Substances cross cell membranes without energy input in different ways. This page focuses on passive transport, especially the difference between simple diffusion and facilitated diffusion, and the roles of channel proteins and carrier proteins.

Passive Transport

Cells often exchange substances with their surroundings by moving them down a concentration gradient. This means there is a higher concentration on one side of the membrane than on the other.

In passive transport, particles still move randomly, but there is a net movement from high concentration to low concentration. This happens because more particles leave the high-concentration side than move back in the opposite direction.

Passive transport is important because it allows cells to take in some substances and remove others efficiently without direct energy expenditure. Two key types are simple diffusion and facilitated diffusion.

Simple Diffusion

Simple diffusion happens when particles pass directly through the membrane itself.

In simple diffusion, molecules move through the phospholipid bilayer. This is most suitable for substances that are small and nonpolar, because they can pass through the hydrophobic interior of the membrane more easily.

Examples include:

• oxygen

• carbon dioxide

A common exam point is that molecules are always moving in both directions. However, if there is a concentration gradient, the overall or net movement is from high to low concentration.

Simple diffusion:

• does not require ATP

• does not require transport proteins

• only works well for certain substances

• continues until there is no net movement across the membrane

If concentrations become equal on both sides, particles still move randomly, but movement in one direction is balanced by movement in the other.

Facilitated Diffusion

Some substances cannot diffuse directly through the phospholipid bilayer. They may be charged, polar, or relatively large, so they need help from membrane proteins.

Facilitated diffusion is still a form of passive transport, so substances move down their concentration gradient. The difference is that the membrane proteins provide an alternative pathway through or across the membrane.

Facilitated diffusion uses membrane proteins to provide a hydrophilic route across the plasma membrane without ATP. This diagram contrasts channel proteins (pores that let specific solutes pass) with carrier proteins (bind-and-change-shape transport), highlighting why charged or polar molecules can cross the bilayer only with protein help. Source

Substances commonly transported by facilitated diffusion include:

• ions, such as sodium or potassium

• glucose

• some other polar molecules

This process is selective. A protein that transports one substance will not necessarily transport another, because the protein has a specific shape and chemical properties.

Facilitated diffusion can also show saturation.

This graph compares how transport rate changes with increasing solute concentration for simple diffusion versus facilitated diffusion. Simple diffusion scales approximately linearly with the driving gradient, whereas facilitated diffusion rises then plateaus at a maximum rate once all transport proteins are occupied (saturation). Source

If all the available transport proteins are in use, increasing the concentration further will not increase transport proportionally.

Channel Proteins

Channel proteins form water-filled pores through the membrane. These pores allow specific ions or other suitable particles to pass through without entering the hydrophobic core of the phospholipid bilayer.

Channel proteins are important because:

• they provide a hydrophilic route across the membrane

• they allow rapid movement of suitable particles

• they are often specific to particular ions or molecules

Selectivity depends on the shape and properties of the channel. For example, the diameter of the pore and the charges on the inside of the channel can determine which ions pass through.

Some channel proteins are always open, while others may open and close. Even when channels change between open and closed states, transport through them is still passive if particles move down their concentration gradient.

Carrier Proteins

Carrier proteins work differently from channel proteins. Instead of forming an open pore, a carrier protein binds to a specific substance on one side of the membrane.

When binding occurs:

• the protein changes shape

• the substance is moved across the membrane

• the substance is released on the other side

This makes carrier proteins highly specific. Only molecules with the correct shape or chemical features will bind to a particular carrier protein.

Carrier proteins are often involved in the facilitated diffusion of larger polar molecules, such as glucose. Because each transport event depends on binding and a shape change, carrier proteins may transport substances more slowly than open channel proteins.

Like channel proteins, carrier proteins do not use ATP in facilitated diffusion as long as movement is down the concentration gradient.

Comparing Simple Diffusion and Facilitated Diffusion

Both processes are forms of passive transport, but they differ in important ways.

Similarities:

• both move substances from high concentration to low concentration

• both involve net movement down a concentration gradient

• neither requires ATP

Differences:

• simple diffusion occurs directly through the phospholipid bilayer

• facilitated diffusion requires membrane proteins

• simple diffusion is mainly for small nonpolar molecules

• facilitated diffusion is used for ions and larger or polar molecules

• facilitated diffusion is usually more selective

• facilitated diffusion can become limited by the number of proteins available

A useful way to distinguish them is to ask whether the substance can cross the membrane unaided. If it can pass through the bilayer, it uses simple diffusion. If it needs a channel protein or carrier protein, it uses facilitated diffusion.