Cell to Cell Transport Facilitated Diffusion

Introduction

Transport of materials across a cell membrane may occur in either active or passive movement. There are two types of cell to cell transport systems(short distance transport) that are namely:

Passive transport: A cell transport system that does not require any energy and proceeds through diffusion is called the passive transport of materials across cell membranes. Processes that involve passive transport are diffusion, facilitated diffusion, filtration, osmosis etc.
Active transport: A cell transport system that requires energy to transport through cell membranes is the active transport system.

We know that some biological/cellular membranes are semi-permeable in nature and some are selective in nature. Semi-permeable membranes allow transport of only certain materials i.e. large and charged molecules can not be transported. However, selective types of cellular membranes allow the movement of materials that can not freely cross.

Active Transport

In an active transport system, there is the movement of materials against a concentration gradient i.e. movement of materials from a low concentration gradient to a region of a high concentration gradient. Since in an active transport system there is a movement against the gradient, it requires energy and thus, active transport is an energy-dependent process. Active transport systems can be primary (direct) active transport or secondary (indirect) active transport. Primary active transport involves the direct use of metabolic energy to mediate transport.

The main features of the active transport process are:

Active transport occurs against a concentration gradient (movement from low concentration to high concentration gradient)
Active transport is an energy-dependent process i.e. it requires the expenditure of energy

Main types of active transport:

Primary Active transport
Secondary Active transport

Inactive transport systems if energy is harnessed by the direct hydrolysis of ATP then such systems are referred to as primary active transport (e.g. Uniport/Cotransport or pump mediated). An example of an antiporter is a sodium-potassium ion pump where both sodium ions and potassium are pumped in two opposite directions. If energy is harnessed by the coupling with the transport of another molecule moving along its electrochemical gradient then such systems are referred to as secondary active transport (Antiport/symport or carrier-mediated). Uptake of glucose molecules in kidneys coupled to the parallel transport of sodium is an example of a symport active transport system. Since sodium moves passively is an example of secondary active transport
If two distinct molecules are coupled and then transported, such active transport is called co-transport
If the movement of transport of two molecules is in the same direction it is called symport and if the movement of transport of two molecules is in opposite direction it is called antiport. The uptake of glucose molecules in kidneys coupled to the parallel transport of sodium is an example of a symporter
An example of an antiporter is a sodium-potassium ion pump where both sodium ions and potassium are pumped in two opposite directions
If there is the movement of a single molecule then it is called uniport

Passive Transport

In Passive transport, the movement of material is along a concentration gradient that proceeds from high concentration to low concentration. Since there is a movement down a concentration gradient, the expenditure of energy (ATP hydrolysis) is not required during passive transport.

The main features of the active transport process are:

Passive transport occurs along a concentration gradient (movement from High concentration to low concentration gradient)
Passive transport is an energy-dependent process i.e. it does not require the expenditure of energy

Simple diffusion:

All of the passive transport systems involve a downhill movement — that is, a movement from an area of high concentration towards an area of low concentration. A passive transport system is an energy-independent movement process. Simple diffusion is the major type of passive transport system.

The motion is reduced as well

The net movement of molecules from a region of high concentration towards a region of low concentration is the process of simple diffusion and this diffusion process will continue until there is a concentration gradient i.e. molecules are evenly distributed (equilibrium). Various small and non-polar (lipophilic) molecules freely diffuse across cell membranes (e.g. O2, CO2, glycerol) is an example of simple diffusion.

There are various factors that affects the process of simple diffusion:

Temperature ( kinetic energy of particles in solution is affected)
Molecular size (within a liquid medium the larger particles undergoes greater resistance)
The steepness of gradient (with a higher concentration gradient rate of diffusion is greater)

Facilitated Diffusion

The diffusion rate depends on the size of the substances; obviously smaller substances diffuse faster. The diffusion of any substance across a membrane also depends on its solubility in lipids, the major constituent of the membrane. Substances soluble in lipids diffuse through the membrane faster. Substances that have a hydrophilic moiety find it difficult to pass through the membrane; their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion. In facilitated diffusion special proteins help move substances across membranes without expenditure of ATP energy. Transport rate reaches a maximum when all of the protein transporters are being used (saturation).Facilitated diffusion is very specific: it allows cells to select substances for uptake. It is sensitive to inhibitors which react with protein side chains.

Examples of such transport proteins which are involved in facilitated diffusion include channel proteins, gated channel proteins and carrier proteins.

Channel Proteins

Transport protein acts like a pore via which water molecules or small ions may cross from one side of the membrane to the other very quickly
Proteins that allow water to diffuse across the membrane fastly are water channel proteins(aquaporins)
Proteins that allow ions to diffuse across the membrane are the ion channel proteins
Channel proteins have a much faster rate of transport and move molecules along a concentration gradient

Gated Channel protein

Transport proteins where there is an ion ‘gate’ which opens to allow a molecule to pass through the membrane or ‘close’ in the effect of the moving ions
These gated channels open or close in response to chemical or electrical signals, temperature or mechanical force, depending on the type of gated channels
These channels have particular binding sites which are specific to ions or molecules
Examples are sodium-gated channels. Large molecules such as glucose which are a major source of energy to cells require gated ion channels to move across membranes

Carrier Proteins(facilitated diffusion via a protein channel)

Transport proteins specific for an ion, molecule,or group of substances act as carrier proteins and this facilitates diffusion via a protein channel
As the word ‘carrier’ suggests it depicts the proteins which can carry the ion or molecule across the membrane by changing their conformation after the binding of the ion or molecule
It is similar to Enzyme substrate interaction. Carrier proteins have a slower rate than channel proteins and movement is against concentration gradients in the presence of ATP

Simple diffusion and facilitated diffusion are related in that both require energy for the transport of substances. Simple diffusion and facilitated diffusion are related in that both are a type of passive transport

Simple diffusion and facilitated diffusion are related in that both involve the movement of substances from regions of lower concentration to higher concentration without the requirement of energy

Simple diffusion and facilitated diffusion differ in that simple diffusion doesn’t require any assistance of transport protein for the movement of molecules across membrane proteins. While in facilitated diffusion molecules diffuse with the assistance of transport proteins (such as carrier protein, gated channels, channel proteins)

Example of simple diffusion: Transport of small nonpolar molecules across the plasma membrane. Example of facilitated diffusion: Transport of glucose and ions into and out of the cell with the assistance of transport proteins

Conclusion

Transport of materials across a cell membrane may occur in either active or passive movement. There are two types of cell to cell transport systems(short distance transport).