through electrons to otherprotein complexes (read: several proteins stuck together). This group of proteins is called the electron transport chain. The proteins operate similarly to a group of dominoes: after the first one has been pushed, each protein transfers energy to each member along down the line.

Water (H2O) is split in this process, releasing oxygen (O2) and hydrogen ions (H+). The electrons from the electron transport chain combine with these H+ ions and nicotinamide adenine dinucleotide phosphate ions (NADP+) to form adenosine triphosphate (ATP) and a reduced unit ofNADP+, called NADPH (NADP plus an electron, or H). These energy storage forms, ATP and NADPH, are used to convert carbon dioxide (CO2) to build carbohydrates during the second phase of

photosynthesis. Plants can then break down these carbohydrates to fuel their existence.

Are you jealous of your green friends? It must be nice to sunbathe all day long. Before you become plant-green with envy (sorry…), remember that plants do not have the ability to eat milkshakes or French fries, and they can't ride a bike along the beach. Does that give you some satisfaction? Good.

And, once you master your knowledge of photosynthesis, you can relax in the sun with a greater appreciation for the O2 you breathe, knowing that it is produced from the photosynthetic process. Don’t forget the sunscreen—at least SPF 30—you aren’t a plant, after all. Finally, you will realize that the beachside smoothie you are sipping, along with everything else that you have ever eaten, or will ever eat, relies on photosynthesis either directly or indirectly as its source of energy. Unless, of course, you have gotten into the habit of snacking on chemoautotrophs that live in rocks or deep-sea hydrothermal vents. They don't use photosynthesis.

The light reactions, or the light-dependent reactions, are up first. We call them either and both names. The whole process looks a little like this:

Do not freak out or fill your head with all the complicated names in that diagram. No—stop right there. All in all, the process is simpler than it looks. In the light-dependent reactions of photosynthesis, the energy from light propels the electrons from a photosystem into a high-energy state. In plants, there are two photosystems, aptly named Photosystem I and Photosystem II, located in the thylakoid membrane of the chloroplast. Thethylakoid membrane absorbs photon energy of different wavelengths of light.

Here again is our friend the chloroplast. All exposed the way he is, he kind of reminds us of a boat with green checkers in it:

Image source

Even though the two photosystems absorb different wavelengths of light, they work similarly. Each photosystem is made of many differentpigments. Some of these pigments can be described as absorption pigments, and others are considered action pigments. 

The absorption pigments transfer the energy from sunlight to another pigment; at each transfer, the absorption pigments pass the photonenergy to another pigment that absorbs a similar or lower wavelength of light. Remember when we said that light is funky and acts like it has both particles and waves? A photon is what we call the particle-like aspect of light. In other words, a photon is the basic unit of light. 

Anyway, eventually, the energy makes it to the reaction center, or action pigment. At this point, the photosystem loses a highly charged electron to adjacent oxidizing agents, or electron acceptors, in the electron transport chain. This transfer all occurs mind-bogglingly quickly at an estimated time of 200 × 10-12 seconds!2 Since the photosystem has lost an electron, it no longer has a neutral charge and has instead become a positively charged photosystem.

The positively charged photosystem creates a scenario similar to one that might occur if Twilight stars Robert Pattinson and Kristen Stewart made a surprise appearance at your local high school. You, like the electrons in the photosystem, would be attracted to their presence even if you hated them. (You would. Admit it.) The positively charged photosystem attracts electrons from water (H2O) that can then be excited by light energy. When exactly four electrons are removed from H2O, oxygen (O2) is generated. Why, you ask? If two water molecules have four hydrogens that lose four electrons, exactly four hydrogen ions (H+) and two oxygens are left. Don't believe us? Count it out for yourself.

Side note: since hydrogen normally only has 1 proton and 1 electron, the four hydrogen atoms that have each lost one electron are each referred to as H+. Since each H+ is now without an electron, there is only one proton remaining in the hydrogen atom. At some point, scientists became lazy and started equating H+ with the word proton. If you think about it, they are in fact equivalent.

