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Describe the formation of a covalent bond.

Forms between non metals. Electrons shared to fill outer shells.


How many bonds do the following elements form: carbon, nitrogen, oxygen and hydrogen?

Carbon: 4
Nitrogen: 3
Oxygen: 2
Hydrogen: 1


How does hydrogen bonding occur between water molecules?

Weak interaction between a slightly positive hydrogen atom and a slightly negative oxygen atom.


Between which other atoms do hydrogen bonds occur?

Any slightly negatively charged atom.


What is a condensation reaction?

The joining of two molecules by the removal of water.


Describe how a condensation reaction occurs.

An OH group is removed from one molecule and a H from the other.


Water property: translucent. What is the role of this?

Allows light through for photosynthesis.
Different depths of water = different wavelengths of light.
Predators/prey can see to hunt/evade.


Water property: ice less dense than water - H bonds form lattice as water cools below 4 degrees. What is the role of this?

Platform to hunt from/hide beneath.
Insulates water beneath - stable environment reduces freezing.


Water property: liquid across range of temperatures. What is the role of this?

Habitat for life - within and on surface.
Site of reactions.
Hydrolysis reactions.
Reactant in photosynthesis.
Transport medium - blood, vascular tissue in plants, food and gametes for sessile organisms.


Water property: solvent - polar. What is the role of this?

Ionic solutes dissolve - positive/negative interactions between dipoles of water.
Ions transported around cells/systems/media, e.g. reactions in cytoplasm, ions in blood, minerals for coral.


Water property: cohesion and surface tension - H bonds between molecules. What is the role of this?

Cohesion - attraction between molecules pulls them together into sphere.
Adhesion - attraction between water and other surface.
Attractions pull molecules together to form surface tension.
Habitat for pond skaters.
Unbroken column of water in xylem.


Water property: high specific heat capacity. What is the role of this?

Thermally stable - lots of energy in/out to raise/cool.
Stable temperature for enzyme catalysed reactions.
Stable environment for aquatic organisms.


Water property: high latent heat of vaporisation. What is the role of this?

Evaporation of water requires high energy so heat is removed from organism.
Cooling through panting/sweating in animals.
Evaporation of water form mesophyll cells.


What are lipids?

Substances composed of large amounts of hydrogen and carbon but fewer carbons than carbohydrates.
Insoluble in water but soluble in ethanol.
Oils, fats and waxes.


How do you test for starch? What is a positive result?

Add iodine (potassium iodide) to a sample. Blue-black colour means starch is present.


How do you test for reducing sugars? What is a positive result?

Heat sample with excess Benedict’s reagent at 80°C for 5 minutes.
Observe colour change from blue to:
Green 0.5%
Yellow 1.0%
Orange 1.5%
Brick red 2.0%


How do you test for non-reducing sugars? What is a positive result?

Test for reducing sugars first.
Take a fresh sample.
Boil sample with dilute HCl to hydrolyse glycosidic bond if present.
Add alkali to solution (i.e. NaOH).
Carry out standard reducing sugars test.


How do you test for lipids? What is a positive result?

Mix a sample with ethanol, then filter into a test tube containing water. A cloudy white precipitate means a lipid is present.


How do you test for proteins? What is a positive result?

Add biuret reagent (equal volumes of sodium hydroxide and copper sulphate) to a sample. Lilac colour means a protein is present.


Describe the role of carbohydrates.

Energy source, energy store, structural units.


What is the role of alpha glucose?

Energy source, monomer of polymers (including amylose, amylopectin and glycogen).


What is the role of beta glucose?

Energy source, monomer of cellulose in plant cell walls.


What is the role of ribose?

Component of RNA, ATP and NAD.


What is the role of deoxyribose?

Component of DNA.


Describe the formation of the bond between di- and polysaccharides.

Glycosidic bond.
Formed through a condensation reaction.
One hydroxyl, OH, group from each molecule align.
HOH removed, bond formed between remaining oxygen.
Bond forms between carbon 1 and carbon 4 or carbon 1 and carbon 6.


Describe the breaking of the bond between di- and polysaccharides.

Hydrolysis reaction.
Water split into H and OH.
Used to break glycosidic bond.


Name polysaccharides, state their role, where they are found and describe their structure and bonds.

Amylose: energy store in plants, combines with amylopectin to form the complex starch, 1,4 glycosidic bonds, coils held by hydrogen bonds, C2 OH groups held within spiral make it less soluble.
Amylopectin: energy store in plants, combines with amylose to form the complex starch, 1,4 and 1,6 glycosidic bonds, coils held by hydrogen bonds, 1,6 bonds form branches.
Glycogen: energy store in animals, stored in liver and muscles, 1,4 and 1,6 glycosidic bonds, more highly branched than amylopectin.


Describe the properties of polysaccharides as an energy store.

Compact for storage, insoluble so do not affect water potential of cells, readily hydrolysed to free monomers for energy source.


Describe the formation of cellulose.

Made of β-glucose monomers.
To form a glycosidic bond, each β-glucose must rotate 180° compared to its neighbour.
β-glucose monomers are joined by 1,4 glycosidic bonds.
Forms straight chains that run parallel to each other.
Hydrogen bonds form between chains, many H bonds add considerable strength.


Describe the role and properties of cellulose.

Found in plants.
Makes up the plant cell wall.
High tensile strength to prevent cells bursting under turgour pressure.
Allows ions and water to permeate.
Can be reinforced with additional substances. Suberin and cutin for waterproofing (e.g. Casparian strip). Lignin for strengthening (e.g. xylem vessels).