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Elements
Although life-forms exist in many diverse forms, they all have one thing in common: they are all made up of matter that is exchanged with the environment to grow, reproduce, and maintain organization. Matter is made up of elements.
Elements, by definition, are substances that cannot be broken down into simpler substances by chemical means.
The Essential Elements of Life
Although there are 92 natural elements, 96% of the mass of all living things? is made up of just four of them: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N) These four elements are used to build biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids. They are also used to form storage compounds and cells in all organisms. Other elements such as calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), chlorine (C!), and magnesium (Mg) are also present, but in smaller quantities. These elements make up most of the remaining four percent of a living thing's weight. Some elements are known as trace elements because they are required by an organism only in very small quantities. Trace elements include iron (Fe), iodine (I), and copper (Cu),
SUBATOMIC PARTICLES
The smallest unit of an element that retains its characteristic properties is an atom. Atoms are the building blocks of the physical world.
Within atoms, there are even smaller subatomic particles called protons, neutrons, and electrons. Let's take a look at a typical atom.
protons
- neutrons
electrons
Protons and neutrons are particles that are packed together in the core of an atom called the nucleus. You'll notice that protons are positively charged (+) particles, whereas neutrons are uncharged particles.
Electrons, on the other hand, are negatively charged (-) particles that spin around the nucleus, Electrons are pretty small compared to protons and neutrons. In fact, for our purposes, electrons are considered massless. Most atoms have the same number of protons and electrons, making them electrically neutral. Some atoms have the same number of protons but differ in the number of neutrons in the nucleus. These atoms are called isotopes.
COMPOUNDS
When two or more individual elements are combined in a fixed ratio, they form a chemical compound. You'll sometimes find that a compound has different properties from those of its elements. For instance, hydrogen and oxygen exist in na-qure as gases. Net when they combine to make water, they often pass into a liquid state, When hydrogen atoms get together with oxygen atoms to form water, we've got a chemical reaction:
2H, (8) + 02 (g) → 2H,0 (D)
The atoms of a compound are held together by chemical bonds, which may ionic bonds, covalent bonds, or hydrogen bonds.
An ionic bond is formed between two atoms when one or more electrons are transferred from one atom to the other. In order for this to occur, first, one atom loses electrons and becomes positively charged, and the other atom gain; electrons and becomes negatively charged. The charged forms of the atoms are called ions.
An ionic bond results from the attraction between the two oppositely charged ions.
For example, when Na reacts with Cl, the charged ions Na* and Cl- are formed.
A covalent bond is formed when electrons are shared between atoms. If the electrons are shared equally between the atoms, the bond is called nonpolar covalent.
If the electrons are shared unequally, the bond is called polar covalent When one pair of electrons is shared between two atoms, the result is a single covalent bond. When two pairs of electrons are shared, the result is a double covalent bond When three pairs of electrons are shared, the result is a triple covalent bond..
WATER: THE VERSATILE MOLECULE
One of the most important substances in nature is water. Water is considered a unique molecule because it plays an important role in chemical reactions.
Just like all biological molecules, the elements that make up water determine the properties of water. Water has two hydrogen atoms joined to an oxygen atom.
In water molecules, the electrons are not shared equally in the bonds between hydrogen and oxygen. This means that the hydrogen atoms have a partial positive charge and the oxygen atom has a partial negative charge. Molecules that have partially positive and partially negative charges are said to be polar. Water is therefore a polar molecule. The positively charged elements of the water molecules strongly attract the negatively charged ends of other /polar compounds (including water).
Likewise, the negatively charged ends strongly attract the positively charged ends of polar compounds, These forces are most readily apparent in the tendency of ater molecules to stick together, as in the formation of water beads or raindrops.
These types of intermolecular attractions are called hydrogen bonds. Hydrogen bonds are weak chemical bonds that form when a hydrogen atom that is covalently bonded to one electronegative atom is also attracted to another electronegative atom. Water molecules are held together by hydrogen bonds.
Although hydrogen bonds are individually weak, they are strong when present in large numbers. Because water reacts well with other polar substances, it makes a great solvent; it can dissolve many kinds of substances. The hydrogen bonds that hold water molecules together contribute to a number of special properties, including cohesion, adhesion, surface tension, high heat capacity, and expansion on freezing.
Cohesion and Adhesion
As mentioned above, water molecules have a strong tendency to stick together.
That is, water exhibits cohesive forces. These forces are extremely important to life.
For instance, during transpiration, water molecules evaporate from a leaf, "pull-ing" on neighboring water molecules. These, in turn, draw up the molecules immediately behind them, and so on, all the way down the plant vessels. The resulting chain of water molecules enables water to move up the stem.
Water molecules also like to stick to other substances-that is, they're adhesive.
Have you ever tried to separate two glass slides stuck together by a film of water?
They're difficult to separate because of the water sticking to the glass surfaces.
These two forces taken together —cohesion and adhesion-account for the ability of water to rise up the roots, trunks, and branches of trees. Since this phenomenon occurs in thin vessels, it's called capillary action.
Surface Tension
The cohesion of water molecules contributes to another property of water, its surface tension. Like a taut trampoline, the surface of water has a tension to it. The water molecules are stuck together and light things like leaves and water striders can sit atop the surface without sinking.
