An overview of the process of transport through cell membranes

Search term Chapter 15Transport across Cell Membranes Three-dimensional structure of a recombinant cardiac gap junction membrane channel determined by electron crystallography.

An overview of the process of transport through cell membranes

It is also possible to move substances across membranes against their concentration gradient from areas of low concentration to areas of high concentration. Since this is an energetically unfavorable reaction, energy is needed for this movement. The source of energy is the breakdown of ATP.

If the energy of ATP is directly used to pump molecules against their concentration gradient, the transport is called primary active transport. Illustration of primary active transport. Note that the substance indicated by the triangles is being transported from the side of the membrane with little of the substance to the side of the membrane with a lot of the substance through a membrane protein, and that ATP is being broken down to ADP.


In some cases, the use of ATP may be indirect. On the left side of the picture below, a substance represented by an X is being transported from the inside of the cell to the outside even though there is more of that substance on the outside indicated by the letter X being larger on the outside of the cell.

This is primary active transport. In the picture on the right side, substance S, already at higher concentration in the cell, is brought into the cell with substance X. Since S is being transported without the direct use of ATP, the transport of S is an example of secondary active transport.

For substance X primary active transport of X is occurring. The high concentration of X outside the cell is being used to bring in substance S against its concentration gradient.Osmosis is a special term used for the diffusion of water through cell membranes.


Although water is a polar molecule, it is able to pass through the lipid bilayer of the plasma membrane. Aquaporins — transmembrane proteins that form hydrophilic channels — greatly accelerate the process, but even without these, water is still able to get.

An overview of the process of transport through cell membranes

Summary of Membrane Transport Processes. Only a limited number of molecules can cross biological membranes without the aid of transport proteins. The majority of biologically relevant molecules and ions utilize membrane transport proteins .

TRANSPORT IN AND OUT OF CELLS Table of Contents Water and Solute Movement | The Cell Membrane Links | References. Water and Solute Movement | Back to Top. Cell membranes act as barriers to most, but not all, molecules. as binding sites for substances to be brought into the cell, through channels that will allow materials into the cell.

Very few molecules enter or leave cells, or cross organellar membranes, unaided by proteins. Even transport of molecules, such as water and urea, that can diffuse across pure phospholipid bilayers is frequently accelerated by transport proteins.

The three major classes of membrane transport proteins are depicted in Figure a. Transport Across Cell Membranes. Facilitated Diffusion of Ions; Ligand-gated ion channels. but this process, called active transport, requires the expenditure of energy (usually from ATP).

2. Lipid bilayers are impermeable to most essential molecules and ions.

An overview of the process of transport through cell membranes

The transport of macromolecules through membranes is described in . In this module you will review mechanisms of membrane transport. The difference is how the substance gets through the cell membrane.

In simple diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels. It is possible for large molecules to enter a cell by a process.

Summary of Membrane Transport