8.2 Flashcards
One molecule of ATP contains…
three covalently bonded phosphate groups – which store potential energy in their bonds
Phosphorylation makes molecules…
less stable and hence ATP is a readily reactive molecule that contains high energy bonds
When ATP is hydrolysed (to form ADP + Pi), the energy stored in the terminal phosphate bond is…
released for use by the cell
ATP has two key functions within the cell:
- It functions as the energy currency of the cell by releasing energy when hydrolysed to ADP (powers cell metabolism)
- It may transfer the released phosphate group to other organic molecules, rendering them less stable and more reactive
ATP is synthesised from ADP using energy derived from one of two sources:
- Solar energy – photosynthesis converts light energy into chemical energy that is stored as ATP
- Oxidative processes – cell respiration breaks down organic molecules to release chemical energy that is stored as ATP
Cell respiration is the controlled release of energy from organic compounds to produce ATP
- Anaerobic respiration involves the incomplete breakdown of organic molecules for a small yield of ATP (no oxygen required)
- Aerobic respiration involves the complete breakdown of organic molecules for a larger yield of ATP (oxygen is required)
The breakdown of organic molecules occurs via a number of linked processes that involve a number of discrete steps:
- By staggering the breakdown, the energy requirements are reduced (activation energy can be divided across several steps)
- The released energy is not lost – it is transferred to activated carrier molecules via redox reactions (oxidation / reduction)
When organic molecules are broken down by cell respiration, the chemical energy is transferred by means of…
redox reactions
- Redox reactions involved the reduction of one chemical species and the oxidation of another (redox = reduction / oxidation)
Most redox reactions typically involve the transfer of electrons, hydrogen or oxygen
- Reduction is the gain of electrons / hydrogen or the loss of oxygen
- Oxidation is the loss of electrons / hydrogen or the gain of oxygen
Redox Mnemonics
LEO goes GER – Loss of Electrons is Oxidation ; Gain of Electrons is Reduction
Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules
- As the organic molecule is losing hydrogen atoms and electrons, this is an oxidation reaction
- Energy stored in the organic molecule is transferred with the protons and electrons to the carrier molecules
The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ ions)
- The most common hydrogen carrier is NAD+ which is reduced to form NADH (NAD+ + 2H+ + 2e– → NADH + H+)
- A less common hydrogen carrier is FAD which is reduced to form FADH2 (FAD + 2H+ + 2e– → FADH2)
The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria
- The cristae is the site of the electron transport chain, which uses the energy transferred by the carriers to synthesize ATP
- This process requires oxygen to function, and hence only aerobic respiration can generate ATP from hydrogen carriers
- This is why aerobic respiration unlocks more of the energy stored in the organic molecules and produces more ATP
The first step in the controlled breakdown of carbohydrates is glycolysis, which occurs in the cytosol of the cell
In glycolysis, a hexose sugar (6C) is broken down into two molecules of pyruvate (3C)
first step of glycolysis: Phosphorylation
- A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form a hexose bisphosphate)
- This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell
second step of glycolysis: Lysis
- The hexose biphosphate (6C sugar) is split into two triose phosphates (3C sugars)
third step of glycolysis: Oxidation
- Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
- Two molecules of NADH are produced in total (one from each 3C sugar)
fourth step of glycolysis: ATP formation
- Some of the energy released from the sugar intermediates is used to directly synthesise ATP
- This direct synthesis of ATP is called substrate level phosphorylation
- In total, 4 molecules of ATP are generated during glycolysis by substrate level phosphorylation (2 ATP per 3C sugar)
At the end of glycolysis, the following reactions have occurred:
- Glucose (6C) has been broken down into two molecules of pyruvate (3C)
- Two hydrogen carriers have been reduced via oxidation (2 × NADH + H+)
- A net total of two ATP molecules have been produced (4 molecules were generated, but 2 were used)
glycose is an…
Glycolysis occurs in the cytosol and does not require oxygen (it is an anaerobic process)
Depending on the availability of oxygen, the pyruvate may be subjected to one of two alternative processes:
- Aerobic respiration occurs in the presence of oxygen and results in the further production of ATP (~ 34 molecules)
