botany - What's the effect of oxygen deficit on plants?

During the daylight, the plant is photosynthesising faster than it is respiring so there is no net uptake of oxygen (the oxygen of course being produced in the as part of the photosynthesis).

Of course, this only applies for tissues where photosynthesis is occurring. In the roots of the plant, oxygen must always be present in the surrounding soil/growth medium for respiration. Therefore if there is no oxygen in the roots then the root cells will be unable to produce ATP from respiration and consequently die. This will eventually lead to the death of the entire plant as it is unable to take up nutrients due to the lack of a functioning root network.

This is the cause of plant death when soil is waterlogged - the water fills spaces in the soil that would otherwise contain air (i.e. oxygen).

Certain plants (often crop plants), rice being given as an example in the comments below, are able to survive being waterlogged and the resulting low oxygen supply. However they can only do so in the short term. Rice plants in particular have adaptations that allow the transport of oxygen from the aerial parts of the plant down to the roots. This does of course mean that the rice can not survive being totally submerged for an extended period of time. Plants such as rice may have better adapted anaerobic fermentation pathways that become the main producer of ATP when aerobic respiration has arrested thus allowing them to better cope when transiently submerged than other plants.

Molecular strategies for improving waterlogging tolerance in plants

stellar evolution - Why does shell fusion produce more energy than core fusion?

Ultimately this is more of an overly long comment, as I think a more satisfying and complete answer would properly explain things in a more concrete fashion—more of a "it has to do this because..." answer than my "it can do this because..." one.

The short of the answer to the first question is that helium fusion needs ~25 times the temperature that hydrogen fusion does. The proton-proton chain initates around $4times 10^6$ Kelvin, whereas helium fusion doesn't begin until around $10^8$ Kelvin. So when the main stage ends and the helium core contracts and the temperature rises, the "edge" of the core can have temperatures well in excess of the minimum hydrogen fusion temperature, and so a shell around it can have temperatures well beyond it. Fusion rates are (approximately) polynomial in temperature, with the degree depending on the reaction in question, so small increases in temperature can produce substantially more fusion. The gravitational force is strong enough to overwhelm the pressure from the induced fusion, and so will contract the surrounding shell to temperatures in excess of the minimum needed. This is basically what happens in the cores of main sequence massive stars (relative to, say, the Sun). Actually, our own Sun's energy is mostly from the proton-proton chain and has a core temperature of around $1.57times 10^7$ Kelvin, nearly four times the minimum necessary. And still, the core needs to be nearly 10 times hotter than that to initiate Helium fusion.

For the second question, the short of the answer is that the core has undergone thermal expansion after the Helium flash, and so occupies the temperature ranges most conducive to a strong hydrogen fusion rate. The material outside the core is now at lower temperatures and pressure, so the fusion rate is reduced substantially. So the energy output comes principally from the core, and the helium fusion at near minimum temperatures releases less energy than the hydrogen shell at (well) beyond minimum temperatures did. Thus the star overall produces less energy and contracts.

The remaining questions are explained in similar fashion: one has to pay attention to the sensitivity of reaction rates to temperature, and what the temperatures in those shells actually are. The sensitivities are different for each reaction chain, and the temperatures can go well beyond the minimum necessary.

cmb - Why can we observe the Cosmic Microwave Background no matter the direction we look?

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and electrons couldn't form neutral hydrogen, since every time it "tried", an energetic photon would knock off the electron.

This gas was everywhere. And photons traveled and scattered in all directions:

^{Photons (purple) scatter on free electrons (green), and both are mixed with protons (red).}

380,000 years after the Big Bang, the temperature had fallen sufficiently that neutral atoms could form (this is called recombination). The radiation, which until now had scattered continously on free electrons, could now stream freely between the atoms (this is called decoupling).

So they did. Still in all directions:

This free streaming is still taking place. Photons travel in all directions, and are everywhere. The photons that you are able to see, are the ones that started out at a particular distance from you, and in a particular direction, but other photons started out at smaller and larger distances, and in other directions. You just don't see them, because you happen to be right here. But a person in another place of the Universe would see the same as you.

