Concept of Movement of Substances Across a Plasma Membrane

3.2

Concept of Movement of Substances Across a Plasma Membrane

The characteristics of substances that are able to move across a plasma membrane:

There are three common factors that determine whether a molecule can pass through a plasma membrane, which are molecule size, polar molecule, and ionic charge.

Characteristics of substances across the plasma membrane
Lipid-soluble substances	Lipid insoluble substances
	Small molecules and ions	Large molecules
Non-polar molecules: Vitamin A, D, E, K Steroid compounds Fatty acids and glycerol	Polar molecules; water Non-polar molecules; oxygen, carbon dioxide, and ion: K+, Na+, Ca2+, Mg2+	Glucose and amino acids

Passive transport:

This process does not require energy
Examples of passive transport are simple diffusion,osmosis and facilitated diffusion.

Simple diffusion:

A process where the substances pass through the plasma membrane follows the concentration gradient.
The substances move from a high concentration region to a low concentration region.
The moving molecules are said to move down the concentration gradient until a dynamic equilibrium is achieved.
This may occur with or without the presence of a plasma membrane.
Lipid soluble molecules (fatty acids and glycerol), oxygen, and carbon dioxide diffuse through the phospholipid bilayer through simple diffusion.

Osmosis:

Osmosis is a passive transport process that is similar to diffusion but it involves only water molecules.
Osmosis refers to the net movement of water molecules from a higher water potential to a low water potential.
Osmosis occurs through the phospholipid bilayer.

Facilitated diffusion:

Lipid-insoluble molecules such as ions, large molecules such as amino acids, and glucose are unable to pass through the phospholipid bilayer.
These substances move across the membrane with the aid of transport proteins (carrier or pore proteins).
Facilitated diffusion does not require energy because the transport proteins transport molecules down a concentration gradient.
The process continues until a dynamic equilibrium is achieved when the concentration of molecules is the same at both sides of membranes.

Active transport:

The movement of a molecule or ion substances across a plasma membrane occurs against a concentration gradient.
It requires energy from ATP (adenosine triphosphate) molecules generated during cellular respiration.
It requires specific carrier proteins with specific active sites to bind with certain molecules or ions.
Carrier proteins also possess receptors to bind with ATP molecules.
Carrier proteins change shape when a phosphate group attaches to it.
As a result, molecules or ions move across a membrane.

The simillarities between passive and active transport:

The differences between passive and active transport:

Passive Transport	Active Transport
Energy does not required	Requires energy
Occurs following the concentration of the gradient.	Occurs againts the concentration of the gradient.
Occurs untill a dynamic equillibrium is achieved.	There are accumulation and disposal of molecule or ions.