1. Explain the function and location of the
pectin molecule shown in the diagrams and how the galacturonic acid
influences its properties.
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The exam is divided into 2 sections: Section A, essay answers, and section B, short answers.
Note that the topics covered in this exam may vary from term to term.
The following instructions are given in the preamble to the exam. Please read the questions carefully and try to organize your answer before you start to write. Please try to be concise and limit your remarks to the question asked.
60 points (15 points each). Each of these answers is restricted to about 1/2 page.
Answer 4 of the following 5 questions (1-5):
1. Explain the function and location of the
pectin molecule shown in the diagrams and how the galacturonic acid
influences its properties.
2. Explain how plant toxins may cause photosensitivity in humans.
3. What are the natural functions of cuticles?
4. Explain the nature of the amino acid deficiency in plant storage proteins and why this might be advantageous to the plants.
5. Explain the mechanisms by which oxalates are toxic.
40 points (8 points each). Each of these answers is restricted to about 4 lines.
6. Briefly, explain the meaning (and if appropriate, give an example) of any 3 of the following terms:
6c. Cis and trans fatty acids.
7. Briefly, explain the function of 2 of the following terms:
7c. Auxin (indoleacetic acid).
2. Explain how plant toxins may cause photosensitivity in humans.
There are two types of photosensitizing compounds in plants. Primary (direct) and secondary (indirect or hepatogenic).
The direct (primary) photosentizers (e.g., hypericin in St. John's Wort) are themselves photodynamic and absorb light causing damage of cells. The indirect (secondary) photosensitizers act on the liver to inhibit the metabolism or removal of photodynamic compounds and cause them to accumulate. For example, pyrrolizidine alkaloids inhibit chlorophyll breadkown in the liver leading the accumulation of phylloerythrin which then causes photosensitivity.
An additional question: How could individuals who need to take drugs that are secondary photosensitizers minimize these secondary effects?
Go back to question 2 or go on to question 3.
6d. Heteropolymeris a polymer made of more than one different type of monomer. Pectin is a good example of a heteropolymer, because it is made up of several different kinds of sugars.
Go back to question 6d or go on to question 7.
3. What are the natural functions of cuticles?
Cuticles are waxy, water-repellent coverings over plant surfaces, and this governs their properties. The foremost function of the cuticle is to prevent water loss from the plants, and thereby, the cuticle is a very important adaptation to the terrestrial environment. Cuticles also repel water, which prevents leaching of plant cell solutes, discourages establishment of pathogens on the surface or even collects/funnels water from condensation for use by the plant.
Cuticles also provide a physical barrier to pathogen invation, and they may aid in collecting needed light or reflecting excess light. Sometimes, cuticles form slippery surfaces that guide insects in pollinating flowers or even trap them for nutrition.
Another approach to answering this question could be forming a numerical list of 5-6 functions.
Go back to question 3 or go on to question 4.
5. Explain the mechanisms by which oxalates are toxic.
Oxalates exert two very different types of toxic effects.
1. Chelation. They may chelate divalent and trivalent cations. Both carboxyl groups binding forming a sort of "claw" which is a strong bond. This tight binding makes the ions unavailable.
2. Mechanical damage. They may form pointed crystals that are capable of inflicting mechanical damage to cells.
Go back to question 5 or go on to question 6.
6c. Cis and trans fatty acids have different configurations around the double bond(s) in unsaturated fatty acids.
The trans configuration produces a straight-chain fatty acid, whereas cis makes a bend in the fatty acid.
Go back to question 6c or go on to question 6d.
7b. Alkaloids. The main function of alkaloids is to deter herbivores from consuming plant parts, but there are some other special functions that we have not covered yet.
Go back to question 7b or go on to question 7c.
4. Explain the nature of the amino acid deficiency in plant storage proteins and why this might be advantageous to the plants.
Animals cannot synthesize all of the amino acids that they need, and therefore, they must get them in their food. Interestingly, these amino acids (called essential amino acids) tend to be deficient (but not absent)in plant proteins, particularly the storage proteins (e.g., seeds) that are prominent in animal diets. This is no problem for the plant which interconvert the amino acids to make whatever they need, but herbivores cannot subsist on these proteins alone and this provides some disincentive to the herbivores.
Go back to question 4 or go on to question 5.
7a. Lignin serves to strengthen the cell walls of xylem cells, and being hydrophobic, it also controls where water flows through the cell walls. In some cases, it may inhibit pathogen invasion of cells.
Go back to question 7a or go on to question 7b.
6a. Terpenoids consist of 5 C units (isopentane) linked together, and they are generally lipids (i.e., soluble in non polar solvents). Examples could include turpentine and rosin in pine trees or gibberellin.
Go back to question 6a or go on to question 6b.
1. Explain the function and location of the pectin molecule shown in the diagrams and how the galacturonic acid influences its properties.
Pectins are components of the middle lamella and the matrix phase of the primary cell wall. They function in binding together cells (middle lamella) and microfibrils (primary cell wall). The galacturonic acid is important in making pectins acidic and making them soluble in hot water. The galacturonic acids in pectins also bind divalent cations (e.g., Ca+2) and may decrease their availability.
Go back to question 1 or go on to question 2.
6b. Lectins are proteins that specifically bind to or recognize specific sugars, but they are not antibodies. Abrin and ricin are examples.
Go back to question 6b or go on to question 6c.
7c. Auxin (indoleacetic acid) is a representative plant hormone, and it may control the outgrowth of lateral buds (apical dominance) or cell wall softening among other things.
Go back to question 7c or go back to question 1. Return to the Biology 415 Home Page.