Fungi are not considered plants because the two groups belong to entirely separate branches of the tree of life, and molecular evidence shows fungi are actually more closely related to animals than they are to plants. The similarities that once fooled biologists, such as growing in soil and having cell walls, turn out to be superficial. Underneath, fungi differ from plants in how they feed, how their cells are built, how they store energy, how they reproduce, and how they grow. These are not minor details but fundamental biological differences that accumulated over more than a billion years of independent evolution.
Fungi Are Closer to Animals Than to Plants
For most of recorded history, fungi were classified as plants. They grow in soil, they do not run away when you approach them, and many of them look vaguely plant-like. This assumption persisted well into the twentieth century. Robert Whittaker’s five-kingdom classification system, published in the late 1960s and a fixture in biology textbooks for decades, was among the first widely adopted frameworks to formally separate fungi from plants into their own kingdom.1Oxford Academic. Five Kingdoms, More or Less: Robert Whittaker and the Broad Classification of Organisms But even Whittaker’s system understated just how different fungi are.
The real shock came from molecular studies in the 1990s. Researchers comparing protein sequences across animals, plants, fungi, and various single-celled organisms found that animals and fungi consistently grouped together as closest relatives, forming their own branch of the evolutionary tree. Not a single analysis supported either an animal-plant or a fungal-plant pairing.2PubMed. Animals and fungi are each other’s closest relatives: congruent evidence from multiple proteins The shared ancestor of fungi and animals is called an opisthokont, and this common heritage means that in evolutionary terms, a mushroom growing on a log is a closer cousin to you than it is to the tree the log came from.
This relationship has been confirmed repeatedly as genome sequencing has expanded. Environmental DNA techniques and increasing microbial genome data have continued to reshape the tree of life, but the animal-fungal sister grouping has held steady across every major reanalysis.3PubMed Central. What Defines the “Kingdom” Fungi?
Fungi Cannot Make Their Own Food
The single most important difference between fungi and plants is how they eat. Plants are autotrophs: they capture sunlight and use it to build sugars from carbon dioxide and water. This is the defining trick of plant life. Fungi cannot do this. They have no chloroplasts, no chlorophyll, and no capacity for photosynthesis. They are heterotrophs, meaning they must get their carbon and energy from organic matter that other organisms have already produced.
But fungi do not eat the way animals do, either. Animals ingest food and then digest it internally. Fungi do the opposite: they secrete enzymes into their environment, break down complex organic material outside their bodies, and then absorb the resulting small molecules through their cell surfaces. This absorptive feeding strategy is why fungi tend to grow through or across their food source, maximizing the surface area in contact with whatever they are decomposing. A mushroom on a rotting log is just the visible reproductive structure; the actual organism is a sprawling network of microscopic filaments threading through the wood.
This ability to break down tough organic molecules is ecologically enormous. White-rot fungi, for instance, produce specialized enzymes including laccases, lignin peroxidases, and manganese peroxidases that can degrade lignin, one of the most chemically stubborn compounds in nature.4FEMS Microbiology Reviews. Lignin-modifying enzymes from selected white-rot fungi: production and role from in lignin degradation Lignin is the polymer that makes wood rigid. Without fungi capable of dismantling it, dead trees and plant debris would pile up far faster than they decompose, and the carbon locked inside them would be trapped for much longer.5PubMed Central. Fungal biodegradation and enzymatic modification of lignin
Different Walls, Different Membranes, Different Chemistry
Both fungi and plants have cell walls, which is one reason they were lumped together for so long. But the materials in those walls are chemically unrelated. Plant cell walls are built primarily from cellulose, a polymer of glucose. Fungal cell walls are built from chitin, a polymer of a modified sugar called N-acetylglucosamine. Chitin is the same structural material found in the exoskeletons of insects and crabs, which fits neatly with the evolutionary link between fungi and animals.6PubMed Central. Chitin and chitin-related compounds in plant–fungal interactions Some fungi also contain chitosan, a deacetylated form of chitin produced by enzymes that plants lack entirely.
The differences extend to the cell membrane itself. The main sterol in fungal membranes is ergosterol, a molecule that plays a key role in membrane fluidity and cell signaling in fungi.7PubMed. Membrane component ergosterol builds a platform for promoting effector secretion and virulence in Magnaporthe oryzae Plant membranes rely on different sterols, and animal membranes use cholesterol. This distinction is not just a curiosity. It is the reason antifungal drugs like amphotericin B and the azole class work: they target ergosterol or its synthesis, disrupting fungal membranes while leaving human cells (which use cholesterol) mostly unharmed. If fungi shared plant membrane chemistry, the entire approach to treating fungal infections would be different.
