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Topic 1 revision summary notes A*/9 (A Level Biology Edexcel B)

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Module
Topic 1 - Biological molecules

Institution
PEARSON (PEARSON)

This document contains in-depth revision notes for topic 1 - biological molecules for the higher grades. These revision notes achieved a 9 (A**) grade. It contains diagrams and tables as needed and in-depth descriptions and definitions.

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Topic 1
Biological molecules
1.1 Chemistry for life
Ionic bonding

• Atoms donate or receive electrons. One atom becomes a positive cation, one a negative anion.

Covalent bonding
• Atoms share electrons.
• Molecules formed are usually neutral. But in some compounds the molecules are slightly polarised.
• The electrons aren’t evenly shared so the molecule has a part that is slightly positive and slightly
negative.

• Dipole: the separation of charge in a molecule when the electrons in covalent bonds are not
evenly shared.
• Polar molecule: molecule containing dipoles that do not cancel

Inorganic ions

Important anions
• Nitrate ions: In plants for formation of proteins and amino acids from the products of
photosynthesis. For formation of DNA.
• Phosphate ions: for formation of ATP, ADP, DNA and RNA.
• Chloride ions: for nerve impulses and secretory systems.
• Hydrogen carbonate ions: needed to bu er the blood to prevent it from becoming too acidic.

Important Cations
• Sodium ions: needed in nerve impulses and secretory systems
• Calcium ions: for formation of calcium pectate for the middle lamella. For bone formation and
muscle contraction.
• hydrogen ions: in cellular respiration and photosynthesis. Needed in pumps and systems in
organelles. pH balance.
• Magnesium ions: production of chlorophyll in plants.
• Middle lamella: rst layer of the plant cell wall to be formed when a plant cell divides, made
of calcium pectate (pectin) that binds the layers of cellulose together.

The chemistry of water

• Electrons are held closer to the oxygen atom than the hydrogen so they’re polar molecules.
• This charge separation leads to hydrogen bonds.
• The negative oxygen atom of one molecule attracts positive hydrogen atoms of other molecules in
a weak electrostatic attraction - hydrogen bond
• Each bond is weak so there are many of them.
• High melting and boiling point- takes high levels of energy to overcome attractive forces of all the
hydrogen bonds.
• Dissolves charged molecules: ions in the salt are pulled apart and surrounded by the oxygen or
hydrogen atoms.
• Hydrogen bond : weak electrostatic intermolecular bonds formed between polar molecules
containing at least one hydrogen atom.

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,The importance of water

• A polar solvent so ionic substances will dissolve in it.
- Covalent substances are polar and will dissolve in water but don’t dissolve in other covalent
substances. So most reactions happen in water.

• A good transport medium as substances will dissolve in it.

• At 4℃ reaches its maximum density.
- At high temperatures, molecules move faster and are far apart,
decreasing density.
- At lower temperatures, molecules slow and get closer- density
increases. At 4℃ ice clusters start forming pushing molecules further
apart and density decreases.
- Ice is less dense than water so oats, forms insulating layer preventing water beneath freezing.

• Slow to absorb and release heat ( high speci c heat capacity): hydrogen bonds take a lot of energy
to break, means temp of large bodies of water don’t change much thought the year-good habitats.

• Can’t be compressed: useful in hydraulic mechanisms in organisms.

• The molecules are cohesive: hydrogen bonds mean they stick together. Important in movement of
water form roots to leaves.

• The molecules are adhesive: attracted to di erent molecules. Important in plant transport systems.

• Has a high surface tension as attraction between water molecules is greater than attraction
between water and air. So water molecules form a skin of surface tension. Important in transport
systems and for life at the surface.

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, 1.2 Biological molecules 1
1. Carbohydrates 1 - monosaccharides and
disaccharides.
Organic compounds

• Contain carbon atoms. Also have hydrogen and oxygen.
• Each carbon atom can make 4 bonds
• The 4 bonds form a tetrahedral shape, leads to the formation of branched chains or rings.
• In some carbon compounds, monomers bond together to form polymers.

Carbohydrates

• Energy source
• Forms part of cell wall in plants, fungi and bacteria.
• Starch: a long chain polymer formed of - glucose monomers
• Sucrose: disaccharide formed by the joining of - glucose and fructose by a glycosidic
bond.

Monosaccharides
• Monosaccharide: simple sugar
• General formula: (CH2O)n - one oxygen atom and two hydrogen atoms for each carbon atom.
• Triose sugars (n = 3): 3 carbon atoms and the molecular formula C3H6O3.
- Important in mitochondria where glucose is broken down into triose sugars during respiration.
• Pentose sugar (n = 5): 5 carbon atoms and molecular formula C5H10O5.
- Ribose and deoxyribose are important in the nucleic acids deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA).
• Hexose sugar (n = 6): 6 carbon atoms and the molecular formula C6H12O6.
- Taste sweet and include: glucose, galactose and fructose.
• Glucose: a hexose sugar.

- glucose and β- glucose

• Glucose has di erent isomers due to di erent
arrangements of the atoms on the side chains of the
molecule.
• The di erent isomers form di erent bonds between
neighbouring glucose molecules.
• Isomer: molecules with the same chemical formula,
but di erent molecular structures.

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