Respiration Flashcards
4 stages of aerobic respiration
Glycolysis
Link reaction
Krebs cycle
Oxidative phosphorylation
Features of aerobic respiration
Produces water & carbon dioxide & more ATP.
Features of anaerobic respiration
In animals, it produces lactate. In plants & fungi, it produces ethanol & carbon dioxide. Less ATP.
What are coenzymes?
Molecules required for the functioning of certain enzymes, e.g., by carrying H atoms between molecules.
3 examples of coenzymes
FAD: KREBS CYCLE.
NADP: PHOTOSYNTHESIS.
NAD: works with DEHYDROGENASE ENZYMES that catalyse the removal of H atoms from substrates and transfer them to other molecules involved in OXIDATIVE PHOSPHORYLATION.
How does glycolysis provide indirect evidence for evolution?
It is a universal feature of all living organisms; it is the 1st step of both aerobic and anaerobic respiration.
Where does glycolysis occur?
In the cytoplasm of all living cells.
Summary of glycolysis
Splitting the hexose glucose molecule into 2 triose pyruvate molecules.
Describe the 1st step of glycolysis.
Phosphorylation of glucose to glucose phosphate: glucose is activated/made more reactive by the addition of 2 phosphate molecules.
In the first step of glycolysis, where do the 2 phosphate molecules for the phosphorylation of glucose come from?
They come from the hydrolysis of 2 ATP molecules to ADP. This provides a lower activation energy for the subsequent enzyme-controlled reactions and the energy to activate glucose.
2nd step of glycolysis
Each phosphorylated glucose molecule is split into 2 triose phosphate molecules.
3rd step of glycolysis: oxidation of triose phosphate
Hydrogen is removed from each of the triose phosphate molecules & transferred to NAD (a hydrogen carrier molecule) to form reduced NAD.
4th step of glycolysis: the production of ATP
Enzyme-controlled reactions convert each triose phosphate molecule into 3-carbon pyruvate molecules. 2 molecules of ATP are regenerated from ADP in the process.
What are the overall products of glycolysis?
2 molecules of pyruvate.
2 molecules of reduced NAD.
Net increase of 2 molecules of ATP.
Summary of the link reaction
The triose pyruvate molecules undergo a series of reactions to form 2-carbon acetylcoenzyme A.
Where does the link reaction occur?
The matrix of mitochondria.
Overall word equation for the link reaction
Pyruvate + NAD + CoA —> Acetylcoenzyme A + reduced NAD + CO2
Via series of reactions.
Describe the link reaction in detail.
- The 2 molecules of pyruvate from glycolysis are actively transported into the matrix of the mitochondria.
- The pyruvate is oxidised to acetate. The 3-carbon pyruvate loses a CO2 + 2H.
- The 2H are accepted by NAD to form reduced NAD.
- The 2-carbon acetate combines with coenzyme A to form acetylcoenzyme A.
Where does the Krebs cycle occur?
The matrix of mitochondria.
Summary of the Krebs cycle.
Acetylcoenzyme A is introduced to a cycle of oxidation-reduction reactions that yield some ATP & lots of reduced NAD & FAD.
1st stage of the Krebs cycle
2-carbon acetylcoenzyme A combines with a 4-carbon molecule to form a 6-carbon molecule.
2nd stage of the Krebs cycle
Via series of reactions, this 6-carbon molecule loses CO2 & hydrogen to give a 4-carbon molecule & a single molecule of ATP, produced by substrate-level phosphorylation.
3rd step of the Krebs cycle
The 4-carbon molecule can now combine again with a new molecule of acetylcoenzyme A. The cycle restarts…
Why is Krebs cycle important?
- It breaks down macromolecules into smaller ones.
- It produces H atoms, which are carried by NAD to the electron transfer chain to provide energy to produce ATP for oxidative phosphorylation.
- It regenerates the 4-carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate.
- It is a source of intermediate compounds used by cells in the manufacture of fatty acids, amino acids & chlorophyll.
How does ATP release energy?
The negative charges mean ATP is unstable. The bonds between the phosphates are weak, so can be easily broken.
Summary of oxidative phosphorylation
Electrons associated with reduced NAD & FAD are released from the Krebs cycle to synthesise ATP. H2O is a by-product.
What is obtained from 1 molecule of pyruvate?
- Reduced coenzymes NAD & FAD, which can provide energy to produce ATP molecules by oxidative phosphorylation.
- 1 ATP molecule.
- 3 CO2 molecules.
1 glucose produces 2 pyruvate, so double the above per glucose.
Where does oxidative phosphorylation occur?
The inner, folded membrane/ cristae where the necessary enzymes & other proteins are.
1st stage of oxidative phosphorylation
The H atoms produced by glycolysis & the Krebs cycle combine with NAD & FAD.
2nd stage of oxidative phosphorylation
The reduced NAD & FAD donate electrons to the 1st molecule in the electron transfer chain.
3rd stage of oxidative phosphorylation
As electrons pass along a chain of electron transfer chain carrier molecules in a series of oxidation-reduction reactions, the energy they release causes the active transport of protons across the inner mitochondrial membrane and into the inter-membranes space.
4th stage of oxidative phosphorylation
The protons accumulate in the intermembranal space before they diffuse back into the mitochondrial matrix through ATP synthase channels embedded in the mitochondrial membrane. The flow of H+ causes the shape of the ATP synthase channel to change.
What happens at the end of the electron transfer chain in oxidative phosphorylation?
The electrons combine with these protons & oxygen to form water. Oxygen is the final acceptor of electrons in the electron transfer chain.
Summary of oxidative phosphorylation
Mechanism whereby some of the energy of the electrons within the H atoms is conserved by ATP formation.
What is the importance of oxygen in respiration?
- To acts as the final acceptor of H atoms produced in glycolysis & the Krebs cycle at the end of the ETC.
- Without oxygen removing H atoms at the end of the chain, protons (H+) & electrons would back up along the chain, and respiration would come to a halt as the Krebs cycle & the ETC cannot combine once all the FAD & NAD are reduced.
What is the purpose of an electron transfer chain?
Each carrier molecule is at a slightly lower energy level, so electrons move down an energy gradient. This releases energy gradually, so more can be used by the organism instead of lost as heat as with large energy changes in a single step.
Mitochondria in more metabolically active cells
There are more mitochondria, & cristae are more densely packed to increase surface area.
How is energy from cellular respiration derived?
Glycolysis & the Krebs cycle involve substrate level phosphorylation, which is the direct transfer of a phosphate from a respiratory intermediate to ADP to produce ATP.
Oxidative phosphorylation in the electron transfer chain: indirect linking of energy from the phosphate to ADP to produce ATP, involving energy from H atom carriers or NAD & FAD.
What is required for glycolysis to continue?
It produces pyruvate & hydrogen that must be constantly removed. The hydrogen must be released from reduced NAD to regenerate NAD in order to take up the hydrogen atom produced in glycolysis.
How is NAD replenished?
The pyruvate molecule from glycolysis accepts the hydrogen from reduced NAD.
Why do lipids release more than twice the energy of the same mass of carbohydrate?
Oxidation of lipids produces 2-carbon fragments of carbohydrate and many hydrogen atoms. The H atoms are used to produce ATP in oxidative phosphorylation.
1st stage in the respiration of lipids
Lipids are hydrolysed to fatty acids & glycerol.
What happens to the glycerol once lipids have been hydrolysed in the respiration of lipids?
The glycerol is phosphorylated & converted to triose phosphate which enters the glycolysis pathway & then Krebs cycle.
What happens to the fatty acid once lipids have been hydrolysed in the respiration of lipids?
The fatty acid is broken into 2-carbon fragments, which are converted into acetylcoenzyme A. This enters the Krebs cycle.
Describe the respiration of proteins.
Protein is hydrolysed into its amino acids. Their amino groups are removed by deamination. They enter the respiratory pathway at different points depending on their number of carbon atoms.
3-carbon compounds are converted into pyruvate.
4- & 5-carbon compounds are converted to intermediates in the Krebs cycle.
Anaerobic respiration in animals to overcome a temporary oxygen shortage
Pyruvate + NADH —> lactate + oxidised NAD
NAD from glycolysis can accumulate and must be removed.
Hence, each pyruvate molecule produced takes up 2 hydrogen atoms from the reduced NAD produced by glycolysis to form lactate.
Lactate
Causes cramp & fatigue, if it accumulates in tissues.
It is also an acid, so causes pH changes to enzymes.
How is lactate removed?
The lactate is oxidised to pyruvate, which is then either further oxidised to release energy or converted to glycogen in the liver.
Anaerobic respiration in some plants, some bacteria & fungi, e.g., yeast.
Pyruvate + NADH —> ethanol + CO2 + oxidised NAD
The pyruvate molecule formed at the end of glycolysis loses a molecule of CO2 & accepts H from reduced NAD to produce ethanol.
Where does the ATP produced in anaerobic respiration come from?
Glycolysis only as the pyruvate is converted into either lactate or ethanol, so cannot take part in the Krebs cycle.
