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www.hoddereducation.co.uk/biologicalsciencesreview
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
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adenosine
triphosphate
ATP: a phosphorylated nucleotide
adenine
three phosphate ions
ribose
adenosine diphosphate (ADP)
adenosine triphosphate (ATP)
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A simpler representation of ATP
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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
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ATP can be hydrolysed
Representation 3:
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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
ADP + Pi
2 × amino acids
dipeptide
This is why ATP is often referred to as the
energy currency of cells.
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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.
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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
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ATP
A-level content
– all the AS content plus what follows
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How is ATP made?
All organisms produce ATP from ADP by one of three basic chemical methods:
• Substrate-level phosphorylation
• Oxidative phosphorylation
• Photophosphorylation
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Substrate-level phosphorylation
ADP
SP
In a chemical reaction, a phosphorylated
substrate (S) loses its phosphate group
to ADP.
In your specification, this occurs in, for
example, glycolysis.
ATP
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S
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
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Some ATP is made during…
glycolysis
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the Krebs cycle
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.
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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.
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Oxidative phosphorylation and
photophosphorylation compared
Feature
Oxidative phosphorylation
Reaction by which ATP is made
ATP synthase
ADP + Pi
ATP synthase
ATP
Location of ATP synthase
Embedded in inner membrane
of mitochondrial cristae
Coenzyme that provides
electrons
Reduced NAD
Notice the similarity between these two processes.
This goes further — both involve chemiosmosis.
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Photophosphorylation
ADP + Pi
ATP
Embedded in thylakoid
membranes of chloroplasts
Reduced NADP
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.
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Chemiosmosis in mitochondria
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Chemiosmosis in chloroplasts
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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