Metabolism, Glycolysis, Fermentation, and Cellular Respiration

General Overview of Metabolism

• Cells harvest chemical energy from organic molecules to generate ATP via catabolic pathways.

• Electron transfer during chemical reactions releases energy stored in organic molecules, which is then used to synthesize ATP.

• Cells continuously regenerate ATP from ADP to perform work.

• ATP is used in anabolic pathways and to drive movement.

• Catabolic pathways break down food, releasing energy and heat, and producing cellular building blocks.

• These building blocks are then used in Anabolic pathways which builds macromolecules.

Relocation of Electrons

• Electron transfers are called oxidation-reduction reactions (redox reactions).

• Oxidation: loss of electrons.

• Reduction: gain of electrons.

• Oxidation and reduction always occur together.

• Reducing agent: electron donor; it reduces the electron acceptor.

• Oxidizing agent: electron acceptor; it oxidizes the electron donor.

• Some redox reactions change electron sharing in covalent bonds instead of fully transferring electrons.

• Oxygen atoms are highly electronegative and attract electrons.

• Partial "gain" of electrons by oxygen and partial "loss" by bonding partners constitutes a redox reaction.

• Oxidation can occur by:

  • Removing a hydrogen atom

  • Adding an oxygen atom

  • Direct electron transfer (involves metal ions)

Catabolic Pathways

• Catabolic pathways are oxidative pathways.

• Electrons are relocated to other molecules, which are reduced (e.g., NAD^+ to NADH).

• NADH is then re-oxidized to produce ATP.

• This can occur anaerobically or aerobically via the electron transport chain (requires oxygen – cellular respiration).

• Dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate.

• Two electrons and one proton are transferred to NAD^+ forming NADH.

• The other proton is released as a hydrogen ion (H^+) into the surrounding solution.

• NADH passes electrons to the electron transport chain, where they are transferred in a series of redox reactions, each releasing a small amount of energy.

• O2, the final electron acceptor, captures the electrons and hydrogen nuclei (H^+, forming H2O).

• The energy yielded is used to regenerate ATP.

Overview of Glycolysis

• Glycolysis: a nearly universal pathway in living cells.

• Its regulation is well understood.

• It plays a central metabolic role in generating both energy and metabolic intermediates for other pathways.

• Early prokaryotes likely used glycolysis to produce ATP before oxygen accumulated in the atmosphere.

• Used in both cellular respiration and fermentation, making it the most widespread metabolic pathway on Earth.

• It is a metabolic heirloom from early cells that continues to function in fermentation and cellular respiration

• Glycolysis is a 10-step pathway.

• One molecule of glucose is converted to two molecules of pyruvate.

• Two distinct phases:

  • Energy investment phase (5 reactions): 2 ATP are used; a 6-carbon molecule is split into two 3-carbon sugar phosphates.

  • Energy generation (payoff) phase (5 reactions): two triose phosphates are converted to energy-rich compounds; these transfer 4 phosphates to ADP.

• Net yield:

  • 2 ATP

  • 2 moles of pyruvate

  • 2 reducing equivalents – NADH

• The fate of pyruvate depends on whether conditions are aerobic or anaerobic.

• All of the carbon originally present in glucose is accounted for in the two molecules of pyruvate.

• Glycolysis does not release any CO2, and occurs whether or not O2 is present.

• Net inputs and outputs:

  • Glucose

  • 4 ATP formed - 2 ATP used = 2 ATP

  • 2 NAD^+ + 4 e^- + 4 H^+ = 2 NADH + 2 H^+

  • 2 Pyruvate + 2 H_2O

Fermentation

• Glycolysis is an ancient metabolic pathway that probably evolved before the earliest known photosynthetic organisms began contributing oxygen to the Earth’s atmosphere.

• Glycolysis had to function under anaerobic conditions.

• For the pathway to continue operating anaerobically, NADH must be re-oxidized to NAD^+ by transferring electrons to an electron acceptor to maintain a steady-state concentration of NAD^+.

• Fermentations use a common strategy to regenerate oxidized NAD^+.

• Fermentation is an energy-yielding metabolic pathway with no net change in the oxidation state of the products compared to that of the substrates.

