DP Biology HL D2.3 Water Potential

DP Biology HL D2.3 Water Potential - Full notes

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DP Biology HL | D2.3 Water Potential

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D2.3

Water Potential

Full notes for HL · IB-style

Contents

1	Solvation and the movement of water
2	Osmosis and solute concentration
3	Hypotonic, isotonic and hypertonic solutions
4	Water movement in animal and plant cells
5	Estimating solute concentration by experiment
6	Applications: isotonic solutions in medicine
7	Water potential: solute and pressure potential (HL only)
8	Solutes and solvents
9	Hypotonic, isotonic and hypertonic
10	Effects on plant cells
11	Medical uses of isotonic solutions
12	Estimating solute concentration of tissue
13	Percentage change calculations
14	Water potential: concept
15	Solute potential and pressure potential
16	Turgor and support in plants
17	Osmoregulation in organisms
18	Water potential in xylem
19	Mixed review: worked problems
20	Additional exam-style practice
21	Water potential and food preservation
22	Kidneys and osmoregulation (link)
23	Practical: plasmolysis and incipient plasmolysis
24	Common misconceptions and links
25	Exam-style question with markscheme
26	Check your understanding
27	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain solvation and why water moves by osmosis
explain the effect of solute concentration on osmosis
use the terms hypotonic, isotonic and hypertonic correctly
explain the effects of osmosis on animal and plant cells, including turgor and plasmolysis
estimate the solute concentration of tissue from experimental data
explain the use of isotonic solutions in medicine
define water potential and explain solute potential and pressure potential
calculate water potential and predict the direction of water movement

Sections marked HL only (Section 7) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

What factors determine the movement of water into or out of cells? Why do plant cells become turgid while animal cells burst?

How this topic is examined

D2.3 is examined through osmosis experiments (potato or beetroot cylinders in different solutions), graphs of percentage change in mass, and questions about cells in different solutions. HL questions require water potential calculations.

Always state the direction of water movement relative to solute concentration or water potential.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Solvation and the movement of water

Solutes hold water molecules.

Think about it

Why do slugs die if you sprinkle salt on them? Where does the water go?

Water is an excellent solvent (A1.1). When a polar or charged solute dissolves, water molecules cluster around each solute particle, forming a hydration shell: this is solvation. Water molecules held in hydration shells are less free to move.

So in a solution with a higher solute concentration, a smaller proportion of water molecules are free to move. Water moves by random motion from where more molecules are free (lower solute concentration) to where fewer are free (higher solute concentration). This net movement across a partially permeable membrane is osmosis.

2 Osmosis and solute concentration

Water moves towards the more concentrated solution.

Osmosis is the net movement of water across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration. It is a passive process (no ATP) and occurs through the phospholipid bilayer and through aquaporins (B2.1).

The membrane is permeable to water but not to the solutes, so the solutes cannot move to even out the concentrations; only water moves. Osmosis continues until the solute concentrations on both sides are equal, or until pressure (such as a plant cell wall) prevents further entry.

Common mistake

Saying solute moves in osmosis, or that water moves from 'high concentration to low concentration' without saying of what. Be explicit: water moves to the region of higher solute concentration.

3 Hypotonic, isotonic and hypertonic solutions

Tonicity compares a solution with the inside of a cell.

Hypotonic solution: lower solute concentration than the cell. Water enters the cell by osmosis. Isotonic solution: same solute concentration as the cell. No net movement of water. Hypertonic solution: higher solute concentration than the cell. Water leaves the cell by osmosis.

Solution	Compared with cell	Net water movement
Hypotonic	lower solute concentration	into the cell
Isotonic	equal	none
Hypertonic	higher solute concentration	out of the cell

4 Water movement in animal and plant cells

The cell wall makes all the difference.

Animal cells have no cell wall. In a hypotonic solution, water enters, the cell swells and may burst (lyse); red blood cells in distilled water haemolyse. In a hypertonic solution, water leaves and the cell shrinks (red blood cells become crenated). Animals must therefore keep their tissue fluid isotonic with their cells (osmoregulation, D3.3).

