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Chapter 5 explores the cell membrane—a dynamic barrier composed of a fluid phospholipid bilayer interspersed with proteins that control what enters and leaves the cell. This "fluid mosaic model" helps us understand how cells maintain their internal environment through processes like diffusion, active transport, and vesicle-mediated bulk transport.
Fluid Mosaic Model
Fluid Mosaic Model
The plasma membrane is a dynamic, fluid structure where proteins float within or on a phospholipid bilayer.
Early models depicted membranes as a “sandwich” of lipids with proteins coating the surfaces, but the modern view is of proteins interspersed throughout a fluid lipid sea.
Key Components :
Phospholipid Bilayer: Forms the basic barrier; it is only 5–10 nm thick yet creates a robust permeability barrier.
Transmembrane Proteins: Integral proteins span the bilayer (with hydrophobic regions inside and polar regions outside) to serve in transport, enzymatic activity, signal reception, and cell recognition.
Interior Protein Network: A scaffold (often involving cytoskeletal elements) helps maintain cell shape and organizes the membrane.
Cell-Surface Markers: Glycoproteins and glycolipids on the outer surface act as identifiers for cell recognition and tissue compatibility.
Evidence from electron microscopy (TEM and SEM) supports this fluid mosaic organization.
Chemical Bonds and Molecules
Chemical Bonds and Molecules
Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions (e.g., NaCl – table salt).
Covalent Bonds: Atoms share electrons to form molecules (e.g., H₂O – water).
Nonpolar covalent bonds: Equal sharing of electrons (e.g., O₂).
Polar covalent bonds: Unequal sharing of electrons, creating partial charges (e.g., H₂O).
Hydrogen Bonds: Weak attractions between molecules, important in DNA and water properties.
Water: The Essential Molecule of Life
Water: The Essential Molecule of Life
Water is a polar molecule, meaning it has a partial positive charge (H side) and a partial negative charge (O side). This allows for hydrogen bonding, leading to water’s unique properties:
Cohesion & Adhesion: Water molecules stick to each other (cohesion) and to other surfaces (adhesion), allowing capillary action in plants.
High Specific Heat: Water absorbs heat slowly and retains heat longer, helping regulate body temperature and climate stability.
High Heat of Vaporization: It takes a lot of energy to evaporate water, allowing for cooling effects like sweating.
Density of Ice: Ice is less dense than liquid water, meaning it floats and insulates aquatic ecosystems.
Water as a Solvent: Water dissolves polar and ionic substances, making it the "universal solvent" in biological systems.
Acids, Bases, and pH
Acids, Bases, and pH
source: https://www.sciencelearn.org.nz/images/4557-ph-scale
Acids donate H⁺ ions, increasing hydrogen ion concentration (pH < 7).
Bases remove H⁺ ions or add OH⁻ ions, decreasing hydrogen ion concentration (pH > 7).
Buffers help maintain a stable pH in organisms by neutralizing acids or bases.
Why This Matters?
Why This Matters?
Understanding chemical interactions helps explain how life works at the molecular level, from DNA replication to digestion.
Water’s unique properties make it essential for all biological processes.
Biological Importance : Cells must maintain a pH balance to prevent damage to proteins and enzymes. Blood uses bicarbonate buffers to maintain a stable pH (~7.4).
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