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

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Covalent bonds – where electrons are shared

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Covalent bonds where electrons are shared

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).


Covalent bonds where electrons are shared

Hydrogen atoms (2 H)

In each hydrogen

atom, the single electron

is held in its orbital by

its attraction to the

proton in the nucleus.

+

+

1

2

3

When two hydrogen

atoms approach each

other, the electron of

each atom is also

attracted to the proton

in the other nucleus.

+

+

The two electrons

become shared in a

covalent bond,

forming an H2

molecule.

+

+

Hydrogen

molecule (H2)

  • Formation of a covalent bond

Figure 2.10


Covalent bonds where electrons are shared

  • 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


Covalent bonds where electrons are shared

Name

(molecular

formula)

Electron-

shell

diagram

Space-

filling

model

Structural

formula

Hydrogen (H2).

Two hydrogen

atoms can form a

single bond.

H

H

Oxygen (O2).

Two oxygen atoms

share two pairs of

electrons to form

a double bond.

O

O

Figure 2.11 A, B

  • Single and double covalent bonds

(a)

(b)


Covalent bonds where electrons are shared

Name

(molecular

formula)

Electron-

shell

diagram

Space-

filling

model

Structural

formula

(c)

Water (H2O).

Two hydrogen

atoms and one

oxygen atom are

joined by covalent

bonds to produce a

molecule of water.

H

O

H

(d)

Methane (CH4).

Four hydrogen

atoms can satisfy

the valence of

one carbon

atom, forming

methane.

H

H

H

C

H

Figure 2.11 C, D

Covalent bonding in compounds


Covalent bonds where electrons are shared

  • 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


A nonpolar covalent bond

A nonpolar covalent bond

  • The atoms have similar electronegativities

  • Share the electron equally

  • Common in hydrocarbons


Covalent bonds where electrons are shared

A polar covalent bond

Because oxygen (O) is more electronegative than hydrogen (H),

shared electrons are pulled more toward oxygen.

d–

This results in a

partial negative

charge on the

oxygen and a

partial positive

charge on

the hydrogens.

O

H

H

d+

d+

H2O

  • The atoms have differing electronegativities

  • Share the electrons unequally

Figure 2.12


Ionic bonds

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


An ionic bond

The lone valence electron of a sodium

atom is transferred to join the 7 valence

electrons of a chlorine atom.

Each resulting ion has a completed

valence shell. An ionic bond can form

between the oppositely charged ions.

+

1

2

Cl

Na

Na

Cl

Cl–

Chloride ion

(an anion)

Na+

Sodium on

(a cation)

Na

Sodium atom

(an uncharged

atom)

Cl

Chlorine atom

(an uncharged

atom)

Sodium chloride (NaCl)

An ionic bond

An attraction between anions and cations

These bonds are strong in crystal form, but weak in water

Figure 2.13


Ionic compounds

Na+

Cl–

Figure 2.14

Ionic compounds

  • Are often called salts, which may form crystals


Weak chemical bonds form due to differences in polarity

H

Water

(H2O)

O

A hydrogen

bond results

from the

attraction

between the

partial positive

charge on the

hydrogen atom

of water and

the partial

negative charge

on the nitrogen

atom of

ammonia.

H

 +

 –

Ammonia

(NH3)

N

H

H

d+

+

H

Figure 2.15

Weak Chemical Bonds – form due to differences in polarity

  • Hydrogen bonds

    • Form when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom

 –

 +

 +


Van der waals interactions

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


Bsc 2010 exam i lectures and text pages

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)


Water the solvent of life ch 3

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 -


Covalent bonds where electrons are shared

Figure 3.1

  • Three-quarters of the Earth’s surface is submerged in water

  • The abundance of water is the main reason the Earth is habitable


The water molecule is a polar molecule

–

Hydrogenbonds

+

H

–

+

H

+

–

 –

+

Figure 3.2

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


Emergent properties of water contribute to life

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.


Cohesion

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.


Cohesion1

Water conducting cells

100 µm

Figure 3.3

Cohesion

Helps pull water up through the microscopic vessels of plants. Water molecules stick to each other and to the walls of the xylem.


Surface tension

Figure 3.4

Surface tension

Is a measure of how hard it is to break the surface of a liquid.


Moderation of temperature

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


Water s high specific heat

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


Water s high specific heat1

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.


Evaporative cooling

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


Ice floats

Hydrogen bond

Liquid water

Hydrogen bonds constantly break and re-form

Ice

Hydrogen bonds are stable

Figure 3.5

Ice Floats

  • The hydrogen bonds in ice

    • Are more “ordered” than in liquid water, making ice less dense


Insulation of bodies of water by floating ice

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.


The solvent of life

The Solvent of Life

  • Water is a versatile solvent due to its polarity

  • It can form aqueous solutions


Forming solutions with ionic solutes

Negative oxygen regions of polar water molecules are attracted to sodium cations (Na+).

Na+

+

+

+

Positive hydrogen regions of water molecules cling to chloride anions (Cl–).

Na+

+

+

Cl –

Cl–

+

+

+

Figure 3.6

Forming solutions with ionic solutes.

  • The different regions of the polar water molecule can interact with ionic compounds and dissolve them.


Forming solutions with polar solutes

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.

(b) Lysozyme molecule (purple) in an aqueous environment such as tears or saliva

(a) Lysozyme molecule in a nonaqueous environment

(c) Ionic and polar regions on the protein’s Surface attract water molecules.

Figure 3.7

Forming solutions with polar solutes.

  • Water can also interact with polar molecules such as proteins


Hydrophilic and hydrophobic substances

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.


Life is sensitive to ph acids and bases

+

H

H

H

+

H

H

H

H

H

Hydroxide

ion (OH–)

Hydronium

ion (H3O+)

Figure on p. 53 of water dissociating

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


Acids and bases

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)


The ph scale

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)


Covalent bonds where electrons are shared

pH Scale

0

1

Battery acid

2

Digestive (stomach)

juice, lemon juice

Vinegar, beer, wine,

cola

3

Increasingly Acidic

[H+] > [OH–]

4

Tomato juice

5

Black coffee Rainwater

6

Urine

Neutral

[H+] = [OH–]

7

Pure water

Human blood

8

Seawater

9

10

Increasingly Basic

[H+] < [OH–]

Milk of magnesia

11

Household ammonia

12

Household bleach

13

Oven cleaner

14

Figure 3.8

  • The pH scale and pH values of various aqueous solutions


Buffers

Buffers

  • The internal pH of most living cells

    • Must remain close to pH 7


Buffers1

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


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