covalent bonds – where electrons are shared
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Covalent bonds – where electrons are shared. Typically the strongest bonds in biological systems. Can be polar (where electrons are not equally shared) or non-polar (electrons are equally shared). Hydrogen atoms (2 H). In each hydrogen atom, the single electron is held in its orbital by - PowerPoint PPT PresentationTRANSCRIPT
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Covalent bonds – where electrons are shared
• Typically the strongest bonds in biological systems.
• Can be polar (where electrons are not equally shared) or non-polar (electrons are equally shared).
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Figure 2.10
• Formation of a covalent bondHydrogen atoms (2 H)
Hydrogenmolecule (H2)
+ +
+ +
+ +
In each hydrogenatom, the single electronis held in its orbital byits attraction to theproton in the nucleus.
1
When two hydrogenatoms approach eachother, the electron ofeach atom is alsoattracted to the protonin the other nucleus.
2
The two electronsbecome shared in a covalent bond,forming an H2
molecule.
3
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• A molecule
– Consists of two or more atoms held together by covalent bonds
• A single bond
– Is the sharing of one pair of valence electrons
• A double bond
– Is the sharing of two pairs of valence electrons
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(a)
(b)
Name(molecularformula)
Electron-shell
diagram
Structuralformula
Space-fillingmodel
Hydrogen (H2). Two hydrogen atoms can form a single bond.
Oxygen (O2). Two oxygen atoms share two pairs of electrons to form a double bond.
H H
O O
Figure 2.11 A, B
• Single and double covalent bonds
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Name(molecularformula)
Electron-shell
diagram
Structuralformula
Space-fillingmodel
(c)
Methane (CH4). Four hydrogen atoms can satisfy the valence ofone carbonatom, formingmethane.
Water (H2O). Two hydrogenatoms and one oxygen atom arejoined by covalent bonds to produce a molecule of water.
(d)
HO
H
H H
H
H
C
Figure 2.11 C, D
Covalent bonding in compounds
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• Electronegativity
– Is the attraction of a particular kind of atom for the electrons in a covalent bond
• The more electronegative an atom
– The more strongly it pulls shared electrons toward itself
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A nonpolar covalent bond
– The atoms have similar electronegativities
– Share the electron equally
– Common in hydrocarbons
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Figure 2.12
This results in a partial negative charge on theoxygen and apartial positivecharge onthe hydrogens.
H2O
–
O
H H+ +
Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.
– The atoms have differing electronegativities
– Share the electrons unequally
A polar covalent bond
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Ionic Bonds
• Electron transfer between two atoms creates ions
• Ions
– Are atoms with more or fewer electrons than usual
– Are charged atoms
– An anion
• Is negatively charged ions
– A cation
• Is positively charged
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An ionic bond
Cl–
Chloride ion(an anion)
–
The lone valence electron of a sodiumatom is transferred to join the 7 valenceelectrons of a chlorine atom.
1 Each resulting ion has a completedvalence shell. An ionic bond can formbetween the oppositely charged ions.
2
Na NaCl Cl
+
NaSodium atom
(an unchargedatom)
ClChlorine atom(an uncharged
atom)
Na+
Sodium on(a cation)
Sodium chloride (NaCl)Figure 2.13
An attraction between anions and cations
These bonds are strong in crystal form, but weak in water
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Ionic compounds
Na+
Cl–
Figure 2.14
– Are often called salts, which may form crystals
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Weak Chemical Bonds – form due to differences in polarity
– +
+
Water(H2O)
Ammonia(NH3)
OH
H
+
–
N
HH H
A hydrogenbond results from the attraction between thepartial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.+ +
Figure 2.15
• Hydrogen bonds
– Form when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
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Van der Waals Interactions
• Van der Waals interactions
– Occur when transiently positive and negative regions of molecules attract each other
• Weak chemical bonds
– Reinforce the shapes of large molecules
– Help molecules adhere to each other
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BSC 2010 - Exam I Lectures and Text Pages• I. Intro to Biology (2-29)
• II. Chemistry of Life
– Chemistry review (30-46)
– Water (47-57)
– Carbon (58-67)
– Macromolecules (68-91)
• III. Cells and Membranes
– Cell structure (92-123)
– Membranes (124-140)
• IV. Introductory Biochemistry
– Energy and Metabolism (141-159)
– Cellular Respiration (160-180)
– Photosynthesis (181-200)
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Water – The Solvent of Life (Ch. 3)
Cells are made of 70-95% water, the “SOLVENT OF LIFE”. All living things require water more than any other substance.
• Solvent -
• Solute -
• Aqueous -
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• Three-quarters of the Earth’s surface is submerged in water
• The abundance of water is the main reason the Earth is habitable
Figure 3.1
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The water molecule is a polar molecule
• The polarity of water molecules
– Allows them to form hydrogen bonds with each other (negative O ends are attracted to positive H ends)
– Contributes to the various properties water exhibits
Hydrogenbonds
+
+
H
H+
+
–
–
– –
Figure 3.2
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Emergent Properties of Water Contribute to Life
• A. cohesion: (related properties: surface tension and adhesion)
• B. Water tends to resist rupturing. (related to cohesion)
• C. Water resists changes in temperature.
