Cellular Respiration

Glucose and ATP | Equation for Respiration | ATP Structure
ADP to ATP | ATP-ADP Cycle | Photosynthesis and Respiration
Aerobic vs Anaerobic | Glycolysis Overview
Glycolysis in Detail | Glycolysis Animated | Anaerobic Respiration
Lactic Acid vs Alcohol | Fermentation Animation | Anaerobic Animated
Mitochondrion | Krebs Cycle | Krebs Cycle Animated | ATP Totals
Hydrogen Ion Pool | Electron Transport Chain | ETS Animated
Respiration Summary | Respiration Animated | Other Fuels | Quiz

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Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
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All living things require a constant input of energy into their cells in order to survive. This energy is needed for cell division, movement, maintenance & repair, and for building new materials. The autotrophs are organisms that can produce their own chemical (food) energy by the use of sunlight. The heterotrophs must eat chemical energy of other organisms to supply themselves with the necessary energy.

Photosynthesis is the process that converts the light energy to chemical energy for a plant. The chemical energy is stored in the molecule glucose. This is the same molecule that is found in the blood of all animals. Glucose is actually a universal food molecule for all organisms. It can easily be used for energy. Plants can make the glucose, animals must eat it.

Glucose and ATP
Glucose can be easily used for energy. Yet, glucose is itself NOT a directly usable form of energy for the organelles of a cell. Organelles cannot use the stored energy of glucose directly because they cannot get the energy out of it. The glucose is actually converted to a more usable form of energy that all organelles can use. That form of energy is in a molecule called ATP.

The energy in glucose is similar to the energy that is stored in gasoline. Although gasoline has much energy in it, it cannot cut the grass or move you safely down a street unless it is converted to a more usable form of energy -- moving mower blades or turning car wheels. Likewise, glucose is an energy rich molecule but it must be converted to a more versatile form of energy that all organelles can use -- ATP.

Equation for Respiration
Respiration is the process where the chemical energy of glucose is put into another molecule - ATP - which can then be used by all of the organelles of a cell. An equation is shown below which summarizes the chemical reactions involved in respiration. Glucose is broken down along with oxygen to form carbon dioxide,water and ATP.

ATP Structure
ATP is a molecule which consists of three smaller types of molecules: 1) adenine, 2) ribose, 3) phosphate. One ATP molecule consists of one adenine, one ribose, and three phosphates bonded together. It is pictured below. The wavy lines between the phosphates represent high energy bonds.

Most of the chemical energy of ATP is stored in the bond between the 2nd and 3rd phosphates. When the energy of ATP is used by an organelle for some activity, the third phosphate is broken off and attached to another molecule on the organelle, thus transferring the energy from the ATP to the organelle. This transfer of the phosphate to another molecule is called PHOSPHORYLATION. Since the ATP has lost one phosphate, it is now called adenosine diphosphate, or ADP. It is drawn below.

Respiration must constantly make more ATP by restoring ADP molecules. It does this by bonding phosphate molecules to the 2nd phosphate of ADP, thus converting it back to ATP, by USING GLUCOSE ENERGY. The ATP would then be ready to be used again by another organelle. ATP is also involved in other chemical reactions, such as photosynthesis, which was covered in a previous program.

The production of ATP and the use of ATP by organelles forms a cycle where ATP is produced, then used, then produced, then used --- over and over again. Essentially what is happening is that glucose energy is being used by organelles via ATP. An animation is shown below to represent this ATP cycle.

Photosynthesis and Respiration
Respiration is the process that releases the energy that was stored in glucose by photosynthesis. All organisms must carry out respiration -- plants and animals -- to harness the stored energy in glucose. So while plants photosynthesize and respire, animals only respire.

There are two types of respiration found in living things. (In other words, there are two different ways to make ATP from glucose energy.) The less efficient of the two is called ANAEROBIC RESPIRATION - which is respiration in the absence of air (oxygen). The second type of respiration is called AEROBIC RESPIRATION -- which is respiration which uses air.

Most organisms (plants and animals) can carry out both types of respiration, although aerobic respiration is the preferred method of making ATP because it is more efficient. Some microbes carry out only anaerobic respiration because oxygen kills them. Yeast is an organism used by man that can carry out both -- when used to make bread they aerobically respire, when used to make alcohol they anaerobically respire. Humans can carry out both types of respiration also.

