Complete Notes
Cellular Respiration is the process which involves oxidation of food (glucose) to release energy, with the breaking of carbon-carbon (C-C) bonds of complex compounds through a series of enzyme-catalysed reactions, resulting in the release of CO₂, water, and energy.
Energy carriers like NADH + H⁺ and FADH₂ are formed during oxidation. These carry electrons to the Electron Transport System (ETS), where they are used to generate ATP.
The molecule which is oxidised during respiration is called the Respiratory Substrate. On the basis of the substrate used, respiration is of types:
Green plant parts convert light energy into chemical energy through photosynthesis. Organisms that make their own food are called Producers, e.g. green plants, BGA (Blue-Green Algae). Other organisms (e.g. Goat, Tiger — Herbivores/Carnivores) depend directly or indirectly on plants for food — these are called Consumers.
Plants need O₂ to oxidise food and make ATP. However, plants do not have a specialised respiratory system like animals — gas exchange occurs by simple diffusion through stomata in leaves, and through the general surface of stem and root.
The early atmosphere on earth lacked free oxygen. Early organisms were anaerobic (living without oxygen). Anaerobes are of two types:
| Type | Description | Example |
|---|---|---|
| Facultative Anaerobes | Can live with or without free oxygen. | — |
| Obligate Anaerobes | Cannot survive in the presence of oxygen. | e.g. Clostridium, Mycoplasma |
Early organisms developed a process called Glycolysis to generate ATP without O₂ — it is considered a universal process, since it is the first step common to both aerobic and anaerobic respiration.
Glycolysis means "breakdown of sugar" (Glycos = sugar, Lysis = breakdown). It is an incomplete oxidation pathway, occurring in the cytoplasm, consisting of 10 steps, also called the EMP pathway (named after Embden, Meyerhoff, and Parnas). It can occur without O₂ (anaerobic) and is also found in eukaryotes.
One molecule of Glucose (6C) [or Sucrose, broken into Glucose + Fructose, both 6C] is converted into 2 molecules of Pyruvate (3C) through 10 sequential steps.
| Step | Conversion | Enzyme/Note | ATP/NADH Change |
|---|---|---|---|
| Step 1 | Glucose → Glucose-6-Phosphate | Hexokinase | −1 ATP (ATP used) |
| Step 2 | Glucose-6-Phosphate → Fructose-6-Phosphate | — | — |
| Step 3 | Fructose-6-Phosphate → Fructose-1,6-bisphosphate | PFK-1 (Phosphofructokinase-1) — Pacemaker enzyme | −1 ATP (ATP used) |
| (Split) | Fructose-1,6-bisphosphate (6C) splits into 2 molecules of 3C — Glyceraldehyde-3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP); DHAP converts to G3P | — | — |
| Step (oxidation) | G3P → 1,3-bisphosphoglycerate (BPG) | Oxidation; +2 NAD⁺ → 2 NADH+H⁺ | +2 NADH+H⁺ (for both 3C molecules) |
| (Phosphorylation) | 1,3-BPG → 3-phosphoglycerate | ADP + Pi → ATP (substrate level phosphorylation) | +2 ATP |
| Step 10 | Phosphoenolpyruvate (PEP) → Pyruvate | Pyruvate kinase; H₂O removed | +2 ATP |
In the absence of O₂, pyruvate produced in glycolysis is further processed by fermentation, regenerating NAD⁺ from NADH+H⁺ (needed to keep glycolysis running).
Aerobic respiration occurs in aerobic conditions, involving complete combustion of glucose:
This reaction connects glycolysis and the Krebs cycle — it occurs in the mitochondrial matrix. Pyruvate (3C) is converted into Acetyl CoA (2C), with release of CO₂.
