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A2 Unit 4: Metabolism, Microbiology and Homeostasis Name: Date: Topic 4.1 The Importance of ATP – Page 2 l. ATP Production Completed 1. Go through the PowerPoint in Class 2. Read through the notes page 2-4 and complete the questions 3. Read through the BioFactsheet on Chemiosmosis and complete the

questions

4. Have a look at some past paper questions on the wikispace.

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The addition of phosphate is known as phosphorylation. ATP acts as an energy carrier in all cells and all cells use it so it is referred to as universal. ATP can be produced within cells by two methods:

• Substrate-Level Phosphorylation

And

• Chemiosmosis (used in both oxidative and photophosphorylation)

Substrate-Level Phosphorylation (SLP) Involves the transfer of phosphate from a high-energy intermediate molecule to ADP, with the aid of an enzyme. The process of forming ATP by the physical addition of a phosphate group to ADP is known as phosphorylation. ADP + Pi ATP This takes place in the cytoplasm during the first stage of aerobic respiration known as glycolysis and also within the mitochondrion in another stage of aerobic respiration known as the Krebs cycle.

Why is the synthesis of ATP described as a phosphorylation reaction? The phosphorylation of ADP to form ATP requires 30.6 kJ mol-1 of energy. The energy to power this reaction must come from somewhere within the cell. Any chemical reaction that occurs within the cell and releases 30.6 kJ mol-1 of energy and can donate a phosphate group could be used to convert ADP and Pi into ATP. What does Pi stand for? Why do you think that the letter P is not used? Using the energy from chemical reactions to produce ATP is known as substrate level phosphorylation. i.e. using the energy from substrate molecules to power phosphorylation.

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Substrate level phosphorylation takes place during glycolysis, the first stage of respiration (in the cytoplasm) and also the Krebs cycle, which takes place in the matrix of the mitochondria.

Glycolysis Krebs Cycle Looking at ATP generation in glycolysis. Using the diagram of glycolysis to outline two reactions that must produce at least 30.6 kJ mol-1 of energy and produce ATP and are hence are examples of substrate level phosphorylation reactions.

1. 2. How many substrate level phosphorylation reactions occur per turn of the Krebs cycle?

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Chemiosmosis Look at the diagram of a hydroelectric power station.

In a hydroelectric power station there is a difference in the level of water on either side of the dam wall and this is a store of potential energy. A gate is opened in a dam wall and the water flows through, turning turbines that turn generators to create electricity. In chemiosmosis ATP is generated in a similar manner. In a cell what structure do you think could act as a dam wall? In cells the difference is not with water but is created using protons (H+), so more protons are on one side of the membrane compared to the other side. This creates a gradient (a difference) in protons on one side of the membrane compared to the other. What do you think could act as a gate in the membrane? This allows the controlled flow of protons through the membrane. Attached to this membrane protein is an enzyme ATP synthetase. As the H+ ions flow through the protein pore energy is released. This energy and the enzyme ATP synthetase are used to attach a Pi group to the molecule ADP to produce ATP.

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Chemiosmosis is used in both aerobic respiration and photosynthesis. In order to create a proton gradient the energy to drive this comes from high-energy electrons. In respiration these high-energy electrons are produced from oxidation reactions and the production of ATP in this manner is known as oxidative phosphorylation. In photosynthesis these high-energy electrons are produced by the absorption of light energy and the production of ATP in this manner is known as photophosphorylation.

Protons have to be pumped into a space for chemiosmosis. In mitochondria this space is known as the intermembrane space and is found between the inner and outer membranes.

• Label this on the diagram on the left

In chloroplasts this space is found inside the thylakoid.

• Label this on the diagram

Summary Chemiosmosis requires a phospholipid bilayer, a proton pump, a proton gradient and the enzyme ATP synthetase. Energy from high-energy electrons is used to pump protons into a space either an inter membrane of a mitochondria or a thylakoid space of a chloroplast. The proton gradient is used to rotate the enzyme ATP synthetase and power the addition of a phosphate group onto ADP.

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