sleep - How are people able to wake themselves up after a pre-specified amount of time?

There is quite a lot of research on self-awakening (see this search on Google Scholar for self awakening). Hopefully someone else more familiar with this literature can add a more authoritative answer about the mechanisms of self-awakening. In the interem I briefly extract some relevant points from Ikeda and Hashi (2012). The study does not directly address biological mechanisms. However, understanding the reliability of self-awakening and the correlates of the ability to self-awaken is presumably relevant to forming such an understanding.

First they summarise some existing research:

Both the accuracy and success rate of self-awakening have been
experimentally examined. More than half of the people who have the
ability to self-awaken suc- cessfully awakened within 30 minutes of
the predetermined time. For example, seven participants succeeded on
nine of 14 days (64%) in a sleep laboratory, and 15 participants
succeeded on 35 of 44 nights (80%) at their homes. Survey studies
indicate that many people habitually self-awaken in daily life; for
example, 52% of 269 adults (aged 21−84 years) and 10.3% of 643
university students6 reported habitu- ally self-awakening. People who
have a habit of self-awakening in the morning have regular sleep/wake
schedules, tended to have a morningness chronotypology, awakened
comfortably in the morning, and had less daytime dozing.

In their own study they found:

The present study investigated self-awakening, both habitual and
inconsistent, compared to awakening by external means in relation to
sleep/wake schedules for five consecutive years in 362 students
(starting at mean age 15.1 ± 0.3 years). Students who self-awakened
consistently for five consecutive years (5% of all students) went to
bed earlier than those who inconsistently self-awakened (mixed group,
40%) or consistently used forced awakening by external means (56%).
Awakening during sleep was more frequent and sleep was lighter in the
consistently self-awakened group than in the mixed and consistently
forced-awakened groups. However, daytime dozing was less frequent and
comfort immediately after awakening was greater for the consistently
self-awakened group than for the mixed and consistently
forced-awakened groups.

Reference

• Ikeda, H., & Hayashi, M. (2012). Longitudinal study of self-awakening and sleep/wake habits in adolescents. Nature, 4, 103-109. PDF


• Bell CR. Awakening from sleep at a pre-set time. Percept Mot Skills. 1980;50(2):503–508.


• Lavie P, Oksenberg A, Zomer J. “It’s time, you must wake up now.” Percept Mot Skills. 1979;49(2):447–450.


• MoorcroftWH,KayserKH,GriggsAJ.Subjectiveandobjectiveconfir- mation of the ability to self-awaken at a self-predetermined time without using external means. Sleep. 1997;20(1):40–45.


• ZepelinH.REMsleepandthetimingofself-awakenings.BullPsychon Soc. 1986;24(4):254–256.


• Zung WW, Wilson WP. Time estimation during sleep. Biol Psychiatry. 1971;3(2):159–164.


• Matsuura N, Hayashi M, Hori T. Comparison of sleep/wake habits of university students with or without a habit of self-awakening. Psychiatry Clin Neurosci. 2002;56(3):223–224.

human biology - Why can't you taste food when you have a cold?

There are many factors that contribute to the flavor of food - the five "traditional" tastes (sweet, sour, bitter, salty, umami), smell, texture, spiciness, "coolness" (like peppermint), temperature, etc. Smell and taste are detected in similar ways, by chemoreceptors expressed in taste pores by specialized cells of the lingual epithelium (tongue), and in the nose by the olfactory epithelium. Both are affected by upper respiratory tract infections like colds and the flu, leading to diminished senses of taste and smell, and a corresponding reduction in the overall flavor of food.

reproduction - Parthenogenesis in Bees

Parthenogenesis is defined as:

"A type of asexual reproduction in which egg develops without fertilization to form a new individual."

If parthenogenesis takes place in bees, a drone or male bee is produced.

My question is that parthenogenesis is the development of EGG into new individual so, how can egg be developed into male because egg won't have that "Y" chromosome required for male offspring?

astrobiology - In our solar system which other planet could have been in the "habitable zone" in the past?

This answer is also speculative, but according to the planetary habitable zone models proposed by "Habitable zone for Earth-like planets in the solar system" (Franck et al. 2000), a key finding was that:

an Earth-like planet at Martian distance
would have been habitable up to about 500 Ma ago while the position of Venus was always outside the habitable zone.

So, Mars could have been habitable, but as it is much smaller than Earth, it has a smaller gravitational field presumably a major factor in how it lost much of its atmosphere.

molecular biology - Termination of translation

Shigeta's got a point: the ribosome is latched onto the mRNA so those two are intrinsically linked. You're really asking whether the ribosome comes off first or whether the tRNA does, but it's actually the new polypeptide, which makes sense:

The stop codon is recognized by a protein, the polypeptide chain release factor (RF), which triggers the hydrolytic release of the nascent polypeptide chain from the P-site-bound peptidyl-tRNA.

This minireview puts forth a model (see below) where, in E. coli at least, the 50S ribosome subunit is then dissociated from the mRNA/30S subunit/tRNA complex, following which the final tRNA is removed. An in-depth review from a few years later gives more context.

Effect of extracellular molecules on membrane potential

The absolute answer would depend on a lot of factors, but the basics of it would be that - Yes, the volume does change and Yes, it would have an affect on the membrane potential.

By adding mass to any liquid solution, you are changing the volume. Plain and simple. Liquids are not compressible, and the only way to maintain volume while adding mass would be to increase the density of the liquid. [Edit] I'm wondering if you don't mean increase the volume of the cell - in which case, in extreme conditions where the cell cannot maintain water and salt concentrations, the cell would lose volume as water and salts move out. In a hypotonic solution (much more water than solutes), the cell would lyse as osmotic pressure favors water moving into the cell

The membrane potential would change, but how much depends on a lot of factors. It would most likely change due to the movement of ionic solvents down their concentration gradients. Osmosis would move water out of the cell and into the extracellular environment, and ultimately reduce the membrane potential as more water outside the cell would essentially decrease the molarity of the solution. The cell would have to pump more ions out in order to regain the membrane potential.

The exception might be if you added so many hydrophobic molecules outside the cell that you basically saturate the environment, which could prevent exchange of all charged or partially charged atoms and molecules.

biochemistry - Why is absorbance at 280 nm for protein solution going up when I measure repeatedly?

It looks like your protein concentrations are right on the limit of detection of the spectrophotometer, and changing the diluent buffer changed their concentrations. The samples may not have been thoroughly mixed after dilution and before measurement, so the varying measurements may simply be the solution coming to equilibrium. Temperature can also affect absorbance, so you should verify that your samples have equilibrated before drawing any conclusions. If the absorbance of your phosphate buffer is 0.03, I'd try to keep the sample absorbances above 0.075 or higher to avoid getting too close to the limit of detection. Also, make sure your buffer isn't too old or contaminated with something which could be affecting its absorbance characteristics.

I would suggest taking one or two protein samples and doing a dilution series (1:1, 1:5, 1:10, 1:20) in a large-ish volume (say 400 ul each, if you can spare it), vortex briefly to mix well, then measure triplicates of each dilution on your reader, along with appropriate blanks (buffer only). You will see differences between each measurement, but it should be quite small, depending on the accuracy and precision of your instrument.

Measured values will not be exactly the same from measurement to measurement, and it would take a lot more than three repetitions to determine if there was an actual drift trend occurring. Measure your sample plate every 5 minutes for an hour and plot the values (don't just eyeball them) to see if the machine may need to be serviced.