(4) The Water Cycle and the importance of water to life in the Earth's biosphere

What happens to water on the Earth's Surface?

The water on the Earth's surface is continually being re-cycled.

Water is the most abundant substance on the surface of our planet and is essential for all life.

Water in rivers, lakes and the oceans is evaporated by the heat energy of the Sun's radiation (liquid ==> gas/vapour, an endothermic process).

Water also evaporates directly from leaves of plants in the process called transpiration.

The water vapour formed rises high in warm air convection currents into the atmosphere, cools and forms clouds of condensed water (gas/vapour ==> liquid/solid, an exothermic process).

Eventually the water falls as rain, hail or snow, collectively called 'precipitation'.

If the precipitation falls on land, and any excess rain/melt water runs off into rivers, lakes, seas and oceans and eventually re-evaporates by the sun's energy to repeat the cycle.

Some water will percolate into soil or porous rocks, most becoming ground water, but this will eventually flow into lakes and rivers. Some water will accumulate in artesian wells.

This is known as the water cycle and is essential for all life based on land.

Water is essential to maintain all habitats.

All organisms need water for cellular fluids, and require it for transport systems.

AND don't forget its importance for generating electricity from hydroelectric power.

When the precipitation of water falls on land it (usually) provides clean fresh water for plants and animals.

This flow of fresh water via the water cycle transports nutrients from location to another i.e. from one ecosystem to another.

Water is an important raw material and has many uses See GCSE Chemistry Notes on Water

 

The importance of water to life

If it wasn't for the water cycle raining down freshwater, life as we know it would not exist.

Soil absorbs fresh water and plants take it in through their roots for photosynthesis and transporting dissolved substances like sugars around the plant.

This means some of the water is incorporated into food chains.

Water in animals is returned to the soil and atmosphere by excretion, respiration and sweating.

Most land or aquatic plants and animals need freshwater, apart from organisms adapted to live in salty water.

Salty water is potentially toxic to many organisms if ingested.

All living things on our planet need water to survive and perform all their biological functions - in fact most biochemical reactions need water as a reaction medium.

Therefore a lack of water makes it difficult for many organisms to exist.

 

Estimating the percentage mass transfer of a plant

A large volume of water can pass through a large tree in just one day.

The water is absorbed through the roots, but most is lost from the leaves by transpiration.

If you know the mass of water absorbed by the roots and the mass evaporated from the leaves, you can calculate the % mass transfer of water e.g.

Suppose an oak tree absorbs 3000 kg of water through its roots and loses 2500 kg through transpiration.

Calculate the % mass transfer of water.

% mass transfer = (2700/3000) x 100 = 90%

 

We need good sources of fresh water

We, and most land based animals, need freshwater from precipitation in the water cycle.

In times of drought e.g. in parts of Africa, animals suffer and die from lack of water - uncertainties in weather patterns, maybe worse due to global warming, make the situation worse.

In 'cooler' developed countries, reservoirs provide a constant supply of potable water - fit for domestic use.

In 'hot' dry countries that can afford it, desalination is an option.

Desalination is the process of obtaining pure water from salty water e.g. removing all the mineral salt ions from sea water.

All desalination processes are costly

they use lots of energy

Method 1. Desalination by distillation

The simplest method is distillation in which you boil off pure water in a large heated flask/tank.

The thermometer monitors the boiling temperature of the liquid.

From the flask/tank the steam is cooled in the condenser so the water vapour condenses and the water runs into a clean tank/flask. All the much higher boiling mineral salts are left behind as a solid residue.

This process is sometimes called thermal desalination.

A simple laboratory demonstration is illustrated in the left diagram (from GCSE chemistry revision notes).

Method 2. Desalination using reverse osmosis

The other principal method used is reverse osmosis which forces water to go in the opposite direction to concentration gradient rule.

Normally, water will pass through a semi-permeable membrane (right diagram of a partially permeable membrane) from a less concentrated solution to a more concentrated solution i.e. from a higher concentration of water to a lower concentration of water.

Think of the purple circles as the salt ion particles.

In reverse osmosis, the water molecules are forced to go in the opposite direction (left diagram).

(i) Salt water is first treated by filtration to remove any insoluble solid debris like sand or twigs. The soluble salts are obviously left in solution.

(ii) The water is then pumped under very high pressure into a vessel partitioned by a semi-permeable membrane.

(iii) The high pressure forces the water to go in the opposite direction to 'normal' osmosis i.e. from a higher salt concentration to lower salt concentration (from a lower concentration of water to a higher concentration).

(iv) Therefore the salt particles concentrate on the input side of the partially-permeable membrane and pure water on the output side - so the salts are removed to give potable water.

The more concentrated salt water produced is steadily replaced by 'fresh' salt water and the purified water drained off to a clean storage tank.

Research is going on to find more effective membranes to increase efficiency and reduce costs e.g. fabricated graphene sheets are being tested for making semi-permeable membranes.

 For more see Water Treatment, pollution and producing potable water (gcse chemistry)

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Key points - Summary of ideas about the water cycle

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

The Water Cycle and Its Importance in Nature

The Water Cycle: An Overview

The water cycle, also known as the hydrological cycle, is the continuous movement of water between the atmosphere, land, and oceans.

It plays a vital role in maintaining life and regulating Earth's climate.

The water cycle consists of several key processes:

1. Evaporation:

   • Water from oceans, lakes, rivers, and soil is heated by the Sun and changes into water vapor.

   • This process removes heat from surfaces, contributing to temperature regulation.

2. Transpiration:

   • Plants lose water through their leaves via tiny pores called stomata.

   • This water evaporates into the atmosphere, adding to humidity levels.

3. Condensation:

   • Water vapor rises and cools in the atmosphere, forming tiny water droplets that cluster to create clouds.

   • This is the reverse of evaporation, where gaseous water becomes liquid again.

4. Precipitation:

   • Water droplets in clouds combine and become heavy enough to fall as rain, snow, sleet, or hail.

   • This replenishes water sources and maintains ecosystems.

5. Runoff and Collection:

   • Water that falls to Earth collects in rivers, lakes, and oceans or seeps into the ground as groundwater.

   • Some of it infiltrates soil, contributing to underground water supplies.

Importance of the Water Cycle in Nature

• Supports Life:

  • Provides freshwater for plants and animals.

  • Distributes nutrients and minerals essential for ecosystems.

• Regulates Climate:

  • Evaporation and transpiration help cool the environment.

  • Cloud formation affects weather patterns.

• Purifies Water:

  • Water is naturally filtered through evaporation, condensation, and infiltration, removing impurities - but not enough for the World's rising population!

• Maintains Ocean and Land Balance:

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Summary of learning objectives and key words or phrases

Be able to construct or interpret a diagram explaining the water cycle and its importance to life in the Earth's biosphere, both plant and animal organisms.

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