What Is Energy?
Energy is defined as the ability to work:
- It takes a variety of forms
- It can be changed from one form to another
- It cannot be created or destroyed
- It is measured in joules (J)
Why Do Organisms Need Energy?
Living organisms are highly ordered systems that require a constant input of energy to prevent them becomg disordered - a condition which would lead to their death.
Specifically energy is required for:
- Active Transport
- Maintenance, Repair and Division of cells and of organelles
- Production of Substances
- Maintenance of Body Temperature
Energy and metabolism
The flow of energy through living systems occurs in three stages:
- Light energy from the sun is converted by plants into chemical energy during photosynthesis
- The chemical energy from photosynthesis, in the form of organic molecules, is converted into ATP during repiration in all cells
- ATP is used by cells to perform useful work
How ATP Stores Energy
ATP has three phosphate groups. The bonds between these phosphate groups are unstable and so have low activation energy, which means they are easily broken. When they do break they do release a considerable amount of energy.
ATP + H20 --> ADP + Pi + Energy
As water is used to convert ATP to ADP, this is known as a hydrolysis reaction.
Synthesis of ATP
The convertion of ATP to ADP is a reversable reaction and therefore energy can be used to add the inorganic phosphate to ADP to reform ATP. As water is removed, it is known as a condensation reaction.
The sythesis of ATP from ADP involves the addition of a phosphate molecule to ADP. It occurs in three ways:
- Photophosphorylation: takes place in the chlorophyll during photosynthesis
- Oxidative Phosphorylation: occurs in the mitochondria of plant and animal cells during the process of electron transport
- Substrate-level Phosphorylation: occurs in plant and animal cells when phosphate groups are transferred from donor molecules to ADP to make ATP
In the first two, ATP is synthesised using energy released during the transfer of electrons along a chain of electron carrier molecules.
Roles of ATP
The feature of ATP that is the instability of phosphate bonds which makes it a good energy donor is also the reason why it is a poor energy store. As a result cells do not store large quantities of ATP, but rather just maintain a few seconds' supply.
ATP is therefore an immediate energy source of a cell.
ATP is a better immediate energy source than glucose for the following reasons:
- Molecules of ATP are smaller than a molecule of glucose, therefore the energy is released in smaller more manageable quantities.
- The hydrolysis of ATP to ADP is a single reaction that releases immediate energy. The breakdown of glucose requires a long series of reactions
As ATP must be constantly made, the cells which require the most energy (such as muscle fibres), contain many large mitochondria to allow for this. ATP is the source of energy for:
- Metabolic Processes
- Active Transport
- Activation of molecules