Energy Storage and Biochemical Pathways
When organisms need to stash energy for later use, the form they store it in reflects their evolutionary heritage. Plants store energy as starch, a relatively simple glucose polymer with sparse branching that is optimized for slow, long-term energy release. Fungi store energy as glycogen, the same molecule animals use. Fungal glycogen has structural features intermediate between plant starch and animal glycogen: its branch density and chain-length distribution produce a molecule well suited for rapid energy mobilization, though not quite as fast-release as the glycogen in animal muscle.8PubMed. Structural evolution and functional adaptation of energy-storage polysaccharides in animals, plants and fungi That fungi and animals both use glycogen while plants use starch is yet another molecular signature of the animal-fungal relationship.
Fungi also differ from plants in how they synthesize certain amino acids. Fungi produce lysine through the α-aminoadipate pathway, a biosynthetic route that is shared with some animals but absent in plants. Plants make lysine through an entirely different biochemical pathway.9PubMed Central. The fungal α-aminoadipate pathway for lysine biosynthesis requires two enzymes of the aconitase family for the isomerization of homocitrate to homoisocitrate The α-aminoadipate pathway is also the route to penicillin production in certain fungi, which gives it medical significance beyond taxonomy.
How Fungi Grow and Reproduce
Plants grow through organized meristems, clusters of dividing cells at the tips of shoots and roots. Fungi grow in a completely different way. Most fungi consist of hyphae, thin tube-like filaments that extend by polarized growth at their tips. This growth is driven by a structure called the Spitzenkörper, which acts as a supply center, concentrating the delivery of building materials to the advancing hyphal tip.10PubMed. Candida albicans hyphae have a Spitzenkörper that is distinct from the polarisome found in yeast and pseudohyphae Some fungi can also grow as oval yeast cells or as chains of elongated cells called pseudohyphae, depending on environmental conditions. This morphological flexibility has no parallel in plant biology.
Fungal reproduction is also distinctive. While plants alternate between a haploid and diploid generation in a well-defined cycle, many fungi, especially the mushroom-forming species, spend much of their life in a state that has no plant equivalent: the dikaryon. In a dikaryotic cell, two genetically distinct haploid nuclei coexist without fusing. The two nuclei divide in sync, each cell maintaining two separate nuclear genomes, but actual nuclear fusion is delayed until the moment just before spore production.11PubMed Central. Modeling the consequences of the dikaryotic life cycle of mushroom-forming fungi on genomic conflict A dikaryotic fungus carries two copies of its genome per cell, like a diploid organism, but those genomes remain compartmentalized and each nucleus retains the ability to mate again with other individuals. This is a genetic arrangement with no real parallel in the plant world.
Mycorrhizal Partnerships and the Plant-Fungus Interface
Although fungi and plants are fundamentally different organisms, their lives are deeply entangled in ways that underscore the distinction rather than blurring it. Mycorrhizal fungi form partnerships with roughly 80 to 90 percent of land plant species. In these relationships, fungal hyphae extend into or around plant roots, dramatically increasing the plant’s access to soil nutrients, especially phosphorus. In return, the plant delivers carbon, in the form of sugars produced by photosynthesis, to the fungus. Researchers tracking this exchange using isotope tracers have confirmed that phosphorus moves from the fungus to plant shoots while carbon flows from the plant to the fungus, with the rates varying depending on soil conditions and the specific plants involved.12PubMed. Carbon and phosphorus exchange rates in arbuscular mycorrhizas depend on environmental context and differ among co-occurring plants
This exchange is not a quirk of a few species. Arbuscular mycorrhizal fungi form symbioses with most crop plants, and research has shown that the carbon-for-nutrient trade varies among cultivars and is affected by atmospheric carbon dioxide levels, making it a significant factor in agricultural sustainability and climate adaptation.13PubMed Central. Carbon for nutrient exchange between arbuscular mycorrhizal fungi and wheat varies according to cultivar and changes in atmospheric carbon dioxide concentration The mycorrhizal partnership is itself strong evidence that fungi and plants evolved as separate organisms with complementary metabolic needs: the plant has carbon from photosynthesis but limited mineral access, and the fungus has unparalleled mineral-mining ability but no capacity to fix carbon from sunlight.
Algal-Fungal Symbioses and Lichens
Mycorrhizae are not the only partnerships fungi form. Lichens, those crusty or leafy patches on rocks and tree bark, are symbiotic composites of a fungus and a photosynthetic partner, usually an alga or a cyanobacterium. The alga provides carbon through photosynthesis while the fungus provides structure, mineral nutrients, and protection from desiccation. These partnerships can be remarkably intimate. In laboratory experiments recreating early-stage symbioses between an alga and a fungus, researchers found that carbon flowed from the alga to the fungus, with nearly 70 percent of the transferred carbon incorporated into the fungal lipid pool. Meanwhile, nitrogen flowed in the opposite direction, from fungus to alga, at roughly double the rate that nitrogen moved from alga to fungus.14PubMed Central. Algal-fungal symbiosis leads to photosynthetic mycelium
Lichens were historically confusing for classification because they look like a single organism and they photosynthesize, which made some early naturalists think they were plants. But the lichen is not a plant performing photosynthesis. It is a fungus that has teamed up with a photosynthetic partner. The fungus remains heterotrophic; it just happens to have a live-in food source. Recognizing this distinction was one of the earlier steps in understanding that fungi and photosynthetic organisms are separate lineages, even when they live in extreme intimacy.
