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Copyright © 2009 Pearson Education, Inc..
Including the lecture Materials of
Gregory AhearnUniversity of North Florida
with amendments andadditions by
John Crocker
Chapter 2pt 2
Atoms, Molecules, and Life
Copyright © 2009 Pearson Education Inc.
2.3 Why Is Water So Important To Life?
Water interacts with many other molecules.• Oxygen released by plants during
photosynthesis comes from water.• Water is used by animals to digest food. • Water is produced in chemical reactions that
produce proteins, fats, and sugars.
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Many molecules dissolve easily in water.• Water is an excellent solvent, capable of
dissolving a wide range of substances because of its positive and negative poles.
• example NaCl dropped into H2O• The positive end of H2O is attracted to Cl–. • The negative end of H2O is attracted to Na+. • These attractions tend to pull apart the
components of the original salt.
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Water as a solvent
Fig. 2-8
Cl–
OCl–
Cl–
H
H
Na+
Na+
Na+
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Copyright © 2009 Pearson Education Inc.
Water-insoluble molecules are hydrophobic• Water molecules repel and drive together
uncharged and nonpolar molecules like fats and oils
• The “clumping” of nonpolar molecules is called hydrophobic interaction
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Copyright © 2009 Pearson Education Inc.
Water molecules tend to stick together.• Surface tension: water tends to resist being
broken• Cohesion: water molecules stick together
Fig. 2-9
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Water Molecules Tend to Stick Together
Hydrogen bonding between water molecules produces high cohesion• Water cohesion explains how water
molecules can form a chain in delivering moisture to the top of a tree
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Copyright © 2009 Pearson Education Inc.
Water Molecules Tend to Stick Together
Cohesion of water molecules along a surface produces surface tension• Fishing spiders and water striders rely on
surface tension to move across the surface of ponds
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Copyright © 2009 Pearson Education Inc.
Water Molecules Tend to Stick Together
Water molecules stick to polar or charged surfaces in the property called adhesion• Adhesion helps water climb up the thin
tubes of plants to the leaves
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Water can form ions.• Water dissociates to become H+ and OH–.• Acid solutions have more H+ (protons). • Alkaline solutions have more OH– (hydroxyl
ions).• A base is a substance that combines with H+,
reducing their numbers.• pH measures the relative amount of H+ and
OH– in a solution.
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Acid, Basic, and Neutral Solutions
A small fraction of water molecules break apart into ions:
H2O OH- + H+
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A water molecule is ionized.
Fig. 2-10
hydrogen ion(H+)
hydroxide ion(OH–)
water(H2O)
+(+)(–)
O
HH
O H
H
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Acid, Basic, and Neutral Solutions
Solutions where H+ > OH- are acidic• e.g. Hydrochloric acid ionizes in water:
HCl H+ + Cl-
• Lemon juice and vinegar are naturally produced acids
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Acid, Basic, and Neutral Solutions
Solutions where OH- > H+ are basic• e.g. Sodium hydroxide ionizes in water:
NaOH Na+ + OH-
• Baking soda, chlorine bleach, and ammonia are basic
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Acid, Basic, and Neutral Solutions
The degree of acidity of a solution is measured using the pH scale• pHs 0-6 are acidic (H+ > OH-)• pH 7 is neutral (H+ = OH-)• pH 8-14 is basic (OH- > H+)
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Buffers Maintain Constant pH
• A buffer is a compound that accepts or releases H+ in response to pH change
• The bicarbonate buffer found in our bloodstream prevents pH change
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Buffers Maintain Constant pH
• If the blood becomes too acidic, bicarbonate accepts (and absorbs) H+ to make carbonic acid
HCO3- + H+ H2CO3
bicarbonate hydrogen ion carbonic acid
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Buffers Maintain Constant pH
• If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH- to form water
H2CO3 + OH- HCO3- + H2O
carbonic acid hydroxide ion bicarbonate water
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Water stabilizes temperature• Temperature reflects the speed of molecular
motion• It requires 1 calorie of energy to raise the
temperature of 1g of water 1oC (specific heat), so it heats up very slowly
• Because it heats up very slowly water moderates the effect of temperature change
• Very low or very high temperatures may damage enzymes or slow down important chemical reactions
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Water Stabilizes Temperature
Water requires a lot of energy to turn from liquid into a gas (heat of vaporization)• Evaporating water uses up heat from its
surroundings, cooling the nearby environment (as occurs during sweating)
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Water Stabilizes Temperature
• Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring
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Water Stabilizes Temperature
Water requires a lot of energy to be withdrawn in order to freeze (heat of fusion)
Water freezes more slowly than other liquids
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Water Forms an Unusual Solid: Ice
• Most substances become denser when they solidify from a liquid
• Water molecules spread apart slightly during the freezing process
• Because of this ice is less dense than liquid water
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Water Forms an Unusual Solid: Ice• Ice floats in liquid water• Ponds and lakes freeze from the top
down and never freeze completely to the bottom
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Frozen water floats (left) and frozen benzene sinks (right)
Figure 2.13x2
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- Lower water is protected by the surface layer of ice.
