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What do biochemists do? Provide new ideas and experiments to understand how life works Support our understanding of health and disease Contribute innovative information to the technology revolution Work alongside chemists, physicists, healthcare professionals, policy makers, engineers and many more professionals Biochemistry, study of the chemical substances and processes that occur in plants, animals, and microorganisms and of the changes they undergo during development and life. It deals with the chemistry of life, and as such it draws on the techniques of analytical, organic, and physical chemistry, as well as those of physiologists concerned with the molecular basis of vital processes. All chemical changes within the organism—either the degradation of substances, generally to gain necessary energy, or the buildup of complex molecules necessary for life processes— are collectively termed metabolism. These chemical changes depend on the action of organic catalysts known as enzymes, and enzymes, in turn, depend for their existence on the genetic apparatus of the cell. It is not surprising, therefore, that biochemistry enters into the investigation of chemical changes in disease, drug action, and other aspects of medicine, as well as in nutrition, genetics, and agriculture. reading Atoms 449
We have now seen that different materials have different properties. Some materials are metals and some are non-metals; some are electrical or thermal conductors, while others are not. Depending on the properties of these materials, they can be used in lots of useful applications. But what is it exactly that makes up these materials? In other words, if we were to break down a material into the parts that make it up, what would we find? And how is it that a material’s microscopic structure is able to give it all these different properties? The answer lies in the smallest building block of matter: the atom. It is the type of atoms, and the way in which they are arranged in a material, that affects the properties of that substance. It is not often that substances are found in atomic form. Normally, atoms are bonded to other atoms to form compounds or molecules. It is only in the noble gases (e.g. helium, neon and argon) that atoms are found individually and are not bonded to other atoms. We will look at the reasons for this in a later chapter. The Atom We have now looked at many examples of the types of matter and materials that exist around us, and we have investigated some of the ways that materials are classified. But what is it that makes up these materials? And what makes one material different from another? In order to understand this, we need to take a closer look at the building block of matter, the atom. Atoms are the basis of all the structures and organisms in the universe. The planets, the sun, grass and trees, the air we breathe, and people are all made up of different combinations of atoms. Models of the Atom It is important to realise that a lot of what we know about the structure of atoms has been developed over a long period of time. This is often how scientific knowledge develops, with one person building on the ideas of someone else. We are going to look at how our modern understanding of the atom has evolved over time. The idea of atoms was invented by two Greek philosophers, Democritus and Leucippus in the fifth century BC. The Greek word __oμo_ (atom) means indivisible because they believed that atoms could not be broken into smaller pieces. Nowadays, we know that atoms are made up of a positively charged nucleus in the centre surrounded by negatively charged electrons. However, in the past, before the structure of the atom was properly understood, scientists came up with lots of different models or pictures to describe what atoms look like. How big is an atom? It is difficult sometimes to imagine the size of an atom, or its mass, because we cannot see them, and also because we are not used to working with such small measurements. How heavy is an atom? It is possible to determine the mass of a single atom in kilograms. But to do this, you would need very modern mass spectrometers, and the values you would get would be very clumsy and difficult to use. The mass of a carbon atom, for example, is about 1.99 x 10−26kg, while the mass of an atom of hydrogen is about 1.67 x 450
10−27kg. Looking at these very small numbers makes it difficult to compare how much bigger the mass of one atom is when compared to another. Molecules Definition: Molecule A molecule is a group of two or more atoms that are attracted to each other by relatively strong forces or bonds. Almost everything around us is made up of molecules. Water is made up of molecules, each of which has two hydrogen atoms joined to one oxygen atom. Oxygen is a molecule that is made up of two oxygen atoms that are joined to one another. Even the food that we eat is made up of molecules that contain atoms of elements such as carbon, hydrogen and oxygen that are joined to one another in different ways. All of these are known as small molecules because there are only a few atoms in each molecule. Giant molecules are those where there may be millions of atoms per molecule. Examples of giant molecules are diamonds, which are made up of millions of carbon atoms bonded to each other, and metals, which are made up of millions of metal atoms bonded to each other. Representing molecules The structure of a molecule can be shown in many different ways. Sometimes it is easiest to show what a molecule looks like by using different types of diagrams, but at other times, we may decide to simply represent a molecule using its chemical formula or its written name. Using formulae to show the structure of a molecule A chemical formula is an abbreviated (shortened) way of describing a molecule, or some other chemical substance. In chapter 1, we saw how chemical compounds can be repre- sented using element symbols from the Periodic Table. A chemical formula can also tell us the number of atoms of each element that are in a molecule, and their ratio in that molecule. For example, the chemical formula for a molecule of carbon dioxide is: CO2 The formula above is called the molecular formula of that compound. The formula tells us that in one molecule of carbon dioxide, there is one atom of carbon and two atoms of oxygen. The ratio of carbon atoms to oxygen atoms is 1:2. Definition: Molecular formula A concise way of expressing information about the atoms that make up a particular chemical compound. The molecular formula gives the exact number of each type of atom in the molecule. A molecule of glucose has the molecular formula: C6H12O6 451
In each glucose molecule, there are six carbon atoms, twelve hydrogen atoms and six oxygen atoms. The ratio of carbon: hydrogen: oxygen is 6:12:6. We can simplify this ratio to write 1:2:1, or if we were to use the element symbols, the formula would be written as CH2O. This is called the empirical formula of the molecule. Definition: Empirical formula This is a way of expressing the relative number of each type of atom in a chemical compound.
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any idea of what the molecule actually looks like, in other words its shape. Another useful way of representing molecules is to use diagrams. Another type of formula that can be used to describe a molecule is its structural formula. A structural formula uses a graphical representation to show a molecule’s structure (figure 2.1). Using diagrams to show the structure of a molecule Diagrams of molecules are very useful because they give us an idea of the space that is occupied by the molecule, and they also help us to picture how the atoms are arranged in the molecule. There are two types of diagrams that are commonly used: 
Figure 2.1: Diagram showing (a) the molecular, (b) the empirical and (c) the structural formula of isobutane • Ball and stick models This is a 3-dimensional molecular model that uses ’balls’ to represent atoms and sticks’ to represent the bonds between them. The centres of the atoms (the balls) are connected by straight lines which represent the bonds between them. A simplified example is shown in figure 2.2. 452 
Figure 2.2: A ball and stick model of a water molecule • Space-filling model This is also a 3-dimensional molecular model. The atoms are represented by multi-
coloured spheres. Space-filling models of water and ammonia are shown in figures 2.3 and 2.4. Figures 2.3 and 2.4 are some examples of simple molecules that are represented in different ways. 
Figure 2.3: A space-filling model and structural formula of a water molecule. Each molecule is made up of two hydrogen atoms that are attached to one oxygen atom. This is a simple molecule. жүктеу/скачать 18,66 Mb. Достарыңызбен бөлісу:
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