Diffusion:
A transport mechanism for moving chemicals into and out of the cell, from an area of higher concentration to an area of lower concentration.
Osmosis:
The movement of a fluid, usually water, across a permeable membrane from high water potential to low.
Concentration:
The amount of a substance (solute) present in a given volume of solution.

Diffusion Diagram

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What can and cannot diffuse?

  • Smaller particles like water and oxygen can easily transport across the cell membrane.
  • Large particles can slowly diffuse but not freely.
  • However charged molecules cannot diffuse through lipid bilayer .

What is it meant by facilitated diffusion? Does it require an energy input? Describe the process.

FACILITATED DIFFUSION --> MOVEMENT OF MOLECULES.

Facilitated diffusion is the transport of substances across a membrane from an area of higher concentration to an area of lower concentration. A special carrier molecule with a central channel acts as a "corridor" which helps the molecules moves across the membrane. Energy is not required in this process because the substances move along the direction of the concentration gradients.

Basically, a molecule from outside of the cell moves into the carrier protein. It then binds with the carrier protein; the protein helps the process along by changing shape and moving the molecule down the concentration gradient, through the membrane and into the cell where it is released. (see diagram below)



The red 'bricks' are the molecules moving through the carrier protein and into the inside of the cell.
The red 'bricks' are the molecules moving through the carrier protein and into the inside of the cell.

Facilitated diffusion is similar to simple diffusion in the sense that both involve the movement of molecules down their concentration gradient and both don't require any energy.

Facilitated diffusion can only take place if there is a carrier protein to help or 'facilitate' the process of difusion/to move the molecules down the concentration gradient.*

The process of faciliated diffusion is reversable; molecules can outside the cell to inside or inside the cell to outside. 

* concentration gradientis when there is an uneven distribution of a substance across a border.
when something moves against its concentration gradient, it is being forced from an area where it is less concentrated into a place where it is more concentrated







See here for a good animated video on how facilitated diffusion works.