3.7 Cell Respiration 2/11/11 9:56 AM

Topic 3: Chemistry of Life3.7 Cell Respiration

3.7.1 Define cell respiration cell respiration is the controlled release of energy from organic compounds in cells to form ATP.

3.7.2 State that, in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP.

3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be converted in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further yield of ATP.

3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken down in the mitochondrion into carbon dioxide and water with a large yield of ATP.

3.7.1 Respiration 2/11/11 9:56 AM

3.7.1 Define cell respiration cell respiration is the controlled release of energy from organic compounds in cells to form ATP.

Orange book pg. 83Green book pg. 47

To do:Watch the animation: “The Big Picture”http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.htmlCopy both the word equation and balanced formula equation for respiration into your greenbooks.Summarise in you own words the purpose of respiration and why enzymes are essential to the process.

Overview of RespirationThe equation for cellular respiration is usually simplified to:

glucose + oxygen carbon dioxide + water (+ energy)C6H12O6 + 6O2 6CO2 + 6H2O (+ energy)

Organic molecules contain energy in their molecular structures. Each covalent bond in a glucose, amino acid or fatty acid represents stored chemical energy. The function of respiration is not to produce carbon dioxide and water but to transfer this energy in glucose to ATP.

If the reactions were carried out in a single step it would generate a short, intense burst of heat – like burning paper, which has the same chemical equation! Not only would this be useless to the body’s metabolism, it would kill the cells.

In the body, the process is carried out in a series of small steps, each controlled by a separate enzyme. This slows oxidation. Energy is released in small manageable amounts and as much as possible is transferred to ATP.

If a cell does not have glucose available, other organic molecules may be substituted, such as fatty acids or amino acids.

Linking respiration and photosynthesisPhotosynthesis and respiration are the reverse of each other, and you couldn’t have one without the other. The net result of all the photosynthesis and respiration by living organisms is the conversion of light energy to heat energy.

The energy released within cells is needed for active transport, movement and the maintenance of body temperature. Biosynthesis is the making of biological molecules e.g. the synthesis of proteins from amino acids.

3.7.2 Glycolysis 2/11/11 9:56 AM

3.7.2 State that, in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP.

Orange book 84Green book 47

To do:Watch the animation: “Glycolysis in 3 Steps”(at this stage do not worry about names)http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.htmlRead the section below “The Basics”. In your green book, using two bullet points only, give a basic description of what occurs during glycolysis.Watch the animation: “Glycolysis”http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.htmlGo through the powerpoint presentation: “Glycolysis” (as a class).From what you have studied, draw a flow chart to show the process of glycolysis in your greenbooks to include: glucose (6C) ATP Pyruvate (3C)

The BasicsAssuming that glucose is the organic nutrient being metabolised, all cells begin the process of cell respiration in the same way. Glucose enters a cell through the plasma membrane and floats in the cytoplasm. The process of glycolysis occurs here in the cytoplasm.

During glycolysis, chemical reactions controlled by enzymes split a six-carbon molecule of glucose (hexose) into two three-carbon molecules of pyruvate.

Even though glycolysis consumes two ATP molecules, it produces four ATP molecules, for a net gain of two ATP molecules.

Study the diagram below and you should recognise the following key points: Glucose (6C) is the starting point 2 x ATP are needed to start the reaction 2 x pyruvate (3C) are the products 4 x ATP are produced (which is a net gain of 2 x ATP, due the two that

were needed initially).

3.7.3 Anaerobic Respiration 2/11/11 9:56 AM

3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be converted in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further yield of ATP.

Orange book pg. 84Green book pg. 47

To do:Read the ‘Introduction’ below and in your greenbooks write a summary to include: how all respiration pathways start using an example, explain under which circumstances anaerobic

respiration will occur.

Read the paragraph on “Lactate Fermentation” and in your green books draw and annotate a flow diagram to show lactate fermentation.

Read the paragraph on “Alcoholic Fermentation” and in your green books draw and annoate a flow diagram to show alcoholic fermentation.

IntroductionThe term ‘cell respiration’ refers to a variety of biochemical pathways that can be used to metabolize glucose. All of the pathways start with glycolysis. In other words, glycolysis is the metabolic pathway that is common to all organisms. There are two types of cellular respiration: aerobic (in the presence of oxygen) and anaerobic or fermentation (in the absence of oxygen).

If oxygen is present, the products of glycolysis will enter the aerobic pathway.

