BSC 314 Lecture Notes - Lecture 20: Ground Tissue, Turgor Pressure, Vacuole
27 Jun 2018
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Movement of Materials in Cells
Cells are bathed in a watery matrix and conduct most of their reactions in a similar
watery fluid—a solution in which water is the solvent and the numerous molecules and
ions dissolved in it are the solutes. The solutes include protons (H +), ions like sodium
(Na +), potassium (K +), calcium (Ca 2+), organic molecules such as sucrose
(C 12H 22O 11), polar and nonpolar molecules, and a host of other substances, the
chemical nature of which determines the ease or difficulty with which they move across
membranes.
Diffusion
All molecules possess kinetic energy and move in a random fashion. In solutions,
solutes become distributed uniformly as they diffuse and occupy all available
space. Diffusion is the net movement of a substance from a region of its higher
concentration to a region of its lower as a result of the random movement of its
individual molecules; or, in other terms, down a concentration gradient. The greater
(steeper) the concentration gradient, the faster the movement. If nothing intervenes, the
movement will continue until the concentration gradient is eliminated, i.e. until the
substance is uniformly distributed. Most movement of materials in cells is by diffusion
although it is neither the most efficient means nor can it be used for long distance
moves.
Osmosis
Osmosis is a special kind of diffusion that pertains specifically to water: the movement
of water across a selectively permeable membrane that permits the passage of water
but inhibits the movement of the solute. The water moves down a concentration
gradient from the region of its higher concentration of free water molecules (less
solutes) to the region of its lower concentration of free water molecules (more solutes),
or from high pressure to low pressure.
In comparing the relationship of the cell contents to those of the surroundings, three
terms are used: 1.) isotonic: The two solutions have the same concentration of solutes,
hence the same amount of water moves into the cell as moves out; 2.) hypotonic: The
water outside the cell has lesssolute (hypo = less), and therefore more free water with
the result that water moves into the cell at a greater rate than it moves out;
3.) hypertonic: The water outside the cell has more solute (hyper = more), and
therefore less free water with the result that water moves out of the cell at a greater rate
than it moves in.
In osmosis, water moves from a hypotonic solution to a hypertonic through a selectively
permeable membrane. Water will diffuse across a selectively permeable membrane
until the concentrations are the same on both sides (i.e. isotonic). If pressure is applied
to the hypertonic side (the side into which the water is moving), it is possible to stop the
inward flow of water. The amount of pressure needed to do so is called the osmotic
pressure of the solution and is determined by the concentration of total solutes in the
solution. Osmosis doesn't depend on the kinds of molecules or ions in solution, only on
the amount of solutes.
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Document Summary
Cells are bathed in a watery matrix and conduct most of their reactions in a similar watery fluid a solution in which water is the solvent and the numerous molecules and ions dissolved in it are the solutes. All molecules possess kinetic energy and move in a random fashion. In solutions, solutes become distributed uniformly as they diffuse and occupy all available space. The greater (steeper) the concentration gradient, the faster the movement. If nothing intervenes, the movement will continue until the concentration gradient is eliminated, i. e. until the substance is uniformly distributed. Most movement of materials in cells is by diffusion although it is neither the most efficient means nor can it be used for long distance moves. Osmosis is a special kind of diffusion that pertains specifically to water: the movement of water across a selectively permeable membrane that permits the passage of water but inhibits the movement of the solute.
