1. Energy for Life: ATP & Metabolism

Every living thing needs energy — to grow, move materials, build molecules, repair damage, and stay warm. But cells can't plug into a wall outlet. Instead they use a tiny energy-carrying molecule called ATP (adenosine triphosphate).

Think of ATP as the cell's rechargeable battery. ATP carries three phosphate groups. When the cell needs energy, it breaks off one phosphate, turning ATP into ADP and releasing energy for work. Later, energy from food is used to reattach the phosphate, recharging ADP back into ATP. This cycle runs constantly.

All of a cell's chemical reactions together make up its metabolism. Some reactions build larger molecules and store energy (for example, linking amino acids into proteins); others break down molecules and release energy (like splitting glucose). Food provides the raw stored energy; ATP is the usable form the cell actually spends. And almost all of that energy traces back to the Sun.

⚠️ Common misconception: the energy in food doesn't simply 'disappear' when you use it. Cells transform it — into ATP, motion, body heat, and stored molecules. Energy is converted, not destroyed.

💡 Tip: ATP = the cell's usable energy 'cash'; food = the larger stored 'savings.'