Plant cells have a strong cellulose cell wall. In a hypotonic solution, water enters, the vacuole and cytoplasm swell, and the cell pushes against the wall. The wall resists further expansion, generating turgor pressure; the cell becomes turgid but does not burst. Turgor supports non-woody plants: when cells lose turgor, the plant wilts. In a hypertonic solution, water leaves, the vacuole shrinks, and the plasma membrane pulls away from the wall: the cell is plasmolysed. The space between wall and membrane fills with the external solution, since the wall is freely permeable.

Freshwater protists such as Paramecium live in a hypotonic environment and expel incoming water using contractile vacuoles, which uses ATP.

Solution	Animal cell	Plant cell
Hypotonic	swells, may burst (lysis)	turgid; wall prevents bursting
Isotonic	normal	flaccid
Hypertonic	shrinks (crenation)	plasmolysed

Quick check

1. Why don't plant cells burst in water?

2. Define plasmolysis.

Answers: 1. the cell wall resists expansion, generating turgor pressure 2. membrane pulls away from the wall as the cell loses water

5 Estimating solute concentration by experiment

Find the concentration that causes no change.

The solute concentration of a plant tissue can be estimated by placing samples (such as potato cylinders of equal size) in a series of solutions of known concentration (for example 0.0 to 0.8 mol dm⁻³ sucrose). After a set time, each sample is blotted and reweighed, and the percentage change in mass is calculated.

Samples in hypotonic solutions gain mass; samples in hypertonic solutions lose mass. Plotting percentage change against concentration and finding where the line crosses zero gives the concentration that is isotonic with the tissue.

Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100

Figure 1. Percentage change in mass of potato cylinders.

WORKED EXAMPLE 1 Estimating isotonic concentration

A potato cylinder of mass 2.50 g has a mass of 2.65 g after 24 hours in 0.2 mol dm⁻³ sucrose. Calculate the percentage change, and use the graph to estimate the isotonic concentration.

Solution

% change = (2.65 − 2.50) ÷ 2.50 × 100 = +6.0%M1 A1

The line crosses zero at about 0.35 mol dm⁻³: the solute concentration of the potato cellsA1

Using percentage change (rather than absolute change) allows comparison of samples of different starting mass. Controls include using the same size samples, the same time, the same temperature, and blotting consistently.

Quick check

1. Why is percentage change used?

2. What does the x-intercept show?

Answers: 1. samples have different starting masses 2. the isotonic concentration

6 Applications: isotonic solutions in medicine

Fluids for patients and organs must match cell solute concentration.

Fluids given to patients by intravenous drip, and solutions used to bathe tissues and organs, must be isotonic with body cells, so cells neither swell and burst nor shrink. Normal saline (0.9% sodium chloride) is isotonic with human blood plasma.

Organs for transplantation (such as kidneys) are stored in cold, isotonic solutions to prevent osmotic damage until they are transplanted. Eye drops and wound washes are also isotonic.

7 Water potential: solute and pressure potential (HL only)

A single measure predicts the direction of water movement.

HL only

This section is Additional Higher Level content. SL students do not need it.

Water potential (Ψ) is the potential energy of water per unit volume, relative to pure water at atmospheric pressure, which is defined as 0 kPa. Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential.

Water potential has two main components: Ψ = Ψs + Ψp. The solute potential (Ψs) is always negative (or zero for pure water): adding solutes lowers the water potential. The more concentrated the solution, the more negative Ψs. The pressure potential (Ψp) is the effect of pressure: positive pressure (such as turgor pressure in a plant cell) raises water potential; negative pressure (tension, as in xylem) lowers it.

In a plant cell placed in pure water, water enters because the cell's Ψ is lower than 0. As water enters, turgor pressure (Ψp) increases, raising the cell's Ψ, until it equals 0, when net movement stops. In a plasmolysed cell, Ψp = 0, so Ψ = Ψs.

