13. Respiration and Energy Transfer - part 07 - Utility of stepwise oxidation

 

13. Respiration and Energy Transfer - part 07 - Utility of stepwise oxidation

Aerobic respiration can be

demonstrated by two simple experiments.

A. A pinch of dry bakers yeast suspended in water or a few ml of yeast suspension used in a bakery is
added to about 10ml of 10 percent glucose solution in a test tube (Tube A).

• The surface of the liquid is carefully covered with oil to prevent contact with air.
• The test tube is closed tightly with rubber stopper.
• One end of a short bent glass tube is inserted through it to reach the air inside the tube. 
• Other end of the glass tube is connected by a polyethylene or rubber tubing to another bent glass tube fitted into a stopper. 
• The open end of the glass tube (delivery tube) is dipped into lime water containing in a test tube (Tube B). 
• Stoppers of both the tubes are fitted tightly to prevent leakage of gases. 
• First test tube is placed in warm water (370C-380C) in a beaker.
• Lime water gradually turns milky, indicating the evolution of carbon dioxide from the yeast preparation.
• Level of the lime water in the delivery tube does not rise, showing that there is no decline in volume of gas in test tube A and consequently no utilization of oxygen by yeast.
• Preparation is stored for a day or two. 
• When you open the stopper of tube A. You will notice a smell of alcohol indicating the formation of ethanol. 
• From this activity it may be inferred that yeast respires anaerobically to ferment glucose to ethanol and carbon dioxide.

B. Seed coats of a few germinating seeds (peas, beans or gram) are removed. 

• Seeds are then put in a test tube filled with mercury.
• After closing the test tube with the thumb, it is vertically inverted in a trough of mercury and the thumb is carefully removed. 
• Being lighter than mercury, the seeds rise to the closed upper end of the test tube. 
• No gas is seen at first in the test tube. 
• As germination proceeds, a gas begins to collect at the top of the mercury in the test tube. 
• On introducing a pellet of potassium hydroxide into the tube, it rises to the top and absorbs the gas. 
• The mercury again fills the tube. The potassium hydroxide reacts with carbon dioxide gas to produce potassium carbonate and water.
• The gas therefore disappears.
•  Evidently germinating seeds produce carbon dioxide by anaerobic respiration in the absence of oxygen in the mercury column.

13.4 Utility of stepwise oxidation :

• A stepwise release of the chemical bond energy facilitates the utilization of a relatively higher proportion of that energy in ATP synthesis.
• Activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. 
• This provides a means of controlling the rate of the pathway and the energy output according to need of the cell.
• The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

always remember

• Removal of Hydrogen from respiratory materials is the primary process in respiration : The fact that during respiration oxygen is taken in and carbon dioxide is given out may give a false impression that respiratory materials directly unite with oxygen. 
• It must be remembered that oxygen does not play such a primary role in the process of respiration.
• The primary process in respiration consists in removal of hydrogen from the respiratory materials. 
• The reactions in which hydrogen is removed are catalyzed by enzymes called dehydrogenases free hydrogen cannot exists in the cell. 
• As soon as it is removed from respiratory material it is picked up by substances known as acceptors. 
• In aerobic respiration this hydrogen is ultimately handed over to oxygen. 
• These two combine with each other and form water.
• Comparison of overall equations of photosynthesis and respiration show that to some extent, two process are reverse of each other. 
• Photosynthesis involves reduction of CO2 and respiration involves oxidation of glucose.