Mineral Nutrition

12.1 DO PLANTS BREATHE?

**Definition**: Plant respiration is the process of oxidative breakdown of food molecules within plant cells to release energy and synthesize ATP. Unlike animals, plants lack specialized respiratory organs like lungs.

**Key Features of Plant Respiration**:

Plants require O₂ for respiration and release CO₂

Gas exchange occurs through **stomata** (in leaves) and **lenticels** (in stems)

No transport of gases between plant parts; each cell is largely self-sufficient

Respiration rates in plants are much lower than in animals

Most plant cells are located close to the plant surface, minimizing diffusion distance

Air spaces in parenchyma tissue (particularly in roots, stems, leaves) create interconnected networks for gas diffusion

**Why Plants Don't Need Respiratory Organs**:

1. Each plant part has direct access to air through stomata and lenticels

2. Living cells are organized in thin outer layers (e.g., bark in woody stems)

3. Interior dead cells provide mechanical support only

4. Loose packing of parenchyma cells enables efficient gas diffusion throughout the plant

5. During photosynthesis, O₂ is produced within cells, eliminating local oxygen scarcity

**Energy Release Strategy**:

Complete combustion of glucose: **C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy**

The problem with single-step combustion is that most energy is released as heat, unusable by the cell. The solution is **stepwise oxidation**: glucose is broken down through multiple small enzymatic steps, allowing energy to be captured incrementally in ATP molecules rather than lost as heat.

**Respiratory Substrates**: Usually carbohydrates (glucose), but proteins, fats, and organic acids can be oxidized under certain conditions in plants.

**ATP as Energy Currency**: Energy released during respiration is trapped as ATP (adenosine triphosphate), which serves as the immediate energy currency for all cellular work.

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12.2 GLYCOLYSIS

**Definition**: Glycolysis (Greek: *glycos* = sugar, *lysis* = splitting) is the partial oxidation of glucose to pyruvic acid, occurring in the **cytoplasm** of all living cells. It is the only respiratory pathway that occurs under both aerobic and anaerobic conditions.

**Historical Basis**: Also called the **EMP pathway** (Embden-Meyerhof-Parnas pathway), named after the scientists who first elucidated it.

**Key Characteristics**:

Occurs in cytoplasm (not requiring mitochondria or oxygen)

Present in all living organisms (prokaryotes and eukaryotes)

Produces 2 molecules of **pyruvic acid** from 1 glucose molecule

In plants, glucose is derived from photosynthetic products (sucrose, starch) transported to non-green cells

**Initial Conversions**:

1. **Sucrose → Glucose + Fructose** (enzyme: invertase)

2. **Glucose → Glucose-6-phosphate** (enzyme: hexokinase; requires 1 ATP)

3. **Fructose → Fructose-6-phosphate** (enzyme: hexokinase; requires 1 ATP)

4. Subsequent reactions are identical for both sugars

**The 10 Steps of Glycolysis** (as shown in Figure 12.1):

| Step | Substrate | Product | Enzyme | ATP/NADH Changes |

|------|-----------|---------|--------|------------------|

| 1 | Glucose | Glucose-6-phosphate | Hexokinase | -1 ATP |

| 2 | Glucose-6-phosphate | Fructose-6-phosphate | Isomerase | — |

| 3 | Fructose-6-phosphate | Fructose-1,6-bisphosphate | Phosphofructokinase | -1 ATP |

| 4 | F-1,6-bisphosphate | PGAL + DHAP | Aldolase | — |

| 5 | PGAL ↔ DHAP | Isomerization | Isomerase | — |

| 6 | PGAL | 1,3-bisphosphoglycerate (BPGA) | GAPDH | +1 NADH+H⁺ |

| 7 | BPGA | 3-phosphoglycerate (PGA) | Phosphoglycerate kinase | +1 ATP (×2) |

| 8 | PGA | 2-phosphoglycerate | Mutase | — |

| 9 | 2-phosphoglycerate | Phosphoenolpyruvate (PEP) | Enolase | — |

| 10 | PEP | Pyruvic acid | Pyruvate kinase | +1 ATP (×2) |

**ATP and NADH Accounting**:

**ATP consumed**: 2 molecules (steps 1 and 3)

**ATP produced**: 4 molecules (steps 7 and 10; each step occurs twice because fructose-1,6-bisphosphate splits into 2 triose phosphates)

