Biochemistry and Cell Biology Overview

Biochemistry

• The study of chemical processes in living organisms.
• It combines biology and chemistry.
• Biochemistry focuses on the structure and interactions of biological macromolecules within cells.
• Biomolecules include carbohydrates, proteins, lipids, and nucleic acids.
• Biochemistry and medicine are closely related.
• Health depends on the balance of biochemical reactions in the body.
• Disease can result from abnormalities in biomolecules and biochemical processes.

Cell Biology

• The branch of biology that studies the structure and functions of cells in living organisms.
• Often referred to as cytology.
• Focuses on cellular processes like respiration, nutrition, cell division, defense mechanisms, and cell death.
• The cell is the fundamental unit of life.
• A tissue is a group of cells with similar structure and function.
• Tissues form organs, and groups of organs work together as an organism.
• Organisms can be unicellular (single-celled) or multicellular (multiple cells).

Composition of Cells

• Cells consist of micromolecules, water, inorganic ions, and carbon-containing molecules.
• Water is the primary component, accounting for 70% or more of a cell's mass.
• Cells have a complex structure comprised of:
  • Cell membrane
  • Cytoplasm
  • Nucleus
  • Organelles (e.g., chromosomes, endoplasmic reticulum, ribosomes, mitochondria, lysosomes).

Types of Cells

• Prokaryotic and eukaryotic cells are the main categories.

Prokaryotes

• Always unicellular.
• Very small (0.2-2.0 μm in diameter).
• Lack a nucleus and nuclear membrane; have a nucleoid region.
• Have ribosomes, cell membranes, but lack organelles like the endoplasmic reticulum and mitochondria.
• Single, circular chromosome for DNA.
• Reproduce through binary fission.
• Flagella are smaller in size.
• Examples include bacteria.

Eukaryotes

• Both unicellular and multicellular.
• Much larger than prokaryotes (10-100 μm in diameter).
• Have a true nucleus with a nuclear membrane.
• Have ribosomes, cell membranes, and a variety of organelles.
• Linear DNA in multiple chromosomes.
• Reproduce through mitosis (asexual) and meiosis (sexual).
• Flagella are larger in size.
• Examples include plant and animal cells.

Cell Membrane

• Also known as the plasma membrane.
• Separates the cell's interior from the external environment.
• Composed of a lipid bilayer with embedded proteins and carbohydrates.

Molecular Structure and Composition

• The lipid bilayer is composed of phospholipids, sterols (cholesterol), and glycolipids.

Phospholipids

• Most abundant lipid in the cell membrane.
• Consist of two fatty acid chains (hydrophobic tails) linked to glycerol and a phosphate group (hydrophilic head).
• Amphipathic molecules with both hydrophobic and hydrophilic regions.
• Form a double layer with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a barrier against water and polar substances.
• Form micelles or liposomes when heated in an aqueous solution.

Cholesterol

• Another lipid component of animal cell membranes.
• Selectively dispersed among phospholipids.
• Contributes to membrane rigidity.
• Not found in plant cell membranes.

Glycolipids

• Located on cell membrane surfaces.
• Have a carbohydrate sugar chain attached.
• Involved in cell recognition.

Proteins

• The second major component of plasma membranes.
• Can extend partway into the membrane, cross it completely, or be loosely attached to the inside or outside.

Membrane Fluidity

• Influenced by:
  • Fatty acid tails of phospholipids:
    • Unsaturated fatty acid chains with double bonds create kinks, increasing fluidity.
    • Saturated fatty acid chains with single bonds pack tightly, making a less fluid membrane.
  • Temperature:
    • Cold temperatures favor tight packing and decrease fluidity.
    • Warm temperatures allow for more movement and increase fluidity.
  • Cholesterol:
    • Holds the membrane in place, preventing excessive separation or compaction.
    • Minimizes the effects of temperature on fluidity.

Functions of Cell Membranes

• Support cell organelles and maintain cell shape.
• Provide a protective barrier and regulate material passage.
• Exhibit selective permeability, allowing certain molecules to enter or leave while restricting others.
• Contain receptors and channels for specific molecules (ions, nutrients, waste products).
• Regulate the concentration of substances inside the cell (ions, nutrients, waste).

Membrane Transport

• Movement of small molecules occurs through passive diffusion or active transport.
• Transport of large molecules occurs through endocytosis or exocytosis.

Passive Transport

• Movement of substances across the membrane without consuming cellular energy.
• Substances move from a high concentration area to a low concentration area (down the concentration gradient).
• Examples: Simple diffusion and facilitated diffusion.

Simple Diffusion

• Small, lipid-soluble molecules (e.g., oxygen, carbon dioxide) cross the membrane directly.
• Follows the concentration gradient.

Facilitated Diffusion

• Transport of substances that cannot cross the membrane directly (e.g., charged or polar molecules).
• Requires the assistance of membrane transport proteins (carrier proteins and channel proteins).

Carrier Proteins

• Bind specific substances and undergo conformational changes to allow passage across the membrane.
• Each carrier protein is specific to one substance.

Channel Proteins

• Form hydrophilic channels through the membrane, allowing polar molecules to bypass the hydrophobic core.

Active Transport

• Movement of substances across the membrane against their concentration gradient.
• Requires energy from ATP.