1.14 Passive Transport

Simple Diffusion Simple diffusion is defined as “the movement of particles from an area where they are more highly concentrated to an area where they are less concentrated” as a result of the random motion of the particles High Concentration  Low Concentration Concentration Gradient

Cell Membrane the cell membrane is selectively permeable tightly-packed phospholipids allow small, nonpolar (i.e. H2O, O2, CO2) molecules and small fatty acids to pass freely but prevent ions and large non polar molecules (i.e. amino acids, carbohydrates, nucleic acids, and large lipids) from passing the fatty acid portion imbedded proteins allow entry of small molecules unable to cross phospholipid bilayer

Passive Transport passive transport always works down a concentration gradient – aka. Osmosis and Diffusion the rate of passive transport is determined by particle size, lipid solubility, distance to travel, and the slope of the concentration gradient i.e. in the lungs, alveoli increase the surface area and capillaries are one cell thick as an adaptation to facilitate diffusion of O2 and CO2.

Facilitated Diffusion specialized transmembrane proteins act as carrier proteins for the diffusion of large or fat-insoluble molecules. This type of transport is passive as it will only go down a concentration gradient

Summary

Glucose Carrier Protein Example Another example is glucose (Fig. 7, p. 62). It attaches to a binding site on a carrier protein at one side of the membrane the structure of the carrier changes in order to carry glucose to the other side of the membrane. It then releases the glucose molecule, and changes back to its original shape, ready to accept another glucose molecule

Differences? the key difference between simple and facilitated diffusion is that facilitated diffusion requires transmembrane carrier proteins, whereas particles pass directly through the phospholipid bilayer or through protein channels in simple diffusion

Osmosis osmosis is the “net movement of water from the side where water is more concentrated to where it is less concentrated” this results in a net (total) diffusion of water from one side of a membrane to another osmotic conditions describe the relative concentration of solutes in the medium around a cell that will determine the movement of water in or out of the cell

Hypertonic Solutions hypertonic solutions have a higher (hyper = above) solute concentration (= lower water concentration) outside the cell resulting in a net movement of water out of the cell

If an animal cell such as red blood cell is placed into a hypertonic solution, water molecules is transported out from the red blood cells by osmosis (as shown in the diagram above). The red blood cells will shrink due to the lost of water from the cell and probably die. The red blood cells are said to undergo crenation .

Hypotonic Solutions hypotonic solutions have a lower (hypo = under) solute concentration (= higher water concentration) outside the cell resulting in a net movement of water in to the cell

If an animal cell such as red blood cell is placed into a hypotonic solution, water molecules is transported into the red blood cells by osmosis (as shown in the diagram above). The red blood cells will inflate and finally burst because the thin membrane cannot withstand the high pressure inside the cell. The red blood cells are said to undergo hemolysis

Isotonic Solutions isotonic solutions have the same (iso = same) solute concentration outside the cell resulting in no net movement of water in or out of the cell

If an animal cell such as red blood cell is placed into a isotonic solution, amount of water molecules is transported into the red blood cells by osmosis is equal to the amount of water molecules transported out from the cell (as shown in the diagram above). Therefore the amount of water in the cell remain unchanged. The red blood cells maintain their shape.

Summary: Osmosis

Examples osmosis is of great importance for freshwater organisms (living in a hypotonic solution) as they have evolved adaptations for expelling water (i.e. contractile vacuoles in Paramecium). Kidneys regulate water balance in terrestrial organisms, adjusting the water level in urine.  Osmosis Lab: Solute Concentration of Potatoes