Question

• Opposite charges attract; like charges repel.
• Like dissolves like.
• “Stable” means “low energy” and unreactive, while “unstable” means “high energy” and reactive; and all things spontaneously become more stable over time (unless there is an input of energy), and in becoming more stable they release energy.

Explain how one or more of those principles apply to each of the following:

1. DNA always assumes a double-stranded, helical shape. (2.5 points)
2. Biological membranes are semi-permeable. (2.5 points)

Answer 1

Question: DNA always assumes a double-stranded, helical shape.

Answer: In biology, "double helix" is a term used to describe the structure of DNA. A DNA double helix consists of two spiral chains of deoxyribonucleic acid. The shape is similar to that of a spiral staircase. DNA is a nucleic acid composed of nitrogenous bases (adenine, cytosine, guanine, and thymine), a five-carbon sugar (deoxyribose), and phosphate molecules. The nucleotide bases of DNA represent the stair steps of the staircase, and the deoxyribose and phosphate molecules form the sides of the staircase.

DNA is coiled into chromosomes and tightly packed in the nucleus of our cells. The twisting aspect of DNA is a result of interactions between the molecules that make up DNA and water. The nitrogenous bases that comprise the steps of the twisted staircase are held together by hydrogen bonds. Adenine is bonded with thymine (A-T) and guanine pairs with cytosine (G-C). These nitrogenous bases are hydrophobic, meaning that they lack an affinity for water. Since the cell cytoplasm and cytosol contain water-based liquids, the nitrogenous bases want to avoid contact with cell fluids. The sugar and phosphate molecules that form the sugar-phosphate backbone of the molecule are hydrophilic, which means they are water-loving and have an affinity for water.

The double-helix shape allows for DNA replication and protein synthesis to occur. In these processes, the twisted DNA unwinds and opens to allow a copy of the DNA to be made. In DNA replication, the double helix unwinds and each separated strand is used to synthesize a new strand. As the new strands form, bases are paired together until two double-helix DNA molecules are formed from a single double-helix DNA molecule. DNA replication is required for the processes of mitosis and meiosis to occur.

In protein synthesis, the DNA molecule is transcribed to produce an RNA version of the DNA code known as messenger RNA (mRNA). The messenger RNA molecule is then translated to produce proteins. In order for DNA transcription to take place, the DNA double helix must unwind and allow an enzyme called RNA polymerase to transcribe the DNA. RNA is also a nucleic acid but contains the base uracil instead of thymine. In transcription, guanine pairs with cytosine and adenine pairs with uracil to form the RNA transcript. After transcription, the DNA closes and twists back to its original state.

Question: Biological membranes are semi-permeable.

Answer: Because of the hydrophobic ( water hating ) tails of the phospholipids.

Cell membrane mostly consists of phospholipids which has hydrophobic tails. This tail does not allow polar molecules to enter or exit the cells. It does not allow glucose, proteins, etc to leave the cell where as prevents unwanted polar molecules to enter the cell.

Also cell membrane consists of cholesterols whose tails are also hydrophobic and restricts water loving molecules .

While water and other small molecules can slip through the gaps between the phospholipid molecules, other molecules like ions and large nutrients cannot force their way into or out of the cell. This makes the phospholipid bilayer an excellent semipermeable membrane that allows cells to keep their contents separated from the environment and other cells. The concentration of the solution bound by a semipermeable membrane can be described by its tonicity as compared to the environment or other cells. Because biological membranes are permeable to water but not solutes, water tends to move into cells that are hypertonic to their environment, while water moves out of cells that are hypotonic.

Due to this, the cell membranes are permeable.