The Abiotic Component of the Ecosystem comprises all of the non-living substances and forces which are eternal to the Biotic Component of the ecosystem that are either taken up by the biotic component or (even if not taken up) either directly or indirectly affect the growth, reproduction, maintenance and distribution of the biotic component of the ecosystem in one way or another. The sub-units of the abiotic component of the ecosystem include energy and chemical substances. Examples of energy forms that constitute an important part of the abiotic component of the ecosystem include light, heat, and wind while the chemical forms that constitute an important part of the abiotic component of the ecosystem, in so far as the biotic component of the ecosystem is concerned are both inorganic and organic substances. The inorganic chemical substances of importance to the biotic component of the ecosystem are water, oxygen, carbon dioxide and mineral nutrients while the organic chemical substances that are important constituents of the abiotic component of the ecosystem include carbohydrates, fats, proteins, and vitamins.
Some constituents of the abiotic environment affect the biotic component of the ecosystem when taken up while others exert their influence without being taken up by the biota. For example, water, oxygen, carbon dioxide and various mineral nutrients enter the biotic component of the ecosystem through uptake by the biota (particularly plants) and, upon entry, influence the physiological activities of all biota. On the other hand, temperature (as a palpable manifestation of the existence a source of heat energy) and pH, though not taken up by the biotic component of the ecosystem, are, nonetheless, important abiotic ecosystem factors because each of them has an influence on some aspect of the physiological activities of plants and, ultimately, those of other biotic components that either directly or indirectly depend on plants as their sources of energy. Temperature affects the rates at which various physiological activities take place in plant tissues while pH influences the availability of various mineral nutrients for plant uptake through controlling their solubility in water. At a pH range of 6.5 to 7.5 many macro-nutrients are highly soluble in water and available for uptake by plants. On the other hand at pH values below 6.5 macro-nutrients like phosphorus and nitrogen become precipitated out of the soil solution and, therefore, unavailable for plant uptake while some micro-nutrients like iron and manganese become highly soluble and, therefore, readily available for plant uptake to the extent of being toxic to plants.
Those constituents of the abiotic component of the ecosystem that are taken up and incorporated in the tissues of the biotic component of the ecosystem are referred to as resources (e.g. water, oxygen, mineral nutrients and carbon dioxide) while those that, though not taken up still influence the growth, reproduction, maintenance and/or distribution of the biotic component of the ecosystem in one way or another are known as regulators (e.g. temperature and pH).
WATER AS PART OF THE ABIOTIC COMPONENT OF THE ECOSYSTEM
Water is a liquid substance at standard temperature and pressure that is essential for the survival of all living organisms. It is composed of molecules that are each made up of a single large atom of oxygen covalently bonded to two comparatively smaller atoms of hydrogen. Covalent bonding is a type of chemical combination where the atoms forming a molecule share electrons. Each of the two hydrogen atoms in a water molecule shares its single electron with the oxygen atom thereby leaving two pairs of electrons on the oxygen atom unshared.
The Structure of Water
The electrons in the covalently bonded atoms of hydrogen and oxygen are not shared equally. The oxygen atom, with its 8 protons, has a greater attraction for the shared electrons than does the hydrogen atom, with its single proton. Therefore, the shared electrons spend more time around the oxygen part of the water molecule than they do around the hydrogen part. As a result, the oxygen end of the water molecule ends up with a net negative charge (ẟ-) i.e. is more negative than the hydrogen end which has a net positive charge (ẟ+). Due to the shared electrons spending more time around the oxygen atom and the repulsive force between the electrons that this phenomenon creates, the hydrogen and oxygen atoms in a water molecule do not occur in a straight line arrangement. The repulsive force between the two pairs of electrons in the outermost shell of the oxygen atom and between them and the electrons in the O – H bonds pushes the O – H bonds towards each other. Thus the hydrogen atoms end up being bonded to the oxygen atom in such a way as to make the water molecule appear more or less like a wide V-shape with an angle of 105 separating the two hydrogen atoms from each other (see Figure 2 below).
Each of the shared hydrogen atoms is closer to the oxygen atom than it is to the other hydrogen atom. The V-shape of the water molecule coupled with the fact that the shared electrons spend more time around the oxygen atom than around the hydrogen atom makes the hydrogen side of the water molecule to be partially positively charged and the oxygen side to be partially negatively charged. The net partial negative charge on the oxygen end of the water molecule and the net partial positive charge on the water molecule’s hydrogen end mean that each water molecule has polarity. This polarity enables neighboring water molecules to have hydrogen bonds between them (Figure 3) and also to have several other special properties (as will be seen later) that make water particularly important for supporting the lives of living organisms. Had it been that the atoms making up a water molecule were linearly arranged, i.e. in an H – O – H fashion, then the negative and positive charges at the ends of the water molecule would cancel each other out and the water molecule would lose its polarity together with all the other properties that enable water to support life on earth.
Hydrogen Bonds
Hydrogen bonds are forces of attraction which can be intermolecular (between molecules) or intra-molecular (within molecules). They occur only either between hydrogen and oxygen or between hydrogen and nitrogen. As intra-molecular forces, hydrogen bonds hold but do not bond molecules or atoms together. Because they do not bond atoms together, hydrogen bonds are not considered to be true chemical bonds. A hydrogen bond attraction is usually diagrammatically represented as three dots between the regions that are attracted to each other.
