Respiration In Plants Class 11 Biology Chapter 12 Notes

The Significance of Breathing and Energy Production in Living Organisms

1. Importance of Breathing and Energy in Life

Breathing is essential for all living organisms.
All living organisms require energy for various life activities.
Questions about energy sources and utilization in different organisms.

2. Energy Sources in Living Organisms

All energy for life processes comes from the oxidation of macromolecules known as ‘food.’
Green plants and cyanobacteria can produce their own food via photosynthesis.
In green plants, photosynthesis occurs in cells containing chloroplasts.
Non-green parts of plants and all animals rely on external food sources.
Saprophytic organisms like fungi obtain energy from dead and decaying matter.
Emphasis on the ultimate source of food being photosynthesis.

3. Cellular Respiration and ATP

Cellular respiration involves the breakdown of complex molecules to release energy.
Photosynthesis occurs in chloroplasts, while respiration occurs in the cytoplasm and mitochondria (eukaryotes).
Respiration involves the oxidation of complex compounds, leading to energy release.
Carbohydrates are typically oxidized for energy, but other substances can also be used.
Energy release occurs in a series of controlled, enzyme-mediated reactions.
Energy is trapped as ATP (adenosine triphosphate), serving as the cell’s energy currency.
ATP is used whenever and wherever energy is required for various cellular processes.
Carbon skeletons produced during respiration are used as precursors for biosynthesis of other molecules in the cell.

Respiration and Gas Exchange in Plants

1. Respiration in Plants

Plants require oxygen (O2) for respiration.
They release carbon dioxide (CO2) during respiration.
Plants lack specialized respiratory organs, unlike animals.

2. Mechanisms for Gas Exchange in Plants

Plants use stomata and lenticels for gas exchange.
Gas exchange needs are met by individual plant parts.
Gas transport between different plant parts is limited.
Gas demands during photosynthesis are met locally.
Cells involved in photosynthesis release O2 within themselves.

3. Gas Exchange in Different Plant Parts

Living cells are located close to the surface of the plant.
Stems have living cells beneath the bark with lenticels.
Loose packing of parenchyma cells in leaves, stems, and roots creates an interconnected network of air spaces.

4. Energy Production and Utilization in Plants

Complete glucose combustion yields CO2, H2O, and energy.
Energy is mostly given out as heat.
Cells need to utilize this energy for other processes.
Glucose oxidation occurs in multiple small steps, coupling energy release to ATP synthesis.

5. Oxygen Utilization in Respiration

Oxygen is used during respiration.
Some cells may live in environments with fluctuating oxygen availability.
Early Earth’s atmosphere lacked oxygen.
Present-day organisms are adapted to anaerobic conditions.
Some organisms are facultative anaerobes, while others are obligate.
Glycolysis is the breakdown of glucose to pyruvic acid and can occur without oxygen.

Glycolysis and its Significance in Cellular Respiration

1. Glycolysis

Origin of the term ‘glycolysis’ from Greek words: ‘glycos’ (sugar) and ‘lysis’ (splitting).
Glycolysis pathway credited to Gustav Embden, Otto Meyerhof, and J. Parnas.
Also known as the EMP pathway.
In anaerobic organisms, glycolysis is the primary respiration process.
Occurs in the cell’s cytoplasm and is found in all living organisms.
In glycolysis, glucose undergoes partial oxidation to produce two pyruvic acid molecules.

2. Source of Glucose for Glycolysis in Plants

In plants, glucose is derived from sucrose, the end product of photosynthesis.
Sucrose is converted into glucose and fructose by the enzyme invertase.
Glucose and fructose enter the glycolytic pathway.
Glucose and fructose are phosphorylated by hexokinase to form glucose-6-phosphate and fructose-6-phosphate.

3. Steps of Glycolysis

Glycolysis comprises a chain of ten reactions, each controlled by different enzymes.
The process results in the formation of pyruvate from glucose.
Certain steps in glycolysis involve the utilization or synthesis of ATP and NADH + H+.