**Net ATP**: 4 - 2 = **2 ATP per glucose**

**NADH produced**: 2 molecules (step 6 occurs twice)

**Pyruvic Acid as Key Junction**: The fate of pyruvate depends on cellular conditions and energy availability:

1. **Under anaerobic conditions**: Converted to lactic acid or ethanol (fermentation)

2. **Under aerobic conditions**: Enters mitochondria for Krebs' cycle

3. **For biosynthesis**: Used as precursor for amino acids, fatty acids, nucleotides

**Exam Important Points**:

Glycolysis is **glycogenesis in reverse** (catabolic, not synthetic)

Occurs in all organisms; universally ancient pathway

Single glucose yields 2 ATP and 2 NADH+H⁺ directly

Rate-limiting enzyme: **phosphofructokinase (PFK)** (step 3)

Temperature and pH significantly affect enzyme activity

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12.3 FERMENTATION

**Definition**: Fermentation is the **incomplete oxidation of glucose under anaerobic conditions**, producing either lactic acid or ethanol (and CO₂) as end products, with no net oxidation of NADH+H⁺.

**Purpose**: To regenerate NAD⁺ from NADH+H⁺, allowing glycolysis to continue and produce minimal ATP even without oxygen.

**Two Main Types**:

**Alcoholic Fermentation** (in yeast, some bacteria)

**Pathway**: Glucose → Glycolysis → Pyruvic acid → **Ethanol + CO₂**

**Enzymatic Steps**:

1. **Pyruvic acid → Acetaldehyde + CO₂** (enzyme: pyruvate decarboxylase)

2. **Acetaldehyde → Ethanol** (enzyme: alcohol dehydrogenase)

Reducing agent: NADH+H⁺ is oxidized to NAD⁺

**Uses**: Beer, wine, sake, and other alcoholic beverages production

**Problem**: Yeast self-poisons at ~13% alcohol concentration (toxic threshold)

**Implication for Beverages**:

Naturally fermented drinks contain max ~13% alcohol

Higher alcohol content requires distillation (physical process, not fermentation)

Example: Wine (12-13%) vs. Spirits like vodka (40%), achieved by distillation

**Lactic Acid Fermentation** (in muscle cells, lactobacilli, some bacteria)

**Pathway**: Glucose → Glycolysis → Pyruvic acid → **Lactic acid**

**Enzymatic Step**:

1. **Pyruvic acid → Lactic acid** (enzyme: lactate dehydrogenase/LDH)

Reducing agent: NADH+H⁺ is oxidized to NAD⁺

**Occurs in**: Muscle cells during intense exercise (oxygen debt), yogurt production, kimchi fermentation

**Consequence**: Lactic acid accumulates → muscle fatigue, soreness (DOMS)

**Energy Efficiency of Fermentation**

| Aspect | Glycolysis Only | Fermentation |

|--------|-----------------|--------------|

| Net ATP produced | 2 ATP per glucose | 2 ATP per glucose |

| Energy yield | ~5% of total glucose energy | ~7% of total glucose energy |

| NADH regeneration | No | Yes (NAD⁺ regenerated) |

| End products | Pyruvic acid | Ethanol/Lactic acid |

| Hazardous byproducts | None | Acid or alcohol |

**Key Points**:

Both fermentation types yield **only 2 net ATP** (same as glycolysis alone)

**Energy released is minimal** (~7% compared to complete oxidation)

Fermentation is essential for **obligate anaerobes** (organisms that cannot tolerate oxygen)

**Facultative anaerobes** (like yeast, muscle cells) can switch between fermentation and aerobic respiration

**Real-Life Application**:

Human muscle during sprinting: Anaerobic metabolism → lactic acid buildup → fatigue

Recovery phase: Oxygen supply restores, lactic acid is oxidized back to pyruvate and enters Krebs' cycle

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12.4 AEROBIC RESPIRATION

**Definition**: Aerobic respiration is the **complete oxidation of organic substrates in the presence of oxygen**, releasing CO₂, water, and large amounts of energy trapped as ATP molecules. It is the most efficient energy-yielding pathway.

