NCERT grounding
NCERT Class 11 Biology, Chapter 8 Cell — The Unit of Life, treats microbodies with extraordinary brevity. The whole of §8.5.11 reads, in essence, that many membrane-bound minute vesicles called microbodies — containing various enzymes — are present in both plant and animal cells. That is the entire NCERT entry: one sentence at the very end of the eukaryotic-organelle list. The NIOS supplement (Senior Secondary Biology, Chapter 4 Cell — Structure and Function, §4.3.4 The microbodies (tiny but important)) is far more explicit. It defines microbodies as small sac-like structures bounded by single membranes and then names the three classes the syllabus expects: lysosomes (already treated under the endomembrane system), peroxisomes, and glyoxysomes.
"Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells."
NCERT Class 11, §8.5.11
For NEET the operational definition is therefore the NIOS one, expanded with the Class 11 chapter-summary statement that the endomembrane system excludes microbodies. Two markers anchor every NEET item on this topic: microbodies are single-membrane structures, and their hallmark enzyme cargo — catalase (peroxisomes) and the glyoxylate-cycle enzymes (glyoxysomes) — defines what they do. The discovery itself is sometimes asked: microbodies were first isolated and named by the Belgian cytologist Christian de Duve in the 1950s using density-gradient cell fractionation, the same technique that gave him the lysosome and earned him the 1974 Nobel Prize.
Structure, types & functions
At the electron-microscope level a microbody is unremarkable: a roughly spherical or ovoid vesicle, 0.2–1.5 µm across — about the size of a mitochondrion or a lysosome — wrapped by one phospholipid bilayer, with a finely granular matrix. Many peroxisomes carry, in addition, a central crystalline core called a nucleoid, packed with urate-oxidase paracrystals. There is no internal cristae system, no second membrane, and no genome. Every microbody protein is imported from the cytosol post-translationally, guided by short peroxisomal targeting signals (PTS1/PTS2) that the microbody surface recognises.
What distinguishes a microbody is therefore not its shape but its enzyme cargo. The single-membrane sac is essentially a sealed reaction chamber that lets cells run chemistry — particularly oxidations that produce hydrogen peroxide — without polluting the rest of the cytoplasm. Two such chambers matter for the NEET botany syllabus: the peroxisome (universal) and the glyoxysome (plant-specific, transient).
membrane
Every microbody — peroxisome, glyoxysome and (historically) lysosome — is bounded by a single phospholipid bilayer. Contrast with mitochondria, plastids and the nucleus (two membranes).
Peroxisomes — the H2O2 chamber
Peroxisomes are present in essentially every eukaryotic cell, plant or animal. They take their name from the chemistry they run: substrates such as fatty acids, amino acids and the photorespiratory intermediate glycolate are oxidised by FAD-linked oxidases that pass their electrons directly to molecular oxygen, generating hydrogen peroxide (H2O2). H2O2 is dangerously reactive — it oxidises lipids, proteins and nucleic acids — so peroxisomes carry a second signature enzyme, catalase, that disposes of it in one of the fastest enzymatic reactions known:
catalase
Decomposes hydrogen peroxide to water and oxygen with a turnover number near 107 s−1. Catalase is the marker enzyme for peroxisomes — its presence is how cytologists identify a fraction as peroxisomal.
products
Water plus molecular oxygen. Coupling H2O2-producing oxidases with H2O2-consuming catalase inside one sealed compartment is the whole engineering point of a peroxisome.
Beyond peroxide chemistry, peroxisomes carry out two further duties of NEET interest. First, β-oxidation of fatty acids. In plant cells β-oxidation is housed almost entirely in peroxisomes (unlike animal cells, where it is shared with mitochondria). Long-chain fatty acids are cleaved two carbons at a time, generating acetyl-CoA. Second, in the green mesophyll cells of C3 plants, peroxisomes are the middle of the three-organelle photorespiration pathway, also known as the C2 or glycolate cycle.
The photorespiration story is worth memorising in full because NEET examiners love its three-organelle handshake. RuBisCO in the chloroplast occasionally fixes O2 instead of CO2, generating one molecule of 3-phosphoglycerate (3-PGA) and one molecule of phosphoglycolate. The phosphoglycolate is dephosphorylated to glycolate, which is exported to the peroxisome. Inside the peroxisome, glycolate oxidase oxidises glycolate to glyoxylate, producing H2O2 (decomposed at once by catalase). Glyoxylate is then transaminated to glycine, and glycine is exported to the mitochondrion. Two glycines condense inside the mitochondrion to release one molecule of CO2 and produce one molecule of serine. Serine returns to the peroxisome, is converted to glycerate, and re-enters the chloroplast as 3-PGA — recapturing 75% of the carbon that was committed to the oxygenation event.
Figure 1. Photorespiration is a three-organelle shuttle. The chloroplast exports glycolate (RuBisCO's oxygenase product) to the peroxisome, which converts it to glycine while catalase neutralises the H2O2 generated. The mitochondrion condenses two glycines to one serine, releasing CO2. Serine returns to the peroxisome and re-enters the chloroplast as 3-PGA. Peroxisomes are the middle organelle in this loop.
Glyoxysomes — the lipid-to-sugar switch
Glyoxysomes are plant-specific microbodies that look just like peroxisomes under the electron microscope. The difference lies entirely in cargo and context. A glyoxysome appears only in the fat-storing tissues of germinating oily seeds — the endosperm of castor (Ricinus), the cotyledons of sunflower, groundnut and cucumber — and only while those tissues are mobilising their stored lipid reserves. Once the reserves are exhausted and the seedling becomes photosynthetic, glyoxysomes disappear, often by re-differentiating into ordinary peroxisomes.
