Adenosine triphosphate (ATP) is an energy storage molecule which is synthesised and utilised during metabolic processes such as photosynthesis and respiration.  ATP is formed from a nitrogenous base (adenine), a pentose sugar (ribose) and three phosphate groups.  

In photosynthesis, ATP synthesis takes place during photophosphorylation.  Photophosphorylation, which involves photolysis, chemiosmosis and reduction, occurs in the photosystems and electron transport chains that are embedded in the granal membranes of chloroplasts.  The products of photophosphorylation are ATP, NADPH and O2.   

ATP and NADPH are used during the Calvin-Benson cycle, which takes place in the stroma, to convert inorganic carbon dioxide into organic assimilates (carbohydrates, proteins and lipids) through the processes of carbon fixation, phosphorylation and reduction.  Regeneration of ribulose 1,5-bisphosphate (RuBP) also takes place. 

Water, which is needed during photophosphorylation, enters plants via root hairs and travels to palisade cells in the leaves via the xylem tissue.  Movement of water into and through the plant is known as the transpiration stream and involves active transport, osmosis, cohesion, adhesion and evaporation.   

Carbon dioxide, which is needed during the Calvin-Benson cycle, enters plants via open stomata and moves through the spongy mesophyll to the palisade cells by simple diffusion.   Opening stomata involves photophosphorylation, active transport and osmosis; stomata close due to water stress through the action of abscisic acid (ABA). 

Assimilates produced during the Calvin-Benson cycle are transported in the phloem tissue from sources to sinks by a process known as the mass flow theory of translocation; this involves differences in hydrostatic pressure. 

In aerobic respiration, ATP is synthesised by substrate level phosphorylation during glycolysis and the Krebs cycle and by oxidative phosphorylation.  Glycolysis takes place in the cytoplasm whereas the link reaction and Krebs cycle take place in the mitochondrial matrix.  Oxidative phosphorylation occurs in the electron transport chains that are embedded in the cristae of mitochondria.  The products of aerobic respiration are CO2, H2O and ATP. 

Oxygen, which is required during aerobic respiration as the final electron acceptor, enters the body by diffusion via alveoli in the lungs.  Alveoli are a specialised exchange surface which are closely associated with blood capillaries.  Once in the blood, oxygen is transported by haemoglobin found in erythrocytes (red blood cells) to respiring tissues.  The uptake and release of oxygen is affected by the concentration of carbon dioxide in the blood; this is known as the Bohr Effect. 

During anaerobic respiration, the lack of oxygen leads to other substances acting as the final electron acceptor.  In the lactate pathway, pyruvate acts as the electron acceptor.  In the ethanol pathway, acetaldehyde acts as the electron acceptor.