Cell Biology: Transport and Communication Notes

  • CELL SOLUTIONS

    • Hypertonic solution: Solution has higher solute concentration outside the cell; water flows out, causing cell to shrivel. In plant cells, cell membrane separates from cell wall (plasmolysis).

    • Hypotonic solution: Solution has lower solute concentration outside the cell; water flows into the cell.

  • MEMBRANE TRANSPORT

    • Facilitated Diffusion (Passive Transport):

    • Does not require cellular energy (ATP).

    • Requires a transport protein.

    • Moves substances down their concentration gradient (from high to low concentration).

    • Channel Proteins: Tunnel-like structures for hydrophilic molecules (e.g., aquaporins for water).

      • Gated Channels: Opened by specific stimuli (e.g., voltage-gated by electricity, chemically-gated by chemical binding).

    • Carrier Proteins: Bind to solute, change shape, and release solute on the other side of the membrane.

    • Active Transport:

    • Requires cellular energy (e.g., ATP, GTP).

    • Requires a transport protein.

    • Moves substances against their concentration gradient (from low to high concentration).

    • Sodium-Potassium ATPase (Pump): An electrogenic pump in animal cells.

      • Uses ATP to pump 3Na+3 Na^+ ions out of the cell and 2K+2 K^+ ions into the cell.

      • Generates and maintains concentration gradients for Na+Na^+ and K+K^+ and contributes to membrane potential.

    • Proton Pump: An electrogenic pump in plant, fungi, and bacteria cells.

      • Uses ATP to pump H+H^+ ions (protons) out of the cell.

      • Generates an electrochemical gradient.

    • Cotransport:

    • Active transport mechanism that uses the downhill diffusion of one solute (established by an active pump) to power the uphill transport of another solute against its concentration gradient.

    • Plant Cells: Proton pumps create an H+H^+ gradient; an H+H^+-sucrose cotransporter uses the influx of H+H^+ down its gradient to bring sucrose into the cell against its gradient.

    • Animal Cells: Sodium-potassium pumps create a Na+Na^+ gradient; Na+Na^+-glucose cotransporters in intestinal epithelial cells use Na+Na^+ influx to absorb glucose against its gradient.

  • MEMBRANE POTENTIAL & ELECTROCHEMICAL GRADIENT

    • Membrane Potential: Voltage difference across a membrane (e.g., −70mV-70 mV inside a neuron).

    • Caused by differences in ion charges across the membrane.

    • Inside of typical animal cells is negative relative to the outside.

    • Electrochemical Gradient: Combines two forces acting on an ion:

    • Chemical force: Ion's concentration gradient.

    • Electrical force: Effect of the membrane potential on the ion's movement.

  • BULK TRANSPORT (VESICLES): For large molecules (polysaccharides, proteins).

    • Exocytosis: Export of macromolecules out of the cell.

    • Macromolecules are packaged into vesicles that fuse with the plasma membrane to release contents (e.g., insulin secretion by pancreas cells).

    • Endocytosis: Import of macromolecules into the cell by forming vesicles from the plasma membrane.

    • Phagocytosis (Cellular Eating): Engulfment of large particles, whole cells (e.g., amoeba engulfing bacteria).

    • Pinocytosis (Cellular Drinking): Non-specific uptake of extracellular fluid and small solutes into small vesicles.

    • Receptor-Mediated Endocytosis: Specific solutes bind to receptors on the cell surface, triggering vesicle formation.

      • Enables selective uptake of specific substances (e.g., human cells importing cholesterol via LDLs; defective in familial hypercholesterolemia).

  • CELL SIGNALING

    • Quorum Sensing: Bacterial cells communicate via secreted signaling molecules to sense population density and coordinate behaviors (e.g., biofilm formation).

    • Direct Contact: Cells communicate via direct contact between surface molecules or through cell junctions.

    • Gap Junctions: In animal cells, allow direct exchange of ions and small molecules.

    • Plasmodesmata: In plant cells, allow cytosol connection between adjacent cells.

    • Local Signaling: Signaling molecules travel short distances to target cells.

    • Synaptic Signaling: Specific type of local signaling where neurons release neurotransmitters across a synapse to a target cell.

    • Long-Distance Signaling: Organisms use hormones as signaling molecules that travel via the circulatory system to target cells throughout the body.