Coupled reactions occur in the same place at the same time. One reaction provides the energy for a second reaction that requires energy.
If you needed to drive to Alaska, this would require quite a bit of energy. How do you get there? Answer- Your car engine burns gasoline and this releases energy. The energy that is released from burning gasoline is used to move your car to Alaska.
The energy-releasing reaction provides the energy to drive the energy-requiring reaction, as in the example below.
If you need to make a 10 hour cell phone call to your friend, this would require electrical energy. The electrical energy is provided by the battery in your cell phone. After 10 hours, you have completed a long conversation with your friend and your cell phone probably has a dead battery.
The Flow of Energy
The cycling of molecules between the chloroplasts and mitochondria is responsible for the flow of energy through the biosphere.
Chloroplasts use solar energy to convert water and carbon dioxide into carbohydrates (sugars).
Cellular respiration in the mitochondria breaks down carbohydrates to yield energy (ATP), releasing carbon dioxide and water.
Humans also contribute to the flow of energy from the sun and through the biosphere.
Humans release carbon dioxide (from our lungs) and water (from our urine) that plants can use for photosynthesis.
The carbohydrates and other nutrients that we eat or drink are broken down in the cytoplasm and mitochondria of our cells to produce the ATP needed for cellular activities.