Unit 2: Topic 6 - Membrane Transport Flashcards
Describe passive transport.
Passive transport is the diffusion of a substance across its membrane across its concentration gradient, which means from an area of high concentration to low concentration. All molecules naturally want to move across their concentration gradient, so passive transport requires no input of energy.
Explain the importance of passive transport in the cell.
Passive transport plays a big role in moving molecules in and out of the cell. Many important and necessary molecules enter the cell through passive transport. For example, oxygen enters the cell through simple diffusion across the cell membrane. Water also enters the cell through diffusion (better known as osmosis) through aquaporins, which are channel proteins specific to water. Many waste molecules, such as urea, leave the cell through diffusion as well.
Describe active transport.
Active transport is the movement of molecules against their concentration gradient (meaning the molecules move from an area of low concentration to an area of high concentration). Molecules naturally want to move with their concentration gradient, so it takes energy to force molecules to move against their concentration gradient.
How does the selective permeability of the cell membrane contribue to the formation of concentration gradients?
Due to the nonpolar tails of the phospholipids that make up the cell membrane, polar molecules aren’t able to pass through without the help of transport proteins because the nonpolar tails repel polar molecules. The cell membrane also does not allow large molecules to pass through since there is limited space between individual phospholipids. These two concepts allow for the cell membrane to have selective permeability. This allows for the formation of a concentration gradient (meaning that one side of the cell membrane has a higher concentration of the molecule than the other) since not all molecules can freely pass through the cell membrane.
Describe the processes of endocyosis and exocytosis.
While small molecules can be transported using transport pumps, large molecules need to be transported through exocytosis or endocytosis. Exocytosis is the process in which a vesicle inside the cell (which can contain either large molecules or proteins that will be secreted) merges with the cell membrane and releases its contents to the outside of the cell. Endocytosis is the process in which the cell membrane pinches inward and forms a vesicle inside the cell containing materials brought in from outside the cell. This process usually depends on receptors on the cell membrane (these receptors are integral membrane proteins located on the outside of the cell membrane), meaning that when a certain molecule binds to the receptors, it triggers the pinching in of the cell membrane and the internalization of the receptors, the ligands that bound to them, and other substances near the site of invagination (the site where the cell membrane pinched in). Two specific types of endocytosis include pinocytosis (the endocytosis of a liquid containing dissolved substances) and phagocytosis (the endocytosis of a large particle or another cell).