–Life can survive in cold water underneath ice.
–Spring thaw pushes nutrient-rich bottom water to surface
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Like no other common substance on earth, water naturally exists in all three physical states:
Figure 2.10B
• solid
• liquid
• gas
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Figure 2.10Bx
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Organic refers to molecules containing a carbon skeleton
Inorganic refers to carbon dioxide and all molecules without carbon
Organic vs. Inorganic in Chemistry
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2.4 Why Is Carbon So Important To Life?
Carbon can combine with other atoms in many ways to form a huge number of different molecules.
Carbon has four electrons in its outermost shell, leaving room for four more electrons from other atoms (4 covalent bonds).
Carbon atoms are versatile and can form up to four bonds (single, double, or triple) and rings.
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Arrangement of atoms determines molecular shape.Shape determines function of molecules
Structuralformula
Ball-and-stickmodel
Space-fillingmodel
Methane
The 4 single bonds of carbon point to the corners of a tetrahedron.
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Butane, ball and stick model
Figure 3.1x3
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Cyclohexane, ball and stick model
Figure 3.1x5
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The great variety of substances found in nature is constructed from a limited pool of atoms.
Organic molecules have a carbon skeleton and some hydrogen atoms.
Much of the diversity of organic molecules is due to the presence of functional groups.
Functional groups in organic molecules confer chemical reactivity and other characteristics
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groups of atoms that participate in chemical reactions
determine the chemical properties of molecules
Examples: acidity, solubility
Functional (R) Groups
-OH -COOH -NH2 -CH3
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What affects solubility in water?
Molecules with +/- charge are usually hydrophilic or “water-loving”
Molecules with no charge and non-polar are usually hydrophobic and not soluble in water
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Copyright © 2009 Pearson Education Inc.
2.5 How Are Biological Molecules Joined Together Or Broken Apart? Biomolecules are polymers (chains) of
subunits called monomers A huge number of different polymers can be
made from a small number of monomers Biomolecules Are Joined Through
Dehydration and Broken by Hydrolysis
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Organic Molecule Synthesis
Monomers are joined together through dehydration synthesis An H and an OH are removed, resulting in the
loss of a water molecule (H2O)
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Copyright © 2009 Pearson Education Inc.
Organic Molecule Synthesis
Polymers are broken apart through hydrolysis (“water cutting”) Water is broken into H and OH and used to
break the bond between monomers
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Copyright © 2009 Pearson Education Inc.
Organic Molecule Synthesis
All biological molecules fall into one of four categoriesCarbohydratesLipidsProteinsNucleic Acids
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Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
2.6 What Are Carbohydrates?
Composition:C, H, and O in the ratio of 1:2:1
Construction: Simple or single sugars are
monosaccharides Two linked monosaccharides are
disaccharides Long chains of monosaccharides are
polysaccharides
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Monosaccharides
Basic monosaccharide structure Backbone of 3-7 carbon atoms Many –OH and –H functional groups Usually found in a ring form in cells
Simple sugars provide important energy sources for organisms.
Most small carbs are water-soluble due to the polar OH functional groups
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A simple sugar
Fig. 2-13
Glucose, linear form Glucose, ring form(a) (b)H
H
CH2OH
HO
OH
OH
O
H H
OH H
2356 4 1
H H H H
H
H
H
H
H H
H
H
O OOOO
O
CCCCCC
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Copyright © 2009 Pearson Education Inc.
Monosaccharides
Example monosaccharides continued Fructose (found in corn syrup and fruits) Galactose (found in lactose) Ribose and deoxyribose (found in RNA and
DNA)
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Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Most small carbs are water-soluble due to the polar OH functional groups
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Disaccharides
Disaccharides are two-part sugars Sucrose (table sugar) = glucose + fructose Lactose (milk sugar) = glucose + galactose Maltose (malt sugar)= glucose + glucose
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Manufacture of a disaccharide
Fig. 2-14
glucose fructose sucrose
dehydrationsynthesis
OHO
OHOCH2
OH
HO
CH2OH
H H
OH
H OH
H
H
O HO
OCH2OH
H H
OH
H OH
H
HH
H
H
HOCH2OHH
HOCH2 H
H
H
HOCH2OH
O
OH
O
+
OHH
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Polysaccharides
Monosaccharides are linked together to form chains (polysaccharides)
Polysaccharides are used for energy storage and structural components
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Polysaccharides
Storage polysaccharides Starch (polymer of glucose)
Formed in roots and seeds as a form of glucose storage
Glycogen (polymer of glucose)Found in liver and muscles
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Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Polysaccharides
Structural polysaccharides Cellulose (polymer of glucose) Found in the cell walls of plants
Indigestible for most animals due to orientation of bonds between glucoses
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Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Polysaccharides
Structural polysaccharides continued Chitin (polymer of modified glucose units)
Found in the outer coverings of insects, crabs, and spiders
Found in the cell walls of many fungi
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Copyright © 2009 Pearson Education Inc.
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