Defensive Measures Prevent Toxicity of Heavy Metals

close up photograph of green plant in a petri dish

Biological Strategy

Thale cress

AskNature Team

Image: HeitiPaves / iStock / Some rights reserved

Regulate Cellular Processes

Cells are the basic building blocks of all living systems, so cellular processes dictate how physiological processes occur within those systems. Cells (whether entire unicellular organisms or parts of multicellular living systems) grow, metabolize nutrients (that is, chemically transform them), produce proteins and enzymes, replicate, and move. Cells as part of multicellular systems rarely act alone, instead having ways to signal to start and complete simple to quite complex interactions. How skin heals is a good example of the role of cellular processes. Blood cells called platelets release clotting factors to stop the bleeding; white blood cells rid the area of foreign materials and release molecules to coordinate healing; cells called fibroblasts start rebuilding using proteins called collagen; new blood vessels form; and skin cells called keratinocytes create the new surface.

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Protect From Chemicals

Chemicals are everywhere in the bodies of living organisms and their external environments. While most chemicals are valuable or benign, some are toxic, including those used for defense (such as the mucus that protects clownfish from an anemone’s stinging tentacles). Even naturally-occurring chemicals, such as arsenic, must be managed to reduce their impact. Some living systems have strategies to break down harmful chemicals, alter them into less toxic forms, physically prevent chemicals from harming sensitive tissues, and more. For example, some herbivorous mammals can digest toxic compounds in plants because they have a particular enzyme that helps them process poisonous plant compounds.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Cells of thale cress protect themselves from dissolved heavy metals by releasing compounds that bind the metal ions or neutralize the destructive substances they spawn.

When exposed to toxic heavy metals like nickel, lead, cadmium, and mercury in the soil, plants initiate a complex sequence of steps to counteract the effects of these poisons. Defensive measures prevent metals from getting inside cells, sequester and neutralize metals that do enter the cell, or mitigate damage caused by the metals that overwhelm these defenses.

Two mechanisms keep metal ions out of plant cells. In one case, the plant releases chelating agents into the soil which form molecular “cages” around the metal ions making them too bulky to fit through slim ion channels in the cells’ protective outer cell wall. Alternatively, compounds in the cell wall material (hystidyl groups, pectic sites, and certain carbohydrates) bind to the metal ions keeping them away from the cells’ internal machinery. Of course, these preventative defensive measures can be overwhelmed by high concentrations of the metals so other means of protection are required. When metal ions do get into the cell, efflux pumps (heavy metal ATPases and HMAs) actively pump metal ions out of the cells and back into the soil or into isolated spaces within the cell (vacuoles) where they are effectively sequestered. If concentrations of the metals are particularly high, the plant cells will pump the metals into its vascular network so that they can be carried to the shoots where they become imprisoned in vacuoles. Metal ions that do get past these defenses can destroy plant cells by producing reactive compounds such as hydrogen peroxide. Presence of these damaging compounds trigger cells to produce defense chemicals that, in turn, scavenge or destroy them.