8.2 Cellular Respiration
Essential idea: Energy is converted to a usable form in cell respiration.
Adenosine triphosphate (ATP) is the energy currency of cells. It is unstable and will breakdown into Adenosine diphosphate (ADP) and a phosphate releasing energy (as heat). The energy released by ATP is held in the bond between the second and the third phosphates. ATP can therefore be used as a coenzyme in many parts of the cells metabolism providing the energy needed for many reactions. Because of it's unstable nature it is only produced when needed carbohydrates, lipids (and sometimes proteins) provide more stable longer term storage for energy. The purpose of cell respiration therefore it to breakdown carbohydrates and lipids so that ATP can be produced from ADP when needed. The image to the above to the right shows the conversion of ADP and a phosphate into ATP.
ATP can be made by substrate level phosphorylation, but most commonly oxidative phosphorylation is used. The key component in oxidative phosphorylation is ATP synthase (above left). This enzyme sits embedded within the inner membrane of mitochondria. When hydrogen ions flow through ATP synthase the motive force is used to convert ADP and phosphate into ATP.
Essential idea: Energy is converted to a usable form in cell respiration.
Adenosine triphosphate (ATP) is the energy currency of cells. It is unstable and will breakdown into Adenosine diphosphate (ADP) and a phosphate releasing energy (as heat). The energy released by ATP is held in the bond between the second and the third phosphates. ATP can therefore be used as a coenzyme in many parts of the cells metabolism providing the energy needed for many reactions. Because of it's unstable nature it is only produced when needed carbohydrates, lipids (and sometimes proteins) provide more stable longer term storage for energy. The purpose of cell respiration therefore it to breakdown carbohydrates and lipids so that ATP can be produced from ADP when needed. The image to the above to the right shows the conversion of ADP and a phosphate into ATP.
ATP can be made by substrate level phosphorylation, but most commonly oxidative phosphorylation is used. The key component in oxidative phosphorylation is ATP synthase (above left). This enzyme sits embedded within the inner membrane of mitochondria. When hydrogen ions flow through ATP synthase the motive force is used to convert ADP and phosphate into ATP.
Understandings
8.2.U1 Cell respiration involves the oxidation and reduction of electron carriers.
8.2.U2 Phosphorylation of molecules makes them less stable.
8.2.U3 In glycolysis, glucose is converted to pyruvate in the cytoplasm.
8.2.U4 Glycolysis gives a small net gain of ATP without the use of oxygen. [The names of the intermediate compounds in gylcolysis is not required.]
8.2.U5 In aerobic cell respiration pyruvate is decarboxylated and oxidized, and converted into acetyl compound and attached to coenzyme A to form acetyl coenzyme A in the link reaction.
8.2.U6 In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide. [The names of the intermediate compounds in the Krebs cycle is not required.]
8.2.U7 Energy released by oxidation reactions is carried to the cristae of the mitochondria by reduced NAD and FAD.
8.2.U8 Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping.
8.2.U9 In chemiosmosis protons diffuse through ATP synthase to generate ATP.
8.2.U10 Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water.
8.2.U11 The structure of the mitochondrion is adapted to the function it performs.
Applications
8.2.A1 Electron tomography used to produce images of active mitochondria.
Skills
8.2.S1 Analysis of diagrams of the pathways of aerobic respiration to deduce where decarboxylation and oxidation reactions occur.
8.2.S2 Annotation of a diagram of a mitochondrion to indicate the adaptations to its function.
8.2.U1 Cell respiration involves the oxidation and reduction of electron carriers.
8.2.U2 Phosphorylation of molecules makes them less stable.
8.2.U3 In glycolysis, glucose is converted to pyruvate in the cytoplasm.
8.2.U4 Glycolysis gives a small net gain of ATP without the use of oxygen. [The names of the intermediate compounds in gylcolysis is not required.]
8.2.U5 In aerobic cell respiration pyruvate is decarboxylated and oxidized, and converted into acetyl compound and attached to coenzyme A to form acetyl coenzyme A in the link reaction.
8.2.U6 In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide. [The names of the intermediate compounds in the Krebs cycle is not required.]
8.2.U7 Energy released by oxidation reactions is carried to the cristae of the mitochondria by reduced NAD and FAD.
8.2.U8 Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping.
8.2.U9 In chemiosmosis protons diffuse through ATP synthase to generate ATP.
8.2.U10 Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water.
8.2.U11 The structure of the mitochondrion is adapted to the function it performs.
Applications
8.2.A1 Electron tomography used to produce images of active mitochondria.
Skills
8.2.S1 Analysis of diagrams of the pathways of aerobic respiration to deduce where decarboxylation and oxidation reactions occur.
8.2.S2 Annotation of a diagram of a mitochondrion to indicate the adaptations to its function.
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8.2.U11 The structure of the mitochondrion is adapted to the function it performs.
Mitochondria are the ‘powerplants’ of the cell – synthesising large amounts of ATP via aerobic respiration
Structure and Function of a Mitochondrion
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
- 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
- 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
Structure and Function of a Mitochondrion
8.2.S2 Annotation of a diagram of a mitochondrion to indicate the adaptations to its function.
Electron micrographs of a mitochondrion may differ in appearance depending on where the cross-section occurs
Typically, mitochondrial diagrams should display the following features:
Mitochondrion Diagrams
Electron micrographs of a mitochondrion may differ in appearance depending on where the cross-section occurs
Typically, mitochondrial diagrams should display the following features:
- Usually sausage-shaped in appearance (though will appear more rounded in perpendicular cross-sections)
- Inner membrane contains many internal protrusions (cristae)
- Intermembrane space is very small (allows for a more rapid generation of a proton motive force)
- Ribosomes and mitochondrial DNA are usually not visible at standard resolutions and magnifications
Mitochondrion Diagrams
8.2.A1 Electron tomography used to produce images of active mitochondria.
Electron tomography is a technique by which the 3-dimensional internal structure of a sample can be modeled
When dealing with biological materials, samples are first prepared by either fixing and dehydrating or freezing (cryogenics)
Using electron tomography, the following features of active mitochondria have been identified:
Overview of Electron Tomography
Electron tomography is a technique by which the 3-dimensional internal structure of a sample can be modeled
- Samples are repeatedly imaged using a transmission electron microscope
- Following each image, the sample is tilted to a different angle relative to the electron beam
- The images are then compiled and used to computationally reconstruct a 3-D representation (called a tomogram)
When dealing with biological materials, samples are first prepared by either fixing and dehydrating or freezing (cryogenics)
- This stabilises the biological structures and prevents aqueous contents (i.e. water) from expanding and exploding
Using electron tomography, the following features of active mitochondria have been identified:
- The cristae are continuous with the internal mitochondrial membrane
- The intermembrane space is of a consistent width thoughout the entire mitochondrion
- The relative shape, position and volume of the cristae can change in active mitochondria
Overview of Electron Tomography
8.2.U2 Phosphorylation of molecules makes them less stable.
Adenosine triphosphate (ATP) is a high energy molecule that functions as an immediate power source for cells
Relationship between ATP and ADP
Adenosine triphosphate (ATP) is a high energy molecule that functions as an immediate power source for cells
- 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
Relationship between ATP and ADP
8.2 U 1 Cell respiration involves the oxidation and reduction of electron carriers.
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)
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)
- Oxidation involves the loss of electrons from an element through the gain of oxygen or the loss of hydrogen.
- Reduction involves the gain of electrons through the addition of hydrogen or the loss of an oxygen molecule.
- These reactions are called Redox reactions (reduction-oxidation) which are chemical reactions in which atoms have their oxidation number changed.
- For example the oxidation of carbon to form CO2 and the reduction of carbon by the addition of hydrogen to yield methane (CH4).
- Electron carriers are specific substances that accept and give up electrons
- The main electron carrier in cellular respiration is NAD (nicotinamide adenine dinucleotide)
- During respiration NAD which actually exists as NAD+ accepts 2 electrons and a proton (H+) from the molecule being oxidized (like pyruvate) to form NADH with one extra H+ leftover as a product.
- After the electron carriers are reduced they transport their electrons and hydrogens to the ETC, where the opposite reaction occurs (oxidation)
- So when NADH is oxidized it donates the electrons and protons to an electron carrier (complex I) in the inner mitochondrial membrane made from conjugated proteins (Fe-S core)
- This carrier is therefore reduced and will be re-oxidized as it passes the electrons down the ETC
Redox Mnemonics
Redox reactions involving electrons can be remembered using any of the following mnemonics:
Redox reactions involving electrons can be remembered using any of the following mnemonics:
- OIL RIG – Oxidation Is Loss (of electrons) ; Reduction Is Gain (of electrons)
- LEO goes GER – Loss of Electrons is Oxidation ; Gain of Electrons is Reduction
- ELMO – Electron Loss Means Oxidation
8.2.U7 Energy released by oxidation reactions is carried to the cristae of the mitochondria by reduced NAD and FAD.
Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules
The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ions)
The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria
Energy Transfer via Hydrogen Carriers
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
Energy Transfer via Hydrogen Carriers
8.2.U3 In glycolysis, glucose is converted to pyruvate in the cytoplasm.
Glycolysis is the process of releasing energy within sugars. In glycolysis, glucose (a six carbon sugar) is split into two molecules of the three-carbon sugar pyruvate. This multi-step process yields two molecules of ATP (free energy containing molecule), two molecules of pyruvate, and two "high energy" electron carrying molecules of NADH. Glycolysis can occur with or without oxygen
Glycolysis is the process of releasing energy within sugars. In glycolysis, glucose (a six carbon sugar) is split into two molecules of the three-carbon sugar pyruvate. This multi-step process yields two molecules of ATP (free energy containing molecule), two molecules of pyruvate, and two "high energy" electron carrying molecules of NADH. Glycolysis can occur with or without oxygen
8.2.U4 Glycolysis gives a small net gain of ATP without the use of oxygen. [The names of the intermediate compounds in gylcolysis is not required.]
Glycolysis involves the breakdown of glucose into pyruvate (× 2), with a small net gain of ATP (two molecules)
Glycolysis involves the breakdown of glucose into pyruvate (× 2), with a small net gain of ATP (two molecules)
- Glycolysis occurs in the cytosol and does not require oxygen (it is an anaerobic process)
- 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
- 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
- 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
- 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
8.2.U5 In aerobic cell respiration pyruvate is decarboxylated and oxidized, and converted into acetyl compound and attached to coenzyme A to form acetyl coenzyme A in the link reaction.
The first stage of aerobic respiration is the link reaction, which transports pyruvate into the mitochondria
The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria
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
The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria
8.2.U6 In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide. [The names of the intermediate compounds in the Krebs cycle is not required.]
The second stage of aerobic respiration is the Krebs cycle, which occurs within the matrix of the mitochondria
In the Krebs cycle, acetyl CoA transfers its acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)
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
8.2.U8 Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping.
The final stage of aerobic respiration is the electron transport chain, which is located on the inner mitochondrial membrane
The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP
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 carriers
8.2.U9 In chemiosmosis protons diffuse through ATP synthase to generate ATP.
- 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
8.2.U10 Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water.
- 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