Water is a polar molecule made of one oxygen atom and two hydrogen atoms. Its polarity allows water molecules to form hydrogen bonds with each other, giving water unique properties important for life. Water can exist in three phases - gas, liquid, and solid - and its ability to absorb large amounts of heat helps regulate Earth's temperature. Water's polarity and hydrogen bonding allow it to dissolve more substances than any other liquid, making it essential for biological functions and chemical reactions in organisms.

1. FUNDAMENTAL BIOLOGY
Ms.L.G Simelane

2. Clitheroe, F & Dilley, L.
(2010),
Natural Sciences
Learner’s Book, Grade 9.
Oxford University Press.
ISBN: 9780195762945

3. UNIT 1: THE ROLE OF
WATER

4.  Water is a chemical substance with
the chemical formula H2O.
 Its molecule contains one oxygen and two
hydrogen atoms connected by
covalent bonds.
 Water molecules are
connected by Hydrogen
bonds.

6. The polarity of water molecules
results in hydrogen bonding
• Water = unique properties that allows it to support
& maintain living systems
• – Due to its structure & interaction with other
molecules
• The water molecule is a polar molecule: The
opposite ends have opposite charges
 Polarity allows water molecules to form
hydrogen bonds with each other
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. Fig. 3-2
Hydrogen
bond
–
H
+
H
O
+
+
+
–
–
–
Slightly + H of 1 molecule
attracted to slightly – O of
nearby molecule –
Hydrogen bond keeps
them together.
In liq water the bond is very
weak -
Continually reforming &
breaking

8. Four properties of water contribute
to Earth’s fitness for life
• 4 of water’s properties that facilitate an
environment for life are:
 1. Cohesive behavior (predictable behavior)
 2. Ability to moderate temperature
 3. Expansion upon freezing
 4. Versatility as a solvent (dissolve various
substances)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. ¾ of the Earth’s surface is
covered by water of the
oceans.
Every cell consists of 50 –
90% water.
Needed to transport
molecules in & out cells.
Many organisms live in water.

10. Water can be found in 3 phases:
1. Gasses
2. Liquids
3. Solids

11.  Consist of water molecules that move around
freely:
 A lot of energy and space between molecules.
 Attraction between molecules very weak.
 Called water vapour.
 Found in atmosphere.

12.  When temperature increases, it causes liquid
water to change to water vapour
(evaporate).

13.  Water molecules move around less
than gasses but more than solids.
 More energy than solids but less
than gas.
 Attraction forces between
molecules is stronger than gas
molecules but weaker than solids.
 It takes on the shape of the
container it is in.
 Found everywhere.
 If solid water is heated, liquid water
forms.

14.  Solid water molecules are called ice.
 Molecules only vibrate at one spot.
Minimal amount of energy but very
strong attraction forces between
molecules.
 Molecules arrange in a structured way
called a crystal lattice.
 Found everywhere.
 If liquid water is cooled down it
becomes a solid.

16.  Water has a high heat capacity
 Amount of heat required to raise the temperature of 1g of
water by 1˚C.
 Prevents water from freezing or boiling if atmospheric
temperature changes protecting aquatic organisms.
 Water has a high heat of evaporation
 Amount of heat required to vaporize a liquid.
 Helps animals, they sweat, water evaporates when they
overheat, to cool them down.
 Water has a high heat of fusion
 Amount of heat required to melt a solid.

18.  More things can dissolve in water than any
other liquid.
 The liquid that a substance dissolves in is
called a solvent.
 Blood contains 55% water called blood
plasma.
 Gasses have to dissolve in the blood plasma
and then diffuse into the red blood cells to
transport the oxygen to the body cells and
and carbon dioxide to the lungs.

20.  All chemical reactions to control
metabolism take place in water. E.g.
Digestion, breathing etc.
 Water takes part in some chemical
reactions e.g. Photosynthesis
 What is photosynthesis? The process
whereby plants use water, sunlight and
carbon dioxide to produce food and
oxygen.
 Enzymes speed up chemical reactions
in the presence of water.

21.  Water helps transport substances in the
body of a plant or animal:
 E.g. In humans, food and gasses are
transported via the blood circulatory
system (consisting of mainly water).
 E.g. In plants, mineral salts, absorbed
from the soil are dissolved in water and
carried to all parts of the plant.

22.  Most organisms need water to
reproduce.
 Mosses and ferns can only
reproduce if they are covered with
a thin film of water.
 All animals need water to
reproduce, the sperm needs water
to swim to the ova (egg cell).
 Aquatic animals reproduce in the
water.

23.  Vertebrates (like humans)have a bony
skeleton to support the shape of their
bodies.
 Invertebrates do not consist of bones,
most of them have water that helps to
support the body shape, called a
hydrostatic skeleton.
 Water pushes against the sides of the body,
exerting pressure to maintain the shape. E.g.
Worms and snails.
 Xylem tubes together with water helps
plants stay upright.
 (Think about a plant deprived of water)

24.  Humans use “water” to cool down.
 This water is known as sweat.
(perspiring)
 Sweat is produced by sweat glands.
 When the sweat evaporates from the
body, it cools down.
 This happens during strenuous exercise,
stress and illness.

25.  Plants lose water through their
leaves.
 Process of transpiration.
 Transpiration helps to cool the
plant down.
 What is transpiration? Loss of
water in the form of water
vapour through openings called
stomata, on the leaves of
plants.
 Helps plants suck water up stem
to leaves
STOMATA
Cross section through leaf to see

26.  Cover pot plant/branch of plant
with a plastic bag
 Seal bag with a string.
 Leave bag for few hours.
 Observe
 Result: Water droplets form on
the sides of the plastic bag.
 What happened? Transpiration
occurred
 You can try this at home