WORKED EXAMPLE 2 Water potential calculations

A plant cell has Ψs = −800 kPa and Ψp = +300 kPa. It is placed in a solution with Ψ = −700 kPa. Calculate the cell's water potential and predict the direction of water movement.

Solution

Ψ cell = Ψs + Ψp = −800 + 300 = −500 kPaM1 A1

Solution Ψ (−700 kPa) is lower than the cell's (−500 kPa)R1

So water moves out of the cell into the solutionA1

Quick check

1. What is the water potential of pure water?

2. Why is solute potential always negative?

3. A cell has Ψs = −600 kPa and Ψp = +200 kPa. What is Ψ?

Answers: 1. 0 kPa 2. solutes lower the potential energy of water 3. −400 kPa

8 Solutes and solvents

Water as a solvent.

A solution consists of solutes dissolved in a solvent (in cells, water). Solute concentration is measured in mol dm−3 or as osmolarity. Water moves by osmosis in response to differences in solute concentration across partially permeable membranes.

WORKED EXAMPLE 3 Concentration (basic)

Calculate the concentration of 0.5 mol of sucrose in 2.0 dm3.

Solution

0.5/2.0M1

= 0.25 mol dm−3A1

9 Hypotonic, isotonic and hypertonic

Comparing solutions.

A hypotonic solution has a lower solute concentration than the cell; water enters the cell. An isotonic solution has the same concentration: no net movement. A hypertonic solution has a higher concentration: water leaves the cell.

WORKED EXAMPLE 4 Animal cells (standard)

Predict what happens to red blood cells in hypotonic and hypertonic solutions.

Solution

Hypotonic: swell and burst (haemolysis)A1

Hypertonic: shrink (crenation)A1

10 Effects on plant cells

Turgor and plasmolysis.

In hypotonic solutions, plant cells take in water and become turgid; the cell wall prevents bursting. In hypertonic solutions, water leaves; the membrane pulls away from the wall (plasmolysis). Wilting occurs when cells lose turgor.

WORKED EXAMPLE 5 Plasmolysis (standard)

Describe what happens to an onion epidermis cell in strong salt solution.

Solution

Water leaves by osmosisR1

Vacuole shrinks; membrane pulls away from wall (plasmolysis)R1

Try this

Mount red onion epidermis in distilled water and in 1 mol dm−3 salt solution and observe plasmolysis under a microscope.

11 Medical uses of isotonic solutions

Saline drips and organ storage.

Intravenous fluids (0.9% saline) are isotonic with blood plasma so that blood cells are not damaged. Donor organs are stored in cold isotonic solutions to prevent osmotic damage.

Quick check

1. Why must donor organs be bathed in isotonic solution?

Answers: 1. prevents cells gaining or losing water by osmosis

12 Estimating solute concentration of tissue

Potato cylinder experiment.

Cylinders of potato are placed in sucrose solutions of different concentrations. Percentage change in mass is plotted against concentration; the point where the line crosses zero gives the concentration isotonic with the tissue.

Figure 2. Percentage change in mass of potato cylinders.

WORKED EXAMPLE 6 Isotonic point (exam level)

Estimate the sucrose concentration isotonic with the potato tissue.

Solution

Line crosses zero at about 0.42 mol dm−3A1

No net water movement at this concentrationR1

Quick check

1. Why is percentage change in mass used rather than change in mass?

Answers: 1. cylinders may have different starting masses

13 Percentage change calculations

Processing data.

Percentage change = (final mass − initial mass)/initial mass × 100. Positive values show water gain; negative values water loss.

WORKED EXAMPLE 7 Calculation (standard)

A cylinder's mass changes from 2.40 g to 2.16 g. Calculate the percentage change.

Solution

(2.16 − 2.40)/2.40 × 100M1

= −10%A1

14 Water potential: concept

Tendency of water to move.

Water potential (ψ) is the potential energy of water per unit volume relative to pure water at atmospheric pressure (ψ = 0 kPa). Water moves from higher (less negative) to lower (more negative) water potential.

