AP Biology

AP Biology

Water— The Elixir of Life!

AP Biology

Why are we studying water?

All life occurs in water inside & outside the cell

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Chemistry of water

Water is a polar molecule A hydration shell

forms as water surrounds an ion

+ & – poles Each water

molecule can form a maximum of four hydrogen bonds at a time

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The Chemistry of Water

Unique physical properties include high specific heat Due to its high specific heat, water is slow to undergo changes in temperature. Much heat must be added or removed before the temperature of water changes much. The temperature of the water within living organisms tends to change more slowly than does that of the surrounding air or soil, so that the living cells are buffered somewhat against temperature fluctuations. This kind of protection is important because many biochemical reactions will take place only within a narrow range of temperatures.

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The Chemistry of Water

Unique physical properties, include a high heat of vaporization. This means that in order for water to reach the gaseous state, it must absorb a great deal of heat from the surroundings.

For many plants and animals, this property is the basis of a natural cooling system. Water evaporating from leaves, skin or lungs uses up heat from the organisms in the process of changing from liquid to gas. That is the reason mammals have evolved sweat glands, when the body is overheated, the glands pour watery "sweat" onto the skin, as the water evaporated, large amounts of body heat are used up and the body is cooled.

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Water Holds Immense Amounts of Heat

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Forecast for San Diego and Gallup 1/11/06-1/15/06

Why does it get so much colder at night in Gallup? See next slide if you can’t figure it out.

Water’s high heat capacity has profound effects on climate and ecology.

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Surface Tension

H bonding between H2O creates cohesion water is “sticky”

Tensile strength is related to cohesion and is a measure of the resistance of molecules to being pulled apart. This phenomena is responsible for "surface tension" that you see above. The water strider can "walk on water" because its feet don’t push hard enough to force the water molecules apart.

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Transpiration

Water has several other properties that make it important to living things. Water molecules exhibit strong cohesion = the tendency of "like" molecules to stick together.

This is the result of the polar nature of water molecules and the resulting hydrogen bonding. In the photos, water is transported up inside trees and other vascular plants inside tiny tubes call "xylem vessels". As water transpires off the leaves the cohesion pulls more water up through the vessels. (remember this when we get to plant form and function)

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The Chemistry of Water

Water shows strong adhesion : tendency of "unlike" molecules to cling together – capillary action

High surface tension allows long water columns to be

drawn from roots to leaves – even in a redwood. (It’s that transpiration thing again!)

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Water is a wonderful solvent

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Dissolving table salt (sodium chloride)

Solution: a mixture where everything is equally distributed

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Hydrophilic

Hydrophilic substances have affinity for H2O

polar or non-polar? ionic

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Hydrophobic

Hydrophobic substances do not have affinity for H2O polar or non-polar? non-ionic

fat (triglycerol)

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The special case of ice

Most substances are more dense when they are solid

But not water…

Ice floats! H bonds form a crystal with

loose structure

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Water Is Lighter as Solid than as a Liquid (so we can go

ice fishing)

This means that ice forms an

insulating blanket over

water.

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Why is “ice floats” important?

Oceans & lakes don’t freeze solid if ice sank…

eventually all ponds, lakes & even ocean would freeze solid

during summer, only upper few inches would thaw

surface ice insulates water below allowing life to survive the winter seasonal turnover of lakes cycling of nutrients

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Water forms ions

Hydrogen ion (H+) splits off from water to leave a hydroxide ion (-OH) H20 ----> H+ + -OH Low pH = lots of H+s = Acid High pH = few H+s = Base

pH scale = how acidic or basic a solution is a measure of proton

concentration H+

In biology, keeping H+ levels within a narrow range is critically important.

Most biological fluids have pH 6 – 8 pH values in human stomach can

reach 2

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pH Scale

Water is neutral!!! pH = 7

In pure water only 1 water molecule in every 554 million is dissociated. very small amount of ions [H+] or [OH-] is 10-7M

[H+] [OH-] = 10-14

pH scale is based on this equation

Buffers are substances that minimize changes in pH. They accept H+ from solution when they are in excess and donate H+ when they are depleted.

Carbonic acid (H2CO3) is an important buffer in living systems. It moderates pH change in blood plasma and the ocean.

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Punchline

Water is a polar molecule

The special properties of water make life on Earth possible

The chemical behavior of water governs how organisms function

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Any Questions??