1. (a) DNA double helix consists of two involute DNA strands. (i) These strands have hydrogen bond between their base pairs. Generally, hydrogen bond is weak but when we have big number of them

Section A

1. (a) DNA double helix consists of two involute DNA strands. (i) These strands have hydrogen bond between their base pairs. Generally, hydrogen bond is weak but when we have big number of them together they hold strands tightly (Alberts et al. 2014). (ii) Hydrophobic interactions causes rearrangement of nonpolar molecules thereby entropy of water molecules, which cover nonpolar molecule, is increased. In DNA strands carbohydrates cycles are hydrophobic so water molecules make ‘cages’ that surround them. Standing in a straight line decreases tense of this ‘cages’ and frees some water molecule so entropy of system increase and it is more energetically preferred (fig.1). That is why sugars form straight backbone of DNA strand (Lodish et al. 2004). (iii). Van der Waals interaction occurs when electrons from different atoms fluctuate instantly. It happens only if nonpolar atoms are close enough to react (van der Waals radius equals 0.14 nm). Strength of this interaction decreases slightly if with the increase of distance (Lodish et al. 2004). DNA molecule bases contain heterocycles and they are slightly aromatic. It means these cycles share their electrons and form π–π stacking between every cycle in a strand. Separately these bonds are weak but together they are strong (Gagné, McGaughey and Rappé 1998).

(b) DNA double helix will be separated into single strands. It occurs because hydrogen bonds can be fairly easy broken. Heat affects these bonds and destroys them. This process is called denaturation. Thus, prolonged heating can cause destruction of DNA double helix (Alberts et. al. 2014).

References

· Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014). Molecular Biology of Cell.Abingdon: Garland Science.

· Gagné, M., McGaughey, G., Rappé, A. (1998). π-Stacking interactions: alive and well in proteins. The Journal of Biological Chemistr y [Online] 273 (25). Available at: http://www.jbc.org/content/273/25/15458.full.pdf+html [Accessed: 15 October 2015].

· Lodish, H., Berk, A., Kaiser, C., Krieger, M., Bretscher, A., Ploegh, H., Amon, A., et al. (2013). Molecular Cell Biology. New York: Freeman.

2. (a) Chitin can be found in insects’ and some of invertebrate organisms’ tissues, to be more precise, in their outer skeleton (Karp 2010). Chitin can also be found in some fungi’s cell walls. It presents there in the fibres and is produced in the plasma membrane (Webster 2007).

(b) Chitin has similar structure to glycose but has acetyl amino group bonded with second carbon instead of hydroxyl group in glucose (fig.1.1). Thus, chitin is also called N-acetylglucosamine. Regularly, chitin forms microfibrils. Their diameter is approximately 3 nm. There are hydrogen bonds between amine and carbonyl groups so structure of microfibril is stabilised. Chitin microfibrils grow up to 0.5 µm in length. There are 3 types of chitin polymer: α, |β and γ-chitin. α-Chitin forms microfibrils that has groups, consisting of 20 or more anti-parallel oriented chitin molecules. Thus, in the α form molecules are arranged tightly into chitin microfibrils with big number of hydrogen bonds within and between molecular chain. This type of formation leads to certain mechanical properties of the cuticle such as solidity and strength. β and γ forms of chitin are formed of parallel oriented chains and two bonded parallel strands, respectively. Both β and γ form can frequently be observed in cocoons. They are packed less tightly than α form and have reduced number of hydrogen bonds so they are soft and can be found in cocoons or peritrophic matrices (Merzendorfer and Zimoch 2003).

References

· Karp, G. (2010). Cell Biology. Singapore: Wiley

· Merzendorfer, H., Zimoch, L. (2003). Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases. Journal of Experimental Biology [Online] 206 (24). Available at: http://jeb.biologists.org/content/jexbio/206/24/4393.full.pdf [Accessed: 16 October 2015].

· Webster, J., Weber, R. (2007). Introduction to Fungi. Cambridge: UP.

3. Cytosolic globular proteins are represented by tertiary protein structures. Cytosolic proteins often exist in aqueous environment so these proteins are folded thereby that water molecules cannot reach amino acids’ hydrophobic side groups in polypeptide chain (Alberts et al. 2014). It is more energetically beneficial to form structures with hydrophobic part inside proteins molecules so there will not be any interactions between water and hydrophobic groups (Pace et al. 1996). Thus, hydrophilic chains are directed diametrically opposite to hydrophobic parts so these groups easily interact with water molecules from outer environment. Tyrosine is para-hydroxyphenylalanine (fig.1.2). Para-hydroxyphenyl side chain is polar and hydrophilic. But phenylalanine consists no hydroxyl groups so it is nonpolar and hydrophobic (fig. 1.3) (Campbell et al. 2014). Thus, Tyrosine is more likely to be found on the exterior of a cytosolic globular protein.

References

· Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014). Molecular Biology of Cell.Abingdon: Garland Science.

· Campbell, N., Reece, J.B., Urry, L.A, Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B (2014). Biology. A Global Approach. Harlow: Pearson Education.

· Pace, N., Shirley, B., Mcnutt, M., Gajiwala, K. (1996) Forces contributing to the conformational stability of proteins. The FASEB Journal [Online] 10. Available at: http://www.fasebj.org/content/10/1/75.full.pdf+html [Accessed: 19 October 2015].

· Wikipedia Commons. (2008). File:D-Tyrosine.svg. [Online]. Available at: https://commons.wikimedia.org/wiki/File:D-Tyrosine.svg Accessed: 19 October 2015].

· Wikipedia Commons. (2007). File:Phenylalanin - Phenylalanine.svg. [Online]. Available at: https://commons.wikimedia.org/wiki/File:Phenylalanin_-_Phenylalanine.svg [Accessed: 19 October 2015].

