Respiration in Plants

1. Chapter at a Glance

Every living cell needs energy for work such as absorbing nutrients, moving materials, building new molecules and dividing. This energy comes from cellular respiration: the breaking of C–C bonds of food molecules such as glucose by oxidation inside the cell, releasing energy that is trapped as ATP (adenosine triphosphate), the energy currency of the cell. The compounds oxidised are called respiratory substrates; carbohydrates are the usual ones, but fats, proteins and even organic acids can be used. This chapter explains glycolysis, fermentation, aerobic respiration (link reaction, Krebs cycle and electron transport system), how much ATP is made, the amphibolic nature of respiration and the respiratory quotient. NEET asks one or two questions every year, usually on the site of each step, the number of ATP/NADH, the enzymes, and RQ values.

2. Do Plants Breathe?

• Plants need O₂ for respiration and give out CO₂, but unlike animals they have no special organs for gas exchange. They use stomata and lenticels.
• Plants manage without lungs because: (i) each part looks after its own gas exchange, and there is little transport of gases from one part to another; (ii) plants do not have great demands for gas exchange — roots, stems and leaves respire at much lower rates than animals, and only during photosynthesis are large volumes of gas exchanged, when leaves are well adapted for it; (iii) the distance gases must diffuse is short, since most living cells are close to the surface (in woody stems, the living cells lie just beneath the bark, and loosely packed parenchyma provides air spaces).
• Complete combustion of glucose in a single step would release most energy as heat, which is useless and could damage cells. So the cell oxidises glucose in many small steps, each catalysed by an enzyme, and couples the energy to ATP synthesis.
• The first stages of respiration evolved when there was no oxygen in the atmosphere, so glycolysis is anaerobic in every organism.

3. Glycolysis (EMP Pathway)

Glycolysis (Greek glycos = sugar, lysis = splitting) was worked out by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. It occurs in the cytoplasm of all living organisms and is the only process of respiration in anaerobic organisms.

• Glucose and fructose are phosphorylated to enter glycolysis. Sucrose from photosynthesis is first broken into glucose and fructose by the enzyme invertase.
• Glucose is converted into two molecules of pyruvic acid through ten reactions, each controlled by a different enzyme.

1. Glucose → glucose-6-phosphate (enzyme hexokinase; uses 1 ATP).
2. Glucose-6-phosphate → fructose-6-phosphate (isomerisation).
3. Fructose-6-phosphate → fructose-1,6-bisphosphate (enzyme phosphofructokinase; uses 1 ATP).
4. Fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL); DHAP is converted into a second PGAL.
5. PGAL → 1,3-bisphosphoglyceric acid (BPGA). Two hydrogen atoms are removed and passed to NAD⁺, forming NADH + H⁺. This happens for each of the two PGAL.
6. BPGA → 3-phosphoglyceric acid (PGA), forming 1 ATP (substrate-level phosphorylation).
7. PGA → 2-PGA → phosphoenolpyruvate (PEP).
8. PEP → pyruvic acid, forming another 1 ATP (enzyme pyruvate kinase).

Pyruvic acid is the key product of glycolysis. Its fate depends on the needs of the cell: lactic acid fermentation, alcoholic fermentation, or aerobic respiration.

4. Fermentation (Anaerobic Respiration)

• In alcoholic fermentation (yeast), pyruvic acid is converted into CO₂ and ethanol. The enzymes are pyruvic acid decarboxylase and alcohol dehydrogenase.
• In lactic acid fermentation (some bacteria, and our muscle cells during vigorous exercise when oxygen is short), pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
• In both, the reducing agent is NADH + H⁺, which is re-oxidised to NAD⁺ so that glycolysis can continue.
• Fermentation is very inefficient: less than 7% of the energy in glucose is released, and not all of it is trapped as ATP. The net gain is only 2 ATP per glucose.
• The products (acid or alcohol) are hazardous. Yeasts poison themselves to death when the alcohol concentration reaches about 13%. To make stronger drinks, the fermented liquid is distilled.

5. Aerobic Respiration

For complete oxidation of glucose to CO₂ and H₂O, organisms use the Krebs cycle, which needs O₂. This is aerobic respiration, and it occurs within the mitochondria. Its crucial events are:

1. Complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, leaving three molecules of CO₂ (in the matrix).
2. Passing the electrons removed as part of the hydrogen atoms to molecular O₂, with simultaneous synthesis of ATP (on the inner mitochondrial membrane).

Link reaction (oxidative decarboxylation of pyruvate)

Pyruvate formed in the cytoplasm enters the mitochondrial matrix and undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex. This reaction needs several coenzymes, including NAD⁺ and Coenzyme A, and Mg²⁺.

