Respiration in Plants: Comprehensive Study Notes

Introduction to Respiration in Plants

The Necessity of Breathing: Breathing is essential to life because it is connected to the process of releasing energy from food. All living organisms, including plants and microbes, require energy to carry out daily activities such as absorption, transport, movement, reproduction, and breathing itself.
Energy Source: Energy required for life processes is obtained through the oxidation of macromolecules collectively known as "food."
Photosynthesis and Energy Storage:
- Green plants and cyanobacteria are autotrophic; they prepare their own food via photosynthesis.
- Light energy is trapped and converted into chemical energy stored within the C-C bonds of carbohydrates like glucose, sucrose, and starch.
- Localization of Photosynthesis: Not all parts of a plant photosynthesise. Only cells containing chloroplasts, typically located in superficial layers, perform this function.
- Translocation: Non-green organs, tissues, and cells do not photosynthesise and therefore require food to be translocated to them for oxidation.
Heterotrophic Nutrition:
- Animals are heterotrophs, obtaining food directly (herbivores) or indirectly (carnivores) from plants.
- Saprophytes (e.g., fungi) depend on dead and decaying matter.
- Ultimately, all food respired for life processes originates from photosynthesis.

Cellular Respiration: Core Concepts

Definition: Cellular respiration is the mechanism of breaking down food materials within the cell to release energy and trapping this energy for the synthesis of Adenosine Triphosphate (ATP).
Breakdown Locations:
- In eukaryotes, photosynthesis occurs in chloroplasts.
- The breakdown of complex molecules to yield energy occurs in the cytoplasm and the mitochondria.
Respiratory Substrates:
- The compounds oxidised during respiration are called respiratory substrates.
- Carbohydrates (specifically glucose) are the most common substrates.
- Proteins, fats, and organic acids can also be used as respiratory substances under certain conditions in some plants.
The Mechanism of Energy Release:
- Energy is not released in a single step or free into the cell.
- It is released in a series of slow, step-wise reactions controlled by enzymes.
- Energy is trapped as chemical energy in the form of ATP.
- The carbon skeletons produced during respiration serve as precursors for the biosynthesis of other molecules within the cell.
ATP as Energy Currency: ATP is not used directly in its stored form but is broken down whenever and wherever energy is required for various energy-requiring processes of the organism.

Gaseous Exchange in Plants

Respiratory Requirements: Plants require $O_2$ for respiration and release $CO_2$ .
Lack of Specialized Organs: Unlike animals, plants do not have specialized organs for gaseous exchange (like lungs). Instead, they utilize stomata and lenticels.
Reasons Plants Lack Specialized Respiratory Organs:
1. Independance of Parts: Each plant part takes care of its own gas-exchange needs. There is very little transport of gases between different parts of the plant.
2. Low Demand: Roots, stems, and leaves respire at much lower rates than animals. High volume gas exchange only occurs during photosynthesis, and leaves are adapted to manage this internally (since $O_2$ is released within the cell during photosynthesis).
3. Proximity to Surface: Even in large, bulky plants, the distance gases must diffuse is small. Each living cell is located close to the surface.
  - In thick woody stems, living cells are organized in thin layers beneath the bark.
  - The interior of woody stems consists of dead cells that provide mechanical support only.
4. Interconnected Air Spaces: The loose packing of parenchyma cells in leaves, stems, and roots provides an interconnected network of air spaces, ensuring most cells have surface contact with air.

The Biochemical Process of Respiration

Overall Reaction (Complete Combustion of Glucose):
- $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$
Energy Strategy: The plant cell catabolises glucose in small steps. This prevents all energy from being lost as heat and allows specific steps to couple the released energy to ATP synthesis.
Anaerobic Origins: It is believed the first cells on Earth lived in an oxygen-lacked atmosphere. Many modern organisms are adapted to anaerobic conditions, either as facultative anaerobes or obligate anaerobes.
Glycolysis (The EMP Pathway):
- Etymology: Derived from Greek words "glycos" (sugar) and "lysis" (splitting).
- Discovered by: Gustav Embden, Otto Meyerhof, and J. Parnas.
- Occurrence: Occurs in the cytoplasm of all living organisms.
- General Process: One molecule of glucose (6C) undergoes partial oxidation to form two molecules of pyruvic acid (3C).
- Starting Material in Plants: Glucose is derived from sucrose (the end product of photosynthesis) or storage carbohydrates. Sucrose is converted to glucose and fructose by the enzyme invertase.

