Eukaryotic cells evolved from symbiosis
Ø Anaerobic cell derived from an archaeon (primitive nucleus) à aerobic bacteria (containing
outer membrane and plasma membrane) à loss of membrane derived from archaeal cell à
early aerobic eukaryotic cell (containing nucleus with internal membranes, and mitochondria
with double membranes).
Ø Anaerobic cell engulfed an aerobic bacterium to become an aerobic eukaryotic cell.
Ø Mitochondria contain own DNA.
, Ø Early eukaryotic cell + photosynthetic bacterium à eukaryotic cell capable of photosynthesis
(containing chloroplasts).
Ø Aerobic eukaryotic cell engulfed photosynthetic bacterium to become eukaryotic
photosynthetic cell.
Ø Chloroplasts contain own DNA.
Model organisms
E. coli
Ø Anaerobic bacteria in colon of mammals
Ø Necessary for digestion
Ø E. coli is able to replicate fast in simple medium
Yeast (S. cerevisiae)
Ø Simple eukaryotic cell
Ø Fungi
Ø Used in genetic experiments
Thale cress (Arabidopsis thaliana)
Ø One plant can have thousands of off springs within 8-10 weeks
Ø Small genome: 125 Mb in 5 chromosomes which all have been mapped
Fruit fly (Drosophila melongaster)
Ø Key to vertebrate development
Caenorhabditis elegans
Ø Adult worm has exactly 959 cells
Ø 70% of human genes are equivalent in C. elegans
Zebrafish (Danio rerio) and frog (Xenopus Laevis)
Ø Vertebrate development
Mouse (Mus musculus)
Ø Almost every human gene has a comparable gene in mice.
Ø More than 95% of the mouse genome is comparable with the human genome
Plasma membrane
Ø Isolation of the cell
Ø Gives shape and strength (together with cytoskeleton)
Involved in:
Ø Intracellular communication
Ø Exchange of substances (import and export)
Ø Cell growth and mobility
Ø Phosphoglycerides and sphingolipids are the major phospholipids in cell membranes.
Sterols
Ø Cholesterol is the most common sterol.
Ø Polar head group, rigid steroid ring structure and a nonpolar hydrocarbon tail.
Ø Cholesterol: polar head group, cholesterol-stiffened region and more fluid bottom region.
Ø Cholesterol stabilizes and stiffens membrane
Lipid bilayer is a fluid
Ø Fluid bilayer
Ø Lateral diffusion possible
Ø Flip-flop occurs rarely (enzymatic).
Ø Fatty acid tails, lipid head groups and water molecules on the
outside.
Fluidity depends on its composition
Ø Unsaturated hydrocarbon chains with cis-double bonds have moving tails.
Ø Saturated hydrocarbon chains have stiff tails.
Ø Saturated hydrocarbon chains make the membrane less fluid.
Ø Membrane composition differs between various cells and organelles depending on which
cell, function or place within a cell
Membrane proteins
Ø Give properties to the cell membrane
Ø Proteins can associate in various ways with the membrane
Ø Amphiphilic
Ø Transmembrane proteins
Ø Membrane-associated proteins or peripheral membrane proteins.
, 1. Single a-helix (transmembrane/integral)
2. Multiple a-helix (transmembrane/integral)
3. b-barrel (transmembrane/integral)
4. Peripheral protein with a-helix attached (perifer)
5. Covalent bound lipid chain (perifer)
6. Covalent bound lipid chain (perifer)
7. Bound via other proteins (perifer)
8. Bound via other proteins (perifer)
Ø Amino acids are hydrophobic/hydrophilic?
Ø Most transmembrane proteins cross the lipid bilayer with a a-helix.
Ø No water present, so H-bonds are formed with each other à most possible with a-helix
Ø Transmembrane part is 20-30 amino acids long
Hydropathy plots predict membrane proteins
Ø Amino acids in the transmembrane region are mostly hydrophobic
Solubilizing membrane proteins
Ø Detergent: small amphiphilic molecules
Ø Soluble in water
Ø At low concentration: monomeric in solution
Ø High concentration: form micelles
Ø Used to lyse cells and solubilize membrane proteins
Ø Hydrophobic end bind to hydrophobic region of membrane proteins
Glycocalyx
Ø The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering
that surrounds the cell membranes of some bacteria, epithelia and other cells.
Ø Contains a carbohydrate layer with glycoproteins/lipids with sugar units attached.
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