Transport Across Membranes: Diffusion and Osmosis

One of the most fundamental functions of the cell membrane is to control the movement of substances into and out of the cell. The Edexcel A-Level Biology specification (9BI0) requires a thorough understanding of passive transport mechanisms — diffusion, facilitated diffusion and osmosis — including the factors that affect their rates and how to investigate them experimentally.

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Diffusion

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process, meaning it does not require metabolic energy (ATP).

Diffusion occurs because all molecules in a liquid or gas are in constant random motion due to their kinetic energy. Although individual molecules move randomly, the overall net movement is from high to low concentration until dynamic equilibrium is reached (where molecules continue to move, but there is no net movement in either direction).

Simple Diffusion

Simple diffusion refers to the direct movement of small, non-polar molecules through the phospholipid bilayer. No membrane proteins are required.

Molecules that can diffuse directly through the bilayer include:

• Oxygen (O$_2$2​) — small, non-polar
• Carbon dioxide (CO$_2$2​) — small, non-polar
• Water (H$_2$2​O) — although polar, water molecules are small enough to pass slowly through the bilayer
• Ethanol and other small lipid-soluble molecules
• Steroid hormones — lipid-soluble, can pass through the hydrophobic core

Molecules that cannot easily diffuse through the bilayer:

• Ions (e.g. Na$^+$+, K$^+$+, Cl$^-$−) — charged particles
• Glucose and other large polar molecules
• Amino acids — polar and relatively large
• Proteins — far too large

Exam Tip: When explaining why a substance can or cannot cross the membrane by simple diffusion, always refer to the hydrophobic core of the phospholipid bilayer. Polar and charged molecules are repelled by this hydrophobic region.

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Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules or ions across the membrane through specific transport proteins. Like simple diffusion, it moves substances down their concentration gradient and requires no ATP.

There are two types of transport protein involved:

Channel Proteins

• Form a hydrophilic pore (channel) through the membrane
• Allow specific ions or small polar molecules to pass through
• Many are gated — they can open or close in response to a stimulus:
  • Voltage-gated channels — respond to changes in electrical potential (e.g. sodium and potassium channels in neurons)
  • Ligand-gated channels — respond to the binding of a specific molecule (e.g. acetylcholine receptor at the neuromuscular junction)
• Channel proteins do not change shape during transport

Carrier Proteins

• Bind to a specific molecule on one side of the membrane
• Change shape (undergo a conformational change) to release the molecule on the other side
• Involved in the transport of larger polar molecules such as glucose and amino acids
• Carrier proteins are specific — each carrier transports only one type of molecule (or a closely related group)

Feature	Channel proteins	Carrier proteins
Mechanism	Form a pore/channel	Bind, change shape, release
Speed	Faster (up to millions of ions per second)	Slower (hundreds to thousands per second)
Specificity	Selective (based on size and charge of the channel)	Highly specific (complementary binding site)
Shape change	No	Yes
Substances transported	Ions, small polar molecules	Glucose, amino acids, larger polar molecules

Exam Tip: Facilitated diffusion is still passive (no ATP required) and still moves substances down their concentration gradient. The only difference from simple diffusion is that it requires specific membrane proteins. Make this distinction clear in your answers.

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Factors Affecting the Rate of Diffusion

Several factors influence how quickly substances diffuse across a membrane or through a medium. These can be remembered using Fick's Law of Diffusion:

$\text{Rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane (diffusion distance)}}$Rate of diffusion∝thickness of membrane (diffusion distance)surface area×concentration difference​

Factors in Detail

Factor	Effect on rate of diffusion
Concentration gradient	The steeper (larger) the gradient, the faster the rate of diffusion. Cells maintain steep gradients by rapidly using up or converting substances that enter the cell
Surface area	The greater the surface area, the faster the rate. Exchange surfaces (e.g. alveoli, villi) are adapted to have large surface areas
Diffusion distance (membrane thickness)	The shorter the distance, the faster the rate. Exchange surfaces are adapted to be thin (e.g. alveolar epithelium is only one cell thick)
Temperature	Higher temperature = more kinetic energy = faster molecular movement = faster diffusion
Size of molecule	Smaller molecules diffuse faster than larger molecules
Polarity of molecule	Non-polar molecules diffuse through the phospholipid bilayer more easily than polar molecules
Number of transport proteins	For facilitated diffusion, more channel/carrier proteins = faster rate (until all proteins are saturated)

Saturation Kinetics in Facilitated Diffusion

An important difference between simple diffusion and facilitated diffusion is the concept of saturation:

• In simple diffusion, the rate of diffusion increases linearly with increasing concentration gradient (no limit, as no proteins are involved)
• In facilitated diffusion, the rate increases with concentration gradient but eventually reaches a maximum rate (V$_{max}$max​) when all available transport proteins are occupied (saturated). At this point, increasing the concentration gradient further has no effect on the rate

This is a key graph you should be able to draw and explain.

