Importance of ATP

ATP can be described as the universal energy currency because it is used by all cells in all metabolic reactions. 

It is characteristic of all living systems and was believed to be present in LUCA (the last universal common ansestor of all cells). 

ATP is suited to its role as the universal energy currency because:

• it is inert, so it doesn't react with many chemicals 
• small enough to pass out fo the mitochondria into the cytoplasm 
• releases energy efficiently as only one enzyme is needed for the reaction
• releases energy in useable quantities so little is wasted as heat
• easily hydrolysed to release energy 
• readily reformed by phosphorylation. 

ADP + pi = ATP

H20 = 

When ATP is produced protons and electrons have different paths they must follow. 

Electrons - 

Electrons produced from hydrogen atoms are passed from one molecule to the next along a chain in a series of reactions. Each transfer is a redox reaction, in which one molecule is oxidised (loses electrons) whilst the next in the sequence is reduced (gains electrons). It is the oxidation reactions that make energy available and used later to synthesis ATP. 

Protons - 

The energy released in oxidation pumps the protons across a membrane so that they found higher in concentration on one side than the other. This difference in concentration gradient constitutes an electrochemical gradient and is a source of energy. The protons then diffuse down this gradient in a process called Chemiosmosis, through the enzyme ATP synthetase. 

A high concentration of hydrogen ions can be found in the inter-membrane space of the mitochondrion. Imbedded in the phospholipid bilayer is a stalked particle, in which hydrogen ions can pass through into the matrix of the mitochondrion, this is the only path hydrogen ions can take as the membrane is impermeable to hydrogen ions.  

Within this stalked particle is an enzyme called ATP synthetase. This enzyme is excited by the passage of a hydrogen ion and converts ADP into ATP:

ADP + Pi = ATP 

The hydrogen ions within the matix are then activly transported back to the inter-membrane space through a proton protein pump which is energised by the transmission of electrons discussed earlier. This is to ensure that the concentration remains higher on one side of the membrane than the other to ensure that the process can continue to occur. 

Proton gradient is undemental characteristic of all living things.

Photosynthesis 

In the light dependent stage of photosynthesis the electrons are excited by energy from light and move through a series of carriers in the thylakoid membranes and pumps protons from the stroma into the inter-membrane space. Creating a proton gradient. 

Respiration 

In respiration, the electrons are excited by energy derived from food molecules. Energy is made available as the electrons move through a series of carriers imbedded on the mitochondrial membrane. This energy is used to activate proton protein pumps which activly transports protons from the low concentration in the matrix to the higher concentration in the inter-membrane space against their concentration gradient. 

By disrupting the proton gradient serious side effects may occur, in severe situations it may lead to death.

• Apoptosis is programmed cell death and occurs during embryonic development. It prevents proton gradients across cell membrances from forming. 
• DNP is a mitochondrial poison which allows the electron transport chain to occur, but not ATP synthesis. DNP has been used in tablets to help lose weight, as the body begins to oxidise fats and carbohydrates which results in weight loss. However, the energy released from fats and carbohydrates is released as heat, as no ATP can be made, and in severe cases overheating can be fatal. 

The electron transport chain is a series of protein carriers on the inner membranes of the mitochondria and chloroplasts. Energy is released from electrons and used to form ATP. 

Respiration

Hydrogen produced from the breakdown of glucose are transferred via dehydrogenase enzymes to the coenzymes NAD and FAD. For every two protons delivered by reduced NAD enough energy is released to create three molecules of ATP. For every two protons delivered by reduced FAD enough energy is released to create two molecules of ATP. 

Energy for the proton pumps and the electron transport chains is made by oxidation reactions.

Phosphorylation is the addition of a phosphate group

Therefore, ATP is created by energy from reactions known as oxidative phosohorylation.

Photosynthesis 

Pigments called photosystems transfer electrons to electron acceptors and then to proton carriers along the thylakoid membranes.

The protons and electrons from water are transferred to the coenzymes NADP

Energy that powers the proton pump and electron transport chain in the chloroplast comes from light, so chloroplasts create ATP by photophosphorylation.