Although the weak hydrogen force of attraction may not be responsible for forming molecules, nevertheless, it is important in determining the three-dimensional shape of a molecule. For example, when a very large molecule, such as a protein, has some regions that are slightly positively charged and others that are slightly negatively charged, these areas attract each other and result in the coiling or folding of these threadlike molecules.
Mixtures and Solutions
A mixture is matter that contains two or more substances that are not in set proportions. A solution is a liquid mixture of ions or molecules of two or more substances. For example, salt water can be composed of varying amounts of Sodium Chloride (NaCl) and Water (H2O). If the components of the mixture are distributed equally throughout, the mixture is homogeneous. The process of making a solution is called dissolving. The amounts of the component parts of a solution are identified by the terms solvent and solute. The solvent is the component present in the larger amount. The solute is the component that dissolves in the solvent. Many combinations of solutes and solvents are possible. If one of the components of a solution is a liquid, it is usually identified as the solvent. An aqueous solution is a solution of a solid, liquid, or gas in water.
When sugar dissolves in water, sugar molecules separate from one another. The molecules become uniformly dispersed throughout the molecules of water. In an aqueous salt solution, however, the salt dissociates into sodium and chlorine ions.The sodium and chloride ions dissociating from the salt crystals that are mixed with water attract the polar ends of water molecules in different ways: while the positively charged sodium ions attract the negatively charged oxygen ends of the water molecules the negatively charged chloride ions attract the positively charged hydrogen ends of the water molecules. In this way, each of the sodium and chloride ions gets surrounded by a shield of water molecules that exclude it from contact with other ions. Such a shield of water molecules surrounding an ion is known as a hydration shell.
The relative amounts of solute and solvent in relation to each other are described by the concentration of a solution. In general, a solution with a large amount of solute is “concentrated” and a solution with much less solute is “dilute” although these are somewhat arbitrary terms.
Ecologically Important Chemical and Physical Properties of Water
Chemical Properties of water
As pointed out earlier, a molecule of water has two small atoms of hydrogen covalently bonded to one large atom of oxygen. Covalent bonding is the sharing of electrons between two atoms. Thus, in the water molecule the lone electron of hydrogen circulates between the hydrogen and oxygen atoms. However, the electron is more attracted towards the oxygen atom than the hydrogen atom due to the fact that oxygen is more electronegative than hydrogen. Covalent bonding between oxygen and hydrogen atoms leaves two pairs of oxygen electrons in the outermost shell unbounded. The repulsive forces between these two electron pairs and between them and the electrons in the O – H bonds push the OH bonds towards each other. As a result, instead of the bonding between hydrogen and oxygen atoms being linearly arranged as H – O – H, they assume a V-shape where the two hydrogen atoms are separated by a wide angle of 105.
Due to the wide angle between them and the strength of the bond of attraction between each of them and the oxygen atom, each of the atoms of hydrogen in the water molecule is closer to the oxygen atom than it is to the other hydrogen atom. Furthermore, as pointed out earlier, because the oxygen atom is more electronegative than the hydrogen atom, the electron shared between the two atoms spends more time closer to the oxygen atom than the hydrogen atom. Because of this and because of the V-shape of the water molecule, the oxygen end of the molecule has a partial negative charge (∂-) while the hydrogen end has a partial positive charge (∂+). In other words, the water molecule is polarized. It is this polarity that enables liquid water molecules to attract each other by hydrogen bonding.
Due to hydrogen bonding, water expands when it freezes and floats on the surface of liquid water, boils at a relatively high temperature for its molecular weight, has high adhesive force of attraction between its molecules and the molecules of other objects and has high cohesive force among its molecules which leads to its stability.
Furthermore, due to its polarity, water is a versatile solvent which means that it is able to dissolve far more substances than any other solvent known on Earth. However, it is not a universal solvent because had it been it would have dissolved all its containers or anything it came into contact with including cell membranes, bones and cellulose which clearly would have made life on Earth impossible. Thus, it is due to being bipolar and having hydrogen bonding that water has the unique properties that make it able to support life on Earth. The hydrogen bonding between water molecules influences its physical properties as shown below.
Specific Heat Capacity
The specific heat capacity of a substance is the amount of heat gained or lost to raise or lower the temperature of one gram of that substance by 1oC. In the case of water, the amount of heat gained or lost to raise or lower the temperature of one gram of that substance by 1oC is one calorie. This amount of heat is high when compared with the amounts of heat energy that have to be gained or lost by most other substances for their temperatures to rise or drop by 1oC. In other words, water has a comparatively high specific heat capacity. The only other substances that have higher specific heat capacities than water are ammonia, liquid hydrogen and lithium. Water owes its high specific heat capacity to its hydrogen bonding. That means that a large amount of heat energy has to be gained before the hydrogen bonds between the water molecules are broken to free the molecules to move rapidly. It is when the water molecules are free to move rapidly that the temperature starts rising. On the other hand, a lot of heat energy would have to be lost to allow the agitated rapidly moving water molecules to calm down and move close together to reform the broken hydrogen bonds.
