← Biology — Std 11
🧬

Respiration and Energy Transfer

Ch. 13Std 11

Easy Overview

You eat food every day. But how does that food actually become energy your body can use? That is respiration — not just breathing (which is ventilation), but the cellular process that converts glucose into ATP, the energy currency of life. This chapter follows the journey of a glucose molecule from the moment it enters a cell until it has been completely broken down, releasing all its stored energy. Cellular respiration is the process by which cells break down organic molecules (usually glucose) to release energy in the form of ATP. The overall equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (about 30-32 ATP). Notice that this is essentially the reverse of photosynthesis. Plants do both — they photosynthesize to make glucose, then respire to extract the energy from it. Respiration can be aerobic (with oxygen) or anaerobic (without oxygen). Aerobic respiration is the main pathway and produces much more ATP. It has four main stages: glycolysis, the link reaction, the Krebs cycle, and the electron transport chain with oxidative phosphorylation. Glycolysis happens in the cytoplasm and splits one glucose (6C) into two pyruvate (3C). It does not require oxygen and produces a net gain of 2 ATP and 2 NADH. The link reaction and Krebs cycle occur in the mitochondrial matrix. The link reaction converts each pyruvate to acetyl-CoA, producing NADH and releasing CO₂. The Krebs cycle completes the oxidation of the carbon atoms, producing ATP, NADH, and FADH₂, and releasing more CO₂. The electron transport chain on the inner mitochondrial membrane uses the NADH and FADH₂ to pump protons and generate a massive amount of ATP — about 28 ATP per glucose. When oxygen is not available, cells can still produce ATP through fermentation. In lactic acid fermentation (human muscles during intense exercise), pyruvate is converted to lactate, regenerating NAD⁺ so glycolysis can continue. This produces only 2 ATP per glucose — much less efficient, but it does not need oxygen. In alcoholic fermentation (yeast and some bacteria), pyruvate is converted to ethanol and CO₂. This is used in making bread, beer, and wine. The efficiency of aerobic respiration is about 40% — the rest is released as heat (which is why you get warm when you exercise). Understanding cellular respiration is crucial for medicine (metabolic diseases like diabetes), agriculture (how crops use energy), and biotechnology (fermentation processes). Every time you take a breath, you are supplying oxygen for this incredible process that powers every cell in your body.

Types of Respiration

Cellular respiration can be aerobic or anaerobic. Aerobic respiration requires oxygen and produces CO₂, H₂O, and a large amount of ATP (about 30-32 per glucose). It occurs in the cytoplasm (glycolysis) and mitochondria (Krebs cycle and ETC). Anaerobic respiration (fermentation) does not require oxygen and produces only 2 ATP per glucose. It occurs entirely in the cytoplasm. Aerobic respiration is the main pathway in most cells under normal conditions. Anaerobic respiration provides a quick but limited energy supply when oxygen is scarce — for example, during intense exercise when your muscles use oxygen faster than it can be delivered. Organisms differ in their oxygen requirements: obligate aerobes need oxygen to live (humans, most animals, plants). Obligate anaerobes cannot tolerate oxygen and are killed by it (some bacteria like Clostridium botulinum — cause botulism). Facultative anaerobes can switch between aerobic and anaerobic respiration depending on oxygen availability (yeast, E. coli, and human muscle cells). The switch between aerobic and anaerobic pathways is tightly regulated — cells prefer aerobic respiration because it produces 15-19 times more ATP per glucose molecule.

Glycolysis — The Universal First Step

Glycolysis (from Greek 'glykys' = sweet, 'lysis' = splitting) is the first stage of cellular respiration, occurring in the cytoplasm of all living cells — from bacteria to humans. It was discovered by Gustav Embden, Otto Meyerhof, and Jacob Parnas (also called the EMP pathway). Glycolysis converts one molecule of glucose (6C) into two molecules of pyruvate (3C). The process has ten steps, divided into two phases. Energy investment phase (steps 1-5): 2 ATP molecules are consumed to phosphorylate glucose (to glucose-6-phosphate and then to fructose-1,6-bisphosphate). This 'investment' of ATP is necessary to destabilize the glucose molecule and prepare it for splitting. The 6-carbon sugar is then split into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) — DHAP is quickly converted to G3P, so both molecules continue through the pathway. Energy payoff phase (steps 6-10): each of the two G3P molecules is oxidized and converted to pyruvate. In this phase, 4 ATP molecules are produced (by substrate-level phosphorylation — direct transfer of phosphate from a high-energy intermediate to ADP) and 2 NAD⁺ are reduced to 2 NADH (per G3P, so 4 ATP and 2 NADH total). Net gain from one glucose: 2 ATP (4 produced minus 2 consumed), 2 NADH, and 2 pyruvate. Key regulatory enzyme: phosphofructokinase-1 (PFK-1), which is inhibited by ATP and citrate (high energy) and activated by AMP and fructose-2,6-bisphosphate (low energy). Glycolysis provides both energy (ATP) and carbon skeletons for other metabolic pathways and does not require oxygen — it is anaerobic.

