Chapter 6: Cells Harvest Chemical Energy – Cellular Respiration and Metabolic Pathways

Chapter 6: Cells Harvest Chemical Energy

Introduction to Cellular Respiration

Cellular respiration is a fundamental process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. Oxygen acts as a key reactant in this process, and the breakdown of food molecules also generates heat.

• Oxygen is required for the complete breakdown of glucose in cellular respiration.

• ATP (Adenosine Triphosphate) is the energy currency produced during cellular respiration.

• Brown fat in some mammals, including humans, can generate heat without producing ATP, due to a 'short circuit' in its cellular respiration pathway.

• Brown fat is especially important for heat production in small mammals and infants.

Overview of Energy Flow in Ecosystems

Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are interconnected processes that provide energy for life on Earth. Energy from the sun is captured by photosynthesis and released by cellular respiration.

• Photosynthesis occurs in chloroplasts, converting carbon dioxide and water into glucose and oxygen using sunlight.

• Cellular respiration occurs in mitochondria, breaking down glucose into carbon dioxide and water, releasing energy to form ATP.

• Most ecosystems ultimately depend on solar energy.

Equation for Cellular Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Breathing supplies oxygen for cellular respiration and removes carbon dioxide, but is not the same as cellular respiration itself.

• Cellular respiration is an aerobic (oxygen-requiring) process.

Stages of Cellular Respiration

Overview of the Three Main Stages

Cellular respiration occurs in three main stages, each with distinct roles and locations within the cell.

1. Glycolysis (in the cytosol): Breaks down glucose into two molecules of pyruvate.

2. Pyruvate Oxidation and the Citric Acid Cycle (in mitochondria): Pyruvate is oxidized to acetyl CoA, which enters the citric acid cycle, producing NADH, FADH2, and CO2.

3. Oxidative Phosphorylation (in mitochondria): NADH and FADH2 donate electrons to the electron transport chain, driving ATP synthesis via chemiosmosis.

Glycolysis

Glycolysis is the first stage of cellular respiration and occurs in the cytosol. It splits one glucose molecule into two pyruvate molecules, generating a small amount of ATP and NADH.

• Consists of two phases: Energy Investment Phase (uses ATP to phosphorylate glucose) and Energy Payoff Phase (produces ATP and NADH).

• Net products per glucose: 2 ATP, 2 NADH, 2 pyruvate.

• ATP is produced by substrate-level phosphorylation (direct transfer of phosphate to ADP).

Pyruvate Oxidation and the Citric Acid Cycle

After glycolysis, pyruvate is transported into the mitochondria, where it is oxidized to acetyl CoA. The citric acid cycle completes the breakdown of glucose derivatives, releasing CO2 and generating high-energy electron carriers.

• Each turn of the cycle processes one acetyl CoA, producing 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (by substrate-level phosphorylation).

• The cycle regenerates oxaloacetate to continue processing acetyl CoA.

Oxidative Phosphorylation

This stage includes the electron transport chain and chemiosmosis, occurring in the inner mitochondrial membrane. Most ATP is produced here.

• NADH and FADH2 donate electrons to the electron transport chain.

• Energy from electron transfer pumps H+ ions into the intermembrane space, creating a gradient.

• ATP synthase uses the H+ gradient to synthesize ATP from ADP and phosphate.

• Oxygen is the final electron acceptor, forming water.

Absence of Oxygen: Without O2, the electron transport chain cannot operate, and oxidative phosphorylation halts.

Summary Table: Stages of Cellular Respiration

Stage	Location	Main Reactants	Main Products	ATP Yield
Glycolysis	Cytosol	Glucose, NAD+, ADP	Pyruvate, NADH, ATP	2 (net)
Pyruvate Oxidation & Citric Acid Cycle	Mitochondrial matrix	Pyruvate, NAD+, FAD, ADP	CO2, NADH, FADH2, ATP	2
Oxidative Phosphorylation	Inner mitochondrial membrane	NADH, FADH2, O2, ADP	ATP, H2O, NAD+, FAD	~28

Fermentation: Anaerobic Harvesting of Energy

Fermentation Pathways

Fermentation allows cells to produce ATP without oxygen by recycling NAD+ through the reduction of pyruvate.

• Lactic acid fermentation: Pyruvate is reduced to lactate (lactic acid), regenerating NAD+. Occurs in muscle cells and some bacteria.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+. Occurs in yeast and some bacteria.

• Fermentation yields only 2 ATP per glucose (from glycolysis).

Comparison Table: Aerobic Respiration vs. Fermentation

Process	Oxygen Required?	ATP Yield (per glucose)	End Products
Aerobic Respiration	Yes	~32	CO2, H2O
Lactic Acid Fermentation	No	2	Lactate
Alcohol Fermentation	No	2	Ethanol, CO2

Connections Between Metabolic Pathways

Other Organic Molecules as Fuel

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration. These molecules enter the pathway at different points.

• Carbohydrates are broken down to glucose or other intermediates.

• Fats are converted to glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA).

• Proteins are deaminated and their carbon skeletons enter glycolysis or the citric acid cycle.

• Fats store more energy per gram than carbohydrates, making them efficient for long-term energy storage in animals.

Biosynthesis and Regulation

Intermediates from cellular respiration are used for biosynthesis of other organic molecules. Metabolic pathways are regulated by feedback inhibition to maintain balance.

• ATP and other molecules can inhibit or activate key enzymes in the pathway.

• Cells use these intermediates to synthesize amino acids, nucleotides, and other essential compounds.

Key Terms and Concepts

• ATP (Adenosine Triphosphate): Main energy carrier in cells.

• NAD+ / NADH: Electron carrier involved in redox reactions.

• FAD / FADH2: Another electron carrier used in the citric acid cycle and electron transport chain.

• Substrate-level phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

• Oxidative phosphorylation: ATP production using energy from the electron transport chain and chemiosmosis.

• Redox reactions: Chemical reactions involving the transfer of electrons.

• Obligate anaerobes: Organisms that cannot survive in the presence of oxygen.

• Facultative anaerobes: Organisms that can survive with or without oxygen.

Summary

• Cellular respiration is essential for extracting energy from food and producing ATP.

• It consists of glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Fermentation provides an alternative pathway for ATP production in the absence of oxygen.

• Cells can use various organic molecules as fuel and for biosynthesis.