GCE Biology CIE 5090

3.1 Diffusion and osmosis

CONTENTS

1 Describe the role of water as a solvent in organisms with reference to

digestion, excretion and transport
2 Understand that the energy for diffusion and osmosis comes from the
kinetic energy of random movement of molecules and ions
3 Understand diffusion as the net movement of molecules or ions from a
region of their higher concentration to a region of their lower concentration
(i.e. down a concentration gradient), as a result of their random movement
4 Investigate the factors that influence diffusion, limited to: surface area,
temperature, concentration gradient and distance
5 Understand osmosis as the net movement of water molecules from a region
of higher water potential to a region of lower water potential, through a
partially permeable membrane
6 Understand that plants are supported by the pressure of water inside the
cells pressing outwards on the cell wall
7 Describe the effects of osmosis on plant and animal tissues and explain the
importance of water potential gradient and osmosis in the uptake and loss of
water
8 Investigate and explain the effects on plant tissues of immersing them in
solutions of different concentrations, using the terms turgid, turgor pressure,
plasmolysis and flaccid
9 Investigate osmosis using materials such as dialysis tubing
3.1 Diffusion and osmosis

3.1 Diffusion and Osmosis

 Role of Water as a Solvent: Water is essential for life as it is a universal

solvent. This means many substances can dissolve in it.
o Digestion: Digestive enzymes work in solution, and digested food
molecules are absorbed in a dissolved state.
o Excretion: Metabolic waste products, such as urea, are dissolved in
water to form urine.
o Transport: Dissolved substances like glucose, amino acids, and
mineral ions are transported around an organism in the blood (in
animals) or xylem and phloem (in plants).
 Source of Energy: The energy for both diffusion and osmosis comes from
the kinetic energy of the random movement of molecules and ions. No
additional energy from the organism is required.

Diffusion

Definition: Diffusion is the net movement of molecules or ions from

a region of their higher concentration to a region of their lower
concentration. This is described as moving down a concentration

Paper 2 Tip: The definition must include the phrase "net movement".
While particles move randomly in both directions, the overall, or net,
movement is from high to low concentration. You must also mention
moving from a higher to a lower concentration.

gradient.

 Factors Influencing Diffusion: The rate of diffusion is increased

by:
1. Higher Temperature: Particles have more kinetic energy
and move faster.
2. Larger Surface Area: Provides more space for particles to
move across (often a larger surface area to volume ratio is
key).
3. Steeper Concentration Gradient: A larger difference in
concentration between two areas results in a faster net
movement.
4. Shorter Diffusion Distance: Particles will diffuse more
quickly over a shorter distance.
Osmosis

 Definition: Osmosis is the net movement of water molecules from

a region of higher water potential to a region of lower water
potential, through a partially permeable membrane.

Paper 1 Tip: Be very precise with the definition. It must include "water
molecules", "higher to lower water potential", and "partially permeable
membrane". Confusing water potential with concentration will lose
marks. A low water potential means a high solute concentration
(concentrated solution).

 Osmosis in Animal Cells:

o In a solution with a higher water potential (e.g., pure water),
animal cells will gain water by osmosis, swell, and burst (lysis)
because they have no cell wall.
o In a solution with a lower water potential (e.g., concentrated
salt solution), they will lose water, shrink, and
become crenated.
 Osmosis in Plant Cells:
o In a solution with a higher water potential, plant cells gain
water and become swollen and firm. The cell is described
as turgid. The outward pressure on the cell wall is
called turgor pressure. The cell wall prevents it from bursting.
This turgor pressure provides support for the plant.
o In a solution with a lower water potential, plant cells lose
water. The vacuole and cytoplasm shrink, and the cell
membrane pulls away from the cell wall. The cell is described
as flaccid

 Osmosis in Animal Cells:

o In a solution with a higher water potential (e.g., pure water),
animal cells will gain water by osmosis, swell, and burst (lysis)
because they have no cell wall.
o In a solution with a lower water potential (e.g., concentrated
salt solution), they will lose water, shrink, and
become crenated.
 Osmosis in Plant Cells:

o In a solution with a higher water

Paper 4 Tip: When describing
potential, plant cells gain water and an osmosis experiment (e.g.,
become swollen and firm. The cell is placing potato strips in
described as turgid. The outward different solutions), you must
pressure on the cell wall is state the variables to control.
Key control variables are: using
potato strips from the same
potato, cutting them to
the same initial length/mass,
and keeping them in the
solutions for the same amount
 called turgor pressure. The cell wall prevents it from bursting.
This turgor pressure provides support for the plant.
o In a solution with a lower water potential, plant cells lose
water. The vacuole and cytoplasm shrink, and the cell
membrane pulls away from the cell wall. The cell is described
as flaccid and the process is called plasmolysis.

Past Paper Practice (Topic 3.1)

Question 1 (MCQ):
What is the definition of osmosis?
A. the net movement of particles from a region of their higher
concentration to a region of their lower concentration
B. the net movement of water molecules from a region of higher water
potential to a region of lower water potential through a partially
permeable membrane
C. the movement of water molecules from a region of lower water
potential to a region of higher water potential through a partially
permeable membrane
D. the movement of particles from a region of their lower concentration to
a region of their higher concentration using energy

 Solution: B
o Explanation: This is the precise definition of osmosis. Option
A is the definition of diffusion. Option C has the direction of
movement wrong. Option D is the definition of active
transport.

