What is it? Why is it important? How is it made? ATP What is it? Why is it important? How is it made? Hodder & Stoughton © 2017

What is ATP? Its full name is adenosine triphosphate Each molecule of ATP contains: one molecule of the purine base, adenine one molecule of the pentose, ribose three phosphate groups triphosphate adenosine Philip Allan Publishers © 2017

ATP: a phosphorylated nucleotide adenine ribose three phosphate ions adenosine diphosphate (ADP) adenosine triphosphate (ATP) Philip Allan Publishers © 2017

A simpler representation of ATP Philip Allan Publishers © 2017

ATP can be hydrolysed Catalysed by ATP hydrolase (ATPase), the hydrolysis of ATP produces: adenosine diphosphate (ADP) a phosphate group (PO43− often represented as Pi) This hydrolysis can be represented in a number of ways: Representation 1: ATP + H2O  ADP + PO43− Representation 2: ATP  ADP + Pi Philip Allan Publishers © 2017

ATP can be hydrolysed Representation 3: Philip Allan Publishers © 2017

Why is the hydrolysis of ATP so important? It can be used to phosphorylate other compounds, making them more reactive. ATP Glucose ADP Glucose phosphate It is exergonic, i.e. releases energy that can be used to drive a coupled energy-requiring (endergonic) metabolic reaction. ATP 2 × amino acids ADP + Pi dipeptide This is why ATP is often referred to as the energy currency of cells. Philip Allan Publishers © 2017

How do cells make ATP? Almost all ATP is made by a condensation reaction catalysed by ATP synthase: ATP synthase ADP + Pi ATP + H2O In animals, this occurs during respiration. In plants, this occurs during respiration and during photosynthesis. Philip Allan Publishers © 2017

This ends the AS content Ofqual accredited GCE board AS content regarding ATP AQA (section 3.1.6) structure of ATP hydrolysis of ATP coupled to energy-requiring reactions or to phosphorylation of other compounds during respiration or photosynthesis, ATP is resynthesised by the condensation of ADP and Pi, catalysed by ATP synthase Edexcel Knowledge of ATP is not included in AS specification Eduqas (sections 5c and 5d) the central role of ATP as an energy carrier and its use in the liberation of energy for cellular activity the structure of ATP OCR (section 2.1.3c) the structure of ADP and ATP as phosphorylated nucleotides Philip Allan Publishers © 2017

A-level content ATP – all the AS content plus what follows Philip Allan Publishers © 2017

How is ATP made? All organisms produce ATP from ADP by one of three basic chemical methods: Substrate-level phosphorylation Oxidative phosphorylation Photophosphorylation Philip Allan Publishers © 2017

Substrate-level phosphorylation ADP SP ATP S In a chemical reaction, a phosphorylated substrate (S) loses its phosphate group to ADP. In your specification, this occurs in, for example, glycolysis. Philip Allan Publishers © 2017

Substrate-level phosphorylation During the final reaction of glycolysis, two phosphate groups from each triose phosphate molecule are transferred to two ADP molecules Ignoring the reduction of NAD also involved, we could represent this as: 2 triose phosphate + 4 ADP → 2 pyruvate + 4 ATP Philip Allan Publishers © 2017

Some ATP is made during… glycolysis the Krebs cycle Philip Allan Publishers © 2017

Oxidative phosphorylation Occurs during aerobic respiration. Electrons from reduced coenzymes (mainly reduced NAD) enter the electron transfer chains in the cristae of mitochondria. Passage of electrons down the electron transfer chains releases the energy that drives the production of ATP from ADP and inorganic phosphate (ADP + Pi  ATP). The synthesis of ATP is catalysed by the enzyme ATP synthase, which is embedded in the inner membranes of the cristae. Philip Allan Publishers © 2017

Photophosphorylation Occurs during the light-dependent reaction of photosynthesis. Absorption of light by chlorophyll results in electrons leaving chlorophyll molecules and being taken up by molecules of coenzyme, NADP: NADP + 2 e− → reduced NADP Electrons from reduced NADP enter the electron transfer chain in the thylakoid membranes of chloroplasts. Passage of electrons down the electron transfer chains releases the energy that drives the production of ATP from ADP and inorganic phosphate (ADP + Pi  ATP). The synthesis of ATP is catalysed by the enzyme ATP synthase, which is embedded in the membranes of the thylakoids. Philip Allan Publishers © 2017

Oxidative phosphorylation and photophosphorylation compared Feature Oxidative phosphorylation Photophosphorylation Reaction by which ATP is made ATP synthase ADP + Pi ATP Location of ATP synthase Embedded in inner membrane of mitochondrial cristae Embedded in thylakoid membranes of chloroplasts Coenzyme that provides electrons Reduced NAD Reduced NADP Notice the similarity between these two processes. This goes further — both involve chemiosmosis. Philip Allan Publishers © 2017

Chemiosmosis Energy released as electrons pass down electron transfer chains and enables proteins embedded in membranes of each thylakoid or crista to pump protons (H+) through the membrane: through inner membrane of crista into space between inner and outer membrane (the intermembrane space) through thylakoid into space in thylakoid (thylakoid space) This creates a proton gradient across these membranes. As a result, protons diffuse down this proton gradient: from intermembrane space into matrix of mitochondrion from thylakoid space into stroma of chloroplast The only place they can diffuse is through ATP synthase — an enzyme embedded in these membranes. The diffusion of protons through ATP synthase provides it with the energy to produce ATP from ADP and Pi. Philip Allan Publishers © 2017

Chemiosmosis in mitochondria Philip Allan Publishers © 2017

Chemiosmosis in chloroplasts Philip Allan Publishers © 2017

Extension reading for A-level chemistry students Try the following accounts of respiration and photosynthesis from the Royal Society of Chemistry website: http://www.rsc.org/Education/Teachers/Resources/cfb/respiration.htm http://www.rsc.org/Education/Teachers/Resources/cfb/photosynthesis.htm   This resource is part of Biological Sciences Review, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to www.hoddereducation.co.uk/biologicalsciencesreview Philip Allan Publishers © 2017