1. Taste sensing is related to smell. Not all tastes have odors, however. In contrast to smell, we only have 5 primary tastes we detect through taste buds located on distinct areas of the tongue. The five primary tastes are sweet, sour, bitter, salty, and umami.
2. Taste buds contain about 150 cells with microvilli projections that are rich in taste receptors. About 50-100 7TM receptors are involved in taste. Not all primary tastes are associated with 7TM receptors, however.
3. Salty tastes are detected by passage of the ions directly through ion channels on the tongue. One class of these channels is sensitive to amiloride, which obscures the taste of salt and lowers neuron activity relating to sodium.
4. A G protein associated with the taste receptors that use 7TM receptors is called gustducin.
5. Whereas individual olfactory receptors are uniquely expressed on each olfactory neuron, with direct 'wiring' to distinct areas of the brain, taste receptors are not uniquely expressed on taste buds and the entire set of taste buds for primary tastes link to a single area of the brain.
6. Vision arises from signaling initiating in rod and cone cells of the eye. Rod cells (sense light, but not distinct for color) contain the pigment rhodopsin, which consists of a 7TM protein called opsin linked to a vitamin A derivative called retinal (linked via a lysine bond). Retinal is light sensitive and can flip between the 11-cis form and the all-trans form when exposed to light. This slight change in structure of retinal also changes the rhodopsin protein, ultimately activating a vision-related G protein called transducin.
7. Transducin binds GTP when active and this causes transducin to activate a specific phosphodiesterase to break down cGMP to GMP. Lowering cGMP concentration causes a cation ion channel to stop the movement ions of ions into the cell, starting the nerve signal.
8. Cone cells of humans have pigment-specific receptors for red, green, and blue light. We differ from more closely related organisms by virtue of the fact that we have evolved red receptors from our green receptor. Dogs and rodents, for example do not have red receptors.
9. The protein that binds the retinal in each of these is similar to the opsin of rod cells (40% identity). Color blindness (usually in males) arises due to recombination between related red and green receptor genes on the X chromosome.
10. Hearing arises as a result of signaling in the ear arising from hair cell micromanipulation (movement) arising from sound waves. Tipping of the cells causes little "tethers" to pull open ion channels that initiate a nerve signal.
11. Touch is the least understood sense. There are receptors for pressure, temperature and other sensations. Touch receptors can be linked to pain centers called nociceptors located in the spinal cord and brain. Interestingly, the compound capsaicin, which causes hot sensations in the mouth stimulates the same nociceptors.
12. The capsaicin receptor is a transmembrane protein. This protein contains a pore that opens to allow calcium ions in when capsaicin binds, thus initiating a nerve signal. Mice lacking the capsaicin receptor do not respond at all to capsaicin.
This course in general biochemistry is intended to integrate information about metabolic pathways with respiration (respiratory control) and initiate the student into a microscopic world where blueprints are made of deoxyribonucleic acids, factories operate using enzymes, and the exchange rate is in ATPs rather than Yens or Euros. Beyond explaining terms, and iterating reactions and metabolic pathways, this course strives to establish that the same principles that govern the behavior of the world around us also govern the transactions inside this microscopic world of the living cell. And by studying and applying these principles, we begin to understand cellular and bodily processes that include sensory mechanisms.
1. Lipids, Membranes and Transport
2. Electron Transport, Oxidative Phosphorylation and Mitochondrial 3. Transport Systems
3. Lipid Metabolism
4. Nucleotide Metabolism
5. DNA Replication