What is water potential in cells?
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Water potential is the tendency of water to move from one place to another. It is typically measured in units of atmospheric pressure: pascals or pounds force per square inch or bars or dynes per square centimeter. It is a measure of the ability of a solution to absorb water by osmosis.
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An animal cell is normally as salty as its external solution. Animal cells do  not have cell walls so have no turgor pressure.  \nAn animal cell transferred from an isotonic to a hypertonic solution will  shrink as water moves out of the cell via osmosis.\nAn animal cell transferred from isotonic to hypotonic will gain water via  osmosis and swell. If the solution is dilute enough, the cell may burst  because it has no cell wall to prevent the cell membrane from stretching as the cell expands.\nTurgid and flaccid are terms that really only apply to cells with cell walls, such as plant cells. Animal cells may shrink or swell when placed in \nhypertonic and hypotonic solutions, respectively, but they never have turgor because the cell membrane is too weak. A cell wall is required for a cell to have a turgor potential.
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The first and most important thing is for the surface area to volume ratio. The smaller the ratio, the more efficient the cell.\n\nAlso, cells are building blocks of something bigger. If we had huge cells, imagine how big we'd have to be in order to hold trillions of them. ;)\n\nThat's about it in the nutshell.
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The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm, a typical cell mass 1 nanogram).\n\nThe cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.\n\nThe word cell comes from the Latin cella, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to the small rooms monks lived in.\n\nAll cells, whether prokaryotic or eukaryotic, have a membrane, which envelopes the cell, separates its interior from its environment, regulates what moves in and out, and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells.
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Increase Co2 result in the more production of sucrose in the guard cells.That increases its osmotic potential.Water from adjoining cells rush in.Resulting increase in it's opening.
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OSMOSIS IS THE MOVEMENT OF WATER MOLECULES ACROSS THE CELL MEMBRANE.  IF THE OSMOTIC PRESSURE IS HIGH i.e THERE ARE MORE WATER MOLECULES INSIDE THE CELL THEN WATER MOVES OUT TO LESSEN THE PRESSURE.  IF THE OSMOTIC PRESSURE IS VERY LOW THEN WATER MOLECULES MOVE INTO THE CELL TO KEEP THE PRESSURE OUTSIDE AND INSIDE EQUAL.\n\n(IT IS RUDE TO WRITE IN ALL CAPITALS)
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http://www.biologyreference.com/Ve-Z/Water-Movement-in-Plants.html\n\nThe Cohesion-Tension Theory\nThe major mechanism for long-distance water transport is described by the cohesion-tension theory, whereby the driving force of transport is transpiration, that is, the evaporation of water from the leaf surfaces. Water molecules cohere (stick together), and are pulled up the plant by the tension, or pulling force, exerted by evaporation at the leaf surface.\n\nWater will always move toward a site with lower water potential, which is a measure of the chemical free energy of water. By definition, pure water has a water potential of 0 MegaPascals (MPa). In contrast, at 20 percent relative humidity, the water potential of the atmosphere is -500 MPa. This difference signifies that water will tend to evaporate into the atmosphere. The water within plants also has a negative potential, indicating water will tend to evaporate into the air from the leaf. The leaves of crop plants often function at -1 MPa, and some desert plants can tolerate leaf water potentials as low as -10 MPa. The water in plants can exist at such low water potentials due to the cohesive forces of water molecules. The chemical structure of water molecules is such that they cohere very strongly. By the cohesion-tension theory, when sunlight strikes a leaf, the resultant evaporation first causes a drop in leaf water potential. This causes water to move from stem to leaf, lowering the water potential in the stem, which in turn causes water to move from root to stem, and soil to root. This serves to pull water up through the xylem tissue of the plant.\nFrom Root to Leaf\nPlants have root hairs and often mycorrhizal fungi at the root surface, both of which serve to filter the soil water as it enters the plant. Mycorrhizae are symbiotic associations between plant roots and fungi. The root cells and mycorrhizal fungi both actively uptake certain mineral nutrients. Mycorrhizae can be particularly important for the uptake of phosphate. The active uptake of minerals by living cells of the root and the subsequent transfer of minerals to the xylem can result in positive root pressures, with water potentials above 0 MPa. This occurs only under certain conditions, such as at night or during rainstorms, when water loss from the leaves is minimal. Such positive root pressures disappear with the onset of leaf transpiration.