Cell Movement

• Plasma membrane of cell selectively permeable (allows some substances to cross more easily than others)

• Made mostly of proteins and lipids (phospholipids).

Phospholipids and proteins create unique physical environment (fluid mosaic model)

• Membrane - bilayer - hydrophilic heads pointing outwards, hydrophobic tails pointing inwards.

  • This makes it an amphipathic molecule.

  • Membrane proteins can also be amphipathic (hydrophobic region is in a nonaqueous environment)

• The fluidity of the membrane means there is lateral movement.

• Membrane - bilayer - hydrophilic heads pointing outwards, hydrophobic tails pointing inwards.

  • This makes it an amphipathic molecule.

  • Membrane proteins can also be amphipathic (hydrophobic region is in a nonaqueous environment)

• The fluidity of the membrane means there is lateral movement.

• As temperatures drop, the phospholipids get more rigid and less fluid (think bacon grease solidifying)

  • This decreases the rate of diffusion, possibly killing the cell (in extreme conditions

  • Cholesterol found in membrane helps with fluidity of membrane.

    • Membranes need to be fluid to work properly.

• Two different types of proteins are found in membrane.

  • Peripheral proteins bound to surface of membrane.

Integral proteins in membrane often spanning entire membrane (transmembrane)

• Membrane helps keep cell’s shape.

• Also aids in cell-to-cell recognition.

• Some substances move steadily across membrane (sugars, ions, and wastes like CO₂)

Hydrophobic molecules (i.e. hydrocarbons, CO₂, and O₂) can dissolve in lipid bilayer and cross easily.

• Charged particles and polar molecules have more difficulty passing.

  • Specific ions and polar molecules can cross lipid bilayer by passing through transport proteins that span membrane.

Diffusion:

• Diffusion - tendency for substance to spread out in open area.

No force acting upon it - substance will tend to move down its concentration gradient from where it is more concentrated to less concentrated (passive transport).

• Diffusion of molecules with limited permeability through lipid bilayer may be assisted by transport proteins (facilitated diffusion)

Water is moved through aquaporins because of its charge.

• Charged particles that cannot pass through membrane - proteins to pass through (facilitated diffusion - diffusion of substance down it’s [ ] gradient with help of transport protein)

Some channel proteins (gated channels) open/close depending on presence/absence of physical or chemical stimulus.

Osmosis:

• Difference in concentration - ions move from one area to other.

• Solution with higher [ ] solutes - hypertonic.

• Solution with lower [ ] solutes -hypotonic.

[ ] equal - isotonic.

• Solution hypertonic - higher solute [ ] but lower H₂O [ ].

• H₂O moves into solution and solute moves out.

• Movement of H₂O across selectively permeable membrane - osmosis.

2 solutions isotonic, H₂O molecules move at equal rates (no net osmosis)

Without cell walls, organisms must osmoregulate (balance out the water and solute concentrations)

• Cell placed in hypertonic solution – H₂O rushes out of cell (cell shrinks).

• Cell placed in hypotonic solution – H₂O rushes into cell (cell swells).

Filtration –molecules forced through membranes (result of blood pressure)

• Plant cells expand when watered causing pressure to be exerted against cell wall.

  • Allows plant to stand up against gravity (turgid cell); not watered, plant will begin to wilt (flaccid cell).

Plant loses enough water, plasma membrane will pull away from cell (plasmolysis).

Active transport requires energy of cell to move substances from an area of low [ ] to an area of high [ ] (i.e. sodium-potassium pump in animal cells)

• Sodium-potassium pump actively maintains gradient of sodium (Na⁺) and potassium ions (K⁺) across membrane.

Sodium-potassium pump uses energy of 1 ATP to pump 3 Na⁺ ions out and 2 K⁺ ions in.

• Cells maintain voltage across plasma membranes.

  • Cytoplasm negative compared to  opposite side of membrane (membrane potential - ranges from -50 to -200 millivolts)

  • Membrane potential favors passive transport of cations (positive ions) into cell and anions (negative ions) out of cell.

    • Creates an electrochemical gradient across membrane.

    • Materials leave membrane through lipid bilayer or through transport proteins (active transport) - this can happen in bulk

• Exocytosis - transport vesicle buds from Golgi apparatus - moved by cytoskeleton to plasma membrane.

• When membranes meet - fuse - material is let out to outside of cell.

• Endocytosis - cell brings in macromolecules and matter by forming new vesicles from plasma membrane.

• Membrane is inwardly pinched off and vesicle carries material to inside of cell.

• Phagocytosis (“cell eating”) - cell engulfs particle by extending pseudopodia around it, packaging it in a large vacuole. 

• Contents of vacuole are digested when vacuole fuses with lysosome.

Pinocytosis (cell drinking) - cell creates vesicle around droplet of extracellular fluid.

• Receptor-mediated endocytosis - specific in transported substances.

• Extracellular materials bind to ligands (receptors) - causes vesicle to form.

• Allows materials to be engulfed in bulk (i.e. cholesterol in humans)