2.2 Data Collection, Measurement, and Lab Safety
Key Takeaways
- SI units (meter, kilogram, second, kelvin, mole, ampere, candela) are the standard measurement system; biology most often uses meters, grams, liters, seconds, and degrees Celsius.
- Accuracy is closeness to the true value; precision is repeatability among measurements. A measurement can be precise without being accurate.
- A serial dilution multiplies the dilution factor at each step: a 1:10 serial dilution across four tubes yields concentrations of 10^-1, 10^-2, 10^-3, and 10^-4 of the stock.
- GHS pictograms (flame, skull-and-crossbones, biohazard, corrosion, exclamation mark) and Safety Data Sheets (SDS) Section 4 (first aid) and Section 8 (PPE) drive emergency response in the classroom lab.
- Total magnification of a compound light microscope equals objective magnification times eyepiece magnification (typically 10x); resolution is limited by wavelength and is roughly 200 nm for light, 0.2 nm for transmission electron microscopy.
Why This Section Matters
ETS expects a Praxis Biology candidate to walk into a classroom on Day 1 and run a safe, quantitative lab. Subarea I items routinely test SI units, dilution math, Safety Data Sheet (SDS) literacy, and microscope operation. Mistakes here are also the most common cause of serious incident reports in school science labs, so the knowledge has direct professional consequence.
SI Units in Biology
The International System of Units (SI) is the metric system used worldwide in science.
| Quantity | SI Base Unit | Common Biology Examples |
|---|---|---|
| Length | meter (m) | seedling height in cm; cell diameter in micrometers (mum) |
| Mass | kilogram (kg) | organism mass in grams (g) or milligrams (mg) |
| Time | second (s) | reaction time in s or min |
| Temperature | kelvin (K) | usually reported in degrees C; K = degrees C + 273.15 |
| Amount of substance | mole (mol) | molarity in mol/L (M) |
| Volume (derived) | cubic meter (m^3) | typically liters (L) or milliliters (mL) |
Useful prefixes: kilo- (10^3), centi- (10^-2), milli- (10^-3), micro- (10^-6), nano- (10^-9). Cell biology lives mostly in mum and nm; molecular work lives in nM to mM.
Accuracy vs. Precision
- Accuracy = closeness to the true value. An archer hitting the bullseye is accurate.
- Precision = closeness of repeated measurements to each other. An archer who clusters all arrows in the same off-center spot is precise but inaccurate.
- Systematic error (an uncalibrated balance always reading 2 g high) damages accuracy.
- Random error (small fluctuations in technique) damages precision.
You can be precise without being accurate, but you cannot be reliably accurate without precision.
Significant Figures
A measurement should be reported with all digits known with certainty plus one estimated digit. A ruler marked in millimeters lets you estimate to about 0.1 mm. When multiplying or dividing, the answer carries the same number of significant figures as the least-precise input (e.g., 4.56 cm x 2.1 cm = 9.6 cm^2, not 9.576 cm^2).
Dilutions and Serial Dilutions
The core dilution formula is:
C_1 x V_1 = C_2 x V_2
where C is concentration and V is volume; subscript 1 is stock, subscript 2 is the diluted solution. Example: to make 100 mL of 0.1 M NaCl from a 1.0 M stock, V_1 = (0.1 x 100) / 1.0 = 10 mL stock, topped up to 100 mL with water.
A serial dilution chains identical dilution steps to span many orders of magnitude — essential for plating bacterial cultures, enzyme kinetics, and standard curves.
| Tube | Action | Resulting Concentration (relative to stock) |
|---|---|---|
| 1 | 1 mL stock + 9 mL water | 10^-1 (1:10) |
| 2 | 1 mL from Tube 1 + 9 mL water | 10^-2 (1:100) |
| 3 | 1 mL from Tube 2 + 9 mL water | 10^-3 (1:1,000) |
| 4 | 1 mL from Tube 3 + 9 mL water | 10^-4 (1:10,000) |
Always change pipette tips between tubes to prevent carryover.
Lab Safety Essentials
Safety Data Sheets (SDS, formerly MSDS)
A Safety Data Sheet is a 16-section document, mandated by OSHA's Hazard Communication Standard, that accompanies every chemical purchased for the lab. The standardized sections you must be able to navigate:
- Section 4 - First-aid measures (what to do if exposed)
- Section 6 - Accidental release (spill cleanup)
- Section 7 - Handling and storage
- Section 8 - Exposure controls / PPE
- Section 10 - Stability and reactivity (incompatibilities)
The SDS file must be accessible to students during a lab activity, not locked in an office.
