In the following flow chart, replace the symbols $\mathrm{a}, \mathrm{b}, \mathrm{c}$ and d with appropriate terms. Briefly explain the process and give any two application of it.
The metabolic pathway given in the figure is fermentation. The products marked as $a, b, c$ and $d$ represents
a-Glyceraldehyde 3 phosphate,
b-Phosphoenol pyruvic acid,
c-Ethanol,
d-Lactic acid
The fermentation is of two types
(i) Alcohol Fermentation in Yeast Fermentation is an incomplete oxidation of glucose under anaerobic condition. Alcohol fermentation in yeast occurs in 2 sets of reaction thus, converting pyruvic acid into ethanol and $\mathrm{CO}_2$.
A. In the first step, pyruvic acid is decarboxylated (equation I), resulting in the formation of acetaldehyde and $\mathrm{CO}_2$.
$$\mathop {2C{H_3}COCOOH}\limits_{Pyruvic\,acid} \buildrel {Pyruvic\,decarboxylase} \over \longrightarrow \mathop {2C{H_3}CHO}\limits_{Acetaldehyde} $$ ..... (i)
B. In the second step acetaldehyde is reduced to alcohol by $\mathrm{NADH}_2$ (equation (ii))
(ii) Lactic Acid Fermentation in Muscles
In animal tissue like muscles, during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase. The reducing agent is $\mathrm{NADH}+\mathrm{H}^{+}$which is reoxidised to $\mathrm{NAD}^{+}$in the subsequent processes.
Two applications of fermentation process are
(i) It helps in manufacture of ethyl alcohol.
(ii) It also helps in curdling of milk to make curd aided by bacteria Lactobacillus.
Given below is a diagram showing ATP synthesis during aerobic respiration, replace the symbols A , $B, C, D$ and $E$ by appropriate terms as given below. F 1 , particle, formation of $\mathrm{Pi}, 2 \mathrm{H}^{+}$, inner mitochondrial membrane, ATP, Fo particle, ADP.
Symbol A, B, C, D and E in the diagram represents
A - ATP
$B-F_1$ particle
$\mathrm{C}-\mathrm{Pi}$
$\mathrm{D}-2 \mathrm{H}^{+}$ E - inner mitochondria membrane.
Role of $\mathrm{O}_2$ in Aerobic Respiration
The respiration of glucose starts with glycolysis in cytoplasm, followed by in Krebs' cycle and finally Electron Transport Chain (ETC) in inner mitochondrial membrane. The requirement of $\mathrm{O}_2$ is at the end of ETC. Where, it acts as final hydrogen acceptor. $\mathrm{O}_2$ is responsible for removing electrons from the system. If oxygen is not available, electrons could not be passed through the co-enzymes, inturn proton pump will not be established and ATP will not be produced via oxidative phosphorylation. Thus Oxygen play a critical role in aerobic respiration in mitochondrial matrix.
The calculations of the net gain of ATP for every glucose molecule oxidised can be made on the following assumptions
(i) There is sequential pathway that follows, i.e., glycolysis, TCA cycle and ETS in cytoplasm, mitochondrial matrix and inner mitochondrial membrane respectively.
(ii) NADH, synthesised in glycolysis enters in to ETC for phosphorylation.
(iii) None of the intermediates in the pathway are utilised to synthesise any other compound.
(iv) Glucose forms respiratory substrate.
These assumptions are not valid for a living system because of following reasons
(i) These all pathways work simultaneously and do not take place one after the other.
(ii) ATP is utilised when needed.
(iii) Rate of enzyme actions is controlled by multiple means.
Comparisan between fermentation and aerobic respiration are as follows
| Fermentation | Aerobic Respiration |
|---|---|
| Fermentation is partial breakdown of glucose. Net gain of only 2 ATP. | It is complete breakdown of glucose. 38 ATP are produced. |
| Oxidation of NADH to NAD ${ }^{+}$is slow process. | It is a vigorous reaction in aerobic respiration. |
Glycolysis occurs in cytoplasm. One glucose molecule forms 2 pyruvic acid molecules.
In anaerobic conditions it forms 2 ATP and ethanol + water.
In aerobic conditions it form 36 ATP + water $+\mathrm{CO}_2$. The steps of glycolysis are as follows
