Respiration and its mechanism
The energy stored in carbohydrate molecules during photosynthesis is released during cellular oxidation of carbohydrates into CO2 and H2O. This is called respiration. In respiration, the oxidation of various organic food substances like carbohydrates, fats and proteins may take place. Among these, glucose is the commonest.
Breakdown of glucose involves many steps to release energy in the form of ATP molecules.
Mechanism of Respiration:
Mechanism of respiration can be divided into the following stages:
A. Glycolysis (Embden-Meyerhof-Paranas- Pathway):
In glycolysis, which can take place even in the absence of O2, 6-carbon compound, glucose is broken down into 3-carbon compound, the pyruvic acid, through a series of steps which are as follows:
(i) Glucose molecule reacts with ATP molecule to form glucose-6-phosphate.
(ii) Glucose-6-phosphate is isomerised into fructose-6-phosphate.
(iii) Fructose-6-phosphate reacts with one mole of ATP to form fructose 1, 6-diphosphate.
(iv) Fructose 1, 6-diphosphate is converted into two 3-phosphoglyceraldehyde and dihydroxyacetone phosphate.
(v) 3-phosphoglyceraldehyde reacts with H3PO4 and forms 1, 3-diphosphoglyceraldehyde
(vi) 1, 3-diphosphoglyceraldehyde is oxidised to form 1, 3-diphosphoglyceric acid. NAD is reduced.
(vii) 1, 3-diphosphoglyceric acid reacts with ADP to form one mol. of ATP and 3-phosphoglyceric acid.
(viii) 3-phosphoglyceric acid is isomerised into 2- phosphoglyceric acid.
(ix) 2-phosphoglyceric acid is converted into 2- phosphoenol pyruvic acid.
(x) 2-phosphoenol pyruvic acid reacts with ADP to form one mol. of ATP and pyruvic acid.
At this step, if molecular oxygen is available, aerobic respiration takes place and the last product of glycolysis, pyruvic acid, enters into Kreb's Cycle for further oxidation.
B. Aerobic Respiration:
The Kreb's Cycle: Kreb's cycle is also known as citric acid cycle or tricarboxylic acid cycle (TCA) and takes place in mitochondria w here all necessary enzymes are found on cristae. The various steps of Kreb's Cycle are as follows:
(i) Pyruvic acid reacts with CoA and NAD to form acetyl CoA.
(ii) Acetyl CoA condenses with oxaloacetic acid to form citric acid. CoA becomes free.
(iii) Citric acid is dehydrated to form cis-aconitic acid.
(iv) Cis-aconitic acid reacts with one mol. of water to form iso-citric acid.
(v) Iso-citric acid is oxidised to oxalosuccinic acid.
(vi) Succinyl CoA reacts with water molecule to form succinic acid.
(vii) Succinic acid is oxidised to fumaric acid.
(viii) One molecule of water is added to fumaric acid to form malic acid.
(ix) In the last step, malic acid is oxidised to oxaloacetic acid.
Complete oxidation of one glucose molecule results in the net gain of 38 ATP molecules which are the real energy generators and are trapped in mitochondria.
C. Anaerobic Respiration (Fermentation):
In the absence of molecular O2, the pyruvic acid (last product of glycolysis) undergoes anaerobic respiration or fermentation by the following two methods:
I. Alcoholic Fermentation:
(i) Pyruvic acid is first decarboxylated to acetyldehyde.
(ii) Acetyldehyde is reduced to ethyl alcohol. During this process, there is net gain of only 2 ATP molecules and most of the energy of glucose molecule is wasted as heat.
II. Lactic Acid Fermentation:
Pyruvic acid is converted into lactic acid and coenzyme NADH2 is oxidised.
The ratio of the volume of CO2 released to the volume of O2 taken in is called respiratory quotient.
Growth in plants is the outcome of cell division, enlargement of new cells and their differentiation into different types of tissues. Growth is confined only to meristems.
The extreme apices of root and shoot are occupied by primary meristems, while the older parts are occupied by secondary meristems which give rise to additional vascular tissues and protective layers of cork-cells.
Course of Growth:
Usually, under favourable conditions, there is a characteristic course of increase in plants in their growing parts. Growth is slow at first, then gains speed and eventually slows down to come to a halt.
The total time during which this course of growth takes place is called the Grand Period of Growth. If this growth rate is plotted against time, an S-shaped curve is obtained which is called Sigmoid Curve or Grand Period Curve.