Cambridge University Press 978-0-521-17690-3 - Biology Unit 1 for CAPE® Examinations Myda Ramesar, Mary Jones and Geoff Jones Excerpt More information Chapter 1 Aspects of biochemistry By the end of this chapter you should be able to: a discuss how the structure and properties of g outline the molecular structure of water relate to the role that water plays as a haemoglobin, as an example of a globular medium of life; protein, and of collagen, as an example of a fibrous protein, ensuring that the relationships b explain the relationship between the structure between their structures and functions are and function of glucose; clearly established; c explain the relationship between the structure h know how to carry out tests for reducing and and function of sucrose; non-reducing sugars (including quantitative d discuss how the molecular structure of starch, use of the Benedict’s test), for starch and lipids glycogen and cellulose relate to their functions and (the biuret test) for proteins; in living organisms; i know how to investigate and compare e describe the generalised structure of an amino quantitatively reducing sugars and starch; acid and the formation and breakage of a j describe the molecular structure of a peptide bond; triglyceride and its role as a source of energy; f explain the meaning of the terms primary, k describe the structure of phospholipids and secondary, tertiary and quaternary structure their role in membrane structure and function. of proteins, and describe the types of bonding (hydrogen, ionic, disulphide) and hydrophobic interactions which hold the molecule in shape; The human body is made of many different types an explosion of knowledge and understanding, of molecule. Most of your body is water, which has and today a major industry has been built on small molecules made of two atoms of hydrogen the many applications to which we can put this combined with one atom of oxygen, formula H2O. knowledge. Biotechnology makes use of molecules The other main types of molecule in organisms and reactions in living organisms, and research are proteins, carbohydrates, fats and nucleic acids. continues to find new information about how These molecules make up your structure, and molecules behave, and new uses for this technology they also undergo chemical reactions – known as in fields including medicine, agriculture, mining metabolic reactions – that make things happen in and food production. and around your cells. In this chapter, we will look at the structures of the Up until the mid 1950s, we did not know very main types of molecule found in our bodies – and much about the structures of these molecules or in those of every other living organism (Table 1.1). how their structures might relate to the ways in We will also see how these structures relate to the which they behave. Since then, there has been functions of the molecules. 1 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-17690-3 - Biology Unit 1 for CAPE® Examinations Myda Ramesar, Mary Jones and Geoff Jones Excerpt More information Chapter 1: Aspects of biochemistry Molecule Percentage of total mass fact that water is often in liquid form allows it to act as the solvent in which metabolic reactions water 60 (the chemical reactions that take place in living protein 19 organisms) can happen. Water provides a means of fat 15 transporting molecules and ions from one place to another, because many of them can dissolve in it. carbohydrate 4 other 2 The structure of a water molecule Table 1.1 Human body composition. Figure 1.1a shows the structure of a water molecule. It is made up of two hydrogen atoms Water covalently bonded to one oxygen atom. The bonds Water is by far the most abundant molecule in our are single covalent bonds, in which the single bodies. It is also one of the smallest and one of the electron of each hydrogen atom is shared along simplest. Yet water is an amazing substance, with a with one of the six outer shell electrons of the collection of properties that is shared by no other. oxygen, leaving four electrons which are organised It is diffi cult to imagine how life of any kind could into two non-bonding pairs. The bonds are very exist without water. The search for other life in the strong, and it is very diffi cult to split the hydrogen universe begins by searching for other planets that and oxygen atoms apart. may have liquid water on them. A covalent bond is an electron-sharing bond, Water is a major component of all cells, often and in this case the sharing is not equal. The making up between 70% and 90% of the cell’s oxygen atom gets slightly more than its fair share, mass. Your body is about 60% water. Water is also and this gives it a very small negative charge. the environment of many living organisms, and life This is written E− (delta minus). The hydrogen must have fi rst evolved in water. atoms have a very small positive charge, E+ (delta Water has very simple molecules, yet has some plus). So, although a water molecule has a neutral surprising properties. Other substances made electrical charge overall, diff erent parts of it do of such small molecules tend to be gases at the have small positive and negative charges. This temperatures found on most of the Earth, whereas unequal distribution of charge in the molecule is water may be found as a solid, liquid or gas. The called a dipole. a Water molecule b Hydrogen bonding E− In a water molecule, the two hydrogen O There is a weak electrical atoms are found to one side of the E+ E+ HH attraction between a hydrogen in oxygen atom. one molecule and the oxygen in E− another. This is a hydrogen bond. O O covalent bond E+ E+ HH HH 104.5° small negative charge hydrogen bond E− E+ E+ small positive charges The oxygen atom pulls the bonding electrons towards it, which makes the oxygen slightly negatively charged. In the liquid state, water molecules undergo The hydrogen atoms have small positive charges. This hydrogen bonding with surrounding molecules. unequal distribution of charge is called a dipole. The bonds break and reform as the molecules move around. Figure 1.1 a The structure of a water molecule; b hydrogen bonding. 2 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-17690-3 - Biology Unit 1 for CAPE® Examinations Myda Ramesar, Mary Jones and Geoff Jones Excerpt More information Chapter 1: Aspects of biochemistry These tiny charges mean that water molecules As the surface water gets colder it gets denser and are attracted to each other – the positively charged sinks to the bottom, displacing warmer water hydrogen atoms in one molecule are attracted which rises to the surface and cools. However, to the negatively charged oxygen atoms in other when water gets colder than 4°C, it becomes less molecules. The attraction is called a hydrogen bond dense than the warmer water below it and so it (Figure 1.1b). Every water molecule is hydrogen stays on the surface where it forms ice. The layer bonded to its four nearest neighbours. of ice insulates the water below it from further Hydrogen bonds are weak, long-distance temperature changes. This water remains at 4°C bonds. As we will see later in this chapter, they are and stays liquid, providing an environment in very common and very important in biological which living organisms can continue to survive. molecules. Changes in density of water as its temperature In the next few pages, we will look in detail changes are the main cause of ocean currents and at some of the special properties of water, and upwellings, which help to maintain the circulation how these make water so important to all living of nutrients in seas and oceans. organisms. Ice, liquid water and water vapour O O In a solid substance, such as ice, the molecules H H H H have relatively little kinetic energy. They vibrate O continuously, but remain in fixed positions. In H H liquid water, the molecules have more kinetic O O energy, moving around past each other, forming H H H H fleeting hydrogen bonds with each other. In water O vapour, the molecules are far apart, scarcely H H hydrogen bond interacting with each other at all. O O In solid water – ice – the hydrogen bonds hold H H H H the water molecules in a rigid lattice formation. As in all solids, the molecules vibrate, but they do not move around. As the temperature of liquid water decreases, the water molecules have less and less Figure 1.2 The structure of ice. Water molecules kinetic energy and they slow down. This allows are held in a regular, fixed arrangement or lattice each molecule to form the maximum number of (shown in only two dimensions here). hydrogen bonds (four) with other water molecules. When this happens, the water molecules spread out Water, heat and temperature to form these bonds. This produces a rigid lattice which holds the water molecules further apart than Specific heat capacity in liquid water (Figure 1.2). Ice is therefore less If you heat water, the temperature of the water dense than liquid water, and so it floats. Hydrogen rises. Temperature relates to the amount of kinetic bonding is responsible for this unique property of energy that the water molecules have. As heat water. energy is added to the water, a lot of the energy The fact that ice floats on liquid water is is used to break the hydrogen bonds between the important for aquatic living organisms.