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