**Fundamental Requirement**: Continuous supply of O₂ to accept hydrogen atoms removed during substrate oxidation.

**Locations in Cell**:

1. **Cytoplasm**: Glycolysis

2. **Mitochondrial matrix**: Pyruvate oxidation and Krebs' cycle

3. **Inner mitochondrial membrane**: Electron Transport System (ETS) and oxidative phosphorylation

**Key Processes of Aerobic Respiration**

**Process 1: Complete oxidation of pyruvate** by stepwise removal of hydrogen atoms

All 3 carbons of pyruvate are released as CO₂

Occurs in mitochondrial matrix

**Process 2: Electron transfer to O₂** with simultaneous ATP synthesis

Occurs on inner mitochondrial membrane

O₂ is final hydrogen/electron acceptor

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12.4.1 TRICARBOXYLIC ACID CYCLE (TCA CYCLE / KREBS' CYCLE)

**Definition**: The TCA cycle (also called **citric acid cycle** or **Krebs' cycle** after Hans Krebs) is a cyclic pathway in which **acetyl-CoA is completely oxidized**, releasing CO₂ and capturing reducing power as NADH and FADH₂.

**Location**: **Mitochondrial matrix**

**Entry Point**: Acetyl-CoA (2-carbon unit) from pyruvate decarboxylation

**Pyruvate Dehydrogenase Complex Reaction** (Bridge to Krebs' Cycle)

**Reactants**: Pyruvic acid + NAD⁺ + CoA + Mg²⁺

**Products**: Acetyl-CoA + CO₂ + NADH + H⁺

**Key Points**:

One glucose molecule produces 2 pyruvate molecules during glycolysis

Thus, 2 Acetyl-CoA molecules enter Krebs' cycle per glucose

2 NADH produced from this step alone

Requires multiple coenzymes: NAD⁺, Coenzyme A, FAD

**Steps of the Krebs' Cycle** (Figure 12.3 Reference)

| Step | Substrate (# Carbons) | Product (# Carbons) | Enzyme | Cofactor Changes | Notes |

|------|----------------------|-------------------|--------|------------------|-------|

| 1 | Acetyl-CoA (2C) + OAA (4C) | Citric acid (6C) | Citrate synthase | CoA released | Cycle begins |

| 2 | Citric acid (6C) | Isocitric acid (6C) | Aconitase | — | Isomerization |

| 3 | Isocitric acid (6C) | α-ketoglutaric acid (5C) | Isocitrate dehydrogenase | NAD⁺ → NADH+H⁺ | 1st decarboxylation |

| 4 | α-ketoglutaric acid (5C) | Succinyl-CoA (4C) | α-ketoglutarate dehydrogenase | NAD⁺ → NADH+H⁺; CoA | 2nd decarboxylation |

| 5 | Succinyl-CoA (4C) | Succinic acid (4C) | Succinyl-CoA synthetase | GTP formed; GDP → ATP | Substrate-level phosphorylation |

| 6 | Succinic acid (4C) | Fumaric acid (4C) | Succinate dehydrogenase | FAD → FADH₂ | Only oxidation without NAD⁺ |

| 7 | Fumaric acid (4C) | Malic acid (4C) | Fumarase | — | Hydration |

| 8 | Malic acid (4C) | Oxaloacetic acid (4C) | Malate dehydrogenase | NAD⁺ → NADH+H⁺ | OAA regenerated; cycle continues |

**Complete Summary for One Glucose Molecule Through TCA Cycle**

Since 2 Acetyl-CoA enter the cycle (from 1 glucose):

| Molecule | Per Acetyl-CoA | Per Glucose (×2) |

|----------|----------------|------------------|

| CO₂ released | 2 | 4 |

| NADH + H⁺ produced | 3 | 6 |

| FADH₂ produced | 1 | 2 |

| ATP (or GTP) produced | 1 | 2 |

**Total from Krebs' Cycle per glucose**:

4 CO₂ (complete oxidation of glucose carbons)

6 NADH + H⁺

2 FADH₂

2 ATP

**Important**: The role of O₂ is indirect at this stage — O₂ is not directly involved in Krebs' cycle reactions but is essential for the ETS to accept electrons, regenerating NAD⁺ and FAD.