4. Foremost, to figure out whether microbes evolved from prokaryotic or eukaryotic organisms differences in structure should be distinguished. First difference is place of DNA molecule within cell. In prokaryotic organisms DNA is located in nucleoid. Nucleoid is certain area of cytoplasm that is not enclоsеd bу а membrаne. However, in eukaryotic organisms organelle called nucleus contains DNA molecule. Thus, relation between microbe from Mars and either eukaryotes or prokaryotes can be identified easily by spotting location of DNA molecule in cell (Campbell et al. 2014). Second significant difference in structure of prokaryotic and eukaryotic organisms is presence of organelle called mitochondrion in eukaryotes. Prokaryotes does not have any mitochondria in their composition so energy synthesis in prokaryotic organisms is less effective than in eukaryotes. Presence (or not) of mitochondrion within Mars’ cells can easily determine whether their ancestors are prokaryotes or eukaryotes (Becker et al. 2012). The third defying characteristic can be presence of only eukaryotes’ organelles such as Golgi apparatus, endoplasmic reticulum, lysosome and vesicles. According to presence of these organelles, ancestor can be explicitly estimated (Alberts et al. 2014).

References

· Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014). Molecular Biology of Cell.Abingdon: Garland Science.

· Becker, W., Hardin, J., Bertoni, G., Kleinsmith, L. (2012) Becker’s World of Cell. San Francisco: Pearson Benjamin Cummings.

· Campbell, N., Reece, J.B., Urry, L.A, Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B (2014). Biology. A Global Approach. Harlow: Pearson Education.

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Section B

Lipids can be found in every cell of human body so it is vital to know their functions and from what they consist. Lipids are heavy organic molecules that are soluble in nonpolar compounds and often are insoluble in polar solvents such as water or acetone, and are located in cells or tissues (Wade 2006). Information from different authoritative sources was collected and analysed in order to make this essay not only truthful, but also scientific. Different groups of lipids and role of phospholipids in cell’s life is described here. Also, interaction between cholesterol and cell membrane is highlighted. This essay uses material from both scientific journals and specific Biology and Organic Chemistry books. Although description of each lipid group will be represented in the essay, biological functions’ mechanisms will not be explained fully. Lipids are essential substances for life located within cellular and subcellular walls. Understanding their structure allows to trace connections between their structure and functions.

To begin with, lipids are divided into two groups, called complex and simple lipids. Complex lipids are hydrolysed effortlessly into simpler substances by adding bases or aqueous acid. Groups of lipids such as waxes and glycerides, belong to complex lipids. On the other hand, simple lipids such as steroids, terpenes and prostgladins cannot be easily hydrolysed by bases or acidic solutions. Despite being called ‘simple’, these lipids often are formed from complicated molecules (Wade 2006). Anyway, structure of lipids from both complex and simple groups will be briefly described.

First group is waxes. Waxes are composed from complex carboxylic acids and fatty alcohols and represented by heavy and insoluble in polar solvents esters (fig. 2.1). These lipids melt at the temperature of 50 ^oC, giving viscous liquids (Wade 2006). Waxes are synthesised within both plants and animals. They are used for protective and coating function (Harborne 1965). The most widely known example of animal wax is beeswax. Honeycombs and simple candles are composed from this substance (Brown 1981).

Second group, called fats, is represented by esters of fatty acids and glycerol molecules. Sometimes they are called glycerides. They composed of glycerol and fatty acid carboxyl chain (fig. 2.2). Fats can be either saturate, required all carbons in chain to be single bounded, or unsaturated (Wades 2006). Unsaturated molecules have geometrical isomers that are called trans that are represented by straight chain and cis isomers. The reason for this is ability of double bound to rotate around its axis so cis isomers can exist. This type of isomerisation affects shape of molecule and slightly alters its melting and vaporization temperatures (Solomons, Fryhle and Snyder 2014). Fats are stored within special cells called adipocyte in cytoplasm. For plants it is essential to produce energy during night when photosynthesis is unavailable. Moreover, storage of fats is important for animal, too. Oxidation of one fat molecule gives twice more energy than one molecule of glucose polymer called glycogen (Alberts et al. 2014). Additionally, fats’ ability to dissolve and transfer some certain vitamins such as vitamins A, D, E, K is essential for life. (McKinley Health Center 2014). Fats can be found in meat, milk, fish, butter, chocolate and other food.

References

· Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P. (2014). Molecular Biology of Cell.Abingdon: Garland Science.

· Brown, R., H. (1981). Beeswax; Burrowbridge, Somerset: Bee Books New & Old.

· Harborne, J.B. (1965). Plant waxes and cutins. Nature [Online] 206 (4981). Available at: http://www.nature.com/nature/journal/v206/n4981/pdf/206247a0.pdf [Accessed: 22 October 2015].

· McKinley Health Center (2014) Handout [Online]. Available at: http://www.mckinley.illinois.edu/handouts/macronutrients.htm. University of Illinois at Urbana-Champaign. [Accessed: 23 October 2015].

· Solomons, T.W.G., Fryhle, C.B., Snyder, S.A. (2014). Organic chemistry. Singapore: Wiley.

· Wades, L.D. (2006). Organic chemistry. Upper Saddle River: Pearson Education.

· Wikipedia Commons. (2012). File: Triacontanyl palmitate.png [Online]. Available at: https://commons.wikimedia.org/wiki/File:Triacontanyl_palmitate.png [Accessed: 22 October 2015].

· Wikipedia Commons. (2006). File:Trimyristin-3D-vdW.png [Online]. Available at: https://commons.wikimedia.org/wiki/File:Trimyristin-3D-vdW.png [Accessed: 23 October 2015].

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