A1 - Water and Nucleic Acids

IB Biology SL Exam Prep 2025 Syllabus

Aqueous Solution

Water with substances dissolved in it.

Medium for Life

A medium is something in which processes can occur. The 8 processes of life happen in water.

The 8 processes of life

• Metabolism

• Nutrition

• Growth

• Reproduction

• Movement

• Response to stimuli

• Excretion

• Homeostasis

Metabolism - Water is essential for life

As an aqueous solution, both water and solutes are free to move and react chemically. Reactants and the products of most chemical reactions are dissolved in water.

Nutrition - Water is essential for life

The reactions of both photosynthesis and digestion take place in aqueous solution.

Growth - Water is essential for life

Cytoplasm is an aqueous solution, so cells must absorb water by osmosis to increase in size

Reproduction - Water is essential for life

Sperm swim to the egg through water and mammalian foetuses are supported by water in the uterus

Movement - Water is essential for life

Aquatic organisms swim through water; pumping of blood and sap transports substances dissolved in water

Response to Stimuli - Water is essential for life

Never impulses are movements of dissolved Na+ and K+ ions; hormone transport is in blood

Excretion - Water is essential for life

Urine is an aqueous solution of waste products; excretion of waste requires moist surface

Homeostasis - Water is essential for life

Blood plasma and tissue fluid are aqueous solutions that are regulated to form a stable and ideal internal environment for cells.

Molecule

Two or more atoms joined together by one or more covalent bonds (sharing or electrons)

Polarity

When one atom in a molecule has a slight positive charge while the other has a slight negative charge. Electrons are not shared equally. (ie. water)

Hydrogen Bond

A bond formed between the positive pole of one water molecule and the negative pole of another water molecule. Individually they are weak but collectively have a large influence on the properties of water.

Cohesion

Water molecules stick to each other because of the hydrogen bonds that form between them

Tensions

Strong pulling forces that are exerted to suck water up to the tops of the tallest trees in tubular xylem vessels. Because of the cohesion from the water bonds, the columns of water molecules in these vessels rarely break.

Surface Tension

The effect of water surfaces forming a cohesive structure that resists breakage. To break through the water surface, hydrogen bonds would have to be broken which require more energy than available.

Adhesion

Water sticking to another substance. Happens if the other substance is hydrophilic.

Hydrophilic Substances

Substances that can make intermolecular bonds with the water molecules

Example of Polar Substance Being Hydrophilic

Cellulose in plant cell walls form hydrogen bonds with water. Cell walls tend to remain saturated with water and draw more water from the nearest supply if unsaturated due to evaporation when a leaf transpires.

Capillary Action - Adhesion

Water being drawn through narrow spaces because it adheres to the surfaces of the spaces. (Ie. water in soil)

Hydrophobic Substances

Water molecules are more strongly attracted to each other than to the polar molecules of the substance. Therefore insoluble in water. (Ie. wax on leaves and oils on human skin)

Physical Properties of Water

• Buoyancy

• Viscosity

• Thermal Conductivity

• Specific Heat Capacity

Buoyancy

Solids floating in fluids if their density is lower

Viscosity

Resistance to flow of fluid due to cohesion between molecules. (Resistance in water vs Air)

Thermal Conductivity

Ability of a material to transfer heat. Water’s thermal conductivity is 20x that of air.

Specific Heat Capacity

The quantity of heat needed to raise the temperature of a gram of a material by one degree. (Water heats up and cools down much more slowly than air because of density)

Nucleotides

• The subunits in both DNA and RNA

• Consist of a pentose sugar, a phosphate, and a base with nitrogen and 1 or 2 rings.

One-Ring Nitrogenous Bases (Pyrimidines)

• DNA: Cytosine, Thymine

• RNA: Cytosine, Uracil

Two-Ring Nitrogenous Bases (Purines)

• Adenine

• Guanine

Sugar-Phosphate “Backbone”

Strands of RNA or DNA linked by the covalent bonds between the pentose sugar of one nucleotide and the phosphate of the next one

Polymers

A molecule composed of repeating subunits (monomers) linked together by covalent bonds

Formation of RNA

• The pentose sugar in RNA is Ribose

• There is a single strand of nucleotides liked by covalent sugar-phosphate bonds

• Water is produced through condensation reaction When bonding sugar to phosphate

DNA as a double helix

• There are two stands of nucleotides in DNA which are linked by hydrogen bonding between their bases

• The two strands are antiparallel - alongside each other but in opposite directions

DNA vs RNA

1. The pentose is ribose in RNA but deoxyribose in DNA

2. DNA has base thymine but RNA has uracil instead

3. RNA has one strand of nucleotides while DNA has two

Roll of Complementary Base Pairing in Cells

• DNA replication

• Transcription

• Translation

DNA Replication

Sequences of base DNA can be copied accurately so the genetic information of a cell can be passed on to daughter cells

Transcription

RNA can be made with the same base sequence as one of the two strands of a DNA molecule. Messenger RNA (mRNA) carries the base sequence of a protein-coding gene to the ribosome.

Translation

A base sequence can be used to determine the amino acid sequence in a polypeptide. mRNA carries a series of 3-base codons then each transfer RNA (tRNA) has one 3-base anticodon that carries the amino acid. Ribosomes link codons to anticodons by complementary base pairing.

Diversity of DNA Base Sequences

The number of possible base sequences is 4^n where n is the number of bases. A typical gene has over a thousand bases while genomes have billions of bases so DNA’s capacity to store information is basically limitless.

Conservation of the Genetic Code

Each of the 64 codons of the genetic code indicates either 1 of the 20 amino acids or the end of the polypeptide. There’s such a large number of ways to assign meaning for the 64 codons but all organisms use the same meanings with only minor variations strongly suggesting that life all evolved from the same original ancestor.

3’ and 5’ ends of DNA and RNA

• The 3’ end has a pentose sugar to which the phosphate of another nucleotide can be linked. The phosphate would bond with the -OH group on the C3 of the deoxyribose

• The 5’ end has a phosphate, attached to the C5 of a pentose

Constructing New Strands of DNA or RNA

• Each extra nucleotide is added to the pentose at the 3’ end of the growing strand

• Done by bonding the phosphate of a free nucleotide which is the nucleotide’s 5’ end.

• Nucleotides are added in a 5’ to 3’ direction and the two strands run in opposite directions