Unit 6- Cellular Respiration

Pathways the Harvest and Store Chemical Energy

Gibb’s Free Energy
- Photosynthesis: ΔG=686 kcal/mol
- Cellular respiration: ΔG-.686 kcal/mole

Energy metabolism:

• Five principles governing metabolic pathways:

  1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway

  2. EAch reaction is catalyzed by a specific enzyme

  3. Most metabolic pathways are similar in all organisms

  4. in eukaryotes. many metabolic pathways occur inside specific organelles

  5. each metabolic pathway is controlled by enzymes that can be inhibited or activated

Biological Energy

• For biological reactions, an exergonic reaction is coupled in time and location to an endergonic reaction

  • exergonic reaction: released energy, catabolism

  • endergonic reaction: requires energy, active transport, anabolism

• Two widely used coupling molecules are ATP and NADH

• NADH: Stands for “nicotinamide adenine dinucleotide (NAD) + hydrogen (H)

  • occurs naturally in the body and plays a role in generating energy

  • NADH produced by the body is involved in making energy in the body

  • NAD plays a crucial role in a wide range of other cellular reactions

  • Conversion of NAD from its oxidized form (NAD+) to its reduced form (NADH), and back, provides cell with a mechanism for accepting and donating electrons

  • Coenzyme NAD+ is a key electron carrier in redox reactions

    • NAD+ (oxidized form)

    • NADH (reduced form)

    • NADH carries 2 high energy electrons

• Redox reactions/ oxidation-reduction or redox reactions transfers electrons

  • can transfer energy by the transfer of electrons

  • oxidation an reduction always occur together

  • transfer of hydrogen atoms involve transfers of electrons (H=H+ + e-)

  • When a molecule loses a hydrogen atom, and with it an electron, it becomes oxidized

  • the more reduced a molecule is, the more energy is stored in its bonds

  • energy is transferred in a redox reaction

  • energy in the reducing agent is transferred to the reduced product

Energy Conversion

• cellular respiration is a major catabolic pathway. Glucose is oxidized

• photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy and carbon dioxide is reduced

Cellular Respiration

• 4 steps

  • 1. Glycolysis

    • anaerobic

    • cytoplasm

  • 2. Pyruvate oxidation

    • eurobic

    • mitochondrial matrix

  • 3. Citric Acid cycle

    • aerobic

    • mitochondrial matrix

  • 4. Electron transport chain

    • aerobic

    • mitochondrial inner membrane

• A lot of energy is released when reduced molecules with many c-c and c-h bonds are fully oxidized to CO2

• The oxidation of glucose occurs in a series of small steps in three pathways

  • glycolysis

  • pyruvate oxidation

  • citric acid cycle

• 1. Glycolysis

  • ten reactions

  • takes place in the cytosol and is anaerobic

  • Final products:

    • 2 molecules of pyruvate (acid)

      • further oxidized in the citric acid cycle

    • net gain of 2 molecules of ATP

      • used by the cell

    • 2 molecules of NADH

      • used in the ETC (electron transport chain)

• Glycolysis: conversion of 1 molecule of glucose in 2 molecules of pyruvate

  1. Hydrolyzes ATP

     3. Hydrolyzes another molecule of ATP

        6. Redox reaction

        7. Substrate level phosphorylation

• Substrate-level phosphorylation metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphate group to ADP or GDP from another phosphorylate

After glycolysis

• Oxygen is required for the final steps of cellular respiration

• because the pathways of cellular respiration require oxygen, they are aerobic

• in the presence of oxygen, pyruvate produced in glycolysis moves into the mitochondria, where it is oxidized to acetate + coenzyme A —> Acetyl-CoA

• Acetyl-CoA enters the Citric Acid Cycle (Kreb's Cycle)

• Both pyruvate oxidation and the citric acid cycle takes place in the matrix of the mitochondria

  2. Pyruvate Oxidation

  • Products: CO2 and AcetylCoA; acetate is bound to form coenzyme A (coA)

    • acetyl CoA

  • Oxidation of (2) pyruvate INTO (2) acetyl CoA and (2) CO_2

    • 1 redox reaction reaction per pyruvate

    • 1 CO2 is released per pyruvate

    • Coenzyme A makes acetate reactive to enter the citric acid cycle

• 3. Citric Acid Cycle (Krebs cycle)