Back to regularly scheduled programming. The protons are then moved into the thylakoid lumen of the chloroplast using the power of the electron transport chain. This move results in a higher concentration of protons in the lumen than in the stroma of the chloroplast. 

With so much positivity around, the protons get a little upset and try to equalize their distribution in the chloroplast by moving from the lumen to the stroma to reach equilibrium (read: equal numbers of protons in both places). The rush of protons moving into the stroma is called a proton gradient. When protons move down the gradient, with down referring to the direction of the area containing fewer protons, the protons are grabbed by enzymes that bring the protons together with the electrons from the electron transport chain. This event ultimately results in the making of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) from the adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate ion (NADP+) that were hanging around nearby. 

Now that everyone is partying it up in the stroma, it becomes the perfect location for the next stage of photosynthesis, the light-independent reactions.

Pictures are worth the thousand words that may or may not have just whizzed by your head as you were reading. Here is a much-simplified version of the earlier picture:

Did you miss something, or do we just suck at drawing these pictures? Nope. Photosystem II is ahead of Photosystem I. You might ask, "What the heck happened, Shmoop?" Well, scientists actually discovered Photosystem II before Photosystem I, and instead of changing the names when they found the other photosystem, they just named them in reverse. We know; it's very annoying.

As you’ve probably gathered by now, the light-dependent reactions fuel the second stage of photosynthesis called the light-independent reactions. Our good buddy carbon dioxide (CO2) provides an excellent source of carbon for making carbohydrates. However, conversion of one mole (one mole is an amount equal to 6.023 × 1023 molecules) of CO2 to one mole of the carbohydrate CH2O requires a lot of energy. And we mean a lot.

Guess what? The ATP and NADPH generated in the earlier light reactionsare strong reducing agents (electron donors) and are able to donate the necessary electrons to make carbohydrates. Altogether, the conversion of one mole of CO2 to one mole of CH2O requires two moles of NADPH and three moles of ATP. If you do the math, that's a heck of a lot of molecules. The cell then uses ATP and NADPH to make carbohydrates in the Calvin cycle. We could use a Calvin and Hobbes cartoon right about now.

A key player in the Calvin cycle is ribulose-1,5-bisphosphate carboxylase oxygenase (affectionately called RuBisCo—thank goodness for nicknames), an enzyme that

"fixes" CO2 to a 5-carbon compound calledribulose-1,5-bisphosphate (RuBP). The oxygen in CO2 is released as H2O. Immediately after RuBisCo catalyzes the attachment of the carbon from CO2 to the 5-carbon RuBP, the new 6-carbon compound is broken down into two 3-carbon compounds called phosphoglycerate (PGA, and no, it does not know how to golf). Since these 3-carbon compounds were the first compounds to be identified in the plants, they were named C3 plants. It was originally thought that RuBisCo was catalyzing the attachment of carbon to a 2-carbon molecule to make a 3-carbon molecule. Oopsies. And we thought RuBisCo was a cookie company at first, too. 

RuBisCo is actually a poor enzyme. Sorry, RuBie. It is slow at catalyzing the attachment of CO2 to RuBP. To make matters worse, RuBisCo is also capable of catalyzing another less-than-beneficial reaction. This reaction is called photorespiration, and it occurs when the concentration of CO2drops too low relative to the concentration of O2 in the cell. Photorespiration begins when RuBisCo uses O2 instead of CO2 and adds it to RuBP.

