CHEM 5006 - First Post-Lab Discussion (Experiments 1-3)
Prokaryote vs Eukaryote
Prokaryote vs Eukaryote. This section likely contrasts basic cellular organization, including presence/absence of a nucleus, membrane-bound organelles, and differences in size and complexity between prokaryotic and eukaryotic cells.
The Living Cell (Experiment No. 1)
Title appears on Page 2: "Experiment No. 1 The Living Cell". Indicates that Experiment 1 focuses on fundamental aspects of the living cell, its structure, and its components.
From the context of the subsequent pages, the experiment covers the distribution of biomolecules within a cell, separation of organelles, and identification/characterization of cellular fractions.
Distribution of Biomolecules in a Cell (Source citation)
Page 3 references a schematic/diagrammatic concept from Campbell, Reece, and Mitchell (Biology, 5th ed., 1999): Distribution of Biomolecules in a Cell. This implies a focus on where carbohydrates, proteins, lipids, nucleic acids, etc., are located within the cell and how they can be partitioned for study.
The content is cited as a source for the understanding of organelle composition and biomolecule localization.
How can the Cell Organelles be Separated?
Core method: Disrupt the cell (or the cell membrane) to release organelles into a homogenate.
Most common disruption: blender or homogenizer.
Gentlest disruption: hand grinding using a mortar and pestle.
Temperature control: Cell disruption procedures are carried out at low temperatures: (or cold temperature) to preserve integrity of organelles.
Homogenization media: Should maintain morphological and functional integrity of organelles.
Differential centrifugation: The homogenate is subjected to differential centrifugation to separate into fractions. Larger/denser organelles sediment faster (lower speeds and shorter times) than smaller ones, enabling sequential separation by size/density. This principle is cited from Lehninger (2003).
Isolation of cell organelles (Overview of the fractionation scheme)
Conceptual pathway (Page 5):
Start with whole tissue or cells → produce a tissue slice → homogenize → filter → buffer.
Potter–Elvehjem homogenizer used to disrupt cells; the homogenate becomes a cytosolic/particulate mixture.
Centrifugation steps create fractions: supernatant (cytosol and small components) and pellets (sediments containing organelles).
Sedimentation sequence (example shown on Page 5):
g = 300,000 × g for 120 min yields a particular sediment (likely nuclei or large structures depending on rotor). Represented as: ,
g = 100,000 × g for 60 min
g = 15,000 × g for 15 min
g = 600 × g for 10 min
Sediment order (from the diagram):
Pellet/Sediment: Nuclei, unbroken cells
Mitochondria, Lysosomes, Peroxisomes
Microsomes, small vesicles, ER fragments
Cytoskeleton, Plasma membrane
Also listed: Ribosomes, Viruses, Mitochondria, etc., depending on the protocol.
Fraction labeling (from the diagram):
Sediment 1: likely nuclei/undisrupted material
Sediment 2: mitochondria, lysosomes, peroxisomes
Sediment 3: microsomes and soluble proteins/enzymes, inorganic ions
Supernatant fractions after successive spins contain smaller components and soluble material
Common theme: Each step increases centrifugal force to pellet progressively smaller organelles.
How can the Cell Organelles be Separated? (Key principle)
Heavier organelles sediment or precipitate at lower speeds (and shorter times) than lighter organelles. This principle guides the choice of speeds and durations in differential centrifugation.
Practical takeaway: By selecting specific speeds (g-forces) and times, you can enrich for particular organelles in the pellet while leaving others in the supernatant for subsequent spins.
Checking the identity of the isolated components
Identity verification is done in two main ways:
Microscopy after treatment with a specific stain (e.g., mitochondria stained by Janus Green B).
Biochemical assay for a characteristic compound or enzyme (e.g., enzymes of the Krebs cycle present in mitochondria).
This two-pronged approach ensures both structural and functional confirmation of organelle enrichment.
Schematic Diagram: Separation workflow (Liver homogenate example)
Stepwise procedure (Page 8):
Homogenize liver tissue in ice-cold homogenizing medium for 5–10 minutes.
Centrifuge at 600 rpm for 10 minutes to remove nuclei and unbroken cells.
Collect supernatant and centrifuge at 8,000–10,000 rpm for 10 minutes to sediment mitochondria, lysosomes, and peroxisomes.
Collect remaining supernatant and centrifuge at 15,000 rpm for 10 minutes to yield microsomes, soluble proteins, enzymes, and inorganic ions.