The Link Reaction — Pyruvate to Acetyl-CoA

After glycolysis, pyruvate (which is produced in the cytoplasm) enters the mitochondrial matrix. It cannot simply cross the mitochondrial membranes — it is transported through specific carrier proteins in the inner mitochondrial membrane. Once in the matrix, pyruvate undergoes oxidative decarboxylation catalyzed by the pyruvate dehydrogenase complex — a large, multi-enzyme complex (about 60 protein subunits, larger than a ribosome). The reaction: pyruvate (3C) + CoA (coenzyme A) + NAD⁺ → acetyl-CoA (2C) + NADH + CO₂. This is called the link reaction because it connects glycolysis to the Krebs cycle. The three steps: (1) decarboxylation — CO₂ is removed from pyruvate (this is the first CO₂ released in cellular respiration — it comes from the carboxyl group of pyruvate). (2) Oxidation — the remaining 2-carbon fragment (hydroxyethyl) is oxidized, and NAD⁺ is reduced to NADH. (3) Transfer to CoA — the oxidized 2-carbon fragment (acetyl group) is attached to coenzyme A, forming acetyl-CoA. For each glucose molecule (which gave 2 pyruvate), the link reaction produces 2 acetyl-CoA, 2 NADH, and 2 CO₂. The NADH enters the electron transport chain. The acetyl-CoA enters the Krebs cycle. The pyruvate dehydrogenase complex is regulated: it is inhibited by its products (NADH and acetyl-CoA) and activated by Ca²⁺ (in muscle cells) and insulin. This ensures that pyruvate is only converted to acetyl-CoA when the Krebs cycle needs it and when energy is required.

Krebs Cycle — The Central Hub

The Krebs cycle (also called the citric acid cycle or tricarboxylic acid (TCA) cycle) takes place in the mitochondrial matrix. It was discovered by Hans Krebs in 1937 (Nobel Prize 1953). The cycle begins when acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Citrate then goes through a series of 8 reactions that gradually oxidize it back to oxaloacetate. For each turn of the cycle: (1) Two CO₂ molecules are released (the original carbon atoms from acetyl-CoA are fully oxidized — these CO₂ are the waste product you exhale). (2) One ATP (or GTP in some organisms) is produced by substrate-level phosphorylation. (3) Three NAD⁺ are reduced to 3 NADH (at isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase steps). (4) One FAD is reduced to 1 FADH₂ (at succinate dehydrogenase step — this enzyme is also part of Complex II of the electron transport chain). Since one glucose produces 2 acetyl-CoA (from 2 pyruvate), the Krebs cycle runs twice per glucose, producing: 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂. Key enzymes: citrate synthase (catalyzes the first condensation step), isocitrate dehydrogenase (catalyzes the first decarboxylation — a key regulatory point), α-ketoglutarate dehydrogenase (structurally similar to pyruvate dehydrogenase — catalyzes the second decarboxylation), succinate dehydrogenase (produces FADH₂, also part of Complex II of ETC), and malate dehydrogenase (last step, regenerates oxaloacetate). The cycle is regulated by the cell's energy status: ATP and NADH inhibit isocitrate dehydrogenase and α-ketoglutarate dehydrogenase; ADP and Ca²⁺ activate them.

