life chemistry and energy 2. chapter 2 life chemistry and energy key concepts 2.1 atomic structure...

Life Chemistry and Energy

2

Chapter 2 Life Chemistry and Energy

Key Concepts

• 2.1 Atomic Structure Is the Basis for Life’s Chemistry

• 2.2 Atoms Interact and Form Molecules

• 2.3 Carbohydrates Consist of Sugar Molecules

• 2.4 Lipids Are Hydrophobic Molecules

• 2.5 Biochemical Changes Involve Energy

Chapter 2 Opening Question

Why is the search for water important in the search for life?

Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry

Living and nonliving matter is composed of atoms.


Like charges repel; different charges attract.

Most atoms are neutral because the number of electrons equals the number of protons.

Dalton—mass of one proton or neutron (1.7 × 10–24 grams)

Mass of electrons is so tiny, it is usually ignored.


Element—pure substance that contains only one kind of atom

Living things are mostly composed of 6 elements:

Carbon (C) Hydrogen (H) Nitrogen (N)

Oxygen (O) Phosphorus (P) Sulfur (S)


• The number of protons identifies an element.

• Number of protons = atomic number

• For electrical neutrality, # protons = # electrons.

• Mass number—total number of protons and neutrons


A Bohr model for atomic structure—the atom is largely empty space, and the electrons occur in orbits, or electron shells.


Behavior of electrons determines whether a chemical bond will form and what shape the bond will have.

Figure 2.1 Electron Shells


Atoms with unfilled outer shells tend to undergo chemical reactions to fill their outer shells.

They can attain stability by sharing electrons with other atoms or by losing or gaining electrons.

The atoms are then bonded together into molecules.


Octet rule—atoms with at least two electron shells form stable molecules so they have eight electrons in their outermost shells.

Concept 2.2 Atoms Interact and Form Molecules

Chemical bond is an attractive force that links atoms together to form molecules.

There are several kinds of chemical bonds.

Table 2.1 Chemical Bonds and Interactions


Ionic bonds

Ions are charged particle that form when an atom gains or loses one or more electrons.

Cations—positively charged ions

Anions—negatively charged ions

Ionic bonds result from the electrical attraction between ions with opposite charges.

The resulting molecules are called salts.

Figure 2.2 Ionic Bond between Sodium and Chlorine


Ionic attractions are weak, so salts dissolve easily in water.


Covalent bonds

Covalent bonds form when two atoms share pairs of electrons.

The atoms attain stability by having full outer shells.

Each atom contributes one member of the electron pair.

Figure 2.3 Electrons Are Shared in Covalent Bonds


Carbon atoms have four electrons in the outer shell—they can form covalent bonds with four other atoms.

Figure 2.4 Covalent Bonding (Part 1)

Figure 2.4 Covalent Bonding (Part 2)


Properties of molecules are influenced by characteristics of the covalent bonds:

• Orientation—length, angle, and direction of bonds between any two elements are always the same.

Example: Methane always forms a tetrahedron.


• Strength and stability—covalent bonds are very strong; it takes a lot of energy to break them.

• Multiple bonds

Single—sharing 1 pair of electrons

Double—sharing 2 pairs of electrons

Triple—sharing 3 pairs of electrons

C H

C C

N N


• Degree of sharing electrons is not always equal.

Electronegativity—the attractive force that an atomic nucleus exerts on electrons

• It depends on the number of protons and the distance between the nucleus and electrons.

Table 2.2 Some Electronegativities


If two atoms have similar electronegativities, they share electrons equally, in what is called a nonpolar covalent bond.

If atoms have different electronegativities, electrons tend to be near the most attractive atom, in what is called a polar covalent bond


Hydrogen bonds

Attraction between the δ– end of one molecule and the δ+ hydrogen end of another molecule forms hydrogen bonds.

They form between water molecules.

They are important in the structure of DNA and proteins.

