B6 - Life in Soil and Water

Question 1

What does soil contain?

Answer 1

• Fragments of rock (minerals) – these are produced when rock is weathered. The size varies, and this determines the type of soil.
• Air spaces – gaps between the particles.
• Water – this fills some of the spaces between particles.
• Dead material – fragments of dead plants, animals, or organic waste.
• Living organisms – there is a huge variety of life in and on the soil. Plants rely on the soil for minerals, water, and to anchor them.

Question 2

Describe the particles sizes, air spaces and permeability to water in clay.

Answer 2

Particle size - small
Aid spaces - few and small
Permeability - low - water is retained in soil; soil can flood

Question 3

Describe the particles sizes, air spaces and permeability to water in loam.

Answer 3

Particle size - small and large
Aid spaces - many and variable in size
Permeability - medium - water retention is good

Question 4

Describe the particles sizes, air spaces and permeability to water in sand.

Answer 4

Particle size - large
Air spaces - many and large
Permeability - poor - little water retained in soil

Question 5

How do do you test for moisture, humus and air?

Answer 5

Moisture - weigh, bake and reweigh sample
Humus - weigh, burn and reweigh sample
Air - weigh the sample and measure its volume

Question 6

Describe oxygen and water levels in different soils.

Answer 6

If the particles are small (clay soil) there will be few air spaces, and they will often be full of water, reducing oxygen levels.

If the particles are big (sandy soil) there will be plenty of oxygen in the air spaces, but the water will drain away.

An ideal soil (loam) has a mixture of particle sizes, providing both air spaces and water retention. Gardeners improve their soil by digging to allow in air and to increase drainage.

Question 7

What is humus?

Answer 7

Dead material decomposes in the soil to produce humus. This releases minerals into the soil which are needed by plants for growth. Humus adds a fibrous quality to soil – it tends to hold soil particles apart, improving aeration. It also helps retain water in the soil.

Question 8

How does soil pH affect the plants?

Answer 8

Some plants such as heathers grow well in acidic soil. Many plants prefer relatively neutral soils. Alkaline soils are rare in the UK. Many farmers add lime to neutralise acidic soils so that they can grow more crops.

Question 9

Why are earthworms so useful?

Answer 9

• worms pull dead leaves down into the soil, burying them. This organic material is then slowly decayed by bacteria and fungi, improving the nutrient content of the soil.
• earthworms burrows create gaps which aerate the soil. As the worms move through the burrows they push air through them. The cavities also allow water to drain more freely, reducing the chance of flooding
• the earthworm mixes the soil by eating it and passing out waste elsewhere in the soil
• earthworms release calcium carbonate into their gut to help the digestion of leaves. This then passes out in their waste and has the added bonus of helping to neutralise acidic soils.

Question 10

What are some advantages of living in water?

Answer 10

• Buoyancy – water is more dense than air, so it gives more
  support to the organisms that live in it. The largest animals e.g. the blue whale could not support their weight on land.
• Removal of waste – animal waste is washed away and does not build up, as it is greatly diluted in the water and broken down.
• Steady temperature – surface waters vary in temperature, but not as much as the air does. Water requires a lot of energy to heat it up. Aquatic organisms do not have to cope with extremes or rapid changes in temperature.
• Ready water supply – living in fresh water means that there is no risk of dehydration.

Question 11

What are the cons of living in water?

Answer 11

• Movement – water is more dense than air, making it harder
to move through, so aquatic animals use more energy.
• Water balance – water is everywhere; the problem is balancing the amount of water in the body.

Question 12

How is water regulated in fresh water?

Answer 12

In fresh water, too much water can enter the body by osmosis. This is not a problem for plants, as cell walls stop cells expanding and prevent excess water getting in. In animals, excess water must be removed from the body.

Freshwater fish urinate frequently, removing the excess.
Microscopic organisms such as amoebae have a cell structure called the contractile vacuole, into which the excess water goes. The vacuole then moves to the cell surface, fuses with it, and releases the water.

Question 13

How is water regulated in salt water?

Answer 13

Sea water is salty, and this affects osmosis. Many invertebrates have bodies at the same salt concentration as the sea and so have no problem, but some larger fish do not. They actively get rid of the salt in the water they drink.

Question 14

How does the contractile vacuole remove excess water in an amoeba?

Answer 14

• the contractile vacuole gets bigger as it fills with excess
  water
• the contents are discharged outside of the cell
• vacuole ready to be filled again

Question 15

What are the two types of plankton?

Answer 15

• Phytoplankton are photosynthetic microorganisms – they are producers.
• Zooplankton are animal-like microorganisms – they are consumers.

Question 16

How does light affect plankton?

Answer 16

Seasonal : As day length increases, the numbers of phytoplankton increase.
Depth : Light only penetrates surface waters, so phytoplankton are limited to the surface waters.

Question 17

How does temperature affect plankton?

Answer 17

Seasonal : As surface water temperatures rise in spring, the numbers of phytoplankton increase.
Depth : At depth the temperature is a constant 4°C, too cold for phytoplankton to grow.

Question 18

How do minerals affect plankton?

Answer 18

Seasonal : Minerals rise to the surface during the winter, so there is a ready supply for phytoplankton in the spring.
Depth : Mineral concentrations increase at depth, but phytoplankton are limited there by light and temperature.

Question 19

Describe the food chains in the sea.

Answer 19

Grazing food webs, based on photosynthetic producers, are common in the surface layers of the oceans.

In deep water food webs do not start with phytoplankton as there is no light. Food chains at these depths rely on dead food falling from above, called marine snow. These are detrital food chains.

Other food chains start with bacteria – producers that get their energy from chemical reactions in a process called chemosynthesis.