What is the biological meaning of water?

The water (biology lecture)

1 Introduction
2. Importance of water
2.1. Lack of water
3. Properties
3.1. Dipole character
3.2. Hydrate shell formation
3.3. Hydrogen bond
3.4. Density anomaly

1 Introduction

"Water, you have neither taste nor color nor aroma. You cannot be described. You can be tasted without knowing you. It's not that you are needed to live; you yourself are life! You permeate us as refreshment, the delicacy of which none of our senses is able to express. Through you all the powers that we have already lost come back to us. One can die of thirst in front of a spring of water containing magnesium. One can languish at a salt lake. And despite two liters of condensation water, you can perish if they contain certain salts. You don't accept every mixture, you don't tolerate every change. You are an easily offended deity! But you give us an indescribable simple and great happiness."

(Antoine de Saint-Expurèy)
  • Water is essential because of its special chemical and physical properties
  • plays the most important role in all metabolic processes or geological and ecological elementary processes in organisms and in inanimate parts of the geosphere (photosynthesis, climate)
  • About 70% of the earth's surface is covered by water, but only 0.3% is drinking water
  • so far only detected in liquid form on earth
  • Water was the most important material in the history of the earth, from which life once originated
  • is the most important element in human history, a symbol of life force, purification, renewal
2. Importance of water
  • Solvents and means of transport (absorption, release and transport of substances usually take place in a dissolved state)
  • Swelling agent (water enables the swelling state of the protein colloids necessary for the life processes)
  • Reaction medium (most of the cell's biological reactions take place in an aqueous environment)
  • Reaction substance in biochemical processes (in many metabolic reactions water occurs as a starting or end substance (photosynthesis, respiration, digestion))
  • Plants also need water for photosynthesis (CO2 + H2O -> glucose + O2)
  • Reaction partner for dissimilation and photosynthesis
Meaning for the cell:
  • Solvent for the conversion of substances in the cell
  • Means of transport for dissolved substances
  • Reaction partner in metabolic reactions
  • Means for regulating the temperature (high heat capacity)
  • In most living cells, water is the most common substance, so the water content of the entire organism is very high
Water content of various organisms in%

