Bio Lab
Scientific Method and Variables
Independent variable: the variable you are manipulating, not the one “unaffected by the experiment”
Common misconception: “unaffected by experiment” is wrong on a short answer; correct answer is “the variable you manipulate”
Dependent variable: changes as a result of the independent variable
Control variables: everything held constant so only one thing is manipulated
Examples from dish soap experiment: type of dish, water, type of stain, wash time
Control group vs. control variable
Control group has all control variables but no independent variable (e.g., water only, no dish soap)
Used as a comparison to determine if the independent variable had an effect
Conclusions: results support or do not support the hypothesis; experiments cannot prove something true
Exception: scientific laws
Lab Equipment to Know
Glassware:
Volumetric flask: precise single-volume measurement only (e.g., 500 mL line only)
Erlenmeyer flask: liquid measurement with multiple graduated markings
Graduated cylinder: read from the bottom of the meniscus
Beaker
Pipette
Thermometers:
Alcohol thermometer: safe, no special protocol if broken
Mercury thermometer: toxic, special cleanup protocol required if broken
Microscope parts to know:
Ocular lens, nosepiece, arm, stage
Objective lenses: 4x (red, scanning), 10x (yellow, low power), 40x (blue, high power), 100x (white, oil immersion)
Coarse and fine adjustment knobs; light source
Macromolecules Lab (Labsters)
Carbohydrates:
Benedict’s reagent: tests for reducing sugars; blue color turns yellow-green if positive
Iodine test: tests for polysaccharides (cellulose, glycogen); turns very dark if positive, light yellow if negative
Proteins:
Biuret reagent: contains copper ions; turns purple in presence of protein
Lipids:
Ethanol test: produces cloudy white droplets if lipids present
These screenshots cover a classic Biological Macromolecules / Food Testing Lab.
Here’s the material I’d want a student to know cold before my test.
BIOLOGY QUICK STUDY GUIDE
Identifying Macromolecules in Food
The Big 4 Tests — Memorize This Table
What are you testing for? Test/Reagent Negative Result Positive Result What positive means
Simple sugars Benedict’s test Blue Green → Yellow → Orange → Red Red/orange = lots of reducing/simple sugar
Complex carbohydrate / starch Iodine test Yellow/amber Blue-black/black Starch is present
Protein Biuret test Blue Purple Protein is present
Lipids (fats) Sudan IV test Orange Yellow-green* Lipid is present
*For this Labster simulation, memorize the Sudan IV colors exactly as the course presents them, even though Sudan tests may be described differently in other lab protocols.
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1. Benedict’s Test — SIMPLE SUGARS
Purpose: Detects reducing/simple sugars such as glucose, fructose, and maltose.
Your simulation specifically emphasizes:
BLUE → RED/ORANGE = POSITIVE
The sample is heated during the Benedict’s test.
Color progression
Blue → Green → Yellow → Orange → Red
Think:
BLUE = none
GREEN/YELLOW = lower amount
ORANGE/RED = higher amount
Test trap
Benedict’s reagent tests for glucose/reducing sugars, but not ordinary sucrose directly under the conditions described in your simulation.
Memory trick:
Benedict likes SWEET things.
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2. Iodine Test — STARCH
Iodine is used to detect complex carbohydrates, especially starch.
Results
Yellow/amber = NEGATIVE
Blue-black/black = POSITIVE
So if I ask:
> A food sample turns black after iodine is added. What macromolecule is present?
Starch / complex carbohydrate
Memory trick:
Iodine + starch → BLACK.
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3. Biuret Test — PROTEIN
This one is especially important because several screenshots focus on it.
Results
Blue = no protein
Pale purple = low protein
Dark purple = high protein
Therefore, the positive control for the Biuret assay should give a purple result.
Test question
> Which color represents a positive Biuret test?
Purple
> Darker purple means what?
Higher protein concentration
Memory trick:
Protein = PURPLE
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4. Sudan IV — LIPIDS
Sudan IV is used to identify lipids/fats.
For this particular Labster exercise, the workflow image gives:
Orange = negative
Yellow-green = positive
Again, use your course's stated color scheme on the test.
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Cellulose & Fiber
Another screenshot gives you a likely test topic.
Cellulose is:
A carbohydrate AND a form of insoluble fiber.
Humans cannot digest cellulose because we lack the enzyme needed to break its bonds efficiently.
Therefore, cellulose travels through much of the digestive tract relatively unchanged and contributes to dietary fiber.
Sources include:
Brown rice, whole-wheat bread, fruits, and vegetables.
Likely exam question
> Which carbohydrate is an insoluble dietary fiber that humans cannot digest?
CELLULOSE
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Positive vs. Negative Controls
This is VERY testable.
Positive control
A positive control contains something known to produce a positive result.
