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The importance of water:
-Water is incredibly important in the world of biology. It allows different types of reactions to occur. It is also a good reaction medium and a universal solvent. It is essential for hydration and survival, and is arguably the most important resource in order for life to flourish.
-It is the environment that many animals live in (amphibians/fish/some reptiles) and forms the basis for temperature regulation owing to its chemical properties.
-Structurally, water is composed of one oxygen atom bound to two hydrogen atoms in an overall polar structure (has charge distribution). Water is also responsible for the hydrophobic effect (protein folding).
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Diffusion definition:
'Diffusion is the movement of a given substance from an area of high concentration to an area of low concentration down the concentration gradient. It is a passive process and does not require energy.'
Osmosis definition:
'Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration down a water concentration gradient through a partially permeable membrane.
Above: A diagram of a water molecule with reference to polarity at play.
Water and temperature regulation:
High specific heat capacity: The SHC is the amount of energy required to raise 1kg of a given substance by 1 degree Celsius. Having a high SHC means that it is difficult to change the temperature of water, something that is an ideal quality for enzymes which operate in aqueous solution for example.
High latent heat of vaporisation: The LHOV is the amount of energy required to change one mole of a given substance at its boiling point. This means that when we sweat the water in our sweat removes a lot of heat when it evaporates.
High boiling point/low freezing point: A boiling point of 100 degrees Celsius makes water hard to boil, whilst sub zero temperatures are required to make it freeze. This makes water an appropriate temperature regulator.
Explained: The thing which holds water together so well and gives it the properties that it has are the intermolecular forces at play. These are called hydrogen bonds, giving water molecules adhesion between each other.
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Plant cells and water:
Plants use water to maintain turgidity in their cell walls. If they do not have enough water, they become flaccid. Turgidity is caused by turgor pressure (the force of water acting outwards on the plant cell wall). Water moves into and out of plant cells by osmosis.
Condensation and hydrolysis reactions:
Water can be used in order to build up or break down biological molecules. In doing so, it can take one of two reactions: hydrolysis or condensation. Hydrolysis (water-hydro to break-lysis) is where water is used to break a chemical bond. Contrastingly, condensation reactions help build up large structures by excluding water to form a bond.
Examples of condensation/hydrolysis reactions:
Making starch: Joining glucose monomers together into a polysaccharide (glycosidic bonds) is an example of multiple condensation reactions occurring.
Protease enzyme action: Enzymes typically use water as a reactive agent to hydrolyse a bond. Proteases target proteins, and therefore hydrolyse the peptide bonds.
Affecting water potential (concentration):
The water concentration, better recognised as the water potential of a given solution can be affected by different things, and establish gradients.
Consider a cell which is isotonic with its environment (meaning the concentration of water within the cell is equal to the water concentration surrounding the cell). If this cell has a higher salt content within it (but the same amount of water) then water will move into the cell by osmosis, causing it to swell. This is because the salt (solute) has lowered the water potential of the cell.
Consider the same cell but instead this time the cell itself is unchanged, with the original water potential and no salt. Place this cell in a now salty solution and water will move out of the cell by osmosis. This is because the salt lowers the water potential of the surrounding solution.
Investigating the water potential of potato tissue:
This practical outlines how you would determine the water potential of a given example of plant tissue. In this instance, potato is used.
Method outlined:
-Using a boring tool, cut cylinders of an equal size and shape from the same potato.
-Dry the cylinders of potato tissue with a paper towel.
-Weigh the cylinders using a mass balance, and record these masses.
-Place cylinders of potato tissue in different beakers of water. The different beakers have differing concentrations of sucrose in solution.
-Leave the tissue in for 1hr
-Remove, dry off water and reweigh potato tissue cylinders. Record these new masses.
-Compare the mass lost or gained from the surrounding solution due to osmosis. Find the solutions at which there is smallest mass change/no change in mass.
-Record the value of the concentration of sucrose in the water.
Method explained:
-cut cylinders of an equal size and shape: This is so that the surface area and volume of each cylinder of tissue is the same. Differing SA:Vol ratios would affect the rate of osmosis.
-Dry the cylinders: This is to remove excess water resting on the tissue itself. Excess water = inaccurate mass balance reading.
-Weigh the cylinders: This is to record their natural mass, including the water inside the tissue itself.
-differing concentrations of sucrose in solution: This is to encourage differing rates of osmosis to occur in each set of cylinders. By having different osmosis rates, a dataset from which the actual water potential of the tissue can be found.
-Leave the tissue in for 1hr: Allowing for enough time for osmosis to occur, with water either entering or leaving the potato tissue.
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