Topic 5: Respiration Flashcards
Why do organisms respire?
Food is a source of energy for all organisms (carbohydrates and fats store chemical energy), but can’t be used directly by cells.
Instead, ATP is formed by cellular respiration to use to release energy.
What are the similarities and differences in respiration between organisms?
- Different organisms use different respiratory substances (e.g glucose)
- Respiration can be aerobic or anaerobic, but is similar in all organisms, providing indirect evidence for evolution
Give the overall equation for aerobic respiration
C6H12O6 + 6O2 –> 6CO2 + 6H2O (+38ATP)
Give a quick overview of the structure of a mitochondrion
Mitochondria consist of a double layer of membrane surrounding a central matrix, with the inner membrane folded into cristae.
What are coenzymes?
Complex organic molecules needed for some enzymes to function. They accept or donate subatomic particles to the substrate
Give the four steps of aerobic respiration and where they occur
- Glycolysis: cytoplasm
- Link reaction: mitochondrial matrix
- Krebs cycle: mitochondrial matrix
- Electron transport chain: inner membrane of cristae
What are carrier molecules used for in respiration?
Used to transport substrates between stages.
The coenzyme NAD accepts hydrogen throughout respiration, and the reduced NAD donates hydrogen in oxidative phosphorylation
What is the yield of glycolysis for one glucose molecule?
- 2x ATP
- 2x reduced NAD
- 2x pyruvate
Describe the process of glycolysis
Phosphorylation:
- One molecule of glucose [6C] is turned into glucose phosphate [6C], as 2ATP are converted to 2ADP, donating their phosphate group
Hydrolysis:
- Water is used to split glucose phosphate into two molecules of triose phosphate [3C]
Oxidation:
- Each triose phosphate is oxidised into pyruvate [3C], as 1NAD accepts 2H from each TP, and 2ADP accepts 2Pi from each TP to form ATP
How do the products of glycolysis enter the link reaction?
Pyruvate from glycolysis enters the mitochondrial matrix by active transport, ready for the link reaction
Give the overall equation for the link reaction
pyruvate + NAD + coenzyme A –> acetylcoenzyme A + reduced NAD + carbon dioxide
What is the yield of the link reaction for one glucose molecules?
Happens twice for one glucose so:
- 2x CO2
- 2x reduced NAD
- 2x acetylcoenzyme A
Describe what happens to the molecules produced in the link reaction
- acetylcoenzyme A enters the Krebs cycle
- reduced NAD enters oxidative phosphorylation
- carbon dioxide leaves the cell as a waste product
Describe the process of the link reaction
-Pyruvate [3C] is oxidised and decarboxylated into acetate [2C]
- During this, 1CO2 is released and NAD is reduced into reduced NAD
- Acetate combines with coenzyme A to form acetylcoenzyme A [2C]
What is the yield of the Krebs cycle for one glucose molecule?
Happens twice for each glucose so:
- 6x reduced NAD
- 2x reduced FAD
- 2x ATP
- 4x CO2
Describe the process of the Krebs cycle
- AcetylCoA [2C] combines with oxaloacetate [4C] to form citrate [6C], releasing CoA back to the link reaction
- Citrate is decarboxylated and oxidised into a 5C compound, releasing 1CO2 and turning NAD into reduced NAD
- The 5C compound is turned back into oxaloacetate in a series of redox reactions:
- CO2 is released
- NAD is turned into reduced NAD
- ADP + Pi is turned into ATP (substrate - level phosphorylation)
- FAD is turned into reduced FAD
- NAD is turned into reduced NAD
What is the most important function of the Krebs cycle?
Producing reduced coenzymes that are passed to the electron transport chain to reduce ATP
It is also a source of intermediate compounds used by cells to manufacture other substances, e.g fatty acids, amino acids, chlorophyll
What is the total yield of respiration for one glucose before oxidative phosphorylation?
- 6x CO2
- 4x ATP
- 10x reduced NAD
- 2x reduced FAD
What is oxidative phosphorylation?
The electron transport chain (final) stage of respiration. It is the mechanism by which some of the energy of the electrons within hydrogen atoms is used to produce ATP.
ATP is produced by oxidative phosphorylation as oxidation reactions are involved in the electron transport chain.
ATP is produced using chemiosmotic theory
Describe the process of the electron transport chain in respiration
- Reduced NAD and reduced FAD donate the electrons of their H atoms to the 1st molecule of the ETC.
- Electrons pass along electron carrier proteins (cytochromes) in a series of redox reactions
- As electrons move down ETC, they release energy used for active transport of H+ from mitochondrial matrix into intermembranal space
- High concentration H+ in intermembranal space, so they move back into matrix by facilitated diffusion through embedded ATP synthase channels
- At end of ETC, electrons combine with H+ and oxygen, forming water. Oxygen is the terminal electron acceptor
Why is oxygen needed for the electron transport chain in respiration?