Electron Transport Chain — The Power Station

The electron transport chain (ETC) is located on the inner mitochondrial membrane. It consists of four protein complexes (Complex I, II, III, IV) and two mobile electron carriers (ubiquinone/CoQ and cytochrome c). The ETC is where most of the ATP from cellular respiration is produced. NADH and FADH₂ donate the electrons they accepted during glycolysis, the link reaction, and the Krebs cycle. NADH donates electrons to Complex I (NADH dehydrogenase). FADH₂ donates electrons to Complex II (succinate dehydrogenase — which is also a Krebs cycle enzyme, so FADH₂ enters the ETC later than NADH and produces fewer ATP). From Complex I/II, electrons pass through a series of carriers: Complex I/II → ubiquinone (CoQ) → Complex III (cytochrome bc₁ complex) → cytochrome c → Complex IV (cytochrome c oxidase). As electrons pass through Complexes I, III, and IV, the energy released is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates a large electrochemical gradient (proton motive force) — high H⁺ concentration in the intermembrane space, low in the matrix. Complex IV is the terminal oxidase — it transfers electrons to molecular oxygen (O₂), the final electron acceptor. The reaction: ½O₂ + 2e⁻ + 2H⁺ → H₂O. This is why you need oxygen — without it, the ETC would back up and stop, and NADH/FADH₂ could not be recycled. Cyanide is deadly because it binds to Complex IV, blocking electron transfer to oxygen and halting the entire ETC.

Oxidative Phosphorylation — ATP Synthesis

Oxidative phosphorylation is the process by which ATP is synthesized using the energy from the proton gradient created by the ETC. The key enzyme is ATP synthase (Complex V), a large, multi-subunit complex embedded in the inner mitochondrial membrane. It has two major parts: F₀ (the proton channel, embedded in the membrane — from 'oligomycin-sensitive,' because the antibiotic oligomycin binds here and blocks ATP synthesis) and F₁ (the catalytic head, projecting into the matrix — contains the active sites that synthesize ATP). The mechanism (binding change mechanism, proposed by Paul Boyer): as protons flow through F₀ down their electrochemical gradient (from the intermembrane space back to the matrix), the energy released causes the central stalk of F₁ to rotate. This rotation causes conformational changes in the three catalytic β subunits of F₁, alternating between three states: open (releases ATP), loose (binds ADP + Pi), and tight (synthesizes ATP). For every 3-4 protons that flow through ATP synthase, one ATP is synthesized. The proton gradient is also used to transport metabolites into the mitochondria (pyruvate, ADP, phosphate) through coupled transporters. The old textbook number of 3 ATP per NADH and 2 ATP per FADH₂ has been revised based on more accurate measurements. Current estimates: about 2.5 ATP per NADH and about 1.5 ATP per FADH₂. From one glucose: glycolysis produces 2 NADH (~5 ATP), the link reaction produces 2 NADH (~5 ATP), the Krebs cycle produces 6 NADH (~15 ATP) and 2 FADH₂ (~3 ATP), plus 2 ATP from glycolysis (substrate-level) and 2 ATP from the Krebs cycle (substrate-level). Total: approximately 30-32 ATP per glucose. The actual yield varies depending on the cell type and the shuttle used to transport NADH from the cytoplasm into the mitochondria.

Fermentation — Anaerobic ATP Production

Fermentation allows ATP production when oxygen is not available. It uses glycolysis (which does not require oxygen) to produce 2 ATP per glucose, and then converts pyruvate to a waste product to regenerate NAD⁺ so glycolysis can continue to run. Without this regeneration, NAD⁺ would be depleted, and glycolysis would stop. Two major types of fermentation are important. Lactic acid fermentation: pyruvate is reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase. This reaction regenerates NAD⁺ from NADH. This occurs in human skeletal muscle cells during intense exercise when oxygen demand exceeds supply — the oxygen debt is paid back later when you rest and the lactate is converted back to pyruvate in the liver (the Cori cycle). Lactic acid fermentation also occurs in Lactobacillus bacteria used to make yogurt, cheese, sauerkraut, and pickles. The acidic environment created by lactic acid preserves the food and gives it the tangy flavor. Lactic acid buildup in muscles causes the burning sensation and fatigue — but recent research suggests that the main cause of muscle fatigue is not lactate itself but the associated acidosis and potassium ion changes. Alcoholic fermentation: pyruvate is first decarboxylated to acetaldehyde (by pyruvate decarboxylase, releasing CO₂), then acetaldehyde is reduced to ethanol (by alcohol dehydrogenase), regenerating NAD⁺. This occurs in yeast (Saccharomyces cerevisiae) and some bacteria. The CO₂ produced is what makes bread rise (the gas bubbles get trapped in the dough) and gives beer and champagne their fizz. The ethanol is what gives alcoholic beverages their intoxicating effect. Alcoholic fermentation produces 2 ATP, 2 CO₂, and 2 ethanol per glucose. Yeast can be switched between aerobic respiration and fermentation depending on oxygen availability (Crabtree effect — yeast ferments even in the presence of oxygen if glucose concentration is high).

