Respiration in Plants

All living organisms require energy for life activities such as absorption, transport, movement, reproduction, and even breathing.
This energy is obtained by the oxidation of macromolecules called 'food'.
Green plants and cyanobacteria create their own food through photosynthesis, converting light energy into chemical energy stored in carbohydrates (glucose, sucrose, starch).
Not all cells in green plants photosynthesize; only those with chloroplasts do.
Non-green parts of plants require food translocation for oxidation.
Animals are heterotrophic, obtaining food directly (herbivores) or indirectly (carnivores) from plants.
Saprophytes (e.g., fungi) depend on dead and decaying matter.
All food respired for life processes comes from photosynthesis.

Cellular Respiration

Cellular respiration is the breakdown of food materials within cells to release energy, which is then trapped for ATP synthesis.
Photosynthesis occurs in chloroplasts (eukaryotes), while breakdown of complex molecules to yield energy occurs in the cytoplasm and mitochondria (eukaryotes).
Respiration involves breaking C-C bonds of complex compounds through oxidation, releasing energy.
Respiratory substrates are compounds oxidized during respiration (usually carbohydrates, but sometimes proteins, fats, or organic acids).
Energy from respiratory substrates is released in slow, step-wise reactions controlled by enzymes and trapped as chemical energy in ATP.
ATP acts as the energy currency of the cell, used in various energy-requiring processes.
Carbon skeletons produced during respiration are used as precursors for biosynthesis of other molecules.

Do Plants Breathe?

Plants require O2 for respiration and release CO2.
Plants lack specialized respiratory organs but use stomata and lenticels for gaseous exchange.
Each plant part manages its own gas exchange with minimal gas transport between parts.
Plants' respiration rates are far lower than those of animals.
During photosynthesis, O_2 availability isn't an issue as it's released within cells.
The distance gases must diffuse is short because each living cell is close to the plant's surface.

Adaptations for Gas Exchange

In stems, living cells are in thin layers inside and beneath the bark and have lenticels.
Interior cells are dead, providing mechanical support.
Loose packing of parenchyma cells in leaves, stems, and roots creates interconnected air spaces.

Oxidation of Glucose

Complete combustion of glucose yields CO2, H2O, and energy, mostly as heat:
C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + Energy
Cells catabolize glucose in small steps to couple released energy to ATP synthesis instead of releasing it all as heat.
Respiration utilizes oxygen and releases carbon dioxide, water, and energy.
Some cells exist in environments lacking oxygen (anaerobic conditions).
First cells likely lived in an atmosphere without oxygen.
Some organisms are facultative anaerobes, while others are obligate anaerobes.
All living organisms retain enzymatic machinery to partially oxidize glucose without oxygen.
Glycolysis is the breakdown of glucose to pyruvic acid.

Glycolysis

Glycolysis (glycos = sugar, lysis = splitting) was described by Gustav Embden, Otto Meyerhof, and J. Parnas (EMP pathway).
In anaerobic organisms, it's the only process in respiration.
Glycolysis occurs in the cytoplasm of all living organisms.
Glucose undergoes partial oxidation to form two molecules of pyruvic acid.
In plants, glucose comes from sucrose (photosynthesis end product) or storage carbohydrates.
Sucrose is converted to glucose and fructose by invertase, entering the glycolytic pathway.
Glucose and fructose are phosphorylated to glucose-6-phosphate by hexokinase, then glucose-6-phosphate isomerizes to fructose-6-phosphate.
Subsequent steps for glucose and fructose are the same.
Glycolysis involves ten reactions controlled by different enzymes to produce pyruvate from glucose (refer to Figure 12.1).
ATP is utilized in two steps: glucose to glucose-6-phosphate and fructose-6-phosphate to fructose 1,6-bisphosphate.
Fructose 1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
NADH + H^+ is formed when 3-phosphoglyceraldehyde (PGAL) is converted to 1,3-bisphosphoglycerate (BPGA).
Two redox-equivalents are removed from PGAL and transferred to NAD^+.
PGAL is oxidized with inorganic phosphate to form BPGA.
Conversion of BPGA to 3-phosphoglyceric acid (PGA) yields energy trapped as ATP.
Another ATP is synthesized during the conversion of PEP to pyruvic acid.
Pyruvic acid is the key product of glycolysis, and its metabolic fate depends on cellular needs.

