MEMBRANES

Campbell and Reece, Chapter 8, plus some references back to earlier chapters

Lipid biochemistry:

Get a good source of membranes: red blood cells (erythrocytes) from adult human have only plasmalemma. Gorter and Grendel showed in1925 that there was enough lipid to make two layers.
Put red blood cells into distilled water, they burst from hyposmotic shock and become only "ghosts" - membrane only. TRANSPARENCY (Fig. 8.12 shows how animal vs plant cells react to hypertonic, isotonic and hypotonic solutions.

TRANSPARENCY (Fig. 8.1) shows hydrophobic vs hydrophilic aspect of polar phospholipid

TRANSPARENCY (Fig. 5.12) [shown earlier] shows the chemical structure of a polar phospholipid)

TRANSPARENCY (Fig. 7.6) 2 two dense lines in EM with osmium (Robertson)

TRANSPARENCY (Fig. 8.2) bilayer (Davson-Danielli)
vs.
TRANSPARENCY (Fig. 8.6) Fluid mosaic Singer and Nicolson

TRANSPARENCY (Fig. 8.7) Here is a famous membrane protein, rhodopsin, the molecule we see with, and how 7 hydrophobic alpha helices fit into the hydrophobic part of the membrane.

TRANSPARENCY (Fig. 8.3) Freeze fracture EM. Membrane is ripped in half, and membrane proteins are shadowed.

Picture I made freeze fracture replicas with this apparatus. Specimen is prepared, frozen to liquid nitrogen temperature, put inside a vacuum, smashed with a razor, blasted from an angle with a platinum gun (to shadow protein with electron dense metal), blasted from above with a carbon gun (to hold replica together), then the tissue is dissolved away.

Here, from my research, is an example of how things look. Picture shows visual membranes in Drosophila. High vitamin A has membranes full of protein (rhodopsin) while vitamin A deprivation eliminates this protein.

Membrane lipids are composed of:
(1) Phospholipids such as phosphatidylcholine (lecithin)
I did some research on the phospholipids of the Drosophila head. Using radioactively lbeled phosphate, many different phospholipids are visualized after they have been separated on a TLC (thin layer chromatography) plate.
Amphipathic
(2) Cholesterol
(3) Glycolipids such as one that accumulates in Tay-Sachs, a hereditary lysosomal storage disease,1/30 Am. Jews carry, recessive, fatal at 6 mo - 5 yr

The sugar groups of glycoproteins and glycolipids are on the outside of the membrane.

TRANSPARENCY (Fig. 8.4) Double bonds make more fluid, cholesterol makes less fluid.

It used to be thought that lipids just sit there. In the 1980's it became clear that they turn over metabolically and that some products of membrane lipid turnover are important mediators of intracellular signalling. This is very fundamental and will come up repeatedly in biology.

TRANSPARENCY (Fig. 8.16) Diffusion
(1) Lipid makes a barrier to anything polar
(LATER: steroid hormones can go in)
(2) Channels (for ions, electrical conductances)
The 1991 Nobel Prize in physiology and medicine was awarded to prize was awarded jointly to: ERWIN NEHER and BERT SAKMANN for their discoveries concerning the function of single ion channels in cells; in 1963 the prize was awarded jointly to: SIR JOHN CAREW ECCLES , SIR ALAN LLOYD HODGKIN and SIR ANDREW FIELDING HUXLEY for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane (in summary, the topic of ion channels is pretty fundamental).
(3) A large fraction of the cell's energy (ATP) goes to pumping ions (active transport)
This creates an ion imbalance, sodium Na+ high outside cell, potassium K+ high inside.
This gives rise to the membrane electrical potential (voltage) important in nerve and muscle cells.

TRANSPARENCY (Fig. 8.15) how pump molecule uses ATP to make sodium and potassium gradients.

run in reverse, makes energy, motor - generator analogy ENERGY - Chap 9

BULK TRANSPORT:
phagocytosis - cell eating
pinocytosis - cell drinking
receptor mediated endocytosis - clathrin coated

Also holes in membranes from one cell to another are important:
(1) Gap junctions (animals)
(2) Plasmodesmata (plants)

TRANSPARENCY (Fig. 8.9) shows some other functions of membrane proteins
(1) in addition to transport,
(2) many enzymes are on the membrane
(3) receptors for hormones, neurotransmitters and developmental signals are on the membrane.
(4) cells are joined by proteins
(5) cells communicate by proteins
(6) cells hook to extracellular proteins by proteins

This page was last updated 6/20/02

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