Summary
Highlights
The lecture begins by introducing the chemical organization of the cell. It clarifies the distinction between cytosol (the fluid containing salts, sugars, and amino acids) and cytoplasm (cytosol plus organelles, excluding the nucleus). The cell is composed of a plasma membrane, cytoplasm, and nucleus.
Living and non-living matter are made of elements. Four key elements—Oxygen, Carbon, Hydrogen, and Nitrogen—constitute about 96% of living matter. The remaining 4% includes Calcium, Phosphorus, Potassium, and Sulfur. The lecture details the symbols, percentages, and functions of these elements in the human body, highlighting elements like Iron's crucial role in hemoglobin formation.
Elements typically exist as compounds, with water (H2O) being one of the most critical. Water makes up about two-thirds (70%) of living organisms, while the remaining 30% consists of macromolecules like carbohydrates, proteins, lipids, and nucleic acids. An example of a bacterial cell's composition is provided, showing 70% water and 30% chemicals, with proteins being the largest organic component.
The lecture explains water's unique properties, starting with its polarity. Water's polarity arises from the polar covalent bonds between oxygen and hydrogen and its bent (non-linear) molecular geometry. Oxygen's higher electronegativity creates partial negative charges on oxygen and partial positive charges on hydrogen. This polarity enables water molecules to form hydrogen bonds with each other, which, despite being individually weak and transient, collectively impart extraordinary physical properties to water and play a role in macromolecular structures like proteins and DNA.
Water acts as a powerful solvent, dissolving substances with full electrical charges (ions) or partial charges (polar molecules). The dissolution process is illustrated with table salt (NaCl), where water molecules surround and separate Na+ and Cl- ions through hydration shells. Similarly, sugar (sucrose), a non-ionic but polar molecule, dissolves in water by forming hydrogen bonds due to its hydroxyl groups. Even large molecules like proteins, with ionic and polar regions, can dissolve in water, as exemplified by lysozyme.
The concepts of hydrophilic and hydrophobic substances are introduced. Hydrophilic substances are 'water-loving' and can be ionic, polar, or capable of forming hydrogen bonds with water. This can mean dissolving in water or simply binding to it, as seen with cellulose in plant cell walls, which helps water ascend against gravity. Hydrophobic substances are 'water-fearing,' lacking attraction to water and unable to form hydrogen bonds, thus repelling water. An example cited is oil, which does not mix with water.