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Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower (more negative) water potential, across a partially permeable membrane.

Water Potential ($\psi$ψ)

Water potential (symbol: $\psi$ψ, the Greek letter psi) is a measure of the tendency of water molecules to move from one region to another. It is measured in kilopascals (kPa).

• Pure water has a water potential of 0 kPa (this is the highest possible water potential)
• Adding solute to water lowers the water potential (makes it more negative)
• Water always moves from a less negative (higher) water potential to a more negative (lower) water potential

The water potential of a cell is determined by two components:

$\psi_{\text{cell}} = \psi_s + \psi_p$ψcell​=ψs​+ψp​

Where:

• $\psi_s$ψs​ = solute potential (always negative; also called osmotic potential) — the effect of dissolved solutes on water potential
• $\psi_p$ψp​ = pressure potential (usually positive in plant cells due to turgor; zero in animal cells) — the effect of physical pressure on water potential

Exam Tip: Always use the term water potential rather than "concentration" when describing osmosis. Describing osmosis as movement from "dilute to concentrated" may not receive full marks at A-Level. The correct terminology is "from higher water potential to lower (more negative) water potential".

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Osmosis in Animal Cells

Animal cells have no cell wall, so they are very sensitive to changes in the water potential of their surroundings:

Solution type	Water potential relative to cell	Effect on animal cell	Term
Hypotonic (dilute)	Higher than the cell	Water enters by osmosis; cell swells and may burst (lysis)	Cytolysis
Isotonic	Equal to the cell	No net movement of water; cell maintains normal shape	—
Hypertonic (concentrated)	Lower than the cell	Water leaves by osmosis; cell shrinks	Crenation

Why Red Blood Cells Are Important Examples

Red blood cells (erythrocytes) are commonly used to demonstrate osmosis because:

• They have no nucleus, so changes in shape are easy to observe
• In a hypotonic solution, they swell and burst (haemolysis)
• In a hypertonic solution, they shrink and develop a spiky appearance (crenation)
• In an isotonic solution (e.g. 0.9% saline), they maintain their normal biconcave shape

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Osmosis in Plant Cells

Plant cells have a rigid cell wall outside the cell membrane, which prevents the cell from bursting. This means they respond differently to changes in water potential:

Solution type	Water potential relative to cell	Effect on plant cell	Term
Hypotonic (dilute, e.g. pure water)	Higher than the cell	Water enters by osmosis; cell swells; the cell membrane pushes against the cell wall; the cell becomes turgid	Turgor / Turgid
Isotonic	Equal to the cell	No net movement of water; cell is flaccid	Incipient plasmolysis
Hypertonic (concentrated)	Lower than the cell	Water leaves by osmosis; the cell membrane pulls away from the cell wall	Plasmolysis

Turgor pressure is the pressure exerted by the cell contents against the cell wall. It is essential for plant support — when cells lose turgor, the plant wilts.

Plasmolysis occurs when so much water is lost that the cell membrane detaches from the cell wall. The point at which the membrane just begins to pull away is called incipient plasmolysis.

Exam Tip: In calculations involving water potential, remember that $\psi_p$ψp​ is usually zero for animal cells and for plasmolysed plant cells. A fully turgid plant cell has $\psi_{\text{cell}} = 0$ψcell​=0 because $\psi_p$ψp​ has become large enough to balance $\psi_s$ψs​.

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Core Practical: Investigating Osmosis

The Edexcel specification includes a practical on investigating the water potential of plant tissue using osmosis (the potato or beetroot method).

Method (Potato Cylinders)

1. Cut potato cylinders of uniform size using a cork borer
2. Measure the initial mass (and/or length) of each cylinder
3. Place cylinders in a range of sucrose solutions of known concentrations (e.g. 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M)
4. Leave for a set period of time (e.g. 30 minutes)
5. Remove, blot dry on paper towel, and measure the final mass
6. Calculate the percentage change in mass:

$\text{Percentage change in mass} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$Percentage change in mass=initial massfinal mass−initial mass​×100

Expected Results and Graph

• In dilute solutions (low sucrose concentration = high water potential): potato gains mass (water enters by osmosis)
• In concentrated solutions (high sucrose concentration = low water potential): potato loses mass (water leaves by osmosis)
• The concentration at which there is no change in mass is where the external solution has the same water potential as the potato cells — this is the estimated water potential of the potato tissue

Plot a graph of percentage change in mass (y-axis) against sucrose concentration (x-axis). The point where the line crosses the x-axis gives the sucrose concentration equivalent to the water potential of the potato cells.

