Biology STPM Trial 1/2012Answer Scheme (964/2)

Section A

Question 1 (2002 Q2)

a. A – pyruvateB – lactateC – ethanolD – carbon dioxide

b. Glycolysis

c. Alcoholic fermentation / ethanol fermentation

d. NAD+ functions as coenzyme. It carries out oxidation- reduction reactions. It acts as a hydrogen acceptor to remove hydrogen atomfrom a substrate. The reduced NAD then donates the hydrogen atom to another acceptor molecule and NAD+ is regenerated.

e. Two examples:i. In anaerobic respiration, the yeast breaks down glucose to produce ethanol and carbon dioxide.

Fermentation of grape sugars by yeast produces alcoholic beverages such as wine.ii. In bread making, carbon dioxide released helps bread dough to rise.

Question 2 (2001 Q2)

a.

b. P: P represents the potential action that occurs when the axon is triggered. During the potential action, the potential of axon membrane decreases, causing the iner side of the membrane to be more positively charged compared with the outer side of the membrane. The potential changes from -70mV to +40mV. At this point the membrane is said to be depolarized. The time taken for this to happen is 2-3 milisecond.

c. Q: Q represents relax potential. Compared with its outer side, the inner side of axon membrane is negatively charged. The membrane is said to be polarized. Polarisation is due to the difference in ion concentration on the inner and outer side of the axon membrane. The sodium-potassium pump plays its role in the existence of this condition. This is the normal condition of plasma membrane in animal cells. The charge is -70 mV.

Question 3 (2004 Q3)

a.i. The plants are short-day plants. Short day plants will only flower if the length of dark period

exceeds the lengths of critical dark/night period.

ii. A flash of red light interrupting the dark period shortens the dark period. It causes rapid conversion of PR to PFR which prevents flowering in short-day plant. The plant will respond to the last flash of light. A flash of far red light after the red light cancels the effect of red light. The critical dark period is long enough for increasing level of PR which stimulates flowering. When a plant is alternately exposed to red and far red light, the plant will respond to the last flash of light whichever it is.

iii. When there is interruption, this shortens the dark period. It causes rapid conversion of P R to PFR. The high level of active PFR stimulates flowering in long day plants.

b.i. PR absorbs red light (with peak absorption at a wavelength of 660nm) and converts PR rapidly

to form PFR. Absorption of far red light by PFR converts PFR rapidly into PR. At night or in darkness PFR reverts spontaneously but slowly to the form PR.

ii. PFR is considered as the active form of phytochrome. In long days plants, high levels of P FR

stimulates conversion of inactive hormone precursor to hormone florigen. Florigen is transported by phloem to the bud and induces flowering.

Question 4 (2004 Q4)

a.i. Graph M. Unlimited growth. The growth of the organism continues throughout life. An

example is woody perennial plants, where unlimited growth pattern follows an annual series of smaller sigmoid curves, each curve represents on year growth. Graph N. Intermittent growth (discontinues growth). This growth pattern shows periods of extremely rapid growth followed by periods where there is little or no growth. During each moult, when arthropods for example insects shed off their old hard inelastic exoskeleton ,there is rapid increase in both size. When the new exoskeleton becomes hardened, growth is prevented.

ii. Growth as representedby increase in organic materials such as proteinsis continuous. Growth curve which uses length as a parameter is therefore not a true reflection of growth.

iii. If dry mass is used as a growth parameter, a normal sigmoid curve is produced.b. Ecdysone activates genes controlling the synthesis of certain enzymes involved in the growth and

formation of new epidermis.c.i. Juvenile hormone (JH) activates the genes that control the retention of larval or nymphal

characteristics during development. The hormone also suppresses the genes which control the production of adult characteristics such as adult cuticle.

ii. Synthetic juvenile hormones can block or prolong growth and development during metamorphosis and inhibit final moult into an adult insect.

Section B

Question 5 (2009 Q5)

a. i)

Starch is a polysacccharide formed from condensation of α – glucose units. It is a polymer of α – glucose units linked by α-1, 4 glycosidic bonds. Cellulose is composed of β – glucose units linked by β – 1, 4 glycosidic bonds in which alternate β-glucose units

in the chain rotate 180°.ii.

Starch is made up of two components, aylose and amylopectin. Amylose is long straight chain carbohydrate which is made up of several thousands of α-glucose units linked

together by α- 1,4 glycosidic bonds. On one side of the amylase chain, the numerous number of hydroxyl groups (-OH) form hydrogen bonds

between the glucose subunits. This causes the chain to coil like a helix into a more compact shape. Amylopectin is a branched polymer of many b.

Question 6 (2008 Q8)Question 7 (2003 Q7)Question 8 (2003 Q8)Question 9 (2002 Q7)Question 10 (1999 Q5)