This week, our learning focuses on the topic of toxicity.  It is natural to wonder whether or not something might be harmful.  From life experience, we know that things we eat, drink, breathe, or touch can injure us or make us sick.  Our bodies are constantly working hard to keep out things that might harm us. Examples include:

• Our skin forms a protective barrier to keep the inside in and the outside out.
• Our mucous membranes are sticky and “catch” bacteria, viruses, and other particles that we inhale.
• Our liver filters our blood, removing toxins from anything we might eat through the collective actions of dozens of enzymes within the Cytochrome P450 family.
• Our innate immune system owes its existence to our evolutionary ancestors.  The innate immune system consists of a variety of molecules that detect common environmental pathogens, like bacterial cell wall components or viral DNA – structures not found in our bodies and therefore indicate an infection.  One important family of cell surface receptors, the Toll-like Receptors (TLRs) were originally discovered in fruit flies, and then scientists found them in humans as well!
• Our adaptive immune system protects us from new threats.  For example, we are currently under attack by COVID-19, a coronavirus never before encountered by humans.  While our innate immune system may recognize aspects of the virus as a threat (resulting in flu-like symptoms), it’s our adaptive immune response that takes a few weeks to really get going.  Our B cells will produce antibodies against the virus, killing it. Our T cells will recognize virus-infected cells, and our T cells will kill those cells, thus preventing the spread of the virus.

After browsing through the links provided, return to the Week 31 – Toxicity post.

For this lesson, we need to develop a basic understanding of how toxins can be compared.  We know that bites from snakes and spiders can sometimes be deadly.  But why only sometimes?  We know eating some berries can be deadly, but not all berries are toxic…or are they?  Can anything be toxic if encountered in large enough amounts?

To talk about toxicity, we need to understand the meaning of LD₅₀ (the lethal dose at which 50% of a population of organisms die after exposure to a given amount of a substance).  LD₅₀ is often expressed in mg/kg, which means the number of milligrams (mg) of a substance per kilogram (kg) of organism body mass.  Before we work through an example, take a look at the Lethal Doses Handout.  The first entry is for aspirin (acetysalicylic acid)  Aspirin has an LD₅₀ of 200 mg/kg when fed orally to a rat.

Important Concept: Rats are commonly used for toxicity studies as the way rats metabolize chemicals is similar enough to humans for rats to be a good model organism for predicting toxicity in humans.  A typical lab rat has a mass of up to 0.5 kg, while a typical human may have a mass of 70 kg (equivalent to 154 pounds, 1 kg = 2.2  pounds, or lbs).  You can imagine an experiment where a scientists feeds aspirin in increasing amounts to various groups of rats.  All of the rats have a mass of 0.5 kg.  The first group of rats eats 1 mg of aspirin and the rats all live.  The next group eats 3 mg of aspirin and the rats all live.  Eventually, a group of rats eats 100 mg of aspirin and half of the rats die.  The scientist just found the LD₅₀ of aspirin: 100 mg of aspirin / 0.5 kg of rat body mass = 200 mg/kg.

To figure out how to apply the LD₅₀ of 200 mg/kg to a human with a mass of 70 kg, we need to multiply the LD₅₀ by 70 kg:

200 mg/kg x 70 kg = 14,000 mg aspirin

Aspirin tablets often come in 81 mg doses.  To figure out how many 81 mg aspirin tablets a person would need to eat to have a 50% chance of dying (assuming a mass of 70 kg), we need to divide 14,000 mg by 81 mg/tablet:

14,000 mg x 1 tablet / 81 mg = 172.8 tablets, (about 173 tablets)

To answer the question of how much is too much when it comes to aspirin, taking more than the recommended dose per day is too much.  Anyone contemplating taking 173 tablets of aspirin needs to get help first!

Additional important information:

• The lower the LD₅₀, the more toxic the substance.
• 1000 milligrams (mg) = 1 gram (g)
• 1000 micrograms (mcg, or μg) = 1 mg
• Water has an LD₅₀ of 90,000 mg/kg = 90 g/kg = 90 mL/kg (water has a density of 1 g/mL) – yes, even drinking too much water can be toxic!  Check out the Material Safety Data Sheet (MSDS) for water.  How much water is toxic to a 70 kg person?  90 mL/kg x 70 kg = 6300 mL = 6.3 L, or a bit over three 2-liter bottles of water.

Before continuing on, complete the Toxicity Exit Task.  Then return to the Week 31 – Toxicity Post.

Welcome to Week 31!  For this week, we will be focusing our learning on the subject of toxicity.  In other words, how much is too much?  Please work through the list of links below.  Each section contains important information and ends with a portion of the weekly assignment.  You can complete it all in one sitting or break it up as needed.  Ready, set, go!

1. Toxicity Entry Task (Entry Task)
2. How do our bodies protect us from toxins?
3. Calculating Toxicity (Exit Task)
4. Bringing it all together (Lesson 74 Exercises)
5. Chemistry Refresher…review now before it’s too late!
6. Unit 4 Honors Project…the wait is over!

You did it!  Just to make sure, here’s a checklist of items you must complete this week by Sunday, April 26 at 11:59pm:

• • Toxicity Entry Task (worth 5 assignment points)
  • Toxicity Exit Task (worth 5 assignment points)
  • Lesson 74 textbook exercises (worth 20 assignment points)

Remember, you can email me any time.  Office hours for Science are Tuesdays from 11am-12pm and Thursdays from 1pm-2pm.  Check your student Gmail for Zoom instructions.

Finally, by popular demand…click here for the Week 31 Bonus Credit Opportunity!

Welcome back from Spring Break!  I hope you are all rested and excited to engage in school via distance learning for our final quarter of the school year.  Please work through the list of links below.  Each section contains important information and ends with an assignment, a quiz, or both.  You can complete it all in one sitting or break it up as needed.  Ready, set, go!

1. Let’s Talk Logistics
2. Balancing Equations
3. Types of Chemical Reactions (end of Week 30 required learning)
4. Chemistry Refresher…review now before it’s too late!
5. Unit 4 Honors Project…the wait is over!

You did it!  Just to make sure, here’s a checklist of items you must complete this week by Sunday, April 19 at 11:59pm:

• Let’s Talk Logistics Quiz (worth 7 quiz points)
• Balancing Equations Gizmo (worth 10 assignment points)
• Types of Chemical Reactions Assignment (worth 10 assignment points)

Finally, by popular demand…click here for the Week 30 Bonus Credit Opportunity!

Over the next few weeks, we will be learning about toxicity (how much is too much?) and integrating the concept of moles into the process of balancing equations (stoichiometry).  The links below will provide a refresher on the core concepts we have learned about this year that you need to understand in order to be successful going forward.  Self-assess your needs and review what you know you need to review.

• Valence and Core Electrons
• Ions, Ionic Compounds, and Formulas
• Polyatomic Ions
• Transition Metal Chemistry
• Electronegativity Scale
• The Mole and Avogadro’s Law