1. CHAPTER 2 The Chemical Basis of Life
Modules 2.1 – 2.8

2. 2.1 The emergence of biological function starts at the chemical level
ATOMS AND MOLECULES
2.1 The emergence of biological function starts at the chemical level
Everything an organism is and does depends on chemistry
Chemistry is in turn dependent on the arrangement of atoms in molecules
In order to understand the whole, biologists study the parts (reductionism)

3. Molecules and ecosystems are at opposite ends of the biological hierarchy
Each level of organization in the biological hierarchy builds on the one below it
At each level, new properties emerge

4. A biological hierarchy
D. Organ: Flight muscle of a moth
A biological hierarchy
Rattlebox moth
C. Cell and tissue: Muscle cell within muscle tissue
Myofibril (organelle)
B. Organelle: Myofibril (found only in muscle cells)
Actin
Myosin
Figure 2.1
Atom
A. Molecule: Actin

5. 2.2 Life requires about 25 chemical elements
A chemical element is a substance that cannot be broken down to other substances by ordinary chemical means
About 25 different chemical elements are essential to life

6. Carbon, hydrogen, oxygen, and nitrogen make up the bulk of living matter, but there are other elements necessary for life
Table 2.2

7. 2.3 Elements can combine to form compounds
Chemical elements combine in fixed ratios to form compounds
Example: sodium + chlorine  sodium chloride

8. 2.4 Atoms consist of protons, neutrons, and electrons
The smallest particle of an element is an atom
Different elements have different types of atoms

9. The nucleus is surrounded by electrons
An atom is made up of protons and neutrons located in a central nucleus
The nucleus is surrounded by electrons 2
Protons
Nucleus 2
Neutrons 2
Electrons
Figure 2.4A
A. Helium atom

10. Neutrons are electrically neutral
Each atom is held together by attractions between the positively charged protons and negatively charged electrons
Neutrons are electrically neutral 6
Protons
Nucleus 6
Neutrons 6
Electrons
Figure 2.4B
B. Carbon atom

11. Atoms of each element are distinguished by a specific number of protons
The number of neutrons may vary
Variant forms of an element are called isotopes
Some isotopes are radioactive
Table 2.4

12. 2.5 Connection: Radioactive isotopes can help or harm us
Radioactive isotopes can be useful tracers for studying biological processes
PET scanners use radioactive isotopes to create anatomical images
Figure 2.5A
Figure 2.5B

13. 2.6 Electron arrangement determines the chemical properties of an atom
Electrons are arranged in shells
The outermost shell (valence shell) determines the chemical properties of an atom
In most atoms, a full outer shell holds eight electrons

14. Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons
Outermost electron shell (can hold 8 electrons)
Electron
First electron shell (can hold 2 electrons)
HYDROGEN (H)
Atomic number = 1
CARBON (C)
Atomic number = 6
NITROGEN (N)
Atomic number = 7
OXYGEN (O)
Atomic number = 8
Figure 2.6

15. 2.7 Ionic bonds are attractions between ions of opposite charge
When atoms gain or lose electrons, charged atoms called ions are created
An electrical attraction between ions with opposite charges results in an ionic bond – + Na Cl Na Cl Na
Sodium atom Cl
Chlorine atom
Na+
Sodium ion
Cl–
Chloride ion
Figure 2.7A
Sodium chloride (NaCl)

16. Sodium and chloride ions bond to form sodium chloride, common table salt
Na+
Cl–
Figure 2.7B
Figure 2.7Bx

17. 2.8 Covalent bonds, the sharing of electrons, join atoms into molecules
Some atoms share outer shell electrons with other atoms, forming covalent bonds
Atoms joined together by covalent bonds form molecules

18. Molecules can be represented in many ways
Table 2.8

19. Hydrogen Bonds
Water is a compound in which the electrons in its covalent bonds are shared unequally.
This causes it to be a polar molecule, one with opposite charges on opposite ends.

20. The polarity of water results in weak electrical attractions between neighboring water molecules.
Hydrogen bond– formed between a hydrogen that is attached to a highly electronegative atom (O, N) and another highly electronegative atom of separate molecule

21. Acknowledgements
Unless otherwise noted, illustrations are credited to Pearson Education have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.