Chemistry of Life

Water: The Primary Molecule of Life

Assessment Statements 3.1.1 to 3.1.6

IB Biology Yr 1

What must cells do to stay alive?Maintain regulation - Get rid of wastes/ingest food and

waterGrow & DevelopRespond to changes in their environmentReproduceProcess energyAdapt

How does each cell do this?

EnergyEach cell uses energy to build the structures it needs out of the materials available in its external

environment – atoms and molecules. The cell maintains a sophisticated barrier between itself

and the outside world via the cell membrane

Living things rely on chemical reactions, which take place at the molecular level.

Cell MembraneRanges in thickness from 3nm to 10 nmAtoms are much smaller than this.

Prior to knowing what the cell membrane actually looked like, scientists used chemical knowledge and procedures to make inferences about the world of the cell, the molecules that living cells use, manufacture, excrete and interact with….

Let me introduce you to this exciting world!

BiochemistryYou may not think of your body

in terms of chemical reactions, yet you rely on your cells to perform trillion of chemical reactions every second.

The study of these reactions and the molecules and processes involved in them is called biochemistry.

Biochemistry

Think about it…You are standing outside in winter waiting for your

bus…when you are “seeing your breath” what is really going on?

Condensed water vapour (H2O) from your lungs Oxygen (O2) and Carbon dioxide (CO2). The oxygen is left over from the previous inhalation

and the carbon dioxide gas is the result of cellular respiration.

Assessment statement 3.1.1State that the most frequently occurring chemical elements in

living things are C, H2, O2, N2

Living organisms are collections of elements in the form of atoms, ions, and molecules.

The 4 most common elements found in living things are

carbon, hydrogen, oxygen and nitrogen. Together these elements are used in the molecular structures

of all carbohydrates, proteins, lipids, and nucleic acids.

See handout – The periodic table for biologists

Carbon and Hydrogen

• Carbon and hydrogen are the backbone of organic molecules, they make up sugars and carbohydrates, amino acids and proteins, and lipids fats and oils

• Hydrogen ions are used in active transport, photosynthesis, and cell respiration

Oxygen

• Main role is to allow aerobic respiration to occur. This is the last stage of cell respiration (much more of this to come!)

• Oxygen is also used in oxidation reactions where oxygen is put into ATP to release energy into tissues.

Nitrogen

The main use of nitrogen is in the production of amino acids. Amino acids are transformed into proteins which are extremely important in the cell.

Nitrogen is also used in chlorphyll.What do you remember about the nitrogen

cycle from Science 10?

Assessment Statement 3.1.2 & 3.1.3State that a variety of other elements are

needed by living organisms, including sulfur, calcium, phosphorous, iron, and sodium.

State one role for each of the elements mentioned in 3.1.2

Assessment Statement 3.1.3Element Example role in plants Example role in

animalsExample role in eukaryotes

Sulfur In some amino acids In some amino acids In some amino acids

Calcium Co-factor in some enzymes

Co-factor in some enzymes and component of bones

Co-factor in some enzymes

Phosphorous Phosphate groups in ATP Phosphate groups in ATP

Phosphate groups in ATP

Iron In cytochromes In cytochromes and in hemoglobin

In cytochromes

Sodium In membrane function In membrane function (Na-K pump)and sending nerve impulses

In membrane function

Water: The primary molecule of Life

Water!

Remains a liquid over a wide temperature range Dissolves most substances involved in living processes,

such as oxygen, carbon dioxide, glucose, amino acids (components of proteins), and sodium chloride (salt)

Gradual temperature change when heated or cooled Expands when it becomes a solid, floats when freezes Clings together which helps water creep up thin tubes,

such as those running from roots to shoots in plants.

• The specific properties of water are determined by its chemical structure. A water molecule has an uneven distribution of electrical charge.

• Animation

The hydrogen and oxygen atoms in a water molecule are held together by polar covalent bonds due to an uneven sharing of electrons. The oxygen has an overall negative charge and the hydrogen atoms a positive.

