Energy in Cells: ATP, Photosynthesis and Respiration
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Photosynthesis has light-dependent reactions (water split, ATP and NADPH made) and the Calvin cycle (carbon dioxide fixed into sugar).
- Respiration transfers energy from glucose to ATP through glycolysis, the Krebs cycle and the electron transport chain.
- Without oxygen, cells rely on glycolysis and fermentation, which yield far less ATP.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Photosynthesis in two stages
Photosynthesis happens in chloroplasts in two linked stages. In the first stage, which needs light, chlorophyll absorbs light energy. That energy is used to split water molecules. This releases oxygen as a waste gas. It also produces energy-carrying molecules, including ATP. In the second stage, called the Calvin cycle, the plant uses these energy carriers to combine carbon dioxide from the air into glucose. This stage does not use light directly. The glucose stores chemical energy. The plant also uses it as a building block for starch, cellulose, fats and proteins. So the oxygen we breathe comes from water.
Respiration and ATP
Every cell needs a constant supply of energy, and it releases that energy from glucose through cellular respiration. Cells cannot use glucose’s energy directly. Instead, they transfer it to a small molecule called ATP. ATP acts like a rechargeable battery, delivering energy wherever it is needed. Respiration begins in the cytoplasm, where glucose is split in half. It continues inside the mitochondria. There, oxygen is used and carbon dioxide and water are released. Each glucose molecule can yield about thirty molecules of ATP. Without oxygen, cells can make only a little ATP, and muscles produce lactic acid.
Curriculum links: NGSS HS-LS1-5 · NGSS HS-LS1-7 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 11