• D. Water expands when it freezes.
• E. Water is a versatile solvent.
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Cohesion
• Water molecules exhibit cohesion
• Cohesion
– Is the bonding of a high percentage of the molecules to neighboring molecules
– Water molecules stick together due to hydrogen bonding
– Causes surface tension and adhesion.
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Cohesion
Helps pull water up through the microscopic vessels of plants. Water molecules stick to each other and to the walls of the xylem.
Water conducting cells
100 µmFigure 3.3
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Surface tension
Is a measure of how hard it is to break the surface of a liquid.
Figure 3.4
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Moderation of Temperature
• Water moderates air temperature
– This is very important for the maintenance of homeostasis by living organisms.
– Also - ~75% of the earth is covered with water, this helps stabilize climate.
– Water absorbs heat from air that is warmer and releases the stored heat to air that is cooler
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Water’s High Specific Heat
• The specific heat of a substance
– Is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC
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Water’s High Specific Heat
• Water has a high specific heat, which allows it to minimize temperature fluctuations to within limits that permit life.
– Heat is absorbed when hydrogen bonds break.
– Heat is released when hydrogen bonds form.
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Evaporative Cooling
• Heat of vaporization
– Is the quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas
• Evaporative cooling
– Is due to water’s high heat of vaporization
– Allows water to cool a surface
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Ice Floats
• The hydrogen bonds in ice
– Are more “ordered” than in liquid water, making ice less dense
Liquid water
Hydrogen bonds constantly break and re-form
Ice
Hydrogen bonds are stable
Hydrogen bond
Figure 3.5
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Insulation of Bodies of Water by Floating Ice
• Solid water, or ice
– Is less dense than liquid water
– Floats in liquid water
• Allows life to exist in frozen lakes and ponds.
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The Solvent of Life
• Water is a versatile solvent due to its polarity
• It can form aqueous solutions
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Forming solutions with ionic solutes.
• The different regions of the polar water molecule can interact with ionic compounds and dissolve them.
Negative
oxygen regions
of polar water molecules
are attracted to sodium
cations (Na+).
+
+
+
+Cl –
–
–
–
–
Na+Positive hydrogen regions
of water molecules cling to chloride anions
(Cl–).
++
+
+
–
–
–
–
–
–Na+
Cl–
Figure 3.6
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Forming solutions with polar solutes.
• Water can also interact with polar molecules such as proteins
This oxygen is
attracted to a slight
positive charge on
the lysozyme
molecule.This hydrogen is attracted to a slight
negative charge on the lysozyme
molecule.(a) Lysozyme molecule
in a nonaqueous
environment
(b) Lysozyme molecule (purple)
in an aqueous environment
such as tears or saliva
(c) Ionic and polar regions on the protein’s
Surface attract water molecules.
+
–
Figure 3.7
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Hydrophilic and Hydrophobic Substances
• Some substances are attracted to water and others are not.
• A hydrophilic substance
– Has an affinity for water. Ions and polar molecules.
• A hydrophobic substance is not attracted to water.
– Nonpolar molecules.
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Life is sensitive to pH (Acids and Bases)
• Water can dissociate
– Into hydronium ions and hydroxide ions
• Changes in the concentration of these ions
– Can have a great affect on living organisms
H
Hydroniumion (H3O+)
H
Hydroxideion (OH–)
H
H
H
H
H
H
+ –
+
Figure on p. 53 of water dissociating
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Acids and Bases
• An acid
– Is any substance that increases the hydrogen ion concentration of a solution (donates protons)
• A base
– Is any substance that reduces the hydrogen ion concentration of a solution (accepts protons)
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The pH Scale
• The pH of a solution
– Is determined by the relative concentration of hydrogen ions
– Is low in an acid
– Is high in a base
Most biological solutions range from pH of 6-8, but there are exceptions (stomach acids pH 1-2)
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• The pH scale and pH values of various aqueous solutions
Incr
easi
ngly
Aci
dic
[H+]
> [
OH
–]
Incr
easi
ngly
Bas
ic[H
+]
< [
OH
–]
Neutral[H+] = [OH–]
Oven cleaner
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pH Scale
Battery acid
Digestive (stomach) juice, lemon juiceVinegar, beer, wine,colaTomato juice
Black coffee RainwaterUrine
Pure waterHuman blood
Seawater
Milk of magnesia
Household ammonia
Household bleach
Figure 3.8
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Buffers
• The internal pH of most living cells
– Must remain close to pH 7
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Buffers
• Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution
• Consist of a weak acid-base pair that reversibly combines with hydrogen ions