The high specific heat capacity of water implies that it can absorb or release relatively high amounts of heat energy without any large changes in its temperature. This property enables large water bodies like oceans, seas and lakes to prevent wide climatic variations and moderate atmospheric and terrestrial temperatures locally, regionally and worldwide.
Latent Heat
Latent heat is the amount of heat energy that has to be lost or gained by a substance before it changes its state, i.e. before it changes from liquid to solid, solid to liquid, liquid to vapor or vapor to liquid.
The amount energy that a substance has to gain for it to change its state from solid to liquid is known as the latent heat of fusion while the amount of heat energy that it has to gain for it to change its state from liquid to vapour is known as its latent heat of vapourization. The latent heat of fusion of water is 80 calories/g/oC for, to convert one gram of ice at 1oC to one gram of liquid water at 1oC requires a supply of approximately 80 calories of heat energy. This amount of heat supply is very large because if the same amount of heat had been supplied to one gram of liquid water it would have raised its temperature from 0oC to 80oC. The latent heat of fusion of water is very high once again because of hydrogen bonding. It requires a relatively high amount of heat energy to break the rigid hydrogen bonds in the solid ice lattice.
When heat is supplied to liquid water, a temperature is eventually reached at which the surface water molecules become sufficiently agitated to break away from the liquid water surface and become water vapour molecules.
Floating of Ice on the surface of Liquid water
Because of its polarity, each molecule of water becomes coupled with its neighbouring molecules through the attraction between the positively charged hydrogen end of one molecule and the negatively charged oxygen end of a neighbouring water molecule. The simultaneous bonding of an H atom to two different water molecules enables the water molecules to be arranged in a lattice structure (Figure 3).*
The lattice structures form and break at frequent intervals in liquid water because the molecules are mobile. However, in ice each water molecule becomes hydrogen-bonded to four neighbouring water molecules at specific distances from each other thereby forming a rigid lattice with large open spaces within. Water molecules that are so rigidly arranged occupy more space than they would in liquid water in which the hydrogen bonds frequently form and break up. As a consequence water expands as it freezes and occupies more volume thereby becoming less dense than liquid water. Therefore, because of its lower density, frozen water (ice) floats on the surface of denser liquid water.
The phenomenon of ice floating on the surface of liquid water is of ecological significance in the sense that as the ice floats on the surface of the liquid water it insulates the water below from further freezing and enables the existence of living aquatic organisms below the frozen ice. Had it been that the frozen water had a higher density than liquid water then it would sink to the bottom of liquid water as it froze. As a consequence, water would have frozen throughout its body, i.e. from the tops to the bottoms of all water bodies including lakes, rivers, streams, oceans, and seas thereby making the existence of aquatic living organisms impossible during the winter period.
Upon the melting of ice, the hydrogen bonds become less rigid and partially break thereby resulting in the partial collapse of the lattice structure with a concomitant decrease in the volume occupied by the solid water (ice) and increase in the density of water until a point is reached where the water attains its greatest density at 3.98oC. At that temperature, contraction of the water molecules brought about by the partial collapse of the ice lattice structure occasioned by the partial breakup of the hydrogen bonds balances the normal thermal expansion of warming water molecules. As the water heats up more hydrogen bonds break up converting the solid water to a liquid state, which is a mixture of individual and aggregate molecules. Further heating of the liquid water results in a complete breakdown of the hydrogen bonds, separating the aggregates of water molecules into individual water molecules which enter the gaseous state.
Seawater is a solution of various salts dominated by sodium chloride and is defined as containing 34.5 grams of salt per 1000 grams of water (34.5o/oo) although it can vary in salinity. It behaves somewhat differently from pure water in the sense that it has no definitive freezing point. Seawater starts freezing at a temperature of about -2oC. As pure water freezes out of seawater, the remaining unfrozen water becomes higher in salinity, its density increases and its freezing point becomes lower. Ultimately, as cold conditions intensify, a solid block of ice crystals mixed with salt crystals forms and floats on the surface of the ocean/sea.
Water and Life—the Most Common Compound of Living Things
Water has a high surface tension. Because water molecules are polar hydrogen bonds form between water molecules, and they stick more to one another than to air molecules. Thus, water tends to pull together to form a smooth surface where water meets air. This layer can be surprisingly strong. For instance, some insects can walk on the surface of a water pond. The tendency of water molecules to stick to each other and to some other materials explains why water can make things wet. It also explains why water climbs through narrow tubes, called capillary tubes against the pull of gravity. This capillary action also helps water move through soil, up the xylem vessels in the plant stems, and through the capillaries (tiny blood vessels) in animals.
Water has unusually high latent heats of vaporization and fusion.
