Valleys are one of the most common landforms on the surface of the planet. They are carved by flowing water or flowing ice through the process of erosion, which is the gradual wearing away of Earth surfaces through the action of wind and water.
The cellular respiration processes of all living cells make use of the coenzyme Nicotinamide adenine dinucleotide (NAD). It plays a key role in energy metabolism by accepting and donating electrons.
The many functions of mitochondria include production of adenosine triphosphate (ATP) via the TCA Cycle (which is also as the Krebs Cycle and the Citric Acid Cycle). That process is an important metabolic pathway that is an essential part of carbohydrate and energy metabolism - see also what is metabolism.