What has happened so far in chemical evolution? Biological monomers formed from interaction of atmospheric gases, water and energy The monomers (ex. nucleotides) stick to clay/sand size particles at coastal areas of the ocean. As waves wash over them carrying other nucleotides, they make physical contact, form chemical bonds and form a chain of nucleotides (RNA or DNA). Problems with this: polymers not stable, likely to break down as exposed to harsh conditions of ocean and air. Not a very efficient way to build polymers (low frequency of monomers interacting this way in open space of coastal waters)

Cell Membranes are Made up of Lipids Phospholipids Important Properties of all lipids Two domains: hydrophobic (H 2 O & Hydrophillic (H 2 O ). Hydrophobic domain has varying length chain of hydrocarbon groups (C-H) 3 classes: steroids, fats and phospholipids Cell membranes made up of phospholipids X

Individual Phospholipids align in sheets to form Lipid Bilayers (the structure of the cell membrane) This is spontaneous: Does NOT require energy

? How could lipid bilayers (membranes) spontaneously form? To understand this question, need to 1st understand polarity

Understanding Polarity Fig also see p25-26 in section 2.2 Water is Polar: electrons are not shared equally creating a partial charge on the O and H atoms H 2 is nonpolar: electrons are shared equally. There are no partial charges on the atoms

Understanding Polarity Molecules with atoms that have different electronegativity will be polar. The more electronegative atoms attract the electrons more strongly and will have a partial charge For example: H-O & H-N When the atoms have the same electronegativity, the molecule Will be nonpolar (equal hogging of electrons). For example: H-H & C-H

Electronegativity Chart

Understanding Polarity: Because water is polar it can interact with other polar molecules like other water molecules by bonding of partially charged O and H atoms 1 water molecule 2 water molecules together via hydrogen bonding Fig 2.12

Understanding Polarity Because water is polar it can dissolve ionic molecules like salt by interacting with the charged Na + and Cl - ions Fig 2.12

Understanding Polarity Nonpolar molecules do not dissolve in water *Notice all of the C-H bonds*

Understanding Polarity Nonpolar molecules do not dissolve in water

ionic polar nonpolar equally shared none no lipids not equally shared partial yeswater, sugar not shared, complete transfer full yessalt (Na + Cl - ) electron sharing charge of atoms soluble in water example class of molecule Understanding Polarity

What about phospholipids, which have both a polar and nonpolar domain? Understanding Polarity polar head (hydrophilic) nonpolar tail (hydrophobic) Fig 6.4

So what happens to lipids in water? Polar (hydrophilic) heads interact with water molecules Nonpolar tails interact with each other to avoid water Fig 6.5 Result= lipid bilayer The polar parts dissolve in water and the nonpolar parts don’t

Recall these bilayers form without energy Individual phospholipids are not stable in water b/c tails disrupt the very stable hydrogen bonds of water

A second kind of lipid structure can also form… Micelles: Tiny droplet of lipids (made up of one layer). Forms with tails that have shorter tails (bilayers have longer tails) Fig 6.5 Soap forms micelles when placed in water

Water filled vesicles made up of lipid bilayers can form when lipid bilayers are agitated. Fig 6.7 Artificial vesicles called liposomes

prebiotic soup This allows for containment of prebiotic soup components -amino acids -nucleotides -sugars -lipids -polymers

Wake up time! Cell Membranes A. Are made up of single sheets of lipids (monolayers) B. Are completely nonpolar and therefore do not dissolve in water C. Have a polar head domain that is not soluble in water D. Have a polar hydrocarbon tail domain that is not soluble in water E. Formed when lipid bilayers formed vesicles by remaining motionless F. Are important because they allow all molecules to enter and leave the cell G. Allow for individual amino acids to form amino acid polymers (proteins) more readily than without a cell membrane

Adding all of this to the chemical evolution story… Simple reduced molecules like H 2 CO and HCN formed from interaction of atmospheric gases, water and energy More complex and more reduced molecules like amino acids, nucleotides, sugars and lipids formed from H 2 CO, HCN, atmospheric gases, water and energy Nucleotide, sugar and amino acid monomers polymerized to form DNA, RNA, polysaccharides and protein (requiring energy) One of these molecules acquired the ability to self-replicate (RNA) Individual lipids formed lipid bilayers, agitation of the bilayers resulted in vesicles filled with water and biological molecules

What characteristics are common to all life? Acquire nutrients and Utilize those nutrients for energy Order: Basic unit of life is the cell. All organisms are made up of cells. Other ex. of order include: symmetry & tissues (not just bags of molecules) Respond to environment (short-term) Evolve: long-term Response to environment Grow and Reproduce

Selectively keep out damaging compounds, admit needed compounds (nutrients), and allow waste to leave Advantages a Cell Membrane Concentrate reactants so chemical reactions are more efficient Make the internal environment different from the external (ex: maintain and internal temp, pH and other conditions that are favorable for reactions) Fluorescent staining of Cell membrane (orange) Invitrogen.com

The cell is surrounded by a membrane so how are the nutrients allowed in through a membrane that also keeps the Riffraff (pathogens, any molecules the cell doesn’t need) out

Factors that determine if a molecule will permeate the cell membrane Polarity / charge of the molecule Size of the molecule Properties of the membrane itself Permeability- measurement of how easily a molecule can cross the cell membrane

What are the natural barriers to permeating the membrane? 1. The bilayer structure of the cell membrane creates a barrier for polar molecules b/c they are repelled by the non-polar membrane interior Polar (hydrophillic) heads Nonpolar (hydrophobic) tails Polar (hydrophillic) heads water

What are the natural barriers to permeating the membrane? 2. The densely packed phospholipids (bonds form between tails of phospholipids) create a barrier for larger molecules b/c they can’t pass between the phospholipids. Polar (hydrophillic) heads Nonpolar (hydrophobic) tails Polar (hydrophillic) heads water

Polarity continuum: Nonpolar (O 2, CH 4 ) Uncharged polar (water, glucose, amino acids) Charged polar (amino acids, ions such as Na +, Ca 2+, K +, Cl - ) Easier passage Harder passage Size continuum: Small (H 2 O ) Medium (glucose) Large (proteins) Take home message: The combination of polarity and size determines the permeability (i.e. small polar molecules cross more easily than medium sized polar molecules). What are the natural barriers topermeating the membrane?

Fig 6.8 fast slow can’t pass Polar charged molecules (amino acids) and ions (Cl - ) (size irrelevant) can’t pass Combination of polarity and size determines permeability What are the natural barriers to permeating the membrane?

Saturated Unsaturated like fig The types fatty acid (property of the lipid) What are the natural barriers to permeating the membrane?

4. Longer hydrocarbon tails provide more opportunities for hydrophobic interactions (bonds) which decrease permeability shorter saturated tail longer saturated tail Fig 6.11 What are the natural barriers to permeating the membrane?

5. Temperature also effects membrane permeability a. Phopsholipids constantly in motion. b. Mobility increases as temperature increases (less rigid membrane) What are the natural barriers topermeating the membrane?