π§ͺ How Do Scientists Analyse the Chemical Composition of Living Cells?
To understand what a living cell is chemically made of, scientists perform a systematic chemical analysis of tissues such as plant leaves or animal liver. This experiment clearly shows that living matter is a complex mixture of organic and inorganic compounds.
πΉ 1οΈβ£ Chemical Analysis Using Trichloroacetic Acid (TCA)
π¬ Step-by-Step Procedure
- Grinding: Fresh living tissue is ground in cold trichloroacetic acid (TCA) using a mortar and pestle.
- Filtration: The thick slurry is filtered through cheesecloth or cotton.
- Separation: Two distinct fractions are obtained.
𧫠Two Fractions Obtained
β (A) Filtrate β Acid-Soluble Pool
- Contains biomicromolecules.
- Molecular weight: less than 1000 Daltons.
- Examples:
- Amino acids
- Simple sugars
- Nucleotides
- Vitamins
- Salts
π Key Point:
Acid-soluble fraction = micromolecules involved in metabolism.
β (B) Retentate β Acid-Insoluble Fraction
- Contains biomacromolecules.
- Molecular weight: 10,000 Daltons or more.
- Examples:
- Proteins
- Polysaccharides (starch, cellulose)
- Nucleic acids
- Lipids
π Key Point:
Acid-insoluble fraction = structural and storage molecules.
β οΈ Special Case of Lipids (Very Important for NEET)
- Lipids have low molecular weight (~800 Da).
- Still found in acid-insoluble fraction.
Reason: When membranes break during grinding, lipids form water-insoluble vesicles that are too large to pass through the filter.
π Exam Line:
Lipids behave like macromolecules due to insolubility, not size.
πΉ 2οΈβ£ Analysis of Inorganic Elements β Ash Analysis
To identify inorganic components of cells, a different method is used.
π₯ Steps in Ash Analysis
- Tissue is first weighed (wet weight).
- Tissue is dried to remove water (dry weight).
- Dry tissue is completely burnt.
- Organic compounds are oxidized to COβ and HβO.
- Remaining residue is called ash.
𧱠Composition of Ash
- Calcium (Ca)
- Magnesium (Mg)
- Sodium (Na)
- Potassium (K)
- Chlorides, phosphates
π Key Concept:
Ash represents the inorganic mineral content of cells.
π§ One-Page Memory Capsule
- TCA grinding separates micromolecules and macromolecules
- Acid-soluble β metabolites
- Acid-insoluble β proteins, nucleic acids, lipids
- Lipids are insoluble, hence trapped
- Ash = inorganic elements
- Primary metabolites = essential
- Secondary metabolites = special products
π‘ Questions and Answers
Q1. Why is living tissue ground in trichloroacetic acid during chemical analysis?
A1. Tissue is ground in trichloroacetic acid to break open cells and separate their chemical components into small soluble molecules and large insoluble molecules.
π Key Points:
- Breaks cells
- Separates micromolecules and macromolecules
Q2. What is the acid-soluble pool? Name two compounds present in it.
A2. The acid-soluble pool contains small organic molecules with low molecular weight, such as amino acids and sugars.
π Key Points:
- Molecular weight < 1000 Da
- Contains metabolites
Q3. Why are lipids found in the acid-insoluble fraction despite being small molecules?
A3. Because lipids are insoluble in water and form membrane vesicles during grinding, which are too large to pass through the filter.
π Exam Line:
Lipids are insoluble, not large.
Q4. What is ash and what does it indicate?
A4. Ash is the inorganic residue left after burning tissue. It indicates the mineral content of the cell.
π Key Points:
- Contains Ca, Mg, Na, K
- Represents inorganic matter
Q5. Distinguish between primary and secondary metabolites.
A5. Primary metabolites are essential for growth and metabolism, while secondary metabolites are not essential but are useful products made by some organisms.
π Key Points:
- Primary β universal, essential
- Secondary β specific, useful
Q6. Identify the secondary metabolite: Glucose, Amino acid, Curcumin, Ribose.
A6. Curcumin is the secondary metabolite.
π Key Tip:
Sugars and amino acids are always primary.