What Are Plants and How Do They Work?

Plants are organisms that capture sunlight and convert it into chemical energy through photosynthesis, using that energy to build their own bodies out of water, air, and soil minerals. That deceptively simple description covers an enormous range of life forms, from single-celled algae floating in the ocean to redwoods towering a hundred meters overhead. What unites them is a piece of ancient biological machinery: the chloroplast, a structure inside plant cells that traces its origin to a free-living photosynthetic bacterium swallowed by another cell over a billion years ago. Everything else about how plants feed, grow, defend, communicate, and reproduce follows from that one event and the innovations that came after it.

Where Chloroplasts Came From

The defining feature of a plant cell is its chloroplast, and its origin story is genuinely strange. Molecular and genomic data now leave little doubt that the chloroplast descended from a cyanobacterium, a photosynthetic microbe that was engulfed by an ancient host cell but never digested.¹ Instead of being broken down for food, the cyanobacterium survived inside its host, eventually becoming a permanent internal partner. Over vast stretches of time, most of the cyanobacterium’s genes migrated into the host cell’s own nucleus, leaving the chloroplast dependent on its host but still carrying a small genome of its own.

This event appears to have happened once. Molecular data from multiple lines of evidence point to a single primary endosymbiosis in the ancestor of a group called the Archaeplastida, which includes green algae, land plants, red algae, and a lesser-known group called glaucophytes.² Every plant you have ever seen descends from that single partnership. The chloroplast retained the cyanobacterium’s ability to harvest light, and the host cell provided a protected environment and access to nutrients. That merger set the stage for photosynthesis to become the engine of plant life.

How Photosynthesis Actually Works

Photosynthesis happens in two connected stages, both inside the chloroplast. The first stage uses light energy directly. Inside a system of internal membranes called thylakoids, light splits water molecules into oxygen, protons, and electrons. Those protons and electrons are shuttled through the membrane to generate two energy-carrying molecules: ATP and NADPH.³ The oxygen is released as a waste product, which is convenient for every oxygen-breathing organism on the planet.

The second stage does not need light directly but depends on the ATP and NADPH produced in the first. Here, a single enzyme grabs carbon dioxide from the air and attaches it to a small sugar molecule, building carbon-based compounds the plant uses to grow. That enzyme, called RuBisCO, is one of the most abundant proteins on Earth.⁴ It is also famously slow and error-prone, sometimes grabbing oxygen instead of carbon dioxide, which wastes energy. Plants have spent hundreds of millions of years evolving workarounds for RuBisCO’s limitations.

Two of the most well-known workarounds are C4 and CAM photosynthesis. In C4 plants like corn and sugarcane, carbon dioxide is first captured in one type of cell and then concentrated around RuBisCO in another, reducing its error rate. In CAM plants like cacti and succulents, carbon dioxide is captured at night when the pores on the leaf surface can open without losing as much water to evaporation, then stored for daytime use. These were long considered completely separate strategies, but researchers studying the plant Portulaca oleracea found that C4 and CAM can be fully integrated in the same cells, with CAM-generated compounds feeding directly into the C4 cycle during drought.⁵ The boundaries between photosynthetic strategies are blurrier than textbooks once suggested.

Moving Water Up and Sugar Down

A tree can be dozens of meters tall, with roots underground and leaves in the canopy. Getting water from one end to the other without a pump or a heart sounds impossible, but plants manage it through elegant physics. The dominant explanation is the cohesion-tension theory: as water evaporates from leaf surfaces (a process called transpiration), it pulls on the continuous column of water in the plant’s xylem, the network of tube-like dead cells that runs from roots to leaves. Water molecules cling to each other tightly enough that the column does not break, so the pull at the top draws water up from the roots. Recent work suggests the full picture may involve additional mechanisms, including contributions from living tissue surrounding the xylem, but cohesion-tension remains the central framework.⁶

Moving sugar is a separate challenge. Photosynthesis produces sugars in the leaves, but every other part of the plant, including roots, flowers, and fruits, also needs energy. Plants transport sugars through a different set of tubes called the phloem. The leading explanation, the Münch hypothesis, says that loading sugar into the phloem at the leaf end draws in water by osmosis, creating pressure. That pressure pushes the sugar-laden fluid through the phloem toward tissues that need it. Studies on morning glory showed that as the transport distance increased, both phloem pressure and conductivity rose accordingly, providing strong support for pressure-driven long-distance transport.⁷

How Plants Grow Without Muscles

Plant cells are enclosed in rigid cell walls, which is why plants hold their shape without a skeleton. But rigidity creates a problem: how does a cell encased in a stiff box expand? The answer is turgor pressure. The cell absorbs water, swells, and pushes outward against its wall. Growth happens when the wall loosens just enough to yield to that internal pressure, allowing the cell to stretch.⁸ The wall then re-stiffens in its new, expanded state. This is why a wilted plant perks up after watering: the cells refill, regain their turgor, and the tissues become rigid again.

