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Lecture 10: Cells
Metabolism
: sum of all chemical reactions in an organism
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Two types of metabolic pathways
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Anabolism
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Larger molecules are assembled from smaller ones – Requires energy (ATP
input)
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Catabolism
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Larger molecules are broken down – Releases energy (ATP output)
Cells Use and Transform Matter and Energy
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Every chemical reaction requires a specific enzyme
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Metabolic activities of
a living cell require a lot of
energy
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Energy is often provided by the catabolism of ATP (energy “cash”)
Glucose Provides the Cell with Energy
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Energy in glucose is used to generate ATP
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One glucose molecule may yield 36 ATP
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In absence of glucose, other carbohydrates, fats, and proteins can be
catabolized to generate ATP
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ATP is a more readily used “pocket change” form of energy
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Glucose and other fuel molecules must be “cashed in” for ATP
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ATP can then be used to do cellular work
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Cellular respiration
: the breakdown of glucose in the presence of
oxygen to yield ATP
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Four stages of cellular respiration
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1.
Glycolysis (cytoplasm)
2.
Preparatory step (interior of mitochondria—matrix)
3.
Citric acid cycle (interior of mitochondria—matrix)
4.
Electron transport system (inner cell membrane of mitochondria)
Glycolysis: Glucose Is Split into Two Pyruvate Molecules
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Occurs in the cytoplasm
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Series of ten reactions that split glucose (six carbon sugar) into two
molecules of pyruvic acid (pyruvate, three carbon molecule)
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Requires investment of two ATP; four ATP are produced
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Net ATP yield: 2
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High-energy electrons and hydrogen ions are removed and picked up
by a coenzyme NAD+, forming NADH
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Later in cell respiration process, the NADH will be used to generate
additional ATP
The Preparatory Step: Pyruvate is Converted to Acetyl
CoA
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Pyruvate (from glycolysis) enters mitochondria
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Pyruvate is converted to acetyl group and CO2
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High-energy elections and hydrogen ions are removed and picked up by
coenzyme NAD+ forming NADH
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Acetyl group is joined to coenzyme A to from acetyl CoA
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Acetyl CoA will enter citric acid cycle
The Citric Acid Cycle (Krebs Cycle) Harvests Energy:
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Occurs in mitochondria
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Cyclic series of eight reactions which break down the acetyl CoA
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Molecule (oxaloacetate) that accepts acetyl CoA is regenerated at the end of
one full turn of the cycle
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High-energy electrons are extracted to form NADH and FADH2
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Produces two ATP and four CO2 per glucose molecule
The Electron Transport System Produces ATP:
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Located in inner mitochondrial membrane
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Takes electrons from NADH and FADH2
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Movement of electrons from one electron carrier to the next releases
energy that is harvested to generate ATP
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Final electron acceptor is O2, which forms water upon receiving
electrons and hydrogen ions
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ATP is generated by ATP synthase enzyme
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Process also known as oxidative phosphorylation
Summary of Energy Production from Glucose:
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Over 20 different enzyme-catalyzed reactions
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Approximately 36 ATP (net) produced from each molecule of glucose
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Oxygen (O2) consumed, carbon dioxide (CO2) produced
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Cellular respiration: cellular process that uses O2 and produces CO2 in
the process of making ATP
Fats and Proteins Are Additional Energy Sources:
Glycogen (storage form of glucose)
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Can be rapidly catabolized to glucose, which then
participates in cellular respiration – 1% of total energy reserves
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Fats: 78% of total energy reserves
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Triglycerides have twice the energy of carbohydrates
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Proteins: 21% of total energy reserves
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Have the same amount of energy as carbohydrates
Anaerobic Pathways Make Energy Available Without
Oxygen:
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Cellular respiration cannot continue in the absence of oxygen
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Glycolysis will continue, pyruvate will build up
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Pyruvate will be converted to lactic acid
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Lactic acid buildup in muscles will cause a burning sensation
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Two ATP produced per molecule of glucose
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When oxygen is available, lactic acid will be metabolized aerobically
