Learning Outline

Cell Transport

BIO 095

 

Transport concepts

• Passive forms of transport — do not require cell expenditure of energy
  
  
  • Particles move down their concentration gradient
    
    
    • That is, particles move from area of high concentration to area of low concentration
      
      
    • If possible, particles eventually reach a dynamic equilibrium in which there is no difference in concentration

Terms related to diffusion
Solution - liquid mixture, usually composed of a liquid solvent and one or more dissolved particles (solutes) Solute - a dissolved particle Solvent - a liquid into which other particles may dissolve <EXAMPLE: seawater is a solution in which sea salts are the solutes dissolved in the solvent water> Permeable - describes a structure through which substances may move; impermeable means that the structure does not permit a substance to pass through Permeant - describes a substance that is able to move through a structure; impermeant means that the substance cannot pass through semi-permeable - describes a structure through which some, but not all, substance may pass selectively permeable - describes a living structure that is able to choose which (and when) particular substances may move through it conductance - the ease with which a substance may pass through a structure < EXAMPLES: A cell membrane may be permeable to oxygen but not to sodium ions, thus we say that the membrane is impermeable to sodium and that sodium is therefore an impermeant solute. Oxygen is a permeant solute. However, the cell may construct sodium channels and choose to open them under certain conditions. Thus we say that this membrane is selectively permeable to sodium. When the sodium channels open to allow sodium ions to pass through (be conducted through), we say that sodium conductance has increased. The more sodium channels open, the greater the sodium conductance. >
Semipermeable membrane. This membrane (yellow) allows some particles to pass through it—but not other particles. (click image to enlarge)

Passive forms of transport

Simple diffusion

Particles pass through cell membrane as they diffuse down their concentration gradient

Depends on how membrane-soluble the particles are

Mediated transport

Not-so-simple diffusion)

Transporters are required

• Cellular membrane structures that serve as gateways to permit movement of particles through the membrane
  
  
• The shape & size of transporters determine what can pass through; if the shape changes, the ability to transport may change
  

Channel-mediated passive transport

• Particles diffuse through membrane channels
  
  
• Some channels are gated (that is, they can open and close to selectively regulate conductance)
  

Carrier-mediated passive transport

• Particles diffuse through carrier mechanisms in a membrane
  

Also called facilitated diffusion

Osmosis

Osmosis = diffusion of water in presence of impermeant solutes

Aquaporins - water channels that facilitate osmosis

Osmotic pressure (actual vs. potential)

• Pressure that develops (actual) or could develop (potential) when water moves (osmoses) across a membrane and changes the volumes (and thus, the pressures) on both sides of the membrane.
  

Isotonic - solution with same potential osmotic pressure as another solution

• There is no net water movement between isotonic solutions
  

Hypertonic - solution with higher potential osmotic pressure than another solution

• There is net movement of water INTO a hypertonic solution
  

Hypotonic - solution with lower potential osmotic pressure than another solution

• There is net movement of water OUT OF a hypotonic solution

Red blood cells in solutions with different osmotic pressures. The labels at the top of the diagram describe the solution and not the intracelluar fluid. In the figure labeled "Hypertonic" the extracellular fluid is hypertonic but the intracellular fluid is hypotonic! Likewise, in the far right figure the cell is hypertonic to the solution outside the cells. In the middle figure, the solution inside the cell is isotonic to the solution outside the cell. Notice that cells placed into a hypertonkic solution shrivel (they are said to be crenated) because water leaves them through osmosis. Cells placed in a hypotonic solution swell because water enters them through osmosis. Such cells may eventually burst (lyse). Cells in isotonic environments experience no net diffusion, and thus remain normal and healthy.

 

Active forms of transport

Requires cell expenditure of energy

Ion pumps

Also called "active transport")

Particles are moved up their concentration gradient

Pumps are carriers that use energy transferred from ATP

Cotransport

• Particles are moved in the same direction by the same mechanism
  
  
• Also called symport
  
  

Countertransport

• Particles are moved in opposite directions by the same mechanism
  
  
• Also called counterport or antiport
  

The sodium-potassium pump. Also called the Na-K pump or Na+-K+ pump, this countertransport (antiport) mechanism is found in all cells. The structure may also be called Na-K ATPase, because it breaks down ATP to get the energy for pumping ions. The continuous opperation of this pump maintains concentration gradients across the plasma membrane for both Na+ and K+ ions. Cells tend to keep Na+ outside the cell and K+ inside the cell. The purpose of this concentration gradient will become apparent in your later studies.

Bulk transport by vesicles

Exocytosis

• Moves large number of molecules OUT OF a cell
  
  
• Internal vesicle moves to plasma membrane and "pops open" releasing material from vesicle
  
  

Endocytosis

• Moves large number of molecules INTO cell
  
  
• Plasma membrane pinches in, trapping extracellular material into a vesicle
  
  
• Two types:
  
  
  • Phagocytosis
    
    
    • Chunks are brought into cell (literally, "cell eating")
      
      
  • Pinocytosis
    
    
    • Fluids are brought into cell (literally, "cell drinking")
      
      
• Endocytosis is often receptor-mediated
  
  
  • Molecules outside the cell trigger receptors in the plasma membrane—this initiates the mechanisms of engulfing the material
    
    
  • The cells "taste" before they swallow, eh?