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

Chapter 6: How Cells Harvest Chemical Energy

6.1 Photosynthesis and Cellular Respiration Provide Energy for Life

All living organisms require energy to sustain life. In nearly all ecosystems, this energy originates from the sun and is transformed through photosynthesis and cellular respiration.

• Photosynthesis: Occurs in chloroplasts, where sunlight energy is captured and used to rearrange atoms of carbon dioxide and water, producing sugar and oxygen.

• Cellular Respiration: Occurs in mitochondria of eukaryotic cells. Sugar is broken down to carbon dioxide and water, and the cell captures some released energy to make ATP. Some energy is lost as heat during these conversions.

Equation for Cellular Respiration:

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

6.2 Breathing Supplies O2 for Use in Cellular Respiration and Removes CO2

• Respiration (Breathing): The exchange of gases—oxygen is brought in from the environment and carbon dioxide is released as waste.

• Cellular Respiration: The aerobic (oxygen-requiring) process by which cells harvest energy from food molecules.

Relationship: Oxygen from breathing is used in cellular respiration, and carbon dioxide produced by cells is expelled through breathing.

6.3 Cellular Respiration Banks Energy in ATP Molecules

• Cellular respiration is an exergonic (energy-releasing) process that transfers energy from glucose bonds to form ATP.

• Up to 32 ATP molecules can be produced per glucose molecule.

• About 34% of glucose's energy is used for ATP production; the rest is released as heat.

• This efficiency is higher than most energy conversion systems (e.g., gasoline engines).

6.4 Connection: The Human Body Uses Energy from ATP for All Its Activities

• The body requires a continuous supply of energy for basic functions (e.g., heart beating, breathing).

• Kilocalorie (kcal): The amount of heat needed to raise 1 kg of water by 1°C; equivalent to a food Calorie.

• The average adult needs about 2,200 kcal/day; 75% for maintenance, 25% for physical activity.

• Energy balance is essential for healthy weight maintenance.

Activity	Kcal/hour (67.5 kg person)
Running (8–9 mph)	979
Dancing (fast)	510
Bicycling (10 mph)	490
Swimming (2 mph)	408
Walking (4 mph)	341
Walking (3 mph)	245
Dancing (slow)	204
Driving a car	61
Sitting (writing)	28

6.5 Cells Capture Energy from Electrons “Falling” from Organic Fuels to Oxygen

• Energy extraction from glucose involves the transfer of electrons during chemical reactions.

• Oxygen is a strong electron acceptor; electrons lose potential energy when transferred to oxygen.

• Direct burning of glucose releases energy as heat and light, not usable by cells.

• Cellular respiration is a controlled descent of electrons, releasing energy in small, usable amounts.

• Redox Reactions:

  • Oxidation: Loss of electrons from a substance.

  • Reduction: Gain of electrons by a substance.

• Glucose is oxidized to CO2; oxygen is reduced to H2O.

• NAD+: A coenzyme that accepts electrons (becomes NADH), acting as a high-energy electron carrier.

• NADH delivers electrons to the electron transport chain, ending with oxygen as the final electron acceptor, forming water.

6.6 Overview: Cellular Respiration Occurs in Three Main Stages

1. Glycolysis: Occurs in the cytosol, does not require oxygen (anaerobic), breaks down glucose into two pyruvate molecules, and produces a small amount of ATP (2 ATP).

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in mitochondria, oxidizes pyruvate, supplies electrons to the next stage, and produces a small amount of ATP (2 ATP).

3. Oxidative Phosphorylation: Uses the energy from electrons carried by NADH and FADH2 to generate a large amount of ATP (about 28 ATP) via the electron transport chain and chemiosmosis.

6.7 Step 1: Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate

• Glucose is enzymatically split into two pyruvate molecules.

• Two NAD+ are reduced to two NADH.

• Net gain of two ATP molecules (produced by substrate-level phosphorylation).

• Glycolysis consists of two phases:

  • Energy Investment Phase: Two ATP are used to energize glucose.

  • Energy Payoff Phase: Four ATP and two NADH are produced.

If oxygen is present, pyruvate enters the mitochondrion for further oxidation in the citric acid cycle and oxidative phosphorylation.

6.8 Step 2: Pyruvate Oxidation and the Citric Acid Cycle (Krebs Cycle)

• Pyruvate is chemically groomed before entering the citric acid cycle:

  • A carboxyl group is removed as CO2.

  • The remaining two-carbon fragment is oxidized, reducing NAD+ to NADH.

  • The resulting acetyl group is attached to coenzyme A, forming acetyl CoA.

• Each turn of the citric acid cycle produces:

  • 1 ATP

  • 3 NADH

  • 1 FADH2

  • 2 CO2

• Since two acetyl CoA are produced per glucose, the cycle turns twice per glucose molecule.

6.9 Step 3: Oxidative Phosphorylation

• Most ATP is produced in this stage via the electron transport chain and chemiosmosis.

• NADH and FADH2 donate electrons to the electron transport chain, which pumps H+ ions across the inner mitochondrial membrane, creating a gradient.

• ATP synthase uses the energy of the H+ gradient to synthesize ATP from ADP.

• Oxygen acts as the final electron acceptor, forming water.

• About 28 ATP are produced in this stage.

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced (per glucose)
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation	~28
Total	~32

Additional info: The exact ATP yield can vary due to differences in shuttle mechanisms and energy used for transporting molecules into mitochondria.