Chapter 2: Chemical Components of Life Living cells are not composed of atoms found only in them. But their compositions do differ from the non-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 shared equally 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 and 2) distance between protons and outer shell e - C and H similar O and H very different Electronegativity

Characteristics of Some Chemical Bonds Found in Cells

Polar H 2 O Accounts for 70% Cell Weight Fig determines behavior of cellular molecules

Ionic Molecules Polar Molecules

-contain mostly non-polar H-C bonds -cannot form H-bonds with H 2 O, so are not soluble in H 2 O Panel 2-2e

Polar Covalent Bonds of H 2 O Can Dissociate Fig. 2-15b at pH 7.0

Acts as Acid in H 2 O (donates H + to H 2 O) Acts as Base in H 2 O (gains H + from H 2 O) N H H N H H amine group amine group hydroxyl ion (protonated) Some Polar Covalent Bonds Can Donate or Gain H + from H 2 O

Noncovalent Bonds Are Important for Associations between Molecules in Cells They’re weakness allows for transient, regulated associations. Fig Complementary charged faces interact

-contain mostly non-polar H-C bonds -cannot form H-bonds with H 2 O, so are not soluble in H 2 O Panel 2-2e

Panel 2-7e

Fatty Acid Hydrocarbon Tails Panel 2-4a The hydrophobic effect causes nonpolar molecules to aggregate to avoid H 2 O

Macromolecules are Constructed of Covalently Linked Subunits Fig. 2-28

Condensation Reactions Link Subunits into Macromolecular Polymers Fig. 2-29

Fig Condensation Reactions Link Monosaccharides Into Polysaccharides and polysaccharides

Nucleic Acids are Built Through Condensation Reactions Between Nucleotides Fig & 2-26

Nucleotides Also Serve as Energy Storage Molecules condensation hydrolysis Fig. 2-25

Proteins Are Built of Amino Acid Subunits

SH CH 2 Panel 2-5e Condensation Reactions Link Amino Acids into Polypeptides 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 bonds participate in hydrophobic and Van der Waals interactions form ionic bonds

Panel 2-5f

Panel 2-5b

Act as Acid (donates H + to H 2 O) Act as Base (gains H + from H 2 O) amine group water amine group hydroxyl ion (protonated) H N H H Polar Charged Amino Acids Can Donate or Gain H + from H 2 O pK ’ : equilibrium pH pK ’ : equilibrium pH protonated deprotonated N H H + O H - H + O H protonated deprotonated

deprotonated at pH 7.0 protonated at pH 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