**Control Points in Krebs' Cycle**:

**Isocitrate dehydrogenase**: Inhibited by NADH, ATP (feedback inhibition); activated by ADP, Ca²⁺

**α-ketoglutarate dehydrogenase**: Inhibited by NADH, ATP, Acetyl-CoA

Regulation ensures cycle operates at pace matching cellular energy demand

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12.4.2 ELECTRON TRANSPORT SYSTEM (ETS) AND OXIDATIVE PHOSPHORYLATION

**Definition**: The **Electron Transport System (ETS)** is a series of membrane proteins in the **inner mitochondrial membrane** that transfer electrons from NADH and FADH₂ to oxygen, releasing energy used to synthesize ATP. This process is called **oxidative phosphorylation**.

**Location**: Inner mitochondrial membrane (cristae)

**Overall Purpose**:

Oxidize NADH and FADH₂ to regenerate NAD⁺ and FAD

Establish proton gradient across inner membrane

Drive ATP synthesis via chemiosmosis

**Electron Transport Chain Complexes** (Figure 12.4 Reference)

**Complex I (NADH Dehydrogenase)**:

Accepts electrons from NADH produced in mitochondrial matrix (glycolysis, pyruvate oxidation, Krebs' cycle)

Electrons are transferred to ubiquinone (CoQ), a lipid-soluble molecule in the membrane

Pumps H⁺ ions from matrix to intermembrane space

Produces 3 ATP per NADH oxidized

**Complex II (Succinate Dehydrogenase)**:

Accepts electrons from FADH₂ (from Krebs' cycle, specifically from succinate → fumarate)

Electrons enter the chain at ubiquinone (bypassing Complex I)

Does not pump H⁺ ions directly

Produces 2 ATP per FADH₂ oxidized

**Complex III (Cytochrome bc₁ Complex)**:

Accepts electrons from reduced ubiquinone (ubiquinol)

Transfers electrons to cytochrome c (a small protein on outer surface of inner membrane)

Pumps H⁺ ions from matrix to intermembrane space

Involved in Q-cycle (for Complex II FADH₂ electrons)

**Complex IV (Cytochrome c Oxidase)**:

Accepts electrons from cytochrome c

Contains cytochromes a and a₃, plus two copper centers (Cu^A and Cu^B)

**Final electron acceptor is O₂**: Electrons + O₂ + 2H⁺ → H₂O

Pumps H⁺ ions from matrix to intermembrane space

Produces 1 ATP per complex IV cycle (contributes to gradient)

**Complex V (ATP Synthase)**:

Not part of electron transport but uses the proton gradient

Uses energy from H⁺ flowing back into matrix to phosphorylate ADP → ATP

Described in detail below

**Electron Carriers and Their Roles**

| Carrier | Function | Produced From | Carries Electrons To |

|---------|----------|---------------|----------------------|

| NADH | Reduces NAD⁺; carries 2e⁻ + H⁺ | Glycolysis, Pyruvate oxidation, Krebs' | Complex I |

| FADH₂ | Reduces FAD; carries 2e⁻ + H⁺ | Krebs' cycle (succinate step) | Complex II |

| Ubiquinone (CoQ) | Lipid-soluble carrier in membrane | Reduced by Complexes I & II | Complex III |

| Cytochrome c | Water-soluble protein; carries 1e⁻ | Complex III | Complex IV |

| O₂ | Final electron acceptor | — | Accepts electrons at Complex IV |

**ATP Yield from ETS**

| Substrate | Electrons Enter At | Complexes Involved | H⁺ Pumped | ATP Produced |

|-----------|-------------------|-------------------|-----------|--------------|

| NADH | Complex I | I, III, IV | 10 H⁺ | 3 ATP* |

| FADH₂ | Complex II | II, III, IV | 6 H⁺ | 2 ATP* |

*Recent data suggests: NADH → 2.5 ATP; FADH₂ → 1.5 ATP (but CBSE uses 3 and 2 respectively)