- Anaerobic respiration (fermentation) occurs in the absence of oxygen and no further ATP is produced
Aerobic Respiration
- If oxygen is present, the pyruvate is transported to the mitochondria for further breakdown (complete oxidation)
- This further oxidation generates large numbers of reduced hydrogen carriers (NADH + H+ and FADH2)
- In the presence of oxygen, the reduced hydrogen carriers can release their stored energy to synthesise more ATP
- Aerobic respiration involves three additional processes – the link reaction, krebs cycle and the electron transport chain
Anaerobic Respiration (Fermentation)
- If oxygen is not present, pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)
- The pyruvate remains in the cytosol and is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)
- This conversion is reversible and is necessary to ensure that glycolysis can continue to produce small quantities of ATP
Anaerobic Respiration
- Glycolysis involves oxidation reactions that cause hydrogen carriers (NAD+) to be reduced (becomes NADH + H+)
- Typically, the reduced hydrogen carriers are oxidised via aerobic respiration to restore available stocks of NAD+
- In the absence of oxygen, glycolysis will quickly deplete available stocks of NAD+, preventing further glycolysis
- Fermentation of pyruvate involves a reduction reaction that oxidises NADH (releasing NAD+ to restore available stocks)
- Hence, anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen
The first stage of aerobic respiration is the…
link reaction, which transports pyruvate into the mitochondria
- Aerobic respiration uses available oxygen to further oxidise the sugar molecule for a greater yield of ATP
link reaction
The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria
- Pyruvate is transported from the cytosol into the mitochondrial matrix by carrier proteins on the mitochondrial membrane
- The pyruvate loses a carbon atom (decarboxylation), which forms a carbon dioxide molecule
- The 2C compound then forms an acetyl group when it loses hydrogen atoms via oxidation (NAD+ is reduced to NADH + H+)
- The acetyl compound then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)
As glycolysis splits glucose into two pyruvate molecules, the link reaction occurs…
twice per molecule of glucose
- Per glucose molecule, the link reaction produces acetyl CoA (×2), NADH + H+ (×2) and CO2 (×2)
The second stage of aerobic respiration is the
Krebs cycle, which occurs within the matrix of the mitochondria
- The Krebs cycle is also commonly referred to as the citric acid cycle or the tricarboxylic acid (TCA) cycle
In the Krebs cycle, acetyl CoA transfers its…
acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)
- Coenzyme A is released and can return to the link reaction to form another molecule of acetyl CoA
Over a series of reactions, the 6C compound is broken down to reform the original 4C compound (hence, a cycle)
- Two carbon atoms are released via decarboxylation to form two molecules of carbon dioxide (CO2)
- Multiple oxidation reactions result in the reduction of hydrogen carriers (3 × NADH + H+ ; 1 × FADH2)
- One molecule of ATP is produced directly via substrate level phosphorylation
As the link reaction produces two molecules of…
acetyl CoA (one per each pyruvate), the Krebs cycle occurs twice - Per glucose molecule, the Krebs cycle produces: 4 × CO2 ; 2 × ATP ; 6 × NADH + H+ ; 2 × FADH2
The final stage of aerobic respiration is the…
electron transport chain, which is located on the inner mitochondrial membrane
- The inner membrane is arranged into folds (cristae), which increases the surface area available for the transport chain
The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP. This is called…
oxidative phosphorylation, as the energy to synthesise ATP is derived from the oxidation of hydrogen carrier
Oxidative phosphorylation occurs over a number of distinct steps:
Proton pumps create an electrochemical gradient (proton motive force)
ATP synthase uses the subsequent diffusion of protons (chemiosmosis) to synthesise ATP
Oxygen accepts electrons and protons to form water
Step 1: Generating a Proton Motive Force
- The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons
- The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins
- As electrons pass through the chain, they lose energy – which is used by the chain to pump protons (H+ ions) from the matrix
- The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)
Step Two: ATP Synthesis via Chemiosmosis
- The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix
- This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
- As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP
Step Three: Reduction of Oxygen
- In order for the electron transport chain to continue functioning, the de-energised electrons must be removed
- Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked
- Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient
- In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted
Summary: Oxidative Phosphorylation
- Hydrogen carriers donate high energy electrons to the electron transport chain (located on the cristae)
- As the electrons move through the chain they lose energy, which is transferred to the electron carriers within the chain
- The electron carriers use this energy to pump hydrogen ions from the matrix and into the intermembrane space
- The accumulation of H+ ions in the intermembrane space creates an electrochemical gradient (or a proton motive force)
- H+ ions return to the matrix via the transmembrane enzyme ATP synthase (this diffusion of ions is called chemiosmosis)
- As the ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP (from ADP + Pi)
- The de-energised electrons are removed from the chain by oxygen, allowing new high energy electrons to enter the chain
- Oxygen also binds matrix protons to form water – this maintains the hydrogen gradient by removing H+ ions from the matrix
Aerobic respiration involves the breakdown of glucose in the presence of oxygen to produce water and carbon dioxide
- It requires the involvement of mitochondria and generates a large yield of ATP (typically 36 ATP per glucose consumed)
- Aerobic respiration may include the following processes: glycolysis, link reaction, krebs cycle, electron transport chain
Types of Aerobic Reactions
Aerobic respiration involves three main types of reactions – decarboxylation, oxidation and phosphorylation
Decarboxylation:
- Carbon atoms are removed from the organic molecule (glucose) to form carbon dioxide
- Aerobic respiration involves the complete combustion of glucose (6C) – so six CO2 molecules are produced
Oxidation:
- Electrons and hydrogen ions are removed from glucose and taken up by hydrogen carriers (NADH and FADH2)
- The hydrogen carriers are in turn oxidised at the electron transport chain (where the energy is used to make ATP)
- The electrons and hydrogen ions are then taken up by oxygen (reduction) to form water molecules
- Twelve hydrogen carriers are produced and so six oxygen molecules are required (12 × O = 6 × O2)
Phosphorylation:
- Energy released from the breakdown of glucose is used to phosphorylate ADP to make ATP
- A net total of four ATP molecules are produced directly via substrate level phosphorylation
- The remaining ATP is produced indirectly via the electron transport chain (oxidative phosphorylation)
Aerobic respiration typically produces a net total of…
36 ATP per molecule of glucose consumed
- A net total of 2 ATP are produced in glycolysis via substrate level phosphorylation (four are produced, but two are consumed)
- A further 2 ATP are similarly produced in the Krebs cycle (one ATP per cycle – two cycles occur per glucose molecule)
- Lastly, 32 ATP are produced in the electron transport chain using energy from hydrogen carriers (oxidative phosphorylation)
Hydrogen carriers produce different amounts of ATP depending on where they donate electrons to the transport chain
- NADH molecules located in the matrix donate electrons to the start of the chain and produce 3 ATP per hydrogen carrier
- Cytosolic NADH (from glycolysis) donate electrons later in the chain and only produce 2 ATP per hydrogen carrier
- FADH2 also donates electrons later in the chain and so only produce 2 ATP per hydrogen carrier
Oxidative phosphorylation:
(8 × matrix NADH) + (2 × cytosolic NADH) + (2 × FADH2) = (8 × 3) + (2 × 2) + (2 × 2) = 32 ATP
Mitochondria are the ‘powerplants’ of the cell – synthesising large amounts of ATP via aerobic respiration
All eukaryotic cells possess mitochondria – aerobic prokaryotes use the cell membrane to perform oxidative phosphorylation
Mitochondria are thought to have once been independent prokaryotes that were internalised by eukaryotes via endosymbiosis
- They have a double membrane structure (due to vesicular coating as part of the endocytotic process)
- They have their own DNA (circular and naked) and ribosomes (70S)
- Their metabolic processes are susceptible to certain antibiotics
The structure of the mitochondrion is adapted to the function it performs:
- Outer membrane – the outer membrane contains transport proteins that enable the shuttling of pyruvate from the cytosol
- Inner membrane – contains the electron transport chain and ATP synthase (used for oxidative phosphorylation)
- Cristae – the inner membrane is arranged into folds (cristae) that increase the SA:Vol ratio (more available surface)
- Intermembrane space – small space between membranes maximises hydrogen gradient upon proton accumulation
- Matrix – central cavity that contains appropriate enzymes and a suitable pH for the Krebs cycle to occur
