A1.1 Water | What the Cell

Guiding questions

Which features of water make life possible?
What does living in water make easier and harder for organisms?

Slides

1 of 17

Notes

Written for this site from the class slides. The chapter is on pages 2–10 of the course textbook. Image credits are on slide 17.

A1.1.1

Water is the medium for life

pp. 2–10

The first cells arose in water, and most processes of life still occur in it. Water makes up much of the cytoplasm, and it is the main part of blood plasma.

A1.1.2

Polar covalent bonds and hydrogen bonds

pp. 2–10

Oxygen attracts the shared electrons in its covalent bonds more strongly than hydrogen does, so oxygen has a slight negative charge, and hydrogen a slight positive charge. The molecule is polar.

The slightly positive hydrogen of one molecule attracts the slightly negative oxygen of another. This is a hydrogen bond. One is weak, but there are very many in water.

Slide: a model of water molecules joined by hydrogen bonds

A1.1.3

Cohesion

pp. 2–10

Cohesion is the attraction between water molecules, due to hydrogen bonds. It keeps a column of water intact under tension in xylem. It also causes surface tension, so a water surface can be a habitat for small animals such as water striders.

A1.1.4

Adhesion

pp. 2–10

Adhesion is the attraction between water and polar or charged materials. It causes capillary action, which draws water up through narrow channels in soil, and through cellulose in plant cell walls.

Watch out: cohesion is between water molecules, and adhesion is between water and something else.

A1.1.5

Water as a solvent

pp. 2–10

A wide variety of hydrophilic substances dissolve in water, and most enzymes catalyse reactions in aqueous solution. Water also transports substances in blood, xylem and phloem.

Substance	Solubility	How it is carried in blood
Glucose	Soluble	Dissolved in plasma
Sodium chloride	Soluble	Dissolved in plasma, as ions
Oxygen	Poorly soluble	Mostly bound to haemoglobin
Fats and cholesterol	Insoluble	Inside lipoprotein particles

Some molecules depend on being hydrophobic. For example, the core of a membrane forms a barrier, and leaf wax reduces water loss.

A1.1.6

Physical properties and aquatic animals

pp. 2–10

Property	Water compared with air	Ringed seal	Black-throated loon
Buoyancy	Much higher	Fat helps it float	Dense bones, and air sacs it can compress to dive
Viscosity	Much higher: more drag	Streamlined shape, flippers	Torpedo shape, webbed feet
Thermal conductivity	Much higher: heat lost fast	Thick blubber	Dense, oiled feathers
Specific heat	Much higher: stable temperature	A stable habitat for both

Videos

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Note-taking sheet

Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.

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Practice questions

A printable worksheet covering all six statements, from quick recall to data questions and a longer answer. It has 36 marks and takes about 35 minutes, or you can do one section at a time.

Worksheet PDF

Your teacher has the mark scheme.

Practical

Cohesion and adhesion of water

Lab30 minA1.1.3, A1.1.4

You will need: Coins, pipettes, water, cooking oil, thin glass tubes, wax paper

Count how many drops of water fit on a coin before they spill, then repeat with oil
Compare the shape of water drops on wax paper and on clean glass
Stand a narrow tube in water and observe the rise
Explain each result using hydrogen bonds

Safety: Low risk. Take care with glass tubes, and wipe up spills.

Carry this out with your teacher's supervision and follow your school's lab rules.

Nature of science and TOK

Nature of science

Falsification

In the late 1960s, scientists reported polywater, an unusual form of water. It was later traced to contamination.

Theory of knowledge

How does science correct its own mistakes?

Researchers around the world tested the claim and rejected it. Consider what this says about the value of replication.

Syllabus statements

Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.

A1.1.1Why life depends on water as its mediump. 2
A1.1.2How polar covalent bonds in water give rise to hydrogen bonds between moleculesp. 3
A1.1.3How hydrogen bonds make water molecules stick together, and why that cohesion matters to living thingsp. 5
A1.1.4How water clings to polar or charged surfaces, and what that adhesion does for organismsp. 7
A1.1.5Water as a solvent: how it supports metabolism and carries substances in plants and animalsp. 8
A1.1.6How water's physical properties affect animals living in aquatic habitatsp. 9

HL extension

AHL · Biomolecules