Chapter 9 Cellular Respiration

Cellular respiration-stepwise series of catabolic reactions that break down complex molecules to CO2 and H2O and release energy.

  • These reactions occur in a controlled manner
  • They are catalyzed by enzymes at every step
  • Some of the energy from these reactions is stored in ATP

Formula for respiration:

C6H12O6 + 6O2------6CO2 + 6H2O + energy

(enzymes)

Single reactions in a cell do not use glucose as a direct source of energy because glucose contains too much energy for one reaction.

THREE METABOLIC STAGES OF CELLULAR RESPIRATION

  • Glycolysis-occurs in cytosol
  • Krebs (Citric Acid) Cycle (TAC cycle) –occurs in mitochondrial matrix
  • Electron Transport Chain and Oxidative phosphorylation- occurs in inner mitochondrial membrane.

Redox reactions: a chemical reaction in which there is a transfer of one or more electrons from one reactant to another (Oxidation-reduction reactions). Be sure to know terms: oxidation and reduction, reducing agent and oxidizing agent)

In respiration Glucose is oxidized to carbon dioxide and oxygen is reduced to water.

For each molecule of glucose broken down into CO2 and H2O the cell makes about 36 to 38 ATP molecules.

  1. Glycolysis- In several steps, glucose (a 6-carbon sugar) splits into two 3-carbon molecules of pyruvic acid and a small amount of ATP forms.
  2. Krebs Cycle (Citric Acid Cycle)-The two molecules of pyruvic acid are broken down to CO2 and additional ATP forms.

Glycolysis and Krebs cycle reactions release hydrogen atoms. Special molecules carry these hydrogen atoms to the third stage of respiration.

  1. Electron Transport Chain-Most ATP is synthesized in this step. Each hydrogen atom is separated into an electron and a proton (H+) and transferred through many small steps to oxygen, finally forming water.

ONLY THE VERY LAST STEP IN CELLULAR RESPIRATION INVOLVES OXYGEN.

Aerobic respiration- with oxygen

Anaerobic respiration-without oxygen

The hydrogen carriers are NADH AND FADH2.

Hydrogen carried by NADH AND FADH2is used to synthesize ATP in the ETC.

The Reactions of Respiration

A. Glycolysis

  1. Molecules of glucose enter a cell.
  2. An enzyme converts glucose to glucose-6-phosphate (ATP provides the phosphate and the free energy to power the reaction).
  3. Another enzyme rearranges the glucose-6-phosphate and a second molecule of ATP donates a phosphate group.
  4. The molecule splits into two 3-carbon phosphates.
  5. The two 3-carbon phosphates go through a series of rearrangements (each catalyzed by specific enzyme).
  6. Four molecules of ATP, two molecules of NADH, and 2 molecules of 3-carbon pyruvic acid are formed.

THE RESULT IS A NET GAIN OF 2 ATP MOLECULES SINCE 2 ATP MOLECULES USED TO BEGIN GLYCOLYSIS.

SUMMARY-The role of glycolysis is to produce:

  • ATP
  • NADH
  • Pyruvic Acid
  • and form carbon skeletons.

AT THE END OF GLYCOLYSIS THE PRESENCE OF ABSENCE OF OXYGEN DICTATES THE FATE OF PYRUVIC ACID.

  • LITTLE OR NO OXYGEN-Lactic acid fermentation occurs. In fermentation animal cells can convert NADH and Pyruvic Acid into NAD+ and Lactate (NAD+ cycles back to glycolysis and the pathway continues to provide a small amount of ATP until oxygen becomes available again.

In plants and bacterial cells fermentation often results in the production of alcohol instead of

lactate. FERMENTATION IS ANAEROBIC RESPIRATION.

IN PLANTSIN ANIMALS

  • OXYGEN PRESENT-Pyruvic acid is transported to the mitochondria.

A complex enzyme system changes pyruvic acid into acetate (2-carbon acid) and CO2. This

step also produces a molecule of NADH from NAD+.

MITOCHONDRIA-Special compartment that house the Krebs Cycle and the ETC.

  • The inner mitochondrial membrane contains so many enzymes that it is more protein than lipid.
  • The outer mitochondria membrane regulates what molecules enter and leave.
  • Cristae-has enzymes of the ETC, the enzymes for ATP formation, and some of the enzymes of the Krebs Cycle (most of the Kreb Cycle enzymes re in the matrix.

LINKING STEP: CONVERSION OF PYRUVATE TO ACETYL CoA: -one Acetyl CoA produced plus one carbon dioxide molecule released , one NADH produced.

KREBS CYCLE

  1. An enzyme combines the acetyl group of acetyl CoA with a 4-carbon acid (Oxaloacetate) making a 6-carbon acid (Citric Acid) and releasing CoA.
  2. In several steps other enzymes rearrange the 6-carbon acid and convert it into a 5-carbon acid (Ketoglutarate). These reactions release a molecule of CO2 and form a molecule of NADH from NAD+.
  3. An enzyme converts the 5-carbon acid into a 4-carbon acid and releases a molecule of CO2 and forms a molecule of NADH.
  4. Enzymes rearrange the 4-carbon acid two times forming one molecule of ATP and FADH2.
  5. Enzymes convert the rearranged 4-carbon acid into Oxaloacetate and form a molecule of NADH.
  6. The cycle continues as Oxaloacetate enters step A.
PRODUCTS OF THE KREBS CYCLE
Reaction / CO2 / NADH / FADH2 / ATP

Pyruvic acid to acetyl CoA

/ 1 / 1 / 0 / 0
Krebs Cycle / 2 / 3 / 1 / 1
TOTAL for linking step & one turn of Krebs cycle / 3 / 4 / 1 / 1
X 2 (per glucose to pyruvate) / 6 / 8 / 2 / 2