Passive Transport

Cellular Transport Mechanism
Examples of molecules that need to cross the cell membrane:

Molecules found outside or inside the cell can cross the cell membrane in three ways:

Passive Transport

• is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement of molecules.

• Substances move from an area of higher concentration to an area of lower concentration.

Three Types of Passive Transport

Simple Diffusion

• A single substance (O2 or CO2) tends to move from an area of high concentration to an area of low concentration until the concentration is equal across space.

  Factors That Affect Diffusion In Liquid

  Extent of the concentration gradient	The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
  Mass of the molecules diffusing	Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules.
  Temperature	Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.
  Solvent density	As the density of a solvent increases, the rate of diffusion decreases.
  Solubility	Nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.
  Surface area and thickness of the plasma membrane	Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it.
  Distance travelled	The greater the distance that a substance must travel, the slower the rate of diffusion.

Osmosis

Tonicity

When comparing two different solutions (inside the cytoplasm and outside the cell), the following terms are used:

• In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out.

•  In a hypotonic solution, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell.

• In a hypertonic solution, the extracellular fluid has a higher osmolarity than the cell’s cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively higher concentration of water, water will leave the cell.

Facilitated Diffusion

• The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane.

• The substances are then passed to specific integral proteins that facilitate their passage.

• Others are carrier proteins that bind with the substance and aid its diffusion through the membrane.

Types of Transport Proteins

• Channel proteins are embedded in the cell membrane & have a pore for materials to cross.

• Carrier proteins can change shape to move material from one side of the membrane to the other.