While CO2 is eventually produced in this reaction, and O2 is consumed, the reaction does not seem to produce any useful energy forms. The origination and purpose of photorespiration is controversial and still under active study by scientists. In an attempt to overcome the deficiency of RuBisCo, the plant cell produces a whopping ton of the enzyme. If this sounds slightly masochistic, it kind of is, which is why photorespiration has been labeled an outdated evolutionary relic. However, RuBisCo is thought to be the most abundant protein on Earth.2 

Right…this not a moan fest about RuBisCo. We were explaining the Calvin cycle. When RuBisCo catalyzes the attachment of CO2 to the 5-carbon RuBP, the Calvin cycle begins. Reactions are initiated to rebuild RuBP from PGA. In this process, 1 molecule of glyceraldehyde-3-phosphate(G3P), a 3-carbon sugar, is removed from the cycle. Altogether, 1 molecule of G3P is produced using 3 molecules of CO2, 9 molecules of ATP, and 6 molecules of NADPH. This 3-carbon sugar can be exported to the cytoplasm to make sucrose (a sugar and a carbohydrate), which is then moved throughout the plant for energy use. Alternatively, sucrose can be converted into another carbohydrate, starch, and then stored in the chloroplast as a type of energy reserve. Smart, plants…real smart.

What's that? You want a picture? Sure thing; glad to be of service.

Brain Snacks

There are carnivores that undergo photosynthesis. Meat-eating plants do not eat for energy; they eat to obtain nutrients, such as nitrogen and phosphate, to build the proteins needed for photosynthesis.

Living organisms besides plants do photosynthesis. One "biological masterpiece," the sea slug Elysia chlorotica, is able to conduct photosynthesis by extracting DNA and chloroplasts from its plant food source. Sneaky, sneaky.

In seeded plants, chloroplasts do not develop unless the seedlings are exposed to light. This process is called photomorphogenesis.3

Weed killers called herbicides work by targeting enzymes used in the light reactions of photosynthesis. Come here, little chloroplasts.

In A NutshellWhat do playing Wii, watching Dancing With the Stars, and doing your homework have in common? At first glance, not much...since two of those things are fun, and one isn't.

Don't worry; we won't judge you on which one of those is the most unpleasant for you. But, they do have something in common: they all require energy. This energy is not just the energy that powers light bulbs and televisions—it is also the energy that powers cells in your body.

Most of us know that we need to eat food for energy, unless you're a plant, in which case you just soak up the Sun and carbon dioxide. If you are a plant, congrats on learning how to read. How does food become useful to cells in the body? Cells can't process pizza, mac and cheese, or Doritos in their natural or intact forms, so the body needs to break them down a bit. Note that we use the term "natural" loosely here. The process of converting food into useful energy at the level of the cells is called cellular respiration.

Cellular respiration is a misleading name: usually "respiration" refers to breathing. In this case, though, cellular respiration refers to converting glucose, a simple sugar (carbohydrate), into energy for cells.

Cellular respiration is a bit like an old-school arcade game. At the arcade, you start with a dollar, change it into tokens (we said old school!), play the game, and get 38 tickets that you can turn in for, at most, one crappy bouncy ball at the exchange counter. Never mind our bitter flashbacks to arcade trips in the early '90s. Carry on.

Your starting input, the dollar bill, is like a glucose molecule, and you end up with 38 tickets, or adenosine triphosphate (ATP) molecules. Glycolysis is the name we would give to the part of the process where you take your dollar bill (glucose) to the change machine and get tokens. You then feed these tokens into the pinball machine, called the citric acid cycle, and it spits out a couple tickets (ATP molecules). Let's just say that, along earning with your tickets, you win a bonus game of Skee-Ball calledoxidative phosphorylation (come to think of it, this is not the coolest arcade we have ever been to...), which gives you even more tickets, or ATPmolecules. Tickets can be exchanged for prizes at the arcade counter, and in the cell, ATP provides the energy for work—the best prize of all!

Not sold on our analogy? Fine.

So, how exactly does ATP provide energy for the cell? ATP is a source ofchemical energy that can be used to

Drive chemical reactions

Transport substances across membranes

Do mechanical work

Chemical energy is the energy released in a chemical reaction, like when certain chemical bonds are broken. One molecule of ATP has three phosphate groups that are bound together. When a bond between phosphates is broken, energy is released. This energy can be used to power another chemical reaction. When a reaction that requires energy is paired with a reaction that releases energy, like the breaking of a phosphate bond in ATP, we call this process energy coupling.

