Energy is defined as 'the ability to do work':
i. It takes a variety of different forms-heat, light, sound, electrical, magnetic, mechanical, chemical, atomic, etc.
ii. It can be changed from one form to another.
iii. It cannot be created or destroyed.
iv. It is measured in joules (J).
The flow of energy through living systems occurs in three stages:
1. Light energy from the Sun is converted by plants into chemical energy during photosynthesis.
2. The chemical energy from photosynthesis, in the form of organic molecules, is converted into ATP during respiration in all cells.
3. ATP is used by all cells to perform useful work.
Living organisms require a constant supply of energy to prevent them from becoming disordered-which would lead to death.
Energy is needed for:
1. Metabolism: All the chemical reactions that take place require energy.
2. Movement: Both internal (e.g. blood flow) and external (e.g. locomotion)
3. Active transport: Movement of ions and molecules against concentration gradients across plasma membranes.
4. Maintenance, repair, division: Of cells and organelles within cells.
5. Production of substances: Such as hormones and enzymes.
6. Maintenance of body temperature: In birds and mammals as they are endothermic and need energy to replace that which is lost as heat to the environment.
Adenosine triphosphate (ATP) has three phosphate groups.
The bonds between the phosphate groups are unstable and therefore have a low activation energy, this means they are easily broken.
When they do break, they release a considerable amount of energy.
Usually in living cells it is only the terminal phosphate that is removed:
ATP + H20 ---> ADP + Pi + E
As water is used to convert ATP to ADP, this is known as a hydrolysis reaction.
The conversion of ATP to ADP is a reversible reaction and therefore energy can be used to add an inorganic phosphate to ADP to reform ATP.
As water is removed in this process, the reaction is known as a condensation reaction.
The synthesis of ATP from ADP involves the addition of a phosphate to ADP:
1. Photophosphorylation: Takes place in the chlorophyll-containing plant cells during photosynthesis.
2. Oxidative phosphorylation: Occurs in the mitochondria of plant and animal cells during the process of electron transport.
3. Substrate-level phosphorylation: Occurs in plant and animal cells when phosphate groups are transferred from donor molecules to ADP.
In 1 and 2, ATP is synthesised using energy released bduring the transfer of electrons along a chain of electron carrier molecules in either the mitochondria or chloroplasts.
Roles of ATP
Instable phosphate bonds makes ATP a good energy donor but also makes it an unsuitable long-term storage molecule.
ATP is therefore an immediate energy source for cells.
As a result, cells do not store large quantities of ATP but rather just maintain a few seconds supply. This is not a problem as ATP is rapidly reformed.
ATP is a better immediate energy source than glucose for the following reasons:
a. Each ATP molecule releases less energy than each glucose molecule.
The energy for reactions is released in small, manageable quantities.
b. Hydrolysis of ATP is a single reaction and releases immediate energy.
The breakdown of glucose is a long series of reactions and therefore energy release takes longer.
Roles of ATP
ATP cannot be stored and so has to be continuously made in the mitochondria. ATP is the source of energy for:
a. Metabolic processes: ATP provides the energy needed to build macromolecules: i. Polysaccharide synthesis from monosaccharides.
ii. Polypeptide synthesis from amino acids.
iii. DNA/RNA synthesis from nucleotides.
b. Movement: It provides energy for muscle contraction by providing energy for the muscle filaments to slide past each other and therefore shorten.
c. Active transport: Provides the energy to change the shape of carrier proteins in plasma membranes.
d. Secretion: Helps to form the lysosomes necessary for secretion.
e. Molecule activation: Phosphates make molecules more reactive and lower activation energy. So enzyme-catalysed reactions occur more readily.