GHS Pictograms
The Globally Harmonized System (GHS) uses red-bordered diamond pictograms to label chemical containers:
- Flame - flammable
- Flame over circle - oxidizer
- Exploding bomb - explosive
- Skull and crossbones - acute toxicity (fatal/toxic)
- Corrosion - corrosive to metal or skin
- Exclamation mark - irritant, harmful (lower severity)
- Health hazard - carcinogen, mutagen, reproductive toxin
- Environment - aquatic toxicity (non-mandatory in the U.S.)
The biohazard symbol (three-crescent trefoil, orange/black) is a separate biosafety marking — not a GHS pictogram.
Personal Protective Equipment (PPE)
Minimum PPE for a wet biology lab: chemical splash goggles, lab apron or coat, closed-toe shoes, nitrile gloves. Add a fume hood for volatile solvents, a face shield for high-splash work, and a respirator only with documented training and fit testing.
Biosafety Levels (BSL)
| Level | Hazard Class | Typical Organisms | Containment |
|---|---|---|---|
| BSL-1 | No known disease in healthy adults | E. coli K-12, Bacillus subtilis, baker's yeast | Standard microbiology, open bench |
| BSL-2 | Moderate, transmitted by ingestion/percutaneous | S. aureus, Salmonella, hepatitis viruses | Biosafety cabinet for aerosols, restricted access |
| BSL-3 | Serious or potentially lethal, respiratory transmission | M. tuberculosis, SARS-CoV-2, Bacillus anthracis | Negative-pressure room, PPE incl. respirator |
| BSL-4 | Life-threatening, no treatment/vaccine | Ebola virus, Marburg virus | Full positive-pressure suit, dedicated facility |
High-school biology labs operate at BSL-1; teacher candidates should know they are not authorized to culture unknown bacteria from environmental swabs, because the resulting colonies cannot be guaranteed BSL-1.
Required Engineering Controls
- Eyewash station capable of 15 minutes of continuous tepid water; tested weekly.
- Safety shower within 10 seconds of any chemical-handling area.
- Fire blanket and ABC extinguisher within easy reach.
- Sharps container (rigid, puncture-resistant, labeled with biohazard symbol) for scalpels, needles, broken glass.
- Chemical Hygiene Plan (CHP) — written, school-specific document required by OSHA 29 CFR 1910.1450; updated annually.
Microscopy Basics
A compound light microscope uses visible light passing through two lens systems: the objective (4x, 10x, 40x, 100x oil-immersion) and the ocular/eyepiece (typically 10x).
Total magnification = objective magnification x eyepiece magnification.
At 40x objective with a 10x eyepiece, total magnification is 400x.
- Magnification is how much bigger the image appears.
- Resolution is the smallest distance between two points still seen as separate. Light microscopy is limited by the wavelength of visible light to about 200 nanometers (0.2 mum). Increasing magnification beyond this point gives "empty magnification" — bigger but blurrier.
- Transmission electron microscopy (TEM) uses electron beams (wavelength about 0.005 nm) and can resolve features about 0.2 nm, revealing ribosomes, viruses, and membrane bilayers. Specimens must be dead, fixed, and sectioned.
- Scanning electron microscopy (SEM) produces a 3D surface image with about 10 nm resolution.
Microscope Operating Reminders
- Always start on the lowest-power objective and focus with the coarse adjustment.
- Switch to higher objectives using only the fine adjustment.
- Carry the microscope with one hand on the arm and one hand under the base.
- Clean objective lenses only with lens paper (never paper towel).
- Park on the lowest-power objective and lower the stage before storing.
A student needs to prepare 250 mL of a 0.20 M glucose solution from a 2.0 M stock. Using C_1 V_1 = C_2 V_2, what volume of stock should she measure before topping up to 250 mL with water?
A teacher demonstrates a 400x view of onion-root tip cells using a 40x objective and a 10x eyepiece. The cells appear sharp. She switches to a 100x oil-immersion objective without changing eyepieces, but the image becomes blurry rather than more detailed at extreme magnifications. What concept best explains this?