Human 60-70
Jellyfish 98
green leaves of plants 80-90

2.1. Lack of water
  • A person weighing 70 kg carries around 42 kg of water with them
  • can go without food for up to 14 days, but dies of thirst after 36 hours
  • 2 - 3 liters of water daily, some of which the body takes in through solid food
  • The same amount of water that a person consumes every day is also given off again
  • when you sweat, up to 1.5 liters of water per hour can be lost
  • in tropical countries people need up to ten times more water
  • Loss of fluid during thirst can reduce the amount of blood by 25 - 40%, in extreme cases up to 60%
  • The heart no longer pumps thick and viscous blood through veins quickly enough
  • only a quarter of the normal amount of blood is pumped through the heart per minute
  • considerable amounts of carbon dioxide are released -> must be exhaled with increased breathing
  • With fluid losses between 15 and 20% of the body weight, the circulatory system is no longer able to adequately supply the heart muscle
  • Metabolic disorders, brain is permanently damaged
  • death from circulatory failure occurs
  • permanent forms in which life processes run at a greatly reduced speed (e.g. seeds of plants) are poor in water
3. Properties of water and their meaning
  • one of the most interesting elements in nature
  • Properties without which life on earth would not be possible
  • Water (also hydrogen oxide, hydrogenium oxide or dihydrogenium oxide) is a chemical compound made up of oxygen and hydrogen
  • Designation water is used especially for the liquid state of aggregation, solid or frozen -> ice, gaseous -> water vapor
  • Formula: H20
  • Pure water is odorless, tasteless and almost colorless
  • Freezing point: 0 ° C; greatest density: 1g / cm3 at 4 ° C
  • Boiling point: 100 ° C
    • Temperature at which a substance boils, d. H. passes from the liquid to the gaseous state of aggregation
    • high boiling temperature of the water indicates strong intermolecular forces of attraction -> hydrogen bond (five times stronger than the sole force of attraction between dipole molecules)
    • Bonds in the water molecule are strongly polar and the water molecule is a dipole
  • Due to the high specific heat capacity, a lot of energy is needed to heat water
    • on earth "almost" constant temperature
    • When the sun is shining, the water absorbs the heat energy during the day and releases it again at night
    • on the moon, which has no water, temperatures of over 150 ° C and -100 ° C at night
  • Water molecules are dipoles with a positive and negative side
    • therefore an excellent solvent
    • dissolves ions of inorganic compounds and organic compounds
  • water can be detected through white copper sulphate, which turns light blue in color, and through blue cobalt (2) nitrate paper, which turns red through water
3.1. Dipole character
  • if water is divided into smaller and smaller parts, one arrives at the smallest part of which the water is composed -> water molecule
  • chem. Formula for water molecule: H2O
  • 2 atoms of hydrogen and 1 atom of oxygen
  • The water molecule is so small that if you poured a shot glass of water over Europe and each molecule were the size of a grain of sand, the whole of Europe would be covered with a 2 meter high layer of sand
  • O2-Atom forms negative pole
  • H2-Atom forms positive pole
  • due to the greater electronegativity of the O2-Atoms (3,5) opposite both H2-Atoms (2,1) become binding electrons stronger from O2-Atom attracted -> charge shift within the molecule
  • polar atomic bond
  • Particles strive to achieve a stable state -> connection
  • Bond is called polar electron pair bond, molecule dipole
3.2. Hydrate shell formation
  • H2O molecules interact with ions and polar groups of other molecules -> other H2O molecules accumulate until ions are enclosed
  • Due to the hydration shell, the cohesion of the ions is lost (ions can move freely) -> solubility of ions and molecules with polar groups (e.g. -OH, -COO, -NH) is made possible -> hydrophilic
  • Substances in which the apolar groups predominate (e.g. -CH) are not soluble in water -> hydrophobic
  • The hydration shell of ions plays a major role in transport through the membrane
  • As the ion size increases, the charge density and the size of the hydration shell decrease with the charge remaining the same (due to the dipole character)
    • Example: NaCl: due to the dipole character, NaCl can be dissolved, surrounded by hydration shells (remain freely movable and can only be transported in this way -> remain in solution)
3.3. Hydrogen bond
  • due to the different charges within a water molecule, the water molecule is a dipole
  • the same charges repel each other, different charges attract each other -> molecules align themselves in patterns (called clusters)
  • these bonds between the molecules are called hydrogen bonds
  • Hydrogen bonding is responsible for the surface tension and the physical state at room temperature.
  • due to the linkage of H2O molecules via hydrogen bridges, ice forms molecular lattices with cavities -> therefore ice has a lower density than liquid water
  • Ionic bonds, hydrogen bonds and VAN DER WAALS forces are weak bonds -> can be easily broken
  • these bonds are not firm and permanent, exist only for a fraction of a second, then the bonds are broken and reconnected with other water molecules
3.4. Density anomaly
  • Due to the chaining of the water molecules via hydrogen bonds, the resulting structure is larger in the solid state than in the liquid -> crystal lattice with cavities
  • Molecules move so slowly that broken hydrogen bonds cannot be reconnected quickly enough
  • When the ice melts, the ordered state of the grid is destroyed and density increases and has the greatest value at 4 ° C -> density anomaly
  • with further heating, the density decreases again, as molecules move faster -> more freedom of movement (hydrogen bonds dissolve very quickly)
  • biological significance of the anomaly of the water: water freezes over from above, because water of 4 ° C is the heaviest and sinks and thus total freezing of deep waters is prevented -> enables the survival of the organisms living in the water
  • Density anomaly of great importance for living beings in the water, since water has the greatest density at + 4 ° C and sinks, this temperature prevails in deep water
  • CAPILLARY EFFECT: is the force that transports the sap upwards within the plant -> the plant can be supplied with water and food
To ask?


- "Cytology", Schroedel-Verlag
- "Biologie heute", Schroedel-Verlag
- Internet, from the information on offer