**Complex V: ATP Synthase** (Figure 12.5 Reference)

**Structure**:

1. **F₀ (membrane-embedded)**: Forms proton channel across inner membrane

2. **F₁ (peripheral headpiece)**: Contains catalytic sites for ATP synthesis (3 catalytic sites arranged around rotating axle)

**Mechanism (Chemiosmotic Hypothesis)**:

1. ETS pumps H⁺ from matrix to intermembrane space → proton gradient

2. H⁺ concentration becomes higher in intermembrane space than matrix

3. H⁺ ions flow back into matrix through F₀ channel

4. **This energy drives rotation of the rotor in the F₀ core**

5. Rotation causes conformational changes in F₁ catalytic subunits

6. Conformational changes promote: ADP + Pi → ATP binding and release

7. Energy from proton gradient is captured in ATP phosphodiester bonds

**Stoichiometry**:

Approximately **3-4 H⁺ ions** flow through ATP synthase per 1 ATP synthesized

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12.5 THE RESPIRATORY BALANCE SHEET

**Complete Oxidation of One Glucose Molecule**:

**Calculation of Total ATP**

| Process | Location | NADH | FADH₂ | ATP |

|---------|----------|------|-------|-----|

| **Glycolysis** | Cytoplasm | 2 | — | 2 (net) |

| **Pyruvate → Acetyl-CoA** | Mitochondrial matrix | 2 | — | — |

| **Krebs' Cycle** | Mitochondrial matrix | 6 | 2 | 2 |

| **Total Reducing Equivalents** | — | **10** | **2** | **4** |

**ATP from ETS**

| Source | Number × ATP per Molecule | Total ATP |

|--------|--------------------------|-----------|

| 10 NADH × 3 ATP | 30 ATP | 30 |

| 2 FADH₂ × 2 ATP | 4 ATP | 4 |

| **Total from ETS** | — | **34 ATP** |

**Grand Total ATP from One Glucose**

**Direct ATP (substrate-level phosphorylation)**: 4 ATP (2 from glycolysis + 2 from Krebs')

**Indirect ATP (ETS)**: 34 ATP

**Total: ~38 ATP per glucose** (theoretical maximum; actual ~30-32 ATP due to costs of transport and leakage)

**Complete Respiratory Equation**

**C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (38 ATP)**

Or in terms of energy:

**C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~2880 kJ/mol** (or ~686 kcal/mol)

**Efficiency**: ATP captures ~40% of energy; rest released as heat

**Comparison of Aerobic vs. Anaerobic**

| Aspect | Glycolysis Alone | Fermentation | Aerobic Respiration |

|--------|-----------------|--------------|-------------------|

| ATP per glucose | 2 | 2 | ~38 |

| Oxygen required | No | No | Yes |

| Complete oxidation | No | No | Yes |

| End products | Pyruvate | Ethanol/Lactate | CO₂ + H₂O |

| Efficiency | ~2% | ~7% | ~40% |

| Time to produce energy | Fast | Fast | Slower but massive yield |

**Example Application**: Marathon runner can sustain aerobic respiration (38 ATP per glucose) far longer than sprinter using anaerobic pathways (only 2 ATP), explaining endurance differences.

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12.6 AMPHIBOLIC PATHWAY

**Definition**: An **amphibolic pathway** is a metabolic pathway that functions in **both catabolism (breakdown) and anabolism (synthesis)** of molecules. The **Krebs' cycle** is the classic example.