Two molecules of NADH are produced from the two pyruvates of one glucose. Acetyl CoA then enters the Krebs cycle.

Tricarboxylic acid (TCA) cycle — Krebs cycle

The cycle was worked out by Hans Krebs and is also called the citric acid cycle.

1. The 2-carbon acetyl group condenses with the 4-carbon oxaloacetic acid (OAA) and water to form the 6-carbon citric acid, catalysed by citrate synthase. CoA is released.
2. Citrate is isomerised to isocitrate.
3. Two successive decarboxylations follow, forming α-ketoglutaric acid (5C) and then succinyl CoA (4C).
4. Succinyl CoA is oxidised to OAA through succinic acid, fumaric acid and malic acid, allowing the cycle to continue.
5. During the conversion of succinyl CoA to succinic acid, one molecule of GTP is made (substrate-level phosphorylation). In a coupled reaction, GTP becomes GDP while ADP becomes ATP.
6. At three points NAD⁺ is reduced to NADH + H⁺, and at one point FAD⁺ is reduced to FADH₂ (succinate → fumarate, by succinate dehydrogenase).

So far, glucose has been broken down to release CO₂, and 8 NADH + 8H⁺, 2 FADH₂ and 2 ATP have been formed in the link reaction and Krebs cycle (for two pyruvates). Where is the oxygen? It is used only in the next stage.

6. Electron Transport System and Oxidative Phosphorylation

The NADH and FADH₂ made so far must be oxidised to release the energy in them. This happens through the electron transport system (ETS), located on the inner mitochondrial membrane.

• Oxidation of one NADH gives 3 ATP; oxidation of one FADH₂ gives 2 ATP (its electrons enter later, at complex II).
• Oxygen is the final hydrogen (electron) acceptor; it combines with electrons and protons to form water. Its role is limited to the end of the process, yet it drives the whole process by removing hydrogen from the system.
• Unlike photophosphorylation, where light energy makes the proton gradient, here the energy of oxidation–reduction is used. So it is called oxidative phosphorylation.

Energy released as electrons pass along the chain is used to pump protons from the matrix into the intermembrane space. The protons then flow back into the matrix through complex V (ATP synthase). ATP synthase has two parts: F₁, a headpiece that contains the site for ATP synthesis, and F₀, an integral membrane protein that forms the proton channel. For each ATP made, 2 H⁺ pass through F₀ down the gradient.

7. The Respiratory Balance Sheet

It is possible to calculate the net gain of ATP for every glucose oxidised, but only on certain assumptions:

• There is a sequential, orderly pathway, with one substrate forming the next, and glycolysis, the TCA cycle and the ETS following one after another.
• The NADH made in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• None of the intermediates is used to make other compounds.
• Only glucose is being respired; no other substrate enters at an intermediate stage.

These assumptions are not really valid in a living system — all pathways work together, substrates enter and leave as needed, and ATP is used as needed. Still, the exercise shows the efficiency of aerobic respiration.

8. Amphibolic Pathway

• Glucose is the favoured substrate, but other molecules can be respired. Fats are first broken into glycerol and fatty acids: fatty acids are degraded to acetyl CoA, while glycerol is converted to PGAL (3-phosphoglyceraldehyde). Proteins are broken into amino acids, which (after deamination) enter at different stages of the Krebs cycle or as pyruvate or acetyl CoA.
• Breaking down substrates is catabolism; respiration is traditionally thought of as catabolic.
• But the same intermediates are withdrawn to build other molecules (anabolism): fatty acids are made from acetyl CoA, and amino acids from Krebs cycle intermediates. When the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway.
• Because the respiratory pathway is involved in both breakdown and synthesis, it is better called an amphibolic pathway rather than a purely catabolic one.

9. Respiratory Quotient (RQ)

The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is the respiratory quotient (or respiratory ratio).

In living organisms, respiratory substrates are often more than one, and pure proteins or fats are never used alone as respiratory substrates.

10. Master Summary Table

11. Mind Map

12. Tips and Tricks

13. Important One-liners

• ATP is called the energy currency of the cell.
• Glycolysis is the only respiratory process in anaerobic organisms.
• Invertase splits sucrose into glucose and fructose.
• Yeast cells die when the alcohol concentration reaches about 13%.
• The link reaction is catalysed by the pyruvate dehydrogenase complex.
• Citrate synthase catalyses the first reaction of the Krebs cycle.
• GTP is formed by substrate-level phosphorylation in the Krebs cycle.
• Cytochrome c oxidase (complex IV) contains copper centres.
• Oxygen is the final acceptor of electrons in aerobic respiration.
• The respiratory pathway is called amphibolic because it serves both catabolism and anabolism.
• RQ of anaerobic respiration is infinity.

14. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

15. Quick Revision Summary