Understanding Biology by Mason, Johnson, Losos, and Singer
Answers to End-of-Chapter Synthesize Questions
Chapter 1
Concept 1.1
1. 1 x 1021 x (0.10) x (0.0001) x (1 x 10-6) = 1,000,000,000 planets support intelligent life
There are many possible explanations for not “hearing from” other intelligent life forms. Some possibilities include: (1) Different forms of intelligent life have different levels of awareness of other possible life forms (so they do not know if we exist either); (2) The reasons we have not made contact with them also explain why they have not made contact with us (cost of space exploration, distance to other life-sustaining planets, etc); (3) Other intelligent life forms constitute a planet full of animals, or dinosaurs, or insects, far, far away. These life forms are not likely try to make “contact”; and (4) The other plants in our universe that support life are still in the earlier stages of developing life—similar to stages on Earth before life as we know it. Just because a planet can support life does not mean that it does, or will.
Concept 1.2
1. The organism level is the lowest level of hierarchical organization that carries out all of the activities we associate with life. An organism might be a single cell (bacterial cells are organized, grow and reproduce, possess genetic information, acquire and use energy, and maintain homeostasis); but this is not characteristic of all individual cells.
Concept 1.3
1. In a classic experiment, Pasteur tested the hypothesis that cells arise from other cells. In this experiment cell growth was looked for following sterilization (killing of all living cells) of broth in a swan-neck flask (the opening faces down) or in a flask with the neck broken off (the opening faces up).
a) The broth, flask, and sterilization step were held the same between the experiments.
b) The shape of the flask influences the experiment because any cells present in the air can enter the flask with the neck broken neck (opening facing up), but are trapped in the flask with the swan-neck (opening facing down).
c) If cells can arise spontaneously, then cell growth will occur in both flasks If cells can only arise from preexisting cells (cells in the air), cells would only grown in the flask with the broken neck since breaking the neck exposes the broth to a source of preexisting cells.
2. If possible, I would create exact models of the equipment in question and test its operation at different temperatures. I would have a control group operating at STP, and experimental groups at several increments of lower temperature (step-wise to a temperature equal to or less than the air temperature at the time of takeoff). I would record the exact same data for each temperature in order to conclude whether temperature has an influence on the proper operation of the equipment.
Concept 1.4
1. Probably not. The plants and animals Darwin saw on the Galapagos Islands were interesting to Darwin because they did not resemble the plants and animals of islands with similar climates. In this way, they provided evidence that plants and animals were not created independently and then placed on the Galapagos Islands, but rather arrived from the South American mainland. The plants and animals on the Cape Verde Islands would be more likely to resemble those on mainland Africa.
Concept 1.5
1. For something to be considered living, it would demonstrate organization, possibly including a cellular structure. The organism would acquire and use materials and energy to maintain homeostasis, respond to its environment, and to grow and reproduce. These latter properties, and evidence of genetic material and evolution, might be difficult to determine if the evidence of life from other planets is fossil evidence.
Chapter 2
Concept 2.1
1. 6, 480 years (The sample would contain 8 mg in 1620 years, 4 mg in 3240 years, 2 mg 4860 years, and 1 mg in 6480 years) .
Concept 2.2
1. This is likely so because so much of the body is composed of water, and most of the mass of that water is attributed to oxygen (16 of 18 g/mol).
Concept 2.3
1. Stable molecules are built from stable covalent bonds. Covalent bonds are stable because they have no net charge, satisfy the octet rule, and result in no unpaired electrons. Ionic bonds are not directional, and thus are not truly between two individual atoms. This means that ionic bonds do not form distinct molecules, and are thus not the basis of stable molecules because they are not the basis of molecules at all.
2. Silicon theoretically has the same bonding chemistry as carbon. Its atomic number is 14; it has four unpaired valence electrons; its atomic mass is less than 21 (like the other elements involved in the formation of biological compounds). However, silicon when silicon forms the structures that carbon does (rings, branched chains, etc.), they are unstable and often reactive. If it was able to form diverse, stable compounds (like carbon), it might be a good candidate for an alternative molecule upon which to base life.
Concept 2.4
1. The key properties of water that would support its role as evidence of life include: 1) its ability to act as a solvent, which can allow for the transport of dissolved molecules that can form other complex molecules; 2) its resistance to changes in temperature, which can serve to protect molecules or organisms within it from temperature extremes; and 3) its ability to form a less dense solid than its liquid form, which can help to maintain a liquid environment beneath a frozen environment.
Concept 2.5
1. When liquids like colas and champagne are consumed, the acids dissolved in them are neutralized by buffers (like bicarbonate) in the body that take up the excess hydrogen to prevent any harm being caused by the acidity.
Chapter 3
Concept 3.1
1. This is an open question in science, with quite a bit of research devoted to answering it. One might suppose that at some point the evolutionary history of life, there might have been some selective pressure for one or the other chiral , which has left us with “right-handed” sugars and “left-handed” amino acids.
Concept 3.2
1. Enzymes are reusable, though they are also specific with respect to the substrates they interact with. Starch and cellulose are both formed from glucose subunits. The molecules that result are different, though: starches are a-glucose chains, while cellulose is composed of b-glucose chains. The formation of each of these macromolecules requires dehydration reactions, but they proceed differently or they would assemble glucose units into identical macromolecules. It is possible that several enzymes are involved in the synthesis of starch and several in the synthesis of cellulose. It is possible that some of these enzymes are the same, perhaps in the early stages. However, on the basis that the resulting molecules are different, it is likely that not all of the enzymes used to produce one polymer are used to produce the other.
