All living organisms require organic matter as food to build up their structure and to provide chemical energy to fuel their activities. Whilst photosynthesis is the crucial process, it should be remembered that a multitude of other processes are occurring all over the plant. Proteins are being produced, many of which are enzymes necessary to speed up chemical reactions in the leaf, the stem, the root, and later in the flower and fruit. The complex carbohydrate, cellulose, is being built up as cell walls of almost every cell. Nucleo-proteins are being provided in meristematic areas to enable cell division. These are three examples of many, to show that growth involves much more than just photosynthesis and respiration.
All the complex organic compounds, based on carbon, must be produced from the simple raw materials, water and carbon dioxide. Many organisms are unable to manufacture their own food, and must therefore feed on already manufactured organic matter such as plants or animals. Since large animals predate on smaller animals, which themselves feed on plants, all organisms depend directly or indirectly on photosynthesis occurring in the plant as the basis of a food web or chain (see p53).
Photosynthesis is the process in the chloroplasts of the leaf and stem by which green plants manufacture food in the form of high energy carbohydrates such as sugars and starch, using light as energy.
A summary of the process of photosynthesis is given in Table 8.1 as a word formula and as a chemical equation. This apparently simple equation represents, in reality, two different stages in the production of glucose. The first, the 'light reaction' occurs during daylight, and splits water into hydrogen and oxygen. The second, the 'dark reaction' occurring at night, takes the hydrogen and joins it to carbon dioxide to make glucose.
Table 8.1 Two ways to represent the chemistry involved in photosynthesis a. Written in a conventional way, the process can be expressed in the following way:
carbon dioxide plus water plus light gives rise to glucose plus oxygen (when in the presence of chlorophyll)
b. Written in the form of a chemical equation, which represents molecular happenings at the sub-microscopic level, the above sentence becomes:
6 C2O molecules + 6 H2O molecules plus light give rise to 1 C6H12O6 molecule + 6 O2 molecules (when in the presence of chlorophyll)
Most plant species follow a 'C-3' process of photosynthesis where the intermediate chemical compound contains three carbon atoms (C-3) before producing the six-carbon glucose molecule (C-6). Many C-3 plants are not able to increase their rate of photosynthesis under very high light levels.
In contrast, a 'C-4' process is seen in many tropical families, including the maize family, where plants which use an intermediate compound containing four carbon atoms (C4) are able to continue to respond to very high levels of light, thus increasing their productivity.
A third process, called 'CAM' (Crassulacean Acid Metabolism) was first discovered in the stonecrop family. Here, the intermediate chemical is a different four-carbon compound, malic acid. This third process has more recently been found in several other succulent plant families (including cacti), all of which need to survive conditions of drought. Such plants need to keep their stomata closed during the heat of the day, but this prevents the entry of carbon dioxide. During the night, carbon dioxide is absorbed and stored as malic acid, ready for conversion to glucose the next day. CAM plants do not normally grow very fast because they are not able to store large quantities of this malic acid, and thus their potential for glucose production is limited.
All living organisms, from viruses to whales, have the element carbon at the centre of their chemistry. The study of this element's chemical activity is called organic chemistry. Originally, it was thought that all organic compounds came from living organisms, but modern chemistry has brought synthetic urea fertilizer and DDT, which are organic.
Unusually in the range of chemical elements, carbon (like silicon) has a combining power, or valency, of four (see p387 for basic chemistry). This means that each carbon atom can react with four other atoms, whether they are atoms of other elements such as hydrogen, or with more carbon atoms. Since the four chemical bonds from the carbon atom point in diametrically opposite directions, it can be seen that molecules containing carbon are three dimensional, a feature which is very important in the chemistry of living things.
Carbon normally forms covalent (non-ionic) bonds in its molecules. Here, the bond shares the electrons, and so there is no chemical charge associated with the molecule (see also ionic bonds). Two of the simplest molecules to contain carbon are carbon dioxide and methane (see Figure 8.2).
Carbon dioxide, or CO2, is a constituent of the air we breathe and the sole source of carbon for almost all plants. In this compound, the carbon attaches to two oxygen atoms, each with a valency of two.
Carbon dioxide molecule
• - carbon electrons o - hydrogen electrons
Carbon dioxide molecule
Figure 8.2 Carbon dioxide and methane
Figure 8.3 Sugar molecule
Methane, or CH4, is often referred to as 'marsh gas' and is the major constituent of the North Sea gas supply. In this compound, the carbon atom is bonded with four hydrogen atoms, each of which has a valency of one. Most fuels, such as petrol, are chemically related to methane, but with more carbon atoms in the molecule. This family of chemicals (containing progressively more carbons in the molecule are methane, butane, propane and octane) is collectively called the hydrocarbons. The molecules in petrol mainly contain eight carbon atoms, hence the term 'octane'.
Slightly more complicated is the glucose molecule (C6H12O6). Here there is a circular molecule and Figure 8.3 indicates its three-dimensional structure. Glucose is the molecule produced by photosynthesis. It is the starting point for the synthesis of all the many molecules used by the plant, i.e. starch, cellulose (see Figure 8.4), proteins, pigments, auxins, DNA, etc. These molecules may contain chains with hundreds of carbon atoms joined in slightly different ways, but the basic chemistry is the same. All the valency (see p388) requirements of carbon, oxygen and hydrogen are still met.
With glucose simplified to
a) amylose starch chain:
100s of glucose b) amylopectin starch branched chain:
b) amylopectin starch branched chain:
J 100s of glucose
c) cellulose fibre
J 100s of glucose c) cellulose fibre
J 100s of glucose
Figure 8.4 Starch and cellulose molecules ch2oh
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