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Metabolism: How Your Body Turns Food Into Energy, Heat & Life - Leejohnston - 11-17-2025
Thread 6 — Metabolism: How Your Body Turns Food Into Energy, Heat & Life
The Complete Guide to Catabolism, Anabolism & Cellular Energy
Every second of your life — even right now — millions of chemical reactions are happening inside
your cells, powering movement, thought, repair, heat, and life itself.
These reactions collectively form something called:
Metabolism
(the entire network of chemical processes inside a living organism).
This thread breaks it down scientifically, clearly, and in a way anyone can understand.
1. What Metabolism Really Is
Metabolism is divided into two giant systems:
• Catabolism — breaking molecules down to release energy
• Anabolism — building molecules using energy
Example:
• Catabolism breaks glucose down → releases ATP
• Anabolism builds proteins, DNA, membranes → requires ATP
Together, they form your metabolic energy economy.
2. ATP — The Universal Energy Currency
Cells use a molecule called ATP (adenosine triphosphate) for energy.
ATP powers:
• muscle contraction
• nerve impulses
• active transport
• repair
• synthesis
• heat generation
ATP is constantly produced and used — you recycle your own body weight in ATP every day.
3. Cellular Respiration — How Cells Make Energy
Food is not directly usable as energy.
It must be processed through a multi-stage biochemical pathway.
The major stages:
1. Glycolysis — glucose → pyruvate (small amount of ATP)
2. Link Reaction — pyruvate → acetyl-CoA
3. Krebs Cycle — energy carriers (NADH, FADH₂) created
4. Electron Transport Chain — large ATP production
With oxygen → ~30–32 ATP per glucose
Without oxygen → only 2 ATP (anaerobic respiration)
Aerobic respiration is what keeps you alive.
4. Metabolism of Fats and Proteins
Fats: broken down into fatty acids → used in respiration
• provide more energy per gram than carbs
• essential for long-term energy storage
Proteins: broken down into amino acids
• used when carbs + fats are low
• not ideal (can produce toxic by-products requiring processing)
Your body switches fuels depending on need and availability.
5. Hormones That Control Metabolism
Several hormones regulate energy balance:
• Insulin — promotes glucose uptake + storage
• Glucagon — releases stored glucose
• Adrenaline — rapid energy availability
• Thyroxine — sets metabolic rate
• Cortisol — regulates long-term glucose availability
These hormones keep blood sugar steady and energy stable.
6. Basal Metabolic Rate (BMR)
BMR = the energy your body uses at rest simply to stay alive.
It powers:
• heartbeat
• breathing
• brain function
• ion balancing
• temperature regulation
• repair
Most energy used in a day comes from BMR — not exercise.
7. What Affects Metabolism?
• age
• muscle mass
• hormones
• genetics
• temperature
• food intake
• activity level
• stress
Muscle mass is the biggest controllable factor — muscles burn more ATP than fat.
8. Metabolic Pathways — Why Life Is So Organised
Reactions must be controlled — otherwise energy would be wasted or toxic molecules would form.
Cells use:
• enzymes
• feedback loops
• compartmentalisation
• gene regulation
to keep metabolism efficient and safe.
9. Metabolic Disorders (Brief Overview)
Examples of disrupted metabolism:
• diabetes (glucose regulation failure)
• hyperthyroidism (excess metabolic rate)
• mitochondrial disorders (energy production failure)
• phenylketonuria (inability to break down phenylalanine)
Modern medicine often targets metabolism at the molecular level.
10. Why Metabolism Is the Core of All Biology
Metabolism is:
• how cells gain energy
• how organisms grow
• how organisms maintain order
• how life fights entropy
• the foundation of physiology
Understanding metabolism = understanding life’s engine.
Written by LeeJohnston & Liora — The Lumin Archive Research Division