Fate of Pyruvic Acid

Pyruvic acid from glycolysis is handled in three major ways by different cells: lactic acid fermentation, alcoholic fermentation, and aerobic respiration.
Fermentation occurs under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
For complete oxidation of glucose to CO2 and H2O, organisms use the Krebs’ cycle (aerobic respiration), which requires O_2 supply.

Fermentation

In fermentation (e.g., by yeast), incomplete glucose oxidation occurs under anaerobic conditions, converting pyruvic acid to CO_2 and ethanol.
Pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions.
Some bacteria produce lactic acid from pyruvic acid (refer to Figure 12.2).
In animal cells (e.g., muscles during exercise), pyruvic acid is reduced to lactic acid by lactate dehydrogenase when oxygen is inadequate.
The reducing agent is NADH + H^+, which is reoxidized to NAD^+ in both processes.
Lactic acid and alcohol fermentation release little energy; less than seven percent of glucose energy is released and not all is trapped as ATP.
These processes can be hazardous due to acid or alcohol production.
Yeasts poison themselves to death at around 13% alcohol concentration.
Complete oxidation of glucose allows organisms to extract more energy, synthesize more ATP.
In eukaryotes, complete oxidation occurs within the mitochondria and requires O_2.

Aerobic Respiration

Aerobic respiration completely oxidizes organic substances in the presence of oxygen, releasing CO_2, water, and a large amount of energy.
This process is common in higher organisms.
Pyruvate, the final product of glycolysis, is transported from the cytoplasm into the mitochondria for aerobic respiration.

Key Events in Aerobic Respiration

Complete oxidation of pyruvate involves stepwise removal of all hydrogen atoms, yielding three molecules of CO_2.
Electrons removed with hydrogen atoms are passed to molecular O_2, with simultaneous ATP synthesis.

Location of Processes

The first process (pyruvate oxidation) occurs in the mitochondrial matrix.
The second process (electron transfer and ATP synthesis) is located on the inner membrane of the mitochondria.

Pyruvate Dehydrogenase Complex

Pyruvate, formed by glycolytic catabolism of carbohydrates in the cytosol, undergoes oxidative decarboxylation in the mitochondrial matrix via pyruvic dehydrogenase.
This process requires several coenzymes, including NAD^+ and Coenzyme A.
Pyruvic\ acid + CoA + NAD^+ \xrightarrow[Mg^{2+}]{Pyruvate\ dehydrogenase} Acetyl\ CoA + CO_2 + NADH + H^+
Two molecules of NADH are produced from two molecules of pyruvic acid (from one glucose molecule during glycolysis).
Acetyl CoA then enters the tricarboxylic acid cycle (Krebs’ cycle).

Tricarboxylic Acid Cycle (Krebs Cycle)

The TCA cycle starts with the condensation of the acetyl group with oxaloacetic acid (OAA) and water to yield citric acid (Figure 12.3).
The enzyme citrate synthase catalyzes the reaction, releasing a molecule of CoA.
Citrate is then isomerized to isocitrate.
Followed by two successive decarboxylation steps, leading to the formation of α-ketoglutaric acid and then succinyl-CoA.
In the remaining steps, 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 (substrate-level phosphorylation).
GTP is converted to GDP with simultaneous ATP synthesis from ADP.
There are three points where NAD^+ is reduced to NADH + H^+, and one point where FAD^+ is reduced to FADH_2.
Continued oxidation of acetyl CoA requires the continued replenishment of oxaloacetic acid and regeneration of NAD^+ and FAD^+ from NADH and FADH_2, respectively.
Summary equation for this phase of respiration:
Pyruvic\ acid + 4NAD^+ + FAD^+ + 2H2O + ADP + Pi \rightarrow 3CO2 + 4NADH + 4H^+ + FADH2 + ATP
Glucose is broken down to release CO2, eight molecules of NADH + H^+, and two of FADH2, plus two ATP molecules in the TCA cycle.

