Unit 3 Cellular Energetics - Cellular Respiration

Cellular Respiration Overview

Daily Energy Use: The body expends energy through various processes and activities:
- EAT (Exercise Activity Thermogenesis): Active exercise, such as running, biking, or sports, significantly contributes to the overall energy expenditure. This includes both structured workouts and physical labor.
- TEF (Thermic Effect of Food): This refers to the energy used for digestion, absorption, and transportation of nutrients. The TEF varies depending on the food composition; protein has the highest thermic effect compared to fats and carbohydrates.
- NEAT (Non-exercise Activity Thermogenesis): This includes all physical movements that are not deliberate exercise, such as fidgeting, walking around the house, and any other daily activities. NEAT can make a substantial impact on total calorie expenditure.
- BMR (Basal Metabolic Rate): This is the rate of energy expended while at rest, which is crucial for maintaining vital body functions, including heartbeat, breathing, and cellular processes. The BMR can be influenced by factors such as body size, age, and genetics.

Humans use approximately 100-150 moles of ATP daily, which corresponds to an astonishing 752 septillion ATP molecules. Each individual cell cycles through about 25 quadrillion ATP molecules each day.
Each ATP molecule is recycled about 1200 times per day, emphasizing the efficiency of ATP usage within cellular processes.
On average, human cells contain between 1000 to 2500 mitochondria per cell, which are critical for ATP generation and energy production.

Definition: Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into ATP while releasing waste products. It consists of both aerobic (oxygen-requiring) and anaerobic (non-oxygen requiring) pathways, with aerobic respiration being far more efficient in ATP production.

Anabolism: This refers to the synthesis of larger molecules from smaller units—specifically, the formation of ATP from ADP through the addition of an inorganic phosphate (Pi).

Substrate-level Phosphorylation: This process occurs during glycolysis, where a high-energy phosphate group is directly transferred from a donor molecule to ADP to form ATP.
Oxidative Phosphorylation: This method occurs in the Electron Transport Chain (ETC), where ATP synthase generates ATP through energy derived from electron transfers, primarily from NADH and FADH2.

Redox Reaction: This is a coupling of oxidation and reduction reactions that are vital in energy transfer.
- Oxidation: The process of losing electrons; energy is released during this phase, which contributes to the generation of ATP.
- Reduction: The gain of electrons, which absorbs energy. This dynamic interplay is crucial for ATP synthesis via the ATP Synthase enzyme located in the ETC.

Chemical Reaction: The overall reaction for glucose oxidation is represented by the equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. This breakdown of glucose in the presence of oxygen releases significant amounts of energy which is captured in the form of ATP.

Glycolysis: Occurs in the cytoplasm and converts one glucose molecule into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH.
Citric Acid Cycle (Krebs Cycle): Takes place in the mitochondrial matrix and processes the pyruvate produced during glycolysis. It generates 2 ATP, along with significant electron carriers NADH and FADH2, which are used in the ETC.
Electron Transport Chain (ETC): Located in the inner mitochondrial membrane, the ETC uses the electrons from NADH and FADH2 to create a proton gradient. This gradient drives ATP synthesis, producing 28-32 ATP per glucose molecule through oxidative phosphorylation.

In the absence of oxygen, humans can undergo lactic acid fermentation to regenerate NAD+, allowing glycolysis to continue.
- Process Details: During intense exercise, NADH produced in glycolysis is converted back to NAD+ by reducing pyruvate to lactic acid, which can accumulate and contribute to muscle soreness and fatigue.

Yeast, such as Saccharomyces cerevisiae, undergoes alcohol fermentation, converting glucose into ethanol and carbon dioxide. This process is critical in the food and beverage industry, especially for bread rising linked to CO2 production.

Glycolysis: Takes place in the cytoplasm, produces 2 ATP and pyruvate.
Krebs Cycle: Occurs in the mitochondria, yields 2 ATP and essential electron carriers (NADH, FADH2).
ETC: Located in the inner mitochondrial membrane, generates a substantial amount of ATP (28-32 ATP).

In the absence of glucose, lipids can be metabolized into Acetyl-CoA for energy production, and proteins can also be broken down under extreme conditions to provide necessary substrates for energy.