Question

Cells use primarily two ways of storing potential energy: 1) as concentration gradients and 2) in molecules. A) Explain how energy can be stored in these two forms and how it can be used to do other work. B) Propose an analogy that can explain these forms of energy storage using everyday items.

Answer 1

1. The main energy utilized by the living systems is in the form of ATP.  coupling the favourable ATP hydrolysis reaction to the movement of molecules against their concentration gradients can also generate energy. The resulting gradient is a form of stored (potential) energy. This energy can be used to move other molecules, that is molecules that are not moved directly by an ATP-driven transporter. Such processes involve what is known as coupled transport. When a substance is in higher concentration at one side of the membrane compared to the other side this substance move along towards the one which is at a lower concentration. During its movement, it forms a concentration gradient driving along with it many other substances. This transport is either cotransport or anti-transport.

energy stored in the molecules - such as ATP where energy is stored in its high energy phosphate bonds. To release the energy ATP is broken down into ADP and Pi. The more organized a molecule is, the more free energy it will store. Energy is stored in the covalent bonds between phosphates, with the greatest amount of energy (approximately 7 kcal/mole) in the bond between the second and third phosphate groups. This covalent bond is known as a pyrophosphate bond.

2. Analogy for concentration gradients - It is just like spilling water from the tank in a terrace. Here water falls down from its higher concentration to its lower concentration.

Analogy for energy stored in a molecule- rechargeable batteries.  The batteries are used, giving up their potential energy until it has all been converted into kinetic energy and heat/unusable energy. Recharged batteries (into which energy has been put) can be used only after the input of additional energy