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Option 9.9 Biochemistry: 7. Structure and function of ATP

Syllabus reference (October 2002 version)
7. ATP is the energy source of every living cell	Students learn to: identify that adenosine triphosphate is an energy source for nearly all cellular metabolic processes explain that the biologically important part of the molecule contains three phosphate groups linked by high energy phosphodiester bonds outline the discovery of ATP synthesis in the mid 20th century in terms of: the discovery of photophosphorylation in chloroplasts of plants the discovery that ATP synthesis involves an electron transfer reaction occurring across a membrane	Students: gather and process information from a diagram or model of the structure of the adenosine triphosphate molecule to discuss the nature and organisation of the phosphodiester bonds between the phosphate groups

Extract from Biology Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit 18 June 09]

Prior learning: Stage 4-5 Syllabus, Structures and Systems 4.8.2 (c), 4.8.4(d) Interactions 4.10 (c);

Preliminary module 8.2 (subsection 2) module 8.3 (subsection 4)

Background: ATP or adenosine triphosphate is the energy currency of every living cell. It powers all the actions found within the cell. The ATP molecule contains three phosphate groups that are linked by phosphodiester bonds. These bonds are high-energy bonds.

gather and process information from a diagram or model of the structure of the adenosine triphosphate molecule to discuss the nature and organisation of the phosphodiester bonds between the phosphate groups

A phosphodiester bond is an ester linkage between two phosphate groups.

• Gather information from various sources including popular scientific magazines, textbooks like the ones previously listed (Option 9.9;Part 6) and the Internet including the sites below to find a diagram or model of ATP.
• Process the information to illustrate patterns in the molecule. Use the secondary sources to draw and label an adenosine triphosphate (ATP) molecule. On the molecule colour the phosphodiester bonds between the phosphate groups. Also label the adenosine molecule and the ribose sugar.
  

ATP Biology Pages, J Kimball

ATP Synthesis Modern Biology/Biochemistry Flash Tutorials, Carnegie Mellon University, Pittsburg, Pennsylvania, USA

identify that adenosine triphosphate is used as an energy source for nearly all cellular metabolic processes

• A cell does three kinds of work:
  1. Mechanical work such as the movement of muscle.
  2. Active transport in which substances are moved against their concentration gradient.
  3. Chemical work that would not occur spontaneously without a supply of energy.
• ATP provides the energy for this cellular work by transferring a phosphate group to an intermediate that is then “phosphorylated”. The molecule that receives the phosphate is then more reactive than the original molecule.

• ATP is regenerated from adenosine diphosphate (ADP) and inorganic phosphate (P_i) in the cellular process of respiration and photosynthesis.

explain that the biologically important part of the molecule contains three phosphate groups linked by high-energy phosphodiester bonds

• The energy used by cells is stored in the bonds between the phosphate groups in ATP. The hydrolysis (splitting) of ATP to give ADP and P_i yields 30.5 kJ of energy per mole of ATP. This large amount of free energy comes from breaking one phosphodiester bond.

outline the discovery of ATP synthesis in the mid 20th century in terms of:

• the discovery of photophosphorylation in chloroplasts of plants
• the discovery that ATP synthesis involves an electron transfer reaction occurring across a membrane

• The light independent reaction of photosynthesis requires considerable energy, 2870kJ per mole of 6-carbon sugar formed. It requires 12 NADPH and 18ATP produced by the light dependent reaction. During the light dependent reaction, electrons are transferred and reducing power in the form of NADPH is generated. (see Biochemistry 9.9: 4 The light reaction.)
• Coupled with the electron transfers of the light dependent reaction is the movement of protons (H⁺, hydrogen ions) from the stroma of the chloroplast, across the thylakoid membrane into the space or lumen inside the thylakoid.
• The thylakoid membrane does not allow the passive diffusion of H⁺ and so an electrochemical gradient builds up, the inside of the thylakoid becoming positive due to the accumulation of hydrogen ions.
• The movement of the protons across the thylakoid membrane back to the stroma in protein channels drives the synthesis of ATP by the enzyme ATP synthase. The protons cause a conformational (or shape) change in the ATP synthase. This light driven synthesis of ATP is called photophosphorylation.
• In 1961 Peter Mitchell proposed the chemiosmotic hypothesis. He proposed the hypothesis for the mitochondrial membrane and the production of ATP during respiration but the first proof of the hypothesis was for photophosphorylation.
• His hypothesis was based on the following observations:
  • ATP synthesis appeared to be membrane based
  • An electrochemical gradient based on a H⁺ concentration gradient was generated when respiration occurred.
• Mitchell proposed that ATP synthesis was driven by an electrochemical gradient across a membrane rather than by the “intermediate” sought by other researchers. He proposed that the gradient was produced by electron transport and that the flow of protons back across the membrane was coupled to ATP synthesis
• In 1966 Jagendorf and Uribe proved that photophosphorylation was chemiosmotic. In the experiment they incubated chloroplasts at pH 4 (hydrogen ion concentration = 1 x 10^{- 4} molL^-1) . They:
  • added ADP and radioactively labeled ³²P, then quickly raised the pH to 8.
  • detected ATP containing radioactive ³²P as the difference in pH on the inside and the outside of the chloroplasts became equal.
  • carried out the experiments in the dark.
• In 1974 Efraim Racker and Walther Stoeckenius confirmed Mitchell’s chemiosmotic hypothesis using light and bacteriorhodopsin, the equivalent of chlorophyll in the bacteria Halobacterium halobium.
• Work by John Walker and Paul Boyer has determined the structure of ATP synthase. They shared the Nobel Prize for their work in 1997.

1. In your research you may have read the terms ”cyclic” and “non-cyclic” when referring to photophosphorylation. Cyclic photophosphorylation is less than 5% of the rate of non-cyclic photophosphorylation, depends only on PS I and does not produce NADPH. The notes provided here, refer to non-cyclic photophosphorylation.
2. Mitchell was not able to prove the proton gradient produced ATP when he proposed his hypothesis. Later experiments using change in hydrogen ion concentration (pH) and mitochondria showed that ATP was produced when the H⁺ crossed the membrane. Peter Mitchell was awarded the Nobel Prize in 1978 for his proposal of what by that time had become the chemiosmotic theory.
3. The increased pH is equivalent to a reduced H⁺ ion concentration on the outside of the chloroplast. The H⁺ ions move out of the chloroplast along the concentration gradient by diffusion (from more concentrated to less concentrated). This change in pH was meant to be equivalent to the effect of the light dependent reaction on the chloroplasts.