Diffusion & Facilitated Diffusion Flashcards
Diffusion is a type of transportation that occurs across the cell membrane
It can be defined as:
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.
How do molecules and ions move in diffusion (think about gradient and energy
The molecules or ions move down a concentration gradient
The random movement is caused by the natural kinetic energy of the molecules or ions
As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are……..
evenly spread within a given volume of space
What is facilitated diffusion and what molecules does it involve
Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)
These substances can only cross the phospholipid bilayer with the help of certain proteins
What proteins facilitate facilitated diffusion
There are two types of proteins that enable facilitated diffusion:
Channel proteins
Carrier proteins
channel proteins and carrier are very …….. to molecule they transport
specific
What are channel proteins and what do they transport
Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane
How do channel proteins transport charged substances
The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
This allows the channel protein to control the exchange of ions
How are carrier proteins different than channel proteins
Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
How do carrier proteins transport sugars, amino acids, and nucleosides
carrier proteins can switch between two shapes
This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
Exam Tip
Remember – the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein (for example because the molecule is charged and cannot pass directly through the phospholipid bilayer) this is facilitated diffusion.