Detailed Steps of Glycolysis

Phosphorylation Steps (ATP Utilization):
1. Glucose is phosphorylated to Glucose-6-phosphate by the enzyme hexokinase, utilizing one ATP molecule.
2. Glucose-6-phosphate isomerises to Fructose-6-phosphate.
3. Fructose-6-phosphate is converted to Fructose 1, 6-bisphosphate, utilizing a second ATP molecule.
Splitting Step:
- Fructose 1, 6-bisphosphate (6C) is split into two 3-carbon compounds: Dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). These are interconvertible triose phosphates.
Oxidation and ATP Synthesis (Energy Generation):
1. NADH Formation: 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA). During this step, two redox-equivalents (hydrogen atoms) are transferred to $NAD^+$ to form $NADH + H^+$ . This involves inorganic phosphate.
2. First Direct ATP Synthesis: 1, 3-bisphosphoglycerate (BPGA) is converted to 3-phosphoglyceric acid (PGA), yielding ATP via substrate-level phosphorylation.
3. 3-phosphoglyceric acid (PGA) is converted to 2-phosphoglycerate.
4. 2-phosphoglycerate is converted to Phosphoenolpyruvate (PEP) with the release of water.
5. Second Direct ATP Synthesis: PEP is converted to Pyruvic acid, yielding another molecule of ATP.
Net Yield of Glycolysis: From one molecule of glucose, the metabolic pathway produces 2 molecules of Pyruvic acid, 2 (net) molecules of ATP, and 2 molecules of $NADH + H^+$ .

Fate of Pyruvic Acid

There are three major pathways for pyruvate depending on cellular needs and oxygen availability:

Lactic Acid Fermentation: Occurs under anaerobic conditions.
Alcoholic Fermentation: Occurs under anaerobic conditions (e.g., in yeast).
Aerobic Respiration (Krebs’ Cycle): Complete oxidation of glucose to $CO_2$ and $H_2O$ in the presence of oxygen.

Fermentation (Anaerobic Respiration)

Alcoholic Fermentation:
- In yeast, glucose is incompletely oxidised to $CO_2$ and ethanol.
- Key enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase.
Lactic Acid Fermentation:
- Occurs in some bacteria and in animal muscle cells during exercise when oxygen is inadequate.
- Pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
Commonalities in Fermentation:
- The reducing agent is $NADH + H^+$ , which is reoxidised to $NAD^+$ in both processes.
- Energy Release: Less than $7\,\%$ of the energy in glucose is released, and not all of it is trapped in ATP.
- Hazards: The products (acid or alcohol) are hazardous. Yeast cells poison themselves to death when alcohol concentration reaches approximately $13\,\%$ .

Aerobic Respiration

Location: Occurs within the mitochondria of eukaryotes.
Process Overview: Complete oxidation of organic substances in the presence of oxygen, releasing $CO_2$ , water, and a large amount of energy.
Key Stages:
1. Oxidative Decarboxylation (Link Reaction): Pyruvate is transported from the cytoplasm to the mitochondrial matrix. It undergoes reaction with Coenzyme A (CoA) and $NAD^+$ catalyzed by pyruvic dehydrogenase and $Mg^{2+}$ .
  - $\text{Pyruvic acid} + \text{CoA} + NAD^+ \xrightarrow[\text{Pyruvate dehydrogenase}]{Mg^{2+}} \text{Acetyl CoA} + CO_2 + NADH + H^+$
  - This produces 2 molecules of NADH for every 1 molecule of glucose.
2. Tricarboxylic Acid Cycle (TCA / Krebs’ Cycle):
  - Elucidated by Hans Krebs.
  - Initiation: Acetyl CoA condensation with Oxaloacetic acid (OAA) and water to form Citric acid (6C), catalyzed by citrate synthase.
  - Steps: Citrate $\rightarrow$ Isocitrate $\rightarrow$ $\alpha$ -ketoglutaric acid (5C) [Yields $CO_2$ + NADH] $\rightarrow$ Succinyl-CoA (4C) [Yields $CO_2$ + NADH] $\rightarrow$ Succinic acid [Yields GTP/ATP] $\rightarrow$ Malic acid [Yields $FADH_2$ ] $\rightarrow$ Oxaloacetic acid [Yields NADH].
  - Substrate Level Phosphorylation: Conversion of Succinyl-CoA to Succinic acid produces GTP, which is coupled to the synthesis of ATP from ADP.
  - Cycle Total (per turn): 3 $NADH + H^+$ , 1 $FADH_2$ , and 1 ATP/GTP.
  - Glucose Total (2 turns): 8 $NADH + H^+$ (including Link Reaction), 2 $FADH_2$ , and 2 ATP.

