Microscopic life in water

Question 1

Advantages of living in water.

Answer 1

• There is no risk of water shortage and dehydration.
• The temperature of the water varies less in water than air temperature.
• Water helps provides support.
• Waste products are easily disposed of into the water.

Question 2

Disadvantages of living in water.

Answer 2

• The water content of the body can vary and needs to be controlled.
• Water is denser than air and so resists movememt.
• If the water is freshwater, then organisms can take up too much water by osmosis.
• In salt water, too much water may be lost to the surroundings by osmosis.

Question 3

Contracile vacuole

Answer 3

Organisms such as amoeba have a contractile vacuole that can store any excess water. The vacuole can then fuse with the cell membrane and empty the water to the outside.

Question 4

Why does the number of phytoplankton and zooplankton vary at different depths and seasons?

Answer 4

• This is because factors that affect the photosynthesis of phytoplankton will vary:
  
  • There will be less light in winter and deeper in the water.
  • The temperature will be lower in winter and deeper in the water.
  • Minerals are used up towards the end of summer.

Question 5

Food webs of marine organism can provide useful information. The webs rely on different sources of food:

Answer 5

• Most rely directly on green plants.
• Others deeper in the ocean feed on dead material called marine snow that floats down.
• Some rely on bacteria, deep in the ocean, acting as producers.

Question 6

Eutrophication

Answer 6

Sewage and fertiliser run-off can cause a process called eutrophication to occur. This involves the rapid growth of algae, which then all die and decay. This uses up oxygen, causing the death of animals because they are unable to respire.

Question 7

Biological indicators

Answer 7

Some species of organism are more sensitive to pollution than others and so they can be used as biological indicators for pH and oxygen.

Question 8

Some chemical such as PCBs and DDT can kill animals at the top of marine food chains. This is because the chemicals:

Answer 8

• Are toxic.
• Do not break down quickly and so accumulate and become concentrated higher up the food chain.
• Affect animals with a long lifespan, such as whales.