Aerobic vs Anaerobic
We will now explain the process of respiration in some detail. Note that the equations shown below appear to show that respiration, aerobic and anaerobic, occur in one step. Respiration, both aerobic and anaerobic,actually occur in many steps and are both quite complex. Aerobic is more complex because it requires more chemical reactions. Both of the types of respiration below occur in humans.

Note that in the chemical reactions of both aerobic respiration, glucose is a reactant, but oxygen is a reactant only in aerobic respiration, while glucose is the only reactant of anaerobic respiration. Aerobic respiration produces carbon dioxide and water while anaerobic produces lactic acid. Note that both processes produce ATP, aerobic - 38 ATP, anaerobic - 2 ATP. Aerobic is more efficient because it produces more ATP from the glucose.

Both aerobic and anaerobic respiration require the use of many different enzymes and the use of a special Hydrogen carrying molecule called NAD. Aerobic respiration also uses a hydrogen carrying molecule called FAD. Each NAD and FAD molecule can hold 2 H-atoms. NAD-H-H represents an NAD carrying 2 H-atoms, FAD-H-H represents FAD carrying 2 H-atoms. The purpose of NAD and FAD is to carry hydrogen atoms away from the glucose as it is broken down so they can be used later to make ATP by a process called chemiosmosis. The enzymes are necessary to allow the chemical reactions of both types of respiration to occur at body temperatures.

Respiration with the use of oxygen involves three main chemical reactions: Glycolysis, the Krebs Cycle, and the Electron Transport System. Together, all three reactions of respiration with the use of oxygen can produce 38 ATP from the breakdown of one glucose. Respiration without the use of oxygen involves only glycolysis and anaerobic respiration. Glycolysis and anaerobic respiration can only produce 2 ATP from the breakdown of one glucose. Note the differences in the animations below. (Note: aerobic respiration is the Krebs Cycle and the ETS combined.)

Glycolysis Overview
Respiration always begins with the same series of reactions called GLYCOLYSIS -- which means "to split glucose". Glycolysis is the chemical reactions of respiration where glucose molecules are split into smaller molecules called pyruvic acid. The purpose of glycolysis is to break the large glucose molecule into smaller molecules and release some energy.

Glycolysis occurs only in the cytoplasm of a cell and does not require any oxygen. It consists of 9 separate chemical reactions that are each controlled by a specific enzyme. We will only consider 4 basic parts of glycolysis. First, the glucose is made more reactive by reacting with ATP. Two ATP are used to make the glucose less stable so it will release its energy, similar to lighting a firecracker with a match - the match makes the firecracker able to "pop".

The new molecule of glucose with 2 phosphates attached then splits in half into two molecules of PGAL (short for phosphoglyceraldehyde).

Each PGAL molecule then reacts and loses two H-atoms while gaining a phosphate molecule. The H-atoms are picked up by NAD, forming 2 NAD-H-H molecules.

Finally, the phosphates on the PGAL are removed and put onto ADP molecules to form ATP molecules, and the PGAL molecules are thus converted to pyruvic acid molecules, abbreviated PA. This marks the END of glycolysis, but not the end of respiration.

Glycolysis In Detail
To summarize, glycolysis split a glucose into 2 pyruvic acid molecules using 2 ATP's , while making 4 ATP's, in the process. 4 H-atoms were produced and picked up by NAD. The H atoms will be used later in aerobic respiration to make more ATP's. Thus, glycolysis results in the NET production of only 2 ATP's. (4-2=2)

Glycolysis Animated
The following links show glycolysis animated. Only one glucose will be shown to break down, although glycolysis occurs constantly on almost all glucose molecules that enter a cell. Below is the first frame of the animation.

Glycolysis Animated

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The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.

In the previous simulation, glucose was activated with ATP, then split in half forming 2 PGAL, which then reacted with phosphates and gave up H atoms, and finally, which then gave up all phosphates forming ATP and turning into pyruvic acid. The
2 NAD-H-H will be used later in aerobic respiration.

