Assessment statements
•
State that the most frequently occurring chemical elements in living things are carbon, hydrogen, oxygen and nitrogen.•
State that a variety of other elements are needed by living organisms, including sulfur, calcium, phosphorus, iron and sodium.•
State one role for each of the above elements.•
Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.•
Outline the thermal, cohesive and solvent properties of water.•
Explain the relationship between the properties of water and its uses in living organisms as a coolant, medium for metabolic reactions and transport medium.3 The chemistry of life
3 THE CHEMISTRY OF LIFE 41 Because of this arrangement, water is said to be a polar molecule. Polar
molecules are those that have an unevenly distributed electrical charge so that there is a positive region and a negative region. Sugars and amino acids are also polar molecules.
A weak bond can form between the negative charge of one water molecule and the positive charge of another, as shown in Figure 3.2. This type of bond, known as a hydrogen bond, is responsible for many of the properties of water.
Hydrogen bonds between water molecules hold them together in a network, resulting in a phenomenon known as cohesion. Cohesive forces give water many of its biologically important properties. For example, they enable water to be drawn up inside the xylem of a plant stem in a continuous column. Strong pulling forces, produced as water evaporates from the leaves at the top of tall trees, draw water and dissolved minerals up great distances to the tips of branches high above the ground. Cohesion is also responsible for surface tension, which enables some small organisms to
‘walk on water’, and contributes to the thermal properties of water too.
Water has unusual thermal properties. A large amount of energy is needed to break the many weak hydrogen bonds between the water molecules. This gives water a high specifi c heat capacity – it can absorb or give off a great deal of heat energy without its temperature changing very much. A stable temperature is important to living things because the range of temperatures in which biological reactions can occur is quite narrow. The thermal properties of water allow it to keep an organism’s temperature fairly constant. Within the body, water can act as a temperature regulator – for example, blood carries heat from warmer parts of the body, such as the liver, to cooler parts such as the feet.
When liquid water evaporates and becomes vapour, many hydrogen bonds between the molecules must be broken, so evaporation requires a lot of energy. As a result, water is a liquid at most temperatures found on Earth, and it has a high boiling point. When it evaporates – for example, when an animal sweats – it carries a great deal of heat with it and thus acts as a coolant for the body.
Element Example of role in prokaryotes
Example of role in plants Example of role in animals sulfur (S) a component of two amino
acids
a component of two amino acids
a component of two amino acids, needed to make some antibodies
calcium (Ca) co-factor in some enzyme reactions
co-factor in some enzyme reactions
important constituent of bones, needed for muscle contraction
phosphorus (P) a component of ATP and DNA a component of ATP and DNA a component of ATP and DNA iron (Fe) a component of cytochrome
pigments
a component of cytochrome pigments
a component of hemoglobin and cytochrome pigments
sodium (Na) important in membranes, changes solute concentration and affects osmosis
important in membranes, changes solute concentration and affects osmosis
important in membranes, changes solute concentration and affects osmosis; also important in transmission of nerve impulses Table 3.1 Roles of inorganic elements in living things.
covalent bonds
hydrogen bonds δ–
δ–
δ–
δ–
δ+
δ+ δ+
δ+
δ+
δ+
δ+
δ+
Figure 3.2 Hydrogen bonding in water.
Water is sometimes known as a universal solvent. Its polarity makes it an excellent solvent for other polar molecules. Most inorganic ions, such as sodium, potassium and chloride ions, dissolve well as their positive or negative charges are attracted to the charges of water molecules (Figure 3.3). Polar organic molecules, such as amino acids and sugars, are also soluble in water. Water is the medium in which most biochemical reactions take place since almost all the substances involved dissolve well in it. Protein synthesis and most of the reactions of photosynthesis and respiration take place in an aqueous (water) solution.
The solvent properties of water also make it an excellent medium for transporting substances around the bodies of all organisms. In plants, the xylem carries dissolved minerals from the roots to the leaves, while the phloem transports soluble sugars up and down the plant. Many animals have blood as their transport medium. Blood is predominantly water, and the blood plasma carries dissolved sugars, amino acids and carbon dioxide as well as many other solutes.
The properties of water are summarised in Table 3.2.
Assessment statements
•
Distinguish between ‘organic’ and ‘inorganic’ compounds.•
Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure.•
List three examples each of monosaccharides, disaccharides and polysaccharides.•
State one function of glucose, lactose and glycogen in animals and of fructose, sucrose and cellulose in plants.•
Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides.•
State three functions of lipids.•
Compare the use of carbohydrates and lipids in energy storage.Figure 3.3 The positive and negative charges of water molecules attract ions with negative or positive charges so that they dissolve.
Property Reason Consequence
cohesion Hydrogen bonds hold water molecules together. Water can travel in continuous columns, for example in the stems of plants, and act as a transport medium.
solvent The polar molecules of water can interact with other polar molecules.
Ions dissolve easily. Large molecules with polar side groups, such as carbohydrates and proteins, can also dissolve. So water acts as an excellent transport medium and as a medium for metabolic reactions.
thermal Water has a high heat capacity. Large amounts of energy are needed to break hydrogen bonds and change its temperature.
The temperature of organisms tends to change slowly.
Fluids such as blood can transport heat round their bodies.
Water has a high boiling point compared with other solvents because hydrogen bonds need large amounts of energy to break them.
Water is liquid at most temperatures at which life exists, so is a useful medium for metabolic reactions.
Water evaporates as hydrogen bonds are broken and heat from water is used.
Sweating and transpiration enable animals and plants to lose heat. Water acts as a coolant.
Table 3.2 Summary of the properties of water.
3.2 Carbohydrates, lipids and proteins
+ –
positive ion
negative ion water molecules
3 THE CHEMISTRY OF LIFE 43