Module 02: Life's Origins
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Food supplies chemical energy. Cells transfer some of it into ATP, use ATP for immediate work, and regenerate ATP from ADP.
Work with one section at a time.
Describe what each label or arrow means.
Explain why the structure or process produces that result.
Answer a practice question and use its explanation to correct your reasoning.
Which components make ATP?
Adenine, ribose, and three phosphates
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Cells use energy to grow, move, transport substances, make proteins and other molecules, digest materials, and respond to changes. Glucose and other food molecules contain chemical energy. ATP is the immediate energy source for much of a cell's work.
ATP stands for adenosine triphosphate. It contains adenine, a five-carbon sugar called ribose, and three phosphate groups. ADP is adenosine diphosphate and has two phosphate groups. Use the phosphate count to distinguish the two.
ATP reacts with water and becomes ADP plus a phosphate. The overall reaction releases energy that can be coupled to movement, active transport, or synthesis. The course focuses on the removal of the terminal phosphate. Energy release belongs to the overall chemical reaction.
Adding a phosphate to ADP requires an energy input. Energy harvested from food can drive that rebuilding. The cell repeats this cycle as it does work: ATP supplies usable energy, then ADP is recharged. ATP is a renewable carrier within the cell.
ATP is useful for immediate work and short-term transfer. Stable molecules hold stored energy longer: plants build sugars and starches, while animal cells store excess energy in fats. Breaking down food molecules through metabolic reactions supplies energy to regenerate ATP.
ATP resembles a charged battery ready to power a task; ADP resembles the lower-energy form that can be recharged. Recharging takes energy. The analogy explains cycling, but ATP is a molecule carrying chemical energy, not a tiny electrical battery.
