Chapter 2: Chemical Components of Life

Living cells are not composed of atoms found only in them. But their compositionsdo differ from thenon-living environment on Earth.

Four atoms (Hydrogen, Carbon, Oxygen, and Nitrogen constitute 96% of living cells.

Fig. 2-4

Fig. 2-5

Chemistry Review - Atoms are most stable with full electron shells. - Inner shells fill first. - They can either donate/gain or share electrons to accomplish this.

Fig. 2-7

Periodic Table of Elements

Cells are composed of the lightest, most stable elements (share electrons that are located close to nucleus).

Atoms in vertical columns share properties

donate e-

gain e-

Covalent vs. Ionic Bonds

Fig. 2-6

can be sharedequally or non-equally

Fig. 2-7

Periodic Table of Elements

donate e-

gain e-

Electronegativity

Electronegativity (Attractive Force on Electrons)depends on: 1) # protons in nucleus and2) distance between protons and outer shell e-

C and H similarO and H very different

Ele

ctro

neg

ativ

ity

Characteristics of Some Chemical Bonds Found in Cells

Polar H2O Accounts for 70% Cell Weight

Fig. 2-14

determines behavior ofcellular molecules

Ionic Molecules

Polar Molecules

- contain mostly non-polar H-C bonds- cannot form H-bonds with H2O, so are not soluble in H2O

Panel 2-2e

Polar Covalent Bonds of H2OCan Dissociate

Fig. 2-15b

10-7 at pH 7.0

Acts as Acid in H2O (donates H+ to H2O)

Acts as Base in H2O (gains H+ from H2O)

N

H H

N H

H

amine group amine group hydroxyl ion(protonated)

Some Polar Covalent Bonds Can Donate or Gain H+ from H2O

Noncovalent BondsAre Important forAssociationsbetween Molecules in Cells

They’re weakness allows for transient, regulated associations.

Fig. 2-13

Complementarycharged faces interact

- contain mostly non-polar H-C bonds- cannot form H-bonds with H2O, so are not soluble in H2O

Panel 2-2e

Panel 2-7e

Fatty Acid Hydrocarbon Tails

Panel 2-4a

The hydrophobic effect causes nonpolar molecules to aggregate to avoid H2O

Macromolecules are Constructed of Covalently Linked Subunits

Fig. 2-28

Condensation Reactions Link Subunits into Macromolecular Polymers

Fig. 2-29

Fig. 2-18

Condensation Reactions Link Monosaccharides Into Polysaccharides

and polysaccharides

Nucleic Acids are Built Through Condensation Reactions Between Nucleotides

Fig. 2-24 & 2-26

Nucleotides Also Serve as Energy Storage Molecules

condensation hydrolysis

Fig. 2-25

Proteins Are Built of Amino Acid Subunits

SH

CH2

Panel 2-5e

Condensation Reactions Link Amino Acids intoPolypeptides through Peptide Bonds

Condensation reaction between Carboxyl group of one amino acid and amine group of second amino acid Peptide bond in red is planar and rigid and does not allow rotation.

Panel 2-5a

form H bondsparticipate in hydrophobic and Van der Waals interactions

form ionic bonds

Panel 2-5f

Panel 2-5b

Act as Acid (donates H+ to H2O)

Act as Base (gains H+ from H2O)

amine group wateramine group hydroxyl ion(protonated)

H

N H

H

Polar Charged Amino Acids Can Donate or Gain H+ from H2O

pK’:equilibrium pH

pK’:equilibrium pH

protonated deprotonated

N

H

H+ O H-

H

+ O H

protonateddeprotonated

deprotonated at pH 7.0

protonated at pH 7.0

- either

equilibrium

< protonated deprotonated >

Acids

Bases

either -

pK’ of Polar Charged R Groups

Panel 2-5h

Panel 2-5g

Noncovalent Bonds Are Also Crucial in Macromolecular 3D Folding and Assembly

Fig. 2-34