Figure 2.5 Hydrogen Bonds Can Form between or within Molecules


Water molecules form multiple hydrogen bonds with each other—this contributes to high heat capacity.


A lot of heat is required to raise the temperature of water—the heat energy breaks the hydrogen bonds.

In organisms, presence of water shields them from fluctuations in environmental temperature.


Water has a high heat of vaporization—a lot of heat is required to change water from liquid to gaseous state.

Thus, evaporation has a cooling effect on the environment.

Sweating cools the body—as sweat evaporates from the skin, it transforms some of the adjacent body heat.


Hydrogen bonds also give water cohesive strength, or cohesion—water molecules resist coming apart when placed under tension.

• This permits narrow columns of water to move from roots to leaves of plants.


Any polar molecule can interact with any other polar molecule through hydrogen bonds.

Hydrophilic (“water-loving”)—in aqueous solutions, polar molecules become separated and surrounded by water molecules

Nonpolar molecules are called hydrophobic (“water-hating”); the interactions between them are hydrophobic interactions.

Figure 2.6 Hydrophilic and Hydrophobic


Functional groups—small groups of atoms with specific chemical properties

Functional groups confer these properties to larger molecules, e.g., polarity.

One biological molecule may contain many functional groups.

Figure 2.7 Functional Groups Important to Living Systems (Part 1)

Figure 2.7 Functional Groups Important to Living Systems (Part 2)


Macromolecules

• Most biological molecules are polymers (poly, “many”; mer, “unit”), made by covalent bonding of smaller molecules called monomers.


• Proteins: Formed from different combinations of 20 amino acids

• Carbohydrates—formed by linking similar sugar monomers (monosaccharides) to form polysaccharides

• Nucleic acids—formed from four kinds of nucleotide monomers

• Lipids—noncovalent forces maintain the interactions between the lipid monomers


Polymers are formed and broken apart in reactions involving water.

• Condensation—removal of water links monomers together

• Hydrolysis—addition of water breaks a polymer into monomers

Figure 2.8 Condensation and Hydrolysis of Polymers (Part 1)

Figure 2.8 Condensation and Hydrolysis of Polymers (Part 2)

Concept 2.3 Carbohydrates Consist of Sugar Molecules

Carbohydrates

• Source of stored energy

• Transport stored energy within complex organisms

• Structural molecules that give many organisms their shapes

• Recognition or signaling molecules that can trigger specific biological responses

nn OHC )( 2

Berry Carroll

Keep formula where it is?


Monosaccharides are simple sugars.

Pentoses are 5-carbon sugars

Ribose and deoxyribose are the backbones of RNA and DNA.

Hexoses (C6H12O6) include glucose, fructose, mannose, and galactose.

Figure 2.9 Monosaccharides (Part 1)

Figure 2.9 Monosaccharides (Part 2)


Monosaccharides are covalently bonded by condensation reactions that form glycosidic linkages.

Sucrose is a disaccharide.


Oligosaccharides contain several monosaccharides.

Many have additional functional groups.

They are often bonded to proteins and lipids on cell surfaces, where they serve as recognition signals.


Polysaccharides are large polymers of monosaccharides; the chains can be branching.

Starches—a family of polysaccharides of glucose

Glycogen—highly branched polymer of glucose; main energy storage molecule in mammals

Cellulose—the most abundant carbon-containing (organic) biological compound on Earth; stable; good structural material

Figure 2.10 Polysaccharides (Part 1)

Concept 2.4 Lipids Are Hydrophobic Molecules

Lipids are hydrocarbons (composed of C and H atoms); they are insoluble in water because of many nonpolar covalent bonds.

When close together, weak but additive van der Waals interactions hold them together.


Lipids

• Store energy in C—C and C—H bonds

• Play structural role in cell membranes

• Fat in animal bodies serves as thermal insulation


Triglycerides (simple lipids)

Fats—solid at room temperature

Oils—liquid at room temperature

They have very little polarity and are extremely hydrophobic.