Electron Transport System (ETS) and Oxidative Phosphorylation

Purpose: To release and utilize energy stored in $NADH + H^+$ and $FADH_2$ by passing electrons to $O_2$ to form $H_2O$ .
Location: Inner mitochondrial membrane.
Complexes of the ETS:
- Complex I: NADH dehydrogenase. Accepts electrons from NADH produced in the matrix.
- Ubiquinone: A mobile carrier located within the inner membrane; receives electrons from Complex I and Complex II.
- Complex II: Receives reducing equivalents via $FADH_2$ generated during succinate oxidation.
- Complex III: Cytochrome $bc_1$ complex. Transfers electrons from Reduced ubiquinone (ubiquinol) to Cytochrome c.
- Cytochrome c: A small protein on the outer surface of the inner membrane; acts as a mobile carrier between Complex III and IV.
- Complex IV: Cytochrome c oxidase complex. Contains cytochromes $a$ and $a_3$ , and two copper centres.
- Complex V: ATP synthase.
The Role of Oxygen: Oxygen acts as the final hydrogen acceptor, removing hydrogen from the system to drive the whole process. It is limited to the terminal stage but vital.
ATP Production Yields:
- Oxidation of 1 molecule of NADH yields 3 molecules of ATP.
- Oxidation of 1 molecule of $FADH_2$ yields 2 molecules of ATP.
Mechanism (Chemiosmotic Hypothesis): Energy from oxidation-reduction is used to create a proton gradient.
- ATP Synthase structure:
  - $F_1$ headpiece: Peripheral membrane protein complex; site for ATP synthesis.
  - $F_0$ : Integral membrane protein complex; forms the proton channel.
- Proton Passage: For every 1 ATP produced, $4H^+$ pass through $F_0$ from the intermembrane space to the matrix, down the electrochemical gradient.

Respiratory Balance Sheet

Theoretical Yield: Aerobic respiration of one molecule of glucose can net a gain of 38 ATP molecules.
Necessary Assumptions:
1. Orderly, sequential pathway (Glycolysis $\rightarrow$ TCA $\rightarrow$ ETS).
2. NADH from glycolysis is transferred to mitochondria for oxidative phosphorylation.
3. No intermediates are withdrawn for synthesis of other compounds.
4. Only glucose is resipred.
Reality in Living Systems: Pathways work simultaneously; substrates are withdrawn/added as needed; ATP is utilized immediately; enzyme rates are regulated.

Comparison: Fermentation vs. Aerobic Respiration

Feature	Fermentation	Aerobic Respiration
Breakdown	Partial breakdown of glucose	Complete degradation to $CO_2$ and $H_2O$
ATP Net Gain	2 ATP molecules	Many more (approx. 38) ATP
NADH Oxidation	Slowly oxidized to $NAD^+$	Very vigorous oxidation

Amphibolic Pathway

Traditional View: Respiration is considered a catabolic (breaking down) process.
The Dual Role:
- Catabolism: Carbohydrates, fats, and proteins are broken down into intermediates (like Acetyl CoA or PGAL) to enter the respiratory pathway.
- Anabolism: When the organism needs to synthesize these substances, the same intermediates (e.g., Acetyl CoA for fatty acids) are withdrawn from the respiratory pathway.
Definition: Because it involves both breakdown (catabolism) and synthesis (anabolism), the respiratory pathway is more accurately described as an amphibolic pathway.

Respiratory Quotient (RQ)

Definition: The ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed.
Formula: $RQ = \frac{\text{volume of } CO_2 \text{ evolved}}{\text{volume of } O_2 \text{ consumed}}$
Values based on Substrate:
- Carbohydrates: $RQ = 1.0$
  - $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$
  - $RQ = 6/6$
- Fats (e.g., Tripalmitin): $RQ < 1$ (approx. 0.7 for Tripalmitin)
  - $2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{energy}$
  - $RQ = 102/145 = 0.7$
- Proteins: $RQ \approx 0.9$
Important Note: Pure proteins or fats are never utilized as respiratory substrates in living organisms; they are typically mixed.", "title": "Respiration in Plants: Comprehensive Study Notes"}