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Tolerance Limit Deviation

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Christian Bien Portrait_edited.jpg

Ben Whitten

Learning Objectives


What are the effects of tolerance limit deviation?

Deviation outside an organism's tolerance limits can affect cell function and general health. Other harmful effects occur when key factors exit the tolerance range.

The key factors include:

  • Temperature

  • Nitrogenous waste

  • Water

  • Salts

  • Gases

The key factors must remain in tolerance ranges to allow for normal cell function and reproduction.


High Temperatures

A small increase in temperature increases enzyme activity and metabolic rate; however, a large increase in temperature causes enzymes to denature, creating critically slow metabolism and the potential for cell death.

When the human body, for example, reaches temperatures of up to 39.2 degrees Celsius, this is harmful;

  • A decline in cognitive skills

  • Heat stress

  • Heat exhaustion

  • Heatstroke (up to 39.6 degrees Celsius)

An extreme increase in temperature causes animal cell membranes to become too fluid, allowing unwanted substances to flow into cells and/or necessary substances out of cells. Photosynthesis in plant cells can slow, impacting plant growth and productivity.

Low Temperatures

A decrease in temperature outside the optimal range,  a decrease in enzyme activity results and metabolic rate decreases. At low temperatures, cell membranes become rigid and transport processes slow.

A decrease in temperature outside the tolerance range results in hypothermia or loss of limbs. Organisms may survive but cannot reproduce until temperatures return to their tolerance range.

Nitrogenous Waste

Nitrogenous Waste Breakdown

Proteins and nucleic acids (containing nitrogen) are necessary for survival. When proteins and nucleic acids break down, ammonia forms; a highly toxic nitrogenous waste. Terrestrial animals convert this to urea but still requires dissolving in water and excretion due to moderate toxicity.

As nitrogenous wastes increase in concentration, they become more toxic.

Increased ammonia in the blood increases pH level; enzymes need a particular tolerance range to function, and hence when cellular pH leaves the optimal range, enzyme activity decreases. When cellular pH exits the tolerance range, enzymes denature resulting in critically slow metabolism.

Increased nitrogenous waste levels affect water balance. Animal cells may lose water to the external environment to dilute the waste for pH homeostasis.


Water: The Universal Solvent

Water is the universal solvent; water dissolves salts and minerals, breaking salts down into ions. Sodium chloride (NaCl) for example can dissociate into Na⁺ and Cl⁻ ions, and the ions can be used for metabolic reactions. Water is usually the medium for metabolic reactions. An increase in water content decreases collision rates of reactants involved in biochemical pathways, slowing metabolism.

Hypo-, Hyper- and Isotonic Solutions

The movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration is called osmosis.

Increasing water content above tolerance ranges leads creates a hypotonic solution surrounding cells in blood and tissues. Water moves into cells down a concentration gradient to restore equilibrium. If too much water enters the cells, animal cells swell and burst (cell lysis). Solute concentration can be too low if cells swell, decreasing collisions of reacting particles and reducing rates of reactions. 

Decreasing water content outside tolerance ranges affects the regulation of solute concentrations, e.g., salts. A hypertonic solution surrounding cells in blood and tissues leads to dehydration as water exits cells via osmosis; animal cells shrink and plant cells undergo plasmolysis. Ions are unable to move reaction sites at a sufficient rate, reducing metabolism.

When cells are surrounded by fluid of the same water concentration, the surrounding solution is described as isotonic; there is no net movement of water.


Salt Ions

Salts dissociate into ions like sodium and calcium, and are concentrations are required to be in  a narrow range for normal functions of;

  • Muscles

  • Neurons (nerve cells)

  • Other body cells

Calcium triggers muscles to contract. Decreased calcium levels in the blood lead to the cramping of muscles in the legs and back.

Increasing external salt concentration may cause osmosis to occur, as water is transported out of cells; cell shrinkage and dehydration result. Salts require regulation for water balance. 

Low blood sodium levels affect water balance, blood pressure and nervous system function. If sodium concentration is too low outside cells, water moves into the cells, the cells swell with too much water, leading to;

  • Weakness

  • Fatigue

  • Confusion

Gas Concentrations

Carbon Dioxide Concentrations

(Note*: pH is a measure of how acidic or alkaline an aqueous solution is.)

Carbon dioxide is a waste product of respiration. Carbonic acid forms when dissolved in water, and further dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO⁻ ₃). Increased carbon dioxide concentration creates a high concentration of hydrogen ions in water, reducing the pH of the blood, and making it more acidic.

Increased carbon dioxide concentrations above the tolerance range increase blood pH, becoming more acidic and affecting enzyme activity/denaturing enzymes.

Decreased carbon dioxide concentrations reduce ventilation/breathing rate in animals, and reduce the rate of photosynthesis in plants.

Oxygen Concentrations

Living cells need a continuous supply of oxygen for cellular respiration to produce energy.

Increased oxygen concentrations above the tolerance range in animals result in cell damage, nausea, dizziness and breathing problems.

Reduced oxygen concentrations reduce the respiration rate and consequently the rate of ATP production (energy).