Concept 3.3
1. The final structure of a protein is determined by its primary structure (the chemical nature of its side groups), but several configurations are still possible. Chaperonin proteins have been shown to ensure that proper folding occurs such that issues like clumping and diseases that might result from improper folding.
Concept 3.4
1. Something like TATATATATATAT; Sequences involving only adenine and thymine because only two hydrogen bonds are formed between these bases, whereas three are formed between guanine and cytosine bases
Concept 3.5
1. A high percentage of unsaturated fatty acids, which resist forming solids because their fatty acid chains bend and cannot become closely associated like saturated fatty acids; Long chains (16-20 carbons), because this is what is generally seen in membrane phospholipids.
Chapter 4
Concept 4.1
1. The flat version of the single-celled organism would have a greater surface area over which to bring materials in and expel wastes out. In the flattened version, the distance from the inside of the cell to the outside would also be decreased, increasing the rate at which this transport could take place.
Concept 4.2
1. Examine the cell wall: if it contains peptidoglycan, it is a bacterium.
Concept 4.3
1. The nuclear envelope would not break down / rebuild during cell division. It is likely that cell division would not proceed normally.
Concept 4.4
1. The modified molecules, or “final products” that leave the Golgi travel in vesicles, which are specialized for transport based upon the final destination of the molecule. They are likely placed into their respective vesicles based upon bearing some molecular marking. So, chemical signals are in control of what goes where.
Concept 4.5
1. Though a chloroplast has its own DNA, external membranes, and is capable of synthesizing sugars, it cannot survive on its own. Some of the genes that code for its components are actually found in the nucleus of the cell it is a part of, so it could not reproduce on its own. The chloroplast also relies on energy produced by the cell’s mitochondria.
Concept 4.6
1. It is possible that mutations amplified the 9 + 2 structure and it then differentiated to function in several structures.
Concept 4.7
1. Integrins and fibronectins connect the extracellular matrix to the cytoskeleton of the animal cell, which provides a protective layer over the cell surface. In plant cells, the rigid cell wall provides this protection.
Concept 4.8
1. Cells membranes are selectively permeable. So, the proteins in the membranes of these cells are specialized for what they can take in and let out. This is likely based on chemical signals, shape, or other factors. The cells in the digestive tract are likely held together by tight junctions that do not allow for leakage between them, so nothing moves in the wrong direction.
Chapter 5
Concept 5.1
1. The polarity of proteins makes this model problematic. If the proteins simply covered the phospholipid bilayer, they would be exposed to a hydrophilic environment on all sides, which is an unstable configuration.
Concept 5.2
1. Since the cell contains the instructions it needs for assembling its own membranes and organelles within its DNA, processes like chemical signaling will determines how lipids are distributed when they are assembled into the membranes of each of these structures. This could be indicated based on molecular markers that originate when the lipid is synthesized in the cell and packaged for transport to its final destination.
Concept 5.3
1. Communication between the internal and external environments of the cell is accomplished via chemical signaling (signal transduction pathways).
Concept 5.4
1. The bilayer is not freely permeable to most polar substances, however water moves by osmosis and through specialized protein channels called aquaporins.
Concept 5.5
1. Perhaps because these pumps are already expending so much of the cell’s energy it was energetically efficient to couple the transport of other molecules to the transport of protons, sodium, and potassium.
Concept 5.6
1. Plant cells use this process to move things from within the cell membrane to the cell wall, for instance, the materials needed to build the cell wall itself.
Chapter 6
Concept 6.1
1. Energy is stored in the bonds of the sugar molecules and is harvested by animals when these molecules are broken down.
Concept 6.2
1. No. Laws of thermodynamics refer to closed systems. Despite the notion that the natural tendency of the universe is an increase in entropy, this increase results from spontaneous reactions (through which energy is lost). However, not every reaction that takes place in the universe is spontaneous. Some reactions actually create order, using energy to reorganize (for instance, those that build macromolecules from smaller subunits) the world. This reorganization does not result in an increase in entropy.
Concept 6.3
1. Yes it is likely that the production of light from the firefly, which involves chemical reaction, requires some form of energy in the form of ATP, which was likely synthesized by the organism’s cells, within the mitochondria, in the same manner in which other organisms’ cells produce energy (using glucose and oxygen). To test this hypothesis, one might attempt to quantify the products of cellular respiration (carbon dioxide, water, and heat) in the firefly or deny the firefly an oxygenated environment to see if it is still able to produce as much light when its cells are forced to undergo anaerobic respiration.
Concept 6.4
1. 40 ° C seems to be the optimum temperature for the protein. Above this temperature, the protein structure is denatured. Since form and function are intertwined, a denatured enzyme does not function, as demonstrated by the drop off in reaction rate above 40° C.
Concept 6.5
1. Phosphofructokinase functions to add a phosphate group to a molecule of fructose-6-phosphate. This enzyme functions early in glycolysis, an energy-yielding biochemical pathway discussed in chapter 7. The enzyme has an active site that binds fructose and ATP. An allosteric inhibitory site also binds ATP when cellular levels of ATP are very high.
a) The reaction rate would be slow because of the low concentration of the substrate (ATP).
b) Above a threshold concentration of ATP, the reaction rate would slow down.