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General Biology Chapter 2

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on 7 September 2016

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Transcript of General Biology Chapter 2

The chemistry of life is sensitive to acidic and basic conditions
A compound that releases H+ ions in solution is an acid
A compound that accepts H+ ions in solution is a base
Acidity is measured on the pH scale from 0 (most acidic) to 14 (most basic)
The pH of most cells is kept close to 7 (neutral) by buffers that resist pH change

Water is the solvent of life
A solution is a homogeneous mixture of a liquid solvent and one or more dissolved solutes
Because water is a polar molecule, it readily forms solutions with many other polar and ionic compounds
A solution in which water is the solvent is an aqueous solution

Water's hydrogen bonds moderate temperature
When water is heated, the heat energy is absorbed, disrupting hydrogen bonds
The water stores a large amount of heat while warming only a few degrees
When water is cooled, heat energy is released as hydrogen bonds are formed
The temperature of the water is lowered slowly

Hydrogen bonds - weak bonds important in the chemistry of life
Created by attraction between slightly positive regions and slightly negative regions
Hydrogen bonding occurs in many biologically important compounds
Water
DNA
Proteins

Ionic bonds - attractions between ions of opposite charge
- An ion is a charged atom that has lost or gained electrons in its outer shell
- A positively charged ion (cation) is an atom that has lost an electron
- A negatively charged ion (anion) is an atom that has gained an electron

2.5 Radioactive isotopes can help or harm us
Useful as tracers to study the fate of elements and molecules in living systems
Tracers are often used in combination with sophisticated imaging instruments for medical diagnosis
Uncontrolled exposure to radioactive material can harm living organisms
Can damage molecules (DNA)
Hiroshima

CONNECTION

Atoms consist of protons, neutrons, and electrons (subatomic particles)
Subatomic particles
- Protons and neutrons occupy the central region (nucleus) of an atom
- A proton has a single positive charge
- A neutron is electrically neutral
- Electrons surround the nucleus
- An electron has a single negative charge

Elements can combine to form compounds
Compounds contain two or more elements in a fixed ratio
More common in nature than elements!!
Molecule – smallest unit of a compound
H2O; NaCl; C6H12O6
Different arrangements of the atoms of elements determine the unique properties of each compound
Elements change their properties when combined in a compound

Nature's Chemical Language
Chemicals play an important role in all organisms
The rattlebox moth provides a good example of chemicals used in mating and defense

Cells are kept close to pH 7 by buffers
Buffers are substances that resist pH change
They accept H+ ions when they are in excess and donate H+ ions when they are depleted
Buffers are not foolproof

Water also moderates temperature by evaporative cooling
The surface cools as the hottest molecules leave

2.8 Covalent bonds join atoms into molecules through electron sharing
Covalently bonded atoms share one or more pairs of outer shell electrons, forming a molecule
In a double bond, two pairs of electrons are shared
Covalent bonds can be represented in various ways

An electrical attraction between ions with opposite charges results in an ionic bond
“opposites attract”!!
Example: sodium chloride (table salt) results from an ionic bond between sodium and chlorine

The chemical reactivity of an atom depends on the number of electrons in the outer shell
Atoms whose outer shells are not full share or transfer electrons to other atoms
Two major types of chemical bonds between atoms form compounds
Ionic bonds
Covalent bonds

Isotopes
Isotopes are atoms with the same atomic number but different atomic masses (different number of neutrons)

Symbol

Differences in Elements
All the atoms of a particular element have the same number of protons
The number of protons-the atomic number-defines the element's unique properties
An atom's mass number (atomic mass) is the sum of its protons and neutrons

Goiter, a physiological response to a deficiency in iodine
Iodine is typically added to food or water to avoid deficiency
What is another popular additive (toothpaste)?

- Trace elements are common additives to food and water
- Trace elements are essential in minute quantities for proper biological functioning
- Example: iodine is a trace element that prevents goiter
Many foods are fortified with trace elements and vitamins (which consist of two or more elements)

CONNECTION

Matter:
- substance of which any physical thing is composed.
Occupies space and has mass
Basic unit of matter = atom
Elements
- substances composed of only 1 type of atom
Represented by 1 – 2 letter symbols
Fe = Ferrous = iron
C = Carbon

Elements, atoms, molecules

Hydrogen bonds make liquid water cohesive
- Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves
- Insects can walk on water due to surface tension created by cohesive water molecules
Cohesion is the tendency of molecules to stick together
- Surface tension results from the cohesion of water molecules

WATER'S LIFE-SUPPORTING PROPERTIES

as a solid
as a liquid
as a gas

Like no other common substance, water exists in nature in all three physical states:

More attraction = strong electronegativity

Electronegativity = force of attraction of an atom for its own
Electrons.

Unequal electron sharing creates polar molecules
A molecule whose covalently bonded atoms share electrons equally is nonpolar
A molecule whose covalently bonded atoms share electrons unequally is polar
Polar = One part of the molecule is slightly positive, and one part is slightly negative

Iron is commonly added by crushing cereal then stirring a magnet through it.
Improves the nutritional value of the product

Living organisms are composed of about 25 chemical elements
- Elements are the basic chemical units that cannot be broken apart by typical chemical processes
- There are 92 naturally occurring elements
- 25 are required by living organisms
- 4 make up 96.3% of the human body
- 6 make up over 98% of the human body

ELEMENTS, ATOMS, AND MOLECULES

Water’s polarity leads to hydrogen bonding and other unusual properties

The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules
This attraction forms weak bonds called hydrogen bonds

Sodium Chloride (table salt)

Chlorine (poisonous gas)

Sodium (metal)

Connection: Acid precipitation threatens the environment

Protons: ?
Neutrons: ?
Electrons: ?

11

23

Sodium

Na

LIQUID WATER
Hydrogen bonds constantly break and re-form

ICE
Hydrogen bonds are stable

Hydrogen bond

Figure 2.13

Ice is less dense than liquid water

Molecules in ice are farther apart than those in liquid water


In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen

This makes the oxygen end of the molecule slightly negatively charged
The hydrogen end of the molecule is slightly positively charged
Water is therefore a polar molecule

OXYGEN (O)
Atomic number = 8

NITROGEN (N)
Atomic number = 7

CARBON (C)
Atomic number = 6

HYDROGEN (H)
Atomic number = 1

Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons

Increasingly BASIC
(Lower concentration of H+)

Oven cleaner

Household bleach

Household ammonia

Milk of magnesia

Seawater

Human blood

Pure water

Human urine

Tomato juice

Grapefruit juice, soft drink

Lemon juice, gastric juice

pH scale

NEUTRAL
[HOH-]

Basic solution

H+

H+

OH

OH

OH

OH

OH

OH

Neutral solution

Increasingly ACIDIC
(Higher concentration of H+)

H+

H+

H+

H+

H+

H+

OH

OH

OH

OH

OH

H+

H+

H+

OH

H+

H+

H+

H+

OH

Acidic solution

Chapter
The Chemical Basis of Life

Acids in the atmosphere can harm the environment
Water vapor combines with sulfuric and nitric acid from industrialization
Living Cells cannot distinguish between isotopes of the same element
In biological systems they can be taken in and used as tracers, being monitored by instruments

Of the 3 subatomic particles, only electrons are involved in chemical activity
- Electrons occur in energy levels called electron shells
The structure of atoms and molecules determines the way the behave.
hydrogen
oxygen
water
reactants
products
Ice is less dense than liquid water....that's why it floats.
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