  • 8 reactions, operates twice for every glucose molecule that enters glycolysis

    • Starts with Acetyl CoA; acetyl group is oxidized to two CO2

    • Oxaloacetate is regenerated in the last step

  • Oxidation of (2) Acetyl CoA to (4) CO_2

    • 4 redox reactions per citrate: 3 NADH and 1 FADH_2 produced per citrate

    • 2CO_2 is released per citrate

    • Substrate-level phosphorylation: 1 ATP produced per citrate

  • Mitochondrial matrix

  • GTP and GDP

    • GTP- Guanine + Ribose + 3p

    • GDP- Guanine + Ribose + 2p

• 4. ETC (Electron Transport Chain)

  • AKA ATP Synthesis

  • matrix to mitochondrial inner membrane

  • NADH is re-oxidized to NAD+ and O2 is reduced to H2O in a series of steps

  • Oxidative Phosphorylation

    • Oxidative phosphorylation uses the high-energy electrons from the citric acid cycle and glycolysis to convert ADP → ATP

    • NADH and FADH_2 molecules are oxidized by O_2 (final electron acceptor)

    • As the high energy electrons pass from one complex to the other, they power the H+ pump

      • this pump creates a proton gradient

    • H+ pass back into the matrix through the ATP-synthase and ADP is converted in ATP

    • Occurs in inner membrane of mitochondria

    • O_2 required to be the final electron acceptor

  • Respiratory chain: series of redox carrier proteins embedded in the inner mitochondrial membrane

  • electron transport: electrons from the oxidation of NADH and FADH_2 pass from one carrier to the next in the chain

  • Chemiosmosis: diffusion of protons across a membrane which drives the synthesis of ATP

    • Chemiosmosis converts potential energy of a proton gradient across a membrane into the chemical energy in ATP

    • In oxidative phosphorylation, electron transport is coupled with chemiosmosis to produce ATP

  • ATP Synthase: membrane protein with two subunits:

    • F_0 is the H_ channel; potential energy of the proton gradient drives H+ through

    • F_1 has active sites for ATP synthesis

about 32 molecules of ATP are produced for each fully oxidized glucose depending on cell type and environmental conditions

Breakdown of other compounds

• Carbohydrates:

  • Glycogen: break down to glucose

• Proteins:

  • Proteins break down to amino acids → amino groups must be removed from amino acids

    • deamination creates nitrogenous waste (ammonia or urea)

      • ammonia is produced from leftover amino acids, must be removed from the body. liver produces several chemicals (enzymes) that change ammonia into urea (contained in urine)

• Fats:

  • Glycerol is converted into glyveraldehyde 3-phosphate, an intermediate in glycolysis

  • fatty acid tails hold the majority of the energy

    • beta oxidation breaks the fatty acids down to 2 carbon molecules

    • the two carbon molecules enter the citric acid cycle as acetyl CoA

    • Beta oxidation produces NADH and FADH2, which go to the electron transport chain

      • one gram of fat produces twice as much ATP as a gram of carbohydrate

Catabolism and anabolism

• Anabolism:

  • many catabolic pathways can operate in reverse

  • Gluconeogenesis: citric acid cycle and glycolysis intermediates can be reduced to form glucose

  • Acetyl CoA can be used to form fatty acids

  • some citric acid intermediates can form nucleic acids

• Amounts of different molecules are maintained at fairly constant levels- the metabolic pools

  • accomplished bu regulation of enzymes, allosteric regulation, feedback inhibition

  • enzymes can also be regulated by altering the transcription of genes that encode the enzymes

• Phosphofructokinase:

  • inhibited by ATP

  • ACtivated my AMP

  • Inhibited by citrate

  • Coordinates glycolysis and the citric acid cycle

• Pyruvate oxidation is regulated by the amount of acetyl CoA present

• The citric acid cycle is predominantly regulated by the amount of ATP and NADH present

Fermentation

• Under anaerobic conditions, NADH is re-oxidized by fermentation

• There are many different types of fermentation, but all operate to regenerate NAD+

• The overall yield of ATP is only 2- the atp made in glycolysis

Lactic Acid Fermentation:

• end product is lactic acid (lactate) and ATP

• NADH is used to reduce pyruvate → lactic acid, this regenerating NAD+

Alcoholic Fermentation

• End product is ethyl alcohol (ethanol)

• Pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD+ for glycolysis

Anaerobic Respiration

• Different from fermentation

• Pyruvate is reduced to lactate or to ethanol in fermentation (after being converted to acetaldehyde, with release of CO2)

• The reducing agent is NADH that is oxidized back to NAD+

• Anaerobic respiration is similar to aerobic respiration, but the final electron acceptor is sulfate, fumarate, sulfur, or nitrate instead of oxygen. Yields LESS ATP