Foundations-Cells-6

From CellBiology

Membrane Structure


This lecture is also an introduction to cell compartments and describes the structure of membranes forming these compartments.


Draft - this online lecture is currently in 2018 draft status. This notice removed when completed.
Cells and Membranes: 1 Cells and sizes | 2 Prokaryotes | 3 Viruses and prions | 4 Plasma membrane | 5 Compartments | 6 Membrane structure | 7 Membrane specialisations | Foundations - Moodle

Terms

  • cholesterol - small lipid molecule required for membrane fluidity. Also the precursor molecule for biosynthesis of steroid hormones,
  • phospholipid - The main lipid component of cell membranes that establishes the bilayer structure of all membranes. Molecules have a polar and non-polar end and there can be many different types of phospholipids in a membrane.
  • protein - A biomolecule consisting of one or more chains of amino acid residues. Proteins are produced within the cytoplasm on ribosomes by translation, from messenger RNA (mRNA).


Membrane Components

Model of Cell (plasma) membrane structure

Cell membrane.png


  • phospholipids, proteins and cholesterol
  • first compartment formed
  • prokaryotes (bacteria) just this 1 compartment
  • eukaryotic cells many different compartments


Phospholipids

  • Membranes contain phospholipids, glycolipids, and steroids
  • Main lipid components include:
  1. phosphatidylcholine (~50%)
  2. phosphatidylethanolamine (~10%)
  3. phosphatidylserine (~15%)
  4. sphingolipids (~10%)
  5. cholesterol (~10%)
  6. phosphatidylinositol (1%).


Phospholipid Orientation

  • A liposome (lipid vesicle) is a small aqueous compartment surrounded by a lipid bilayer.
  • A micelle is a small compartment surrounded by a single lipid layer.


Links: MBoC - Three views of a cell membrane | MBoC - Phospholipid structure and the orientation in membranes

Phospholipids aqueous.jpg

Phospholipid Orientation

Membranes History  
Background only, you do not need to know the details.
  • 1890 Charles Overton - selective permeation of membranes, non-polar pass through (lipid soluble), polar refractory
  • 1905 Irving Langmuir - lipids faced with heads towards water away from organic solvents
  • 1925 Gorter and Grendel - monolayer of lipid isolated from rbc
  • 1930-40 Danielle-Davson - Proteins coat a bilayer with polar “pores”
  • 1960s Robertson - Modification with glycoprotein on one side, therefore asymmetric
  • 1972 Singer and Nicholson - proteins “floating” within lipid bilayer like a “liquid” surface
  • 1975 Unwin and Henderson - integral membrane proteins, glycoprotein carbohydrate groups on outer surface
  • 1997 Simons - cholesterol to form "rafts" that move within the fluid bilayer PMID 9177342

Lipid rafts.jpg

Membrane Proteins

Membrane model 02.jpg

Model of membrane proteins and dimensions

Links: image - Membrane model A,B,C | image - Membrane model dimensions | image - Membrane model outside | image - Membrane model inside
  • 20-30% of the genome encodes membrane proteins PMID 9568909
  • Proteins can be embedded in the inner phospholipid layer, outer phospholipid layer or span both layers
  • Some proteins are folded such that they span the membrane in a series of “loops”
  • Membrane Protein Functions - transport channels, enzyme reactions, cytoskeleton link, cell adhesion, cell identity


Links: Figure 17-21. Topologies of some integral membrane proteins synthesized on the rough ER

  • Membrane Glycoproteins - Glycoproteins are proteins which have attached carbohydrate groups (sugars)
  • produce these proteins go through a very specific cellular pathway of organelles (secretory pathway)
  • reach the cell surface where they are either secreted (form part of the extracellular matrix)
  • or are embedded in the membrane with the carbohydrate grouped on the outside surface (integral membrane protein)


Two major protein transmembrane structures: α-helical - ubiquitously distributed; β-barrel - outer membranes of Gram-negative bacteria, chloroplasts, and mitochondria.

Membrane Cholesterol

  • small molecule regulates lipid mobility (MH - lipid rafts)
  • embedded between the phospholipid molecules, different concentrations in different regions of plasma membrane
  • lateral organization of membranes and free volume distribution
  • may control membrane protein activity and "raft” formation

Note - bacterial membranes (except for Mycoplasma and some methylotrophic bacteria) have no sterols, they lack the enzymes required for sterol biosynthesis.


Cell membrane.png

Model of Cell (plasma) membrane structure


Links: MBoC Figure 10-9. Cholesterol in a lipid bilayer

Additional Concepts

Note that while I state that all membranes have the same "basic structure" this is a simplification, as internal membranes within the cell surrounding each organelle are different in the specific structural components from that found in the plasma membrane.


Bacterial Membranes

Gram positive bacteria

Gram Negative

  • inner membrane is the cell's plasma membrane
  • do not retain dark blue dye used in gram staining
  • Bacteria with double membranes (Example: Escherichia coli, Salmonella, Shigella,)

Gram Positive

  • because they do retain blue dye, thicker cell walls
  • single membrane comparable to inner (plasma) membrane of gram negative bacteria
  • Bacteria with single membranes (Example: staphylo-cocci and streptococci)

(Named after - Hans Christian Gram (1853–1938), a Danish scientist.)


Membrane Fluidity

Neutrophil activation membrane reorganisation

Membranes can demonstrate both high fluidity and fixed domains (regions)???

  • Experiments wit fusion of 2 cells, FRAP
  • membrane domains (polarized cells) - epithelia have apical, basal and lateral domains



Links: MBoC - Membrane Fluidity movie FRAP | MBC - Membrane Fluidity|