Lecture #7 – 9/19 – Dr. Hirsh
Microtubules, continued
Image from molecular motors page – tubulin alpha-beta dimers added at the + end, lost at the –
end of hollow microtubule structure. Note the alpha-beta dimers add 2 rows at once to form a
kind of double helix – hollow, about 25 nm in diameter.
Cell Membranes
Conflict: a “static” cytoskeleton versus a dynamic, moving membrane
The membrane is FLUID – fluid mosaic
Phospholipid bilayer is an excellent insulator
Phosphatidyl “head” groups are hydrophilic
Hydrophobic “tails”
Intrinsic or integral membrane proteins span the membrane
Extrinsic proteins are linked to above but are not within the membrane
Phospholipids move in 2 dimensions, NOT in 3 dimensions – only in plane of the membrane.
No “flipping” unless specifically designed to do so (rare).
Proof of motion through Fluorescence recovery after photobleaching (FRAP)
Label cell exterior proteins with GFP – cell glows when hit with UV light
Bleach a zone with a laser beam -> black zone in a green fluorescing cell
Wait
Black zone diffuses over space!
Recovery is fast (under 50 seconds); recovery is about 50%...therefore about half the
proteins are mobile.
Fluid mosaic = much motility but some parts of the membrane are fixed, implying some degree
of cabling by cell cytoskeleton.
Membranes are barriers:
Intracellular domain
Extracellular domain
Manufactured proteins have trafficking signals to direct them to precise location in the
membrane.
There is a set orientation for proteins in the membrane – no flipping! See the diagram
representation of dDAT, a neurotransporter for dopamine in the fly. It has 12 alpha helices
embedded in the membrane; there are many hydrophobic groups in the alpha helices (sticking
outwards!). Note the glycosylation sites are always at the extracellular surface.
Proteins with extracellular domains can control binding of cells to each other.
Homotypic binding -> same type of protein interacting – identical domains hood to themselves
Heterotypic binding -> different domains stick together (think of Velcro!)
In a developing neuron cell, the growing axon binds to cells in space as it projects towards its
ultimate goal. Both hetero and homotypic binding of adjacent cells to the axon process “guide”
the developing axon towards its ultimate site.
Cell-Cell connections
Tight junctions are necessary to seal adjacent cells when there may be consequences from
material leakage along the exterior surfaces of the cells. Good example is intestinal walls, where
leakage from the apical (top) surface to the basal (bottom) surface could prevent proper digestion
and nutrient acquisition.
Other connections – desmosomes (localized strong adhesion sites), gap junctions (allow
communication from cell to cell through a channel formed by connexons).
Osmosis
Remember this: water moves passively; ions are moved actively by the cell!
Diffusion is driven by Entropy (S).
Entropy is a measure of disorder; it tends to increase spontaneously.
In diffusion, random motion drives diffusion leading to an increase in Entropy.
All systems move from a state of high energy (much order) towards a state of low energy (more
disorder)
Osmosis is driven by diffusion of water down a concentration gradient through holes in a
membrane. The water can move through the membrane, but solute ions cannot.
The side of the membrane with the most solute is called hypertonic.
Hypertonic or hypotonic refers to the concentration of solute!
Water is driven in and out of a cell based on the relative solute concentration in and out of the
cell.
Transport
Facilitated diffusion
Takes no energy!
Pore in membrane specific for a type of solute. Channel may be regulated through
activation binding of agonist (sometimes called ligand) molecule.
Specific glucose carrier -> no energy needed.
Active transport – requires energy
Can work against a concentration gradient. Hydrolysis of an energy molecule creates a
coupled reaction.
Different types of active transporters
Uniport
Symport – transports the desired molecule along with another molecule that drives the
reaction
Antiport – one molecule going in drives the transport of different molecule going out
Good example of an Antiport is the Na+/K+ ATPase pump.
3 Na+ ions bind inside the cell membrane, along with ATP
2 K+ ions bind on the outside of the cell membrane
The ATP is hydrolyzed to ADP and Pi (inorganic phosphate group)
The 3 Na+ ions are transported out of the cell; the 2 K+ ions are transported into the cell.
This 3+ ions out, 2+ ions in transport generates an electro-motive force (voltage difference); the
inside of the cell is more negative (-) than the outside of the cell.
LOTS of ATP is used by cells to do this (between 20-40%). If you stop this process, the cell is
dead.
Secondary active transport
This is an indirect result of the NaK ATPase system. The Na+/K+ pump develops a chemical
and ionic gradient.
Na+ ions are more concentrated outside of the cell – the cell couples transport of Na+ to the
transport of Glucose into the cell. The dopamine transporter also couples Na+ ions to the import
of dopamine.
Chart of typical intra and extracellular ion concentrations. Note the dramatic difference in Ca++
concentration: this is a neural cell, and must carefully control Ca++ because it is involved in the
cascade that leads to neurotransmitter release.
Bulk Transport
Endocytosis functions through vesicle manufacture and transport, as does exocytosis.
Vesicles fuse with cell membrane to form a contiguous membrane and release the carried
substances. Endocytosis and exocytosis are the reverse of each other.
These processes are strongly conserved throughout evolution. See neurotransmitter release in
the neuron – similar genetic components in yeast exocytosis except for Ca++ involvement.
Hypercholesterolemia
Major cause of atherosclerosis and heart disease
Image of LDL particle
LDL Particle
Vesicle of cholesterol esters with a major protein, ApoB – docks with LDL receptor
localized to the region of membrane coated with clathrin
Clathrin = protein, forms a spherical cage, pulls membrane down to form a vesicle. Clathrin is
then dissociated, frees up cholesterol for use in cell.
Information Processing by cell membranes
A signal molecule binds on a membrane protein, causing change on the inner membrane zone of
the cell protein, leading to cascade signaling. Often causes the manufacture of cyclical AMP
(cAMP), a so-called second messenger molecule.
Energy transformation by cell membranes
A membrane protein complex can capture light energy, converting it into an energy-rich protein
that can be utilized to manufacture ATP within the cell.