Key Concept 2: Large Molecules Are Made of Chains of Smaller Units

Molecules (7.6C)

Overview

Student Expectation

The student is expected to recognize how large molecules are broken down into smaller molecules such as carbohydrates can be broken down into sugars.

Key Concepts

 Key Concept 1: A molecule is formed when two or more atoms join together. Molecules can consist of two or more atoms of the same element or from more than one type of element.  Key Concept 2: Large molecules are made of chains of smaller units.  Key Concept 3: Our bodies take larger molecules and break them down through chemical changes during digestion to create smaller molecules that can be absorbed by the body.  Key Concept 4: Carbohydrate chains are broken into simple sugars.  Key Concept 5: Protein chains are broken into amino acids.  Key Concept 6: Lipid chains (fats) are broken into fatty acids.

Fundamental Questions

 What is a molecule?  What does our body do with large molecules we consume?  When an organism consumes a carbohydrate, what molecule(s) do carbohydrates become during digestion?  When an organism consumes a protein, what molecule(s) do proteins become during digestion?  When an organism consumes a lipid (fat), what molecule(s) do lipids become during digestion?

Background Information (same as physical and chemical changes, as they are related) Chemical changes in matter produce new substances with properties different from the original substances. In a chemical change, compounds may change into other compounds. Then these compounds may be broken down into elements. Alternatively, elements may combine to form compounds. In all of these types of changes, new substances with new properties form. Various types of evidence are used to identify the formation of a new substance through a chemical change.

Gas production is a form of evidence indicating a chemical change taking place. For example, when baking soda and vinegar react one of the products formed is carbon dioxide gas. When the two reactants combine, bubbles form indicating the production of CO2 gas. Another way to see this gas formation is to put the two reactants in a sealed flask with a deflated balloon around the top of the flask. When the two reactants combine, they form bubbles and the balloon inflates.

Temperature changes are sometimes used as evidence to indicate a chemical reaction has occurred. However, a temperature change does not always indicate a chemical change. Thermal energy is transferable in many processes, including physical changes. For example, when ice is placed in warm water, the temperature changes but no chemical reaction takes place. However, when two combined substances with similar beginning temperatures produce (exothermic) or lose (endothermic) heat, then a temperature change indicates a chemical reaction.

The formation of a precipitate can be evidence of a chemical change. A precipitate is an insoluble compound formed out of solution in precipitation reactions. A common example of precipitate formation is seen when mixing two clear solutions, silver nitrate and sodium chloride. The result is a chemical change in which two products, sodium nitrate and silver chloride, a white solid, are produced. Silver chloride is the precipitate. It is a white solid.

Color changes, as with temperature changes, are sometimes used as evidence of a chemical change. Not all color changes, however, indicate chemical changes. For example, adding food color to water might appear to change the color of the water, but no chemical reaction takes place. Many times a chemical indicator changes color when the pH of a solution changes. When the two clear solutions, silver nitrate and potassium iodide are mixed, a bright yellow compound called silver iodide precipitates. This color change is an indicator of a chemical change, as well as a sediment formation.

In the digestive system, both physical and chemical changes occur as food digests. When food first enters the digestive system in the mouth, it changes physically through the process of chewing. Food then physically changes into smaller pieces that are easy to swallow. Most of the physical change (mechanical digestion) is done by chewing. Saliva mixes with food to moisten it making it easier to swallow. Digestive enzymes are present in saliva beginning the process of chemically changing the food (chemical digestion).

Food passes from the mouth to the stomach through the esophagus. The walls of the esophagus produce mucus, which lubricates the food making it easier to move. Next, food enters the stomach where the breakdown of protein in food begins. The muscles in the stomach churn the food contributing to mechanical digestion (physical change). Food is also mixing with digestive juice containing mucus, pepsin (an enzyme that digests proteins) and hydrochloric acid. These help to break food down chemically. As food leaves the stomach and passes into the small intestine, the food becomes a thick liquid called chyme. Muscles move food through the small intestine by peristalsis. Most chemical digestion occurs in the small intestine. Food mixes with several digestive juices produced by the small intestine, liver and pancreas.

Once food is digested, it absorbs into the bloodstream in the small intestine. This is where nutrients pass through all body parts. When food enters the small intestine, the pancreas releases enzymes that help digest starch, proteins and fats. The liver produces bile, which helps break apart large clusters of fat into tiny droplets making it easier for enzymes to digest. The last part of the digestive system is the large intestine. Undigested food entering the large intestine contains lots of water and minerals. After the water and minerals are reabsorbed by the large intestine, feces remain as waste until it is eliminated from the body.

As food is digested, larger molecules break down chemically into smaller molecules with the help of digestive enzymes. Catabolism is a process that helps break down larger molecules into smaller units and releases energy. In this process, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into monosaccharides, fatty acids, nucleotides and amino acids.

Carbohydrates create smaller subunits called saccharides. This includes sugars, starches and fiber. Through hydrolysis, water breaks long chains of polysaccharides into smaller chains or simpler carbohydrates. The body converts digestible (non-fiber) carbohydrates into glucose, which cells use as fuel. Some carbohydrates (simple) break down quickly into glucose while others (complex) slowly break down and enter the bloodstream gradually. During digestion, all carbohydrates break down into glucose before entering the bloodstream where insulin helps the glucose enter the body’s cells. Some glucose is stored as glycogen in the liver and muscles for future use. Simple carbohydrates are composed of 1 or 2 sugar units. Simple carbohydrates are digested quickly. Complex carbohydrates (starches) are made up of many smaller units and take longer to break down and be digested.

