The Chemical Basis of Life Why are we studying chemistry in a biology course??? At the base of the hierarchy of life are atoms and molecules! Many biological processes take place on a molecular level Biosphere Ecosystem Florida coast Community All organisms on the Florida coast Population Group of brown pelicans Nucleus Nerve Spinal cord Cell Nerve cell Tissue Nervous tissue Organ Brain Organelle Nucleus Molecule DNA Atom Organism Brown pelican Organ system Nervous system Brain

The food we eat is made up of chemical elements, many of which are essential to life

Elements, Atoms and Molecules Living organisms are made up of 25 chemical elements An element is a ‘substance’ that cannot be broken down to any other substance by ordinary chemical means –Remember your Periodic Table of Elements? –Oxygen (O), Carbon (C), Nitrogen (N), Hydrogen (H) = ~96% of the weight of the human body Main ingredients of proteins, sugars and fats

Elements, Atoms and Molecules The next 3.99% is made up of –Calcium (Ca)  bone formation –Phosphorus (P)  DNA synthesis –Potassium (K)  cell signaling, nervous system –Sulfur (S) –Sodium (Na) –Chlorine (Na) –Magnesium (Mg)

Elements, Atoms and Molecules The remaining 0.01% are represented as trace elements –very small quantities, but extremely important Examples of trace elements include: –Iron (Fe) – hemoglobin, binds oxygen –Copper (Cu) – enzyme function –Fluorine (F) – prevents tooth decay –Iodine (I) – thyroid hormones

Elements, Atoms, and Molecules An element is a unique substance that cannot be broken down to into smaller substances An atom is the smallest unit of matter (stuff that the universe is made of) that still retains the properties of an element A molecule is a substance consisting of 2 or more elements (either the same or different) –Sodium chloride (NaCl; “table salt”) –Water (H 2 O) –O 2 (oxygen)

The Periodic Table of Elements lists all of the known elements and their atomic numbers

Atoms An atom is made up of: −Proton: (+) charge −Electron: (-) charge −Neutron: neutral, no charge Each elements differs by the number of protons, neutrons and electrons found in their respective atoms

Elements An element is an atom with a specific number of protons in its nucleus Every element has an atomic number. The atomic number is the number of protons in the nucleus of the atom

Structure of an Atom An atom is made up of a nucleus which contains protons (positive charge) and neutrons (no charge) and is surrounded by a cloud of electrons (negative charge) nucleus proton neutron electron = + = = = = =

An Introduction to Chemistry The electrons are much, much smaller than the nucleus and circle around the atom at the speed of light in energy layers, or shells = = = = = = nucleus proton neutron electron ATOM

The first shell of an atom always contains up to two electrons The outer shells always contain up to eight electrons. An atom is stable when the outermost shell of electrons is complete = = = = = = electron ATOM

Let’s take a look at Oxygen Oxygen has an atomic number of 8 so it has: –8 protons –8 neutrons –8 electrons 2 of the 8 electrons are in the first electron shell, leaving 6 for the outermost electron shell (8-2 = 6) Its outermost electron shell is not complete; it wants 8 electrons and only has 6 ( so it needs 2 more) O

H Now let’s look at Hydrogen Hydrogen has an atomic number of 1 so it has only 1 proton and 1 electron With only 1 electron, it has only 1 electron in its first electron shell Like oxygen, is outermost electron shell is not complete; it wants 2 electrons and only has 1 ( so it needs 1 more)

But what if oxygen – which needs 2 more electrons – could share an electron with 2 hydrogen atoms? The shared electrons would orbit around the atoms completing the outermost electron shells for all 3 atoms HH O

O This is exactly what happens….when oxygen shares its electrons with 2 hydrogen atoms, theresulting molecule is water, or H 2 O WATER H H

The sharing of outer shell electrons is called a covalent bond. The resulting bonding of atoms to one another forms a molecule. But covalent bonds are not the only types of bonds that atoms can form. Let’s look at another example; this time with Sodium and Chlorine Covalent Bonds

Sodium (Na) has 11 electrons: 2 in the first shell, 8 in the second shell and 1 in the outermost shell

Chlorine (Cl) has 17 electrons: 2 in the first shell, 8 in the second shell and 7 in the outermost shell

If Sodium donated its extra electron to Chlorine, its outermost electron shell would be complete

When Chlorine accepts this extra electron, it’s outer electron shell will also be complete

Since Sodium lost an electron, it has one more proton than it does electrons (11 protons, only 10 electrons); this gives Sodium a positive (+) charge

Since Chlorine gained an electron, it has one more electron than it does protons (17 protons, and 18 electrons); this gives Chlorine a negative (-) charge

The resulting atoms are now called ions, since they have an electrical charge. When two atoms exchange electrons, they form an ionic bond

Na + (sodium ion) is attracted to the Cl - ion (chloride ion) and they bond to one another, forming NaCl (sodium chloride)

Hydrogen Bonds Covalent bonds exist between atoms sharing electrons Ionic bonds exist between ions, atoms that have exchanged (gained or lost) electrons, and as a result have a positive or negative charge Hydrogen bonds are a type of bond that exist between molecules, and result from an unequal sharing of electrons within the molecule

Recall that in a water molecule, two hydrogen atoms share electrons with an oxygen atom, completing the outer shell of all 3 atoms O H H

But the electrons do not spend an equal amount of time with hydrogen as they do oxygen O H H

This is because oxygen has 8 protons in its nucleus, while hydrogen only has 1 Remember that opposites attract. The difference in positive charges pulls the shared electrons toward oxygen, and away from the two hydrogen atoms O H H

The abundance of electrons near oxygen makes the oxygen atom in a water molecule slightly negative Likewise, the 2 hydrogen atoms become slightly positive since their shared electrons spend so little time near them (-) (+)

A water molecule is said to be polar in that it has two ends of opposing charges: a slightly positive charge near the hydrogen atoms, and a slightly negative charge near the oxygen atom O H H (-) (+)

The bonds that form between the positive and the negative ends of two or more polar molecules are called hydrogen bonds

Water is the solvent of life Because of its polar properties, water is an incredible solvent A solvent is an agent that dissolves substances or solutes, together making up a solution

The positive and negative ends of a water molecule attract ions, even those once joined by ionic bonds For example, table salt, NaCl is readily separated back into Na + and Cl - ions in water Table salt, NaCl Cl - ions in solution Na + ions in solution

Water is essential to life Because of its polar properties, water is also cohesive, or “sticky” Cohesion exists because the positive (hydrogen) end of one water molecule is drawn towards the negative (oxygen) end of another water molecule

Water: Heat Capacity Because of its hydrogen bonds, water is said to have a high heat capacity, or ability to absorb a large amount of heat energy without an increase in its own temperature –To increase the temperature of water, heat energy must first disrupt the hydrogen bonds that have formed between water molecules

Water: Heat Capacity We experience the benefits of water’s high heat capacity every time we sweat. Our body provides the heat to evaporate water from a liquid to a gas (water vapor), keeping us cool This is the same reason why dogs pant and pigs wallow in the mud!

Water’s amazing properties Water is unique in that it naturally exists in all phases (gas, liquid, solid) on Earth Water is also unique in that its solid phase (ice) is less dense than its liquid phase (water)

Water’s amazing properties At low temperatures, the hydrogen bonds between water molecules stabilize This creates a 3-dimensional shape, or crystal that traps air between the water molecules The trapped air causes ice to take up more space than liquid water, making it less dense and causing ice to float