Resulting fractions:
Sediment 1: Nuclei & unbroken cells
Sediment 2: Mitochondria; Lysosomes; Peroxisomes
Sediment 3: Microsomes; soluble proteins; enzymes; inorganic ions
Supernatants contain progressively smaller components and soluble material
Definition of key terms (Experiment 1 context)
Homogenization: The process of disrupting cells under mild conditions using a pestle rotated within a glass container. Key points:
The pestle imposes mechanical shear forces to disrupt cells.
Resulting suspension is called a homogenate.
The instrument is a homogenizer.
Differential Centrifugation: Sub-fractionation of a homogenate by a series of centrifugation steps at successively greater speeds (rpm), each yielding a sediment and a supernatant. Key points:
At lower rpm, coarser particles settle.
At higher rpm, finer particles settle.
The centrifuge is the instrument used.
Supernatant: The upper liquid part after centrifugation.
Sediment: The residue that settles after centrifugation.
Qualitative Analysis of the Cell Fractions (Tests and interpretation)
Tests used to characterize fractions include:
Molisch Test: general test for presence of carbohydrates. Reagent: Molisch reagent (α-naphthol in 95% alcohol) + H₂SO₄. Positive result: purple ring at the interface of two immiscible layers. Chemistry: dehydration of carbohydrates by sulfuric acid to produce furfural or hydroxymethylfurfural.
Biuret Test: test for peptide bonds in proteins. Reagents: sample + 10% NaOH + 0.5% CuSO₄. Positive: violet/purple color. Mechanism: under alkaline conditions, peptide bonds form a coordination complex with Cu²⁺; the color intensity is proportional to peptide bond content.
Sudan IV Test: test for lipids. Positive: reddish-orange color. Lipid-soluble dye that stains lipids/triglycerides/lipoproteins; does not stain polar compounds or aqueous solutions.
Dische Diphenylamine (Dische) Reaction: test for DNA. Reagents: diphenylamine + concentrated H₂SO₄. Positive: blue coloration. Mechanism: DNA depurination and dehydration of deoxyribose to ω-hydroxylevulinic aldehyde which reacts with diphenylamine to form a blue complex. Distinguishes DNA from RNA via sugar differences (deoxyribose vs ribose).
Feulgen’s Test: nuclear stain for DNA. Reagents: Schiff’s reagent (fuchsin red + KHSO₃ in water). Positive: magenta/reddish-violet coloration. Mechanism: aldehydes produced by hydrolysis of DNA react with Schiff’s reagent; semi-quantitative for DNA content.
Orcinol Test: test for pentose rings (ribose) in RNA. Reagent: orcinol. Positive: blue-green complex. Mechanism: pentoses dehydrated to furfural react with orcinol; iron in reagent forms a blue-green complex.
Additional qualitative results table (Page 21-22): Summary of which fractions (Sediment 1/2/3, Supernate 3) tested positive for each test (Molisch, Biuret, Sudan IV, Dische, Feulgen, Orcinol) with intensity indicators (++++ to +).
Qualitative results trend: All fractions were positive for all tests, but with varying intensities, indicating distribution of carbohydrates, proteins, lipids, and nucleic acids among fractions. Example qualitative trends from Page 22:
Carbohydrates: Sediment 1 > Sediment 2 > Sediment 3 > Supernate 3
Proteins: Supernate 3 > Sediment 3 > Sediment 2 > Sediment 1
Lipids: Sediment 2 > Supernate 3 > Sediment 1 > Sediment 3
DNA: Sediment 1 > Sediment 2 > Sediment 3 > Supernate 3
RNA: Sediment 1 > Sediment 2 > Sediment 3 > Supernate 3
Experiment No. 2: Proteins and Amino Acids
Focus: characterization of proteins and amino acids using qualitative color reactions and precipitation/denaturation behavior.
Example proteins:
Albumin: egg protein; amino acid composition diverse; Ovalbumin is ~75% of egg white; Albumin family includes serum albumin, lactalbumin, grain and soybean albumin; Found in nearly all living tissues.
Sample used: Egg albumin (egg white) as protein source.
Sample preparation (Page 25):
Separate egg white from yolk.
Beat egg white with 6–10 volumes of water.
Strain through cheesecloth.
Filtrate used as protein sample.
Color reactions (Page 26): A matrix for qualitative protein analysis:
Biuret test: Purple coloration → Peptide bonds.
Xanthoproteic test: Yellow or yellow-orange coloration → Tyrosine and Tryptophan.
Millon’s test: Old rose ppt → Phenolic group (Tyr).
Hopkin’s-Cole reaction: Purple or violet ring → Indole group (Trp).