**Krebs' Cycle as Amphibolic Pathway**:

**Catabolic Functions** (Breaking down glucose)

Oxidation of acetyl-CoA to CO₂

Extraction of energy as NADH, FADH₂, ATP

Releases all 6 carbons of glucose as CO₂

**Anabolic Functions** (Synthesizing new molecules)

Krebs' cycle intermediates serve as **precursors for biosynthesis**:

| Intermediate | Biosynthetic Product | Pathway |

|--------------|---------------------|---------|

| **Acetyl-CoA** | Fatty acids, Cholesterol, Amino acids | Lipogenesis |

| **Citrate** | Fatty acids (via export to cytoplasm) | Fatty acid synthesis |

| **α-ketoglutarate** | Glutamate, Glutamine, Proline, Arginine | Amino acid synthesis |

| **Succinyl-CoA** | Heme (porphyrin ring), Nucleotides | Porphyrin & nucleotide synthesis |

| **Oxaloacetate (OAA)** | Aspartate, Methionine, Lysine, Threonine | Amino acid synthesis |

| **Malate** | Pyruvate (anaplerotic), Oxaloacetate | Gluconeogenesis |

**Regulation of Amphibolic Function**

**When energy is abundant** (high ATP/ADP ratio, high NADH/NAD⁺):

Krebs' cycle slows down

Acetyl-CoA diverts to fatty acid synthesis

Cell builds reserves (anabolism dominates)

**When energy is scarce** (low ATP, high ADP):

Krebs' cycle accelerates

Molecules are fully oxidized for maximum ATP

Anabolic reactions slow down

Catabolism dominates

**Anaplerotic Reactions**:

Restore OAA when intermediates are removed for biosynthesis

Example: Pyruvate carboxylase reaction: **Pyruvate + CO₂ → OAA** (requires ATP)

Ensures Krebs' cycle can continue even when intermediates are withdrawn

**Practical Implication**:

Protein diets increase amino acid catabolism (more OAA, α-ketoglutarate needed)

Fat diets increase Acetyl-CoA production

Carb-rich diets support both pathways efficiently

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12.7 RESPIRATORY QUOTIENT (RQ)

**Definition**: The **Respiratory Quotient (RQ)** is the **ratio of CO₂ released to O₂ consumed** during respiration:

**RQ = CO₂ Released / O₂ Consumed**

**Significance**: RQ indicates which type of organic substrate is being oxidized.

**RQ Values for Different Substrates**

| Substrate | RQ Value | Equation | Example |

|-----------|----------|----------|---------|

| **Carbohydrates** | **1.0** | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O | Glucose RQ = 6CO₂/6O₂ = 1.0 |

| **Fats/Lipids** | **0.7** | 2C₅₁H₁₀₂O₆ + 145O₂ → 102CO₂ + 98H₂O | Palmitic acid RQ = 0.7 |

| **Proteins** | **0.8-0.9** | C₁H₁.₅N₀.₃O₀.₅ + 1.4O₂ → 1CO₂ + N products + H₂O | Protein RQ ≈ 0.9 |

| **Organic Acids** | **>1.0** | Malic acid RQ = 1.4 | Succulents during acid metabolism |

**Explanation of Different RQ Values**

**RQ = 1.0 (Carbohydrates)**:

General formula: Cₙ(H₂O)ₘ — already contains oxygen in correct ratio

Complete oxidation requires just enough O₂ to produce CO₂ and H₂O

Example: Glucose C₆H₁₂O₆ has O already; no excess hydrogen

**RQ = 0.7 (Fats)**:

Fats are highly reduced (more H, C; less O)

Require excess O₂ to oxidize the abundant hydrogen atoms

Produce fewer CO₂ relative to O₂ consumed

Formula: CₓHᵧOz where y >> z (e.g., C₁₆H₃₂O₂ for palmitic acid)

Example: 2C₁₆H₃₂O₂ (palmitic acid) + 145O₂ → 102CO₂ + 98H₂O

RQ = 102/145 ≈ 0.7

**RQ = 0.8-0.9 (Proteins)**:

Intermediate oxidation state

Contain nitrogen (removed as urea, not counted in CO₂/O₂)

