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Mike O.
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Basic Chemistry
TutorMe
Question:

Why is solid water less dense than liquid water?

Mike O.
Answer:

Water is a polar liquid that is able to hydrogen bond with itself. Hydrogen bonding occurs when a hydrogen atom is bound to a highly electronegative atom (nitrogen, oxygen, or fluorine). This creates a strong negative and positive pole. For example, in water, the oxygen is very electronegative, so it pulls the molecule's electrons really strongly and gives the oxygen a partial negative charge, while the hydrogens have a partial positive. These charges can then attract the opposite partial charge on other water molecules. Usually, solids are more dense than liquids since the molecules are so much more tightly packed in a solid. However, water is the opposite due to hydrogen bonding. When water molecules get tightly packed, hydrogen bonding makes the water form a lattice structure, which leaves big gaps in the structure of ice, making it actually less dense than liquid water, where molecules can simply slide past each other.

Biology
TutorMe
Question:

What is the difference between complete dominance, codominance, and incomplete dominance.

Mike O.
Answer:

In genetics, genes can be inherited by multiple patterns. You will see the differences between these patterns in the heterozygous genotypes, since both alleles are present. In complete dominance (like in Mendel's pea plant experiment), one allele is dominant, and one is recessive. In the heterozygous genotype (ex: Aa), the dominant phenotype will be exhibited. Codominance occurs when neither allele is dominant over the other. In the heterozygous phenotype, both alleles will be exhibited at the same time (ex: if A= black, B= white, AB genotype would give black and white striped phenotype). Incomplete dominance is similar to codominance, but in the heterozygous phenotype a mixture of the two alleles will be expressed (ex: if R=red and r= white, then the Rr genotype would give a pink phenotype, a mix of red and white).

Anatomy
TutorMe
Question:

What is the difference between a positive and negative feedback loop? Give examples of each.

Mike O.
Answer:

These two types of feedback are opposites. A positive feedback loop works to amplify a change in the body's normal functioning. So, something changes, and the body works to make that change happen more and more strongly. Negative feedback loops are more common in the body, and work to lessen or reverse the effect of a change in the body's functioning. So, something changes, and the body wants to stop that change and return things to normal. An example of positive feedback occurs during labor. The presence of the baby's head in the cervix causes a nerve impulse telling the pituitary to release oxytocin. This oxytocin causes uterine contractions, moving the baby down the birth canal. The baby's head continuing to press against the cervix causes the release of more oxytocin, which then increases uterine contractions more and more until the baby is born, when no more oxytocin is released. So, the change to the body in this case is the uterine contractions pushing the baby out, and the body uses oxytocin to strengthen that change so the baby can be born. One negative feedback loop involves blood sugar. If blood sugar changes from the norm, then the body works to lessen that change and return the levels to normal. If blood sugar levels get too high, the pancreas releases insulin to reduce it, while if they get too low, glucagon is released to make it higher.

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