A2 Unit 3.1 &3.3: ATP & Respiration

Energy Transfer

Energy cannot be created or destroyed, only converted from one form to another. Food contains chemical energy.

Glucose & Fatty Acids are energy rich respiratory substances that are broken down during respiration.

C-C, C-H & C-OH bonds are the high energy bonds broken during respiration.

Lower energy bonds are formed and the difference is released. Energy from respiration is transferred to ATP & released as heat.

Define Endergonic & Exergonic

Endergonic - A reaction in which energy is absorbed and stored in the products.

Exergonic - A reaction in which energy is released from the products.

What is ATP & Synthesis of ATP?

ATP is referred to as the ‘Universal Energy Currency’ as ATP is present in all living organisms and used to provide energy for all cell reactions.

ATP Synthesis - Condensation reaction forms a bond between phosphate groups (Phosphorylation) when ADP reacts with inorganic phosphate during respiration - ADP + Pi (inorganic phosphate) —> ATP (Endergonic)

How is energy released from ATP?

ATP Breakdown - Hydrolysis reaction that breaks a bond between phosphate groups (Dephosphorylation) of ATP - ATP —> ADP + Pi + 30.6kJmol-1 (Exergonic - happens when energy is used, e.g during active transport)

• High energy bond broken between 2nd & 3rd Phosphate group by enzyme ATPase to release energy from ATP to provide energy for different chemical reactions.

Define Anabolism & Catabolism

Anabolism - Smaller molecules built up to form larger molecules.

Catabolism - Complex molecules broken down to form smaller molecules.

Advantages of ATP as an energy carrier

• 1 enzyme needed to break down ATP (ATPase)

• Single step reaction (only 1 bond broken)

• Releases small amounts of energy when and where needed

• Less energy wasted and energy release controlled

Respiration

Aerobic Respiration: Glucose + Oxygen —> Carbon Dioxide + Water & 38 ATP Molecules

Anaerobic Respiration: Glucose —> Lactic Acid

Enzymes such as ATPase catalyse the series of reactions in respiration.

Respiration is Catabolic.

ATP production in mitochondria & chloroplasts

ATP is produced during:

• Respiration (chemical energy from glucose is transduced to chemical energy in ATP)

• Photosynthesis (light energy is transduced to chemical energy in ATP)

What is Chemiosmosis?

Chemiosmosis: Flow of protons down an electrochemical gradient across membranes through a stalked particle with ATP synthase (which catalyses phosphorylation of ADP into ATP)

• ATP is synthesised by the enzyme ATP synthase

• ATP synthetase is located in stalked particles

• Stalked particles are present in the inner membranes (cristae) of mitochondria & thylakoid membranes of chloroplasts

• Proton pumps are driven by potential energy from excited electrons in an electron transport chain (an alternate arrangement of proton pumps and electron carriers)

Chemiosmosis Labelled Diagram

Describe Chemiosmosis

1. A molecule in a reduced form (has extra hydrogens attached) becomes oxidised.

2. The hydrogen atom passes through a proton pump. As it is passed through, an electron is removed, and the remaining proton (H+) is pumped to the other side of the membrane.

3. The electrons are passed from one proton pump/ electron carrier to another. The energy released in this process is used by the proton pumps to pump more protons across the membrane.

4. A higher concentration of protons builds up on one side of the membrane - they cannot diffuse across the membrane as the membrane is impermeable to H+. The difference in H+ concentration creates a difference in the pH. (more H+ = more acidic) and a big difference in charge across the membrane.

Embedded in the membrane are numerous stalked particles which contain the enzyme ATP synthetase.

Chemiosmosis

5. The protons can pass down an electrochemical gradient through a channel in the enzyme.

6. The protons have high energy. This energy is used by the ATP synthetase to combine ADP with organic phosphate to form ATP.

7. Some Hydrogen ions are pumped back across the membrane to maintain the electrochemical gradient across the membrane.

8. The electrons passed through the electron carrier proteins are picked up by a final electron acceptor - it also picks up some protons and is reduced in the process.

Labelled Diagrams - Electron Transport Chain, Mitochondria & Chloroplast

Features of Mitochondria & Chloroplasts

Respiration

What are the 4 Stages of Aerobic Respiration?

1. Glycolysis

2. Link reaction

3. Krebs cycle

4. Electron transport chain

Glycolysis

Glycolysis occurs in the cytosol (cytoplasm)

Stages:

• Phosphorylation of glucose into 6 carbon hexose bisphosphate using phosphate from hydrolysis of 2 ATP molecules

• Hexose bisphosphate is unstable & has a lower activation energy & splits into two 3C triose phosphate molecules

• Oxidation (dehydrogenation) of each of triose phosphate to 3C pyruvate through loss of hydrogen by a dehydrogenase enzyme reduces 2 NADs (1 per triose phosphate).

4 ATP are produced (2 per triose phosphate) by substrate level phosphorylation.

Substrate level phosphorylation: energy released in a reaction is used to produce ATP directly from ADP + Pi & phosphate groups are transferred from donor molecules

Glycolysis Labelled Diagram

Glycolysis - Products & Reactions per Glucose

Link Reaction

The Link reaction occurs in the mitochondrial matrix. Only occurs in the presence of oxygen.

Stages (for each of the 2 pyruvates from glycolysis):

• Pyruvate transported into mitochondrial matrix

• 3C pyruvate is decarboxylated (releasing 1 molecule of carbon dioxide per pyruvate) by a decarboxylase enzyme producing 2C acetate

• Oxidation (dehydrogenation) of each pyruvate through loss of hydrogen by a dehydrogenase enzyme reduces NAD (1 per pyruvate)

• Acetate is activated by combining with co-enzyme A to produce acetyl co-enzyme A

(Oxidative decarboxylation of pyruvate to form acetyl coenzyme A)

Link reaction labelled diagram

Link Reaction - Products & Reactions per Glucose

Krebs Cycle (citric acid cycle)

Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix. Only occurs in the presence of oxygen.

Stages (for each of the 2 acetates from the link reaction):

• Acetate from acetyl coenzyme A combines with a 4C compound the form a 6C acid

• 6C acid is decarboxylated (releasing 1 molecule of carbon dioxide) by a decarboxylase enzyme & oxidised (degydrogenated) through loss of hydrogen by a dehydrogenase enzyme, reducing NAD (1 per 6C acid) producing a 5C acid

• 5C acid is decarboxylated (releasing 1 molecule of carbon dioxide) by a decarboxylase enzyme & oxidised (dehydrogenated) through loss of hydrogen by a dehydrogenase enzyme, reducing 2 NAD & 1 FAD (per 5C acid) to produce a 4C acid. 1 ATP is produced by substrate level phosphorylation.

• The 2C acetate fragment that entered the Krebs cycle is completely broken down by the oxidative decarboxylation reactions & the 4C acid is regenerated & combines with acetyle CoA to repeat the cycle.

NB: H2O is fed in at 3 reactions. The oxygen that combines with carbon (from decarboxylation) to form carbon dioxide comes from water molecules.

Krebs Cycle Labelled Diagram

Krebs Cycle - Products & Reactions per Glucose

Electron Transport Chain