Untitled

Chapter 13: How Cells Obtain Energy from Food

Overview

• This chapter discusses the following primary concepts:
  • The breakdown and utilization of food
  • Regulation of metabolism
  • Major biosynthetic pathways:
  • Glycolysis
  • Citric acid cycle
  • Oxidative phosphorylation (electron transport chain)

Food Molecule Breakdown Stages

• Food molecules undergo breakdown in three main stages:
  1. Glycolysis
  • Extracts energy from the splitting of glucose into pyruvate.
  • Requires an initial investment of ATP.
  • Produces both ATP and NADH.
  1. Citric Acid Cycle
  • (also known as the TCA cycle or Krebs cycle)
  • Converts Acetyl CoA into energy carriers (NADH, FADH2) while releasing CO2.
  1. Oxidative Phosphorylation
  • Utilizes high-energy electrons from NADH and FADH2 to synthesize ATP via the electron transport chain.

Glycolysis

• Location: Cytoplasm
• Glycolysis splits a molecule of glucose to form two molecules of pyruvate.
• Energy Investment:
  • The process requires an input of energy in the form of ATP, which is used at the start.
  • Subsequent phases yield 2 NADH and 2 ATP, thus recouping the initial energy investment.
• Fermentation:
  • In the absence of oxygen, pyruvate can undergo fermentation:
  • In Muscle Cells:
    • Pyruvate is converted to lactate to regenerate NAD+ needed for glycolysis.
  • In Microorganisms:
    • Pyruvate is converted to carbon dioxide and ethanol.

Intermediate Step

• Conversion of Pyruvate to Acetyl CoA:
  • In the mitochondrial matrix, pyruvate is converted into Acetyl CoA and CO2 by the Pyruvate Dehydrogenase Complex.
  • This enzyme complex catalyzes the removal of CO2 from pyruvate and generates NADH along with Acetyl CoA.

The Citric Acid Cycle

• Overview of Cycle:
  • Each turn of the cycle (per Acetyl CoA) yields:
  • 3 NADH
  • 1 FADH2
  • 2 CO2
  • 1 GTP (or ATP)
• The cycle includes a series of enzymatic reactions:
  • Step 1: Acetyl CoA combines with oxaloacetate to form citrate through action of Citrate Synthase.
  • Step 2: Citrate is isomerized to isocitrate by Aconitase via a dehydration-hydration reaction.
  • Step 3: Isocitrate is converted to α-ketoglutarate by Isocitrate Dehydrogenase, producing NADH and CO2.
  • Step 4: α-Ketoglutarate is oxidized to succinyl CoA by α-Ketoglutarate Dehydrogenase Complex, generating NADH and CO2.
  • Step 5: Succinyl CoA is converted to succinate while generating GTP (or ATP) by Succinyl CoA Synthetase.
  • Step 6: Succinate is oxidized to fumarate by Succinate Dehydrogenase, producing FADH2.
  • Step 7: Fumarate is hydrated to malate via Fumarase.
  • Step 8: Malate is oxidized back to oxaloacetate by Malate Dehydrogenase, producing NADH and completing the cycle.

Summary of the Citric Acid Cycle Steps

1. Citrate Synthase: Acetyl-CoA + Oxaloacetate + H2O → Citrate + CoA-SH
2. Aconitase: Citrate → Isocitrate
3. Isocitrate Dehydrogenase: Isocitrate + NAD+ → α-Ketoglutarate + CO2 + NADH + H+
4. α-Ketoglutarate Dehydrogenase Complex: α-Ketoglutarate + NAD+ + CoA-SH → Succinyl-CoA + CO2 + NADH + H+
5. Succinyl CoA Synthetase: Succinyl-CoA + GDP + Pi → Succinate + CoA-SH + GTP
6. Succinate Dehydrogenase: Succinate + FAD → Fumarate + FADH2
7. Fumarase: Fumarate + H2O → Malate
8. Malate Dehydrogenase: Malate + NAD+ → Oxaloacetate + NADH + H+

Oxidative Phosphorylation

• Definition: Oxidative phosphorylation is the process that completes the catabolism of food molecules and generates most of the ATP produced by the cell.
• High-energy electrons from NADH and FADH2 are transferred to an electron transport chain located in the inner mitochondrial membrane.
• This electron transfer (depicted by blue arrows) is coupled to the pumping of protons (H+) across the mitochondrial inner membrane, creating a proton gradient (depicted by red arrows).
• The resulting proton gradient facilitates ATP synthesis through the process of oxidative phosphorylation, harnessing the energy released by the electron transport chain.

Energy Storage Mechanisms

• Glycogen Storage:
  • Animals store glucose as glycogen in the liver.
  • This process is referred to as glycogenesis.
  • Glycogen can be broken down back to glucose in a process called glycogenolysis.
  • The formation of glucose from non-carbohydrate sources is called gluconeogenesis.
• Fat Storage:
  • Fats are stored in adipocytes (fat cells) in the form of fat droplets.
  • The storage of fats in adipocyte cells is referred to as lipogenesis.
  • The breakdown of stored fat for energy release (into fatty acids and glycerol) is called lipolysis.