Bio1441 Chapters 2-5 Review
Chapter 2: The Chemical Context of Life
I.
Matter consists of chemical elements in pure form and in combinations called compounds
a.
Elements and compounds
i.
Element
: substance that cannot be broken down into other substances by chemical rxns
(92 that occur in nature)
ii.
Compound
: substance made of 2 or more different elements combined in a fixed ratio
1.
Ex: NaCl—1:1, pure Na is metal (corrosive) and Cl is a gas (toxic)—combined they
make table salt (emergent properties)
b.
The elements of life
1.
Of the 92 in nature, approx. 25% are essential for life (humans- 25, plants- 17)
2.
Humans: C, H, O, and N make up 96.3% of living matter; Ca, P, K, S, Cl, Mg =
3.7%
ii.
Trace elements
: needed in small quantities
1.
Ex: Fe needed in all lifeforms
2.
Ex: vertebrates need iodine b/c it is essential to hormones produced by the thyroid
gland (goiter—enlarged gland due to iodine deficiency)
c.
Evolution of tolerance to toxic elements
i.
Ex: serpentine plants—can survive in enviro with heavy cobalt, chromium, and nickel
deposits
II.
An element’s properties depend on the structure of its atoms
i.
Atom
: smallest unit of matter that still retains the properties of an element
1.
Charge is neutral (protons = electrons) unless indicated otherwise
b.
Subatomic particles
i.
Neutrons
: no charge, found in nucleus, approx. 1 Dalton
ii.
Protons
: positive, found in nucleus, approx. 1 Dalton
iii.
Electrons
: negative, surround nucleus in a cloud and travel at speed of light (centrifugal
force), approx. 1/2000
th
Dalton
c.
Atomic number and atomic mass
i.
Atomic number
: number of protons (and electrons if neutral), written as subscript to the
left-down of an element (Ex:
2
He)
ii.
Mass number
: sum of protons and neutrons, written as subscript to the left-up of the
element (
23
11
Na—mass number is 23 [protons + neutrons] and atomic number [protons] is
11 so 23-11= number of neutrons =
12
iii.
Atomic mass
: total mass (only slightly more than mass number because electron mass is
negligible)
d.
Isotopes
: different number of neutrons of same element
i.
Ex: carbon-12 (
12
6
C) which is 99% of carbon in nature and has 6 neutrons;
12
C (6
neutrons) and
13
C (7 neutrons) are stable isotopes (their nuclei don’t tend to lose
subatomic particles—a process called
decay
);
14
C (8 neutrons) is radioactive (unstable)
ii.
Radioactive isotope
: one in which the nucleus decays spontaneously, giving off particles
and energy; when radioactive decay leads to a change in number of protons, it transforms
the atom to an atom of another element (neutron splits into 1 electron and 1 proton)
1.
Ex:
14
C (6 protons) decays and becomes
14
N (7 protons)
1
Bio1441 Chapters 2-5 Review
e.
Radioactive tracers:
radioactive isotopes usually used as diagnostic tools in medicine
i.
Cells use radioactive atoms same as non-radioactive isotopes of same element;
radioactive isotopes are incorporated into biologically active molecules which are then
used as tracers to track atoms during metabolism
1.
Ex: certain kidney disorders diagnosed by injecting small amount of radioactively
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- Fall '09
