Chapter 12: Respiration in Plants

Respiration

Combustion

It occurs inside living cells (cellular process).

It is a non-cellular process

Respiration is a biochemical process.

Combustion is a physio-chemical process.

Chemical bonds are broken down into steps, as a result, energy is released in stages.

All chemical steps occur simultaneously, as a result, energy is released in a single step.

A considerable amount of energy is stored in ATP molecules.

ATP is not formed.

Oxidation occurs at the end of the reaction (terminal oxidation) between reduced coenzymes and oxygen.

During combustion, the substrate is oxidized directly.

Several intermediates are formed. They are utilized in the synthesis of various organic compounds

No intermediates are produced in combustion.

Less than 50% of energy is liberated in the form of heat energy. Light is rarely produced.

Energy is liberated in the form of both light and heat energy.

Temperature is not allowed to rise.

Temperature becomes very high.

Several enzymes are needed, one for each step or reaction.

Burning is a non-enzymatic process.

Glycolysis

Krebs’ Cycle

It occurs inside the cytoplasm.

Krebs’ cycle operates inside mitochondria.

Glycolysis is the first step in respiration where glucose is broken down to pyruvate.

Krebs' cycle is the second step in respiration in which an active acetyl group is broken down completely.

This process is common in both aerobic and anaerobic respiration.

It only occurs during aerobic respiration.

It degrades a molecule of glucose into 2 molecules of pyruvate, an organic substance.

It degrades pyruvate completely into inorganic substances i.e., \[{\text{C}}{{\text{O}}_{\text{2}}}\] and ${{\text{H}}_{\text{2}}}{\text{O}}$.

Glycolysis requires two ATP molecules for the initial phosphorylation of the substrate molecule.

It does not require ATP molecules.

One glucose molecule yields four ATP molecules in glycolysis through substrate-level phosphorylation.

Two acetyl residues in the Krebs cycle liberate two ATP or GTP molecules through substrate-level phosphorylation.

The net gain is 2 molecules of NADH and 2 molecules of ATP for every molecule of glucose broken down.

Krebs’ cycle produces 6 molecules of NADH, and 2 molecules of ${\text{FAD}}{{\text{H}}_{\text{2}}}$ for every two molecules of ${\text{acetyl}}\;{\text{CoA}}$oxidized by it. Two molecules of NADH are released during the conversion of two pyruvates to ${\text{acetyl}}\;{\text{CoA}}$.

The net gain of energy during glycolysis is equal to 8 ATP molecules.

In krebs' cycle, the net gain of energy is equal to 24 ATP molecules. Six molecules of ATP can be produced from 2 molecules of ${\text{NAD}}{{\text{H}}_{\text{2}}}$ formed during the dehydrogenation of 2 pyruvates.

In glycolysis, no \[{\text{C}}{{\text{O}}_{\text{2}}}\] is evolved.

\[{\text{C}}{{\text{O}}_{\text{2}}}\] is evolved during Krebs’ cycle.

Oxygen is not required for glycolysis.

Oxygen is used as a terminal oxidant during krebs' cycle.

Aerobic Respiration

Fermentation

It uses oxygen for breaking the respiratory material into simpler substances.

Oxygen is not used in the breakdown of the respiratory substrate.

Respiratory material is completely oxidized.

Respiratory material is incompletely broken.

The end products are inorganic i.e., \[{\text{C}}{{\text{O}}_{\text{2}}}\]and ${{\text{H}}_{\text{2}}}{\text{O}}$.

Small, reduced organic molecules (ethanol or lactic acid) are produced as end products. Inorganic substances (\[{\text{C}}{{\text{O}}_{\text{2}}}\]) may or may not be produced.

Aerobic respiration is known for the normal mode of respiration in both plants and animals.

It occurs in yeast cells, bacteria and in the muscle cells of animals during vigorous exercise.

Aerobic respiration consists of three-step: - Glycolysis, Krebs’ cycle and terminal oxidation.

Anaerobic respiration or fermentation consists of two steps: - Glycolysis and incomplete breakdown of pyruvic acid.

Every carbon atom in the food is oxidised, releasing a substantial amount of carbon dioxide.

Less quantity of carbon dioxide is evolved.

Water is formed.

Water is usually not formed.

$686\;{\text{kcal}}$ of energy is produced per gm mole of glucose.

$39 - 59\;{\text{kcal}}$ of energy is produced per gm mole of glucose.

It continues indefinitely.

It cannot be continued indefinitely (except in some microorganisms) due to the accumulation of poisonous compounds and the reduced availability of energy per gram mole of food broken.

Aerobic Respiration

Anaerobic respiration

It occurs in the presence of oxygen.

It occurs in the absence of oxygen.

It is a type of respiration in which food (generally carbohydrates) is completely oxidized to carbon dioxide and water with the release of chemical energy.

It is a type of respiration in which food (generally carbohydrates) is partially oxidized with the release of chemical energy.

Since the substrate is completely oxidized, the energy yield of this type of respiration is more than that of anaerobic respiration.

Since the substrate is oxidized partially, the energy yield of this type of respiration is lower than that of aerobic respiration.

Complete oxidation of one molecule of glucose leads to a net gain of 38 ATP molecules.

Partial oxidation of one molecule of glucose leads to a net gain of 2 ATP molecules.

The end product of aerobic respiration is ${\text{C}}{{\text{O}}_{\text{2}}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$ (all higher organisms).

The end product of anaerobic respiration is lactic acid (animal cells), ethanol and ${\text{C}}{{\text{O}}_{\text{2}}}$ (lower organisms like bacteria and yeast).

Some reactions of aerobic respiration occur in the cytoplasm (glycolysis) and the mitochondria (Krebs’ cycle and ETS).

All reactions of anaerobic respiration occur in the cytoplasm. There is no involvement of mitochondria in this process.

${{\text{C}}_6}{{\text{H}}_{12}}{{\text{O}}_6} + 6{{\text{O}}_2} \to 6{\text{C}}{{\text{O}}_2} + 6{{\text{H}}_2}{\text{O}} + 686{\text{Kcal}}$

${{\text{C}}_6}{{\text{H}}_{12}}{{\text{O}}_6} \to 2{\text{C}}{{\text{O}}_2} + 2{{\text{C}}_2}{{\text{H}}_5}{\text{OH}} + 59{\text{Kcal}}$

Glycolysis

Fermentation

Glycolysis is a common pathway for both aerobic and anaerobic modes of respiration.

Fermentation is a type of anaerobic respiration which occurs in the absence of oxygen.

Glucose is split into two molecules of pyruvic acid during glycolysis.

During fermentation, pyruvic acid is converted to ethyl alcohol (yeast and some other microbes) or lactic acid (muscle cells of humans).

Glycolysis results in a net gain of 2 molecules of ATP.

No ATP is produced during fermentation.

It gives out two molecules of NADH per glucose molecule.

It generally utilizes NADH produced during glycolysis.

Glycolysis

Citric Acid Cycle

Glycolysis is the first step of respiration.

The citric acid cycle (Krebs’ cycle or TCA Cycle) is the second step of respiration.

This process happens in the cytoplasm.

This process occurs in the matrix of mitochondria.

It occurs both aerobically and anaerobically.

It occurs anaerobically.

Two ATPs are consumed during this process.

No consumption of ATP in the citric acid cycle.

The total gain of ATP is 8 (which includes NADH).

The net gain of ATP is 24.

Oxidative phosphorylation is not involved.

Oxidative phosphorylation is involved.

It is a linear pathway.

It is a circular pathway.

Carbon-di-oxide has not evolved.

Carbon-di-oxide is evolved.