The first step when going to an old-school arcade is breaking your dollar bills down into tokens; something similar can be said for cellular respiration, where the glucose (C6H12O6) molecule needs to be broken down into a substance called pyruvate (CH3COCOO−). Instead of four tokens to the dollar, one glucose molecule is converted to two pyruvate molecules. Sounds like a bit of a ripoff, if you ask us. That's not all, though. Glycolysis also results in two ATP molecules, and two molecules of another compound, NADH. Note: You won't get a better deal if you move to Canada and try glycolysis up there.

Normally, a bill changer in an arcade wouldn't give you tickets (ATP and NADH) for just converting your dollars to tokens. That's one way glycolysis is a little different than a machine at an arcade. To better understand glycolysis, we must get up close and personal. Ready to dive into the inner workings of the machine?

Glycolysis takes place in the cytoplasm. If we rewind our brains a bit back to the Cells unit, we remember that the cytoplasm is the material inside the cell but not in any particular organelle—like a change machine in the lobby of an arcade. From the Overview section of this unit, we know that the conversion of one compound to another in the steps of cellular respiration happens due to reduction and oxidation, or redox reactions. Watch out for these guys.

One more important fact about glycolysis: it can happen with or without oxygen. 

What?! That's right, ladies and gentlemen, no oxygen needed. But, don't let that fool you—later steps of cellular respiration (the citric acid cycle andoxidative phosphorylation) do require oxygen. So demanding, those two.

We start with a glucose molecule, which has six carbons in it.

Step 1. Two phosphates are added to the glucose molecule. In other words, the glucose is phosphorylated, which is a fancy way of saying it had phosphate added to it. The two phosphates come from two ATP molecules, which each had three phosphates—that's the triin adenosine triphosphate—and now only have two. They are now called adenosine diphosphate, or ADP. Not very creative,

these science-y naming people. Since two ATP were used, this reaction actually takes energy to occur. But hey, no pain no gain, right?

Step 2. The glucose-phosphate molecule splits into two sugar molecules, each with three carbons and one phosphate. Eventually, these molecules will be converted into pyruvate molecules. Thissplitting, or lysis, of glucose is where glycolysis gets its name.

Step 3. The three-carbon phosphate sugars each lose an electron, or are oxidized, which goes to the NAD+ molecules waiting in the wings. These two NAD+ become NADH molecules after they each gain an electron, which counter-intuitively, is called reduction. While NADH is not exactly ATP, it can be converted into ATP later. This part would be analogous to the arcade machine giving you euros instead of tickets, but you could convert the euros into tickets by playing another arcade game.

Step 4. An enzyme attaches phosphates to the two oxidized sugars. In step 1, phosphates came from ATP, but in this case, the phosphates come from a supply in the cytoplasm.

Step 5. The two phosphates that were just added to our sugars are abruptly taken away and added to two ADP…sad, because they were just getting to know each other. The two ADP of course, with the additional phosphates, become ATP. Sweet.

Step 6. The remaining phosphates on the three-carbon sugars are taken off to make two more ATP. The two remaining molecules arepyruvate. Pyruvate molecules are like our game tokens that we get to put into the pinball machine to win tickets.

In pictures, glycolysis looks something like this:

Now that we got through the inner workings of glycolysis, let's see what we ended up with. We have

2 pyruvate

2 NADH

2 ATP

Wait, didn't we make four ATP? Yes, but remember, in step 1, two ATP were used for their phosphates, so those two even out two of the four that were made, and the net gain is two ATP.

The next steps in cellular respiration take place in a mitochondrion (that's the singular form of mitochondria, one of the cell's organelles). Here's a quick review of what a mitochondrion looks like. For cellular respiration, just pay attention to these three important parts:

• Outer membrane: Just what it sounds like. A membrane on the outside of the mitochondrion. This membrane, and the inner membrane, is made up of a phospholipid bilayer, just like the

plasma membrane that surrounds the cell.• Inner membrane: A second membrane, on the inside of the outer membrane. The space between the inner and outer membranes is called the inter membrane space. The space on the inside of the inner membrane is the mitochondrial matrix.• Cristae: These are wrinkles formed by folding of the inner membrane. The cristae provide a large surface area for cellular respiration, particularly the electron transport chain.