Assessment statement 3.1.4 (draw and label a water molecule to show it’s polarity and hydrogen bonds)

Assessment statement 3.1.5 – Outline the thermal, cohesive and solvent properties of water.

Assessment statement 3.1.6 – Explain the relationship between the properties of water and its uses in living organisms as a coolant, mediumfor metabolic reactions and transport medium.

High specific heat - allows water to give off or absorb heat without changing a great deal in temperature. This means that the temperature of water can remain relatively stable.

This unique property of water is important to living things as many organisms are adapted to a narrow range of conditions. The slow heating and cooling of water are ideal for these organisms and loessesns the risk of death due to extreme temperature ranges.

Thermal properties of water

• High heat of vaporization - Water has the ability to absorb a lot of heat when it evaporates.

• It takes a lot of energy to make water evaporate, it is thus a good coolant – as evaporating water removes a lot of heat energy from the organism.

Thermal properties of water

• A single hydrogen bond is not that strong, but a large number of hydrogen bonds is very strong! Each water molecule bonds with 4 others in a tetrahedral arrangement.

• Due to this tetrahedral arrangement, water iscohesive and water molecules “cling” to eachother in groups. Water is also adhesive and will

cling to other surfaces.

• Demonstration “The Floating Paper Clip!”

Cohesive properties of water

• Capillary Action: water will move againstgravity up xylem from roots to shoots!

• Surface Tension: the surface of water is strong enough to support insects and causesdrops to form.

– E.g. Pondskaters walk on water

– E.g. Cohesion forms droplets, surface tension keeps them spherical and adhesion sticks them to the leaf! HOW COOL IS THAT!

Cohesive properties of water

Solvent PropertiesMost molecules in the body are

polar too!Water is a good solvent because it

is a polar molecule. It will dissolve polar solutes easily.

Remember that like dissolves like and water makes an excellent solvent for the chemical properties to occur in your body.

Aq = aqueous solution (latin for “dissolved in water”)

Animation!

• Ionic solids dissociate in water (they break into their ions)

• Polar attractions cause water molecules to surround and isolate the solute molecules.

• The more soluble a solute is, the easier it is for the ions to be isolated from each other.

Water is a good solvent: It dissolves nutrients, gases, and waste products. These can be carried in the circulatory systems of animals or through xylem and phloem in plants through the water in soil or aquatic habitats.

Water as a solvent

Solvent properties table (p. 49)Aqueous Solution Location Common Reactions

Cytoplasm Fluid inside cell but outside organelles

Glycolysis/protein synthesis reactions

Nucleoplasm Fluid inside nuclear membrane DNA replication/transcription

Stroma Fluid inside chloroplast Light-independent reactions of photosynthesis.

Blood plasma Fluid in arteries, veins and capillaries

Loading and unloading of respiratory gases/clotting

• Water is a good solvent – dissolved particles move around and diffuse.

• Moving particles are likely to collide with one another leading to a reaction.

• All metabolic reactions (reactions in living things) occur in solution – the reactants are dissolved.

• Membranes and biological surfaces are wet allowing molecules to dissolve, including gases, so they can diffuse through more easily. (lungs are wet allowing for oxygen diffusion into the bloodstream)

Water as a medium for metabolic reacti ons

• Water is dense: allows for large mammals to be supported in it.

• Water can diffuse across the cell membrane, allowing it to carry small molecules from cell to cell or inside the cell itself from one location to another.

AND YET STILL MORE PROPERTIES OF WATER!

BloodThe most common transport medium in animals

composed mostly of water – known as plasma.Transports – red blood cells, white blood cells, platelets,

dissolved molecules, glucose, amino acids, fibrinogen, CO2

More than water?Water of the future?Breathing water? Is it

possible?

http://www.youtube.com/watch?v=Z6G4s8-upGk

So…what do you think?http://www.guardian.co.uk/education/2004/jan/22/research.badscience