Processes by Which Substances are Transferred Across Cell Membranes

**PROCESSES BY WHICH SUBSTANCES ARE TRANSFERRED ACROSS CELL MEMBRANES** **Diffusion** -- the movement of any substance from an area where it is more concentrated to an area where it is less concentrated. **Osmosis** -- a type of diffusion, but involving only the movement of water across a membrane...

**PROCESSES BY WHICH SUBSTANCES ARE TRANSFERRED ACROSS CELL MEMBRANES** **Diffusion** -- the movement of any substance from an area where it is more concentrated to an area where it is less concentrated. **Osmosis** -- a type of diffusion, but involving only the movement of water across a membrane and therefore proceeds from the more dilute to the more concentrated solution. **Facilitated diffusion --** moves substances in and out of cells from higher to lower concentration of a particular solutes **Active transport** -- moves substances across the cell membrane against the concentration gradient, - from lower to higher concentration. This results in the accumulation of substances on one side of the cell membrane. ATP is required. The movement of AA from the SI to the blood is an example of active transport. The Na-K pump is another example. **Phagocytosis** -- is the term used when solid particles are ingested. It is the process by which a cell engulfs a foreign substance or body. **Pinocytosis** -- the pumping of water across the membrane. It is similar to phagocytosis, except the smaller vesicles containing liquids are formed. **Question:** Why is meant by selectively permeable, and why is this term applied to the cell membrane? **Answer:** Permeable means that substances can pass through. Selectively permeable means that certain substances can pass through but not others... One of the important functions of the cell membrane is to regulate which substances are admitted into and out of the cell.