The photons that we observe as the CMB come from a region we call the surface of last scattering, because it corresponds to the surface of a shell centered at us. But there is nothing special about this "surface", except that is consists of all points in the Universe that are roughly 47 billion lightyears away from us.

exoplanet - Which is the largest planet ever observed?

Yes, there is a limit. Anything with a mass larger than about 13 times that of Jupiter would be called a brown dwarf (a failed star), though whether such an object would consist entirely of gas, or had a rocky/icy core as is probable for most giant planets, is not presently observable. Any larger than about 75 Jupiter masses and we would just call it a star. The exact definition of what a planet is (especially, the 13 Jupiter mass boundary) is still disputed.

Of the bonafide planets that have been detected and confirmed, the catalogue at exoplanets.org lists Kepler-435b as the one with the largest measured radius (although its radius error bar overlaps with that of other planets). The quoted radius is $1.99 pm 0.18$ times that of Jupiter.

Most giant planets have very similar radii for masses between about 0.5 and 10 times that of Jupiter. The reason for this is that they are largely supported by electron degeneracy pressure. The diversity in the radii (between about 0.7 and 2 times that of Jupiter) of such planets is not yet fully understood.

The plot below shows mass vs radius for "planets". The smaller (probably rocky/icy) planets do show a trend of increasing radius with mass (the solid line is where a theoretical relationship for rocky/icy planets has been used to estimate the mass from the radius). The gas giants above about 0.5 Jupiter masses show no trend and a small scatter.

cell biology - Significance of basal lamina for outer layers of epithelium

The basal lamina, a specialised type of extracellular matrix (ECM) that differs between cell types, acts as a base for stratified epithelia cells to layer on top of and therefore has a supportive role as well as providing a base for attachment (for the layer of cells immediately on top of it) 1. Layers of epithelial cells on top of further strata of similar cells with no differing tissue composition underneath would have no support and cell types would be able to mix freely within tissues. The different cell types within tissues are kept seperate by this ECM to maintain areas for specific functions. The importance of this can be seen through studying epithelial cancers, where the epithelial cell layer breaks through the entire basement membrance and invades the surrounding tissue. This can cause massive disruption of tissue function.

Yes, it seems that the layers of cells are attached by various junctional complexes, the desmosomes (maculae adherentes) being the most abundant for stratified epithelia.

Referenes

1 http://www.histology.leeds.ac.uk/tissue_types/connective/con_basal_lam.php

Is Jupiter just a super earth with hydrogen atmosphere?

Does it originate from super earth with excessive collection of
hydrogen?

Basically no, and this is why:

Source from the water article linked below.

Jupiter likely formed outside the frost-line, so it never had a rocky or even a molten magma surface. It formed with much too high a percentage of ices, which, under pressure, became hot gas and perhaps, at one time, a very deep ocean around the whole planet.

As Rob Jeffries said, the formation of Jupiter isn't well understood, so there's some uncertainty there, but I don't see how a planet that forms with as much water, CO2, NH3 and other gases as Jupiter did could ever form what we would consider a solid surface. It's large size and heat of formation probably kept it highly gaseous during it's entire formation. Perhaps it was once a water world of sorts, but my guess is it always resembled a gas giant once it was recognizable as a planet.

The Earth is about 0.02% water (some websites say 0.05% but whichever percentage, the Earth is still almost entirely Rocky Mantle & Metallic core. I don't think a planet that forms outside the frost line would ever become Earth like or a super-earth.

Inside the frost line, A super earth could in time become a gas giant, but outside, I don't think so.

That said, Jupiter might well have an earth like ratio of elements at it's core, but I don't believe that means it was ever a super-earth.

temperature - How cold is interstellar space?

