Chapter 2 - ATP

Energy is defined as 'the ability to do work'. Some facts are listed below:

• Takes many different forms (e.g. chemical, light & heat)
• It can be changed from one form to another
• It can't be created or destroyed
• It's usually measured in Joules (J)

Why do organisms need energy?

• Metabolism - All reactions occuring in living things need energy
• Movement - Both within an organism (e.g. blood circulation) & movement of the whole organism (locomotion)
• Active Transport - of ions & molecules against a concentration gradient across membranes
• Maintenance, repair and division - of cells & organelles within them
• Production of substances - E.g. enzymes and hormones
• Thermoregulation - In birds & mammals. These organisms are endothermic & need energy to replace that lost as heat to the environment

The flow of energy through living systems occurs in 3 stages:

1. Light energy from the sun is converted into chemical energy during photosynthesis

2. The chemical energy from photosynthesis (in the form of organic molecules) is converted into ATP during respiration

3. ATP is used by cells to perform useful tasks

How ATP stores energy

• ATP has 3 phosphate groups - these are key to understanding how ATP stores energy.
• The bonds between P groups are unstable, so have a low activation energy & are easily broken
• When they do break (hydrolysis reaction) they release a considerable amount of energy. 
• Usually it is only the terminal P that is removed, according to the equasion:

ATP + H20 -------------> ADP + Pi + Energy

The conversion of ATP to ADP is a reversible reaction, therefore energy can be used to reattach P to ADP. Water is removed, so this is a condensation reaction. 

Synthesis of ATP from ADP involves addition of a phosphate molecule to ADP, it occurs in 3 ways:

• Photophosphorylation - takes place in chlorophyll containing plant cells during photosynthesis
• Oxidative phosphorylation - Occurs in mitochondria (plant & animal) during electron transport
• Substrate - level phosphorylation -  Occurs in plant & animal cells when P groups are transferred from donor molecules to ADP to make ATP. For example, in the formation of pyruvate at the end of glycolysis. 

In the first two cases, ATP is synthesised used energy released during the transfer of electrons along a chain of electron carrier molecules in either the chloroplasts or mitochondria. 

The instability of the P bonds makes ATP a poor long term energy store. Fats & carbs (e.g. glycogen) perform this role much better. ATP is an immediate energy source - cells don't store much ATP, just a few seconds' supply. 

ATP is a better immediate energy source than glucose for the following reasons:

• ATP releases less energy than glucose. Energy is released in smaller, manageable amounts rather than the larger & harder to manage energy from a glucose molecule
• Hydrolysis of ATP is a single reaction that releases energy immediately - breakdown of glucose takes much longer.

ATP can't be stored for long, so must be continually made in the mitochondria of the cells that need it. Cells such as muscle fibres & the epithelium of the small intestine (that need energy for movement) have lots of large mitochondria.