Water – important points Water is a polar molecule (there is an uneven distribution of ………… across the molecule). The oxygen attracts the pair of electrons in the O-H bond more ………………. than the hydrogen atom. This leaves the …………… atom with a delta negative (δ-)charge and the ……………..a delta positive(δ+) charge. The oxygen with a delta negative (δ-) charge in one …………… will attract and the hydrogen with a delta positive(δ+) charge in …………………molecule. These attractions are called ……………….bonds and are VERY IMPORTANT to water’s properties. charge strongly oxygen hydrogen H2O molecule anothrt H hydrogen O H

Why is water important for living organisms? Discuss this with the person next to you. H O H

Water quick quiz AS Biology Give the chemical formula of water. [1] Is water polar or non-polar? [1] Name the bonds that form between water molecules. [1] Ice is more dense that water, true or false? [1] Water molecules can stick to each other, true or false? [1] Is water used or made in photosynthesis? [1] The temperature of large bodies of water fluctuates dramatically, true or false? [1] Small organisms can “walk on water”, true or false? [1] Are ionic compounds soluble or insoluble in water? [1] Name the process by which water moves into and out of cells. [1] H2O (l) Polar hydrogen False True Used False True Soluble Osmosis

Biological importance of water AS Biology Biological importance of water Property of water - SOLVENT Polar molecules/ions dissolve (are soluble) in water (e.g. nitrates, NO3- ions). Water is attracted to the ions/polar molecules, clusters around them and separates them. Once dissolved, chemicals can react together in solution. Importance/examples Organisms can take up dissolved minerals/ions (nitrates used to make amino acids). 70-95% of the cytoplasm is water with dissolved chemicals (solutes). Respiration and photosynthesis relies on reactants being dissolved in water and, therefore, being able to react.

Biological importance of water AS Biology Biological importance of water Property of water - LIQUID Water is a liquid at room temperature. Hydrogen bonds form between water molecules, forming a network that allows the molecules to move around, continually making and breaking hydrogen bonds. Importance/examples Water is used as a liquid transport medium, moving materials around living organisms. Blood in animals uses water as a liquid transport medium. Vascular tissues in plants uses water as a liquid transport medium.

Biological importance of water AS Biology Biological importance of water Property of water - COHESION Hydrogen bonds between water molecules results in water molecules “sticking” to each other = COHESION. Importance/examples Cohesion means that long, thin water columns of water are very strong and difficult to break. Water molecules in the xylem stick to each other and are pulled up the xylem in the transpiration stream. Cohesion also creates surface tension at the water surface. Some small invertebrates (insects) make use of this to “walk on water” and this provides a habitat.

Biological importance of water AS Biology Biological importance of water Property of water - FREEZING As water is cooled below a temperature of 4C, its density decreases, the molecules spread out more. This is due to more and longer hydrogen bonds forming between H2O molecules, creating an open lattice structure. Ice is less dense that liquid water, and ice floats on liquid water. Importance/examples When water freezes, ice forms on the surface of water. Organisms (e.g. Polar bear) live in an environment of floating ice packs. Lakes tend not to freeze completely, but ice insulates water below, and this means that aquatic organisms do not freeze, can move about and survive the winter. Also allows nutrients to circulate.

Biological importance of water AS Biology Biological importance of water Property of water – THERMAL STABILITY A relatively large amount of energy is needed to raise the temperature of water. There are many stable hydrogen bonds between water molecules that need to be broken. So water has a (high specific heat capacity). Importance/examples The temperature of large bodies of water, such as lakes and oceans, remains stable even when the outside temperature changes dramatically. This provides a stable environment in terms of temperature. Gases will remain dissolved (e.g. oxygen). Organisms temperature changes very slowly and they use less energy on temperature control. The evaporation of water (liquid to gas) uses up a relatively large amount of energy (latent heat of vaporisation). Water evaporating from a surface “removes” heat from the surface, using heat energy in evaporation. Organisms use evaporation as a cooling mechanism (e.g. panting and sweating).

Biological importance of water AS Biology Biological importance of water Property of water – METABOLIC Water takes part as reactant in chemical processes inside cells (metabolism). Importance/examples Water is used in photosynthesis. Water molecules are used in hydrolysis reactions.