WORKED EXAMPLE 8 Direction (standard)

Cell A has ψ = −400 kPa; cell B has ψ = −700 kPa. Which way does water move?

Solution

From A to B (higher to lower ψ)A1

15 Solute potential and pressure potential

ψ = ψs + ψp.

Water potential has two components: solute potential (ψs, always negative, more negative with more solute) and pressure potential (ψp, usually positive in turgid plant cells due to the wall pushing back; negative under tension in xylem).

WORKED EXAMPLE 9 Calculation (exam level)

A plant cell has ψs = −800 kPa and ψp = +300 kPa. Calculate ψ and predict water movement into a solution of ψ = −600 kPa.

Solution

ψ = −800 + 300 = −500 kPaA1

Water moves out of the cell (−500 to −600)R1

Quick check

1. A plasmolysed cell has ψp = 0 and ψs = −900 kPa. What is ψ?

2. Find ψp for a cell with ψ = −200 kPa and ψs = −650 kPa.

Answers: 1. −900 kPa 2. +450 kPa

16 Turgor and support in plants

Hydrostatic skeletons.

Turgor pressure pushes cell membranes against walls, keeping non-woody plants upright. Guard cells change turgor to open and close stomata. Loss of turgor causes wilting.

Quick check

1. Explain how guard cells open stomata.

Answers: 1. potassium ions enter, lowering ψ; water enters; cells become turgid and bow apart

17 Osmoregulation in organisms

Freshwater and marine.

Freshwater protists (Paramecium) use contractile vacuoles to expel excess water. Freshwater fish excrete large volumes of dilute urine; marine fish drink seawater and excrete salt through gills. Osmoconformers (many marine invertebrates) match their surroundings.

WORKED EXAMPLE 10 Contractile vacuole (standard)

Explain why freshwater protists need contractile vacuoles.

Solution

Water enters by osmosis from hypotonic surroundingsR1

Vacuole expels excess water, preventing burstingR1

18 Water potential in xylem

Negative pressure.

Transpiration creates tension in the xylem, so ψp is negative. Water potential decreases from soil to root to leaf to atmosphere, which drives water movement through the plant.

Quick check

1. Arrange in order of decreasing water potential: leaf, soil, atmosphere, root.

Answers: 1. soil > root > leaf > atmosphere

19 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 11 Mixed problem (exam level)

Explain why plant cells do not burst in pure water but animal cells do.

Solution

Both take in water by osmosisR1

Plant cell wall resists expansion: pressure potential builds upR1

Animal cells lack a wall and burstR1

20 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 12 Paper 1 question (standard)

What is the water potential of pure water at atmospheric pressure? A. −100 kPa B. 0 kPa C. +100 kPa D. depends on temperature only

Solution

BA1

WORKED EXAMPLE 13 Paper 2 question (exam level)

Describe how to determine the water potential of potato tissue.

Solution

Cylinders in a range of sucrose solutions; same size; blottedR1

Measure % mass change after a set time; plot graphR1

Find concentration of zero change; convert to ψR1

21 Water potential and food preservation

Salt and sugar.

High concentrations of salt or sugar (jam, salted fish) lower water potential, drawing water out of microorganisms by osmosis and preventing their growth.

Quick check

1. Why does jam not go mouldy quickly?

Answers: 1. high sugar concentration draws water out of microbes

22 Kidneys and osmoregulation (link)

Controlling body water.

Kidneys regulate blood water potential: ADH increases water reabsorption when the blood is concentrated. Desert animals have long loops of Henle to produce very concentrated urine.

Quick check

1. What happens to urine when ADH levels rise?

Answers: 1. smaller volume, more concentrated

23 Practical: plasmolysis and incipient plasmolysis

Estimating solute potential.

Incipient plasmolysis occurs when 50% of cells are plasmolysed; at this point ψp = 0, so the cells' solute potential equals that of the external solution.

WORKED EXAMPLE 14 Incipient plasmolysis (standard)

Why does incipient plasmolysis give the solute potential of the cells?

