respiration (glycolysis, krebs)

What is cellular respiration

The process by which chemical energy in organic molecules is released by oxidation → Production of ATP

When is ATP required?

• Muscle contraction

• Active transport of substances into and out of cells

• Synthesis of substances for growth and repair

• Electrical transmission of nerve impulses

• Maintenance of constant body temperature

• Bioluminescence by fireflies

• Transcription, translation, photosynthesis

What are the stages of aerobic respiration?

• Glycolysis in cytosol

• Link reaction in mitochondrial matrix

• Krebs cycle in mitochondrial matrix

• Oxidative phosphorylation involving electron transport chain on cristae

Why is ATP a suitable energy source?

• Universally used by all cells and organisms

• Soluble, highly mobile, can be transported readily

• Easy interconversion and rapid turnover

• Intermediary between energy yielding and energy requiring reactions

What does glycolysis form?

• 2 pyruvate (3C) without the requirement and release of O₂

• 4 ATP (net gain of 2 due to initial investment) and 2 NADH (each G3P molecule contributes half)

What happens in glycolysis?

1. Phosphorylation of glucose

2. Lysis

3. Oxidation by dehydrogenation

4. Substrate-level phosphorylation

What happens at the during the start of phosphorylation of glucose?

• Initial investment of 2 ATP molecules

• 1 phosphate group from each ATP → Phosphorylate glucose → Produce fructose 1,6-biphosphate

What does phosphorylation of glucose do in glycolysis?

• Activates the sugar → Makes sugar more reactive and commits it to the glycolytic pathway

• Confers negative charge to glucose → Makes glucose impermeable → Cannot diffuse across cell membrane → Trapped within cytosol

What is phosphofructokinase (PFK)?

• Catalyses the addition of the 2nd phosphate group in phosphorylation of glucose

• Rate of glycolysis is regulated according to energy demand of cells

  • Inhibited by excess ATP or citrate in cell → End product inhibition

  • Stimulated by AMP and ADP (allosteric activators)

What happens during lysis?

• Fructose 1,6-biphosphate is split into 2 3C sugar phosphates

  • Glyceraldehyde-3-phosphate (G3P) / Triose phosphate (TP) / Phosphoglyceraldehyde (PGAL) [favoured, constantly removed by the next step of the reaction]

  • Dihydroxyacetone phosphate

• 2 sugar phosphates are isomers or each other and can be converted from one form to another by isomerase

What happens during oxidation by dehydrogenation?

• Highly exergonic redox reaction

  • G3P is oxidised by dehydrogenation

  • Coenzyme NAD⁺ is reduced to NADH

• Energy released is used to add a second phosphate group to G3P → 1,3-bisphosphoglycerate

What happens during substrate-level phosphorylation?

• 1,3-bisphosphoglycerate is dephosphorylated → Pyruvate

• 2 phosphate groups on 1,3-bisphosphogylcerate are transferred by enzymes to 2 ADP molecules → 2 ATP

How does pyruvate enter the mitochondrion for link reaction?

A transport protein embedded into the mitochondrial membrane translocates pyruvate from cytosol to mitochondrion via active process, consuming energy

What happens to pyruvate during link reaction?

Each pyruvate (3C) is decarboxylated → Removal of C from pyruvate through loss of CO₂

How is acetyl coenzyme A (acetyl CoA) formed in before Krebs Cycle?

• Oxidation by dehydrogenation occurs, yielding NADH and a 2C compound

• 2C compound combines with coenzyme A to form acetyl CoA

What happens at the start of Krebs Cycle?

Acetyl CoA (2C) enters the cycle by combining with oxaloacetate (4C) to form citrate (6C)

What happens to citrate in the Krebs Cycle?

• Citrate is decarboxylated and dehydrogenated to form alpha-ketoglutarate (5C) and NADH

• Oxidative decarboxylation results in a loss of carbon in the form of CO₂

What happens to oxaloacetate (4C) during Krebs Cycle?

• Combines with acetyl CoA to form citrate (6C)

• Regenerated through

  • 1 decarboxylation step to yield 1 CO₂

  • 3 dehydrogenation steps to yield 2 NADH, 1 FADH₂

  • 1 substrate-level phosphorylation to yield 1 ATP

How many rounds of Kreb cycle are needed to completely oxidize molecule of glucose?

• 2

• Glycolysis yields 2 pyruvate and hence 2 acetyl CoA

What happens to each carbon in 6C glucose molecule?

• All 6 carbons released

• 4 CO₂ released through Krebs Cycle

• 2 CO₂ released in link reaction

What are the products of glycolysis?

• 2 ATP

• 2 NADH

What are the products of link reaction?

• 2 NADH

• 2 CO₂

What are the products of Krebs Cycle?

• 2 ATP

• 6 NADH

• 2 FADH₂

• 4 CO₂