CELL MEMBRANE
AND PASSIVE TRANSPORT NOTES
Function
The cell membrane provides protection and support for the cell. In addition, the cell membrane regulates what enters and leaves the cell.
The cell membrane is SEMI-PERMEABLE. This means that only some materials are allowed to pass through it.
Structure
The cell membrane is composed phospholipids. Each phospholipid consists of a polar phosphate head, and two non-polar lipid tails.
Structure
These phosphates are arranged in two layers. The polar phosphate heads face the inside and outside of the cell, while the polar tails face each other.
For this reason, the cell membrane is referred to as the PHOSPHOLIPID BILAYER.
It’s arrangement creates the barrier that keeps the cytoplasm and organelles inside of the cell, and materials that should remain in the external environment out.
INSIDE OF CELL
OUTSIDE OF CELL
(CYTOPLASM)
(EXTERNAL ENVIRONMENT)
Passive Transport Across the Cell Membrane
Passive transport is the movement of molecules across the cell membrane without the need for energy.
The three types of passive transport are:�1. DIFFUSION
2. OSMOSIS
3. FACILITATED DIFFUSION
DIFFUSION
In diffusion, molecules move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules).
This difference in concentration is known as the CONCENTRATION GRADIENT.
Because all molecules have random motion, they will eventually move down the concentration gradient until the molecules are in equal concentration!
Example: Diffusion across a membrane.
In the beaker to the right:
There are many more molecules on the left side of the membrane (dotted line) than the right.
Because of the random motion of molecules, molecules will move from the left side of the membrane to the right side of the membrane until the number of molecules on each side of the membrane are equal!
DIFFUSION ACROSS CELL MEMBRANES
Diffusion also occurs across your cell membranes.
Molecules like oxygen and carbon dioxide are small enough to pass through the phospholipid bilayer. Since your cell membranes are selectively permeable, these small molecules are allowed to pass back and forth across your cell membrane.
If there is a concentration gradient, molecules will move from an area of high concentration to an area of low concentration.
ENERGY REQUIRED? NO.
DIRECTION OF FLOW: HIGH TO LOW.
EXAMPLE MOLECULES: O2, CO2.
OUTSIDE OF CELL
INSIDE OF CELL
OSMOSIS
Osmosis is similar to diffusion, except that it occurs with water molecules.
The molecules of water still experience a concentration gradient (an area of high and low concentration).
The water will move accordingly, until the concentration of water molecules are distributed evenly.
VISIBLE DIFFERENCE IN
CONCENTRATIONS
OSMOSIS OCCURS UNTIL
CONCENTRATIONS ARE EQUAL
OSMOSIS ACROSS CELL MEMBRANES
Just like diffusion, osmosis also occurs across your cell membranes.
Water molecules can easily pass through your cell membranes, so water will osmose as long as there is a difference in concentrations on both sides of the cell membrane.
ENERGY REQUIRED? NO.
DIRECTION OF FLOW: HIGH TO LOW.
EXAMPLE MOLECULES: H2O.
OUTSIDE OF CELL
INSIDE OF CELL
FACILITATED DIFFUSION
Occasionally, large molecules need to pass back and forth across the cell membrane.
Since these molecules are large and cannot fit through the phospholipid bilayer, your cell membranes contain special protein channels that these large molecules can pass through.
This process does not require energy because molecules are still moving from high to low concentration.
ENERGY REQUIRED?: NO.
DIRECTION OF FLOW: HIGH TO LOW.
EXAMPLE MOLECULES: C6H12O6 (GLUCOSE).
Active Transport Across the Cell Membrane
Active transport is the movement of molecules across the cell membrane while requiring energy
The three types of Active transport are:�1. Sodium-Potassium Pump
2. Endocytosis
3. Exocytosis
Sodium-Potassium pump
Endocytosis
Exocytosis