Because polar water molecules stick to one another, an unusually large amount of heat energy is required to separate them. Water resists changes in temperature. It takes 540 calories of heat energy to convert 1 gram of liquid water to its gaseous state, water vapor. This means that large bodies of water, such as lakes, rivers, seas and oceans must absorb enormous amounts of heat energy before they will evaporate and leave the life within them high and dry. Usually such high amounts of heat energy are never absorbed by such water bodies. That a lot of heat energy has to be absorbed for water to change from the liquid state to the gaseous state also means that humans can get rid of excess body heat by sweating because, when the water evaporates, it removes heat from the skin. On the other hand, a high latent heat of fusion means that this large amount of heat energy must be removed from liquid water before it changes from a liquid to a solid state, ice. Therefore, water can remain liquid and a suitable home for countless organisms long after the atmospheric temperature has reached the freezing point, 0°C (32°F). All Wed Groups - 21/11/2018
Water has unusual density characteristics. Water is most dense at 3.98°C. As heat energy is lost from a body of water and its temperature falls below 3.98°C, its density decreases and this less dense, colder water is left on top. As the surface water reaches the freezing point and changes from its liquid to its solid phase, the molecules form new arrangements, which resemble a honeycomb. The spaces between the water molecules make the solid phase, ice, less dense than the water beneath and the ice floats. It is the surface water that freezes to a solid, covering the denser, liquid water and the living things in it.
Water’s specific gravity is also an important property.
Water has a density of 1 gram/cubic centimetre at 3.98°C. Anything with a higher density sinks in water, and anything with a lower density floats. Specific gravity is the ratio of the density of a substance to the density of water. Therefore, the specific gravity of water is 1.00. Any substance with a specific gravity less than 1.00 floats to the surface of water. If you mix water and gasoline, the gasoline (specific gravity of 0.75) floats to the top. People also vary in the specific gravity of their bodies. Some persons find it very easy to float in water, whereas others find it impossible. This is directly related to each person’s specific gravity, which is a measure of the person’s ratio of body fat to muscle and bone.
Water is considered the universal solvent, because most other chemicals can be dissolved in water. This means that wherever water goes—through the ground, in the air, or through an organism—it carries chemicals. Water in its purest form is even capable of acting as a solvent for oils.
Water comprises 50–60% of the bodies of most living things. This is important, because the chemical reactions of all living things occur in water.
Some constituents of the abiotic environment affect the biotic component of the ecosystem when taken up while others exert their influence without being taken up by the biota. For example, water, oxygen, carbon dioxide and various mineral nutrients enter the biotic component of the ecosystem through uptake by the biota (particularly plants) and, upon entry, influence the physiological activities of all biota. On the other hand, temperature (as a palpable manifestation of the existence a source of heat energy) and pH, though not taken up by the biotic component of the ecosystem, are, nonetheless, important abiotic ecosystem factors because each of them has an influence on some aspect of the physiological activities of plants and, ultimately, those of other biotic components that either directly or indirectly depend on plants as their sources of energy. Temperature affects the rates at which various physiological activities take place in plant tissues while pH influences the availability of various mineral nutrients for plant uptake through controlling their solubility in water. At a pH range of 6.5 to 7.5 many macro-nutrients are highly soluble in water and available for uptake by plants. On the other hand at pH values below 6.5 macro-nutrients like phosphorus and nitrogen become precipitated out of the soil solution and, therefore, unavailable for plant uptake while some micro-nutrients like iron and manganese become highly soluble and, therefore, readily available for plant uptake to the extent of being toxic to plants.
Those constituents of the abiotic component of the ecosystem that are taken up and incorporated in the tissues of the biotic component of the ecosystem are referred to as resources (e.g. water, oxygen, mineral nutrients and carbon dioxide) while those that, though not taken up still influence the growth, reproduction, maintenance and/or distribution of the biotic component of the ecosystem in one way or another are known as regulators (e.g. temperature and pH).
WATER AS PART OF THE ABIOTIC COMPONENT OF THE ECOSYSTEM
Water is a liquid substance at standard temperature and pressure that is essential for the survival of all living organisms. It is composed of molecules that are each made up of a single large atom of oxygen covalently bonded to two comparatively smaller atoms of hydrogen. Covalent bonding is a type of chemical combination where the atoms forming a molecule share electrons. Each of the two hydrogen atoms in a water molecule shares its single electron with the oxygen atom thereby leaving two pairs of electrons on the oxygen atom unshared.
The Structure of Water
The electrons in the covalently bonded atoms of hydrogen and oxygen are not shared equally. The oxygen atom, with its 8 protons, has a greater attraction for the shared electrons than does the hydrogen atom, with its single proton. Therefore, the shared electrons spend more time around the oxygen part of the water molecule than they do around the hydrogen part. As a result, the oxygen end of the water molecule ends up with a net negative charge (ẟ-) i.e. is more negative than the hydrogen end which has a net positive charge (ẟ+). Due to the shared electrons spending more time around the oxygen atom and the repulsive force between the electrons that this phenomenon creates, the hydrogen and oxygen atoms in a water molecule do not occur in a straight line arrangement. The repulsive force between the two pairs of electrons in the outermost shell of the oxygen atom and between them and the electrons in the O – H bonds pushes the O – H bonds towards each other. Thus the hydrogen atoms end up being bonded to the oxygen atom in such a way as to make the water molecule appear more or less like a wide V-shape with an angle of 105 separating the two hydrogen atoms from each other (see Figure 2 below).