1 Glucose → 2 Pyruvate → 2 Acetyl CoA + 2 CO₂ + 2 NADH+H⁺.
The Krebs cycle is also called the Citric Acid Cycle or Tricarboxylic Acid (TCA) Cycle, because the first product of this cycle is Citric acid (Citrate), which has 3 carboxylic acid (-COOH) groups.
| Step | Conversion | Enzyme / Product |
|---|---|---|
| 1 | Oxaloacetic Acid (OAA, 4C) + Acetyl CoA (2C) → Citrate (6C) | Citrate Synthase |
| 2 | Citrate → cis-Aconitate → Isocitrate (6C) | — |
| 3 | Isocitrate (6C) → Alpha-Ketoglutarate (5C) + CO₂ | Oxidation — produces NADH+H⁺ |
| 4 | Alpha-Ketoglutarate (5C) → Succinyl CoA (4C) + CO₂ | Oxidative decarboxylation — produces NADH+H⁺ |
| 5 | Succinyl CoA → Succinate (4C) | Substrate level phosphorylation — produces GTP/ATP |
| 6 | Succinate → Fumarate (4C) | Produces FADH₂ |
| 7 | Fumarate → Malate (4C) | + H₂O |
| 8 | Malate → Oxaloacetic Acid (OAA, 4C) — regenerated | Produces NADH+H⁺ |
ETS is located on the Inner Mitochondrial Membrane (IMM). NADH+H⁺ and FADH₂ produced during glycolysis, link reaction, and Krebs cycle donate electrons to the ETS, which are passed through a series of carriers/complexes to finally combine with O₂, forming water.
| Complex | Name / Components |
|---|---|
| Complex I | NADH Dehydrogenase — contains FMN (Flavin Mononucleotide) and Fe-S (Iron-Sulphur) centers |
| Complex II | Succinate Dehydrogenase — contains FAD (Flavin Adenine Dinucleotide) and Fe-S centers |
| Complex III | Cytochrome bc1 Complex — contains Cytochrome b, Cytochrome c1, and Fe-S centers |
| Complex IV | Cytochrome c oxidase — contains Cytochrome a, a3, and Cu centers |
| Complex V | ATP Synthase — has F0 and F1 subunits |
In the electron carriers: Cu centers are stable in +1 and +2 oxidation states; Fe centers (in Fe-S clusters and cytochromes) are stable in +2 and +3 oxidation states — these changes in oxidation state allow them to accept and donate electrons.
| Electron Donor | ATP Produced (via ETS) |
|---|---|
| 1 NADH+H⁺ | 3 ATP |
| 1 FADH₂ | 2 ATP |
| Stage | NADH+H⁺ | FADH₂ | ATP (Direct/Substrate level) |
|---|---|---|---|
| Glycolysis | 2 | 0 | 2 ATP (net) |
| Link Reaction (Oxidative decarboxylation, ×2) | 2 | 0 | 0 |
| Krebs Cycle (×2 turns) | 6 | 2 | 2 ATP (GTP/ATP via substrate level) |
| Total | 10 | 2 | 4 |
Respiration is not solely a catabolic process — it is called an Amphibolic Pathway, because the respiratory pathway is involved in both Catabolism (breakdown) and Anabolism (synthesis). Intermediates of the respiratory pathway (glycolysis and Krebs cycle) are utilised to synthesise other compounds.
| Respiratory Substrate | RQ Value | Reason |
|---|---|---|
| Carbohydrates | RQ = 1 | Equal volumes of CO₂ produced and O₂ consumed (e.g. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) |
| Fats | RQ < 1 (e.g. ~0.7) | More O₂ is required relative to CO₂ produced, since fatty acids are more reduced (more H, less O) than carbohydrates |
| Proteins / Organic acids | RQ > 1 (e.g. ~1.33, for some organic acids) | Organic acids are more oxidised — less O₂ is required relative to CO₂ released |
Q1. Which enzyme is considered the "pacemaker" of glycolysis, controlling its rate-limiting step?
(a) Hexokinase (b) Phosphofructokinase-1 (PFK-1) (c) Pyruvate kinase (d) Lactate dehydrogenase
Answer: (b) Phosphofructokinase-1 (PFK-1)
Explanation: PFK-1 catalyses step 3 of glycolysis, converting Fructose-6-phosphate to Fructose-1,6-bisphosphate, using 1 ATP. This step is irreversible and tightly regulated, making PFK-1 the "pacemaker" enzyme that controls the overall rate of glycolysis. Steps 1, 3, and 10 of glycolysis are the only irreversible steps, while all other steps are reversible.
Q2. What is the net ATP gain from glycolysis (per glucose molecule)?
