It is a REACTANT in photosynthesis and many other reactions.
It transports dissolved substances around organisms.
It provides structure to plant cells (turgor pressure).
It regulates body temperature (through sweating and evaporation).
Like carbon, water is continuously RECYCLED through the environment in the WATER CYCLE (also called the HYDROLOGICAL CYCLE).
The total amount of water on Earth is effectively fixed — it moves between stores: oceans, atmosphere, freshwater lakes and rivers, ice caps, groundwater, and living organisms.
Stages of the Water Cycle
EVAPORATION:
The sun's heat energy causes water to evaporate from the surfaces of oceans, lakes, rivers and soil.
Water changes from liquid to water vapour — rising into the atmosphere.
TRANSPIRATION:
Plants release water vapour through their stomata — a process called TRANSPIRATION.
In tropical rainforests, transpiration adds enormous quantities of water to the atmosphere — maintaining local rainfall patterns.
CONDENSATION:
As water vapour rises, it cools.
Cooling causes it to CONDENSE — changing from vapour back to tiny liquid water droplets.
These droplets form CLOUDS and mist.
PRECIPITATION:
When water droplets in clouds collide and join to form larger drops, they fall as PRECIPITATION — rain, snow, sleet or hail.
RUNOFF AND GROUNDWATER:
Precipitation that falls on land either:
Flows across the surface as RUNOFF — into streams and rivers → back to the sea.
SOAKS INTO the ground to form GROUNDWATER — extracted by plant roots or flowing slowly to rivers and sea.
Living Organisms and the Water Cycle
Living organisms play an important role in the water cycle:
PLANTS:
Absorb water from the soil through root hair cells.
Transport water up through xylem to leaves.
Release water vapour through stomata (transpiration) — returns water to the atmosphere.
Soil absorption is helped by the structural effects of plant roots.
ANIMALS:
Obtain water by drinking, and from food.
Return water to the environment through:
Urination and defaecation.
Exhaling water vapour.
Sweating.
HUMAN IMPACTS on the water cycle:
DEFORESTATION — removing trees reduces transpiration → less water returned to atmosphere → reduced local rainfall → increased risk of drought.
URBANISATION — impermeable surfaces (roads, buildings) prevent water soaking into soil → increased runoff → flooding.
IRRIGATION — withdrawing groundwater for farming can deplete underground reserves.
CLIMATE CHANGE — warming increases evaporation → more intense rainfall events and more severe droughts.
⚠️ Common Mistake
TRANSPIRATION is the loss of water vapour from PLANT LEAVES through stomata — it is NOT the same as evaporation from the soil or water surface. Both contribute to atmospheric water, but transpiration is specifically the biological process in plants. Also: precipitation is NOT just rain — it includes snow, sleet and hail.
📌 Key Note
Water cycle: evaporation + transpiration → condensation (clouds) → precipitation (rain/snow) → runoff/groundwater → back to sea. Plants contribute via transpiration. Deforestation reduces transpiration → less rainfall. Key: water is continuously recycled.
🎯 Matching Activity — Match the Stage of the Water Cycle
Match each stage to its correct description. — drag the symbols on the right to match the component names on the left.
Evaporation
Drop here
Transpiration
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Condensation
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Precipitation
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Runoff
Drop here
Heat energy causes water to change from liquid to vapour from oceans, lakes and soil
Water flows across the land surface into streams, rivers and eventually back to the sea
Rising water vapour cools and forms tiny liquid droplets — creating clouds
Water vapour released by plants through stomata into the atmosphere
Water droplets in clouds fall as rain, snow, sleet or hail
🎯 Test Yourself
Question 1 of 2
1. How do plants contribute to the water cycle?
2. Why does deforestation in tropical areas often lead to reduced local rainfall?
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