Question 2 (MCQ):
A plant cell is placed in a solution with a lower water potential than the
cell contents. What will happen to the cell?
A. The cell will become turgid.
B. The cell will become flaccid and plasmolysed.
C. The cell will burst.
D. There will be no change to the cell.

 Solution: B
o Explanation: Placing a cell in a solution with a lower water
potential means water will move out of the cell by osmosis.
This causes the cell to lose turgor and become flaccid. The
membrane will pull away from the wall (plasmolysis).
Question 3 (Paper 2):
The red blood cells in the 0.10 mol dm−3 salt solution burst. Explain why
the red blood cells burst? [3]

 Solution:
1. Distilled solution or 0.10 mol dm-3 salt solution has a higher
water potential than the cytoplasm of the red blood cells.
2. Water moves by osmosis from the region of higher water
potential (the distilled water) to the region of lower water
potential (inside the cell), down a water potential gradient.
3. The red blood cell does not have a cell wall, so it cannot
withstand the increase in pressure and bursts.

Examiner's View (P2): The 3 marks are awarded for:

4. Correctly identifying the water potential gradient (high in

water, low in cell).
5. Stating that water moves into the cell by osmosis.
6. Explaining that the cell bursts because it has no cell wall to
withstand the pressure.
3.2 Active transport

CONTENTS
1 Understand active transport as the movement of molecules or ions into or
out of a cell through the cell membrane, from a region of their lower
concentration to a region of their higher concentration (i.e. against a
concentration gradient), using energy released during respiration
2 Explain the importance of active transport in ion uptake by root hair cells
3.2 Active transport

3.2 Active Transport

 Definition: Active transport is the movement of molecules or ions

into or out of a cell through the cell membrane, from a region of
their lower concentration to a region of their higher concentration.
This is against a concentration gradient.

 Energy Requirement: This process requires energy, which is

released by the cell during respiration. It uses special carrier
proteins in the cell membrane to move the substances.

Paper 1 Tip: The key features that distinguish active transport are
that it moves substances against a concentration gradient and that
it requires energy. Diffusion and osmosis do not require energy from
the cell.

 Importance in Plants: Root hair cells absorb mineral ions (e.g.,

nitrates) from the soil. The concentration of these ions is often much
higher inside the root hair cell than in the soil water. Therefore, the
cell must use active transport to pump the ions into the root against
the concentration gradient. This is why root hair cells have many
mitochondria—to provide the energy from respiration needed for
active transport.
Past Paper Practice (Topic 3.2)

Question 1 (MCQ): [Nov 2020, Paper 12, Q5]

Which process requires energy from respiration?
A. movement of carbon dioxide into a leaf
B. movement of ions into a root hair cell
C. movement of oxygen into a red blood cell
D. movement of water into a root hair cell

 Solution: B
o Explanation: The movement of ions into a root hair cell from
the soil occurs by active transport, as the concentration of
ions is usually higher in the cell than in the soil. Active
transport requires energy from respiration. A, C, and D are all
examples of diffusion or osmosis, which are passive
processes.

Question 2 (MCQ): [June 2019, Paper 12, Q6]

The table shows the concentration of a mineral ion in a plant root and in
the soil.

Location Concentration of mineral ion /

ppm
Plant 80
root
Soil 5

By which process does the plant root absorb this mineral ion?
A. active transport
B. diffusion
C. osmosis
D. translocation

 Solution: A
o Explanation: The concentration of the ion is much higher
inside the root than in the soil. To move the ion from a lower
concentration (soil) to a higher concentration (root), the cell
must move it against the concentration gradient. This process
is active transport.
Past Paper Practice (Topic 2.2)

Question 1 (MCQ): [June 2021, Paper 12, Q2]

Which type of organism has a body consisting of a mycelium of hyphae?
A. an animal
B. a bacterium
C. a fungus
D. a plant

 Solution: C
o Explanation: This is a key structural feature of the Fungi
kingdom. The network of thread-like hyphae is called a
mycelium.

Question 2 (MCQ): [Nov 2021, Paper 12, Q3]

The diagram shows an animal.

Using the dichotomous key, what is this animal?

1 (a) has six legs......................... go to 2
(b) has eight legs...................... go to 3
2 (a) has wings............................. A
(b) has no wings....................... B
3 (a) has a segmented body........ C
(b) has an unsegmented body... D

 (The diagram provided would show an insect like a beetle or

fly)
 Solution: A (assuming the diagram showed a winged insect).
o Explanation: This tests the use of a key and knowledge of
arthropod features. An insect has six legs (go to 2). If it has
wings, it is organism A. This is a typical dichotomous key
question.

Question 3 (Paper 2): [Based on 5090 Syllabus]

State two structural features that are present in viruses but not in
bacteria. [2]

 Solution: Viruses do not have any features that are not also
present in bacteria in some form. The question is likely intended to
be reversed.
  Revised Question: State two structural features that are present
in bacteria but not in viruses.
 Solution:
1. Cell wall
2. Cell membrane / Cytoplasm / Ribosomes / Plasmids

Examiner's View (P2): The key difference is that viruses are non-
cellular. Therefore, any cellular component (cell wall, cell membrane,
cytoplasm, ribosomes) is a valid feature that bacteria have and viruses
lack.