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Cellular Respiration

Definitions of respiration

•  Respiration happens in all cells of all organisms - bacteria, protists, plants, animals, etc.

•  It is not the same as “breathing.” That is a different kind of respiration.

•  The breathing kind of respiration is the exchange of oxygen and carbon dioxide that happens in an animal’s lungs.

•  Cellular respiration is a metabolic process (series of chemical reactions) that happens in every cell.

•  Respiration can be aerobic (oxygen-requiring) or anaerobic (no oxygen needed). It depends on the kind of organism.

•  Obligate aerobes are organisms that must have oxygen - you, for example.

•  Obligate anaerobes are organisms that die in the presence of oxygen. They have to have an oxygen-free environment.

•  Some species are facultative - they can use oxygen when it’s present, but can still survive using anaerobic respiration if no oxygen is available.

General overview of aerobic respiration

•  Importance

•  Cells require ATP to drive uphill reactions.

•  Food compounds, such as sugars, are oxidized during respiration.

•  Energy released from the oxidation used to synthesize ATP from ADP and phosphate.

•  This process is called oxidative phosphorylation.

•  The overall process

•  Aerobic respiration can be summarized as the oxidation of sugars and other food molecules by oxygen to produce carbon dioxide and water while releasing energy that is used to synthesize ATP.

•  Summary of aerobic respiration

•  glucose + O2 -> CO2 + H2O + energy (ATP)

•  This does not occur in a single reaction, but in many smaller steps.

•  Each step is catalyzed by a specific enzyme.

•  The overall process can be divided into three sets of reactions:

•  Glycolysis - occurs in the cytosol

•  Citric acid cycle (Krebs cycle) - occurs in mitochondrion matrix

•  Electron transport - occurs on mitochondrion cristae membranes


•  Glycolysis literally means "sugar splitting.”

•  A molecule of glucose (6-carbons) is split into 2 molecules of pyruvic acid (3-carbons each).

•  A small amount of ATP is produced.

•  Produces reduced NADH which can be used to make ATP by the mitochondrion.

•  Glycolysis enzymes are located in the cytosol.

•  No oxygen is required (these steps are anaerobic).

•  Summary of glycolysis:

•  glucose + ADP + phosphate + NAD -> 2 pyruvic acid + ATP + NADH

•  Details of glycolysis (actually about a dozen reactions or so)

The major reactions of the glycolysis pathway
Copyright McGraw-Hill companies

Possible fates of pyruvic acid

•  Fermentation

•  Yeasts and some bacteria do fermentation reactions

•  For example: pyruvic acid -> CO2 + ethanol (in yeast)

•  The reaction is used in baking - CO2 bubbles cause dough to rise

•  Also used in brewing - ethanol makes beer and wine alcoholic

•  Fermentation is anaerobic - no oxygen needed

•  Does not produce any additional ATP

•  Aerobic respiration

•  Glycolysis (and fermentation) is very inefficient - does not make much ATP for each glucose molecule used.

•  Pyruvic acid still contains much potential energy.

•  If pyruvic acid can broken down further, much more ATP can be produced, but this takes oxygen.

•  So aerobic respiration is much more efficient than fermentation.

•  You get many more ATPs per glucose oxidized.

•  The remainder of the reactions of aerobic respiration occur in the mitochondrion (in eukaryotes).

•  Pyruvic acid from glycolysis enters the mitochondrion.

•  There it is completely oxidized to carbon dioxide and water.

•  Lots more ATP is produced.

The mitochondrion

•  Nearly all eukaryotic cells contain mitochondria

•  Double membrane envelope

•  The outer membrane is smooth.

•  The inner membrane is folded (cristae) to increase its surface area.

•  Matrix

•  The space within the inner membrane is called the matrix

•  The matrix contains enzymes that oxidize pyruvic acid (citric acid cycle)

•  Cristae

•  Each fold of the inner membrane is called a crista

•  Cristae membranes have a high protein content

•  These proteins are involved in electron transport and phosphorylation, which makes most of the ATP

•  Mitochondrion as seen with the electron microscope

Electron micrograph of mitochondria with detail showing matrix and cristae

Pyruvic acid oxidation and the citric acid cycle (Krebs cycle)

•  Pyruvic acid enters the matrix of the mitochondrion from the cytosol.

•  It is oxidized to acetyl-Coenzyme A and CO2.

•  Acetyl-CoA is then oxidized to CO2 by the citric acid cycle.

•  This produces a small amount of ATP, but most of the energy released is now stored as NADH and FADH, which are used in electron transport.

•  Summary:

•  Pyruvic acid + ADP + phosphate + NAD + FAD -> 3 CO2 + ATP + NADH + FADH

•  Note that oxygen has still not been needed up to this point.

•  Details of citric acid cycle (review, but don't memorize!)

Krebs cycle
The reactions of the citric acid cycle (Krebs cycle)
Copyright McGraw-Hill companies

Electron transport and phosphorylation

•  In electron transport, electrons flow from NADH and FADH to oxygen.

•  The electron transport proteins are attached to the cristae membranes

•  The energy released from the electron flow drives ATPases also located on the cristae membranes.

•  Summary of electron transport

•  NADH + FADH + ADP + phosphate + O2 -> ATP (lots) + NAD + FAD + H2O

•  Electron transport causes hydrogen ions (H+) to be pumped across the cristae membranes from the matrix to the inter-membrane space.

•  This creates a gradient of H+ that provides the energy to make ATP

•  ATP-ases allow H+ to move back to the matrix. The release energy drives ATP synthesis

•  This mechanism is referred to as chemiosmosis.

•  Details of electron transport and ATP synthesis

Electron transport
The respiration electron transport system
Copyright McGraw-Hill companies

•  Electron transport (animation)

To play the animation, visit the Brooker textbook web site, respiration chapter then click "Electron transport" from the chapter contents. Although this animation shows how electron transport works in the plasma membrane of a bacterium, the process is basically similar in the cristae membranes of mitochondria of eukaryotic cells.

Overall process of respiration showing how main steps are connected

Respiration summary
A summary of the reactions of aerobic respiration
Copyright McGraw-Hill companies

Metabolic map (shown in lecture)

If you would like to study the metabolic map in detail, you can download it from the web using the link below. (WARNING, the file is a large [over 1MB] PDF file). This is "just for fun." You won't need to know any more details of the respiration pathway than we have discussed in class, but looking at the more complete process should give you an appreciation for how complex a cell really is!

Metabolic Pathways Map

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