4. Utilization of ATP and NADH + H+ in Glycolysis

ATP is utilized in two steps: the conversion of glucose to glucose 6-phosphate and fructose 6-phosphate to fructose 1,6-bisphosphate.
NADH + H+ is formed when 3-phosphoglyceraldehyde is converted to 1,3-bisphosphoglycerate.
Two redox equivalents (hydrogen atoms) are transferred from PGAL to NAD+.
ATP synthesis occurs during the conversion of BPGA to 3-phosphoglyceric acid.
Calculate the number of ATP molecules directly synthesized in glycolysis from one glucose molecule.

5. Metabolic Fate of Pyruvate

Pyruvic acid is the key product of glycolysis.
Its metabolic fate depends on cellular needs.
Three major pathways for handling pyruvic acid: lactic acid fermentation, alcoholic fermentation, and aerobic respiration.
Fermentation occurs under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
Complete glucose oxidation to CO2 and H2O requires the Krebs’ cycle, also known as aerobic respiration, which necessitates oxygen supply.

Fermentation and its Role in Energy Production

1. Fermentation Process

Yeast and certain organisms carry out fermentation.
It’s an incomplete oxidation of glucose under anaerobic conditions.
Involves the conversion of pyruvic acid to CO2 and ethanol in yeast.
Catalyzed by enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase.
Some bacteria produce lactic acid from pyruvic acid.
In animal cells, like muscle cells during strenuous exercise, pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
In both processes, NADH+H+ is the reducing agent, which is reoxidized to NAD+.

2. Energy Production in Fermentation

Both lactic acid and alcohol fermentation release minimal energy (less than seven percent of glucose’s energy).
Not all of this energy is trapped as high-energy ATP bonds.
These processes have hazards, producing acid or alcohol.
Calculate the net ATP synthesized when one glucose molecule is fermented to alcohol or lactic acid.
Yeasts are poisoned when alcohol concentration reaches about 13 percent.

3. Maximum Alcohol Concentration in Naturally Fermented Beverages

Determine the maximum alcohol concentration in naturally fermented beverages.
Consider how alcoholic beverages with greater alcohol content are produced.

4. Complete Oxidation of Glucose

To carry out complete glucose oxidation and generate a larger number of ATP molecules needed for cellular metabolism, eukaryotes utilize mitochondria.
This process is called aerobic respiration.
Requires the presence of oxygen.
Leads to the complete oxidation of organic substances, releasing CO2, water, and a substantial amount of energy.
Aerobic respiration is common in higher organisms.

Aerobic Respiration and the Role of Mitochondria

1. Aerobic Respiration

In aerobic respiration, the final product of glycolysis, pyruvate, is transported into the mitochondria.
The key events in aerobic respiration:
- Complete oxidation of pyruvate by the removal of hydrogen atoms, resulting in the formation of three molecules of CO2.
- Transfer of the electrons, removed during hydrogen atom removal, to molecular O2, leading to the synthesis of ATP.
The two processes occur in different mitochondrial compartments.

2. Oxidative Decarboxylation of Pyruvate

Pyruvate, formed from the glycolytic breakdown of carbohydrates in the cytosol, undergoes oxidative decarboxylation in the mitochondrial matrix.
This process is catalyzed by pyruvate dehydrogenase.
Several coenzymes, including NAD+ and Coenzyme A, are involved in these reactions.
The reaction results in the formation of acetyl CoA, NADH, and CO2.
Each glucose molecule, during glycolysis, yields two molecules of pyruvic acid, generating two molecules of NADH.

3. Tricarboxylic Acid Cycle (Krebs’ Cycle)

Acetyl CoA enters the tricarboxylic acid cycle, commonly known as the Krebs’ cycle.
Named after scientist Hans Krebs.
The Krebs’ cycle is a cyclic pathway within the mitochondria.

Aerobic respiration involves the complete oxidation of glucose, yielding CO2, water, and a significant amount of energy. The mitochondria play a critical role in this process, ensuring efficient energy production in eukaryotic cells.