Triglycerides consist of:

• Three fatty acids—nonpolar hydrocarbon chain attached to a polar carboxyl group (—COOH) (carboxylic acid)

• One glycerol—an alcohol with 3 hydroxyl (—OH) groups

Synthesis of a triglyceride involves three condensation reactions.

Figure 2.11 Synthesis of a Triglyceride


Fatty acid chains can vary in length and structure.

In saturated fatty acids, all bonds between carbon atoms are single; they are saturated with hydrogens.

In unsaturated fatty acids, hydrocarbon chains contain one or more double bonds. These acids cause kinks in the chain and prevent molecules from packing together tightly.

Figure 2.12 Saturated and Unsaturated Fatty Acids (Part 1)

Figure 2.12 Saturated and Unsaturated Fatty Acids (Part 2)


Fatty acids are amphipathic; they have a hydrophilic end and a hydrophobic tail.

Phospholipid—two fatty acids and a phosphate compound bound to glycerol

The phosphate group has a negative charge, making that part of the molecule hydrophilic.

Figure 2.13 A Phospholipids


In an aqueous environment, phospholipids form a bilayer.

The nonpolar, hydrophobic “tails” pack together and the phosphate-containing “heads” face outward, where they interact with water.

Biological membranes have this kind of phospholipid bilayer structure.

Figure 2.13 B Phospholipids

Concept 2.5 Biochemical Changes Involve Energy

Chemical reactions occur when atoms have enough energy to combine, or change, bonding partners.

sucrose + H2O glucose + fructose

(C12H22O11) (C6H12O6) (C6H12O6)

reactants products


Metabolism—the sum total of all chemical reactions occurring in a biological system at a given time

Metabolic reactions involve energy changes.


All forms of energy can be considered as either:

Potential—the energy of state or position, or stored energy

Kinetic—the energy of movement (the type of energy that does work) that makes things change

Energy can be converted from one form to another.


Two basic types of metabolism:

Anabolic reactions link simple molecules to form complex ones.

• They require energy inputs; energy is captured in the chemical bonds that form.

Catabolic reactions break down complex molecules into simpler ones.

• Energy stored in the chemical bonds is released.

Figure 2.14 Energy Changes in Reactions (Part 1)

Figure 2.14 Energy Changes in Reactions (Part 2)


The laws of thermodynamics apply to all matter and energy transformations in the universe.

First law: Energy is neither created nor destroyed.

Second law: Disorder (entropy) tends to increase.

When energy is converted from one form to another, some of that energy becomes unavailable for doing work.

Figure 2.15 The Laws of Thermodynamics (Part 1)


If a chemical reaction increases entropy, its products are more disordered or random than its reactants.

If there are fewer products than reactants, the disorder is reduced; this requires energy to achieve.


As a result of energy transformations, disorder tends to increase.

• Some energy is always lost to random thermal motion (entropy).


Metabolism creates more disorder (more energy is lost to entropy) than the amount of order that is stored.

Example:

• The anabolic reactions needed to construct 1 kg of animal body require the catabolism of about 10 kg of food.

Life requires a constant input of energy to maintain order.

Answer to Opening Question

One way to investigate the possibility of life on other planets is to study how life may have originated on Earth.

An experiment in the 1950s combined gases thought to be present in Earth’s early atmosphere, including water vapor. An electric spark provided energy.

Complex molecules were formed, such as amino acids. Water was essential in this experiment.

Figure 2.16 Synthesis of Prebiotic Molecules in an Experimental Atmosphere (Part 1)

Figure 2.16 Synthesis of Prebiotic Molecules in an Experimental Atmosphere (Part 2)

life chemistry and energy 2. chapter 2 life chemistry and energy key concepts 2.1 atomic structure...

Documents

form moleculescarbon

number of electrons

links atoms

form molecules2

pairs of electrons

gramsmass of electrons

form moleculesionic

form moleculeschemical