Its job is essentially to prove:
> "My test actually works."
For example, if you're performing a Biuret test, your positive control should contain known protein and therefore turn purple.
If the positive control doesn't work, you cannot trust a negative experimental result.
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Negative control
A negative control contains something known NOT to produce the reaction.
Its purpose is to make sure an apparent positive result isn't being caused by the procedure, contamination, or another source of error.
Think:
Positive control → proves the test CAN detect it.
Negative control → proves the test isn't falsely detecting it.
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THE CONTROL QUESTION THAT CAN TRICK YOU
Suppose your experimental sample is negative.
Can you immediately conclude that the macromolecule isn't present?
Not necessarily.
You first need a successful positive control.
Why?
If:
Sample = negative
AND
Positive control = positive
You can have confidence that the sample's negative result is valid.
But if:
Sample = negative
AND
Positive control fails
You cannot tell whether the sample is truly negative or whether the experiment simply failed.
Memorize:
> A positive control validates negative results.
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THE 30-SECOND EXAM CRAM
If you remember nothing else, remember this:
Benedict → SUGAR → heat → green/yellow/orange/red
Iodine → STARCH → BLACK
Biuret → PROTEIN → PURPLE
Sudan IV → LIPID
Cellulose → insoluble carbohydrate fiber → humans can't digest it
Positive control → confirms the test WORKS
Negative control → checks for false positives/procedural error
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Mini Practice Test
1. A Biuret test turns dark purple. What does that indicate?
Answer: High protein concentration.
2. Biuret solution remains blue. What does that mean?
Answer: No detectable protein.
3. Benedict's solution changes from blue to red after heating. What is present?
Answer: A high level of reducing/simple sugars.
4. Which test detects starch?
Answer: Iodine.
5. What does a black iodine result indicate?
Answer: Starch/complex carbohydrate is present.
6. What macromolecule does Sudan IV test for?
Answer: Lipids.
7. What type of molecule is cellulose?
Answer: A carbohydrate; specifically an insoluble dietary fiber.
8. Why can't humans digest cellulose?
Answer: Humans lack the enzyme needed to break its characteristic bonds.
9. Why do you need a positive control when your experimental sample is negative?
Answer: To demonstrate that the assay worked, allowing you to trust the negative result.
10. What's the easiest four-word color mnemonic?
Benedict = Sugar
Iodine = Starch
Biuret = Protein
Sudan = Lipid
If those four associations are automatic, a big chunk of this Labster assessment becomes straightforward.
Lab 1- scientific method
The scientific method steps
Observing and identifying a problem
Question based off what's identified
Hypothesis - an educated guess
Null hypothesis - your experiment will not work
Experiment
Variables considered when having an experiment
Independent variable - the variable that you are manipulating in the experiment
Dependent variable - the variable that is being changed by the experiment
Controlled variable - the variables that stay the same
Analysis
Analytical tools
Graph
Line
Bar
Pie
Results
Did it support the hypothesis
You cannot always prove that something is true
Measurements
We use the metric system
Volumetric flask
Graduated cylinder
Measure the bottom of the meniscus
Earlynmyer flask
Pipet
Beaker
Two types of thermometer
Alcohol
Mercury
Microscopy
Parts of a microscope
Arm - where you hold the microscope
Objective lenses - red- 4x, yellow(low power)- 10x, blue(high power)- 40x, white(oil magnification)- 100x
Stage
2 kinds of adjustment knobs
Coarse
Fine
Light
Base
The cell
Prokaryote - no nucleus or membranes that have organelles
Eukaryote - has nucleus
Parts of the cell
Cell membrane
Made of a phospholipid bilayer
Phospholipid head - polar
Fatty acids - nonpolar
Glycerol
Nucleus
Nucleolus - makes ribosomal RNA
Endoplasmic Reticulum
Rough ER
Has ribosomes that makes proteins
Smooth ER
Makes fats and lipids
Golgi apparatus
Modifies, stores and packages
Glycoproteins
Glycolipid
Lipoprotein
Mitochondria
ATP production
Aerobic cellular respiration
glucose + O2 -> CO2 + H2O + 36-38 ATP + heat
Lysosome
Breaks down waste/bacteria
Centrioles
Cell division
Made of microtubules
Peroxisome
Catalase enzymes break down fatty acids that produce hydrogen peroxide
Cell wall
Plasmodesmata
Chloroplast
Vacuole
Labster
Macromolecules
Carbohydrates
Benedict's reagent
The actual reagent is a dark/royal blue color
Essentially, the electrons ion the sugar molecules go onto the Benedict's reagent
Iodine test: also tests for polysaccharides
Proteins
Amino acids
To test for proteins, you use the buret’s reagent (light/turquoise/aqua blue)
Lipids
Ethanyol
Sudan IV