Detailed notes on each test (Pages 27-31):
Biuret Test (as above) – mechanism and meaning.
Xanthoproteic Test – reagents: concentrated HNO₃; nitration of aromatic rings (Tyr, Trp); Phenylalanine not readily nitrated.
Millon’s Test – detects phenolic group (tyrosine) via a red/rose color change using Millon’s reagent (Hg²⁺-Hg⁺ nitrate in nitric acid).
Hopkin’s-Cole Reaction – detects indole group (tryptophan); glyoxylic acid reacts with indole to form a purple/violet ring; note that glyoxylic acid is a contaminant in some concentrated acetic acid; used historically to identify tryptophan.
Precipitation Reactions (Denaturation context) – Denaturation vs precipitation (Pages 32-41):
Denaturation: destruction of 3D structure of proteins (secondary/tertiary) without breaking peptide bonds; disrupts functional activity; examples: enzyme loss, antibody binding loss, collagen fiber synthesis disruption, hemoglobin function loss.
Denaturation effects: changes in particle size, viscosity, surface tension, reactivity of side chains; reversible under mild conditions; irreversible under drastic conditions leading to coagulation/precipitation.
Precipitation: coagulation/insoluble solid formation; caused by denaturation or interactions with precipitants; various methods described (acidic, heavy metals, alkaloidal reagents, salting out, etc.).
Hydrolysis vs Denaturation definitions: Hydrolysis breaks peptide bonds; Denaturation disrupts higher-order structure without breaking peptide bonds.
Denaturation and precipitation experiments (Pages 35-41):
Example: Denaturation by heat and extreme pH; mild heating can be reversible; vigorous heating leads to irreversible denaturation and coagulation; HAc (acetic acid) can increase denaturation by breaking salt bridges.
Precipitation with concentrated mineral acids (e.g., HNO₃, HCl, H₂SO₄): disrupts H-bonds and salt linkages; long exposure can hydrolyze peptide bonds.
Precipitation with heavy metal ions (HgCl₂, AgNO₃, Pb(C₂H₃O₂)₂): disrupts salt bridges; forms metal-proteinates with carboxylate groups; potential toxicity; antidote: milk/egg white or chelating agents like EDTA.
Precipitation with alkaloidal reagents (picric acid, TCA, K₄Fe(CN)₆): disrupts salt bridges; formation of protein salts (protein picrate, tannate).
Salting out (NH₄)₂SO₄, NH₄Cl, etc.: reduces protein solubility with increasing salt concentration; reversible precipitation; practical use to separate proteins.
Practical results (Table-like content on Page 39-41):
Denaturation by heat and extreme pH: gentle heating is reversible; HAc increases denaturation; heat coagulation is used in cooking and sterilization contexts.
Heavy metal/alkaloidal/acidic treatments produce varying degrees of precipitation or dissolution depending on concentration.
Experiment No. 3: Enzymes and Enzyme Tests
Concept: Enzymes as biological catalysts that accelerate biochemical reactions without being consumed; the structure and activity of enzymes are central to metabolism.
Key ideas from Pages 41-44:
Active site concept: enzymes have specific active sites where substrates bind.
Examples: Catalase catalyzes the decomposition of hydrogen peroxide (H₂O₂) into water and oxygen.
Experimental setup includes demonstrating enzyme action via catalase activity and starch digestion using ptyalin (salivary amylase).
Catalase activity (Page 43):
Reaction:
Detection can involve a glowing splinter test or formation of oxygen gas bubbles.
Tissue activity: tissues that are physiologically active have higher catalase activity; liver is a common source for catalase.
A qualitative test also mentions a filtrate that is positive to Biuret test after reacting with NaOH and CuSO₄, indicating protein presence.
Ptyalin digestion of starch (Page 44):
Ptyalin (salivary amylase) hydrolyzes starch into dextrins and maltose, eventually yielding glucose (a simple reducing sugar).
Saliva composition: ~99.5% water; mucin (glycoprotein) acts as lubricant; inorganic salts act as buffers; enzymes like amylase catalyze hydrolysis.
Iodine test for starch (Page 44-45):
Amylose forms a helical structure that iodine can insert into, giving a blue-black color (diagnostic for starch).
Time-course of digestion: as ptyalin digests starch, the color of the starch-iodine complex fades (blue) and reverts toward lighter colors as hydrolysis proceeds; Benedict’s test becomes positive for reducing sugars as starch is broken down.
Enzymatic hydrolysis schematic (Page 46):
Pathway: Starch --(ptyalin)--> soluble starch -->(P/SA)--> amylodextrin --> erythrodextrin --> maltose --> glucose.