• Pyruvate is reduced in the final step – so there is no net oxidation of glucose.

• Dozens of different fermentation processes have been discovered in different organisms, many of them play major roles in the food and chemical industries.

• Alcoholic fermentation by yeasts is an important process in brewing, wine-making, and baking, generating ethanol and CO_2.

• In baking, the CO_2 causes bread to rise, while the ethanol evaporates.

• Other useful fermentations lead to acetic acid (vinegar manufacture) and propionic acid (Swiss cheese manufacture).

• Alcohol fermentation:

  • Pyruvate is converted to ethanol in two steps.

  • The first step releases CO_2 from pyruvate.

  • The second step produces NAD^+ and ethanol.

• Lactic acid fermentation:

  • Pyruvate is reduced directly by NADH to form lactate and NAD^+.

  • There is no release of CO_2 in lactic acid fermentation.

  • Lactic acid fermentation by fungi and bacteria is used to make cheese and yogurt.

Other Fates of Pyruvate

• Aerobic respiration:

  • When oxygen is present, pyruvate in eukaryotic cells enters a mitochondrion, where the oxidation of glucose is completed.

  • In aerobically respiring prokaryotic cells, this process occurs in the cytosol.

• Anaerobic respiration:

  • Takes place in certain prokaryotic organisms that live in environments without oxygen.

  • These organisms have an electron transport chain but do not use oxygen as a final electron acceptor.

  • Some “sulfate-reducing” marine bacteria use SO4^{2-} at the end of their respiratory chain resulting in H2S instead of H20

Anaerobes

• Obligate anaerobes:

  • Use only fermentation or anaerobic respiration.

  • Cannot survive in the presence of oxygen.

• Facultative anaerobes:

  • Examples: yeasts and many bacteria (consume sugar much faster when fermenting than when respiring).

  • On a cellular level – muscle cells behave as facultative anaerobes – form lactic acid when not sufficient oxygen for aerobic respiration.

  • A complex series of fermentation and aerobic respiration carried out by yeasts and bacteria on cacao beans is responsible for chocolate production.

• Though previously thought that human muscle cells produced lactate only when O_2 was in short supply, recent research indicates a more complicated story

  • One type of skeletal muscle (red) oxidizes glucose completely to CO_2; the other (white) produces lactate even under aerobic conditions

  • The term fermentation is no longer used because lactate production is not anaerobic

Aerobic Respiration

• Most of the energy from glycolysis remains stockpiled in the 2 molecules of pyruvate.

• Pyruvate enters the mitochondrion via active transport.

• Pyruvate is converted to acetyl CoA by a multienzyme complex that catalyzes 3 reactions:

Oxidation of Pyruvate to Acetyl CoA

• Pyruvate is converted to acetyl coenzyme A (acetyl CoA) before entering the citric acid cycle.

• Pyruvate dehydrogenase catalyzes three reactions:

  1. Oxidation of pyruvate’s carboxyl group, releasing the first CO_2 of cellular respiration.

  2. Reduction of NAD^+ to NADH.

  3. Combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA.

Stages of Cellular Respiration

• Harvesting energy from glucose by cellular respiration has three stages:

  1. Glycolysis: breaks down glucose into two molecules of pyruvate.

  2. Pyruvate oxidation and the citric acid cycle: complete the breakdown of glucose to CO_2.

  3. Oxidative phosphorylation: the electron transfer chain and chemiosmosis facilitate synthesis of most of the cell’s ATP.

ATP Production

• Oxidative phosphorylation generates almost 90% of the ATP and is powered by redox reactions.

• Some ATP is also formed in glycolysis and the citric acid cycle by substrate-level phosphorylation.

• Substrate-level phosphorylation occurs when an enzyme transfers a phosphate group directly from a substrate to ADP.

• For each molecule of glucose degraded to CO2 and H2O by cellular respiration, up to 32 molecules of ATP are produced.

The Citric Acid Cycle

• The citric acid cycle, also called the Krebs cycle, or the tricarboxylic acid cycle, oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH_2 per turn.

