Chemical Composition of the Cell - Part 1

Chemical Composition of the Cell - Part 1

• The cell is a complex system composed of billions of different chemical compounds and molecules.
• Despite variations in cell shapes, their chemical compositions are remarkably similar.
• The environment contains ninety-two major chemical elements.
• Carbon, oxygen, hydrogen, nitrogen, and calcium make up approximately 98% of living matter.
• Chemical elements in the cell exist as ions or are bonded together in various proportions to form small molecules (e.g., water, sodium chloride).
• Complex molecules (e.g., proteins, carbohydrates, lipids, nucleic acids) are composed of numerous smaller molecules.

Classification of Chemical Compounds

• Chemical compounds in living matter are categorized into:
  • Inorganic compounds (water and mineral salts)
  • Organic compounds (proteins, carbohydrates, lipids, and nucleic acids)

Water

• Water is crucial for all cellular chemical reactions.

• Most nutrients enter the cell dissolved in water.

• Water loss can result in cell damage.

• Water content varies among tissues, ranging from 80% in blood tissue to 20-27% in bone.

• Water content varies with organism type and cell activity.

• Water content differs between cells, and within an organism.

• Water molecule consists of two hydrogen atoms bonded to one oxygen atom by single covalent bonds.

• Water is polar; oxygen has a partial negative charge, and hydrogens have a partial positive charge

• The polarity of water gives it several important properties for living organisms.

• Water exists in two forms inside the cell:

  • Free water: Essential for cellular metabolism, acts as the cytoplasm environment, facilitates chemical reactions, and dissolves substances like dissolved salts for better chemical interactions.
  • Bound water: Participates in hydrogen bonding with large molecules like DNA and proteins, contributing to cell structure.

Functions of Water in Cells

Water's role in cells includes:

• Temperature regulation: Water has a high specific heat capacity which maintains stable internal cell temperatures.
• Solvency: Water dissolves many ions and compounds in cells.
• Transport: Water transports nutrients from the external environment into the cell and waste products out.
• Providing oxygen and hydrogen: Participating in the formation of many organic compounds.
• Neutral environment for cellular reactions: Supports many chemical reactions in cells due to its neutral pH of 7.
• Cohesion: Water molecules stick together (cohesion) due to hydrogen bonds, influencing properties like surface tension.

Mineral Salts (Inorganic Salts)

• Salts are dissolved in the cell's watery environment in all cell types.
• They exist in body fluids like blood and lymph.
• Salt composition and concentration vary by cell type.
• Salts contribute to cell survival and function.

Functions of Mineral Salts (Inorganic Salts)

• Components of important substances: E.g., amino acids, hemoglobin, containing iron, sodium, and potassium.
• Influence cell membrane polarization and depolarization (e.g., in nerve impulse conduction).
• Activators of enzymes.
• Transport: Example is iron carrying oxygen in hemoglobin.
• Maintain internal pH balance.

Organic Compounds (Carbohydrates)

• Carbohydrates are crucial components of living cells.
• Plant cells primarily contain 90%, and human cells contain 2%.
• Carbohydrates consist of carbon, oxygen, and hydrogen (in a 1:2:1 ratio).
• Their main function is to provide energy through oxidation.
• They also act as building blocks for other organic compounds.

Classification of Carbohydrates

• Simple sugars (monosaccharides): basic units (e.g., glucose, fructose, ribose, deoxyribose), major energy source, soluble in water, sweet taste.
• Double sugars (disaccharides): formed by joining two monosaccharides via a glycosidic bond (e.g., lactose (glucose + galactose), sucrose (glucose + fructose)).
• Complex sugars (polysaccharides): long chains formed by numerous monosaccharides linked through glycosidic bonds, insoluble, tasteless, and odorless. Two types:
  • Structural polysaccharides (e.g., cellulose (in plant cell walls), chitin (in insect exoskeletons)).
  • Storage polysaccharides (e.g., starch, glycogen (stored in the liver and muscles, easily converted to glucose when needed)).

Organic Compounds (Lipids)

• Lipids are a diverse group of organic compounds with similar solubility properties but different compositions.
• They are insoluble in water but soluble in organic solvents (alcohol, acetone, etc).
• Lipids serve as energy sources in cells and are components of cell membranes.
• Lipids are part of some vitamins and hormones (e.g., steroid hormones that originate from cholesterol).

Classification of Lipids

• Fatty acids: basic building blocks of many lipids, with a hydrophilic (water-loving) carboxyl group at one end and a hydrophobic (water-fearing) hydrocarbon tail at the other end. They consist of saturated or unsaturated fatty acids.
  • Saturated (e.g., palmitic acid): single bonds.
  • Unsaturated (e.g., oleic acid): double bonds which cause kinks.
• Triglycerides: three fatty acids attached to a glycerol molecule by ester bonds, main energy storage in adipose cells. Solid at room temperature if made of saturated fatty acids; Liquid at room temperature (oils) if made of unsaturated fatty acids.
• Phospholipids: glycerol, two fatty acids, a phosphate group, and sometimes other polar groups. A fundamental component of all cell membranes. Hydrophilic heads face the watery environment, and hydrophobic tails face inward.
• Glycolipids: Similar to phospholipids but with a carbohydrate group instead of phosphate. Components of some cell membranes (e.g., those in the nervous system).
• Steroids (e.g., cholesterol, hormones): four interconnected carbon rings, e.g., cholesterol is vital in cell membranes, and is a precursor for the synthesis of bile acids, vitamin D, and steroid hormones.