Tutor profile: My Q.
How do elements obtain a valence shell filled with eight electrons?
Elements can obtain a valence shell filled with eight electrons by forming bonds. There are two major types of bonds: covalent and ionic. In covalent bond, participating elements have electronegativity values of approximately the same magnitude. Therefore, electrons are shared between the two participating elements to form a complete octet. (EX: CH4 - methane - each C-H bond is a covalent). In ionic bonds, the participating elements have electronegativity values that are very different. The element with a higher electronegativity value completely steals the electron(s) from the second element with lower electronegativity value. As a result of this transfer, the element with an extra electron now carries a negative charge while the second element possesses as positive charge. The opposite charges are attracted to each other. (EX: NaCl Na gives up one electron to Cl, forming Na+ and Cl-. These two ions have complete octet and a full valence shell. Na+ is attracted to the negative charge on Cl-, creating salt NaCl).
How can gel electrophoresis be used to separate DNA strands?
In DNA electrophoresis, the gel is made by heating and cooling agarose dissolved in various buffers. The size of the pores in the gel matrix can be controlled by the type of buffer and the concentration of agarose. The largest DNA fragments have to weave through more pores, thereby requiring a longer time to travel through the gel matrix. On the hand, smaller fragments weave through less pores, therefore traveling faster through the matrix. In other words, if the gel electrophoresis is operated for 25 minutes. During that time span, larger DNA fragments will cover less distance while smaller DNA fragments will travel a greater distance.
How does a tree deliver water absorbed from its roots to the leaves?
Water is drawn from the roots and moves up the stem through a specialized tissue called xylem. There are three mechanisms by which water moves throughout the plant. 1) Osmosis - the difference in concentrations of nutrients between the soil and the roots drives water into the plant. The center of plant called stele or vascular cylinder has a higher concentration of minerals and nutrients than that of outside of the root; therefore, the water is driven into the roots. 2) Capillary action (adhesion) - Adhesion occurs when water molecules "stick" to the xylem tissue. Coupled with cohesion, adhesion provides the force to pull water along the inside of the xylem. 3) cohesion - Cohesion is the attractive force between water molecules. It is the "glue" that holds water molecules together. When water evaporates from the leaves into the atmosphere, a process called transpiration, the leaving water molecules tug neighboring water molecules to fill in the gap that will form. As more water molecules gets "pulled out" of the leaves by evaporation, water moves up the xylem in the plant.
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