Cellular Energy Metabolism Overview
📌 Cells derive necessary energy from glucose ($\text{C}_6\text{H}_{12}\text{O}_6$), a sugar synthesized from inorganic molecules like $\text{CO}_2$ and $\text{H}_2\text{O}$ via photosynthesis in plants.
🌿 Autotrophs (like plants) produce their own glucose using photosynthesis in chloroplasts, while heterotrophs (like animals) must obtain it by consuming other organisms.
🔬 Energy extracted from glucose's chemical bonds is stored in ATP molecules, which fuel the cell's vital processes.

Photosynthesis in Autotrophs
☀️ Autotrophs use photosynthesis in chloroplasts to convert light energy, $\text{CO}_2$, and $\text{H}_2\text{O}$ into glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) and oxygen ($\text{O}_2$).
⚛️ The overall chemical reaction is: $6\text{H}_2\text{O} + 6\text{CO}_2 \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$.
🌱 This process forms the basis of almost all food chains by converting inorganic matter into organic matter and releasing crucial oxygen.

Cellular Respiration: Aerobic Pathway
🌬️ Aerobic respiration, which requires oxygen, breaks down glucose over three phases: Glycolysis, the Krebs cycle (Citric Acid Cycle), and Oxidative Phosphorylation.
💥 Glycolysis (anaerobic, in the cytoplasm) splits one $\text{C}_6$ glucose molecule into two $\text{C}_3$ pyruvate molecules, yielding a net gain of 2 ATP.
🌀 The Krebs cycle (aerobic, in the mitochondria) completely demolishes the products of glycolysis, generating 2 ATP, 6 $\text{NADH}$, and 2 $\text{FADH}_2$ per glucose molecule.
💧 Oxidative Phosphorylation (aerobic, inner mitochondrial membrane) includes the electron transport chain and chemiosmosis, producing the majority of ATP—up to 28 ATP per glucose.

Cellular Respiration: Anaerobic Pathways
🦠 In the absence of oxygen, organisms rely solely on Glycolysis, yielding 2 net ATP.
🥛 The pyruvate can undergo Lactic Acid Fermentation (e.g., in human muscle cells under exertion), producing lactate.
🍺 Alternatively, Alcoholic Fermentation produces ethanol and $\text{CO}_2$, a process historically used to create alcoholic beverages.
♻️ Both fermentation pathways feature energy coupling, recycling electron carriers ($\text{NADH}$) necessary to sustain glycolysis.

Detailed Look at Photosynthesis Phases
💡 The Light-Dependent Phase occurs in the thylakoids, using light energy absorbed by photosystems to generate ATP and $\text{NADH}$ carriers, splitting $\text{H}_2\text{O}$ and releasing $\text{O}_2$.
🌑 The Light-Independent Phase (Calvin Cycle), occurring in the stroma, uses the $\text{ATP}$ and $\text{NADH}$ from the light phase to synthesize sugar from $\text{CO}_2$.
🔄 The Calvin Cycle fixes $\text{CO}_2$ onto ribulose-1,5-bisphosphate, ultimately generating $\text{G3P}$ ($\text{C}_3$) using 9 ATP and 6 $\text{NADH}$ per net synthesized sugar molecule.

Key Points & Insights
➡️ The total maximum yield for aerobic respiration is 32 ATP per molecule of glucose metabolized.
➡️ The fundamental difference between animal and plant life lies in autotrophy vs. heterotrophy regarding glucose acquisition.
➡️ Energy transfer between trophic levels follows an approximate 1:10 ratio, with significant energy loss (increasing universal entropy) at each transfer.
➡️ Early life forms evolved anaerobic metabolism (glycolysis only) before the oxygenation of Earth's atmosphere spurred the evolution of complex aerobic life.

📸 Video summarized with SummaryTube.com on Nov 26, 2025, 16:06 UTC