Internal versus external respiration
- Internal (cellular) respiration is the enzyme-controlled release of energy from food.
- External respiration (breathing) is the exchange of gases with environment.
Aerobic versus anaerobic respiration
- Aerobic respiration is the enzyme-controlled release of energy from food using oxygen
- Anaerobic respiration is the enzyme-controlled release of energy from food without the use of oxygen
Aerobic respiration occurs in the mitochondrion.
Structure of the mitochondrion
Aerobic respiration consists of two stages:
- Stage 1 (Glycolysis)
- Stage 2 (Krebs cycle & Electron Transport Chain)
Stage 1: Glycolysis
- Oxygen-independent (can occur in presence or absence of oxygen)
- Occurs in cytosol
- Glucose (a 6-carbon molecule) is changed into 2 three-carbon molecules (pyruvate)
- This breaking down of glucose releases high energy electrons and protons – they are captured by NAD+ to become NADH
- Glycolysis also produces two molecules of ATP directly
Stage 2: Krebs cycle
- Oxygen-dependent (can only occur in the presence of oxygen).
- Occurs in the lumen of the mitochondrion (matrix).
- Pyruvate enters the mitochondrion and is converted to two-carbon molecule (acetyl-coA) with release of NADH and carbon dioxide.
- The acetyl-coA then joins with four-carbon molecule from the previous Krebs cycle to form six-carbon molecule.
- The six-carbon molecule is then broken down into five-carbon molecule with release of carbon dioxide and NADH.
- The five-carbon molecule is then broken down into four-carbon molecule with release of ATP, carbon dioxide, and 2NADH.
- The four-carbon molecule goes into the next Krebs cycle.
- The ATP goes to power metabolism (chemical reaction in cells).
Stage 2 (continued): Electron Transport Chain
- Oxygen dependent (can only occur in the presence of oxygen).
- Occurs in the inner membranes of the mitochondria (cristae).
- The NADH is an energy carrier that travels the short distance to the cristae and releases its high energy electrons and protons.
- The energy carried by the high energy electrons is used to power the production of three ATP molecules from ADP molecules and phosphates.
- The resultant low energy electrons, along with the proton, are combined with oxygen gas to form water.
- The three ATP molecules are then used within the cell for other reactions (e.g. anabolic reactions such as protein synthesis).
- NAD+ returns to the lumen of the mitochondrion to take part in another reaction.
Anaerobic respiration occurs when no oxygen or limited oxygen is present.
- Lactic acid fermentation/Alcohol fermentation
Lactic acid fermentation:
- Pyruvic acid is reduced by NADH (in other words, NADH gives its electron and proton to pyruvic acid) and forms lactic acid. NADH is oxidised (loses its electrons and proton).
- Both pyruvic acid and lactic acid have three carbon atoms, so NO carbon dioxide is produced.
- Lactic acid fermentation occurs in all animals (when there is a lack of oxygen) and in some bacteria and fungi.
- Pyruvic acid is reduced by NADH (in other words, NADH gives its electron and proton to pyruvic acid) and forms ethanol. NADH is oxidised (loses its electrons and proton).
- Ethanol has two carbon atoms and pyruvic acid has three carbon atoms, so one carbon atom is lost, in the form of carbon dioxide (which is then released/excreted).
- Ethanol fermentation occurs in all plants (when there is a lack of oxygen) and in some bacteria and fungi.
Role of microorganisms in industrial fermentation:
Industrial fermentation is a type of bioprocessing.
Bioprocessing: use of living cell, or their components (such as enzymes) to make useful products or carry out useful procedures.
Bioprocessing is carried out in bioreactors.
A bioreactor is a vessel in which a product is formed by a cell or cell component (such as an enzyme).
Example of bioprocessing:
- Brewing alcohol
- Producing dairy products
- Making vinegar
Bioprocessing is carried out in two ways:
- Batch culture
- Continuous flow culture
1. Batch culture:
- Nutrient medium, microorganisms/enzymes and substrate are added at the start.
- The microorganisms are allowed to proceed through the lag, log and stationary phases and then product is removed at the end of the process.
- Sometimes microorganisms are allowed to proceed through all of the stages of the microorganism growth curve and the product is removed at the end of the process.
2. Continuous flow culture:
- Nutrient medium, microorganisms/enzymes and substrate are added all the time (continuously).
- The microorganisms are kept in the log phase of the microorganism growth curve.
- Product is removed all the time (continuously).
Bioprocessing with immobilised cells/enzymes:
Microorganisms and enzymes can be immobilised (trapped in a gel) and then used in bioprocessing.
- Produces a pure, uncontaminated product
- Immobilised enzymes/cells can be reused many times
Uses of immobilised cells/enzymes:
- Immobilised yeast cells are used in the industrial fermentation of beers, wines and spirits.
- Pencillin G is produced using immobilised Penicillium fungal cells
- Immobilised E. coli are used in water treatment plants.
Mandatory Experiment: to prepare alcohol using yeast
- Deionised water
- Conical flasks
- Vegetable oil
- Fermentation lock
- Filter paper
- Potassium iodide
- Sodium hypochlorite
- Water bath
- Dissolve 50 g glucose in 500 ml deionised water and bring to the boil (to exclude oxygen).
- Separate into two conical flasks (250 ml glucose solution in each).
- Once cooled add 5 g yeast to one of the conical flasks and label this “TEST’ – the other flask remains as the control (without yeast).
- Slowly pour a layer of vegetable oil on top of each glucose solution (keeps the oxygen out).
- Place a stopper in each and attach fermentation locks each containing limewater.
- Leave in a 25˚C incubator for 3-4 days.
Test for alcohol (Iodoform test):
- Filter the solutions obtained from each conical flask.
- Place 3 ml of each in two separate test tubes.
- Add 3 ml of a potassium iodide solution and 5 ml of a sodium hypochlorite solution to each test tube.
- Warm the test tubes gently in a hot water bath and observe any changes.
- Appearance of pale yellow crystals in the ‘TEST’.
- No appearance of pale yellow crystals in the control.
- Yeast produced alcohol by anaerobic respiration in the ‘TEST’.
- Oxygen must be lacking or limited in order for alcohol to be produced.