Goo, gunge, gunk... while there are many names for the stuff that makes things slippery or sticky, slime isn’t a single material but a label for a variety of substances with similar physical properties.
Those qualities are desirable to many living things, which is why slime is made by such a wide range of organisms.
What is slime?
There’s no exact definition: the word reflects characteristics. So you could say (perhaps unhelpfully!) slime is any substance that feels slimy – it’s something that will slip or ooze from your hand when you hold or squeeze it, for instance.
What is mucus?
The best-known slime is mucus, a material that covers many external and internal body surfaces. Mucus keeps an amphibian’s skin moist, for example, and prevents the lining of a mammal’s lungs from drying out. Technically a viscous (thick) liquid, mucus doesn’t flow in its steady state and behaves like a solid – it’s a gel.
How does mucus protect animals?
The gel’s main function is to serve as a barrier that lines a surface to protect an animal’s outermost cells from infection or damage. In the human body, snot (nasal mucus) captures invaders such as viruses, while the mucus within the digestive system stops your stomach digesting itself.
All complex creatures secrete mucus and, importantly, the gel is a selective filter: it traps disease-causing microbes (pathogens) while housing a body’s inner ecosystem of friendly bacteria. So a mucosal surface extends an animal’s innate immune system and supports its microbiome.
How else is mucus used?
It can be a dedicated lubricant. Bony fish are covered in a thin layer of slick mucus or ‘slime coat’ that helps to smooth the rougher texture of scales (which improves hydrodynamics for swimming), while gastropods (such as slugs and snails) exploit mucus to reduce friction during locomotion, which leaves a slime trail.
Mucus can serve as a glue, too, enabling molluscs to climb walls or travel upside-down. In fact, garden snails make three distinct secretions: lubricating, adhesive and protective mucus. The qualities are different due to the molecules they contain.
What is mucus actually made of?
The key ingredients are molecules called mucins. Each mucin is a ‘glycoprotein’ with a long protein backbone and sugars (glycans) that radiate outward from its core, resembling a bottlebrush structure. Mucins cross-link with one another or other molecules.
These molecular interactions give mucus its properties of a gel. More specifically, mucus is a ‘hydrogel’ – once released from special ‘goblet cells’ in the epidermis, mucin molecules combine with water (hydration) and suddenly expand to several hundred times their original volume through an almost explosive process.
Do plants make slime?
Yes, though it’s not always called slime. Most plants and some single-celled organisms produce mucilage, a substance mainly composed of long sugar chains or ‘polysaccharides’. Like mucus, mucilage is a multipurpose material. While it’s often used to store energy, carnivorous plants use mucilage as a sticky glue in flypaper traps and certain bacteria make it to glide over substrates using a slime layer.
When formed by a microbial community attached to a surface, an extracellular matrix of polysaccharides is known as a biofilm. Biofilms include the plaque on your teeth and the ‘pond slime’ that grows on submerged rocks. They help buffer environmental changes that might harm the community and enable communication to coordinate behaviour so its members
