Download bio 12 chpt 2 prop of water

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
UNIT A Chapter 2: The Molecules of Cells
Chapter 2: The Molecules of Cells
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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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
2.1 Basic Chemistry
Matter—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.
Figure 2.1 Elements that make
up Earth’s crust and its
organisms.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
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)
Figure 2.2 Model of helium
(He).
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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
Atomic Structure
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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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
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.
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.
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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
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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UNIT A Chapter 2: The Molecules of Cells
Section 2.1
High Levels of Radioactivity
• High radiation levels: Because radiation can damage cells,
it can be used for sterilizing medical and dental products.
Radiation can also
damage DNA and cause
cancer, however its
ability to kill cells can be
applied to cancer cells.
Figure 2.4b High levels of
radiation. Physicians use
radiation therapy to kill
cancer cells.
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UNIT A Chapter 2: The Molecules of Cells
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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Section 2.1
UNIT A Chapter 2: The Molecules of Cells
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Section 2.1
UNIT A Chapter 2: The Molecules of Cells
Section 2.2
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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UNIT A Chapter 2: The Molecules of Cells
Section 2.2
Ionic Bonding
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+.
Figure 2.5
Formation of
sodium chloride
(table salt)
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UNIT A Chapter 2: The Molecules of Cells
Section 2.2
Covalent Bonding
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)
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Figure 2.6a, b, and c Covalently
bonded molecules.
UNIT A Chapter 2: The Molecules of Cells
Section 2.2
Shapes of Molecules
• 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.
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Figure 2.6d Covalently bonded molecules.
UNIT A Chapter 2: The Molecules of Cells
Section 2.2
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 in
a 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.
UNIT A Chapter 2: The Molecules of Cells
Section 2.2
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.
UNIT A Chapter 2: The Molecules of Cells
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.
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Section 2.2
UNIT A Chapter 2: The Molecules of Cells
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Section 2.2