a. Importance of the following in the Nitrogen Cycle
(i) Lightning
- When lightning occurs, nitrogen reacts with oxygen in air to form nitrous oxide/oxides of nitrogen;
- which is further oxidised to form nitrogen (IV) oxide /nitrogen dioxide;
- which dissolves in rain water to form nitric and nitrous acids;
- (that fall to the ground) when it reaches the soil combines with bases/mineral salts to form nitrates;
- which the plants absorbs.
(ii) Nitrosomonas
- Is a nitrifying bacterium in the soil;
- that converts ammonia to nitrites in the soil.
(iii) Azotobacter
- Is a nitrogen-fixing bacterium in the soil;
- that converts nitrogen in the soil;
- into ammonium compounds/amino compounds and protein in the soil;
- converts nitrites to nitrates in the soil.
b. Excretory organs
Insects – malpighian tubules;
Earthworm – nephridium
c. of diseases
(i) Hepatitis
- Is the inflammation/destruction of liver cells;
- hepatitis can arise from bacteria/virus/an autoimmune disease;
- or from excess consumption of alcohol.
The symptoms include fatigue;
- loss of appetite/anorexia;
- muscle aches/pains;
- flu;
- fever;
- nausea;
- jaundice;
- and dark coloured urine/stool.
(ii) Kidney stones
- Are small solids/crystals of concentrated minerals/uric acid found in the urine;
- they are formed if there is an excessive loss of water/insufficient water intake;
- so that the solubility of minerals/calcium phosphate/calcium oxalate in the urine is reduced;
- high levels of uric acids/amino acids can lead to kidney stones;
- bacteria may be trapped in the kidney tubules as the kidney stones get bigger;
- symptoms are sharp pain at the flank of back;
- frequent urination;
- nausea;
- sweating;
- kidney stones can be treated by medications;
- change in diet of food rich in calcium;
- surgery;
- drinking plenty of water.
d. (i) Effects of lack of sense receptors in the skin
- Humans will not be able to sense/feel/perceive external stimuli in the environment;
- pain;
- touch;
- a change in temperature;
- pressure;
- danger which could lead to death.
(ii) Layers of the epidermis
- cornified layer/stratum corneum;
- granular layer/stratum granulosum;
- malpighian layer/stratum Malpighi.
e. (i) Metamorphosis
Is the development of some/certain organisms from the egg; to an adult stage; involving three to four stages.
(ii) Type of metamorphosis of listed insects

Insect	Type of metamorphosis
Grasshopper	Incomplete
Cockroach	Incomplete
Butterfly	Complete
Mosquito	Complete
Housefly	Complete