Comparison of Aerobic and Anaerobic Respiration

Aerobic respiration and anaerobic respiration (fermentation) differ in several important ways. Oxygen requirement: aerobic requires O₂; anaerobic does not. Location: aerobic occurs in the cytoplasm + mitochondria; anaerobic occurs only in the cytoplasm. Glucose breakdown: aerobic completely oxidizes glucose to CO₂ and H₂O; anaerobic partially oxidizes glucose to lactate or ethanol + CO₂. ATP yield: aerobic produces about 30-32 ATP per glucose; anaerobic produces only 2 ATP per glucose. The ratio is about 15:1 in favor of aerobic. Efficiency: aerobic captures about 40% of the energy in glucose as ATP (the rest is released as heat); anaerobic captures about 2% (very inefficient). Waste products: aerobic produces CO₂ and H₂O (harmless); anaerobic produces lactate (causes muscle fatigue/pain) or ethanol (toxic to cells at high concentrations). Sustainability: aerobic can be sustained for long periods (hours of walking, running, resting); anaerobic can only be sustained for short periods (minutes at most — a 400-meter sprint is mostly anaerobic). NAD⁺ regeneration: in aerobic, NADH donates electrons to the ETC, regenerating NAD⁺; in anaerobic, NADH reduces pyruvate, regenerating NAD⁺. In summary: aerobic is the 'fuel-efficient highway driving'; anaerobic is the 'emergency sprint' — fast but unsustainable and inefficient. Most cells use aerobic respiration as the default and switch to anaerobic only when oxygen is limited.

Regulation of Respiration

Cellular respiration is tightly regulated to match the cell's energy needs. The key regulatory points are irreversible, rate-limiting steps catalyzed by allosteric enzymes. In glycolysis: phosphofructokinase-1 (PFK-1) is the main control point. It is the 'gatekeeper' of glycolysis. PFK-1 is inhibited by ATP and citrate (high energy charge — the cell has enough energy, slow down) and activated by AMP and fructose-2,6-bisphosphate (low energy charge — the cell needs more ATP, speed up). PFK-1 is also activated by ADP and inorganic phosphate (Pi). The ratio of ATP to AMP is the most important signal — when ATP is abundant, it inhibits; when ATP is being used and AMP accumulates, it activates. Hexokinase (the first enzyme of glycolysis) is inhibited by its product glucose-6-phosphate (product inhibition). Pyruvate kinase (the last enzyme of glycolysis) is activated by fructose-1,6-bisphosphate (feedforward activation) and inhibited by ATP and alanine. In the link reaction: pyruvate dehydrogenase is inhibited by its products (NADH and acetyl-CoA) and activated by Ca²⁺ (which signals muscle contraction and energy demand) and insulin. In the Krebs cycle: isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are inhibited by ATP and NADH (high energy) and activated by ADP and Ca²⁺ (low energy). In the electron transport chain: the rate is regulated primarily by substrate availability (NADH, FADH₂, O₂) and the size of the proton gradient. If the gradient is high (high energy charge), proton pumping slows because it requires more energy to pump against the gradient, and electron flow through the ETC slows down. This is called respiratory control or acceptor control. The overall coordination ensures that glucose is only metabolized when ATP is needed — preventing wasteful consumption of fuel.

Respiratory Quotient

The respiratory quotient (RQ) is the ratio of the volume of CO₂ produced to the volume of O₂ consumed during respiration: RQ = CO₂ released / O₂ consumed. RQ values indicate which substrate is being metabolized. For carbohydrates (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O): RQ = 6/6 = 1.0. For fats (e.g., tripalmitin — 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O): RQ = 102/145 = 0.7. Fats have lower RQ because they contain less oxygen relative to carbon and hydrogen — they require more O₂ for complete oxidation. For proteins: RQ is approximately 0.8-0.9, varying with the amino acid composition. For organic acids: RQ > 1 (more CO₂ produced than O₂ consumed). For example, oxalic acid (2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O) gives RQ = 4/1 = 4.0. For anaerobic respiration: RQ is undefined or infinite because no O₂ is consumed but CO₂ may be produced (as in alcoholic fermentation). In germinating seeds, RQ indicates which food reserve is being used — castor seeds (rich in fats) show RQ < 1; wheat seeds (rich in carbohydrates) show RQ = 1. During starvation in animals, the body shifts from carbohydrate to fat metabolism, and the RQ decreases from about 1.0 to about 0.7. RQ is measured using a respirometer — an apparatus that measures gas exchange. In plants, RQ can be measured using a Ganong's respirometer or similar device. RQ measurements have practical applications in determining the metabolic substrate being used and in assessing metabolic disorders.

