Cell Respiration

Energy Transformation from Food to Cells

• Introduction

  • The process of turning food into chemical energy recognizable and usable by cells involves several steps.

  • The first step is to break down the food into its component chemical compounds.

  • This initial breakdown is called digestion.

Digestion Process

• Objectives of Digestion

  • To remove carbohydrates and other molecules from consumed food.

  • To transport these nutrients into the bloodstream.

• Transport to Cells

  • Nutrients, particularly glucose (a carbohydrate), exit the bloodstream through capillary walls and enter tissue cells.

Cellular Respiration

• Overview

  • Once inside the cell, nutrients undergo a series of processes to become useful energy via cellular respiration.

  • The primary goal is to produce ATP (adenosine triphosphate), which serves as the energy currency of the cell.

• Stages of Cellular Respiration

  • Cellular respiration consists of four stages:

  1. Glycolysis

  2. Pyruvate oxidation

  3. Citric acid cycle (Krebs cycle)

  4. Electron transport chain

Glycolysis

• Definition and Location

  • Glycolysis literally means "splitting sugars".

  • It occurs in the cytoplasm of the cell.

• Phases of Glycolysis

  • Comprises two distinct phases:

  1. Energy Investment Phase

     • Two ATP molecules provide energy to the glucose molecule.

     • This forms a six-carbon sugar diphosphate molecule.

  2. Energy Harvesting Phase

     • The six-carbon sugar splits into two three-carbon molecules (pyruvate).

     • ATP is produced during this conversion.

• Key Outputs of Glycolysis

  • Net products are:

  • 2 ATP

  • 2 Pyruvate

  • 2 NADH (high-energy electron carriers)

Pyruvate Oxidation

• Process

  • Pyruvate molecules move into the mitochondria.

  • Each pyruvate undergoes oxidation, leading to:

  • Conversion into acetyl CoA (acetyl coenzyme A).

  • Transfer of electrons to form NADH.

  • Loss of carbon as carbon dioxide (CO₂).

Citric Acid Cycle (Krebs Cycle)

• Functionality

  • Acetyl CoA combines with oxaloacetate to initiate the cycle.

  • This cycle utilizes enzymatic redox reactions where carbons, hydrogens, and oxygens from pyruvate are converted into carbon dioxide and water.

• Cycle Characteristics

  • Coined a cycle because oxaloacetate is both the starting and ending compound.

  • Each glucose molecule entering glycolysis leads to two turns of the cycle (once for each pyruvate).

• Net Outputs of the Krebs Cycle:

  • From one glucose:

  • 8 NADH

  • 2 FADH₂

  • 2 ATP

  • 6 CO₂

Electron Transport Chain

• Overview

  • Follows the Krebs cycle and involves a series of membrane-bound carriers located in the mitochondria.

• Mechanism of Action

  • Electrons are passed from carrier to carrier, enabling the cell to capture energy:

  • Proteins in the chain pump hydrogen ions (H⁺) across a membrane.

  • H⁺ returns through an ATP synthase complex, leading to the synthesis of ATP.

• Role of Oxygen

  • Oxygen acts as the terminal electron acceptor and gets reduced to form water as a byproduct of the chain.

• Outputs

  • The process leads to a net production of 32 to 36 ATP molecules.

Conclusion: Energy Capture from Cellular Respiration

• Summary of Cellular Respiration

  • The four stages of cellular respiration effectively convert energy from glucose into ATP, the energy currency of the cell.

  • On average, around 36 ATP molecules are generated per glucose molecule utilized in the process.

• Additional Notes

  • Oxygen intake from bloodstream is crucial as it serves as the final electron acceptor.

  • Carbon dioxide produced as a byproduct is released from the body following cellular respiration.

  • Ultimately, the goal of cellular respiration is to efficiently transfer energy from consumed food into ATP that the body can utilize for various functions, culminating from the breakdown of nutrients into energy and carbon dioxide.