Biological Molecules

Biological Molecules Summary

The Importance of Water

  • Role of Water in Cells

    • Water is the medium for all metabolic reactions in cells.

    • It facilitates the transport of substances throughout the body.

  • Composition of Water

    • Water is made up of hydrogen and oxygen atoms (H2O).

    • Covalent Bonding: One oxygen atom shares electrons with two hydrogen atoms, resulting in the formation of covalent bonds.

  • Polarity of Water

    • The sharing of electrons between oxygen and hydrogen is uneven; oxygen has a higher electronegativity, leading to:

    • A weak negative charge (δ−) on the oxygen atom.

    • A weak positive charge (δ+) on the hydrogen atoms.

    • Dipole Nature: Water has a dipole nature, making it a polar molecule.

  • Hydrogen Bonding

    • Hydrogen bonds form between the positively charged regions of hydrogen and the negatively charged regions of oxygen in water molecules.

    • These bonds are weak and constantly break and reform in the liquid state.

  • Properties of Water

    • Cohesion: The attraction between water molecules due to hydrogen bonds allows for surface tension.

    • Adhesion: The attraction between water molecules and other materials facilitates capillary action, aiding in the movement of water in plants.

    • Solvent Properties:

    • Water is a polar solvent, capable of dissolving many ionic compounds (e.g., sodium and chloride ions) and polar molecules (e.g., glucose).

    • Water molecules surround these particles, allowing chemical reactions within cells, facilitated by dissolved solutes.

Saccharides

Types of Saccharides

  • Carbohydrates: Composed of carbon (C), hydrogen (H), and oxygen (O). Key characteristics include:

    • Carbon atoms can form covalent bonds, leading to stable structures.

    • They can exist as monomers (single units) or polymers (long chains).

  • Types of Carbohydrates

    • Monosaccharides (simple sugars, e.g., glucose): The monomers of carbohydrates.

    • Disaccharides: Formed from two monosaccharides through condensation reactions. For example:

    • Maltose: Two glucose molecules.

    • Sucrose: Glucose and fructose.

    • Lactose: Glucose and galactose.

    • Polysaccharides: Long chains of monosaccharides, examples include starch, glycogen, cellulose.

Monosaccharides

  • Structural Characteristics:

    • Types:

    • 3 carbons: Triose (e.g., glyceraldehyde)

    • 5 carbons: Pentose (e.g., ribose)

    • 6 carbons: Hexose (e.g., glucose).

  • Function of Monosaccharides:

    • Primary role is to serve as energy sources during respiration and to act as building blocks for polymers.

Disaccharides

  • Formation:

    • Formed from two monosaccharides via a condensation reaction, creating a glycosidic bond (e.g., a 1,4 or a 1,2 bond).

  • Functional Characteristics:

    • Provide quick-release energy and are easily digestible due to their simple structure.

Polysaccharides

  • Starch: The storage polysaccharide in plants, consisting of:

    • Amylose: Unbranched, helical structure with 1,4 glycosidic bonds.

    • Amylopectin: Branched structure with 1,4 and 1,6 glycosidic bonds.

  • Glycogen: The storage polysaccharide in animals, significantly branched and more compact than starch, allowing quick energy access.

  • Cellulose: A structural component in plant cell walls, but details are not required for this topic.

Core Practical 1: Estimating the Concentration of Sugars & Starch

Concentration of Sugars

  • Sugar Classification: Sugars fall into reducing (e.g., glucose) and non-reducing (e.g., sucrose) categories, based on their ability to donate electrons.

Benedict’s Test for Reducing Sugars

  • Materials: Benedict's reagent (contains copper (II) sulfate), beakers, test tubes, water bath.

  • Method:

    • Mix sample with Benedict's reagent and heat in a water bath.

    • A positive result is indicated by a color change from blue (no sugar) to brick-red (high concentration).

Testing for Non-Reducing Sugars

  • Method:

    • Hydrolyze non-reducing sugars by boiling with dilute hydrochloric acid, then neutralizing with sodium hydrogen carbonate before testing with Benedict’s reagent.

Semi-Quantitative Measurement using a Colorimeter

  • Setup: Prepare standard solutions and measure absorbance to create a calibration curve for determining concentrations of reducing sugars.

  • Colorimeter Functionality: Measures absorbance of light, allowing quantification of sugar concentrations based on color intensity.

Starch Testing

  • Iodine Test: Use iodine solution; a blue-black color indicates the presence of starch.

  • Semi-Quantitative Measurement: Similar to sugar testing using standard solutions of starch and colorimeter.

Condensation & Hydrolysis

  • Condensation Reaction: Formation of glycosidic bonds between monosaccharides occurs during the condensation reaction by releasing a water molecule.

  • Hydrolysis Reaction: Breaking of glycosidic bonds occurs in the presence of water, essential for digestion and energy release in metabolism.

Triglycerides & Ester Bonds

Lipids Overview

  • Structure: Lipids contain lower oxygen content than carbohydrates and are non-polar and hydrophobic.

Triglyceride Formation

  • Components: Composed of glycerol (an alcohol) and fatty acids.

  • Ester Bond Formation: An ester bond forms when the hydroxyl group of glycerol bonds with the carboxyl group of a fatty acid, resulting in the release of water during a condensation reaction.

  • Triacylglycerols: Formed from one glycerol molecule and three fatty acids, releasing three water molecules in the process.

Fatty Acids

  • Types:

    • Saturated Fatty Acids: No carbon-carbon double bonds, resulting in straight chains.

    • Unsaturated Fatty Acids: Contain one or more double bonds; can be mono- or polyunsaturated.

    • Cis and Trans Configurations: Determined by the positioning of hydrogens around the double bond; cis fats are metabolizable, while trans fats are not and are linked to health risks.