Respiration vs. Photosynthesis

Respiration and photosynthesis are complementary processes that form the basis of the carbon cycle. Photosynthesis: consumes CO₂ and H₂O, produces glucose and O₂, requires light energy, occurs in chloroplasts, stores energy (overall endergonic — requires energy input), reduces carbon (CO₂ → carbohydrate). Respiration: consumes glucose and O₂, produces CO₂ and H₂O, releases energy, occurs in mitochondria (and cytoplasm for glycolysis), releases energy (overall exergonic — releases energy), oxidizes carbon (carbohydrate → CO₂). The products of photosynthesis are the reactants of respiration (glucose and O₂), and the products of respiration are the reactants of photosynthesis (CO₂ and H₂O). In plants, both processes occur simultaneously in the same cells — but photosynthesis only during daylight, while respiration occurs 24/7. During the day, photosynthesis usually exceeds respiration (net CO₂ uptake — the plant is a carbon sink). At night, only respiration occurs (net CO₂ release — the plant is a carbon source). The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration — at this point, net gas exchange is zero. Below the compensation point, the plant cannot grow because it is using more energy (respiration) than it is producing (photosynthesis). On a global scale, the two processes roughly balance each other, keeping atmospheric CO₂ and O₂ levels relatively stable over long time scales. However, human activities (burning fossil fuels, deforestation) are releasing CO₂ faster than photosynthesis can remove it, leading to the increase in atmospheric CO₂ that drives climate change. Understanding the balance between these two processes is essential for addressing global environmental challenges.

Key Points

  • •Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30-32 ATP.
  • •Glycolysis: cytoplasm, splits glucose into 2 pyruvate, net 2 ATP + 2 NADH (anaerobic).
  • •Link reaction: pyruvate → acetyl-CoA + CO₂ + NADH (in mitochondrial matrix).
  • •Krebs cycle: matrix, oxidizes acetyl-CoA to CO₂, produces 2 ATP, 6 NADH, 2 FADH₂ per glucose.
  • •ETC: inner mitochondrial membrane, NADH/FADH₂ donate electrons, protons pumped, O₂ final acceptor → H₂O.
  • •Oxidative phosphorylation: ATP synthase uses proton gradient to make ATP (chemiosmosis).
  • •Aerobic respiration: ~30-32 ATP per glucose. Anaerobic (fermentation): only 2 ATP per glucose.
  • •Lactic acid fermentation: pyruvate → lactate (human muscles, Lactobacillus — yogurt/cheese).
  • •Alcoholic fermentation: pyruvate → ethanol + CO₂ (yeast — bread, beer, wine).
  • •PFK-1 is the main regulatory enzyme of glycolysis — inhibited by ATP, activated by AMP.
  • •Oxygen is the final electron acceptor in the ETC — essential for aerobic respiration.
  • •Respiratory Quotient (RQ) = CO₂ released / O₂ consumed. RQ = 1 for carbs, ~0.7 for fats.
  • •Respiration and photosynthesis are complementary — products of one are reactants of the other.
  • •ETC complexes I, III, IV pump protons. Complex II (succinate dehydrogenase) does not pump protons.
  • •ATP yield per NADH ≈ 2.5 ATP. Per FADH₂ ≈ 1.5 ATP (updated from old values of 3 and 2).
  • •Cyanide poisons by binding Complex IV — blocking electron transfer to oxygen.

Practice Questions

  • Explain glycolysis — where does it occur? What is the net gain of ATP and NADH?
  • Describe the Krebs cycle. Where does it occur? What are its products per turn and per glucose?
  • Explain the electron transport chain and oxidative phosphorylation. What is the role of oxygen?
  • Differentiate between aerobic and anaerobic respiration with at least six points of comparison.
  • What is fermentation? Explain lactic acid and alcoholic fermentation with examples and applications.
  • Calculate the total ATP yield from one molecule of glucose in aerobic respiration.
  • What is the respiratory quotient (RQ)? For what substrates do you get RQ = 1, < 1, and > 1?
  • How is cellular respiration regulated? Explain the roles of PFK-1, pyruvate dehydrogenase, and isocitrate dehydrogenase.