Respiration in Plants: NEET notes

1. Respiration and ATP

NCERT §12 (opening text, before §12.1)

• Every life activity, from absorbing water to moving and reproducing, runs on energy that comes from oxidising food. Green plants and cyanobacteria make that food by photosynthesis; animals eat plants directly (herbivores) or indirectly (carnivores); saprophytes such as fungi feed on dead and decaying matter. So all respired food traces back to photosynthesis.
• Only chloroplast-containing cells, mostly in the outer layers, photosynthesise. Every non-green organ, tissue and cell of the same plant needs food translocated to it for oxidation.
• In eukaryotes, photosynthesis happens in chloroplasts, while the breakdown of food to release energy happens in the cytoplasm and in mitochondria.
• Cellular respiration is the oxidation of complex compounds inside the cell, breaking their C–C bonds and releasing a considerable amount of energy. The compounds oxidised are called respiratory substrates.
• Carbohydrates are the usual respiratory substrates, but in some plants, under certain conditions, proteins, fats and even organic acids are respired.
• The energy is not released all at once or set free into the cell. Enzymes release it in a series of slow, small steps and trap it as chemical energy in ATP.
• The cell cannot use the energy of oxidation directly; it makes ATP, and ATP is broken down wherever and whenever work must be done. That is why ATP is called the energy currency of the cell.
• Respiration also leaves carbon skeletons behind, and the cell uses these as precursors to build other molecules.

2. Do plants breathe?

NCERT §12.1

• Plants do take in O₂ and give out CO₂ during respiration, but they have no specialised respiratory organs; gases move in and out through stomata and lenticels.
• Reason one: each part of the plant looks after its own gas exchange, and very little gas is carried from one part to another.
• Reason two: demand is low. Roots, stems and leaves respire far more slowly than animals. Large volumes of gas move only during photosynthesis, when each leaf handles its own needs, and O₂ is released right inside the photosynthesising cells.
• Reason three: diffusion distances are short, because every living cell lies close to the surface. In woody stems and roots the living cells form thin layers inside and just beneath the bark, which has lenticels; the cells deep inside are dead and only give mechanical support.
• Loosely packed parenchyma in leaves, stems and roots leaves a connected network of air spaces, so most cells have part of their surface touching air.
• Complete combustion of glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Burnt in one go, most of this energy would be lost as heat.
• The cell instead oxidises glucose in many small steps, some of them just large enough for the energy they release to be coupled to ATP synthesis.
• The first cells probably lived in an atmosphere without oxygen, and many organisms today are facultative or obligate anaerobes. All living organisms keep the enzymes to partly oxidise glucose without O₂; this breakdown of glucose to pyruvic acid is glycolysis.

3. Glycolysis

NCERT §12.2

• The name comes from Greek: glycos (sugar) and lysis (splitting). Gustav Embden, Otto Meyerhof and J. Parnas worked out the scheme, so it is also called the EMP pathway.
• Glycolysis takes place in the cytoplasm and occurs in all living organisms; in anaerobic organisms it is the only process of respiration. One glucose is partly oxidised to two molecules of pyruvic acid through a chain of ten enzyme-controlled reactions.
• In plants the glucose comes from sucrose, the end product of photosynthesis, or from stored carbohydrate. Invertase splits sucrose into glucose and fructose, and both enter glycolysis.
• Hexokinase phosphorylates glucose to glucose-6-phosphate, which isomerises to fructose-6-phosphate; from here glucose and fructose follow the same steps.
• ATP is used at two steps: glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose-1,6-bisphosphate.
• Fructose-1,6-bisphosphate (6C) splits into two 3-carbon triose phosphates: dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
• NADH + H⁺ is formed at one step only: PGAL is oxidised, taking up inorganic phosphate, to 1,3-bisphosphoglycerate (BPGA); two redox equivalents (two hydrogen atoms) pass from PGAL to NAD⁺.
• ATP is made at two steps: BPGA to 3-phosphoglyceric acid (PGA), and phosphoenolpyruvate (PEP) to pyruvic acid. As each happens once per triose and there are two trioses, 4 ATP are made per glucose.
• Per glucose: 2 ATP used, 4 ATP made directly, so a net gain of 2 ATP, plus 2 NADH + H⁺ and 2 pyruvic acid. What happens to pyruvate next depends on what the cell needs.

