Importance

• ATP (adeninosine triphosphate) is described as the universal energy currency, as it is used in all reactions in all organisms. ATP is:

  • Inert - Meaning it is not chemically reactive.

  • Can pass out of mitochondria into the cytoplasm.

  • Releases energy in small quantities to avoid waste.

  • Is easily hydrolysed and easily goes through phosphorylation, which are reversible.

  • Efficient release of energy.

Structure

• ATP is a nucleotide, meaning it requires three things:

  • A nitrogenous base - adenine.

  • A pentose sugar (five carbon) - ribose.

  • Phosphate - x3.

Reactions

Hydrolysis

• This is the addition of water to split the second and third phosphate in the ATP molecule.

  • This releases 30.6kjs of energy.

  • It also releases ADP (adeninosine diphosphate) and an inorganic phosphate.

  • It requires the enzyme ATPase.

• It is an exergonic and reversible reaction.

Phosphorylation

• This is a reaction that adds phosphate.

  • ADP + P = ATP.

  • It is a condensation reaction, meaning it has a product of water.

  • It requires the enzyme ATP synthetase.

• It is an endergonic and reversible reaction.

• There are 3 types of phosphorylation:

  • Oxidative - Occurs during aerobic respiration on the inner membranes (cristea) of the mitochondria.

    • The energy to make this ATP comes from oxidation and reduction reactions, and is released in the transfer of electrons along a chain of electron carrier molecules.

  • Photophosphorylation - Occurs in the light-dependent stage of photosynthesis on the thylakoid membranes of the chloroplasts.

    • The energy to make ATP comes from light and is released in the transfer of electrons along a chain of electron carrier molecules.

  • Substrate-level - Occurs when phosphate groups are transferred to donor molecules during glycolysis or in the Krebs cycle.

Membranes

• ATP is synthesised using proton gradients across membranes.

• It is done in different ways:

  • Photosynthesis uses the thylakoid membranes of the chloroplasts.

  • Respiration uses the inner membranes of the mitochondria.

  • Bacteria use their cell membrane, as they do not have inner membranes, and pump protons out into the cell wall.

• This supports the endosymbiotic theory.

• Membranes are watertight to fit this purpose, as protons are small and easily pass through water molecules. This leads them to become known as sealed membranes.

Proton gradients

• Proton gradients occur in non-living systems, such as oceanic alkaline hydrothermal vents.

  • This shows they may even have a significant role in the origin of life, as they are a fundamental characteristic of living things.

• In respiration, electrons are excited by energy derived from food molecules, which drives protons across a membrane to create a gradient.

  • Energy is then released in chemiosmosis, which creates ATP.

  • Energy not used in this process is lost as heat.

Electron transport chain

• This is where oxidative respiration occurs in respiration, via the transport of hydrogen atoms across a gradient.

• In Bacteria, Archeae and Eukaryotes ATP synthetase still occurs, so it likely evolved very early in life’s history.