Energy flows from the sun and through all life on earth.

• 4.1 Cars that run on french fry oil? Organisms and machines need energy to work.
• 4.2 Energy has two forms: kinetic and potential.
• 4.3 As energy is captured and converted, the amount of energy available to do work decreases.
• 4.4 ATP molecules are like free-floating rechargeable batteries in all living cells.

Photosynthesis uses energy from sunlight to make food.

• 4.5 Where does plant matter come from? Photosynthesis: the big picture.
• 4.6 Photosynthesis takes place in the chloroplasts.
• 4.7 Light energy travels in waves: plant pigments absorb specific wavelengths.
• 4.8 Photons cause electrons in chlorophyll to enter an excited state.
• 4.9 Photosynthesis in detail: the energy of sunlight is captured as chemical energy.
• 4.10 Photosynthesis in detail: the captured energy of sunlight is used to make food.
• 4.11 The battle against world hunger can use plants adapted to water scarcity.

Cellular respiration converts food molecules into ATP, a universal source of energy for living organisms.

• 4.12 How do living organisms fuel their actions? Cellular respiration: the big picture.
• 4.13 The first step of cellular respiration: glycolysis is the universal energy-releasing pathway.
• 4.14 The second step of cellular respiration: the Krebs cycle extracts energy from sugar.
• 4.15 The third step of cellular respiration: ATP is built in the electron transport chain
• 4.16 This is how we do it: Can we combat the fatigue and reduced cognitive functioning of jet lag with NADH pills?

There are alternative pathways to energy acquisition.

• 4.17 Beer, wine, and spirits are by-products of cellular metabolism in the absence of oxygen.
• 4.18 Eating a complete diet: cells can run on protein and fat as well as on glucose.