4. Fermentation

NCERT §12.2–§12.3

• Cells handle pyruvic acid in three main ways: lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation runs under anaerobic conditions in many prokaryotes and unicellular eukaryotes, and also in germinating seeds.
• In yeast, glucose is incompletely oxidised without O₂: pyruvic acid is converted to CO₂ and ethanol by the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase.
• Some bacteria make lactic acid from pyruvic acid. In animal cells such as muscles during exercise, when O₂ falls short, lactate dehydrogenase reduces pyruvic acid to lactic acid.
• Both kinds of fermentation use NADH + H⁺ as the reducing agent and turn it back into NAD⁺, which lets glycolysis keep going.
• Fermentation frees under seven per cent of the energy stored in glucose, and only part of even that is trapped as ATP. The net gain is only 2 ATP per glucose, all from glycolysis.
• Fermentation is hazardous to the cell because it produces acid or alcohol. Yeasts poison themselves to death when alcohol reaches about 13 per cent.
• Fermentation versus aerobic respiration: glucose is only partly broken down rather than fully to CO₂ and H₂O; the net gain is 2 ATP rather than many more; and NADH goes back to NAD⁺ only slowly in fermentation but very vigorously in aerobic respiration.

5. Aerobic respiration: pyruvate to acetyl CoA

NCERT §12.4

• Aerobic respiration completely oxidises organic substances in the presence of O₂, releasing CO₂, water and a large amount of energy. It is the most common type in higher organisms, and in eukaryotes it takes place inside mitochondria.
• Pyruvate, the end product of glycolysis, is carried from the cytoplasm into the mitochondrion.
• Two crucial events follow: all the hydrogen atoms are removed from pyruvate step by step, leaving three molecules of CO₂; and the electrons taken with those hydrogens are passed to O₂ while ATP is made.
• The first event happens in the mitochondrial matrix; the second is located on the inner mitochondrial membrane.
• In the matrix, pyruvate undergoes oxidative decarboxylation, catalysed by pyruvic dehydrogenase, which needs coenzymes such as NAD⁺ and coenzyme A: pyruvic acid + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺ (Mg²⁺ is needed).
• One glucose gives two pyruvic acids, so this step makes 2 NADH and releases 2 CO₂ per glucose. The acetyl CoA then enters the tricarboxylic acid cycle.

6. Tricarboxylic acid cycle

NCERT §12.4.1

• The cycle is also called Krebs' cycle, after Hans Krebs, who first worked it out, or the citric acid cycle. It runs in the mitochondrial matrix.
• It starts when the acetyl group (2C) condenses with oxaloacetic acid (OAA, 4C) and water to give citric acid (6C). Citrate synthase catalyses this, and CoA is set free.
• Citrate is isomerised to isocitrate. Two decarboxylations follow in succession, giving α-ketoglutaric acid (5C) and then succinyl-CoA; each releases one CO₂.
• Succinyl-CoA is converted to succinic acid, making one GTP. This is substrate-level phosphorylation; in a coupled reaction the GTP becomes GDP as ADP becomes ATP.
• The rest of the cycle oxidises succinic acid, via malic acid, back to OAA so the cycle can go on.
• Each turn has three steps where NAD⁺ is reduced to NADH + H⁺ and one where FAD⁺ is reduced to FADH₂.
• For the cycle to keep running, OAA must be continually replenished, and NAD⁺ and FAD⁺ must be regenerated from NADH and FADH₂.
• Summary per pyruvic acid (matrix): pyruvic acid + 4NAD⁺ + FAD⁺ + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP. This includes the link reaction.
• Per glucose, counting the link reaction, the story so far is 8 NADH + H⁺, 2 FADH₂ and 2 ATP (from the cycle), with 6 CO₂ released. Neither O₂ nor the large ATP yield has appeared yet.