Solution

Pressure potential is zeroR1

So ψ of cells equals ψs, which equals the external ψR1

24 Common misconceptions and links

Students often think…	Actually…
water moves from high to low solute concentration	water moves to the higher solute concentration
plant cells burst in water	the wall prevents bursting
isotonic means no water movement at all	there is no net movement; molecules still move both ways
higher water potential means more negative	higher water potential is less negative

How D2.3 connects to the rest of the course

Topic	Connection
A1.1 Water	solvation and hydrogen bonding
B2.1 Membranes (HL)	aquaporins and osmosis
B3.2 Transport	root uptake and xylem
D3.3 Homeostasis	osmoregulation

25 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 3. Percentage change in mass of potato cylinders.

(a) Calculate the percentage change in mass for a cylinder that went from 3.20 g to 2.92 g. [2]

(b) Estimate the isotonic concentration from the graph. [1]

(d) Explain why red blood cells burst in distilled water but plant cells do not. [3]

(e) A cell has Ψs = −900 kPa and Ψp = +350 kPa. State its water potential and the direction of water movement into a solution of Ψ = −400 kPa. [2]

Markscheme

Part	Answer	Marks
(a)	(2.92 − 3.20) ÷ 3.20 × 100	M1
	= −8.75%	A1
(b)	about 0.35 mol dm⁻³	A1
(c)	solution hypertonic	R1
	water leaves cells by osmosis	R1
	mass decreases; cells may plasmolyse	R1
(d)	water enters both by osmosis	R1
	red cells have no wall; swell and lyse	R1
	plant wall resists; turgor pressure	R1
(e)	Ψ = −550 kPa	A1
	water moves into the cell (higher Ψ outside)	A1

Examiner's comments. In (c), answers referring to 'sugar entering' scored nothing: water leaves by osmosis.

26 Check your understanding

Section A: multiple choice

1. In a hypotonic solution, a red blood cell

A. swells and may burst B. shrinks C. stays the same D. plasmolyses

2. A plasmolysed cell has

A. burst B. high turgor C. membrane pulled away from the wall D. a larger vacuole

3. Osmosis is the movement of

A. solute B. water C. ions D. glucose

4. Normal saline is

A. hypertonic B. hypotonic C. pure water D. isotonic with blood

5. A potato in 0.0 mol dm⁻³ sucrose will

A. lose mass B. gain mass C. stay the same D. dissolve

6. Wilting occurs when plant cells

A. lose turgor B. gain water C. burst D. become turgid

7. If Ψs = −700 kPa and Ψp = +200 kPa, Ψ is

A. −900 kPa B. +500 kPa C. −200 kPa D. −500 kPa

8. Water moves from

A. lower to higher water potential B. −1000 to −200 kPa C. higher to lower water potential D. cells to pure water

Section B: structured

9. Explain osmosis in terms of solute concentration. [3]

10. Describe how to find the solute concentration of potato tissue. [4]

11. Explain why isotonic solutions are used for organ storage. [2]

12. Explain how turgor pressure affects water potential. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Partially permeable membrane; water moves from lower to higher solute concentration; solutes held in hydration shells.

10. Cylinders in a range of solutions; weigh before and after; % change; plot; find x-intercept.

11. No net osmosis; cells neither swell and burst nor shrink.

12. Water entering raises turgor pressure (Ψp); raises Ψ; net entry stops when Ψ equals that outside.

27 Summary and topic map

Idea	Remember
Solvation	hydration shells reduce free water
Osmosis	water to higher solute concentration
Tonicity	hypotonic, isotonic, hypertonic
Animal cells	lysis or crenation
Plant cells	turgid or plasmolysed
Experiment	% change in mass; x-intercept isotonic
Medicine	isotonic saline; organ storage
HL	Ψ = Ψs + Ψp; water moves to lower Ψ

Before the exam, make sure you…

•state direction of water movement explicitly
•use tonicity terms relative to the cell
•calculate % change in mass
•explain turgor and plasmolysis

Water Potential on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 4. Topic map for Water Potential.

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