Tricarboxylic Acid Cycle (TCA) – A Closer Look

1. Initiation of the TCA Cycle

The TCA cycle begins with the condensation of the acetyl group, OAA (oxaloacetic acid), and water, forming citric acid.
Enzyme citrate synthase catalyzes this reaction, releasing a CoA molecule.

2. Progression of the TCA Cycle

Citrate is isomerized to isocitrate.
Two consecutive decarboxylation reactions result in the formation of α-ketoglutaric acid and then succinyl-CoA.
In the remaining steps of the cycle, succinyl-CoA is oxidized to OAA, allowing the cycle to continue.
During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesized via substrate-level phosphorylation.
In a coupled reaction, GTP is converted to GDP, and ATP is synthesized from ADP.

3. Reduction of NAD+ and FAD+

Three points in the TCA cycle lead to the reduction of NAD+ to NADH + H+, and one point results in the reduction of FAD+ to FADH2.

4. Replenishment of Oxaloacetic Acid

Continuous oxidation of acetyl CoA in the TCA cycle requires the constant replenishment of oxaloacetic acid, the initial component of the cycle.

5. Summary of Respiration

The summary equation for this phase of respiration involves the conversion of pyruvic acid to CO2, the generation of NADH, FADH2, and ATP.

The discussion has so far focused on the breakdown of glucose, with the release of CO2 and the production of NADH + H+, FADH2, and limited ATP in the TCA cycle. To understand the role of oxygen (O2) in respiration and how ATP synthesis occurs, we will now delve into the final stages of cellular respiration.

Electron Transport System (ETS) and Oxidative Phosphorylation

1. Energy Release from NADH+H+ and FADH2

Energy stored in NADH+H+ and FADH2 is released and utilized in the respiratory process.
This occurs through oxidation in the electron transport system (ETS) with the electrons being transferred to O2, forming H2O.

2. Electron Transport System (ETS)

ETS is present in the inner mitochondrial membrane.
Electrons from NADH produced in the mitochondrial matrix are oxidized by NADH dehydrogenase (complex I).
Electrons are transferred to ubiquinone within the inner membrane.
Ubiquinone also receives reducing equivalents from FADH2 (complex II) generated during the oxidation of succinate in the citric acid cycle.
Reduced ubiquinone (ubiquinol) is then oxidized with electron transfer to cytochrome c via cytochrome bc1 complex (complex III).
Cytochrome c acts as a mobile carrier for electron transfer between complex III and complex IV (cytochrome c oxidase complex).

3. ATP Synthesis

As electrons pass through the electron transport chain from complex I to IV, they are coupled to ATP synthase (complex V) for ATP production from ADP and inorganic phosphate.
The number of ATP molecules synthesized depends on the nature of the electron donor: NADH oxidation results in 3 ATP molecules, while FADH2 oxidation produces 2 ATP molecules.

4. Role of Oxygen

Oxygen plays a crucial role in the terminal stage of respiration, as the final hydrogen acceptor.
Oxygen is necessary to drive the entire process by removing hydrogen from the system.

5. Oxidative Phosphorylation

In respiration, the energy of oxidation-reduction is utilized for ATP synthesis, different from photophosphorylation that uses light energy.
The process is termed oxidative phosphorylation.

6. Mechanism of ATP Synthesis

The membrane-linked ATP synthesis mechanism is explained by the chemiosmotic hypothesis.
ATP synthase (complex V) consists of two major components, F1 and F0.
F1 is a peripheral membrane protein complex containing the ATP synthesis site.
F0 is an integral membrane protein complex forming a proton channel through which protons cross the inner membrane.
Proton passage through F0 is coupled to the catalytic site of F1 for ATP production.
Four protons (4H+) pass through F0 for the production of one ATP, moving from the intermembrane space to the matrix along the electrochemical proton gradient.

The Respiratory Balance Sheet: Fermentation vs. Aerobic Respiration

1. Theoretical Net Gain of ATP

Calculations can be made to estimate the net gain of ATP for every glucose molecule oxidized during respiration.
Assumptions include a sequential pathway of glycolysis, TCA cycle, and ETS, with proper NADH transfer to mitochondria, no intermediate utilization for other compounds, and only glucose being respired.