The nitrogenous part is not oxidized to CO₂, reducing the RQ

Variable based on amino acid composition

**RQ > 1.0 (Organic Acids)**:

Acids are highly oxidized already

Require less O₂ for complete oxidation

Produce more CO₂ relative to O₂

Common in **succulent plants** (Crassulacean Acid Metabolism - CAM) during dark hours

Example: Malic acid C₄H₆O₅ has RQ ≈ 1.4

**Determining Substrate from RQ**

**Practical Application** (in exams and research):

Measure O₂ consumed and CO₂ produced

Calculate RQ ratio

Identify which substrate the organism is metabolizing

**Example**:

If a seed initially uses stored fats, RQ ≈ 0.7

As it grows and develops photosynthetic tissues, shifts to carbohydrate use → RQ → 1.0

If stressed and catabolizing proteins, RQ ≈ 0.9

**Multi-Substrate Scenario**:

If an organism simultaneously respires carbs (RQ 1.0) and fats (RQ 0.7) in equal proportion:

**Overall RQ = (1.0 + 0.7) / 2 = 0.85**

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SUMMARY TABLE: RESPIRATION OVERVIEW

| Aspect | Glycolysis | Fermentation | Aerobic Respiration |

|--------|-----------|--------------|-------------------|

| **Location** | Cytoplasm | Cytoplasm | Mitochondria (matrix + membrane) |

| **O₂ requirement** | No | No | Yes |

| **ATP per glucose** | 2 (net) | 2 (net) | ~38 |

| **NADH regeneration** | Via fermentation | Yes (reoxidized) | Via ETS |

| **Key products** | Pyruvate | Ethanol/Lactate | CO₂, H₂O |

| **Percentage energy recovered** | ~2% | ~7% | ~40% |

| **Rate of ATP production** | Fast | Fast | Slower |

| **Common in** | All organisms | Anaerobes; muscle during exercise | Aerobes at rest/moderate activity |

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KEY DEFINITIONS FOR BOARD EXAMS

1. **Respiration**: Breaking of C-C bonds of complex compounds through oxidation within cells to release energy trapped as ATP.

2. **Cellular Respiration**: The mechanism of breakdown of food materials within the cell to release energy and synthesis of ATP.

3. **Respiratory Substrate**: The compound being oxidized during respiration (usually carbohydrates, but can be proteins, fats, organic acids).

4. **ATP**: Adenosine triphosphate; the energy currency of the cell; energy from respiration is trapped in ATP and released when ATP → ADP + Pi.

5. **Glycolysis**: Partial oxidation of glucose (or other hexoses) to pyruvic acid in the cytoplasm, yielding 2 ATP and 2 NADH per glucose.

6. **Fermentation**: Anaerobic partial oxidation of glucose producing ethanol or lactic acid; regenerates NAD⁺ without O₂.

7. **Krebs' Cycle (TCA Cycle)**: Cyclic oxidation of acetyl-CoA in mitochondrial matrix, producing CO₂, NADH, FADH₂, and ATP.

8. **Electron Transport System**: Series of protein complexes in inner mitochondrial membrane transferring electrons from NADH/FADH₂ to O₂, establishing proton gradient.

9. **Oxidative Phosphorylation**: ATP synthesis powered by energy released in electron transport and the proton gradient across inner mitochondrial membrane.

10. **Amphibolic Pathway**: A pathway functioning in both catabolism and anabolism (Krebs' cycle serves this dual function).

11. **Respiratory Quotient (RQ)**: The ratio CO₂ released / O₂ consumed; indicates substrate type being respired.

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EXAM QUESTION TYPES & STRATEGIES

**Short Answer (2-3 marks)**:

Define respiration vs. photosynthesis

Why do plants need to respire despite photosynthesis?

How is ATP the energy currency?

Calculate ATP from glycolysis step

List the 3 main fates of pyruvate

What is RQ and its value for carbohydrates?

**Long Answer (5 marks)**:

Describe glycolysis with diagram and ATP/NADH calculations

Explain