Each of the shared hydrogen atoms is closer to the oxygen atom than it is to the other hydrogen atom. The V-shape of the water molecule coupled with the fact that the shared electrons spend more time around the oxygen atom than around the hydrogen atom makes the hydrogen side of the water molecule to be partially positively charged and the oxygen side to be partially negatively charged. The net partial negative charge on the oxygen end of the water molecule and the net partial positive charge on the water molecule’s hydrogen end mean that each water molecule has polarity. This polarity enables neighboring water molecules to have hydrogen bonds between them (Figure 3) and also to have several other special properties (as will be seen later) that make water particularly important for supporting the lives of living organisms. Had it been that the atoms making up a water molecule were linearly arranged, i.e. in an H – O – H fashion, then the negative and positive charges at the ends of the water molecule would cancel each other out and the water molecule would lose its polarity together with all the other properties that enable water to support life on earth.
Hydrogen Bonds
Hydrogen bonds are forces of attraction which can be intermolecular (between molecules) or intra-molecular (within molecules). They occur only either between hydrogen and oxygen or between hydrogen and nitrogen. As intra-molecular forces, hydrogen bonds hold but do not bond molecules or atoms together. Because they do not bond atoms together, hydrogen bonds are not considered to be true chemical bonds. A hydrogen bond attraction is usually diagrammatically represented as three dots between the regions that are attracted to each other.
Although the weak hydrogen force of attraction may not be responsible for forming molecules, nevertheless, it is important in determining the three-dimensional shape of a molecule. For example, when a very large molecule, such as a protein, has some regions that are slightly positively charged and others that are slightly negatively charged, these areas attract each other and result in the coiling or folding of these threadlike molecules.
Mixtures and Solutions
A mixture is matter that contains two or more substances that are not in set proportions. A solution is a liquid mixture of ions or molecules of two or more substances. For example, salt water can be composed of varying amounts of Sodium Chloride (NaCl) and Water (H2O). If the components of the mixture are distributed equally throughout, the mixture is homogeneous. The process of making a solution is called dissolving. The amounts of the component parts of a solution are identified by the terms solvent and solute. The solvent is the component present in the larger amount. The solute is the component that dissolves in the solvent. Many combinations of solutes and solvents are possible. If one of the components of a solution is a liquid, it is usually identified as the solvent. An aqueous solution is a solution of a solid, liquid, or gas in water.
When sugar dissolves in water, sugar molecules separate from one another. The molecules become uniformly dispersed throughout the molecules of water. In an aqueous salt solution, however, the salt dissociates into sodium and chlorine ions.The sodium and chloride ions dissociating from the salt crystals that are mixed with water attract the polar ends of water molecules in different ways: while the positively charged sodium ions attract the negatively charged oxygen ends of the water molecules the negatively charged chloride ions attract the positively charged hydrogen ends of the water molecules. In this way, each of the sodium and chloride ions gets surrounded by a shield of water molecules that exclude it from contact with other ions. Such a shield of water molecules surrounding an ion is known as a hydration shell.
The relative amounts of solute and solvent in relation to each other are described by the concentration of a solution. In general, a solution with a large amount of solute is “concentrated” and a solution with much less solute is “dilute” although these are somewhat arbitrary terms.
Ecologically Important Chemical and Physical Properties of Water
Chemical Properties of water
As pointed out earlier, a molecule of water has two small atoms of hydrogen covalently bonded to one large atom of oxygen. Covalent bonding is the sharing of electrons between two atoms. Thus, in the water molecule the lone electron of hydrogen circulates between the hydrogen and oxygen atoms. However, the electron is more attracted towards the oxygen atom than the hydrogen atom due to the fact that oxygen is more electronegative than hydrogen. Covalent bonding between oxygen and hydrogen atoms leaves two pairs of oxygen electrons in the outermost shell unbounded. The repulsive forces between these two electron pairs and between them and the electrons in the O – H bonds push the OH bonds towards each other. As a result, instead of the bonding between hydrogen and oxygen atoms being linearly arranged as H – O – H, they assume a V-shape where the two hydrogen atoms are separated by a wide angle of 105.
Due to the wide angle between them and the strength of the bond of attraction between each of them and the oxygen atom, each of the atoms of hydrogen in the water molecule is closer to the oxygen atom than it is to the other hydrogen atom. Furthermore, as pointed out earlier, because the oxygen atom is more electronegative than the hydrogen atom, the electron shared between the two atoms spends more time closer to the oxygen atom than the hydrogen atom. Because of this and because of the V-shape of the water molecule, the oxygen end of the molecule has a partial negative charge (∂-) while the hydrogen end has a partial positive charge (∂+). In other words, the water molecule is polarized. It is this polarity that enables liquid water molecules to attract each other by hydrogen bonding.
Due to hydrogen bonding, water expands when it freezes and floats on the surface of liquid water, boils at a relatively high temperature for its molecular weight, has high adhesive force of attraction between its molecules and the molecules of other objects and has high cohesive force among its molecules which leads to its stability.
Furthermore, due to its polarity, water is a versatile solvent which means that it is able to dissolve far more substances than any other solvent known on Earth. However, it is not a universal solvent because had it been it would have dissolved all its containers or anything it came into contact with including cell membranes, bones and cellulose which clearly would have made life on Earth impossible. Thus, it is due to being bipolar and having hydrogen bonding that water has the unique properties that make it able to support life on Earth. The hydrogen bonding between water molecules influences its physical properties as shown below.
