Examples of Chemical Elements and Their Roles Used in neurons (nerve cells) for the transmission of nerve impulses Sodium Needed to make hemoglobin and thus to carry oxygen in blood Iron Part of the phosphate groups in ATP and DNA molecules Phosphorus Needed to make the mineral that strengthens bones and teeth Calcium Part of the amine groups of amino acids and therefore proteins Nitrogen Role in Plants or Animals Element
The Significance of Water to Living Organisms
Water is of immense importance to all living organisms
It is used by them in many different ways
These uses can be explained by referring to the properties of water
Water is used a coolant
Refer to thermal properties
Water is used as a transport medium
Refer to cohesion, solvent properties, and thermal properties
Water is used as a habitat
Hydrogen bonding, refer to cohesion, transparency, and thermal properties
Polarity and Hydrogen Bonding in Water
Water molecules consist of 2 hydrogen atoms bonded to an oxygen atom
The hydrogen atoms have a slight positive (+) charge and the oxygen atom has a slight negative (-) charge
So, water molecules have 2 poles (they are dipoles) – a positive hydrogen pole and a negative oxygen pole
This feature of a molecule is called polarity
A bond can form between the positive pole of one water molecule and the negative pole of another
This is called a hydrogen bond
In liquid water, many of these bonds form
Hydrogen bonds provide water with much of its basic properties
Water molecules form Hydrogen bonds slightly positive charge slightly negative charge hydrogen bond between (+) and (-) areas of different water molecules
Thermal Properties: Heat Capacity
Water has a high heat capacity
This means that large amounts of energy are needed to raise its temp
The energy is needed to break one of the hydrogen bonds
This heat energy is given out again when the water is cooled
The temperature of water tends to remain quite stable
This is useful for organisms such as fish that use water as a habitat
Blood, which is mainly composed of water, can carry heat from warmer parts of the body to cooler parts
The Importance of Ice Floating
If ice sank to the bottom of a body of water as it cooled…
Ponds and lakes would freeze up from the bottom
Fish and other organisms would be trapped in the small amount of water that is left
There would be no access to the nutrients are the muddy bottom
Ice atop a body of water, insulates the water below
Life persists under the frozen surface
Thermal Properties: Boiling and freezing points
The boiling point of water is relatively high because to change it from a liquid to a gas all of the hydrogen bonds between the water molecules have to be broken
In natural habitats on Earth, water rarely boils
Living organisms could not survive if the water inside them boiled
Water also freezes at a relatively high temperature but because it becomes less dense as it cools to freezing point, ice forms at the surface first
The ice that forms on the surface of lakes or seas insulates the water underneath, so living organisms can survive there
Thermal Properties: The Cooling Effect of Evaporation
Water can evaporate at temps below boiling point
Hydrogen bonds have to be broken to do this
The heat energy needed to break the bonds is taken from the liquid water, cooling it down
Evaporation of water from plant leaves ( transpiration ) and from human skin ( sweat ) has useful cooling effects
Water molecules stick to each other because of the hydrogen bonds that form between them
Strong pulling forces can be exerted to suck columns of water up to the tops of the tallest trees in their transport systems
These columns of water rarely break
Trees have specialized structures to transport water: xylem and phloem “plumbing”
Water molecules are “dragged” from the roots to the top of the tree by capillary action (adhesion) and cohesion: hydrogen bonds help water molecules hydrogen bond to each other
Adhesion refers to attraction to other substances .
Water is adhesive to any substance with which it can form hydrogen bonds .
Think water climbing up capillary tubes…
At a surface, the cohesion of water molecules can make it difficult for small objects to break through
a measure of the force necessary to stretch or break the surface of a liquid
Some animals such as mosquito larvae use the surface of water as a habitat
Though they are denser than water they remain on the surface and do not sink because of the high surface tension of water caused by cohesion
Many different substances dissolve in water because of its polarity
Inorganic particles such as sodium ions and organic substances such as glucose can dissolve
Water’s polarity “pulls” apart the ions of a particular substance (above NaCl). The positive hydrogen ends are attracted to the anions of a substance and the negative oxygen ends are attracted to the cations of a substance
Like dissolves like: water can interact with other polar molecules
Water transports molecules dissolved in it
Blood , a water-based solution, transports molecules of nutrients and wastes within organisms
Nutrients dissolved in water get transported through plants
Unicellular organisms that live in water absorb needed dissolved substances
The fact that water is clear allows light to pass through it
Aquatic plants can receive sunlight
Light can pass through the eyeball to receptor cells in the back
pH, Acids, Bases, Buffers
An acid is a substance that increases the hydrogen ion concentration in a solution.
When hydrochloric acid is added to water, hydrogen ions dissociate from chloride ions:
HCl -> H + + Cl -
Addition of an acid makes a solution more acidic.
1. Organisms are sensitive to changes in pH
Any substance that reduces the hydrogen ion concentration in a solution is a base .
Some bases reduce H+ directly by accepting hydrogen ions.
Ammonia (NH 3 ) acts as a base when the nitrogen’s unshared electron pair attracts a hydrogen ion from the solution, creating an ammonium in (NH 4 + ).
NH 3 + H + <=> NH 4 +
Other bases reduce H+ indirectly by dissociating to OH - that combines with H+ to form water.
NaOH -> Na + + OH - OH - + H + -> H 2 O
Solutions with more OH - than H + are basic solutions .
Some acids and bases (HCl and NaOH) are strong acids or bases.
These molecules dissociate completely in water.
Other acids and bases (NH 3 ) are weak acids or bases.
For these molecules, the binding and release of hydrogen ions are reversible.
Carbonic acid (H 2 CO 3 ) is a weak acid:
H 2 CO 3 <=> HCO 3 - + H +
the environment needs to be at a specific pH for many organisms to survive
Ex: most cells have an internal pH of 7 (neutral)
If the pH changes, even slightly, this can disrupt the chemical reactions inside the cell
Blood pH =7.4, a person will die if pH reaches 7.8 or 7 for even a little while!
The chemical processes in the cell can be disrupted by changes to the H+ and OH- concentrations away from their normal values near pH 7.
To maintain cellular pH values at a constant level, biological fluids have buffers .
Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution.
Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.
Buffers typically consist of a weak acid and its corresponding base.
One important buffer in human blood and other biological solutions is carbonic acid .
The chemical equilibrium between carbonic acid and bicarbonate acts at a pH regulator.
The equilibrium shifts left or right as other metabolic processes add or remove H+ from the solution.
Acid rain and acid fog can significantly affect life in streams, rivers and ponds
Their effects upon forest and terrestrial life are still controversial– acid rain can affect soils and mineral content.