Their job is to run the glyoxylate cycle, the metabolic route that converts stored triglyceride into sucrose. Stored fat is first hydrolysed to glycerol and free fatty acids; the fatty acids enter the glyoxysome and undergo β-oxidation two carbons at a time, generating acetyl-CoA. The acetyl-CoA then feeds into the glyoxylate cycle — a streamlined variant of the Krebs cycle that bypasses both decarboxylation steps and uses two distinctive enzymes, isocitrate lyase and malate synthase, to convert two acetyl-CoA molecules into one succinate. Succinate moves to the mitochondrion, malate exits the mitochondrion to the cytosol, and gluconeogenesis converts malate to sucrose — the export sugar that feeds the rest of the growing embryo.
From stored fat to sucrose in a germinating oily seed
-
1
Lipid hydrolysis
Stored triglycerides in oleosomes are cleaved to glycerol + free fatty acids by lipase.
Oil body -
2
β-oxidation
Inside the glyoxysome, each fatty acid is shortened by 2 C per turn, generating acetyl-CoA.
Glyoxysome -
3
Glyoxylate cycle
Isocitrate lyase + malate synthase bypass the two CO2-releasing steps of Krebs. Net: 2 acetyl-CoA → 1 succinate.
Glyoxysome -
4
Gluconeogenesis
Succinate enters the mitochondrion → malate exits → cytosol converts malate to sucrose for export to the embryo.
Mito + cytosol
The glyoxylate cycle is the engineering trick that lets a plant turn fat into sugar — something animals cannot do, because animal cells lack the two key enzymes (isocitrate lyase and malate synthase) and therefore lack glyoxysomes altogether. This is also the reason glyoxysomes are transient: there is no fat reserve to mobilise once the embryo is photosynthesising, so the organelle that exists for that single purpose vanishes when the job is done.
Peroxisome vs glyoxysome — the side-by-side
Peroxisome
Universal
Plant + animal · permanent
- Catalase + flavin-oxidases — H2O2 metabolism
- β-oxidation of fatty acids (whole job in plants)
- Photorespiration — glycolate → glycine in green leaves
- Often has a crystalline urate-oxidase nucleoid
- Marker enzyme: catalase
Glyoxysome
Plant only
Germinating oily seeds · transient
- Glyoxylate cycle — fat to sugar
- β-oxidation of seed-storage fatty acids
- Absent from mature, photosynthetic tissues
- Disappears once seed lipid reserves are consumed
- Marker enzymes: isocitrate lyase, malate synthase
Figure 2. Both microbodies are bounded by a single membrane and look almost identical under EM. The peroxisome characteristically shows a crystalline nucleoid (urate oxidase) and runs catalase and glycolate oxidase. The glyoxysome runs β-oxidation plus the glyoxylate-cycle enzymes (isocitrate lyase and malate synthase) and lies next to a stored-lipid droplet from which it draws its fatty-acid substrate.
Who discovered them? — Christian de Duve
The cytologist Christian de Duve (1917–2013) isolated the first peroxisomes in 1965 by centrifuging rat-liver homogenates on a sucrose density gradient — the same fractionation technique he had used a decade earlier to discover lysosomes (1955). He coined the term peroxisome for the H2O2-handling fraction and the umbrella term microbody for the morphological class to which peroxisomes, glyoxysomes and lysosomes all belong. De Duve shared the 1974 Nobel Prize in Physiology or Medicine with Albert Claude and George Palade — incidentally the same Palade who first imaged the ribosome — for these structural-cytology contributions. NEET has not yet asked his name directly, but he routinely appears in the lysosome/microbody discovery options of state-level entrance papers.
Worked examples
Which one of the following pairs of cell organelles is enclosed by a single membrane only?
Solution. Microbodies — including peroxisomes and glyoxysomes — are single-membrane organelles. So are lysosomes, vacuoles and the cisternae of Golgi/ER. Mitochondria, plastids and the nucleus are double-membrane organelles. The 2016 NEET item on this point (Q.122) used lysosome as the correct option among chloroplasts, lysosomes, nuclei and mitochondria; the same logic applied to a peroxisome/glyoxysome distractor gives the same answer pattern.
In a germinating castor seed, the metabolic conversion of stored fat to carbohydrate takes place primarily in which organelle?
Solution. Glyoxysomes. The seed stores energy as triglyceride. After lipase hydrolysis, fatty acids enter glyoxysomes where β-oxidation generates acetyl-CoA. The glyoxylate-cycle enzymes isocitrate lyase and malate synthase then channel acetyl-CoA into succinate, which the mitochondrion and cytosol convert to sucrose. Mitochondria, peroxisomes and plastids alone cannot do this conversion in plants — the glyoxylate-cycle enzymes are restricted to glyoxysomes.
During photorespiration in a C3 leaf, glycine is shuttled from which organelle to which other organelle?
Solution. From the peroxisome to the mitochondrion. The glycolate exported by the chloroplast is oxidised in the peroxisome to glyoxylate and then transaminated to glycine. Two glycines then enter the mitochondrion, where they are condensed to one serine plus one CO2. The serine returns to the peroxisome and eventually re-enters the chloroplast as 3-PGA. This three-organelle shuttle is one of NEET's favourite photosynthesis traps.
Identify the marker enzyme of peroxisomes and write its catalytic reaction.
Solution. Catalase. It decomposes hydrogen peroxide to water and molecular oxygen: 2 H2O2 → 2 H2O + O2. The biological role is to neutralise the H2O2 continuously generated by FAD-linked oxidases (glycolate oxidase, urate oxidase, fatty-acyl-CoA oxidase) housed inside the same peroxisomal compartment. Sequestering both the producer and the destroyer of H2O2 inside one single-membrane sac is the entire design logic of a peroxisome.