Oxygen is the terminal electron acceptor. Without oxygen removing the H atoms at the end of the chain, H+ ions and electrons would back up down the chain and respiration would stop
How much ATP does one molecule of reduced NAD and reduced FAD produce?
- 1 reduced NAD = 3ATP
- 1 reduced FAD = 2ATP
How is energy released during the electron transport chain in respiration and why is it done this way?
Electrons are passed along a series of transfer carrier molecules, each at a slightly lower energy level than the last.
This means electrons move down an energy gradient, releasing energy gradually.
This happens because releasing the energy all in one step would mean more is wasted as heat, so this way more energy can be harvested
What are the two alternative respiratory substances?
- Lipids
- Protein
Describe how lipids can be used as an alternative respiratory substrate
- Can be hydrolysed to glycerol and fatty acids before respiration
- Glycerol is phosphorylated and turned into triose phosphate
- Fatty acid turn into acetylcoenzyme A. Each enters respiration at the appropriate stage
- Lipid oxidation produces [2C] fragments of carbohydrate and many H atoms, which are used to produce ATP in the ETC
- This is why lipids release more than double the energy than the same mass of carbohydrate
Describe how proteins can be used as an alternative respiratory substrate
Protein is first hydrolysed into amino acids, then deaminated (NH2 removed).
[3C] compounds are turned into pyruvate, while [4C] and [5C] are converted to intermediates in the Krebs cycle
Why can aerobic respiration not continue in the absence of oxygen?
How is ATP produced in these conditions?
The Krebs cycle and electron transfer chain can’t continue as FAD and NAD will all be reduced, so nothing is available to accept the H+ produced in the Krebs cycle, so enzymes stop working. Oxygen is also the terminal electron acceptor in the ETC.
ATP is produced by anaerobic respiration. The only way to produce ATP is through glycolysis, so the H atom from reduced NAD must be regenerated, otherwise glycolysis would stop
What plants and microorganisms respire anaerobically?
What is this process useful for?
Occurs in certain bacteria and fungi (e.g yeast) and some plant cells, e.g root cells in waterlogged conditions.
The ethanol produced by yeast is often used in breweries
Give the overall equation for anaerobic respiration in plants and microorganisms
pyruvate + reduced NAD –> carbon dioxide + ethanol + NAD
Describe the process of anaerobic respiration in plants and microorganisms
Glycolysis occurs, then:
- Pyruvate [3C] is decarboxylated into ethanol
- This releases one molecule of CO2
- During this, reduced NAD is converted into NAD, which returns back to glycolysis so the reaction can continue. Without regenerating NAD, glycolysis would stop too.
- The ATP is produced during glycolysis.
When does anaerobic respiration occur in animals?
Occurs as a means of overcoming a temporary oxygen shortage.
Most common in muscles during strenuous exercise, as there is an oxygen debt - oxygen is used quicker than it can be replaced.
Give the overall equation for anaerobic respiration in animals
pyruvate + reduced NAD –> lactate + NAD
Describe the process of anaerobic respiration in animals
Glycolysis occurs, then pyruvate is reduced into lactate. During this, reduced NAD is re-oxidised into NAD for use again in glycolysis
In animals, what happens when oxygen becomes available again after anaerobic respiration? Why?
Lactate is oxidised back to pyruvate, then either further oxidised to release energy or converted to glycogen by the liver
If left to accumulate, lactate will cause cramp and muscle fatigue, and will decrease pH, affecting enzyme function
Is aerobic or anaerobic respiration more efficient and why?
Aerobic respiration is much more efficient because it requires much less glucose for the same quantity of ATP
What equipment would you use to measure respiration and how does it work in general?
Uses a respirometer to measure the rate of exchange of oxygen / carbon dioxide.
Oxygen is taken in by the organism and carbon dioxide is absorbed by potassium hydroxide, decreasing pressure in the boiling tube and so a drop of coloured liquid in the capillary tube moves towards the organism
What measurements do you need to perform respiration rate calculations using a respirometer?
- Distance the drop moves and the time taken
- Mass of organism
- Diameter of tube
What is the purpose of the syringe in a respirometer?
- You can pull up the syringe to reset the liquid drop to the original position
- The volume increase inside the syringe is the amount of oxygen used - measured more easily and accurately than calculations involving the diameter of the tube
What are some limitations of using a respirometer to measure rate of respiration?
- Temperature / pressure changes affect gas volume
- Things like movement change the respiration rate of the organism
- Leaks in the system affect pressure
- Chemicals used might affect gas composition
- Difficult to restart without taking apart the apparatus
- Volume change calculated using tube diameter may be inaccurate