Benedict’s test: positive when reducing sugars are produced; the color shifts from blue to brick-red precipitate for reducing sugars.
Salivary amylase (Ptyalin) context (Page 47-49):
Function: hydrolyzes starch to dextrins and maltose, and eventually to glucose.
Source: saliva; composition of saliva includes amylase which catalyzes hydrolysis.
Vitamins – General context (Pages 49-63)
General characteristics of vitamins
Organic micronutrients body cannot synthesize in sufficient amounts; must be obtained via diet.
Vitamins are needed in micro- to milligram quantities; many enzymes use vitamins as cofactors (coenzymes for B vitamins).
Two major classes based on solubility:
Water-soluble vitamins: rapidly replenished, excreted by kidneys, highly polar; mostly act as coenzymes (except vitamin C with broader roles).
Fat-soluble vitamins: dissolve in lipids; stored in fat tissues; transported by carrier proteins; required periodically; higher risk of toxicity with excess; do not act as cofactors in the same general sense as B vitamins.
Fat-Soluble Vitamins and Sources (Overview)
Major fat-soluble vitamins: A, D, E, K (plus emerging/adjunct mentions in slides like MK-1 as a mnemonic).
Typical dietary sources and general roles:
Vitamin A (retinol): synthesized visual pigments; sources include eggs, butter, cheese, dark green and deep orange vegetables; deficiency can cause night blindness and skin issues.
Vitamin D (calciferol): regulation of calcium and phosphorus metabolism; sources include fish-liver oils, fortified milk; deficiency leads to rickets.
Vitamin E (tocopherol): antioxidant; sources include whole-grain cereals, margarine, vegetable oil; protects lipids from oxidation; deficiency can cause red blood cell fragility.
Vitamin K: blood clotting factor synthesis; sources include leafy greens; deficiency can cause bleeding disorders.
A concise overview is provided in Table 22.3 (Pages 52-53) with dietary sources, functions, and deficiency conditions.
Water-Soluble Vitamins – Overview and Specific B vitamins (Pages 53-57)
B vitamin group overview: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), Pyridoxine (B6), Biotin (B7), Folate (B9), Cobalamin (B12).
General note: B vitamins serve as coenzymes and have diverse structural forms; they participate in energy metabolism, amino acid metabolism, and other essential cellular processes.
Specific references in the slides include:
Vitamin B1 (Thiamine): coenzyme in decarboxylation reactions; sources include bread, beans, nuts, milk, peas, pork, rice bran; deficiency: beriberi (nausea, exhaustion, paralysis).
Vitamin B2 (Riboflavin): coenzyme forms (FMN and FAD); sources include milk, meat, eggs, dark green vegetables, bread, beans; deficiency symptoms include dermatitis, nervous disorders.
Vitamin B3 (Niacin): sources include meat, whole grains, poultry, fish; role as part of coenzymes NAD+ and NADP+ (hydride transfer).
Vitamin B6 (Pyridoxine): forms FMN and FAD; involved in amino acid metabolism; deficiency: dermatitis, nervous disorders.
Vitamin B12 (Cobalamin): coenzyme in amino acid metabolism; deficiency risk in vegetarians; pernicious anemia.
Folic acid (B9): coenzyme in methyl group transfers; deficiency leads to anemia.
Pantothenic acid (B5): component of coenzyme A (CoA); involved as acyl carrier; deficiency leads to anemia.
Biotin (B7): coenzyme form used in fatty acid synthesis; deficiency leads to dermatitis, muscle weakness.
Vitamin C (Ascorbic acid) is listed under water-soluble vitamins with its own notes (Page 60) on collagen synthesis, antioxidant roles, and deficiency (scurvy).
Vitamin tests: colorimetric/color-based methods (Pages 57-61)
Vitamin tests listed as color tests with observed colors and brief descriptions:
Vitamin A: blue coloration with antimony chloride (SbCl₃) in CHCl₃; blue color is transient and fades to violet and red as retinol oxidizes.
Vitamin D: reddish-brown precipitate that can turn blue in solution; indicates presence via a color shift.
Vitamin C: cloudy green solution with a dark brown precipitate; test indicates ascorbic acid presence.
Vitamin B1: orange precipitate or orange solution indicates presence.
Additional notes pair these tests with alternative methods for vitamin detection and mention cod liver oil as a natural source for vitamins A and D.
More specific observations tie each vitamin to its role in health and the consequences of deficiency (e.g., scurvy for vitamin C).
