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Wize Grade 12 Biology Textbook > Transport Across Membranes

Passive Transport: Simple and Facilitated Diffusion

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Simple and Facilitated Diffusion

Cells need to interact with their external environment in order to get what they need or to remove what they don't need. This is all done in a controlled manner by the cell membrane. The simplest form of transport across the membrane is passive transport.


During passive transport, molecules move down their concentration gradient; from a region of their
high
concentration to a region of their
low
concentration, resulting in the molecules becoming evenly distributed. For this type of transport, the cell does not have to use any energy to move these molecules.
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Form Fits Function

The cell membrane is selectively permeable, only allowing certain molecules to enter or exit the cell. Its structure is well suited for this function in many ways:

  1. Tight packing of phospholipids: Only allows small molecules can easily pass across.
photo by OpenStax / CC BY

  1. Amphiphatic phopholipids: Phospholipid bilayers form, with a hydrophobic core and a hydrophillic exterior.
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Types of Passive Transport

There are 3 types of passive transport: Simple diffusion, facilitated diffusion and Osmosis
  1. Simple Diffusion

photo by Mariana Ruiz Villarreal / CC BY
During simple diffusion, molecules will have a net movement from a high concentration to a low concentration. until their concentration is equal on both sides of the membrane.

Factors Affecting the Rate of Diffusion

  1. Concentration gradient: The greater the difference between the 2 concentrations, the faster the rate.
  2. Diffusing Distance: The shorter the diffusing distance, the faster the rate.
  3. Surface Area: The larger the surface area, the faster the rate of diffusion.
  4. Temperature: The higher the temperature, the faster the rate.
  5. Size and type of diffusing molecule: The smaller and non-polar (fat soluble) the molecule, the faster the rate.
  • Small, non-polar molecules
  • Example: Gases like oxygen and carbon dioxide, pass across the most readily; easily and quickly
  • Small, polar molecules
  • Example: Water and ethanol (alcohol) are just small enough that they can pass across but since they are polar, they do so slowly
  • Large, non-polar molecules
  • Example: Molecules like benzene can pass across but slowly since they are large .
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  1. Facilitated Diffusion
Molecules that are either too large or polar or both (large and polar) can only move down their concentration gradient with some help. This is known as facilitated diffusion.

photo by OpenStax / CC BY

  • The proteins that assist during facilitated diffusion are known as transport proteins; they are large, transverse the entire lipid bilayer and tend to be specific for the certain substance they translocate.
  • Two types of transport proteins: Channel proteins and Carrier Proteins
  1. Channel proteins: Fixed-shaped proteins that form a hydrophilic tunnel enabling water-soluble, polar molecules to travel across the membrane. Many channel proteins are gated, allowing the passage of ions only under particular conditions.
  • Example: Aquaporins are specific for only water molecules and are found in large numbers in kidney cells.
  • Example: Ion channels are specific for charged ions, many are gated.
  1. Carrier proteins- These proteins can rapidly change their shape in a way to get a specific molecule across the membrane, this may result in a slower transport of molecules across the membrane.
  • Example: GLUT1 helps to transports glucose across the cell membrane of many animal cells.

Example: Passive Transport

During a simple experiment you decide to compare cells which contain a transmembrane protein in question, with cells that do not have it. When the cells were placed in a water solution, those that had the protein in their membranes absorbed water and swelled up, while those that lacked the protein were not affected at all. What can you deduce about the identity of this protein?

Practice: Passive Transport

Why do cells not need to expend any energy during passive transport?

Practice: Diffusion

What property of gases, such as O2 and CO2, allows them to readily pass across the cell membrane?

Practice: Diffusion

The disease cystic fibrosis is caused by a defect in a protein that helps chloride ions move down their concentration gradient out of cells that line the lung, what type of passive transport mechanism is this?