\n\nWater molecules move from the soil into living cells of the root, and eventually into the transport cells of the xylem, known as tracheids and vessels. These xylem cells are dead and hollow, allowing rapid water transport. They also have hardened cell walls to help them resist the tendency to collapse as water is sucked through them. Both tracheids and vessels \n\n \nA drop of guttation, water extruded by a plant's root pressure, on a blade of grass. have pits on the sides of their walls, which include porous areas for side-to-side transport. Unlike tracheids, a vessel is composed of many cells stacked end to end, with perforations between cells, allowing for more efficient transport.\n\nThe long-distance transport of the water molecule occurs first within the xylem cells of the root, then the xylem of the stem and branch, and then into the xylem of a leaf midrib and vein. Driven by transpiration, the water molecule is pulled from the nonliving tracheids and vessels of the xylem in the living cells of the leaf mesophyll (middle layer) and to the surface of mesophyll cell walls. The water molecule then evaporates into a leaf inter-cellular air space and finally out of a stomatal pore and into the atmosphere. Though photosynthetic action consumes some water, only a small fraction of the water that travels through the plant is used directly for the photo-synthetic reaction, which occurs in leaf mesophyll cells. Instead, most water is lost by transpiration through the stomates.\nThe Role of Stomates\nLeaves of land plants are covered with a waxy cuticle that pr
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Given that water in an hypotonic environment would tend to enter the cell, causing it to swell, it would constitute an expanding force aiding cell´s growing. Think of a baloon when you blow air into it: it kindda grows.
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The necessary organelle is the contractile vacuole.  Water will enters the cell by osmosis (freshwater is a hypotonic environment for the paramecium).  Since the paramecium doesn't have a cell wall, they would burst without a contractile vacuole.
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I assume you mean resting potential of the cell, for example a neuron.  It is created by biological pumps that pump ions into or out of the cell at to set up a specific electrochemical gradient.  So you get a voltage across the cell membrane along with a concentration gradient of sodium, potassium, chloride, and calcium.  When a signal is present it opens differnt pores that allow ions to cross the membrane quickly which causes an action potential and the desired effect, whether it be neurotransmitter being released, or calcium from the Sarcoplasmic reticulum.  The resting potential is the standard state for a cell maintained by the Na/K pump as well as calcium and chloride pumps that are generally ATP dependent.  ATP is the energy of the cell.  I can't remember off the top of my head if extracellular/intracellular concentrations of K,Na, Ca, and Cl but this shoudl give you the general idea.
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I'd have to say water (controls concentration gradients) and proteins (which form channels for things that can't cross the membrane by themselves).
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Cell surface hydrophobicity
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the more solute...the higher the solute concentration which lowers the water potential (concentration of water)
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While a cell is in it's resting membrane potential, the amount of K+ inside the cell is greater than extracellular potassium; at the same time, Na+ is in higher concentration outside of the cell than inside. While in this "resting" state, the cell is more negative inside the cell due to the mechanism of the Na+/K+ pump. 3 Na+ are pumped out of the cell while 2 K+ are pumped into the cell. Since both Na+ and K+ have plus 1 charges, the cell inside will be more negative than outside because of the difference in charge. \n\nWhen the cell is "excited", something called sodium gated channels open and Na+ shoots into the cell, making it more positive. This increase of positive charge shoots past the threshold potential (the amount of energy required for the action potential to be initiated), and the action potential is passed. Once it reaches a peak, potassium channels open, causing the cell to become more negative once again inside the cell. As soon as the potential goes back to it's resting potential, the cell is ready for another response.
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CELL MEMBRANE IS THE BOUNDARY BETWEEN IT'S CELL AND IT ENVIRONMENT
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Most professional papers that I read refer to it as just water ion (in its charged form).  This a water molecule with an over-all positive charge.  I do not believe that its structure is known, but I could be wrong about this.\nSee example reference below:
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Animal cells have little protection from water induced osmosis.  Leaving a cell in such an environment will surely cause the cell to absorb water until its pressure equalizes with it's surroundings.  Once the pressure reaches this critical level, the cell wall is compromised and it will burst.