The pyruvic acid produced by glycolysis will then be further broken down by aerobic or anaerobic respiration. In anaerobic respiration the pyruvic acid is broken down only slightly, usually forming lactic acid (in animals) or alcohol in yeast or bacteria. In aerobic respiration the pyruvic acid is broken down as far as possible, all the way down to carbon dioxide and water. So anaerobic respiration only breaks down glucose part way, while aerobic respiration breaks down glucose all of the way.

Fermentation another name for the anaerobic respiration of pyruvic acid, the breakdown of pyruvic acid without the use of oxygen. The fermentation of pyruvic acid does not produce any ATP so it is not useful for energy production. The pyruvic acid is broken down to lactic acid or alcohol for another reason.

Anaerobic Respiration
To stay alive, a cell must have a constant uninterrupted supply of ATP. Usually a cell makes its ATP aerobically by breaking down glucose fully to carbon dioxide and water. If the cell should be deprived of oxygen temporarily, it could easily die if it could not make ATP anaerobically. This is why the pyruvic acid must be broken down to lactic acid or alcohol.

Anaerobic Respiration
Glycolysis requires empty NAD molecules (not carrying H-atoms) in order to occur. NAD-H-H (loaded NAD) are only used in aerobic respiration where the H-atoms are unloaded from the NAD. A cell has a limited supply of empty NAD molecules and if no air is present, all of the NAD molecules quickly become loaded with H-atoms from glycolysis. Without empty NAD molecules, glycolysis cannot occur and the cell would quickly run out of ATP and die.

Anaerobic Respiration
Instead, the pyruvic acid accepts H-atoms from the loaded NAD-H-H molecules, emptying the NAD molecules to keep glycolysis working on more glucose molecules and making more ATP. This produces some ATP (although not much) and will keep the cell alive for a short time. The purpose of fermentation is thus to empty NAD molecules so glycolysis can still occur.

Lactic Acid vs Alcohol
There are two types of fermentation. One is called LACTIC ACID FERMENTATION where pyruvic acid is anaerobically broken down to lactic acid. This occurs in animals (especially muscle) and some microbes. The other type of fermentation is called ALCOHOLIC FERMENTATION where pyruvic acid is anaerobically broken down to alcohol and carbon dioxide. This occurs in some bacteria and fungi. Both types of fermentation are represented below. Note that neither produces any ATP, the purpose of both is to empty NAD-H-H so glycolysis can continue making ATP.

Lactic Acid Fermentation Animated
In the next animation, lactic acid fermentation is shown. Below is the first frame of the animation.

Fermentation Animation

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Simple Fermentation Animation
In the previous animation, pyruvic acid molecules received H-atoms from NAD molecules to become lactic acid molecules. The only purpose of lactic acid fermentation is to empty the NAD-H-H molecules for more glycolysis ATP production. This happens in human muscle whenever the muscle uses up all of its available supply of oxygen. In order to keep contracting, it must keep producing ATP. To do this, it empties its NAD molecules onto pyruvic acid and builds up lactic acid. Too much lactic acid will actually damage a muscle, and soreness will result. Another animation below displays how NAD is important to understanding glycolysis and anaerobic respiration.

Anaerobic Respiration
In alcoholic fermentation, alcohol and carbon dioxide are made instead of lactic acid, but the purpose is still the same. Neither lactic acid nor alcoholic fermentation supply the cells directly with any ATP and most of the energy that was originally in the glucose is still locked up in the lactic acid or alcohol. This is why fermentation is inefficient. The energy of the glucose is not completely released.

Anaerobic Respiration
Fermentation only occurs because a cell would die if it could not make ATP under conditions of temporary oxygen loss. All cells experience times of temporary oxygen loss. So, fermentation keeps a cell alive under those conditions, but only for a short time. If oxygen is not eventually supplied, the cell will die. Fermentation is too inefficient in making ATP to keep a cell with its organelles alive for long. Glycolysis alone only produces 2 ATP, aerobic respiration produces 36 ATP.

Aerobic Respiration
The equation shown below represents the result of glycolysis and aerobic respiration, the chemicals used and the chemicals produced. Obviously aerobic is more efficient than anaerobic respiration because aerobic respiration does produce ATP while anaerobic makes no ATP, it only keeps inefficient glycolysis working.

