CELLULAR TRANSPORT

• The cell membrane separates the cell from its surroundings with a structure described by the fluid mosaic model
• The cell membrane allows for the movement of materials into and out of the cell by diffusion, facilitated diffusion, osmosis, active transport, and vesicular transport (endocytosis/exocytosis)
• Diffusion is a passive transport process where molecules move from regions of high concentration to areas of low concentration along a concentration gradient
• Factors affecting diffusion rate across a membrane include concentration gradient, diffusion distance, surface area, and physical barriers
• Facilitated diffusion occurs when a substance is aided across a membrane by ionophores, selectively increasing the diffusion rate of specific molecules without requiring energy
• Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher to lower concentration of water molecules
• Water potential (Ψ) of a solution describes the tendency for water molecules to enter or leave a solution by osmosis
• Animal cells lack a cell wall and may change shape depending on the cellular environment, which can be isotonic, hypotonic, or hypertonic
• Hypotonic environment:

  • Water potential of the extracellular environment is less negative than that of the cell
  • Water enters the cell because the cell cytoplasm has a higher solute concentration
  • In animal cells, this causes cell lysis (cell bursting open)
  • In cells with a cell wall, the cell wall prevents cell rupture by providing rigidity and wall pressure
• Hypertonic environment:

  • Water potential of the extracellular environment is more negative than that of the cell
  • Cell loses water because the cellular environment has a higher solute concentration
  • In animal cells, this process is called crenation
  • In plant cells, the process is termed plasmolysis
  • Cells shrink due to a net loss of water across the plasma membrane
• Active transport processes:

  • Require energy expenditure to move materials against their concentration gradient
  • Performed by specific carrier proteins in the membrane
  • Transport proteins use ATP to pump molecules from low to high concentration
  • ATP transfers a phosphate group to the carrier protein, changing its shape for molecule movement
• Sodium-Potassium Pumps:

  • Exchange sodium ions (Na+) for potassium ions (K+) across the membrane
  • ATP is used as the energy source for the exchange
  • Creates large gradients in ion concentration for driving other active transport mechanisms
• Membrane pumps:

  • Proteins that require energy (often ATP) to transport molecules across the cell membrane
  • Pump activity may be coupled, e.g., H+ accumulation from the proton pump drives sucrose transport against its concentration gradient
• Cytosis:

  • Form of active transport involving membrane-bound vesicles or vacuoles
  • Plasma membrane regions infold or outfold to transport substances
  • Cells carry out various forms of cytosis, including endocytosis and exocytosis
• Endocytosis:

  • Incorporation of substances from outside the cell into the cell as a membrane-bound vesicle or vacuole
  • Three forms: phagocytosis (solid particles), pinocytosis (liquid particles), receptor-mediated (specific particles)
• Phagocytosis:

  • Engulfment and transport of solid particles into the cell
  • Particles contained within a membrane-enclosed sac (vacuole)
  • Digestion occurs when the vacuole fuses with a lysosome containing digestive enzymes
• Pinocytosis:

  • Engulfment of liquids or fine liquid suspensions into the cell
  • Plasma membrane invaginations enclose liquid droplets within small vesicles
  • Non-specific form of endocytosis, taking in all solutes suspended in the liquid medium
• Receptor-Mediated Endocytosis:

  • Involves engulfment and transport of specific molecules into the cell
  • Cell membrane has receptor proteins with binding sites for specific molecules
  • Clusters of receptor proteins called coated pits bind specific molecules for transport
• Exocytosis:

  • Releases substances from inside the cell to outside the cell
  • Occurs by fusion of a vesicle membrane with the plasma membrane
  • Vesicle contents are expelled into the intercellular space