7. Electron transport system and oxidative phosphorylation

NCERT §12.4.2

• The energy held in NADH + H⁺ and FADH₂ is released when they are oxidised through the electron transport system (ETS), a chain of carriers in the inner mitochondrial membrane, with the electrons finally passed to O₂ to form H₂O.
• Complex I (NADH dehydrogenase) oxidises NADH made in the matrix and passes the electrons to ubiquinone in the inner membrane.
• Ubiquinone also receives reducing equivalents from FADH₂ through complex II, which is generated when succinate is oxidised in the citric acid cycle.
• Reduced ubiquinone (ubiquinol) passes its electrons to cytochrome c through the cytochrome bc₁ complex (complex III).
• Cytochrome c, a small protein sitting on the outer face of the inner membrane, shuttles electrons as a mobile carrier between complexes III and IV.
• Complex IV is cytochrome c oxidase, containing cytochromes a and a₃ and two copper centres.
• Electron flow through complexes I to IV is coupled to ATP synthase (complex V). The yield depends on the donor: one NADH gives 3 ATP, one FADH₂ gives 2 ATP.
• O₂ acts only at the very end, but it is vital: as the final hydrogen acceptor it pulls hydrogen out of the system and so drives the whole process.
• In photophosphorylation, light energy builds the proton gradient; in respiration the energy of oxidation-reduction builds it, so ATP synthesis here is called oxidative phosphorylation.
• ATP synthase has two parts: F₁, a peripheral headpiece with the site where ATP is made from ADP and Pi, and F₀, an integral membrane complex forming the proton channel. For each ATP made, 4 H⁺ pass through F₀ from the intermembrane space into the matrix, down the electrochemical proton gradient.

8. The respiratory balance sheet

NCERT §12.5

• The net ATP from one glucose can be calculated, but only as a theoretical exercise resting on four assumptions.
• Assumption one: the pathway runs as one orderly sequence, glycolysis then the TCA cycle then the ETS, each substrate forming the next.
• Assumption two: the NADH made in glycolysis is moved into the mitochondria and undergoes oxidative phosphorylation.
• Assumption three: no intermediate is drawn off to make another compound. Assumption four: only glucose is respired, with no other substrate entering at any intermediate stage.
• None of these holds in a living cell: all the pathways run at once, substrates enter and leave as needed, ATP is used as needed, and enzyme rates are controlled in many ways.
• On these assumptions the net gain is 38 ATP per glucose in aerobic respiration: 10 NADH × 3 = 30, 2 FADH₂ × 2 = 4, plus 2 net ATP from glycolysis and 2 from the TCA cycle.
• Fermentation, by contrast, nets only 2 ATP per glucose.

9. Amphibolic pathway

NCERT §12.6

• Glucose is the favoured respiratory substrate, and other carbohydrates are usually converted to glucose first. Other substrates can be respired, but they join the pathway at later points, not at the first step.
• Fats are first split into glycerol and fatty acids. Fatty acids are broken down to acetyl CoA before entering; glycerol enters after being converted to PGAL.
• Proteins are broken down by proteases into amino acids, which after deamination enter according to their structure: as pyruvate, as acetyl CoA, or at some stage within Krebs' cycle.
• The same intermediates are withdrawn from the pathway when the cell builds these molecules; for example, acetyl CoA is taken out of the pathway to synthesise fatty acids.
• Catabolism is breaking down and anabolism is building up. Since the respiratory pathway serves both, it is better called an amphibolic pathway than a purely catabolic one.

10. Respiratory quotient

NCERT §12.7

• The respiratory quotient (RQ), also called respiratory ratio, is the volume of CO₂ evolved divided by the volume of O₂ consumed in respiration.
• RQ depends on which substrate is being respired.
• Carbohydrates completely oxidised give RQ = 1: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, so RQ = 6CO₂ / 6O₂ = 1.0.
• Fats give RQ below 1. For tripalmitin, 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy, so RQ = 102CO₂ / 145O₂ = 0.7.
• Proteins as substrate give an RQ of about 0.9.
• In living organisms more than one substrate is usually respired at a time; pure proteins or pure fats are never the only substrate.

