A suspension in biology is a mixture where particles, such as cells or other solid material, are dispersed throughout a liquid but not dissolved in it. Unlike a true solution where everything blends at the molecular level, the particles in a suspension are large enough to eventually settle out if left undisturbed. This concept shows up across biology, from the way blood cells float in plasma to how researchers grow cells in a lab.
How a Suspension Differs From a Solution or Colloid
The key distinction comes down to particle size and behavior. In a true solution, dissolved particles are extremely small (under 1 nanometer) and never separate on their own. A colloid contains mid-range particles (1 to 1,000 nanometers) that stay dispersed and scatter light, a phenomenon called the Tyndall effect. A suspension contains the largest particles, over 1,000 nanometers, and those particles will settle to the bottom over time if nothing keeps them mixed.
Suspensions are also the easiest to separate mechanically. You can filter suspended particles out of a liquid, which you can’t do with a solution or colloid. And while colloids scatter a beam of light in a visible path, suspensions may scatter light or simply look opaque, depending on how dense and large the particles are. In biology, this matters because cells, microorganisms, and tissue fragments all fall into the suspension size range.
Blood: The Classic Biological Suspension
Blood is the most familiar example of a biological suspension. Plasma, the liquid portion, makes up more than half of blood’s volume and consists mostly of water with dissolved salts and proteins. Red blood cells, white blood cells, and platelets are all suspended in that plasma. Red blood cells alone account for about 40% of blood’s total volume.
These cells don’t dissolve into plasma the way sugar dissolves in water. They remain as distinct, intact particles floating in the liquid medium. And true to the definition of a suspension, they will settle if blood is left standing in a tube. This settling behavior is actually the basis of a common medical test.
The Sedimentation Rate Test
The erythrocyte sedimentation rate, or ESR, measures how quickly red blood cells sink to the bottom of a blood sample over one hour. Under normal conditions, red blood cells settle slowly. When inflammation is present in the body, certain proteins increase in the blood, causing red blood cells to clump together and form heavier aggregates that sink faster.
Normal ESR values depend on age and sex. For men under 50, a normal result is less than 15 millimeters per hour. For women under 50, it’s less than 20 mm/hr. After age 50, the upper limit rises slightly for both groups. A high ESR doesn’t point to a specific disease, but it signals that inflammation is happening somewhere. Doctors often use it alongside other tests to monitor conditions like infections, autoimmune disorders, or chronic inflammatory diseases.
What Controls How Fast Particles Settle
Whether you’re looking at blood cells in plasma or algae in ocean water, the same physics governs settling speed. A principle called Stokes’ law describes the relationship: sedimentation rate depends on particle size, the density difference between the particle and the surrounding fluid, and the fluid’s viscosity (how thick or resistant to flow it is).
Size has an outsized effect. Even small increases in a particle’s radius produce large, quadratic jumps in how fast it sinks. A cell that doubles in diameter doesn’t just settle twice as fast; it settles roughly four times faster. Density matters too: particles sitting near the buoyancy threshold, where they’re almost the same density as the fluid around them, can shift from floating to sinking with only tiny changes in their composition. Research on single-celled organisms closely related to animals found over 300-fold variation in sedimentation rates across species, driven primarily by differences in cell size and density during different life stages.
Suspension Cultures in the Lab
In biotechnology and cell biology, “suspension culture” refers to growing cells that float freely in a liquid nutrient medium rather than attaching to a surface. Some cell types naturally prefer to grow this way, while others must be adapted to it. The practical advantage is scalability. Suspension cultures can be grown in large stirred-tank reactors as closed systems with automated monitoring, producing far higher cell yields for a given physical footprint compared to cells that need to attach to flat surfaces.
This approach is central to modern drug manufacturing. CHO cells (a workhorse cell line originally derived from hamster ovaries) grown in suspension are widely used to produce monoclonal antibodies, which treat conditions ranging from cancer to autoimmune diseases. Insect cells grown in suspension have enabled rapid vaccine production, including recombinant protein vaccines against viruses like human papillomavirus. The tradeoff is that adapting cells to suspension growth requires significant upfront time and optimization, but the efficiency gains at production scale are substantial.
Suspensions in Drug Formulation
Many medications are formulated as suspensions rather than solutions for a straightforward reason: some drug compounds simply won’t dissolve in water. When an active ingredient has poor water solubility, pharmacists suspend fine particles of it in a liquid medium instead. This is especially important for pediatric medications, where patients need a liquid form but the drug itself resists dissolving at the required concentration.
Pharmaceutical suspensions typically use very fine particles, usually smaller than 25 microns, to slow settling and improve consistency. The main challenge is uniformity. Because the drug particles can separate during storage or during the manufacturing process, these formulations need careful quality control. This is why liquid medications that are suspensions carry the familiar instruction to “shake well before use,” redistributing the settled particles so each dose contains the right amount of drug.
Suspensions in Aquatic Environments
In ecology, suspended particles in water, often called suspended solids or suspended sediment, play a major role in aquatic ecosystems. Soil runoff, stirred-up bottom sediment, and organic debris all create natural suspensions in rivers, lakes, and coastal waters. These particles reduce how deeply light penetrates the water, which directly affects photosynthesis by aquatic plants and phytoplankton.
For aquatic animals, suspended sediment affects respiration and feeding. Fish and bivalves like mussels show sublethal stress responses (changes in breathing rate and enzyme activity) at concentrations below 100 milligrams per liter, a level commonly reached near construction sites, dredging operations, or after heavy storms. Phytoplankton tend to tolerate much higher concentrations, often exceeding 1,000 mg/L before showing measurable effects. The biological impact depends not just on concentration but on particle size, duration of exposure, and the species involved.