Specific Heat Capacity
The specific heat capacity of a substance is the amount of heat gained or lost to raise or lower the temperature of one gram of that substance by 1oC. In the case of water, the amount of heat gained or lost to raise or lower the temperature of one gram of that substance by 1oC is one calorie. This amount of heat is high when compared with the amounts of heat energy that have to be gained or lost by most other substances for their temperatures to rise or drop by 1oC. In other words, water has a comparatively high specific heat capacity. The only other substances that have higher specific heat capacities than water are ammonia, liquid hydrogen and lithium. Water owes its high specific heat capacity to its hydrogen bonding. That means that a large amount of heat energy has to be gained before the hydrogen bonds between the water molecules are broken to free the molecules to move rapidly. It is when the water molecules are free to move rapidly that the temperature starts rising. On the other hand, a lot of heat energy would have to be lost to allow the agitated rapidly moving water molecules to calm down and move close together to reform the broken hydrogen bonds.
The high specific heat capacity of water implies that it can absorb or release relatively high amounts of heat energy without any large changes in its temperature. This property enables large water bodies like oceans, seas and lakes to prevent wide climatic variations and moderate atmospheric and terrestrial temperatures locally, regionally and worldwide.
Latent Heat
Latent heat is the amount of heat energy that has to be lost or gained by a substance before it changes its state, i.e. before it changes from liquid to solid, solid to liquid, liquid to vapor or vapor to liquid.
The amount energy that a substance has to gain for it to change its state from solid to liquid is known as the latent heat of fusion while the amount of heat energy that it has to gain for it to change its state from liquid to vapour is known as its latent heat of vapourization. The latent heat of fusion of water is 80 calories/g/oC for, to convert one gram of ice at 1oC to one gram of liquid water at 1oC requires a supply of approximately 80 calories of heat energy. This amount of heat supply is very large because if the same amount of heat had been supplied to one gram of liquid water it would have raised its temperature from 0oC to 80oC. The latent heat of fusion of water is very high once again because of hydrogen bonding. It requires a relatively high amount of heat energy to break the rigid hydrogen bonds in the solid ice lattice.
When heat is supplied to liquid water, a temperature is eventually reached at which the surface water molecules become sufficiently agitated to break away from the liquid water surface and become water vapour molecules.
Floating of Ice on the surface of Liquid water
Because of its polarity, each molecule of water becomes coupled with its neighbouring molecules through the attraction between the positively charged hydrogen end of one molecule and the negatively charged oxygen end of a neighbouring water molecule. The simultaneous bonding of an H atom to two different water molecules enables the water molecules to be arranged in a lattice structure (Figure 3).*
The lattice structures form and break at frequent intervals in liquid water because the molecules are mobile. However, in ice each water molecule becomes hydrogen-bonded to four neighbouring water molecules at specific distances from each other thereby forming a rigid lattice with large open spaces within. Water molecules that are so rigidly arranged occupy more space than they would in liquid water in which the hydrogen bonds frequently form and break up. As a consequence water expands as it freezes and occupies more volume thereby becoming less dense than liquid water. Therefore, because of its lower density, frozen water (ice) floats on the surface of denser liquid water.
The phenomenon of ice floating on the surface of liquid water is of ecological significance in the sense that as the ice floats on the surface of the liquid water it insulates the water below from further freezing and enables the existence of living aquatic organisms below the frozen ice. Had it been that the frozen water had a higher density than liquid water then it would sink to the bottom of liquid water as it froze. As a consequence, water would have frozen throughout its body, i.e. from the tops to the bottoms of all water bodies including lakes, rivers, streams, oceans, and seas thereby making the existence of aquatic living organisms impossible during the winter period.
Upon the melting of ice, the hydrogen bonds become less rigid and partially break thereby resulting in the partial collapse of the lattice structure with a concomitant decrease in the volume occupied by the solid water (ice) and increase in the density of water until a point is reached where the water attains its greatest density at 3.98oC. At that temperature, contraction of the water molecules brought about by the partial collapse of the ice lattice structure occasioned by the partial breakup of the hydrogen bonds balances the normal thermal expansion of warming water molecules. As the water heats up more hydrogen bonds break up converting the solid water to a liquid state, which is a mixture of individual and aggregate molecules. Further heating of the liquid water results in a complete breakdown of the hydrogen bonds, separating the aggregates of water molecules into individual water molecules which enter the gaseous state.
Seawater is a solution of various salts dominated by sodium chloride and is defined as containing 34.5 grams of salt per 1000 grams of water (34.5o/oo) although it can vary in salinity. It behaves somewhat differently from pure water in the sense that it has no definitive freezing point. Seawater starts freezing at a temperature of about -2oC. As pure water freezes out of seawater, the remaining unfrozen water becomes higher in salinity, its density increases and its freezing point becomes lower. Ultimately, as cold conditions intensify, a solid block of ice crystals mixed with salt crystals forms and floats on the surface of the ocean/sea.