ACIDS AND BASES
We just said that water is important because most reactions occur in watery solu-tions. Well, there's one more thing to remember: reactions are also influenced by whether the solution in which they occur is acidic, basic, or neutral.
What makes a solution acidic or basic? A solution is acidic if it contains a lot of hydrogen ions (H*). That is, if you dissolve an acid in water, it will release a lot of hydrogen ions. When you think about acids, you usually think of substances with a sour taste, like lemons. For example, if you squeeze a little lemon juice into a glass of water, the solution will become acidic. That's because lemons contain citric acid, which releases a lot of H* into the solution.
Bases, on the other hand, do not release hydrogen ions when added to water.
They release a lot of hydroxide ions (OH-). These solutions are said to be alkaline (the fancy name for a basic solution). Bases usually have a slippery consistency.
Common soap, for example, is composed largely of bases.
The acidity or alkalinity of a solution can be measured with a pH scale. The pH scale is numbered from 1 to 14. The midpoint, 7, is considered neutral pH. The concentration of hydrogen ions in a solution will indicate whether it is acidic, basic, or neutral. If a solution contains a lot of hydrogen ions, then it will be acidic and have a low pH. Here's the trend:
An increase in H* ions causes a decrease in the pH
pH = - log [H*]
One more thing to remember: the pH scale is not a linear scale-it's logarithmic.
That is, a change of one pH number actually represents a tenfold change in hydrogen ion concentration. For example, a pH of 3 is actually ten times more acidic than a pH of 4. This is also true in the reverse direction: a pH of 4 represents a tenfold decrease in acidity compared to a pH of 3.
Therefore, as the concentration of H* ions increases by a factor of 10, the pH becomes one number smaller. For example, stomach acid has a pH between 1 and 3, and if we use the equation, we discover that the concentration of H* ions in stomach acid is 10-2 M This is pretty high when you consider the other extreme is lye, which has a pH of 14, a concentration of H* ions around 10-14 M! Use your calculator to double-check that these numbers are correct.
You'll notice from the scale below that stronger acids have lower pHs. If a solution has a low concentration of hydrogen ions, it will have a high pH.
The equation for pH is listed on the AP Biology Equations and Formulas sheet.
However, you will not be expected to perform calculations using this equation. In-stead, you should understand how the equation works and when pH calculations are useful.
ORGANIC MOLECULES
Now that we've discussed chemical compounds in general, let's talk about a special group of compounds. Most of the chemical compounds in living organisms contain a skeleton of carbon atoms surrounded by hydrogen atoms and often other elements. These molecules are known as organic compounds. By contrast, molecules that do not contain carbon atoms are called inorganic compounds. For example, salt (NaCI) is an inorganic compound.
Carbon is important for life because it is a versatile atom, meaning that it has the ability to bind not only with other carbons but also with a number of other elements including nitrogen, oxygen, and hydrogen. The resulting molecules are key in carrying out the activities necessary for life. Most macromolecules are chains of building blocks, called polymers. The individual building blocks of a polymer are called monomers.
Dehydration Synthesis and Hydrolysis Reactions
Polymers are formed through dehydration synthesis (or condensation) reactions.
In these reactions, a water molecule is lost, and a larger compound is formed. Polymers can also be broken down into monomers. That's called hydrolysis, which means "water" (hydro-) and "breaking" (lysis). The water breaks the bond between the two monomers.
To recap:
• Organic compounds contain carbon atoms AND hydrogen atoms (and sometimes other things too)
• Inorganic compounds don't contain both carbon atoms and hydrogen atoms.
• Polymers are formed through dehydration synthesis reactions and broken down through hydrolysis reactions.
Now let's focus on the following four classes of organic compounds central to life on Earth:
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•
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carbohydrates proteins lipids nucleic acids
Carbohydrates
Carbohydrates are organic compounds that contain carbon, hydrogen, and oxy-gen. They usually contain these three elements in a ratio of approximately 1:2:1, respectively. We can represent the proportion of elements within carbohydrate molecules by the formula C,H, n, which can be simplified as (CH,O), Most carbohydrates are categorized as either monosaccharides, disaccharides, or polysaccharides. The term saccharides is a fancy word for "sugar." The prefixes mono-, di-, and poly- refer to the number of sugars in the molecule. Mono- means
"one," di- means. "two," and poly- means "many." A monosaccharide is therefore a carbohydrate made up of a single saccharide.
Monosaccharides: The Simplest Sugars
Monosaccharides, the simplest sugars, serve as an energy source for cells. The two most common sugars are (1) glucose and (2) fructose. Galactose, ribose, and deoxyribose are also monosaccharides.
Both glucose and fructose are six-carbon sugars with the chemical formula
C,H,,% Glucose, the most abundant monosaccharide, is the most popular sugar around. Glucose is an important part of the food we eat, and it is the favorite food made by plants during photosynthesis. Living organisms break down glucose in order to provide cells with energy. Almost all living organisms can perform this ancient biochemical process (which is called cell respiration; you'll learn more about this process in Chapter 6). In fact, the evolution of glycolysis, photosyn-thesis, and oxidative metabolism is thought to have occurred about 3 billion years ago! Fructose, the other monosaccharide you need to know for the test, is a common sugar in fruits.