Chapter 2: The Chemistry and Energy of Life
Modified from Principles of Life: Hillis, Sadava, Hill and Price.
2.1 Atomic Structure Is the Basis for Life’s Chemistry
1. For the following elements, draw the electron shells and place electrons in the appropriate shells based on their atomic numbers: carbon (6C), nitrogen (7N), sodium (11Na), chlorine (17Cl), argon (18Ar) and oxygen (8O).
2. Compare and contrast the atomic number and the mass number of an element.
The atomic number of an element reflects the number of protons in the nucleus, and thus us unique for each element. The mass number is the mass of the atom – includes the number of protons and the number of neutrons.
2.2 Atoms Interact and Form Molecules
3. Calcium has an atomic number if 20. Draw structures Ca and Ca2+. What is the difference between these structures?
the difference between these structures is that calcium (Ca) has two electrons in its outer shell. Calcium (Ca2+) has lost its two outer electrons and therefore has a positive charge of 2 because it has more protons than electrons.
4. Water is a polar molecule. This property contributes to cohesion and surface tension. Draw six water molecules. In your drawing, indicate how hydrogen bonding between molecules contributes to cohesion and surface tension. Be sure to include the appropriate covalent bonds in each molecule.
The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen atoms of another molecule of water. This attraction tends to cause water molecules to “stick” together, creating surface tension.
2.3 Carbohydrates Consist of Sugar Molecules
5. Complex carbohydrates should be the mainstay of one’s diet. What properties of carbohydrates make them excellent food sources?
Complex carbohydrates are easily broken down into glucose monomers, which provide nearly all cellular energy. By storing glucose molecules in large complex carbohydrates, the osmotic strain on any given cell is reduced without sacrificing availability of energy.
6. Examine the structures of glucose polymers in Figure 2.10. Hypothesize why cellulose is a better structural polysaccharide than either starch or glycogen.
Cellulose is an unbranched, linear structure. Both starch and glycogen are branched molecules. The individual cellulose fibers can pack together very tightly and the hydrogen bonds between the different fibers also provide stability. The branched starch and glycogen do not pack as tightly as cellulose does, and thus are not as favorable for providing structure to a cell.
2.4 Lipids are Hydrophobic Molecules
7. Consider the following triglyceride:
a. Circle the remnant of the
glycerol portion of the triglyceride.
b. How many water molecules result
from the formation of this triglyceride
from glycerol and three fatty acids?
Three water molecules will result – one for each
of the fatty acid chains
8. Which triglyceride (A or B) is probably a solid at room temperature? Explain your answer.
A will be solid at room temperature. Its fatty acid chains are saturated (no double bonds) and relatively long, both of which are characteristics of solid, animal derived triglycerides.
9. Draw a phospholipid and a bilayer. What characteristics of phospholipids make them perfectly suited for membranes? What do you think might happen if phospholipids did not for a bilayer? How might they arrange themselves in an aqueous environment?
10. Dietary guidelines encourage people to stay away from saturated fats. What is meant by the term “saturated fat”? Why is this type of fat of more concern than unsaturated fats in the diet? What is the structural difference between saturated and unsaturated fats?Saturated fats contain only single bonds and are “saturated” with hydrogen, which allows fat molecules to pack together densely. This is the reason that most saturated fats are solid at room temperature. Unsaturated fats contain double bonds that affect the shape of the molecule and are not “saturated” with hydrogen. This characteristic keeps them from packing together tightly, and they tend to be liquid at room temperature. Saturated fats are dangerous because of this “packing” ability, which can affect membrane function.
2.5 Biochemical Changes Involve Energy
11. It is estimated that approximately 90 percent of energy that passes between levels in the food web is “lost” at each level. Explain the first law of thermodynamics and discuss why this apparent loss of energy does not contradict the law.
The first law of thermodynaomcs states that energy cannot be created or destroyed, but that it may be converted from one form to another. In the transfer between levels of a food web, approximately 90 percent of the energy is converted to unusable heat energy. There is a met loss of usable energy during each conversion, but the total amount of energy (usable and unusable) remains the same.
12. You decide to purchase a new water heater and start looking at the energy efficiency ratings. You find one unit that is labeled as 100 percent energy efficient, and the salesperson says that the more efficient the appliance is, the more money you will save. However, you don’t trust that the store is providing accurate information, and you do not buy the product. Was this decision correct?The decision was correct. An appliance with 100 percent energy efficiency is not possible. The second law of thermodynamics indicates that every time energy is transformed, some is lost in the form of entropy. An appliance with no energy lost to entropy therefore does not exist.
Additional Resources: use the web addresses listed
- Animated Tutorial 2.1 Chemical Bond Formation PoL2e.com/at2.1
- Animated Tutorial 2.2 Macromolecules: Carbohydrates and Lipids PoL2e.com/at2.2
- Animated Tutorial 2.3 Synthesis of Prebiotic Molecules PoL2e.com/at2.3
- Activity 2.1 Functional Groups PoL2e.com/ac2.1
- Activity 2.2 Forms of Glucose PoL2e.com/ac2.2