TOPIC 3.1

Chemical Elements and Water

How do we organize living things?

BiosphereEcosystemCommunityPopulationOrganismOrgan SystemOrganTissueCellOrganelleMoleculeAtom

Why Chemistry?

To understand the structures and functions of living organisms, it’s essential to know the basis of all living things.

The properties of life emerge from the arrangement of its chemical parts into higher and higher levels of biological organization.

At the base of the hierarchy of living things: Elements, atoms, molecules

What’s the Matter?

Matter Anything that occupies space and has mass. In forms as diverse as rock, water, air, and living organisms Composed of chemical elements

Element A substance that cannot be broken down to other substances

by ordinary chemical means. Life requires about 25 chemical elements:

4 make up about 96% of the human body: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N) Main ingredients in biological molecules such as proteins,

sugars, and fats A few others make up the remaining 4%: Calcium (Ca),

phosphorous (P), sulfur (S), sodium (Na), Chlorine (Cl), and Magnesium (Mg) Important in bone formation, nerve signaling, and DNA

synthesis

Electronegativity

Electronegativity An atom’s attraction for its electrons, including

shared electrons. Molecules like 02 consist of two identical atoms, and

thus exert an equal pull on the electrons. Covalent bonds are said to be nonpolar because the

electrons are shared equally between the atoms. Some compounds, like CH4, also have nonpolar

bonds.

Electronegativity and Water

Electronegativity and waterUnequal sharing of electrons produces what

is called a polar covalent bond. The pulling of shared electrons closer to the more

electronegative atom makes that atom partially negative and the other atom partially positive.

Oxygen is one of the most electronegative elements, and thus attracts the shared electrons in H2O much more strongly than H does.

the O atom has a slight negative charge and each H atom a slight positive charge.

The V shape and its polar covalent bonds make water a polar molecule

Water

Hydrogen bonds

Since the positively charged region in H20 is always a hydrogen atom, this polar covalent bond is called a hydrogen bond.

O on each water molecule can form hydrogen bonds with two H atoms

Each H20 molecule can hydrogen-bond to as many as four partners.

The hydrogen bond is a weak bond.

Water

Properties of Water

CohesiveThermalSolvent

Properties of Water

Cohesive Tendency of molecules to stick together Much stronger for water than for most other liquids. Hydrogen bonds give water unusually high surface

tension, a measure of how difficult it is to stretch or break the surface of a liquid Water looks as though it were coated with an invisible

film. Very important in the living world:

Trees depend on cohesion to help transport water and nutrients from their roots to their leaves.

Properties of Water

Thermal Because of hydrogen bonding, water has a better ability to

resist temperature change than most other substances. Because of this, Earth’s giant water supply moderates

temperatures, keeping them within the limits that permit life: When water is heated, the heat energy first disrupts hydrogen

bonds and then makes water molecules move faster. Because heat is absorbed as the bonds break, water absorbs and stores a large amount of heat while warming up only a few degrees.

When water is cooled, more hydrogen bonds form. Heat energy is released when the bonds form, slowing the cooling process.

Properties of Water

Thermal (continued) A large body of water can store a huge amount of heat from

the sun during warm periods. At cooler times, heat given off from the gradually cooling

water can warm the air. That’s why coastal areas generally have milder climates than inland regions. Water’s resistance to temp. change also stabilizes ocean temperatures, creating a favorable environment for marine life.

And at 66% of your body weight, water helps moderate your internal temperature (HOMEOSTASIS!)

Hydrogen bonds also decrease water’s tendency to evaporate, or vaporize.

Another way temperature is moderated by water is via evaporative cooling. When a substance evaporates, the surface of the remaining

behind cools down as the molecules with greatest energy leave.

Helps prevent overheating

Properties of Water

Solvent Water is known as the universal solvent, or dissolving agent. When it is the solvent, the result is an aqueous solution. Water’s versatility as a solvent results from the polarity of

its molecules: For example, Na+ attract the electronegative O of water

molecules, and Cl- attract the positively charged H regions. As a result, H2O molecules surround and separate NaCl, dissolving the crystal in this process.

Blood and most other biological fluids are classified as aqueous solutions.

Properties of Water

Other things about water… Water exists in nature as a gas (water vapor, liquid,

and solid) Water is less dense as a solid than as a liquid.

Ice crystals have more spaciously arranged molecules, and water is more tightly packed. Therefore, there are fewer molecules in an ice cube than an equal volume of water. Ice Liquid water