Toxicants enter organisms through various routes, including ingestion, inhalation, and skin absorption. These pathways determine how quickly and effectively harmful substances reach target organs, influencing their toxic effects.
Once inside the body, toxicants must cross cell membranes to cause damage. This happens through processes like diffusion and active transport, which depend on the chemical properties of the toxicant and cellular mechanisms.
Routes of Exposure
Oral Ingestion
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Ingestion occurs when a substance is consumed and absorbed through the digestive tract (gastrointestinal tract)
Common route of exposure for many organisms, especially those that consume contaminated food or water
Ingested toxicants can be absorbed into the bloodstream from the gut and distributed throughout the body
Factors affecting uptake include the chemical properties of the toxicant, such as solubility and lipophilicity, and the physiology of the digestive system
Inhalation and Gill Uptake
Inhalation involves the uptake of toxicants through the respiratory system, such as the lungs in terrestrial animals or gills in aquatic organisms
Toxicants can be absorbed directly into the bloodstream from the respiratory surface due to the high surface area and rich blood supply
Particle size, solubility, and volatility of the toxicant influence the extent of uptake via inhalation
Gill uptake is a major route of exposure for aquatic organisms, as gills are in constant contact with the surrounding water and have a large surface area for absorption
Dermal Absorption
Dermal absorption occurs when a substance penetrates the skin and enters the body
The skin acts as a barrier, but certain toxicants can pass through, especially those that are lipophilic or have small molecular sizes
Factors affecting dermal absorption include the concentration of the toxicant, duration of exposure, and the condition of the skin (e.g., cuts, abrasions, or inflammation can increase absorption)
Some pesticides and organic solvents are readily absorbed through the skin, posing a significant risk of exposure
Cellular Uptake Mechanisms
Passive and Facilitated Diffusion
Passive diffusion is the movement of substances across a cell membrane from a region of high concentration to a region of low concentration, without requiring energy input
Facilitated diffusion also follows the concentration gradient but involves the use of transport proteins to move substances across the membrane more efficiently
Lipophilic substances, such as many organic pollutants, can readily diffuse across cell membranes due to their affinity for the lipid bilayer
Factors affecting diffusion include the concentration gradient, membrane permeability, and the size and polarity of the molecule
Active Transport and Endocytosis
Active transport involves the movement of substances across a cell membrane against the concentration gradient, requiring energy input in the form of ATP
Transport proteins, such as pumps and carriers, selectively bind to specific molecules and transport them across the membrane
Endocytosis is the process by which cells engulf extracellular materials, including toxicants, by invaginating the cell membrane to form vesicles
Phagocytosis (cell eating) and pinocytosis (cell drinking) are forms of endocytosis that can lead to the uptake of particulate matter and dissolved substances, respectively
Some toxicants, such as heavy metals, can be actively transported into cells via specific membrane transporters, leading to accumulation within the cell
Key Terms to Review (1)
Biomagnification: Biomagnification is the process by which the concentration of toxic substances increases as they move up the food chain, affecting organisms at higher trophic levels more severely. This phenomenon connects various aspects of ecological interactions and highlights the importance of understanding how pollutants behave in ecosystems and impact wildlife health.