Role of Oxygen

The following steps in the respiratory process are to release and utilise the energy stored in NADH+H^+ and FADH_2.
This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2 resulting in the formation of H2O.
The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) (Figure 12.4) and it is present in the inner mitochondrial membrane.

Electron Transport System (ETS) and Oxidative Phosphorylation

Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidized by NADH dehydrogenase (complex I) and transferred to ubiquinone within the inner membrane.
Ubiquinone also receives reducing equivalents via FADH_2 (complex II) that is generated during succinate oxidation in the citric acid cycle.
Reduced ubiquinone (ubiquinol) is oxidized, transferring electrons to cytochrome c via the cytochrome bc1 complex (complex III).
Cytochrome c is a small protein attached to the outer surface of the inner membrane, acting as a mobile carrier between complex III and IV.
Complex IV (cytochrome c oxidase complex) contains cytochromes a and a3, and two copper centers.
Electrons pass from one carrier to another (complex I to IV), coupling 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.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH_2 produces 2 molecules of ATP.
- The role of oxygen, though limited to the terminal stage, is vital as it drives the entire process by removing hydrogen from the system, acting as the final hydrogen acceptor.
- In respiration, the energy of oxidation-reduction is utilized for the production of proton gradient required for phosphorylation.
- The process called oxidative phosphorylation.

Chemiosmotic Hypothesis

The energy released during the electron transport system is utilized in synthesising ATP with the help of ATP synthase (complex V).
This complex comprises two major components: F1 and F0 (Figure 12.5).
F_1 headpiece is a peripheral membrane protein complex containing the site for ATP synthesis from ADP and inorganic phosphate.
F_0 is an integral membrane protein complex forming the channel through which protons cross the inner membrane.
Proton passage through the channel is coupled to the catalytic site of the F_1 component for ATP production.
For each ATP produced, 4H+ pass through F_0 from the intermembrane space to the matrix down the electrochemical proton gradient.

The Respiratory Balance Sheet

Net ATP calculations are theoretical due to:
- Sequential, orderly pathway functioning.
- NADH synthesized in glycolysis being transferred into the mitochondria for oxidative phosphorylation.
- Intermediates not being utilized to synthesize other compounds.
- Only glucose being respired.
A net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose is possible.

Fermentation vs. Aerobic Respiration

Fermentation involves partial glucose breakdown, whereas aerobic respiration involves complete degradation to CO2 and H2O.
Fermentation yields a net gain of two ATP molecules per glucose molecule, while aerobic respiration generates many more ATP molecules.
NADH is oxidized to NAD^+ slowly in fermentation but vigorously in aerobic respiration.

Amphibolic Pathway

Glucose is the favored substrate for respiration; other carbohydrates are usually converted into glucose first.
Other substrates can be respired but enter the pathway at different steps (Figure 12.6).
Fats break down into glycerol and fatty acids; fatty acids degrade to acetyl CoA, and glycerol converts to PGAL.
Proteins degrade into amino acids (after deamination), entering the pathway at various points, including pyruvate or acetyl CoA.
The respiratory pathway is both catabolic (breakdown) and anabolic (synthesis), making it an amphibolic pathway.

Respiratory Quotient (RQ)

During aerobic respiration, O2 is consumed, and CO2 is released.
Respiratory quotient (RQ) is the ratio of CO2 evolved to O2 consumed:
RQ = \frac{Volume\ of\ CO2\ evolved}{Volume\ of\ O2\ consumed}
RQ depends on the respiratory substrate.
- For carbohydrates (completely oxidized), RQ = 1:
  C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O
  RQ = \frac{6}{6} = 1
- For fats, RQ is less than 1. For example, tripalmitin:
  2C{51}H{98}O6 + 145O2 \rightarrow 102CO2 + 98H2O
  RQ = \frac{102}{145} = 0.7
- For proteins, RQ is about 0.9.
Living organisms often use multiple respiratory substrates.