Must-know facts

1. Respiration: oxidative breaking of C–C bonds inside the cell, releasing energy trapped as ATP, the energy currency.
2. Plants exchange gases through stomata and lenticels; they have no specialised respiratory organs.
3. Glycolysis = EMP pathway (Embden, Meyerhof, Parnas); cytoplasm; all living organisms; ten reactions; glucose → 2 pyruvic acid.
4. Invertase: sucrose → glucose + fructose. Hexokinase: glucose → glucose-6-phosphate.
5. Glycolysis ATP used: glucose → G-6-P and F-6-P → F-1,6-bisP. ATP made: BPGA → PGA and PEP → pyruvic acid.
6. Only NADH step of glycolysis: PGAL → BPGA. Per glucose: net 2 ATP, 2 NADH.
7. Alcoholic fermentation (yeast): pyruvic acid decarboxylase + alcohol dehydrogenase → CO₂ + ethanol. Lactic acid: lactate dehydrogenase.
8. Fermentation releases less than 7% of glucose energy; yeast dies at about 13% alcohol.
9. Link reaction (matrix): pyruvic dehydrogenase, NAD⁺, CoA, Mg²⁺; pyruvic acid → acetyl CoA + CO₂ + NADH.
10. Krebs' cycle (matrix): acetyl CoA + OAA + H₂O → citric acid, by citrate synthase.
11. Krebs' cycle per turn: 3 NADH, 1 FADH₂, 1 GTP (→ ATP, substrate-level), 2 CO₂.
12. ETS on inner membrane: complex I NADH dehydrogenase, II (FADH₂), III cytochrome bc₁, IV cytochrome c oxidase (cyt a, a₃, 2 copper centres), V ATP synthase.
13. Cytochrome c: small mobile protein on the outer surface of the inner membrane, between complexes III and IV.
14. NADH → 3 ATP; FADH₂ → 2 ATP. O₂ is the final hydrogen acceptor.
15. ATP synthase: F₁ headpiece (catalytic site), F₀ proton channel; 4 H⁺ per ATP from intermembrane space to matrix.
16. Net gain in aerobic respiration: 38 ATP per glucose; fermentation: 2 ATP.
17. Entry points: fatty acids → acetyl CoA; glycerol → PGAL; amino acids (after deamination) → pyruvate, acetyl CoA or Krebs' cycle.
18. Respiratory pathway is amphibolic (both catabolic and anabolic).
19. RQ = CO₂ evolved / O₂ consumed: carbohydrate 1.0, fat (tripalmitin) 0.7, protein about 0.9.

Common traps

Key terms

Respiratory substrate

Any compound oxidised in respiration to release energy; usually carbohydrate.

ATP

The molecule that stores the energy released in respiration and gives it up where work is done: the cell's energy currency.

Lenticel

An opening in the bark of woody stems through which gases reach the living cells beneath.

Glycolysis

The ten-step breakdown of glucose to two pyruvic acids in the cytoplasm, without O₂.

EMP pathway

Glycolysis, named after Embden, Meyerhof and Parnas.

Facultative anaerobe

An organism that can live with or without oxygen.

Obligate anaerobe

An organism that must have anaerobic conditions.

Fermentation

Anaerobic incomplete oxidation of glucose, ending in ethanol and CO₂ or in lactic acid, with a net gain of 2 ATP.

Oxidative decarboxylation

Removal of CO₂ together with hydrogen, as when pyruvic acid becomes acetyl CoA.

Acetyl CoA

The 2-carbon acetyl group on coenzyme A that feeds Krebs' cycle.

Substrate-level phosphorylation

ATP (or GTP) made directly in a pathway reaction, without the ETS.

Ubiquinone

The carrier in the inner membrane that takes electrons from complexes I and II to complex III.

Cytochrome c

A small mobile protein carrying electrons from complex III to complex IV.

Oxidative phosphorylation

ATP synthesis driven by the energy of oxidation-reduction in the ETS.

F₀ and F₁

The proton channel in the inner membrane and the headpiece that makes ATP, together forming ATP synthase.

Amphibolic pathway

A pathway used both for breaking molecules down and for building them up.

Respiratory quotient

Volume of CO₂ evolved divided by volume of O₂ consumed in respiration.