Cell Membranes and Cell Organization

• Cell membranes and cell organization are the topics covered

Housekeeping

• Gauri holds office hours Wednesdays 4-5 pm in Ritter 106, Katie 5-6 pm in Ritter 323
• Betsy holds office hours Thursdays 11-12 pm in Ritter 314
• Lyon holds office hours Fridays 10-11 am in Macelwane 100
• Attendance/participation will start being tracked this week
• Questions can be posted to the discussion page or after class iClicker poll

Polarity

• Oxygen is most likely to form polar bonds

Hydrophobicity

• A substance with only non-polar covalent bonds would be least likely to dissolve in water

Learning Objectives

• To explain how the structure and properties of phospholipids determine their tendency to form bilayers in water
• To describe the structure and properties of biological membranes (phospholipid bilayers), and the structure of prokaryotic and eukaryotic cells
• To describe the basic function of cell walls in prokaryotes, plants and fungi
• To predict how the structure and properties of amino acid side chains will affect their localization within transmembrane proteins

Carbon

• Organic molecules contain at least one carbon atom, usually with many C-C and C-H bonds
• Most biomolecules contain oxygen (O) and nitrogen (N)
• Nucleic acids (DNA and RNA) also contain phosphorus (P)
• Proteins can also contain sulfur (S)
• Nucleic acids have a 5'-phosphate group
• Proteins can contain methionine and cystine side-chains

Biomolecules

• Lipids are mostly nonpolar and are grouped together because of their hydrophobicity
• Lipids primarily contain a large number of carbon and hydrogen atoms
• Lipids are the only major biomolecule category not made of polymers
• Major categories of lipids include steroids, fats and phospholipids

Fat Saturation

• Type of C-C bonds in hydrocarbon chains influences form and function
• Double bonds cause a rigid bend
• Unsaturated fats can be mono- or poly-unsaturated
• Saturated fatty acids have a high melting temp, and are a solid at room temp
• Unsaturated fatty acids have a low melting temp, and are a liquid at room temp

Water interactions

• The atoms present dictate how a molecule interacts with water

Phospholipids

• The polar group and phosphate group are polar
• Glycerol
• Fatty acid tail

Amphipathic Molecules

• Amphipathic molecules have hydrophilic and hydrophobic regions
• The polar "head" is hydrophilic
• The fatty acid tails are hydrophobic

The Shape of Water

• Molecules containing water orient and arrange so that the hydrophilic end faces water
• The hydrophobic end is secluded from water, which drives structure formation
• Lipids form lipid micelles, lipid bilayers and liposomes in water

Micelles and Liposomes

• Detergents form micelles around hydrophobic particles
• Liposomes have many uses in biomedical research

Phospholipid Bilayers

• Bilayers ("two layers") are fluid, phospholipids can move laterally (sideways) but not flip between layers

Permeability

• Small, nonpolar molecules can pass through phospholipid bilayers easily
• Small, uncharged polar molecules can pass through
• Large, uncharged polar molecules permeate with difficulty
• Small ions cannot pass

Saturation

• Shorter and unsaturated hydrocarbon tails in the lipid bilayer increase fluidity
• Longer and saturated hydrocarbon tails in the lipid bilayer reduce fluidity

Boundaries of Life

• All cells are separated from their environment by plasma membranes
• Biological membranes are made of phospholipids with embedded proteins

Fluid Mosaic Model

• Plasma membranes contain both lipids and proteins
• Plasma membranes keep unwanted materials out
• Plasma membranes allow specific materials in
• The fluid mosaic model facilitates chemical reactions
• Proteins in the membrane have a large range of functions

Amino Acids

• Amino acids contain an Amino Group, a "Variable Side Chain" (R), and a Carboxylic Acid Group bound surrounding an alpha - carbon
• At pH 7 both the amino and carboxyl groups ionize
• Charged ends help solubilize the molecule
• Foundation for polymerization

Side chains - Hydrophilic Amino Acids

• There are 20 common amino acids grouped by side chain properties
• Polar and charged R groups are hydrophilic and face water in the cell fluid (cytosol) or outside
• Nonpolar R groups can't Hydrogen bond with water, and are hydrophobic, and usually packed inside or in membranes

Form and Function

• The amino acid hydrophobicity can predict which parts of a protein are in the membrane
• Hydrophobic parts tend to be in the tails
• Hydrophilic parts stick out of the membrane

Cell Walls

• Cell walls are found in all prokaryotes, fungi, plants, and some protists, but not in animals
• Cell walls are located outside of the plasma membrane and provide structural support
• Cell walls have different compositions in each group
• Plants and Algae have pectin, cellulose microfibril, and cross-linking glycan
• Fungi have B-glucan, chitin, and lipoprotein

Prokaryotes

• Prokaryotes include Archaea and Bacteria, and are simple cells
• They do not contain membrane-bound organelles
• Prokaryotes are unicellular or colonial
• They are 5-100x smaller than Eukaryotic cells
• They have a cell wall and plasma membrane
• They lack a nucleus and have a single circular chromosome
• They have specialized protein-based structures

Eukaryotes

• Eukaryotes have a cell wall, and contain membrane-bound organelles with specific specialized functions
• They are unicellular, colonial, or multicellular
• Multiple linear chromosomes are stored in a nucleus
• Multicellular organisms can have many specialized cell types

Cell Wall Functions

• Cell walls keep unwanted materials out
• Cell walls allow specific materials in
• Cell walls provide more rigid structure
• Cell walls facilitate chemical reactions

Study of cell membranes and cell organization. Includes information about office hours, attendance tracking, polarity, and hydrophobicity. Also, a discussion of learning objectives.