Key Variables

Variable	Details
Independent	Concentration of sucrose solution
Dependent	Percentage change in mass of potato cylinders
Control variables	Size/dimensions of potato cylinders, volume of solution, temperature, time, same potato source

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Summary

• Diffusion is the passive net movement of molecules from high to low concentration
• Simple diffusion of small, non-polar molecules occurs directly through the phospholipid bilayer
• Facilitated diffusion uses channel or carrier proteins to move molecules down their concentration gradient; it shows saturation kinetics
• Fick's Law relates the rate of diffusion to surface area, concentration difference and distance
• Osmosis is the movement of water from higher to lower water potential across a partially permeable membrane
• In animal cells, osmosis can cause lysis or crenation; in plant cells, it causes turgor or plasmolysis
• The water potential of plant tissue can be estimated experimentally using potato cylinders in sucrose solutions

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A-Level Deep Dive: Transport Across Membranes — Diffusion and Osmosis

Spec mapping

The Edexcel 9BI0 specification places passive transport within Topic 2: Membranes, Proteins, DNA and Gene Expression, building directly on the fluid mosaic model of the previous lesson. Candidates must: define simple diffusion as the net movement of molecules or ions down a concentration gradient through the phospholipid bilayer; define facilitated diffusion as the passive movement of polar solutes through channel or carrier proteins; define osmosis as the net movement of water from a region of higher (less negative) water potential to a region of lower (more negative) water potential across a partially permeable membrane; apply Fick's law qualitatively (rate $\propto$∝ surface area $\times$× concentration difference / diffusion distance); and use the relation $\psi = \psi_s + \psi_p$ψ=ψs​+ψp​ to predict water movement between cells and surroundings. The same content is examined for both core practical literacy (the potato-cylinder serial-dilution practical) and synoptic application: forward to active and bulk transport (the next lesson), to Topic 5/7 (electron-transport metabolite movement across mitochondrial and chloroplast membranes), to Topic 7 (gas exchange and Fick's law applied to alveoli, gills and leaves) and to Topic 8 (kidney osmoregulation and plant root water uptake) — refer to the official Pearson Edexcel 9BI0 specification document for exact wording.

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Worked example with full mark scheme

Question (8 marks):

A plant cell has a solute potential ($\psi_s$ψs​) of $-800\,\text{kPa}$−800kPa and a pressure potential ($\psi_p$ψp​) of $+300\,\text{kPa}$+300kPa. It is placed in a sucrose solution of water potential $-400\,\text{kPa}$−400kPa.

(a) Calculate the water potential ($\psi$ψ) of the cell. (2)

(b) State and explain the direction of net water movement. (3)

(c) Predict, with reasoning, what would happen to the cell if the bathing solution were replaced with one of water potential $-700\,\text{kPa}$−700kPa. (3)

Solution with mark scheme:

(a) Step 1 — recall the relation. $\psi = \psi_s + \psi_p$ψ=ψs​+ψp​.

M1 (AO1.2) — correct equation written and solute and pressure potentials correctly identified.

Step 2 — substitute and evaluate. $\psi = (-800) + (+300) = -500\,\text{kPa}$ψ=(−800)+(+300)=−500kPa.

A1 (AO2.1) — correct numerical answer with sign and unit. Many candidates lose A1 by dropping the negative sign or by giving the answer as $+500$+500. The credited language is "the water potential is $-500\,\text{kPa}$−500kPa, which is more negative than the bathing solution".

(b) M1 (AO2.1) — correct comparison: the bathing solution has $\psi = -400\,\text{kPa}$ψ=−400kPa; the cell has $\psi = -500\,\text{kPa}$ψ=−500kPa; the cell is more negative.

M1 (AO2.1) — correct direction: water moves by osmosis from the bathing solution (higher / less negative $\psi$ψ) into the cell (lower / more negative $\psi$ψ), through the partially permeable membrane.

A1 (AO3.2a) — explicit framing as net movement of water down a water-potential gradient, not of solute, and via aquaporins as well as the bilayer. A common pitfall is to write "water moves into the cell because the cell is more concentrated" — examiners credit the water-potential framing, not solute-concentration framing.

(c) Step 1 — compare the new bathing solution to the cell. Bathing $\psi = -700\,\text{kPa}$ψ=−700kPa; cell $\psi = -500\,\text{kPa}$ψ=−500kPa; the cell is now less negative.

M1 (AO3.1a) — correct prediction: water moves out of the cell by osmosis.

M1 (AO3.2a) — correct mechanism: the protoplast loses water and shrinks; the pressure potential falls towards zero; if water loss continues, the protoplast pulls away from the cell wall — incipient plasmolysis when $\psi_p \to 0$ψp​→0, then plasmolysis when the protoplast has detached.

A1 (AO3.2a) — full reasoning: as $\psi_p$ψp​ falls, the cell's $\psi$ψ becomes more negative, narrowing the gradient until equilibrium ($\psi_\text{cell} = \psi_\text{bath}$ψcell​=ψbath​). A common pitfall is to state "the cell bursts" — animal cells lyse, plant cells plasmolyse because the cellulose wall prevents bursting.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare and contrast simple diffusion, facilitated diffusion and osmosis as mechanisms of passive transport across the cell-surface membrane.

Mark scheme decomposition by AO: