respiration and biological processes Flashcards
Why is respiration a metabolic pathway?
It is a sequence of reactions controlled my enzymes
Why are respiration reactions catabolic?
They break down energy-rich macromolecules, such as glucose and fatty acids
What is oxidative phosphorylation and where does it occur?
- It occurs on the inner membrane of the mitochondria in aerobic respiration
- The energy for making ATP comes from oxidation-reduction reactions and is released in the transfer of electrons along a chain of electron carrier molecules
What is photophosphorylation and where does it occur?
- Occurs on the thylakoid membrane of the chloroplasts in the light-dependent stage of photosynthesis
- The energy for making the ATP comes from light and is released in the transfer of electrons along a chain of electron carrier molecules
What is substrate level phosphorylation?
- It occurs when phosphate groups are transferred from donor molecules, e.g. glycerate-3-phosphate to ADP to make ATP in glycolysis or when enough is released for a reaction to bind ADP to inorganic phosphate e.g. in the Krebs cycle
Three groups of organisms are recognised, depending on their respiration: Living organisms
Most living organisms use aerobic respiration, and break down substrate using oxygen, with the release of a relatively large amount of energy. These are obligate anaerobes.
Three groups of organisms are recognised, depending on their respiration: microorganisms
Some microorganisms including yeast and many bacteria, respire aerobically, but can also respire without oxygen; these are facultative anaerobes
Three groups of organisms are recognised, depending on their respiration: some species of bacteria
Some species of bacteria and archea use anaerobic respiration. They respire without oxygen and cannot grow in its presence. They are obligate anaerobes
Aerobic respiration can be divided into 4 distinct but linked stages
- Glycolysis which occurs in solution in the cytoplasm and generates pyruvate, ATP and reduced NAD
- The link reaction , in solution in the matrix of the mitochondrion. Pyruvate is converted into acetyl coenzyme A
- The Krebs cycle, in solution in the mitochondrial matrix generates carbon dioxide and reduced NAD and FAD
- The electron transport chain, on the cristae of the inner mitochondrial membrane, in which the energy from protons and electrons generates ATP from ADP and inorganic phosphate Pi
What is glycolysis?
Where does it occur and why?
- The initial stage of both anaerobic and aerobic respiration
- It occurs in the cytoplasm because glucose cannot pass through the mitochondrial membrane
Why can glucose not be broken down in the mitochondria?
- The enzymes for its breakdown are not present in the mitochondria and so it could not be metabolised there
Explain the process of glycolysis:
- A glucose molecule is phosphorylated by the addition of two phosphate groups , using two molecules of ATP, making a hexose phosphate called glucose diphosphate
- The glucose diphosphate splits into two molecules of a triose phosphate, a 3-carbon sugar, glyceraldehyde-3-phosphate
- The two triose phosphate molecules are dehydrogenated, i.e. hydrogen is removed from each of them, oxidising them to pyruvate, the hydrogen atoms are transferred NAD, a hydrogen carrier molecule, making reduced NAD.
- these steps release enough energy to synthesise 4 ATP molecules. The ATP is formed by substrate-level-phosphorylation
- The phosphate from the triose phosphate converts ADP to ATP, without the involvement of an electron transport chain producing pyruvate
- Of the 4 ATPs made by substrate-level-phosphorylation, 2 were used to phosphorylate the glucose molecule. Net production of 2 ATPSs
- 2 molecules of NADH is produced
What is the link reaction?
The link reaction links glycolysis to the Krebs cycle
Explain the process of the link reaction
NAD + CoA—> AcCoA + reduced NAD + CO2
- Pyruvate diffuses from the cytoplasm into the mitochondrial matrix
- The pyruvate is dehydrogenated and the hydrogen released is accepted by NAD to form NADH
- The pyruvate is decarboxylated i.e. a molecule of carbon dioxide is removed from it. All that remains of the original glucose molecule is a 2-carbon acetate group which combines with coenzyme A (CoA), making acetyl coenzyme A (AcCoA) which enters the krebs cycle
Describe the processes in the Krebs cycle
A means of liberating from C-C, C-H and C-OH bonds. It produces ATP, containing the energy which was held in the chemical bonds of the original glucose molecule
It also produces reduced NAD and reduced FAD which deliver hydrogen atoms to the electron transport chain on the inner mitochondrial membrane . Three molecules of water are used in reactions in the Krebs cycle. Carbon dioxide is released as a waste product
Explain the processes in the Krebs cycle
- Acetyl CoA enters the Krebs cycle by combining with a 4-carbon acid to form a 6-carbon compound and the CoA is regenerated
- The 6-carbon acid is dehydrogenated, making reduced NAD, and decarboxylated to make carbon dioxide and a 5-carbon chain
- The 5-carbon acid is dehydrogenated, making reduced NAD and FAD, and decarboxylated to make carbon dioxide and to regenerate the 4-carbon acid
- The 4-carbon acid can combine with more AcCoA and repeat the cycle
In the Krebs cycle, there are two significant types of reaction
- Decarboxylation happens twice. Decarboxylases remove carbon dioxide from the -COOH groups of Krebs cycle intermediates. They collected by hydrogen carriers giving three molecules of reduced NAD and one molecule of reduced FAD
- The acetate group from the original glucose molecule is now entirely broken down to carbon dioxide and water. The energy in the bonds of the glucose molecule is carried by electrons in the hydrogen atoms in the reduced NAD and FAD
The Krebs cycle: Summary
- One ATP produced by substrate-level-phosphorylation
- Three molecules of reduced NAD
- One molecule of reduced FAD
- Two molecules of carbon dioxide
The passage of electrons
2H+ + 2e- + 1/2 O2—-> H2O
- The reduced NAD donates the electrons of the hydrogen atoms to the first of a series of electron carriers in the electron transport chain
- The electrons from these atoms provide energy for the fist proton pump and protons from the hydrogen atoms are pumped into the inter-membrane space
- The electrons pass along the chain of carrier molecules providing energy to pump the hydrogen ions (protons) across to the intermembrane space
- This builds up a proton gradient, which is also a pH gradient and electrochemical gradient.
- At the end of the chain, the electrons combine with protons and oxygen to form water
2H+ + 2e- +1/2O2—>H2O
where is the electron transport chain located?
On the cristae of the inner mitochondrial membranes
What is the electron transport chain?
A series of protein molecules that are carriers and pumps, which are sometimes called ‘respiratory enzymes’
The carrier molecules include cytochromes and are proteins conjugated to iron or copper and the metal ions are oxidised and reduced by electron transport
What is the role of coenzymes?
NAD feeds electrons and protons into the electron transport chain earlier than FAD does
Each pair of hydrogen atoms carried by reduced NAD provides enough energy to synthesise how many molecules of ATP?
3 molecules of ATP
Each pair of hydrogen atoms carried by reduced FAD provides enough energy to synthesise how many molecules of ATP?
2 molecules of ATP
FAD passes hydrogen atoms directly to the second proton pump
Explain the passage of protons
- The inner membrane is impermeable to protons and so the protons accumulate in the inner membrane space
- The concentration of protons in the intermembrane space becomes higher than the matrix, so a gradient of concentration and charge is set up, and maintained by proton pumps
- In the membrane are protein complexes, which are channels through which protons flow back into the mitochondrial matrix. The enzyme ATP synthase is associated with each channel
-Protons diffuse back through these channels, and as they do so their electrical potential energy produces
ADP+Pi—> ATP + H2O - At the end of the chain, the protons combine with electrons and oxygen to form water
Why is oxygen referred to as ‘the final electron acceptor’ ?
The electrons are passed along a chain of electron carriers and then donated to molecular oxygen
oxygen is reduced by the addition of hydrogen ions and electrons, making water.
2H+ +2e-+1/2O2–>H2O
How does cyanide act as a non-competitive inhibitor of the final carrier in the electron transport chain?
- In its presence, electrons and protons cannot be transferred to water.
- They accumulate, destroying the proton gradient.
- ATP synthase cannot operate and the cell dies very quickly
For each molecule of glucose entering the Krebs cycle, the electron transport system receives 10 reduced NAD and how many molecules of ATP does this generate?
10 x 3 = 30 ATP
For each molecule of glucose entering the Krebs cycle, the electron transport system receives 2 reduced FAD and how many molecules of ATP does this generate?
2 x 2 = 4 ATP
When does anaerobic respiration take place?
- If there is no oxygen to remove hydrogen atoms from reduced NAD, and make water, the electron transport chain cannot function
- Without oxygen, reduced NAD cannot be reoxidised and no NAD is regenerated to pick up more hydrogen
- Consequently, the link reaction and the Krebs cycle cannot take place and only the first stage of respiration , glycolysis is possible
- For glycolysis to continue, pyruvate and hydrogen must be constantly removed and NAD must be regenerated
- This is done by pyruvate accepting the hydrogen from reduced NAD
ATP can be made without oxygen by the process of?
substrate level phosphorylation
There are two different anaerobic pathways to remove hydrogen from reduced NAD: In animals
- Muscle cells may not get sufficient oxygen during vigorous exercise
- When deprived of oxygen, pyruvate is the hydrogen acceptor and is converted to lactate, regenerating NAD
- If oxygen subsequently becomes available, the lactate can be respired to carbon dioxide and water, releasing the energy it contains
There are two different anaerobic pathways to remove hydrogen from reduced NAD: In microorganisms
- pyruvate is converted to carbon dioxide and to ethanol, a hydrogen acceptor, by decarboxylase
- Ethanal is reduced to ethanol and NAD is regenerated, in alcoholic fermentation
- This pathway is not reversible, so even if oxygen becomes available again, ethanol is not broken down. It accumulates in the cell and can rise to toxic concentrations
How may ATP molecules are produced per molecule o glucose respired?
38
why is the figure 38 a theoretical total?
- ATP is used to move pyruvate, ADP, reduced NAD and reduced FAD across the mitochondrial membrane
- The proton gradient may be compromised by proton leakage across the inner mitochondrial membrane, rather than passing through ATP synthase
- Molecules may leak through membranes
How do you calculate the efficiency of ATP production?
energy made available through ATP/ energy released in combustion
How many ATP molecules are produced during anaerobic respiration?
2
Why is the Krebs cycle sometimes called a ‘metabolic hub?’
the metabolic pathways of carbohydrates, lipids and proteins can feed into it and in some situations, fats and proteins can be used as respiratory substraes
Alternative respiratory pathways: Lipids
- Fat provides an energy store and is used as respiratory substrate when carbohydrate in the body, such as glycogen and blood glucose are low
- First, fat is hydrolysed into its constituent molecules, glycerol and fatty acids
- Then the glycerol is phosphorylated with ATP, dehydrogenated with NAD and converted in to a 3 carbon sugar, triose phosphate, which enters the glycolysis pathway
The long chain fatty acid molecules are split into 2-carbon fragments that enter the Krebs cycle as AcCoA. Hydrogen is released, picked by NAD and fed into the electron transport chain. This produces very large numbers of ATP molecules but the precise number depends on the length of the hydrocarbon chain of the fatty acid. Longer fatty acid chains have:
- More carbon atoms, so more carbon dioxide is produced. Muscles have a limited blood supply and if the respired fat, rather than glucose, such as the liver, respire fat, rather than glucose, they would produce more carbon atoms than could be removed quickly enough
- More hydrogen atoms, so more NAD is reduced, so more ATP is produced. This explains why tissue with a rich blood supply, such as the liver, respire fat: the large amount of ATP they produce is readily transported around the body
- More hydrogen atoms, so more water is produced. This metabolic water is very important for desert animals and explains why they respire fat
Alternative respiratory pathways: Proteins
- Protein can be used as a respiratory substrate whenever dietary energy supplies are inadequate ; the protein component of the food is diverted for energy purposes if carbohydrate and fat are lacking in the diet
What happens during prolonged starvation
- In prolonged starvation, tissue protein is mobilised to supply energy. Heart muscle and kidney tissue are among the first the body breaks down to release protein.
- Protein is hydrolysed into its constituent amino acids, which are deaminated in the liver
- The amino group is converted into urea and excreted
- The residue is converted to acetyl CoA, pyruvate or some other Krebs cycle intermediate, and oxidised