Aerobic respiration produces a much larger amount of ATP, (up to 20 time more ATP).

There are two types of anaerobic respiration or fermentation: lactic acid fermentation and alcoholic fermentation.

Lactate FermentationOrganisms that use an aerobic cell respiration pathway sometimes find themselves in a metabolic situation where they cannot supply enough oxygen to their cells. A good example of this is a person pushing beyond their normal exercise pattern. In this situation, the person’s pulmonary and cardiovascular systems supply as much oxygen to their cells as is physically possible. If the person’s exercise rate exceeds their capability of supplying oxygen, then at least some of the glucose entering into cell respiration will follow the anaerobic pathway called lactic acid fermentation.

Once again, remember that glycolysis is used by all cells to begin the cell respiration sequence. Also remember that glycolysis: Takes place in the cytoplasm Results in the net gain of 2 x ATP per glucose Results in 2 x pyruvate

Cells that are aerobic normally take the two pyruvate molecules and further metabolize them in an aerobic series of reactions (covered in HL). In a cell that is not receiving sufficient oxygen for the aerobic pathway, it will instead enter the anaerobic pathway. In low-oxygen situations, excess pyruvate molecules are converted to lactate. Like pyruvate, lactic acid molecules are 3C so there is NO production of CO2.

What benefit does this serve?It allows glycolysis to continue with the small gain of ATP generated.The conversion of pyruvate to lactic acid is reversible and so the lactate can be converted back to pyruvate to enter the aerobic pathway when oxygen is available.

Alcoholic FermentationYeast is a common single-celled fungus that uses alcoholic fermentation for ATP generation. Remember that all organisms use glycolysis to begin the cell respiration sequence. Yeast cells take glucose from their environment and generate a net gain of 2 x ATP by way of glycolysis. The product of glycolysis is 2 x pyruvate molecules. Yeast then converts both of the 3C pyruvates to molecules of ethanol. Ethanol is 2C, so a carbon atom in ‘lost’ in this conversion. The ‘lost’ carbon atom is given off in a carbon dioxide molecule. Both the ethanol and carbon dioxide that are produced are waste produces to the yeast and are simply given off into the environment.

Bakers; yeast is added to bread products for baking as the generation of carbon dioxide helps the dough to rise. It is also common to use yeast in the production of ethanol as drinking alcohol.

3.7.4 Aerobic Respiration 2/11/11 9:56 AM

3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken down in the mitochondrion into carbon dioxide and water with a large yield of ATP.

Orange book pg. 86Green book pg. 47

To do:Answer the following question to make a summary in you greenbooks”1. Where is the pyruvate from glycolysis further oxidized?2. What are the end products of aerobic respiration?3. Explain why aerobic respiration releases more energy than anaerobic.4. Find out the total number of ATP produced in Aerobic respiration compared to Anaerobic.

Draw a simple flow diagram to show what happens to pyruvate in the Link Reaction and the Krebs cycle.

Aerobic cell respiration is the most efficient pathwayCells that have mitochondria use an aerobic pathway for cell respiration.

The diagram below is a high-resolution, false-colour SEM showing a single mitochondrion. Any cell containing mitochondria uses aerobic cell respiration as its primary cell respiration pathway when oxygen is available.

Aerobic respiration starts with glycolysis and thus a net gain of 2 x ATP is generated as well as 2 x pyruvate. The 2 x pyruvate molecules now enter a mitochondria and are further metabolized.

Link Reaction Each pyruvate first loses a carbon dioxide molecule and becomes

known as acetyl (2C). It is picked up by Co-Enzyme A so the resulting molecule is called acetyl-CoA.

Each acetyl-CoA carries the acetyl to the mitochondria and the acetyl (2C) enters the Krebs cycle.

Krebs Cycle The acetyl (2C) is picked up by a 4C molecule to form a 6C it then loses

CO2 to form a 5C and loses an addition CO2 to form the 4C again.

Hence two more CO2 molecules are produced from each original pyruvate.

It is a cycle as the 4C molecule in the Kreb cycle goes round in a circle undergoing changes and eventually picking up the acetyl (2C) molecule again as it enters the cycle.

Some ATP is directly generated during the Krebs cycle.

OutcomeAerobic respiration completely oxidizes (breaks down) a glucose molecule and the end-products are carbon dioxide and water.

The reason aerobic respiration is so much more efficient than anaerobic respiration is that in anaerobic respiration is due to glucose not being completely broken down.