Biological Molecules - ATP Flashcards
What is the structure of ATP?
The ATP molecule is a phosphorylated macromolecule. It has three parts:
- adenine: a nitrogen-containing organic base
- ribose: a sugar molecule with a 5-carbon ring structure (pentose sugar) that acts as the backbone to which the other parts are attached
- phosphates: a chain of three phosphate groups (phosphorylated nucleotide)
How does ATP store energy?
Adenosine triphosphate (ATP) is a nucleotide and has three phosphate groups. These are the key to how ATP stores energy. The bonds between these phosphate groups are unstable and so have a low activation energy, which means they are easily broken. Usually in living cells, it is only the terminal phosphate that is removed.
As water is used to convert ATP to ADP, this is known as a hydrolysis reaction. The reaction is catalysed by the enzyme ATP hydrolase (ATPase).
What is the equation for the hydrolysis of ATP?
ATP + H2O -> ADP + Pi (+ energy)
How is ATP synthesised?
The conversion of ATP to ADP is reversible reaction and therefore energy can be used to add an inorganic phosphate to ADP to re-form ATP.
This reaction is catalysed by the enzyme ATP synthase. As water is removed in this process, the reaction is known as a condensation reaction.
What does the synthesis of ATP from ADP involve? What are the three ways in which this occurs?
The synthesis of ATP from ADP involves the addition of a phosphate molecule to ADP. It occurs in three ways:
- in chlorophyll-containing plant cells during photosynthesis (photophosphorylation)
- in plant and animal cells during respiration (oxidative phosphorylation)
- in plant and animal cells when phosphate groups are transferred from donor molecules to ADP (substrate-level phosphorylation)
Why is ATP not a good long-term energy store?
The same feature that makes ATP a good energy donor, namely the instability of its phosphate bonds, is also a reason why it is not a good long-term energy store. Fats, and carbohydrates such as glycogen, serve this purpose far better. ATP is therefore the immediate energy source of a cell. 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 re-formed from ADP and inorganic phosphate (Pi) and so a little goes a long way.
Why is ATP a better immediate energy source than glucose?
- Each ATP molecule releases less energy than each glucose molecule. The energy for reactions is therefore released in smaller more manageable quantities rather than the much greater, and therefore less manageable, release of energy from a glucose molecule.
- The hydrolysis of ATP to ADP is a single reaction that releases immediate energy. The breakdown of glucose is a long series of reactions and therefore the energy release takes longer.
- ATP can be used by many energy requiring process and it isn’t stored in large quantities as ADP can be quickly recycled back to ATP.
When is ATP synthesised and hydrolysed?
ATP is synthesised during reactions that release energy and it is hydrolysed to provide energy for reactions that require it.
Where is ATP made?
ATP cannot be stored and so has to be continuously made within the mitochondria of cells that need it. Cells, such as muscle fibres and the epithelium of the small intestine, which require energy for movement and active transport respectively, possess many large mitochondria.
What energy-requiring processes in cells is ATP used in?
- metabolic processes
- movement
- active transport
- secretion
- activation of molecules
How is ATP used in metabolic processes?
ATP provides the energy needed to build up macromolecules from their basic units. For example, making starch from glucose or polypeptides from amino acids.
How is ATP used in movement?
ATP provides the energy for muscle contraction. In muscle contraction, ATP provides the energy for the filaments of muscle to slide past one another and therefore shorten the overall length of a muscle fibre.
How is ATP used in active transport?
ATP provides the energy to change the shape of carrier proteins in plasma membranes.
ATP adds its third phosphate to the carrier protein. ATP becomes ADP. The phosphorylated protein changes shape and energy is released. The phosphate is released from the protein (Pi). The protein then returns to its original shape.
This allows molecules or ions to be moved against a concentration gradient.
How is ATP used in secretion?
ATP is needed to form the lysosomes necessary for the secretion of cell products.
How is ATP used in the activation of molecules? What can the inorganic phosphate released do?
The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds in order to make them more reactive, thus lowering the activation energy in enzyme-catalysed reactions. For example - the addition of phosphate to glucose molecules at the start of glycolysis.