An up-close diagram of a mitochondrion:

Real-life mitochondria from a paramecium, seen under a microscope:

(synonym for Krebs cycle)

At the arcade, after you change your dollar bill into tokens, you set your sights on a game. In a cell, if oxygen is present, the pyruvate from glycolysis heads over to a mitochondrion for the citric acid cycle. If no oxygen is in the house, then skip right along to fermentation. We know all these biology terms and processes are a lot to handle, so don't worry, we got you covered.

In this installment of the cellular respiration saga, we will finish up the oxidation of the glucose molecule we started with by moving into the citric acid cycle. Sometimes the citric acid cycle is called the Krebs cycle, after Hans Krebs, the scientist who discovered it and won a Nobel Prize in 1953. Krebs shared the Nobel Prize that year with Fritz Albert Lipmann, who discovered coenzyme A, which will enter the pinball machine soon.

In glycolysis, the glucose was oxidized to pyruvate. In the citric acid cycle, the pyruvate will be further broken down to carbon dioxide. And it will result in more prize tickets (ATP), but we'll also end up with two other compounds that can be converted to ATP: NADH and FADH2. More on that later.

As we fire up the old pinball machine, we drop a token in and get a ball to start. In the same way you trade the token for a ball to bounce around the machine, a mitochondrian changes pyruvate to a compound called acetyl CoA, short for acetyl coenzyme A. A coenzyme is a helper molecule that binds to an enzyme and is necessary for that enzyme to function. Acetyl CoA is the compound that enters the citric acid cycle. This step of converting the pyruvate to acetyl CoA, is sometimes called the transition or bridge step.

The transition step:

1. Pyruvate loses a molecule of CO2.2. NAD+ oxidizes pyruvate, which makes acetate.3. Acetate is joined by coenzyme A, making acetyl CoA.

Now for the pinball game…er, citric acid cycle. Get those reflexes ready. At each stage of the cycle, the carbon compound we're calling the pinball has a different name. We're not going to worry about those names here, except for at the beginning and end.

Step 1: Our pinball, acetyl CoA, drops its acetyl group, which has two carbons, and the two carbons combine with a four-carbon compound called oxaloacetate (we won't quiz you on that name), which make, ta-da, citric acid.

Step 2: Citric acid is oxidized and loses a carbon dioxide (CO2) molecule. The electron it loses goes on to reduce NAD+ to NADH. Then this happens again. Yup, it happens two times, meaning two CO2 molecules are lost and 2 NADH are formed.

Step 3: The carbon compound pinball resulting from step 2 briefly joins up with coenzyme A, which is then replaced with a phosphate group. But the phosphate group doesn't stick around long either--one ATP forms. This is the only ATP made in the citric acid cycle.

Step 4: Our four-carbon pinball loses two hydrogens (which includes their electrons, so it is oxidized). The hydrogens reduce FAD to FADH2. FAD is a coenzyme (like coenzyme A), and is short for flavin adenine dinucleotide.

Step 5: The pinball bonds with a water molecule, then gets oxidized with NAD+ on the reducing side, forming NADH. After all these changes, the pinball is back to a form we saw at the beginning of the citric acid cycle: oxaloacetate. That's right. All we have to do is drop in another acetyl CoA to spin the wheel around again.

So what did we get from all this? Only one ATP. But the winnings are better than they seem at first, because we got three NADH, plus the NADH from converting pyruvate to acetyl CoA. We also got one FADH2, and we get to multiple everything by two. Why? These compounds came from one pyruvate molecule, but glucose was broken down into two pyruvate molecules. So our totals for the citric acid cycle are

• 2 ATP• 8 NADH• 2 FADH2

• 6 CO2 (these won't be used for energy like the other products – but don't worry, they don't contribute much to global warming.)The NADH and FADH2 are the bonus tokens for the next round of cellular respiration pinball, oxidative phosphorylation.

