fluid mosaic model
The currently accepted model of cell membrane structure, depicting the membrane as a mosaic of protein molecules suspended in a fluid bilayer of phospholipid molecules.
selective permeability
A property of biological membranes that allows them to regulate the passage of substances across them.
diffusion
The random movement of particles that results in the net movement of a substance down its concentration gradient from a region where it is more concentrated to a region where it is less concentrated.
concentration gradient
A region along which the density of a chemical substance increases or decreases.
passive transport
The diffusion of a substance across a biological membrane, with no expenditure of energy.
tonicity
The ability of a solution surrounding a cell to cause that cell to gain or lose water.
isotonic
Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell.
hypotonic
Referring to a solution that, when surrounding a cell, will cause the cell to take up water.
hypertonic
Referring to a solution that, when surrounding a cell, will cause the cell to lose water.
osmoregulation
The homeostatic maintenance of solute concentrations and water balance by a cell or organism.
osmosis
The diffusion of free water across a selectively permeable membrane.
facilitated diffusion
The passage of a substance through a specific transport protein across a biological membrane down its concentration gradient.
aquaporin
A transport protein in the plasma membrane of an animal, plant, or microorganism cell that facilitates the diffusion of water across the membrane (osmosis).
Can you identify six different types of functions of proteins in a plasma membrane?
Attachment to the cytoskeleton and ECM, signal reception and relay, enzymatic activity, cell-cell recognition, intercellular joining, and transport.
Why is diffusion across a membrane called passive transport?
The cell does not expend energy to transport substances that are diffusing down their concentration gradients.
Predict the net water movement between two solutions—a 0.5% sucrose solution and a 2% sucrose solution—separated by a membrane not permeable to sucrose.
Water will move from the 0.5% sucrose solution (lower solute concentration) to the 2% sucrose solution (higher solute concentration).
How do transport proteins contribute to a membrane’s selective permeability?
Because they are specific for the solutes they transport, the numbers and kinds of transport proteins affect a membrane’s permeability to various solutes.
Explain the function of the contractile vacuoles in a freshwater Paramecium in terms of what you have just learned about water balance in cells.
The pond water in which Paramecium lives is hypotonic to the cell. The contractile vacuoles expel the water that constantly enters the cell by osmosis.
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