While glycolysis and anaerobic respiration occur in the cytoplasm of a cell, aerobic respiration occurs in the mitochondria of a cell. A mitochondria is shown below. It consists of 2 membranes, an outer smooth membrane and an inner folded membrane. The inner membrane is folded to maximize surface area for chemical reactions to occur.

Aerobic Respiration
The fluid within the inner membrane is called the MATRIX. The fluid between the inner and outer membranes is called the INTERMEMBRANE SPACE. Note that the intermembrane space extends into the inner part of the mitochondria because of the folds of the inner membrane. The inner membrane and the matrix are the site of many chemical reactions.

Aerobic Respiration
There are two main groups of chemical reactions that occur in aerobic respiration. One is the KREBS CYCLE, the other is THE ELECTRON TRANSPORT SYSTEM. The krebs cycle occurs in the matrix of the mitochondrion, and the electron transport occurs on the inner membrane. The electron transport chain is often abbreviated "ETS".

The Krebs Cycle
The first step of aerobic respiration converts pyruvic acid to acetic acid (vinegar). Then the Krebs Cycle completely breaks down the acetic acid to carbon dioxide and H-atoms, which are picked up by NAD molecules. The electron transport chain pumps the H-atoms into the intermembrane space of the mitochondria (as H-ions) and chemiosmosis makes ATP from the hydrogen ions.

Now we will cover aerobic respiration in detail, starting with the pyruvic acid that was produced by glycolysis. Up to this point, only 2 ATP have been made.
Pyruvic acid is made in the cytoplasm. It then diffuses into the mitochondrion where it is broken down to acetic acid in the matrix. This chemical reaction produces carbon dioxide. It is shown below.

The carbon dioxide is a waste product and is eventually exhaled from the body. The acetic acid then enters the krebs cycle reactions in the matrix.

The acetic acid is picked up by a special coenzyme called coenzyme A, or CoA for short. (A coenzyme is a special molecule that carries other molecules.) This forms a new molecule called acetyl-CoA, meaning "acetic acid bonded to coenzyme A." This reaction also forms H-atoms which are picked up by NAD molecules. The reaction is shown below.

The acetyl-CoA then reacts with a 4-carbon acid (its name is oxaloacetic acid - too complicated to worry about), bonding the acetic acid to a 4-carbon acid to form a 6 carbon acid. The CoA is then freed to pick up another acetic acid to repeat the process. Basically, the acetic acid is a 2 carbon acid, and when it bonds to a 4-carbon acid it forms a (4+2) 6-carbon acid.

Krebs Cycle
What happens next is a series of complicated reactions where the
6-carbon acid reacts with various molecules, including water, to reproduce the original 4-carbon acid that started the process, plus 1 ATP and 8 H-atoms; 6 of the H-atoms are carried by NAD, 2 are carried by FAD.

Even though the Krebs Cycle is complex, its purpose is simple -- to completely break down acetic acid, extracting its energy in the form of Hydrogen atoms and ATP's. The FAD and NAD only serve to carry the energy rich H-atoms.

The Krebs cycle is a cycle, it does remake part of what it used. The 4-carbon acid is always recycled, it is easier to recycle than to make from scratch.

The acetic acid that originally reacted with the 4-carbon acid is essentially broken down to carbon dioxide. Its H-atoms end up on the NAD and FAD. Water molecules are used as the Krebs Cycle occurs to supply some H-atoms.

Krebs Cycle Animated
In the following animation, the breakdown of pyruvic acid is shown, as well as all important Krebs Cycle reactions. The cycle is shown going around twice because 2 pyruvic acid are made from the glycolysis of one glucose. Try and only watch one pyruvic acid being broken down at a time, it is much easier to understand. Below is the first frame of the animation.

Krebs Cycle Animation

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The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.

In the previous simulation, 2 pyruvic acid molecules were broken down all of the way to carbon dioxide and H-atoms. The NAD and FAD molecules carried the H-atoms away and the carbon dioxide is eventually exhaled from the lungs. Remember, 2 pyruvic acid molecules were shown passing through the cycle because 2 pyruvic acids are formed from the breakdown of one glucose.

All totaled, for 2 pyruvic acids aerobically respired through the Krebs Cycle, 6 carbon dioxide are formed and 20 H-atoms are released, 16 are carried by 8 NAD's, 4 are carried by 2 FAD's. Also, 2 ATP were formed. Note: Glycolysis is considered separate from aerobic respiration.

ATP Totals
For every glucose that is broken down for energy, 2 pyruvic acids are formed, which form 2 acetic acids, which each are broken down in the Krebs Cycle. Thus, so far, the following ATP have been made: 2 ATP ---- Glycolysis
2 ATP ---- Krebs Cycle
4 ATP ---- TOTAL
Also, so far, the following H-atoms have been made:
2 NAD-H-H ---- Glycolysis

pyruvic acid -> acetic acid
6 NAD-H-H ---- Krebs Cycle
2 FAD-H-H ---- Krebs Cycle
24 H-atoms -- TOTAL

Hydrogen Ion Pool
The H-atoms are used in the final phase of aerobic respiration - the electron transport system. These hydrogen atoms are passed onto the electron transport chain on the inner membrane of the mitochondria. There they pump Hydrogen IONS into the intermembrane space -- the fluid between the inner and outer walls of the mitochondria.

The Hydrogen ions are then used to make ATP by chemiosmosis. Basically, for each 2 H-atoms carried by NAD, 3 ATP are made and for each 2 H-atoms carried by FAD, only 2 ATP are made. All of the H-atoms made by aerobic respiration so far are worth 34 ATP to the electron transport chain.

Electron Transport Chain
Below is an enlargement of a section of the inner membrane containing the electron transport chain. Note that the chain goes back and forth from matrix side to intermembrane space side 3 times.

The H atoms from NAD and FAD are dropped off on the matrix side of the chain and pass down the chain to the intermembrane space side. Note that FAD actually drops its Hydrogen atoms off further down the chain.

Only NAD's Hydrogen atoms are shown passing down the chain to simplify things somewhat. Below the hydrogen atoms are shown passing from matrix side to intermembrane space side.

The hydrogen atoms are then split and Hydrogen ions are released to the intermembrane space while electrons pass back to the matrix side. A hydrogen atom consists of a hydrogen ion and electron.

The electrons pick up more H-ions on the matrix side and then go back to the intermembrane space side, to drop them off.

After the hydrogen ions are dropped off to the intermembrane space, the electrons again pass back to the matrix side to pick up more Hydrogen ions.

After the Hydrogen ions are picked up, the electrons pass back to the intermembrane space side where they drop them off.

Once again, the electrons pass back to the matrix side, where they will react with hydrogen ions and oxygen to form water. This is the only place oxygen is ever needed, at the end of the chain to accept the electrons.

Below, the electron have reacted with with Hydrogen ions and oxygen to form water. The oxygen is only needed to accept electrons a the end of the chain. The Hydrogen ions in the intermembrane space will next be used in chemiosmosis.

The twelve Hydrogen ions have enough energy to make 6 ATP from ADP and P.

The six ATP shown below will then diffuse to the cytoplasm where an organelle will use them for their energy.

ETS Animated
Using the links below, the electron transport chain is animated. 24 hydrogen atoms will pass down the chain to produce 34 ATP. You will notice that turning on the animation makes oxygen present, turning off the simulation takes the oxygen away. The electron transport chain only works if oxygen is present.

ETS Animated

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The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.

In the previous animation, the electron transport chain used
24 H-atoms (carried by NAD and FAD) to pump 68 Hydrogen ions into the intermembrane space. 6 oxygen molecules were used and 12 water molecules were produced. The oxygen is supplied by the air you are breathing in and its use in the electron transport chains of your body is THE ONLY REASON you need oxygen. Also, and most importantly, 34 ATP were produced which will diffuse to the cytoplasm and supply the organelles of the cell with the energy they need.

From the electron transport simulation, it should be apparent that the purpose of the chain is to transport hydrogen ions to the intermembrane space, so they can be used to make ATP, 34 total for all H-atoms from the breakdown of one glucose molecule.

Most cells aerobically respire because they can make so many more ATP for their organelles by breaking down the pyruvic acid all the way to carbon dioxide and water. The oxygen provides the electron transport chain a place to deposit the used electrons and without the oxygen, the chain stops and will not operate.

Respiration Summary
The purpose of glycolysis was to break the glucose down to pyruvic acid and make some H-atoms and ATP. The purpose of the krebs cycle was to make many H-atoms and a few ATP. The purpose of the electron transport chain was to make "mucho" ATP from all of those hydrogen atoms of glycolysis and the krebs cycle.

Respiration Summary
Thus, the purpose of respiration was to release the energy of glucose, (stored solar energy) into a form the organelles of a cell can use. That form was ATP.

Aerobic Respiration Animated
The final animation shows the whole of aerobic respiration, from glycolysis through ETS. A mitochondrion is shown highly magnified, with its appropriate parts labeled. Glycolysis occurs outside of the cytoplasm, and the rest of aerobic respiration occurs in the mitochondrion. Below is the first fram of the animation.

Respiration Animated

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The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.

In the previous animation, only one glucose was shown to be broken down. 6 oxygen were used and 6 carbon dioxide were produced. The ETS produced 12 water molecules but the Krebs Cycle actually uses 6 so the actual water production is only 6 molecules (12-6). The six water molecules made by the Krebs Cycle were shown in this and the previous Krebs Cycle animation. All totaled, 38 (40-2) ATP are made for the organelles of a cell from a single glucose respired aerobically.

Other Fuels
Lipids and proteins can also be used for energy. They are broken down to acetic acid and sent through the Krebs Cycle to make H-atoms for the ETS. A single large fat molecule is worth about 350 ATP molecules to a cell. Fats are an efficient way for animals to store energy for this reason. Glucose is usually stored in the form of a polysaccharide (starch or glycogen) and broken down as needed for energy.

The ATP's that are made by respiration will be broken down to ADP and P when used by an organelle, and the trapped light energy that was passed from glucose to ATP will then be put to use by the organelle. Almost all life on the planet processes fuel in this way, and they almost all ultimately depend on the sun for the energy.


  1. What molecule serves as an immediate source of energy to an organelle?

  2. In what organelle does aerobic respiration occur?

  3. What 3-carbon molecule is produced by glycolysis?

  4. How many ATP does glycolysis produce? (minus ATP used)

  5. How many molecules of oxygen are used for every glucose burned in glycolysis and aerbobic respiration?

  6. Does glycolysis require oxygen?

  7. Do plants respire glucose just as animals do?

  8. What does the ETS pump into the intermembrane space of the mitochondrion?

  9. What acid is produced by anaerobic respiration in human muscle?

  10. What hydrogen carrying molecule is used during glycolysis to pick up hydrogen atoms?

  11. How many ATP are produced by the aerobic respiration of 2 pyruvic acids?

  12. What gaseous waste molecule is produced by the Krebs Cycle?

  13. What coenzyme carries acetic acid in the Krebs Cycle?

  14. What is the name of the process involving ATP production by use of hydrogen ions?

  15. Does anaerobic respiration produce any additional ATP molecules after glycolysis?

  16. How many water molecules are produced by the ETS from the complete breakdown of 1 glucose?

  17. What is the major and most important product of the Krebs cycle?

  18. What is the name of the fluid in the center of the mitochondrion?

  19. Where does glycolysis occur in a cell?

  20. What phase of aerobic respiration produces the most ATP?

  21. How many ATP are required in glycolysis to activate a glucose molecule?

  22. What is another name for anaerobic respiration?

  23. Onto what molecule are Hydrogen atoms dumped to produce lactic acid in anaerobic respiration?

  24. What is the name of the chemical reactions where glucose is split into two pyruvic acid molecules?

  25. How many Hydrogen ions are required to make one ATP molecule in chemiosmosis?

  26. Where does the Krebs Cycle occur in the mitochondrion?

  27. What type of molecule can store up to 350 ATP's worth of energy?

  28. What is the ultimate source of energy which respiration releases for a cell?

  29. How many net ATP can a cell actually produce from the complete breakdown of a single glucose molecule?

  30. Where does anaerobic respiration occur in a cell?

Name the parts of the mitochondrion below, and state the functions of the molecules represented in the molecular diagrams below.