Brain Snack

In addition to figuring out the citric acid cycle, Hans Krebs also discovered the urea cycle, which produces urea. Urea is excreted from animals' bodies in pee. It is also used as a fertilizer. We won't ask how Hans discovered it.

This is our bonus game, but also the more important one, because we will convert the tokens from the citric acid cycle (NADH and FADH2) to tickets (ATP). This is where the bulk of ATP comes from in cellular respiration—not glycolysis or the citric acid cycle, but oxidative phosphorylation.

If we break it down, it is not too hard to figure out what this long phrase means: oxidative must have something to do with oxidation, which involves a transfer of electrons. Phosphorylation is a term we saw earlier, and it means adding phosphate. So put them together, and we are talking about redox reactions and phosphate additions, which is not all that different than what we dealt with in glycolysis and the citric acid cycle.

The main point of oxidative phosphorylation is the transfer of electrons from NADH and FADH2 to power ATP production. Similarly, the main purpose of playing arcade games is to win tickets for prizes (okay, and also maybe to have fun and earn high scores in the games). NADH is more often the electron donor because more NADH than FADH2 result from the citric acid cycle.

Oxidative phosphorylation happens in two steps:

1. The electron transport chain: no ATP here, just lots of electrons being transported.2. Chemiosmosis: a gradient of protons (H+) drive the enzyme ATP synthase to make ATP.

We like to call the electron transport chain the ETC. It is sort of like the EAC (East Australian Current) in Finding Nemo, except that instead of transporting fish and sea turtles hundreds of miles the ETC transports hundreds of electrons.

In the ETC, NADH (or less frequently FADH2) starts the electrons down the chain. NADH and FADH2 donate electrons to protein complexes, which are basically blobs of protein. These blobs are numbered I, II, III and IV, and they pass electrons down the chain from protein to protein in the blobs I to IV. One of the electron carriers, ubiquinone, is not a protein, but a lipid. Yet another example of protein jobs getting outsourced to lipids. Oxygen is the terminal electron acceptor, which means it takes the electrons from the last protein complex. Oxygen takes electrons from NADH or FADH2, then also picks up two hydrogen ions to make water.

When FADH2 is the electron donor, it binds to complex II, later in the chain than where NADH starts. This means fewer protons are released (no protons go through complex I) which makes FADH2 a less energetic electron carrier and results in fewer ATP molecules being made.

Since electrons and protons travel together, all this electron shuffling results in some loose protons (H+) floating around. The protons end up on the other side of the inner mitochondrial membrane that the NADH and FADH2 (and their reduced forms) are.

At the end of the ETC, the protons flow back across the membrane, which powers the enzyme ATP synthase to make ATP. This flowing of protons across the inner mitochondrial membrane is chemiosmosis. The wordchemiosmosis means ions moving across a membrane. When water flows across a membrane, it is called osmosis, and chemiosmosis is a very similar process. In both cases, particles diffuse from an area of high concentration to an area of lower concentration. The gradient of high to low proton concentration has potential energy, and it can be used by ATP synthase.

ATP synthase makes ATP from ADP and inorganic phosphate (Pi). The enzyme ATP synthase is like a motor driven by protons because a steady stream of H+ moving back across the inner

membrane of the mitochondria provides the power to make ATP. In the same way that flowing water can move a water wheel, protons move the ATP synthase wheel.

Go ATP synthase, go!

In this round of pinball, we won 34 tickets. This means the process of oxidative phosphorylation produces 34 molecules of ATP per glucose molecule.

Brain Snack

Chemiosmosis happens not just in mitochondria, but also in bacteria and chloroplasts.

