Section 3
Transport Across Membranes
By Boundless
![Thumbnail](../../../../../../figures.boundless-cdn.com/18631/square/figure-05-02-02.jpeg)
Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18634/square/figure-05-02-06.jpeg)
Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18635/square/figure-05-02-07.jpg)
Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18632/square/figure-05-02-04.jpeg)
Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18622/square/figure-05-02-02.jpeg)
Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18641/square/figure-05-03-03.jpeg)
The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18639/square/figure-05-03-01.jpeg)
To move substances against the membrane's electrochemical gradient, the cell utilizes active transport, which requires energy from ATP.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18642/square/figure-05-03-04.jpg)
In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
![Thumbnail](../../../../../../figures.boundless-cdn.com/18643/square/figure-05-04-01.jpeg)
Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell.