Biochemistry Water and the Fitness of the Environment

Water: The molecule that supports life on this planet! Water is the biological medium for all life on Earth All living organisms require water more than any other substance 2

Water: The molecule that supports life on this planet! Most cells are surrounded by water, and cells themselves are about 70–95% water The abundance of water is the main reason the Earth is habitable 3

FOUR Emergent Properties of water contribute to Earth’s suitability for life 1.Water’s cohesive & adhesive behavior 2.Water’s ability to moderate temperature 3.Water’s expansion upon freezing 4.Water’s versatility as a solvent. 4

What causes water molecules to both cohere and adhere? Yep! Our good buddy the intermolecular force (IMF) named hydrogen bonding Cohesion is when water molecules stick to each other. Adhesion is when water molecules stick to some other type of substance like plant cell walls. 5 Animation by scrolling over image

What causes water molecules to both cohere and adhere? Surface tension is a measure of how hard it is to break the surface of a liquid. Surface tension is a consequence of cohesion. What causes it? Go ahead, guess! Hydrogen bonding, yet again! 6

How does water moderate temperature? Water has a high heat capacity. Heat capacity is the amount of heat required to raise 1 gram of water by 1 o Celsius (1 calorie = J) What does that mean? It means water can absorb large quantities of heat without much change in its own temperature, thus it’s a good thermo regulator. But, why? Go, ahead…guess! 7

How does water moderate temperature? Yep! Hydrogen bonding again! The fact that each water molecule can make TWO H-bonds each means they are really, really attracted to each other, thus more ENERGY must be added to the water sample to increase their molecular motion. Remember, temperature is a measure of the average kinetic energy of a sample of molecules! 8

How does water moderate temperature? We should also discuss this effect as it relates to “air” with regard to the amount of water vapor in the air or humidity. The more water vapor in the air, the more heat the air can absorb. Large bodies of water absorb heat from warmer air and release stored heat to cooler air. 9

It’s a 2-way street! Water’s high specific heat can be traced to hydrogen bonding (IMFs) – Heat is absorbed when hydrogen bonds break – Heat is released when hydrogen bonds form The high specific heat of water minimizes temperature fluctuations to within limits that permit life 10

Care to examine some data? Formulate an explanation for the temperature data presented below. Your explanation should address the properties of water as they relate to thermoregulation. 11

Ever sweat? Ever perspire? Evaporation is transformation of a substance from liquid to vapor Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to vapor As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms (sweating, panting, etc.) and bodies of water 12

Water is a freak! It expands upon freezing! 13 Ice: stable hydrogen bonds Liquid water: transient hydrogen bonds

Water is a freak! It expands upon freezing! 14 Ice floats in liquid water because hydrogen bonds in ice are more “ordered” forming a hexagonal shape with a hole in the middle, making ice less dense. Water reaches its greatest density at 4°C, which is excellent news if you’re a fish! If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

Water is a freak! It expands upon freezing! 15 Hydrogen bond Ice: Hydrogen bonds are stable Liquid water: Hydrogen bonds break and re-form

Water’s Versatility as a Solvent A solution is a liquid that is a homogeneous mixture of substances A solvent is the dissolving agent of a solution or “dissolver” The solute is the substance that is dissolved or “disolvee” An aqueous solution is one in which water is the solvent 16

Water’s Versatility as a Solvent Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily. When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell 17

Water’s Versatility as a Solvent Water can also dissolve compounds made of nonionic polar molecules Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions 18

Hydrophilic vs. Hydrophobic A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water; “fears water” Oil molecules are hydrophobic because they have relatively nonpolar bonds A colloid is a stable suspension of fine particles in a liquid (like milk—fat suspended in water—Yummy!) 19

Acids, Bases and Buffers. Oh, my! ACIDIC AND BASIC CONDITIONS AFFECT LIVING ORGANISMS A bonded hydrogen atom within a water molecule can shift between two water molecules (from one molecule to the other). 20

Acids, Bases and Buffers. Oh, my! –The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H + ) –The molecule with the extra proton is now a hydronium ion (H 3 O + ), though it is often represented as H + –The molecule that lost the proton is now a hydroxide ion (OH – ) 21

Acids, Bases and Buffers. Oh, my! Though statistically rare, the dissociation of water molecules has a great effect on organisms About 2 in every billion water molecules exist as H + and OH – Changes in concentrations of H + and OH – can drastically affect the chemistry of a cell 22

Acids, Bases and Buffers. Oh, my! 23 Concentrations of H + and OH – are equal in pure water Adding certain solutes, called acids and bases, modifies the concentrations of H + and OH – Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic; also known as alkaline)

Acids, Bases and Buffers. Oh, my! Simply put, An acid is any substance that increases the H + concentration of a solution A base is any substance that reduces the H + concentration of a solution 24 Bleach

Acids, Bases and Buffers. Oh, my! Buffers The internal pH of most living cells must remain close to pH 7 Buffers are substances that RESIST changes in concentrations of H + and OH – in a solution, therefore they RESIST a change in pH Most buffers consist of an acid-base pair that reversibly combines with H + 25

Buffers in Action 26 Human activities such as burning fossil fuels threaten water quality CO 2 is the main product of fossil fuel combustion About 25% of human-generated CO 2 is absorbed by the oceans CO 2 dissolved in sea water forms carbonic acid; this process is called ocean acidification Acidification: A Threat to Water Quality

Buffers in Action 27 As seawater acidifies, H + ions combine with carbonate ions to produce bicarbonate Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals

The pH Scale 28 An acid is any substance that increases the H + concentration of a solution A base is any substance that reduces the H + concentration of a solution The scale was designed to compare WEAK acids and bases.

The pH Scale 29 In any aqueous solution at 25°C the product of H + and OH – is constant and can be written as the autoionization constant of water The pH of a solution is defined by the negative logarithm of H + concentration, written as For a neutral aqueous solution, [H + ] is 10 –7, so [H + ][OH – ] = 10 –14 pH = –log [H + ] pH = –(–7) = 7

The pH Scale 30 Acidic solutions have pH values less than 7 Basic solutions have pH values greater than 7 Most biological fluids have pH values in the range of 6 to 8