Suppose a cell does not have oxygen available. Maybe the cell happens to be on the moon, or maybe the cell's owner is sprinting away from a lion and is using up all the oxygen at the moment. Rule #1 of oxidative phosphorylation – stay away from lions. The cell can do glycolysis as usual, but can't run the citric acid cycle or electron transport chain as described before since oxygen isn't around to accept extra electrons.

If the cell is on the moon, or some other environment without a lot of oxygen (like your bedroom – seriously, clean it up), say in the deep ocean, then it uses anaerobic respiration to break down food into chemical energy. This type of metabolism uses a reactant other than oxygen to accept electrons in its electron transport chain.

Microorganisms are the only things that use anaerobic respiration, as far as we know. Some bacteria that live in vents near the ocean floor are called sulfate-reducers, since they use sulfate instead of oxygen as their electron receptor. Microbes called methanogens also use anaerobic respiration—they use carbon dioxide as an electron receptor and make methane in the process.

Brain Snack

Methanogens are from the third domain of life, the Archaea. Some methanogens can be found in the guts of cows, where they contribute to methane-filled cow burps. 

Others can live in "extreme environments" where most other life cannot exist, such as hot springs, hydrothermal vents, and the Earth's crust.

If a cell has a short supply of oxygen because happens to be in a body running at full speed away from a lion, however, it will not use anaerobic respiration but will start fermentation. Fermentation extends glycolysis with extra reactions that replenish NAD+, so that glycolysis can

keep running and keep producing small amounts of ATP. Fermentation comes in two types:

• Lactic acid fermentation• Alcohol fermentation

Lactic acid fermentation happens in bacteria, fungi, and animal cells. It is a pretty simple follow-up to glycolysis: the pyruvate molecules are reduced to lactate, while NADH is oxidized to NAD+. In this way, NAD+ is replenished for glycolysis. Lactic acid fermentation happens in some fungi and bacteria, but also in animal muscle cells. The dairy industry uses bacteria and fungi for their fermentation to make cheese and yogurt.

Alcohol fermentation is pretty similar to lactic acid fermentation. Instead of the pyruvate being reduced to lactate, it is reduced to ethanol, and lets off two molecules of CO2 along the way.

Two kinds of organisms use alcohol fermentation: bacteria and yeast (yeast are fungi, btw). Humans "use" alcohol fermentation in another way, by using it to make bread, beer and wine. We'll talk more about that in another section.

GlycolysisSometimes, it can be confusing to keep track of what is being oxidized and what is being reduced. Using the trick about the phrase "LEO the lion says GER" helps us remember which is which…but looking at the charge of a molecule can also clue you in about where electrons are going. If a molecule is sporting a + sign (such as NAD+), that is a good sign that it needs an electron. It will likely be reduced, gain an electron, and become neutrally charged, or, not charged at all.

Glycolysis breaks down glucose, so you might think that glucose is the only form of energy acceptable to animals, including humans. On the contrary, glucose is rare in our diets. We eat lots of other sugars (despite the dentist's advice), including sucrose, fructose, and lactose, but not a lot of plain old glucose. The good news is that our bodies can convert other compounds into glucose, including proteins and fats.

Glycolysis is presented here as the first step in cellular respiration, which it is. However, it can also exist without the rest of cellular respiration—organisms that undergo fermentation in anaerobic environments undergo glycolysis, too.

Citric Acid CycleBecause the citric acid cycle makes many different compounds, it is easy to lose track of the main product of the citric acid cycle: energy. The main point of the citric acid cycle is to change pyruvate into ATP, NADH, and FADH2. Pyruvate that originally came from glucose is oxidized in a series of steps that result in ATP, NADH, and FADH2. ATP is energy, and NADH and FADH2 equal energy, too, after they go through the electron transport chain.

FermentationAnaerobic sounds a little bit too much like aerobic for some people's liking. The "aerobic" part means oxygen, and "an" means "no," so anaerobic means "no oxygen." Just remember: you don't have the guts to do anaerobic respiration, but cow guts do.

Carbon dioxide is only produced in alcohol fermentation and not lactic acid fermentation.