A source is any part of a system that produces or releases a substance, while a sink is any part that absorbs, stores, or consumes it. These complementary terms appear across biology, environmental science, and physics, but they always describe the same basic relationship: material or energy flows from a source to a sink. The most common contexts are plant biology (where sugars move from leaves to fruits and roots) and climate science (where carbon moves between the atmosphere, oceans, and land).
Source and Sink in Plant Biology
In plants, “source” and “sink” describe how sugars get distributed from the organs that make them to the organs that need them. A source is any tissue that produces more sugar than it uses, primarily through photosynthesis. Mature leaves are the main source organs in most plants, and up to 80% of the carbon they fix through photosynthesis gets exported to other parts of the plant.
A sink is any tissue that consumes or stores those sugars rather than producing its own. Which organs act as sinks changes over a plant’s life cycle. During early growth, roots and young developing leaves are the major sinks. Once the plant shifts to reproduction, fruits, seeds, and storage organs like tubers take over as the dominant sinks, all competing for the available sugar supply.
The interesting part is that a single organ can switch roles. A young leaf starts as a sink, importing sugar to fuel its own growth. Once it matures and begins photosynthesizing efficiently, it becomes a source that exports sugar to the rest of the plant. A potato tuber acts as a sink while it’s growing and storing starch, but if that tuber sprouts the next season, it becomes a source, releasing its stored energy to feed new shoots.
How Sugars Actually Move Between Them
Plants transport sugars through the phloem, a network of tube-like cells that runs from leaves to roots and everything in between. The mechanism that drives this flow, first proposed by Ernst Münch in 1930, relies on pressure differences rather than a pumping organ like a heart.
At the source end, cells actively load sugar into the phloem tubes. This high sugar concentration draws in water through osmosis, building up pressure inside the tubes. At the sink end, cells pull sugar out of the phloem for use or storage, which lowers the sugar concentration and reduces pressure. The result is a pressure gradient: high at the source, low at the sink. Fluid flows along this gradient without requiring any additional energy input along the way, carrying dissolved sugars from where they’re made to where they’re needed.
This loading process at the source end takes energy and can happen through two main routes. In some plants, sugars pass directly between connected cells through tiny channels. In others, sugars are first released into the space outside cells and then pumped into the phloem by specialized transport proteins. The route a plant uses depends largely on its species and the types of sugars it transports.
Source and Sink in Climate Science
In the carbon cycle, the same logic applies at a planetary scale. A carbon source is anything that releases carbon dioxide into the atmosphere. A carbon sink is anything that pulls it back out and stores it.
The largest anthropogenic carbon source is fossil fuel combustion. Electricity and heat production alone accounts for 34% of global greenhouse gas emissions. Industry adds another 24%, agriculture and land use changes contribute 22%, and transportation accounts for 15%. These are all sources because they transfer carbon that was locked underground (in coal, oil, and gas) or in biomass back into the atmosphere.
Natural sources exist too. Volcanic eruptions, wildfires, and the decomposition of dead organisms all release carbon dioxide. But these natural flows were roughly balanced before industrialization because natural sinks absorbed about as much carbon as natural sources released.
Where the Carbon Goes
Of the carbon dioxide humans emit, roughly 25% is absorbed by the ocean and just under 30% is taken up by land ecosystems like forests and soils. The remaining 45% or so stays in the atmosphere, which is why atmospheric CO2 concentrations keep climbing despite these enormous natural sinks.
The ocean acts as a giant carbon sink, dissolving CO2 directly from the air. Coastal ecosystems are particularly efficient at this. Seagrasses, mangroves, and salt marshes capture what’s known as “blue carbon,” storing it in sediments where it can remain for centuries. When these coastal systems are damaged or destroyed, that stored carbon gets released back into the atmosphere, effectively converting a sink into a source.
On land, forests are the most significant sink. Trees pull CO2 from the air during photosynthesis (acting as sources and sinks at the individual organ level, just like all plants) and lock carbon into wood, roots, and soil. Deforestation reverses this process. A standing forest is a sink; a burned or cleared forest becomes a source.
Source and Sink in Physics
In thermodynamics and engineering, the terms describe heat flow. A heat source is any object or process that generates thermal energy. A heat sink is a device or material that absorbs and dissipates that heat. The classic example is a computer processor: the chip is the heat source, and the metal fin assembly mounted on top of it is the heat sink.
Heat sinks are typically made from materials with high thermal conductivity, like aluminum or copper, and are designed with fins or other structures that maximize surface area. The principle is passive: heat naturally flows from a hotter object (source) to a cooler one (sink), and the sink’s job is to spread that heat out and transfer it to the surrounding air or water as efficiently as possible. This same source-to-sink logic governs everything from power plant design to refrigeration systems.
The Common Thread
Across all these fields, source and sink describe a directional flow. Something is produced, released, or generated at the source. Something is consumed, absorbed, or stored at the sink. The gradient between them, whether it’s sugar concentration in a plant, CO2 in the atmosphere, or temperature in an electronic device, is what drives the movement. And in every case, the balance between sources and sinks determines whether a system stays stable or shifts toward accumulation or depletion.