Controlling where and when cells grow is how plants develop shape. A seedling stem grows upward, roots grow downward, and leaves flatten out, all without the cell migration that animal embryos rely on. Instead, plant cells are cemented in place and can only divide and expand. All the sculpting happens by controlling which cells divide, in which direction they elongate, and how much they expand. The hormone auxin plays a central role in directing this. When auxin accumulates on one side of a stem, the cells there grow faster, causing the stem to bend toward light or away from gravity.⁹

Gatekeepers on Every Leaf

Scattered across the surface of nearly every land plant leaf are tiny pores called stomata. Each stoma is flanked by a pair of guard cells that can swell or shrink to open or close the pore. When open, stomata let carbon dioxide in for photosynthesis and let water vapor out through transpiration. The evolution of stomata was a pivotal innovation: together with a waxy cuticle and an internal water-conducting system, stomata allowed the earliest land plants to manage water loss while still exchanging gases with the atmosphere, enabling plants to colonize dry land and diversify across the planet.¹⁰

During drought, a plant hormone called abscisic acid (ABA) surges, triggering a signaling cascade in guard cells that causes stomata to close.¹¹ This reduces water loss but also cuts off the carbon dioxide supply, so the plant essentially holds its breath. The trade-off between conserving water and taking in carbon is one of the most fundamental tensions in plant life, and every land plant is constantly negotiating it.

Sensing Light and Measuring the Seasons

Plants cannot see, but they are remarkably sophisticated light sensors. They carry families of photoreceptor proteins that detect different wavelengths. Phytochromes sense red and far-red light, while cryptochromes sense blue and ultraviolet-A light. These receptors do far more than orient a plant toward the sun. They measure day length, which is how many plants decide when to flower. In the model plant Arabidopsis, the blue-light receptor cryptochrome 2 acts as a day-length sensor: its protein levels show a daily rhythm under short days but not under long days, driven by light-dependent breakdown of the protein.¹² Plants mutant for this receptor flower late and are only partially responsive to changes in day length.¹³

This explains why the same plant flowers in spring in one latitude but in summer in another, and why artificial lighting near a field can confuse crop timing. The plant is not responding to temperature or some vague sense of “season.” It is literally counting the hours of light and dark each day, using molecular machinery that has been fine-tuned over hundreds of millions of years.

Chemical Conversations Between Plants

Plants may not talk, but they do send chemical messages. When a plant is attacked by a chewing insect, the damaged tissue releases volatile organic compounds into the air. Undamaged leaves on the same plant can detect these airborne chemicals and ramp up their own defenses before the herbivore reaches them. In lima bean plants, researchers showed that undamaged leaves exposed to volatiles from damaged leaves on the same shoot increased their secretion of extrafloral nectar, a substance that attracts predatory insects that eat herbivores. When the volatile signal was experimentally removed, the defensive response disappeared.¹⁴

Neighboring plants can eavesdrop on these signals too. Citrus trees exposed to volatile compounds emitted by nearby guava plants showed stronger defenses against a damaging insect pest. The guava volatiles triggered changes in the citrus trees’ defense-related pathways, boosted the accumulation of defensive hormones, and even attracted parasitic wasps that prey on the pest.¹⁵ The guava plants were not even under attack themselves; they were constitutively releasing these compounds. The citrus trees were, in effect, using the chemical background of their neighborhood as an early-warning system.

Underground Partnerships

What happens below the soil surface is just as dynamic as what happens above it. Most land plants form symbiotic relationships with mycorrhizal fungi, threadlike organisms that colonize root tissue and extend far out into the surrounding soil. The fungi are much better than roots at extracting mineral nutrients, especially phosphorus, from the soil, and they deliver these to the plant. In return, the plant feeds the fungi with sugars produced by photosynthesis. These fungal networks can connect multiple plants, forming what is sometimes called a common mycorrhizal network through which nutrients and signaling compounds can be exchanged.¹⁶

Legumes, the family that includes beans, peas, and clover, have an additional underground partnership. Their roots host nitrogen-fixing bacteria called rhizobia, which live inside specialized root structures called nodules. These bacteria convert atmospheric nitrogen gas into ammonium, a form of nitrogen the plant can use to build proteins and DNA. The plant provides the bacteria with carbon in return.¹⁷ This is why farmers have rotated legumes with other crops for centuries: the legumes enrich the soil with nitrogen, reducing the need for fertilizer.

How Plants Fight Back

Plants cannot run from a threat, so they have evolved a sophisticated immune system. It operates on two main levels. The first is a broad surveillance system: receptors on the plant cell surface detect molecular patterns common to many microbes, triggering a baseline defensive response that slows down most would-be invaders. Successful pathogens get past this layer by injecting proteins into plant cells that suppress the initial defense. The second level responds to that sabotage: intracellular receptors detect the disruption caused by these pathogen proteins and mount a much stronger, localized defense that can stop the infection entirely.¹⁸ This strong local response often involves deliberately killing the plant’s own cells at the infection site, sacrificing a small patch of tissue to prevent the pathogen from spreading.