2. Limitations of Assumptions

In reality, pathways operate simultaneously, substrates enter and exit as needed, ATP is utilized when required, and enzymatic rates are multifactorially controlled.
Despite these limitations, theoretical calculations help understand the efficiency of the living system in energy extraction and storage.

3. Net Gain of ATP in Aerobic Respiration

A net gain of 38 ATP molecules can be achieved during aerobic respiration for one glucose molecule.
This process involves complete degradation of glucose to CO2 and H2O, harnessing a substantial amount of energy.

4. Comparison: Fermentation vs. Aerobic Respiration

Fermentation leads to only partial glucose breakdown, while aerobic respiration fully degrades glucose to CO2 and H2O.
Fermentation results in a net gain of only two ATP molecules for each glucose molecule degraded to pyruvic acid, while aerobic respiration generates a significantly higher number of ATP molecules.
NADH oxidation to NAD+ is slow in fermentation, while it’s highly efficient in aerobic respiration.

Amphibolic Pathway: A Dual Role in Respiration

1. Preferred Substrate: Glucose

Glucose is the primary substrate for respiration, favored for its efficiency in energy extraction.
Carbohydrates are typically converted into glucose before entering the respiratory pathway.

2. Entry Points for Different Substrates

Fats require breakdown into glycerol and fatty acids before entering, with fatty acids degrading into acetyl CoA.
Glycerol enters the pathway after conversion to PGAL.
Proteins are broken down by proteases and individual amino acids enter at different stages within the Krebs’ cycle or as pyruvate or acetyl CoA.

3. Is Respiration Purely Catabolic?

Respiration is often viewed as a catabolic process due to the breakdown of substrates.
However, a deeper understanding reveals that the same intermediates in the respiratory pathway are withdrawn for the synthesis of various substrates.
For example, fatty acids are broken down into acetyl CoA before entering the pathway for respiration but withdrawn for fatty acid synthesis when needed.
Respiratory intermediates play a pivotal role in both breakdown (catabolism) and synthesis (anabolism) processes within the organism.

4. Amphibolic Pathway

Because the respiratory pathway serves dual roles in catabolism and anabolism, it is better described as an amphibolic pathway.
This term emphasizes the pathway’s involvement in both the breakdown and synthesis of various compounds, highlighting its versatility and central role in metabolism.

Respiratory Quotient (RQ): Substrate-Dependent Measurement

1. Respiratory Quotient

In aerobic respiration, oxygen (O2) is consumed, and carbon dioxide (CO2) is released.
The ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration is known as the respiratory quotient (RQ) or respiratory ratio.
The RQ depends on the type of respiratory substrate used during respiration.

2. RQ Values for Different Substrates

When carbohydrates are the substrate and are entirely oxidized, the RQ is 1. This is because equal amounts of CO2 and O2 are evolved and consumed.
The balanced equation for carbohydrate oxidation shows this equilibrium: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
The RQ for carbohydrates is calculated as follows: RQ = (6CO2 evolved) / (6O2 consumed) = 1

3. RQ Values for Fats

When fats are used as substrates, the RQ is less than 1. For instance, the RQ for the fatty acid tripalmitin is about 0.7.
Calculations for tripalmitin are as follows: C51H98O6 + 145O2 → 102CO2 + 98H2O + Energy
The RQ for fats is calculated as follows: RQ = (102CO2 evolved) / (145O2 consumed) ≈ 0.7

4. RQ Values for Proteins

When proteins are used as substrates, the RQ is approximately 0.9.

5. Real-World Application

In living organisms, respiratory substrates are usually a combination of carbohydrates, fats, and proteins.
Pure proteins or fats are seldom used as exclusive respiratory substrates in physiological conditions.

The respiratory quotient (RQ) provides valuable insights into the type of substrates being oxidized for energy in living organisms. It allows for a better understanding of metabolic processes and the role of different substrates in respiration.