Iodine/Starch and Benedict’s test – Summary of enzyme-related experiment (Page 44-46)
The iodine test for starch is used to monitor starch digestion by ptyalin (salivary amylase) in the presence of saliva. The test shows a color change from deep blue (starch) to progressively lighter blue as digestion proceeds, eventually leading to color loss when starch is completely hydrolyzed and Benedict’s test becomes positive for reducing sugars (glucose, maltose, etc.).
Benedict’s test: indicates reducing sugars in solution; presence of reducing sugars results in a brick-red precipitate after heating.
Practical lab topics and knowledge-rich points
Laboratory gear and safety
Common glassware, apparatus, and instruments are listed as essential knowledge (Page 62). Students should be familiar with standard lab equipment and safety practices.
Experimental interpretation and real-world relevance
The process of organelle separation by differential centrifugation has broad applications in cell biology, biochemistry, and molecular biology for isolating mitochondria, lysosomes, peroxisomes, microsomes, and other fractions for functional assays.
Qualitative tests provide quick, classroom-scale means to infer the presence of biomolecules in fractions, enabling cross-validation with microscopy and enzyme assays.
Understanding denaturation/precipitation is crucial for protein chemistry, food science, and biotechnology, including how processing conditions affect protein structure and functionality.
Connections to foundational principles
Cell fractionation is grounded in the physical principles of sedimentation under centrifugal force, which relates to density, size, shape, viscosity of the medium, and rotor geometry.
The sequence of organelle enrichment relies on differences in buoyant density and size, which in turn connects to foundational biochemistry and cell biology concepts: membranes, organelle membranes, enzyme localization, and macromolecular complexes.
Colorimetric tests connect to classic qualitative chemistry, reagent chemistry, and the interpretation of color changes as proxies for molecular functional groups (e.g., peptide bonds, aromatic amino acids, indole groups, nucleic acids).
Enzyme testing emphasizes the concept of an active site and substrate specificity, as well as the difference between enzymatic activity and protein presence as detected by general protein assays (e.g., Biuret test).
Vitamin classification and functions reinforce the importance of micronutrients as cofactors and coenzymes in metabolic pathways, including energy metabolism, nucleotide synthesis, and collagen formation.
Equations and numerical references (LaTeX)
Sedimentation principles (example values and units):
Centrifugal force representation:
Alternating spins:
and for subsequent spins, with corresponding values as shown.
Enzymatic reactions (stoichiometry):
Catalase:
Molecular biology color test indicators (qualitative, not quantitative):
Molisch: Purple ring at interface indicates carbohydrates.
Biuret: Violet color intensity proportional to peptide bonds.
Feulgen: Magenta coloration indicates DNA content after hydrolysis.
Orcinol: Blue-green complex indicates ribose-containing sugars.
Referenced chemical formulas and structures appear throughout (e.g., CuSO₄, NaOH, H₂SO₄, HgCl₂, picric acid, TCA, K₄Fe(CN)₆). When used in notes, these are treated as standard reagents with their typical reaction contexts.
Notes on ethical, philosophical, or practical implications
Safety and toxicity: Several reagents used in protein precipitation (heavy metals like Hg²⁺, Pb²⁺), acids, and alkaloidal reagents can be hazardous. Students should apply standard safety hygiene, PPE, and proper waste disposal. The presence of heavy metals in certain reagents (e.g., HgCl₂) highlights the importance of safe handling and the need for antidotes/chelators when discussing potential exposure.
Biochemical experimentation emphasizes reproducibility and interpretation caution: colorimetric tests provide qualitative data and can be influenced by concentration, pH, and the presence of interfering substances. Cross-validation with microscopic or enzymatic assays strengthens conclusions.
Biological sample handling: Egg white proteins, liver tissue, and other biological materials require ethical sourcing and proper handling in line with institutional guidelines.
Quick reference: key terms to remember
Homogenization, Differential Centrifugation, Supernatant, Sediment
Nuclei, Mitochondria, Lysosomes, Peroxisomes, Microsomes, ER fragments, Plasma membrane, Cytoskeleton
Molisch, Biuret, Sudan IV, Dische, Feulgen, Orcinol, Xanthoproteic, Millon’s, Hopkin’s-Cole
Denaturation, Precipitation, Hydrolysis, Salting Out
Catalase, Ptyalin (salivary amylase), Iodine test for starch, Benedict’s test
Vitamins: Water-soluble vs Fat-soluble; B vitamins list (B1, B2, B3, B5, B6, B7, B9, B12); Vitamins A, C, D, E, K; sources and deficiency signs
Common lab safety and glassware knowledge (as a general reminder)