Water and Life—the Most Common Compound of Living Things
Water has a high surface tension. Because water molecules are polar hydrogen bonds form between water molecules, and they stick more to one another than to air molecules. Thus, water tends to pull together to form a smooth surface where water meets air. This layer can be surprisingly strong. For instance, some insects can walk on the surface of a water pond. The tendency of water molecules to stick to each other and to some other materials explains why water can make things wet. It also explains why water climbs through narrow tubes, called capillary tubes against the pull of gravity. This capillary action also helps water move through soil, up the xylem vessels in the plant stems, and through the capillaries (tiny blood vessels) in animals.
Water has unusually high latent heats of vaporization and fusion.
Because polar water molecules stick to one another, an unusually large amount of heat energy is required to separate them. Water resists changes in temperature. It takes 540 calories of heat energy to convert 1 gram of liquid water to its gaseous state, water vapor. This means that large bodies of water, such as lakes, rivers, seas and oceans must absorb enormous amounts of heat energy before they will evaporate and leave the life within them high and dry. Usually such high amounts of heat energy are never absorbed by such water bodies. That a lot of heat energy has to be absorbed for water to change from the liquid state to the gaseous state also means that humans can get rid of excess body heat by sweating because, when the water evaporates, it removes heat from the skin. On the other hand, a high latent heat of fusion means that this large amount of heat energy must be removed from liquid water before it changes from a liquid to a solid state, ice. Therefore, water can remain liquid and a suitable home for countless organisms long after the atmospheric temperature has reached the freezing point, 0°C (32°F). All Wed Groups - 21/11/2018
Water has unusual density characteristics. Water is most dense at 3.98°C. As heat energy is lost from a body of water and its temperature falls below 3.98°C, its density decreases and this less dense, colder water is left on top. As the surface water reaches the freezing point and changes from its liquid to its solid phase, the molecules form new arrangements, which resemble a honeycomb. The spaces between the water molecules make the solid phase, ice, less dense than the water beneath and the ice floats. It is the surface water that freezes to a solid, covering the denser, liquid water and the living things in it.
Water’s specific gravity is also an important property.
Water has a density of 1 gram/cubic centimetre at 3.98°C. Anything with a higher density sinks in water, and anything with a lower density floats. Specific gravity is the ratio of the density of a substance to the density of water. Therefore, the specific gravity of water is 1.00. Any substance with a specific gravity less than 1.00 floats to the surface of water. If you mix water and gasoline, the gasoline (specific gravity of 0.75) floats to the top. People also vary in the specific gravity of their bodies. Some persons find it very easy to float in water, whereas others find it impossible. This is directly related to each person’s specific gravity, which is a measure of the person’s ratio of body fat to muscle and bone.
Water is considered the universal solvent, because most other chemicals can be dissolved in water. This means that wherever water goes—through the ground, in the air, or through an organism—it carries chemicals. Water in its purest form is even capable of acting as a solvent for oils.
Water comprises 50–60% of the bodies of most living things. This is important, because the chemical reactions of all living things occur in water.
Water vapor in the atmosphere is known as humidity, which changes with environmental conditions. The ratio of how much water vapor is in the air to how much water vapor could be in the air at a certain temperature is called relative humidity. Relative humidity is closely associated with your comfort. When the relative humidity and temperature are high, it is difficult to evaporate water from your skin, so it is more difficult to cool yourself and you are uncomfortably warm.
Water’s specific gravity changes with its physical phase.
LIGHT
Visible light is the energy that propels life on earth. Green plants capture some of the visible light and, through the process of photosynthesis convert it into biochemical energy for use in their own metabolic processes and the excess energy from photosynthesis is converted into plant biomass for later use by heterotrophs.
Photosynthetically Active Radiation (PAR)
Not all the light from the sun reaches the Earth. Ozone in the upper atmosphere absorbs light in most of the wavelengths but most especially in the blue, violet and ultraviolet wavelengths. Also light in the blue wavelength is scattered into the sky by molecules of atmospheric gases thereby giving the sky its bluish colour. Furthermore, water molecules in the atmosphere scatter lights of all wavelengths thereby making the atmosphere with moderate amounts of water vapour appear whitish in colour. But the sky with a lot of water vapour appears gray in colour. Dust particles in the atmosphere scatter lights with long wavelengths thereby giving the sky its reddish and yellowish colours particularly in the evenings and early mornings on clear days.
Some of the sunlight scattered by water vapour and dust particles in the atmosphere reaches the Earth as skylight. Also some of the light from the sun filters through the sky and reaches the Earth’s surface. However, from an ecological point of view, the light of most interest is that with a wavelength range of 0.40 to 0.70 microns in the visible range of the solar radiation spectrum which is the photosynthetically active radiation (PAR). About 6 – 12% of the PAR that strikes the surface of a leaf is reflected back into space with green colour being the most reflected and the degree of reflection of incoming light varying with the nature of the leaf surface. Almost all of the photosynthetically active red light that strikes the leaf surface is absorbed by the leaf mesophyll chloroplasts and used in photosynthesis. A little percentage of the incoming light is transmitted through the leaf. However, the percentage of the incoming light transmitted through the leaf depends on the thickness and structure of the leaf. Most leaves transmit 10 – 20% of the light that strikes the leaf surface but some leaves may transmit as much as 40% of the incoming light.