A chemical formula is a combination of element symbols and subscript numbers used to show the composition of a substance, either as a compound or in pure element form. Chemical formulas make it easier to describe substances by providing information about the number and kinds of atoms creating a molecule of that substance. These formulas identify each component element by its chemical symbol and indicate the atoms’ number of each element found in each molecule of that substance. If a molecule contains more than one atom of a particular element, this quantity is indicated by using a subscript after the chemical symbol. For example, a molecule of carbon dioxide consists of one carbon atom and two oxygen atoms. Its chemical formula is CO2. If there is only one atom of an element in a molecule of a compound, then there is no subscript number written. Another example, glucose, is represented by the chemical formula C6H12O6. For each molecule of glucose, there are six carbons atoms, twelve hydrogen atoms, and six oxygen atoms. Some elements occur naturally in pairs of atoms rather than as single atoms. These are called diatomic molecules. Hydrogen gas (H2), nitrogen gas (N2), and oxygen gas (O2) are common diatomic molecules.

Chemical formulas are used in chemical equations to describe chemical reactions. Chemical equations represent chemical reactions symbolically with the reactants written on the left hand side of an arrow (represented by an arrow symbol and usually read as “yields”) and the products written on the right hand side. If there is more than one reactant or product, then they are separated by a plus sign.

The Law of Conservation of Mass states the quantity of each element does not change in a chemical reaction. Thus, each side of a chemical equation must represent the same quantity of each element in the reaction. Chemical equations are balanced by adjusting the number of each chemical formula by placing a number, the coefficient, in front of the formula. Most of the time equations are balanced using the smallest whole number coefficient for a reactant or product.

For example, hydrogen gas (H2), can react with oxygen gas (O2), to form water (H2O).

H2 + O2 → H2O

This equation, however, is not balanced. Since the quantity of each element cannot change according to the Law of Conservation of Mass, coefficients must be used to “balance” the equation. In order for the amount of oxygen to be the same on each side a “2” is placed in front of H2O. This balances the number of oxygen atoms on each side, but also changes the number of hydrogen atoms on the right hand side. Another “2” is then placed in front of the reactant H2 to balance both sides of the equation giving the same quantity of hydrogen and the same quantity of oxygen on both sides. The new balanced equation is written as: 2H2 + O2 → 2H2O

Two molecules of hydrogen and one molecule of oxygen yield two molecules of water. Subscripts never change when balancing equations. For the example above, adding a subscript “2” to H2O changes the compound completely (in this case from water H20 to hydrogen peroxide H2O2). Only coefficients are used when balancing chemical equations.

A chemical reaction occurs when the chemical identity of the reactants is different than that of the products. A new substance or substances forms with properties different from the original reactants. Often times, chemical reactions result in changes easily observed. Evidence of chemical reactions includes temperature changes, formation of a precipitate, color changes and gas production. During chemical reactions involving the gain or loss of heat, temperature changes indicate the formation of new substances. In endothermic reactions, heat is absorbed when transforming from reactants to products (gain in enthalpy). When it takes more energy to break the bonds of the reactants than is released when new chemical bonds form, the reaction is endothermic (feels colder). A reaction is exothermic (loss in enthalpy) when more energy is released in forming new bonds than it takes to break the original bonds of the reactants (feels warmer).

In precipitation reactions, a precipitate is a solid formed out of solution. A precipitate forms because the solid produced is insoluble in aqueous solutions. This is a new substance formed with new properties and is evidence of a chemical change. When gas is produced during a reaction, it is also evidence of a new substance formed. For example, when baking soda (solid) is mixed with vinegar (liquid), carbon dioxide (gas) is one of the products formed. The new products formed from this reaction, including CO2 gas, have properties different from the original reactants.

A color change represents more than just a physical change. This is the same as diluting a substance or adding food color to a substance, is used to identify a chemical change. The color of a substance may change when its chemical composition changes. An example of a color change as evidence of a chemical reaction is seen when crystal violet dye is added to a solution containing hydroxide ion. The intensity of the color decreases over time until it disappears. The product of the reaction is colorless in water solution.

The density of a substance is a physical property defined as mass per unit volume. Density is calculated by dividing the mass of an object by its volume. For solids, the units are usually expressed as g/cm3. For liquids and gases, the units are usually expressed as g/ml. Each element and compound has a unique density associated with it. Often times in chemistry, the density of a substance is compared to the density of water (1.0 g/ml). Objects that sink in water have density values greater than 1 (g/ml or g/cm3) and objects that float on water have density values less than 1 (g/ml or g/cm3).

Calculating the density of an unknown substance is sometimes used to identify the substance by comparing it to known density values of elements and compounds. Density is just one of many physical and chemical properties used to identify an unknown substance. To identify an unknown substance using density, the mass and volume of an unknown are measured. For an irregular shaped solid, the volume is found by placing the object in a known amount of water and determining the volume of the displaced water to find the volume of the object. The mass of the object is then divided by the volume to determine the density. That density value is then compared to a list of known density values to help identify the object. Density alone may or may not be enough to determine the identity of an unknown. Other physical and chemical properties may be needed as well.