MEMBRANES

Assignment
Audesirk, Audesirk & Byers Chapter 5

Today's musical selection
Ray Charles America the Beautiful

Lipid biochemistry:

Figure 5-10
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.

Membrane structure

Figure 5-3
shows hydrophobic vs hydrophilic aspect of polar phospholipid
bilayer (Davson-Danielli)

Figure 5-2
shows the chemical structure of a polar phospholipid)

Figure 5-1
Fluid mosaic Singer and Nicolson
Proteins (channels, pumps, receptors, etc.) among lipids

Figure 5-4
Double bonds make more fluid, cholesterol makes less fluid.

Techniques to study membranes

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 lipid types

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.

Signal transduction

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.


How do molecules get across the membrane?

Diffusion
Lipid makes a barrier to anything polar
(steroid hormones can go in)
Channels (for ions, electrical conductances)

Figure 5-12
how pump molecule uses ATP to make sodium and potassium gradients.

Bulk transport

Figure 5-15
phagocytosis - cell eating
pinocytosis - cell drinking

Figure 5-16
Exocytosis

Figure 5-14
receptor mediated endocytosis - clathrin coated

Figure 5-19
Also holes in membranes from one cell to another are important:
(1) Gap junctions (animals)
(2) Plasmodesmata (plants)

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.

Figure 5-18
(4) cells are joined by proteins
(5) cells communicate by proteins
(6) cells hook to extracellular proteins by proteins

Questions used in 2007 relating to this outline

A red blood cell ghost was used to study
(a) lysosomal storage diseases.
(b) nuclei.
*(c) membranes.
(d) the genetic code.
(e) the action potential.

A TLC (thin layer chromatography) plate could be used to
(a) model continental drift.
(b) allow Cl- (chloride ions) to pass out of the cell.
(c) demonstrate phagocytosis.
(d) visualize proteins in a carbon-platinum replica of the membrane.
*(e) separate membrane lipids chemically.

Clathrin coated pits are part of what process?
(a) meiosis
(b) apoptosis
(c) hydrolysis
*(d) endocytosis
(e) transcription

It would take energy in the form of ATP to transport Ca2+ ions from [A] to [B].
(a) [A] coated pit; [B] coated vesicle
*(b) [A] inside the cell; [B] outside the cell
(c) [A] microfilament; [B] microtubule
(d) [A] omega figure; [B] coated pit
(e) [A] nucleus; [B] rough endoplasmic reticulum

The function of a gap junction is
(a) to hold cells together.
*(b) allow confluence of cytoplasm of adjacent cells.
(c) receptor mediated endocytosis.
(d) to allow a white blood cell to "eat" bacteria.
(e) to make the plant cell wall permeable.

Pseudopods might be used by Amoebae to eat Paramecia by a process called
(a) exocytosis.
(b) phosphorylation
(c) hydrophobicity.
(d) autoradiography.
*(e) phagocytosis.

Which is true about glycolipids?
(a) They are the rigid motile elements of cilia and flagella.
(b) They are the universal currency of biological energy.
(c) They are phosphorylated peptides.
*(d) They accumulate in victims of Tay-Sachs disease.
(e) Phosphatidylcholine is the predominant type.

Tight junctions are used
*(a) to keep layers of cells from leaking.
(b) to hold the tRNA to the DNA.
(c) in the Kreb's cycle.
(d) in flagella.
(e) to coat vesicles used in bulk transport.

The central concept of the fluid mosaic model is that
*(a) there is enough lipid in membranes to make two layers.
(b) mRNA can pass freely from the cytoplasm to the nucleus.
(c) ions can pass through the hydrophobic layer.
(d) 38 pyruvates are made from one glycogen.
(e) the cell membrane is the only membrane in eukaryotic cells.

Questions used in 2002 relating to this outline (and other outlines)

One reason that membrane lipids are arranged the way they are is because of
(a) essential amino acids.
(b) ionic bonds.
*(c) hydrophobic fatty acids.
(d) hydrolysis of bonds.
(e) the half life.

A cell's membrane consists principally of two layers of
(a) hydrocarbon.
(b) disaccharides.
(c) cellulose.
*(d) lipid.
(e) collagen.


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