Of the amount of sunlight reaching the top of a vegetation canopy, the percentage that reaches the ground floor of that vegetation varies depending on the type of vegetation under consideration. In an equatorial rainforest only about 0.25 – 2% of light reaching vegetation canopy reaches the forest floor whereas in a temperate hardwood forest the range is only between 1 and 5%. On the other hand, grasslands may allow between 10 – 15% of incoming solar radiation to reach the ground floor. In grasslands most of the incoming light is intercepted by middle and lower leaves.
An indication of the amount of solar radiation intercepted by vegetation foliage is given by the leaf area index (LAI) which is a ratio of the total area of the leaves of the vegetation on a given piece of land to the total ground surface area covered by that vegetation, i.e.
LAI = Total area of the leaves/Area of ground
TEMPERATURE
Temperature is a palpable manifestation of the existence of heat energy in a given place. It is a measure of the degree of kinetic motion of the motion of the substance being felt to find out whether it is warming up or not. Temperature influences the rates of enzyme-controlled physiological reactions in the bodies of living organisms as long as it remains within the range that can be tolerated by the enzymes. Also, through controlling the solubility of mineral nutrients in water, the temperature of a soil or aquatic system in which plants grow is an important regulatory abiotic factor in the sense that it influences the physiological activities of plants and, ultimately, those of other biota that either directly or indirectly depend on plants as their sources of energy. When the temperature of the plant growth medium is around 15 – 30oC many nutrients are highly soluble and therefore available for plant uptake. But, when the temperature is below 15oC the plants die due to chilling or if it is above 30oC the plants immediately die due to excessive water loss.
Ice is also more likely to change from a solid to a liquid (melt) as conditions warm. If the specific gravity of water did not decrease when it freezes, then the ice would likely sink and never thaw. Our life-giving water would be trapped in ocean-sized icebergs. Ice also provides a protective layer for the life under the ice sheet.
LIGHT
Visible light is the energy that propels life on earth. Green plants capture some of the visible light and, through the process of photosynthesis convert it into biochemical energy for use in their own metabolic processes and the excess energy from photosynthesis is converted into plant biomass for later use by heterotrophs.
Photosynthetically Active Radiation (PAR)
Not all the light from the sun reaches the Earth. Ozone in the upper atmosphere absorbs light in most of the wavelengths but most especially in the blue, violet and ultraviolet wavelengths. Also light in the blue wavelength is scattered into the sky by molecules of atmospheric gases thereby giving the sky its bluish colour. Furthermore, water molecules in the atmosphere scatter lights of all wavelengths thereby making the atmosphere with moderate amounts of water vapour appear whitish in colour. But the sky with a lot of water vapour appears gray in colour. Dust particles in the atmosphere scatter lights with long wavelengths thereby giving the sky its reddish and yellowish colours particularly in the evenings and early mornings on clear days.
Some of the sunlight scattered by water vapour and dust particles in the atmosphere reaches the Earth as skylight. Also some of the light from the sun filters through the sky and reaches the Earth’s surface. However, from an ecological point of view, the light of most interest is that with a wavelength range of 0.40 to 0.70 microns in the visible range of the solar radiation spectrum which is the photosynthetically active radiation (PAR). About 6 – 12% of the PAR that strikes the surface of a leaf is reflected back into space with green colour being the most reflected and the degree of reflection of incoming light varying with the nature of the leaf surface. Almost all of the photosynthetically active red light that strikes the leaf surface is absorbed by the leaf mesophyll chloroplasts and used in photosynthesis. A little percentage of the incoming light is transmitted through the leaf. However, the percentage of the incoming light transmitted through the leaf depends on the thickness and structure of the leaf. Most leaves transmit 10 – 20% of the light that strikes the leaf surface but some leaves may transmit as much as 40% of the incoming light.
Of the amount of sunlight reaching the top of a vegetation canopy, the percentage that reaches the ground floor of that vegetation varies depending on the type of vegetation under consideration. In an equatorial rainforest only about 0.25 – 2% of light reaching vegetation canopy reaches the forest floor whereas in a temperate hardwood forest the range is only between 1 and 5%. On the other hand, grasslands may allow between 10 – 15% of incoming solar radiation to reach the ground floor. In grasslands most of the incoming light is intercepted by middle and lower leaves.
An indication of the amount of solar radiation intercepted by vegetation foliage is given by the leaf area index (LAI) which is a ratio of the total area of the leaves of the vegetation on a given piece of land to the total ground surface area covered by that vegetation, i.e.
LAI = Total area of the leaves/Area of ground
TEMPERATURE
Temperature is a palpable manifestation of the existence of heat energy in a given place. It is a measure of the degree of kinetic motion of the motion of the substance being felt to find out whether it is warming up or not. Temperature influences the rates of enzyme-controlled physiological reactions in the bodies of living organisms as long as it remains within the range that can be tolerated by the enzymes. Also, through controlling the solubility of mineral nutrients in water, the temperature of a soil or aquatic system in which plants grow is an important regulatory abiotic factor in the sense that it influences the physiological activities of plants and, ultimately, those of other biota that either directly or indirectly depend on plants as their sources of energy. When the temperature of the plant growth medium is around 15 – 30oC many nutrients are highly soluble and therefore available for plant uptake. But, when the temperature is below 15oC the plants die due to chilling or if it is above 30oC the plants immediately die due to excessive water loss.
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