• Another 2 CO_2 are produced as a waste product.

• Because 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule consumed.

• 2 C compound + 4 C compound = 6 C compound.

• It’s a cycle, so need to end up with 4 C compound (oxaloacetate).

• During the cycle, 2 carbons are lost as CO_2.

• The citric acid cycle has eight steps, each catalyzed by a specific enzyme.

• First, the acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate.

• The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle.

• The NADH and FADH_2 produced by the cycle carry electrons to the electron transport chain.

Oxidative Phosphorylation

• Molecules of NADH and FADH_2 produced during glycolysis and the citric acid cycle account for most of the energy extracted from glucose.

• NADH and FADH_2 donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.

• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis.

• Molecules of the electron transport chain are embedded in the inner mitochondrial membrane in eukaryotic cells.

• The membrane is folded into cristae to increase surface area for electron transport chains.

• In prokaryotes, the electron transport chain is embedded in the plasma membrane.

• Most of the molecules in the electron transport chain are proteins, which exist in multi-protein complexes.

• NADH and FADH_2 donate electrons to different electron acceptors early in the chain.

• Electrons are passed through a number of carrier molecules including several cytochromes (proteins with heme groups containing an iron atom).

• Electron carriers alternate between reduced and oxidized states as they accept and donate electrons.

• Electrons drop in free energy as they are transferred down the chain, finally passing to O2 to form H2O.

• The electron transport chain breaks the large free-energy drop from glucose to O_2 into smaller steps, releasing energy in manageable amounts.

• No ATP is produced directly by the chain.

• If NADH transferred electrons directly to oxygen, energy would be released in one explosive reaction.

• Instead, cellular respiration uses an electron transport chain to break the fall of electrons to O_2 into several energy-releasing steps.

• An electron transport chain consists of a series of molecules built into the inner membrane of the mitochondria (or plasma membrane of prokaryotes).

• Certain electron carriers in the electron transport chain accept and release H^+ along with the electrons.

• Maintaining the H^+ gradient couples redox reactions of the electron transport chain to ATP synthesis.

• The H^+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work.

• During cellular respiration, most energy flows in this sequence: glucose → NADH → electron transport chain → proton-motive force → ATP.

• About 34% of the energy in a glucose molecule is transferred to ATP, making about 32 ATP.

• The rest of the energy is lost as heat.

• There are three reasons why the exact number of ATP produced is not known:

  1. Photophosphorylation and the redox reactions are not directly coupled; the ratio of NADH to ATP molecules is not a whole number.

  2. ATP yield varies depending on whether electrons are passed to NAD^+ or FAD.

  3. The proton-motive force is also used to drive other kinds of work.

• The energy released as electrons are passed down the electron transport chain is used to pump H^+ from the mitochondrial matrix to the intermembrane space.

• H^+ then moves down its concentration gradient back across the membrane, passing through the protein complex ATP synthase.

• H^+ moves into binding sites on the rotor of ATP synthase, causing it to spin in a way that catalyzes phosphorylation of ADP to ATP.

• This is an example of chemiosmosis, the use of energy in a H^+ gradient to drive cellular work.

Glycolysis and the Citric Acid Cycle

• Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways.

• Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration.

• Glycolysis can use many carbohydrates, including starch, glycogen, and several disaccharides.

• Proteins used for fuel must be digested to amino acids, and their amino groups must be removed in a process called deamination.

• Nitrogenous waste is excreted as ammonia (NH_3), urea, or other products.

• Fats are digested to glycerol (used to produce compounds needed for glycolysis) and fatty acids.

• Fatty acids are broken down by beta oxidation and yield acetyl CoA, NADH, and FADH_2.

• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate.

• Organisms use small molecules from food to build macromolecules, such as proteins from amino acids.

• These small molecules may come directly from food, from glycolysis, or from the citric acid cycle.

Regulation of Cellular Respiration

• Feedback inhibition is the most common mechanism for metabolic control because it prevents wasteful production.

• If ATP concentration drops, respiration speeds up; if there is plenty of ATP, respiration slows down.

• Catabolism is controlled by regulating the activity of enzymes at strategic points in the pathway.