Plants That Act Like Fungi, Fungi That Act Like Plants
Nature loves blurring categories. Some plants have abandoned photosynthesis entirely and now depend on fungi for all their nutrition. The ghost plant, Monotropa uniflora, is a flowering plant with no chlorophyll. It is obligately mycotrophic, meaning it obtains all of its carbon and nutrients from mycorrhizal fungi rather than from sunlight.15Canadian Journal of Botany. Fungal calcium oxalate in mycorrhizae of Monotropa uniflora It taps into the mycorrhizal network connecting a fungus to a nearby photosynthetic tree, essentially stealing from the fungal middleman. Despite looking eerily fungus-like with its pale, waxy stems and complete lack of green pigment, Monotropa is still unambiguously a plant by every molecular and anatomical criterion: it has cellulose cell walls, plant-type embryonic development, and a clear position among the heaths and blueberries on the evolutionary tree.
Conversely, some fungi can exploit light-dependent partnerships so effectively that it is tempting to describe them as photosynthetic. In the lichen symbioses mentioned earlier, the fungal mycelium becomes photosynthetically active in a functional sense: it gains carbon from its algal partner and channels it into growth. But the fungus itself never produces chlorophyll or conducts photosynthesis. It remains a heterotroph with a clever supply chain.
These boundary cases are useful because they show that the distinction between fungi and plants is not about lifestyle or appearance. It is about evolutionary ancestry, cellular architecture, and the molecular toolkit encoded in the genome. A plant that stops photosynthesizing is still a plant. A fungus that hitches a ride with a photosynthetic partner is still a fungus.
Why the Confusion Persisted So Long
There are historical reasons the misclassification stuck for centuries. Before microscopy and molecular tools existed, organisms were sorted by what they looked like and how they lived. Fungi grow from the ground. They do not move. Many of them are rooted in soil or wood. Mushrooms even seem to have stems and caps, vaguely reminiscent of plant structures. Without the ability to examine cell chemistry or sequence DNA, it was reasonable to assume fungi were unusual plants, perhaps parasitic or degenerate ones that had lost the ability to photosynthesize.
Even after Whittaker separated fungi into their own kingdom in the late 1960s, the conceptual link between fungi and plants lingered in textbooks and popular culture. Many biology courses continued to teach fungi as part of botany departments. Mycology, the study of fungi, was traditionally housed within plant science faculties at universities. Some of this institutional inertia continues today: you can still find fungi covered in the plant biology section of introductory courses, which perpetuates the misconception for new generations of students.
The shift in understanding has been dramatic but uneven. Professional biologists and ecologists now treat fungi as firmly belonging to the opisthokont supergroup alongside animals, and molecular phylogenetics has made this position essentially unassailable.2PubMed. Animals and fungi are each other’s closest relatives: congruent evidence from multiple proteins But in grocery stores, in cooking, and in everyday language, mushrooms still get shelved next to the vegetables. That cultural habit is not likely to change soon, even as the science has moved on completely.
Counting the Differences at a Glance
It can help to see the major distinctions collected in one place. None of these alone would be enough to separate fungi from plants, but together they paint a picture of two groups that diverged very early and evolved in fundamentally different directions.
- Nutrition: Plants photosynthesize; fungi absorb pre-made organic compounds from their environment.
- Cell wall material: Plant walls are cellulose; fungal walls are chitin.
- Membrane sterol: Plants use phytosterols; fungi use ergosterol; animals use cholesterol.
- Energy storage: Plants store starch; fungi store glycogen, the same molecule animals use.
- Lysine synthesis: Plants use the diaminopimelate pathway; fungi use the α-aminoadipate pathway, which is shared with some animals.
- Growth form: Plants grow from organized meristems; fungi grow by extending hyphae from polarized tips.
- Reproduction: Many fungi maintain a dikaryotic state with two unfused nuclei per cell, a condition not found in plants.
- Evolutionary placement: Fungi and animals share a more recent common ancestor with each other than either shares with plants.
Each of these differences is independently well supported, and they all point in the same direction. Fungi are not plants that lost their chlorophyll. They are a separate kingdom of life that happens to share a few convergent features with plants, like growing in place and having cell walls, while being fundamentally more similar to the animal lineage at the molecular level. The fact that it took science until the late twentieth century to fully appreciate this is a good reminder that looking like something and being related to it are very different things.