IB Topic 3.7

Every living cell must carry out cell respiration Converts energy into a form that can be used within the cell Cells require energy for: Synthesizing large molecules like DNA, RNA, and proteins Pumping molecules or ions across membranes by active transport Moving things around inside the cell, such as chromosomes, vesicles or in muscle cells the protein fibers that cause muscle contraction

The energy for these processes is supplied by ATP (adenosine triphosphate) When ATP is split into ADP (adenosine diphosphate) and phosphate, energy is released ADP + phosphateATP Cell respiration Active cell processes

ATP cannot usually be absorbed through the plasma membrane So, every cell must produce its own supply Organic compounds containing energy are broken down by enzymes Very controlled So that as much as possible of the energy released can be used to form ATP from ADP Cell respiration defined as the controlled release of energy from organic compounds to form ATP

A: taking air into the lungs B: removing carbon dioxide from the lungs C: Use of ATP in cells D: Controlled release of energy in cells

Cell respiration can release energy form a variety of organic compounds, but carbohydrates and lipids are the usual substrates If glucose is being used, almost all organisms begin cell respiration the same way Before we get to the process, think about what the word means … Glyco = sugar Lysis = split

Overview: A chain of reactions takes place in the cytoplasm of the cell Converts glucose to pyruvate This chain of reactions is called glycolysis

Glucose enters the cell through the plasma membrane (into the cytoplasm) An enzyme modifies the glucose (slightly) A second enzyme modifies the molecule even more Followed by a series of reactions (biochemical …) Which you do not need to know Ultimately, the reactions cleave the 6-carbon glucose molecule into 2, 3-carbon molecules called pyruvate

Some of the covalent bonds in the glucose were broken Some of the energy that was released from the breaking of these bonds was used to form a small number of ATP molecules 2 ATP molecules are needed to get the reaction started A total of 4 ATP molecules are produced So there is a net gain of 2 ATP molecules

No oxygen is used in glycolysis A small amount of ATP is produced If no oxygen is available, then this is the only ATP that can be produced Anaerobic conditions Alcoholic fermentation Lactic acid fermentation

A: glucose B: glucose and ATP C: ATP and pyruvate D: glucose, ATP, and pyruvate

Cell respiration  variety of biochemical pathways that metabolize glucose All of the pathways start with glycolysis Common to all organisms Some organisms derive all of their ATP completely without oxygen (anaerobic) Also called fermentation Two main anaerobic pathways Alcoholic fermentation Lactic acid fermentation

Yeast – common, single-celled fungus that uses alcoholic fermentation for ATP generation Use glycolysis Produce 2 net ATP molecules Produce 2, 3-carbon pyruvate molecules Yeast then converts the pyruvate molecules to ethanol Ethanol is a 2-carbon molecule So a carbon molecules is “lost” Given off as carbon dioxide Baking – helps dough rise Ethanol is drinking alcohol

Glucose 6C Pyruvate 3C Ethanol 2C CO2 Glycolysis (net gain 2 ATP)

A: carbon dioxide and lactate B: carbon dioxide and ethanol C: Lactate and ethanol D: carbon dioxide, lactate, and ethanol

Organisms that use aerobic respiration sometimes find themselves in a metabolic situation where they cannot supply enough oxygen to their cells Example: a person pushing beyond their physical limits; their pulmonary & cardiovascular systems supply as much oxygen to their cells as is physically possible If a person’s exercise rate exceeds their capability of supplying oxygen, then some of the glucose entering the cell respiration will follow lactic acid fermentation

Low-oxygen setting (not a normal setting) Excess pyruvate molecules are converted into lactic acid molecules Lactic acid molecules 3-carbon molecules No production of carbon dioxide What’s the benefit? Allows glycolysis to continue with the small gain of ATP generated in addition to the ATP which is being generated through aerobic respiration

Glucose 6C Pyruvate 3C Lactate 3C Glycolysis (net gain 2 ATP) Aerobic pathway (Reaction reversible when oxygen available)

A: the person has been drinking too much milk and ingested large amounts of lactose B: the person is lactose intolerant and should change to drinking lactose-free milk C: the person has eaten live yogurt, containing bacteria that have carried out anaerobic cell respiration D: the person has exercised vigorously and carried out anaerobic cell respiration

Cell Respiration = glucose  pyruvate Occurs in almost all organisms Aerobic or anaerobic respiration Lactic Acid Fermentation = pyruvate  lactate Occurs in humans Occurs in some bacteria Anaerobic respiration Alcoholic Fermentation = pyruvate  ethanol + CO2 Occurs in yeast Anaerobic respiration ADPATP

The most efficient pathway Takes place in the mitochondria Begins with glycolysis 2 net ATP & 2 pyruvate molecules Enter the mitochondrion Further metabolized

Each pyruvate first loses a carbon dioxide molecule Becomes a molecule called acetyl-CoA Each acetyl-CoA enters into a series of reactions called the Krebs cycle During this series, two more CO2 molecules are produced from each original pyruvate that enter See Figure 3.24 in your textbook (page 82)

Some ATP is directly generated during the Krebs cycle Some ATP is indirectly generated through a later series of reactions directly involving oxygen Aerobic respiration breaks down (completely oxidizes) a glucose molecule and the end product are carbon dioxide and water In most organisms, carbon dioxide is a waste product that must be excreted The water is useful – in humans, about ½ a liter of water is produced per day Some desert animals never have to drink water and eat only dry foods because cell respiration supplies almost all their water needs

A: oxygen B: ATP C: Heat D: organic compounds

Anaerobic respiration does not completely oxidize the glucose molecule Ethanol and lactic acid are generated because they represent portions of the original glucose that were not oxidized Aerobic respiration leaves no such by-products Aerobic respiration results in a much higher yield of ATP