Ganongs respiroscope, Pentose phosphate pathway

Demonstration of respiration by Ganongs respiroscope

The aim of this experiment is to demonstrate liberation of carbon
dioxide during respiration.

The respiroscope is a glass apparatus consisting of a bulb like part with a bent neck and vertical tube.

Germinating seeds are taken in the bulb and the mouth of the tube is kept immersed in the beaker containing KOH solution as shown in the figure.

The respiroscope is fixed in the vertical position with the help of a stand.

Thus, the enclosed air in the bulb is completely cut off from the atmosphere.

The apparatus is kept undisturbed for few hours.

It is observed that the level of KOH solution in the limb is raised.

The KOH solution absorbs carbondioxide released by the seeds and a vacuum is created. It results in the raise of KOH level.

Pentose phosphate pathway

Generally, majority of living organisms obtain energy for various biochemical activities from glucose.

In aerobic organisms, it is degraded in three major phases namely, glycolysis, Krebs cycle and electron transport system.

In anaerobes, glucose is partially degraded by glycolysis and fermentation.

In 1938, Dickens discovered an alternate pathway for the utilization of glucose by the living cells.

This pathway is called pentose phosphate pathway or hexose monophosphate pathway or direct oxidation pathway.

This pathway consists of major phases – oxidative and nonoxidative phases.

Pentose phosphate pathway takes place in the cytoplasm only.

Oxidative phase

In this phase, glucose is oxidized and decarboxylated with the
formation of pentose through phosphogluconic acid as shown in the flow
chart.

The essential feature of this phase is the production of NADPH2.

1. Glucose is phosphorylated to glucose-6-phosphate by hexokinase.

2. The glucose-6-phosphate is oxidized to 6-phospho-gluconolactonate
in the presence of NADP+ by enzyme glucose-6-phosphate dehydrogenase.

NADP+ is reduced to NADPH2.

3. The 6-phosphogluconolactone is hydrolysed by gluconolactonase
to form 6-phosphogluconic acid.

4. The 6-phosphogluconic acid undergoes oxidative decarboxylation
again in the presence of NADP+ to form Ru5P. This reaction is catalyzed by 6-phosphogluconic dehydrogenase. NADP+ is reduced to NADPH2.
In this reaction CO2 is released.

Nonoxidative phase

In this phase, various intermediates such as 3C, 4C, 5C and 7-carbon
phosphorylated sugars are produced.

They are phosphoglyceraldehyde (3C), erythrose phosphate (4C), xylulose phosphate (5C) and sedoheptulose phosphate (7C).

To summarize, six molecules of glucophosphate enter this pathway.

After oxidation, six molecules of CO2 are released as shown in the step 4
and twelve molecules of NADPH2 are produced as shown in the steps 2
and 4.

In other words, after oxidation one molecule of glucose produces
six molecules of CO2 and twelve molecules of NADPH2.

Out of six glucose molecules one is completely oxidized and other five molecules are involved in the formation of 3C, 4C, 5C, and 7- carbon sugar intermediates.

From these intermediates, five molecules of glucose-6-phosphate are regenerated.