Beyond immune receptors, plants deploy a vast arsenal of chemical weapons. Secondary metabolites, compounds that are not directly involved in growth or reproduction, serve as toxins, feeding deterrents, and signals that recruit the natural enemies of herbivores.¹⁹ The major classes include alkaloids (think caffeine and nicotine), terpenoids (the scents of pine and mint), and phenolics (the astringent tannins in tea). Some of these are always present in the tissue, acting as a standing garrison. Others are manufactured on demand when the plant detects damage.²⁰ Many of the flavors, aromas, and medicinal properties humans value in plants originally evolved not for our benefit but to poison or repel something trying to eat the plant.

Sex, Seeds, and Double Fertilization

Plant reproduction is wildly varied. Mosses and ferns still rely on water for fertilization: their sperm physically swim to reach the egg cell, which is part of why these plants tend to live in moist habitats. The evolution of pollen freed seed plants from this constraint. A pollen grain is a tiny, tough package that can travel by wind or animal to reach a compatible flower, germinate a pollen tube, and deliver sperm directly to the egg without needing a film of water.

Flowering plants added another twist: double fertilization. When a pollen tube reaches the female part of a flower, it releases two sperm cells. One fuses with the egg cell to form the embryo. The other fuses with a second female cell, the central cell, to form the endosperm, a nutrient-rich tissue that feeds the developing embryo inside the seed.²¹ This is unique to flowering plants and is one of the reasons they dominate most terrestrial ecosystems. The endosperm is also, incidentally, what makes wheat flour, corn meal, and rice nutritious for humans: we are eating the food the plant packed for its embryo.

Early land plants had life cycles very different from what we see today. Fossil evidence shows that some of the earliest vascular plants had gametophyte generations (the phase that produces eggs and sperm) that were nearly as large and complex as the sporophyte generation (the phase that produces spores), a striking departure from modern ferns and seed plants, where the gametophyte is typically tiny or hidden inside the flower.²² The trend over evolutionary time has been toward the sporophyte becoming dominant and the gametophyte shrinking, but that was not where the story started.

Plants That Break the Rules

Not all plants play by the standard photosynthesis-and-soil-minerals playbook. Carnivorous plants like sundews and Venus flytraps supplement their nutrition by capturing and digesting insects. Isotopic analysis shows that carnivorous plants register at a trophic level consistent with organisms feeding on animal diets, around 2.1, compared to the 1.0 typical of ordinary photosynthetic plants.²³ They are still photosynthetic, still making their own sugars from sunlight, but they get their nitrogen from prey rather than from the soil. This strategy tends to evolve in nutrient-poor environments like bogs, where the soil cannot supply enough nitrogen or phosphorus for normal growth.

Parasitic plants take a different approach: they tap directly into the vascular systems of other plants and steal water, sugars, or both. Some, like mistletoe, still photosynthesize and only steal water and minerals. Others, like dodder, have lost chlorophyll entirely and depend on their host for everything. Mycoheterotrophic plants take this even further, parasitizing the mycorrhizal fungi that partner with other plants and extracting nutrients through the fungal network rather than photosynthesizing at all. Despite their radically different lifestyle, isotopic data place these parasitic-mycoheterotrophic plants at the same trophic level as ordinary photosynthesizers, suggesting they draw on fungal-derived ammonium rather than consuming animal tissue.²³

Stress Memory Without a Brain

One of the more surprising discoveries in plant biology is that plants can “remember” stressful experiences and respond more effectively to repeated stress. This is not memory in the animal sense, with neurons storing information. Instead, it involves epigenetic changes: chemical modifications to DNA and the proteins that package it, which alter how genes are read without changing the genetic code itself. These modifications can persist through cell divisions, meaning that a plant cell’s descendants can inherit a changed pattern of gene activity from a stress that happened days or weeks earlier. In some cases, the changes can be transmitted to the next generation through seeds, giving offspring a head start in dealing with the same stress their parent faced.

This phenomenon, sometimes called “priming,” has practical implications. A plant that has been mildly stressed by drought, for instance, may close its stomata faster and lose less water when the next dry spell arrives. The volatile signaling described earlier feeds into this as well: undamaged leaves exposed to airborne alarm chemicals from wounded neighbors do not just mount an immediate defense, they become primed to respond more strongly to any future damage.¹⁴ Agriculture is beginning to explore whether deliberate, mild stressing of crops early in their growth can improve their resilience later, essentially training plants the way you might train an immune system with a vaccine. The science is still young, but the principle that plants are not passive, static organisms is now thoroughly established. They sense, respond, communicate, defend, and adapt, all without a single nerve cell.