Biology May / June 2008

ANSWER

In stating the functions of the arteries and veins the candidates merely stated carrying blood away from the heart and carrying blood towards the heart. They did not state the exceptions namely the pulmonary artery and veins which led to loss of marks.

Most candidates stated all the components of blood including plasma rather than just the cellular components and since there was no penalty they scored full marks.

Only an insignificant number of candidates could describe the changes in blood as it passed through the organs listed in 7(b). Expected answers include:

Changes in blood composition as it passes through the
Lungs
– Oxygen is added to the blood;
– Carbon dioxide is removed from the blood;
– Water is removed from the blood.

Liver
– Urea is added to the blood;
– Hormonal level is reduced;
– Glucose level is reduced;
– Worn out erythrocytes are removed from the blood.

Kidneys
– Water, urea, some glucose, dissolved salts, mineral salts are reduced in the blood.

Small Intestine –
– simple sugars/glucose, amino acids, water; salts and vitamins are added to the blood.

ANSWER

Most candidates who attempted this question were able to correctly define transpiration and list four factors which affect the rate of transpiration as well as name the types of transpiration. However, in naming the types of transpiration ( Stomatal, Lenticular and Cuticular) a good number of candidates lost marks due to spelling errors.

Most candidates performed poorly in describing the mechanism of opening and closing of the stomata and where they were able to state some of the processes they did so in a non sequential manner. The proper order of the mechanism is as follows.

– When the solute concentration of the guard cells is high;
– water flows from neighbouring epidermal/cells into the guard cells;
– by osmosis;
– the volume and turgidity of the guard cells increase;
– the thin outer walls of the guard cells stretch more than the thicker inner walls;
– causing the guard cells/stomata to open;
– when the solute concentration is low;
– water flows out of the guard cells by osmosis;
– the volume and turgidity decrease causing the guard cell to become flaccid;
– and the stomatal pore is closed.

In stating the similarities and differences between transpiration and sweating most candidates could only state one similarity namely both processes involve loss of water. Other expected answers include the following:

Similarities between transpiration and sweating
– Both involve loss of water from the body of the organism;
– Both processes result in cooling;
– Water is lost through pores.

Differences:

ANSWER

Most candidates who attempted this question could explain the terms population and competition but performed poorly in trying to explain Succession. 
The expected answers include:
Succession  –   This is the step by step/orderly; and gradual replacement of communities; arising from a change in the environment/  environmental factors; which leads to a climax stable community.

The candidates equally performed well in stating the sources of noise pollution as well as the means by which they can be prevented but some candidates listed factories as sources of noise pollution instead of factory machineries and so lost the mark.

Most candidates also performed well on effects of smoke and sewage on humans.

ANSWER

Main Stages of Mitotic Division
Prophase –  Chromosomes appear as two threads/chromatids; chromatids are long thin and/later become shorter and thicker; and joined together at the centromere; the nuclear membrane; and the nucleolus disappear; spindle fibres form in the cytoplasm; with a pair of centrioles; one at each pole.

Metaphase – Spindle formation is completed; the chromosomes are arranged at the equator of the spindle; attached by their centromeres; centromeres divide; and begin to move apart;

Anaphase – The paired chromatids are pulled apart/separated; migrate/move towards opposite poles; spindle fibres shorten; chromatids reach the poles;

Telophase – Fibres disappear; nuclear materials and nucleous are formed; bounded by nuclear membrane; the cell constricts; to form two daughter cells.

Most candidates who attempted 4(b) could only state two importance of mitosis. The answers include:

Importance of mitosis                                                                                                                                                                     Promotes cell growth; replacement/repair of damaged tissues; basis of a sexual/vegetative reproduction; produces genetically identical offspring; which are identical to the parents; maintains the diploid number of the chromosome of the cell.

Questions Not Available.

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NOT 

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ANSWER

  In defining hormones many candidates just stated that they were chemical substances. They omitted to state their origin  i.e. that they are secreted by the endocrine glands and so they did not score the full marks.  Most candidates tabulated the differences between nervous coordination and hormonal coordination but could only give two differences. Expected answers include:

Differences between hormonal coordination and nervous coordination in humans

Candidates could not state the effects of over secretion and under secretion of thyroxine. Very poor diagrams of the reflex arc was made. Many of the diagrams were not titled, some candidates gave the title as diagram of a reflex action rather than the diagram of a reflex arc. In many cases the guidelines were drawn with free-hand and did not touch the structure to be labelled. Only two neurones were shown, in many cases the intermediate neurone was omitted and the motor neurone did not end up in an effector organ.
The expected answers and diagram are as follows:

Effects of over – secretion
– Over active/Hyperactive/irritable/nervous
– Excessive growth /gigantism;
– Loss of weight;
– Enlarged thyroid gland/exothermic goitre;
– Accelerated/increased heartbeat;
– Protrusion/bulging out of eyeball.
Effect of under – secretion
– Cretinism/dwarfism in children;
– Overweight/increase in subcutaneous fat/obesity;
– Physical//mental sluggishness in adults;
– Reduced heart rate;
– Lowered/reduced metabolic rate/myxoedema;
– Goitre formation/hyperplastic goiter.

ANSWER

Chemosynthetic –  These plants/bacteria manufacture their food; using atmospheric carbon dioxide; and energy from the breakdown of inorganic compounds.

Carnivorous      –    These plants in addition to photosynthesizing have special  devices for trapping and digesting insects and other small animals; to balance a deficiency in nitrogen.
Examples
Chemosynthetic
Nitrosomonas, Nitrobacter, Fungi, Nitrococcus.

Carnivorous
Pitcher Plants, Bladderwort, Sundew, Venus flytrap, Butterwort.

Candidates responded poorly to the aspect that dealt with ways by which nitrogen is added and lost from the soil. Most candidates could only state the actions of bacteria in nodules of leguminous plants as well as the action of thunderstorms respectively. Expected answers include.

Addition of nitrogen to soil
Decomposition of dead organic matter         
Dead organic matter; are decomposed by putrifying bacteria; into ammonium compounds, ammonium compounds are converted to nitrites by nitrifying bacteria, in the soil; nitrites are converted into nitrates by nitrifying bacteria in the soil.

Nitrogen fixation in root nodules
Nitrogen fixing bacteria’ living in the root nodules of leguminous plants;Convert gaseous nitrogen in the soil/atmosphere/air-spaces; into nitrates;

During thunder storms; the energy from electrical discharge/action of lightning; enables atmospheric nitrogen to react with oxygen to form oxides of nitrogen/nitric oxides; which dissolve in rain water to form weak nitric acid; which is washed in to the soil and converted into nitrites and nitrates.

Loss of nitrogen from the soil