UNIT A: Cell Biology

Chapter 2: The Molecules of Cells: Sections 2.1, 2.2

Chapter 3: Cell Structure and Function

Chapter 4: DNA Structure and Gene Expression

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis

In this chapter, you will learn how basic chemistry is used in biology.

What life processes might be affected by a problem with protein structure?

How are biological molecules involved in energy use in the body?

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Chapter 2: The Molecules of Cells

2.1 Basic ChemistryMatter—anything that takes up space and has mass—consists of chemical elements, which are composed of atoms. Only 92 naturally occurring elements serve as the building blocks of all matter.

•The primary elements in Earth’s crust are Si, Al, and O

•The primary elements in organisms are O, N, C, and H; with S and P, these elements make up biological molecules.

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Figure 2.1 Elements that make up Earth’s crust and its organisms.

Atomic Structure

Sub-atomic particles that make up atoms include•protons (in the nucleus, positive charge)•electrons (in the nucleus, uncharged)•neutrons (shell surrounding the nucleus, negative charge)

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Figure 2.2 Model of helium (He).

All atoms of an element have the same number of protons, called the atomic number.

Each atom has its own mass number.

mass number = number of protons + number of neutrons

Carbon is represented by the symbol C.

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Atomic Structure

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, and, therefore, a different mass number.

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These are different isotopes of carbon.

Unstable isotopes change over time into stable isotopes, releasing various types of energy in the form of rays and subatomic particles during the process. These are called radioactive isotopes.

Radioactivity

Radioactive isotopes have different uses, depending on their level of radioactivity.

•Low radiation levels: Because these isotopes have the same chemical behavior as stable isotopes, they can be used as tracers in medical testing and imaging.

Figure 2.3 a Low levels of radiation. The missing area in this thyroid scan indicates the presence of a tumour that does not take up the radioactive iodine.

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High Levels of Radioactivity

• High radiation levels: Because radiation can damage cells, it can be used for sterilizing medical and dental products.

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Figure 2.4b High levels of radiation. Physicians use radiation therapy to kill cancer cells.

Radiation can also damage DNA and cause cancer, however its ability to kill cells can be applied to cancer cells.

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Check Your Progress

1. Explain why the key elements in Earth’s crust would differ from those present in living organisms.

2. Explain how radiation can be both beneficial and harmful to humans.

3. Explain the differences between oxygen 16 and oxygen 18.

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2.2 Molecules and Compounds

When atoms of two or more different elements bond together, a compound forms. Two types of bonds that form are

•ionic bonds: electrons transfer between atoms to form ionic compounds

•covalent bonds: electrons are shared between atoms to form molecules

Molecules also form when two or more of the same atom are covalently bonded (e.g., H2, O2, O3).

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Ionic Bonding

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Figure 2.5 Formation of sodium chloride (table salt)

Ions are charged particles that form when electrons transfer from one atom to another.

•For example, Na donates electrons and Cl accepts electrons

Ionic compounds are held together by attraction between the positive and negative ions, called an ionic bond.

Biologically important ions include Na+, Cl−, and K+.

Covalent Bonding

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Figure 2.6a, b, and c Covalently bonded molecules.

A covalent bond forms when two atoms share electrons.

•A common way to show a covalent bond is a line between atoms (a)

•A double covalent bond is two shared pairs of electrons (b)

•A triple covalent bond is three shared pairs of electrons (c)

• All molecules have three-dimensional shapes.

• Molecules consisting of two atoms are always linear. Molecules such as methane, with five atoms and four single covalent bonds, have a tetrahedral shape.

• Three-dimensional shapes of biological molecules are related to their structural and functional roles.

Figure 2.6d Covalently bonded molecules.

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Shapes of Molecules

Nonpolar and Polar Covalent Bonds

• When sharing of electrons between atoms is equal, the bond is a nonpolar covalent bond. (All bonds on the previous figure are nonpolar.)

• In some compounds, such as water, the sharing is not equal. Oxygen has greater electronegativity than hydrogen (it attracts electrons to a greater extent).

• Unequal sharing of electrons results ina polar covalent bond.

• The more electronegative atom will have a partial negative charge; the other atom will have a partial positive

charge.

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Figure 2.7a Water molecule.

Hydrogen Bonding

• Polarity within a water molecule causes hydrogen atoms of one molecule to be attracted to the oxygen atom of another molecule. This attraction is called a hydrogen bond.

• Hydrogen bonds are weaker than ionic and covalent bonds and are often represented as a dotted line. Many hydrogen bonds can collectively be very strong.

• Hydrogen bonding occurs in many biological molecules, which have polar covalent bonds between hydrogen and oxygen or nitrogen.

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Figure 2.7b Water molecule.

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Check Your Progress

1. Explain whether carbon dioxide (CO2) and nitrogen gas (N2) are considered to be molecules, compounds, or both.

2. Explain why hydrogen ions form polar bonds that have a partially positive charge.

UNIT A Chapter 2: The Molecules of Cells Section 2.2

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