Cellular Respiration Overview

Overview

This lecture introduces cellular respiration, focusing on the breakdown of glucose to produce ATP, the main energy currency in cells, through a series of metabolic pathways.

Introduction to Cellular Respiration

• Cellular respiration is the process of breaking down glucose to generate ATP for cellular energy.
• The energy in glucose's carbon bonds is captured in ATP, rather than released as heat.
• Overall reaction: glucose + oxygen → carbon dioxide + water + ATP.

Energy Transfer & Electron Carriers

• Cellular respiration involves multiple small steps to efficiently capture energy.
• High-energy electron carriers (NADH, FADH₂) transport electrons and store potential energy.
• Electron carriers prevent sudden energy loss as heat by transferring energy gradually.
• Redox reactions (oxidation-reduction) involve the transfer of electrons between molecules.

Redox Reactions Explained

• Oxidation: loss of electrons (OIL—Oxidation Is Loss).
• Reduction: gain of electrons (RIG—Reduction Is Gain).
• Reducing agents donate electrons and become oxidized; oxidizing agents accept electrons and become reduced.
• Acronym "LEO the Lion says GER" helps remember: Lose Electrons—Oxidation, Gain Electrons—Reduction.

ATP: The Energy Currency

• ATP (adenosine triphosphate) stores energy in its phosphate bonds.
• Hydrolysis of ATP releases energy, forming ADP and inorganic phosphate (dephosphorylation).
• Adding a phosphate to a molecule (phosphorylation) is often done by enzymes called kinases.

ATP Generation Methods

• Substrate-level phosphorylation: transfers a phosphate directly from a substrate to ADP to make ATP.
• Chemiosmosis: uses ATP synthase and is the main method (≈90%) for ATP production in cells.
• In eukaryotes, chemiosmosis occurs in mitochondria; in prokaryotes, it happens in the plasma membrane.

Glycolysis Overview

• Glycolysis: first stage of glucose catabolism; splits glucose (6C) into two pyruvate (3C each).
• Occurs in the cytoplasm and does not require oxygen (anaerobic).
• Inputs: glucose, NAD⁺, ATP, ADP; Outputs: pyruvate, NADH, ATP, ADP.
• First phase (investment): uses ATP to phosphorylate glucose and trap it in the cell.
• Second phase (payoff): produces a net gain of 2 ATP (4 made, 2 used) and 2 NADH.
• ATP in glycolysis is made via substrate-level phosphorylation.

Regulation of Glycolysis

• Phosphorylation of glucose regulates glycolysis by trapping it in the cell.
• Glycolysis slows or stops if NAD⁺ is depleted or if ATP levels are high.

Key Terms & Definitions

• Cellular Respiration — multi-step process breaking down glucose to make ATP.
• ATP (Adenosine Triphosphate) — main cellular energy carrier.
• NAD⁺/NADH — electron carrier; NAD⁺ accepts electrons (oxidized), NADH donates electrons (reduced).
• Redox Reaction — chemical reaction involving electron transfer.
• Phosphorylation — addition of a phosphate group to a molecule.
• Kinase — enzyme that adds phosphate groups.
• Substrate-level phosphorylation — ATP formation by direct phosphate transfer.
• Chemiosmosis — ATP production using a proton gradient and ATP synthase.
• Glycolysis — metabolic pathway splitting glucose into two pyruvate molecules.

Action Items / Next Steps

• Review video links in the textbook for visual summaries of cellular respiration pathways (optional).
• Prepare to learn about the steps following glycolysis in the next lecture.