You can stick a thermometer in space, and if it is a super-high-tech one, it might show you the temperature of the gas. But since the interstellar medium (ISM) is so dilute, a normal thermometer will radiate energy away faster than it can absorb it, and thus it won't reach thermal equilibrium with the gas.
It won't cool all the way to 0 K, though, since the cosmic microwave background radiation won't allow it to cool further than 2.7 K, as described by David Hammen.

The term "temperature" is a measure of the average energy of the particles of a gas (other definitions exist e.g. for a radiation field). If the gas is very thin, but particles move at the same average speed as, say, at the surface of Earth, the gas is still said to have a temperature of, say, 27º C, or $ 300,mathrm{K}$.

The ISM consists of several different phases, each with their own physical characteristics and origins. Arguably, the three most important phases are (see e.g. Ferrière 2001):

Molecular clouds

Stars are born in dense molecular clouds with temperatures of just 10-20 K. In order for a star to form, the gas must be able to collapse gravitationally, which is impossible if the atoms move too fast.

The warm neutral medium

The molecular clouds themselves form from gas that is neutral, i.e. not ionized. Since most of the gas is hydrogen, this means that it has a temperature of roughly $10^4,mathrm{K}$, above which hydrogen tends to get ionized.

The hot ionized medium

Gas that accretes onto the galaxy in its early phases tend to have much larger temperature, of roughly $10^6,mathrm{K}$. Additionally, the radiative feedback from the hot stars (O and B), and the kinetic and radiative energy injected by supernova explosions ionize and heat gas bubbles that expand. This gas comprises the hot ionized medium.

Cooling

The reason that the ISM is so sharply divided into phases, as opposed to just being a smooth mixture of particles of all sorts of energies, is that gas cools by various physical processes that have a rather temperature-specific efficiency.
"Cooling" means converting the kinetic energy of particles into radiation that is able to leave the system.

Hot gas

Very hot gas is fully collisionally ionized and thus cools mainly through free electron emitting Bremsstrahlung. This mechanism becomes inefficent below $sim10^6,mathrm{K}$.

Warm gas

Between $10^4,mathrm{K}$ and $10^6,mathrm{K}$, recombinations (i.e. electrons being caught by ions) and collisonal excitation and subsequent de-excitation lead to emission, removing energy from the system.
Here the metallicity$^dagger$ of the gas is important, since various elements have different energy levels.

Cool gas

At lower temperatures, the gas is almost fully neutral, so recombinations cease to have any influence. Collisions between hydrogen atom become too weak to excite the atoms, but if molecules or metals are present, it is possible through fine/hyperfine lines, and rotational/vibrational lines, respectively.

The total cooling is the sum of all these processes, but will be dominated by one or a few processes at a given temperature. The figures below from Sutherland & Dopita (1993) shows the main cooling processes (left) and the main cooling elements (right), as a function of temperature:

The thick line show the total cooling rate. The figure below, from the same paper, shows the total cooling rate for different metallicities. The metallicity is a logarithmic scale, so [Fe/H] = 0 means Solar metallicity, and [Fe/H] = –1 means 0.1 times Solar metallicity, while "nil" is zero metallicity.

Since these processes don't cover equally the full temperature range, the gas will tend to reach certain "plateaus" in temperatures, i.e. it will tend to occupy certain specific temperatures. When gas cools, it contracts. From the ideal gas law, we know that the pressure $P$ is proportional to the product of the density $n$ and the temperature $T$. If there's pressure equilibrium in the ISM (which there isn't always, but in many cases is a good assumption), then $nT$ is constant, and thus if a parcel of hot ionized gas cools from $10^7,mathrm{K}$ to $10^4,mathrm{K}$, it must contract to increases its density by a factor $10^3$. Thus, cooler clouds are smaller and denser, and in this way the ISM is divided up in its various phases.

So, to conclude, interstellar space is not as cold as you may think. However, being extremely dilute, it is difficult to transfer heat, so if you leave your spaceship, you will radiate away energy faster than you can absorb it from the gas.

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$^dagger$_{In astronomy, the term "metal", refers to all elements that are not hydrogen or helium, and "metallicity" is the fraction of gas that consists of metals.}