Respiration in Plants: common doubts, answered
The questions students ask most often about Respiration in Plants, each with a short answer. For the full chapter, read the Respiration in Plants notes.
Respiration and ATP
Read this section in the notes →Why is ATP called the energy currency of the cell?
ATP stores the energy released during the breakdown of food in its phosphate bonds and gives it up when the cell needs to do work. Cells do not use glucose energy directly, because respiration releases it in small steps and traps it in ATP, which is then spent for growth, transport and other energy-requiring processes.
Do plants breathe?
Read this section in the notes →Do plants breathe like animals?
No. Plants have no specialised organs for gas exchange. Each part takes in oxygen and releases carbon dioxide on its own, and gases move through stomata in leaves and lenticels in woody stems, and by diffusion in roots. The distance between living cells and the surface is small, so diffusion is enough.
Glycolysis
Read this section in the notes →Why is glycolysis also called the EMP pathway?
It is named after Gustav Embden, Otto Meyerhof and J. Parnas, the scientists who worked out its scheme. Glycolysis occurs in the cytoplasm of all living organisms, runs through ten reactions and converts one glucose into two molecules of pyruvic acid. The word glycolysis itself comes from Greek roots meaning sugar and splitting.
How many ATP are made and used in glycolysis?
Glycolysis uses 2 ATP in the early steps and makes 4 ATP directly, so the net gain is 2 ATP per glucose, along with 2 NADH. ATP is used when glucose becomes glucose-6-phosphate and when fructose-6-phosphate becomes fructose-1,6-bisphosphate. ATP is made when BPGA becomes PGA and when PEP becomes pyruvic acid.
Does glycolysis need oxygen?
No. Glycolysis does not require oxygen and takes place in the cytoplasm, not in the mitochondria. This is why it is shared by all organisms, whether they respire aerobically or not. What happens to pyruvic acid afterwards depends on whether oxygen is available: it enters fermentation or the mitochondria.
Fermentation
Read this section in the notes →What is the difference between alcoholic fermentation and lactic acid fermentation?
In alcoholic fermentation, as in yeast, pyruvic acid is converted by decarboxylase and alcohol dehydrogenase into carbon dioxide and ethanol. In lactic acid fermentation, as in muscle cells during heavy exercise, pyruvic acid is reduced by lactate dehydrogenase to lactic acid with no carbon dioxide released.
Why is fermentation so much less efficient than aerobic respiration?
Fermentation frees under seven per cent of the energy stored in glucose and yields only 2 ATP, since the glucose is only partly broken down. In aerobic respiration the pyruvic acid is completely oxidised, giving a net of 38 ATP. Also, yeast poisons itself, because alcohol above about 13 per cent kills it.
Aerobic respiration: pyruvate to acetyl CoA
Read this section in the notes →Where does the link reaction take place and what does it produce?
The link reaction takes place in the mitochondrial matrix, where pyruvic acid is oxidatively decarboxylated to acetyl CoA by pyruvic dehydrogenase. It needs NAD⁺, coenzyme A and Mg²⁺ and releases one carbon dioxide and one NADH per pyruvic acid. Acetyl CoA then enters the Krebs cycle.
Tricarboxylic acid cycle
Read this section in the notes →Where does the Krebs cycle occur and what starts it?
The Krebs cycle occurs in the mitochondrial matrix. It starts when acetyl CoA combines with oxaloacetic acid and water to form citric acid, catalysed by citrate synthase. Because the first product is citric acid, which has three carboxyl groups, the cycle is also called the tricarboxylic acid cycle.
What is produced in one turn of the Krebs cycle?
One turn gives 3 NADH, 1 FADH₂, 1 GTP and 2 carbon dioxide molecules. The GTP forms by substrate-level phosphorylation and is converted to ATP. Since one glucose gives two acetyl CoA, the cycle turns twice per glucose, and the totals have to be doubled.
Electron transport system and oxidative phosphorylation
Read this section in the notes →What is the difference between substrate-level phosphorylation and oxidative phosphorylation?
Substrate-level phosphorylation makes ATP directly from a substrate in a reaction, as in glycolysis and in the step of the Krebs cycle that gives GTP. Oxidative phosphorylation makes ATP using the energy of the electron transport chain and oxygen, through ATP synthase on the inner mitochondrial membrane, and it yields most of the ATP.
What are the five complexes of the electron transport system?
Complex I is NADH dehydrogenase, complex II is succinate dehydrogenase carrying FADH₂, complex III is cytochrome bc₁, complex IV is cytochrome c oxidase, and complex V is ATP synthase. Cytochrome c is a small mobile carrier between complexes III and IV. They lie on the inner mitochondrial membrane.
What is the role of oxygen in respiration?
Oxygen is the final hydrogen acceptor at the end of the electron transport chain, where cytochrome c oxidase passes electrons to it to form water. It is not used in glycolysis or the Krebs cycle directly. Without oxygen, the chain stops, NADH cannot be reoxidised, and the cycle stalls.
The respiratory balance sheet
Read this section in the notes →How many ATP does each NADH and FADH₂ give?
Each NADH gives 3 ATP and each FADH₂ gives 2 ATP in the electron transport system. NADH hands its electrons in at complex I, whereas FADH₂ enters at complex II, bypassing one of the proton-pumping steps, so it yields less ATP. These values are the basis of the 38 ATP total for one glucose.
What is the net ATP gain from one glucose in aerobic respiration?
The net gain is 38 ATP per glucose in the NCERT balance sheet. This comes from the 2 net ATP of glycolysis, 2 GTP from the Krebs cycle and the ATP from the oxidation of NADH and FADH₂ in the ETS. Fermentation, by contrast, gives only 2 ATP, from glycolysis alone.
Amphibolic pathway
Read this section in the notes →Why is the respiratory pathway called amphibolic?
It is amphibolic because it serves both breakdown and building. It is catabolic in oxidising glucose, but its intermediates, such as acetyl CoA, are also drawn off to make fatty acids and amino acids, which is anabolic. Fats, proteins and carbohydrates all feed into the same pathway at different points.
Where do fats and proteins enter the respiratory pathway?
Fatty acids break down to acetyl CoA, and glycerol is converted to PGAL, which enters glycolysis. Proteins are first broken to amino acids, which after deamination enter as pyruvate, acetyl CoA or intermediates of the Krebs cycle, depending on the amino acid. So the main pathway handles all three types of food.
Respiratory quotient
Read this section in the notes →What is the respiratory quotient and what are its values?
The respiratory quotient is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed. It is 1.0 for carbohydrates, about 0.7 for fats like tripalmitin, and about 0.9 for proteins. It depends on the respiratory substrate, so it shows what is being burnt.
Why is the RQ of fats less than 1?
Fats contain little oxygen relative to carbon and hydrogen, so they need more oxygen to be oxidised than the carbon dioxide they release. For tripalmitin, 145 molecules of oxygen are used and 102 of carbon dioxide are produced, giving 0.7. Carbohydrates, which are richer in oxygen, give exactly 1.
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