"ADP and ATP are involved in energy transfer in cells." I know what is ATP and I know how ADP is formed, but I don’t understand what “energy transfer in cells” means.
Cells require a constant supply of energy to generate and maintain the biological order that keeps them alive. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells.
Cellular nutrients (components in foods that an organism uses to survive and grow) come in many forms, including sugars and fats, so food molecules are rich sources of energy for cells because much of the energy used to form these molecules is literally stored within the chemical bonds that hold them together.
Cells release the energy stored in their food molecules through a series of oxidation reactions. Oxidation describes a type of chemical reaction in which electrons are transferred from one molecule to another, changing the composition and energy content of both the donor and acceptor molecules. Food molecules act as electron donors. During each oxidation reaction involved in food breakdown, the product of the reaction has a lower energy content than the donor molecule that preceded it in the pathway. At the same time, electron acceptor molecules capture some of the energy lost from the food molecule during each oxidation reaction and store it for later use. Eventually, when the carbon atoms from a complex organic food molecule are fully oxidized at the end of the reaction chain, they are released as waste in the form of carbon dioxide.
They (Cells) do not use the energy from oxidation reactions as soon as it is released. Instead, they convert it into small, energy-rich molecules such as ATP and nicotinamide adenine dinucleotide (NADH), which can be used throughout the cell to power metabolism and construct new cellular components.
For more informations :
Lets imagine the energy budget of a cell as the economy.
Using this analogy a glucose molecule is like a 5kg bar of silver. It is very valuable, but it is hardly convenient if you want to pay for chocolate milk or buy a cup of coffee. What you need is an exchange that will break that silver bar into smaller units of currency like a stack of $1 dollar bills. The smallest and most common currency of energy in a cell is ATP.
Most (but not all) chemical reaction in a cells that requires an input of energy uses ATP. This is why ATP is said to be used in energy transfer within the cell.
Glucose is a 5kg bar of silver. The mitochondria and the TCA cycle is the exchange. ATP is a $1 dollar note.