~ SCIENCE PSLE Answer ~

20a) The direction of flow of blood in a fish is single while the direction of flow of blood in a human is double.

b) The blood flowing at P has a high concentration of carbon dioxide and a low concentration of oxygen (deoxygenated blood) while the blood flowing at Q has a high concentration of oxygen and a low concentration of carbon dioxide. (oxygenated blood)


21a) Black colour is a better absorber of heat than white colour.

b) Bird P wants to absorb more heat from the sun to keep itself warm.

c) Staying very close to one another helps the birds to conserve heat, thus keeping them warm.


22a) Substance Q was most efefctive in keeping cockroaches away from the food as the time taken for the cockroach to leave the box was the shortest.

b) The packets of sugar will be contaminated by the insecticides and be harmful to Paul when he eats the sugar.


23a) P did not produce oxygen in the presence of light, thus it could not photosynthesise and was concluded to be an animal.

b) The process is photosynthesis. During photosynthesis, plants take in carbon dioxide and give out oxygen. Thus, there was more oxygen than carbon dioxide presnet in beaker 4 and the colour of liquid X was blue.


24a) Animals are attracted by the juicy fuit of platn A. They may carry and store the fruit at other places. As the seeds are hard and stone-like, animals spit out the seeds when they eat the fruit and disperse the seeds over a wide area.

b) The squirrel is less likely to lose all its store of food if it buries each seed in a different place.

c) Benefit 1: The seedlings of plant B will have less competition for water sunlight, space and minerals.
    Benefit 2: The squirrel helps to disperse the seeds of plant B over a larger area and allows plant B to colonise more areas.


25a) As the light intensity increase, the average size of stomata increases.

b) The larger the size of the stomata in the presence of light, more carbon dioxide can diffuse into the leaves at a higher rate. Thus, the rate of photsynthesis increases.

c) As the stomata increases in size, more water vapour move out of the stomata which may cause the plant to lose too much water and wilt.


26a) Many eggs are laid so that at least some of the eggs will not be eaten by preators and will hatch into tadpoles and grow into adults.

b) Advantage 1: If there is an unfavourable change to the surroundings of the young, at least the adults will still survive and continue to reproduce. (Or vice versa)

Advantage 2: There is less competition for food sources and living space between the young and the adults, thus ensuring better surival of the species.


27a) Photosynthesis

b) Burning of trees produces carbon dioxide. The loss of trees will result in fewer trees photosynthesizing, removing less carbon dioxide from the air. Carbon dioxide is a greenhouse gas that traps heat. As a result, the surrounding temperature will rise and this will lead to global warming.

28a) Insect G is attracted to the smell of flower F because the strong rotting meat smell misleads it into thinking that the flower is rotting meat and can be a food source for its larvae when it lays eggs on the flower.

b) Insect G helps to pollinate flower F when the pollen grains from another flower stuck on its body drop onto the stigma of flower F.


29) When the temperature of the water increased, the amount of oxygen dissolved in the water decreased. Thus, in order to get enough oxygen for respiration, the breathing rate of the fish increased to take in more oxygen.