(a) 4 ATP (b) 2 ATP (c) 6 ATP (d) 8 ATP
Answer: (b) 2 ATP
Explanation: During glycolysis, 2 ATP are consumed (in steps 1 and 3, by hexokinase and PFK-1 respectively), while 4 ATP are produced via substrate level phosphorylation (2 ATP for each of the two 3-carbon molecules produced from the splitting of fructose-1,6-bisphosphate). The net ATP gain is therefore 4 − 2 = 2 ATP, along with 2 NADH+H⁺ and 2 pyruvate molecules.
Q3. During alcoholic fermentation in yeast, what happens to the NADH+H⁺ produced during glycolysis?
(a) It is sent to the ETS to produce ATP
(b) It is used to convert acetaldehyde to ethanol, regenerating NAD⁺
(c) It accumulates and is not reused
(d) It is converted to FADH₂
Answer: (b) It is used to convert acetaldehyde to ethanol, regenerating NAD⁺
Explanation: In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde (releasing CO₂), and then acetaldehyde is reduced to ethanol by alcohol dehydrogenase. This reduction step uses the NADH+H⁺ produced during glycolysis, converting it back to NAD⁺. This regeneration of NAD⁺ is essential because glycolysis cannot continue without a fresh supply of NAD⁺. Importantly, this NADH+H⁺ is NOT sent to the ETS in fermentation, so no additional ATP is generated from it.
Q4. The Krebs cycle is also called the Citric Acid Cycle because:
(a) It occurs only in citrus fruits
(b) The first stable product of the cycle is citrate, which has three carboxylic acid groups
(c) It produces citric acid as a final waste product
(d) Citrate is the substrate that enters the cycle from glycolysis
Answer: (b) The first stable product of the cycle is citrate, which has three carboxylic acid groups
Explanation: The Krebs cycle begins when Acetyl CoA (2C) combines with Oxaloacetic Acid (OAA, 4C) via the enzyme citrate synthase, forming Citrate (6C) — a tricarboxylic acid (having three -COOH groups). This is why the cycle is also called the Citric Acid Cycle or Tricarboxylic Acid (TCA) Cycle. Through a series of steps, citrate is eventually converted back to OAA, regenerating it for the next turn of the cycle.
Q5. In the Electron Transport System (ETS), at which point is molecular O₂ directly utilised?
(a) During the link reaction (oxidative decarboxylation of pyruvate)
(b) During the Krebs cycle, when succinate is converted to fumarate
(c) As the terminal electron acceptor at the end of the ETS, forming water
(d) During glycolysis, when glucose is phosphorylated
Answer: (c) As the terminal electron acceptor at the end of the ETS, forming water
Explanation: A common misconception is that O₂ is used directly in the link reaction or Krebs cycle — but it is not. O₂ is used only at the very end of the Electron Transport System, as the final/terminal electron acceptor. Electrons that have passed through Complexes I to IV combine with H⁺ ions and O₂ to form water (4H⁺ + O₂ + 4e⁻ → 2H₂O), via Complex IV (cytochrome c oxidase). This step is essential to keep the ETS running, since it regenerates the electron carriers (NAD⁺, FAD) for further oxidation steps.
Q6. A respiratory substrate gives a Respiratory Quotient (RQ) of less than 1. What can be inferred about the substrate?
(a) It is a carbohydrate, since RQ = 1 for carbohydrates
(b) It is likely a fat, since fats require more O₂ relative to the CO₂ produced
(c) It is an organic acid, since organic acids are highly oxidised
(d) RQ less than 1 indicates anaerobic respiration is occurring
Answer: (b) It is likely a fat, since fats require more O₂ relative to the CO₂ produced
Explanation: Respiratory Quotient (RQ) = Volume of CO₂ evolved / Volume of O₂ consumed. For carbohydrates, RQ = 1, since equal volumes of CO₂ are released and O₂ consumed. For fats, RQ is less than 1 (around 0.7), because fatty acid molecules are more reduced (containing more hydrogen relative to oxygen) than carbohydrates, and therefore require relatively more O₂ to be fully oxidised compared to the CO₂ they release. For proteins/organic acids, RQ is often greater than 1, since these substrates are more oxidised to begin with and require relatively less O₂.
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