B. Biochemistry · Biochemistry

Passive Transport: Diffusion, Osmosis and Facilitated Diffusion

B3.6 B2.2 B2.1 A1.2 A1.5 A1.6 A1.8 A1.13About 75 minutes

Learning goals

I can explain diffusion, osmosis and facilitated diffusion as passive transport down a concentration gradient.
I can predict how animal and plant cells respond to hypotonic, isotonic and hypertonic solutions.
I can compare channel proteins and carrier proteins and explain why facilitated diffusion can become saturated.
I can plan and conduct an investigation of osmosis in potato tissue, calculate percentage mass change, and find the isotonic concentration from a graph.

Words to know

passive transport
Movement of substances across a membrane down their concentration gradient, with no energy (ATP) supplied by the cell.

concentration gradient
A difference in concentration between two regions. Substances diffuse from higher to lower concentration ("down" the gradient).

diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration, caused by their random motion.

osmosis
The diffusion of water across a selectively permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

hypotonic / isotonic / hypertonic
Describing a solution compared with a cell: lower, equal or higher solute concentration than the cell's contents.

facilitated diffusion
Passive transport through a channel or carrier protein, used by ions and polar molecules that can't cross the bilayer.

channel protein
A transmembrane protein that forms a water-filled pore for specific ions or water (aquaporins). Many are gated: they open and close.

carrier protein
A transmembrane protein that binds a specific molecule, changes shape and releases it on the other side.

plasmolysis
Shrinking of a plant cell's cytoplasm away from the cell wall when the cell loses water in a hypertonic solution.

Why salt the road but not the lawn?

After a Canadian winter, the grass next to salted roads and sidewalks is often brown. Road salt dissolves in meltwater and soaks into the soil, making the water around the roots saltier than the inside of the root cells. Water then moves out of the roots instead of in, and the plants suffer, even though the soil is wet.

That is osmosis, one of three ways substances cross membranes without the cell spending energy. This lesson covers all three, then has you measure osmosis yourself.

Remember from Grade 10

In SNC2D you learned that the cell membrane is selectively permeable. In Grade 11 SBI3U you saw diffusion at work in gas exchange and absorption. Now you will explain these processes with the fluid mosaic model.

Diffusion

All particles in a liquid or gas are in constant random motion. Where there are more particles, more of them happen to move toward places with fewer particles than the other way, so there is a net movement down the concentration gradient until the concentration is even (dynamic equilibrium: particles still move, but equally in both directions).

The rate of diffusion is faster with a steeper gradient, a higher temperature, smaller particles, a larger surface area, and a shorter distance. Small non-polar molecules (O₂, CO₂) and lipid-soluble ones (steroid hormones) diffuse directly through the bilayer: this is simple diffusion.

Facilitated diffusion

Ions and polar molecules are repelled by the bilayer's non-polar core. They cross by facilitated diffusion, still down their gradient and still passive (no ATP), but through proteins:

Channel proteins form a pore lined with polar R groups. Each is specific: a K⁺ channel passes K⁺ but not Na⁺. Many are gated, opening in response to a voltage change or a signal molecule (important for nerve impulses in Homeostasis). Aquaporins are channels for water; a single aquaporin can pass billions of water molecules per second. Peter Agre discovered aquaporins and shared the 2003 Nobel Prize in Chemistry with Roderick MacKinnon, who worked out the structure of ion channels.
Carrier proteins bind a specific molecule, change shape, and release it on the other side. Glucose enters red blood cells this way, through a carrier called GLUT1. Because a cell has a limited number of carriers, facilitated diffusion has a maximum rate: like an enzyme, it can be saturated.

Three kinds of passive transport, all down the concentration gradient (top = higher concentration). Small non-polar molecules cross the bilayer directly; ions and water use channel proteins; molecules such as glucose use carrier proteins that change shape. Not to scale.

Osmosis

Water can cross membranes (slowly through the bilayer and quickly through aquaporins), but many solutes cannot. Osmosis is the net diffusion of water across a selectively permeable membrane toward the side with the higher solute concentration (where there is, in effect, less free water).

We describe the outside solution compared with the cell:

Hypotonic (lower solute concentration than the cell): water moves in. Animal cells swell and may burst (lysis). Plant cells swell until the cell wall pushes back: they become turgid (firm), which is the healthy state for plant cells.
Isotonic (equal): no net water movement. Animal cells keep their normal shape; this is why IV fluids are made isotonic, with about 0.9% salt ("normal saline"). Plant cells become flaccid (limp).
Hypertonic (higher): water moves out. Animal cells shrivel (crenation). In plant cells, the cytoplasm pulls away from the wall: plasmolysis.

Freshwater protists such as Paramecium live in a hypotonic world; they use contractile vacuoles to pump extra water back out.

How animal cells (top) and plant cells (bottom) respond to solutions of different tonicity. The plant cell wall (thick outline) prevents bursting, so the turgid plant cell is the healthy one.

Investigating osmosis (B2.2)

In the school lab, two classic set-ups are:

Dialysis tubing (an artificial selectively permeable membrane): fill a bag with a starch and glucose solution, tie it, and put it in water containing iodine. After 30 minutes, the inside turns blue-black (iodine, a small molecule, diffused in), while the water outside gives a positive Benedict's test (glucose diffused out). Starch, a huge molecule, stays inside. If you weigh the bag before and after, it gains mass: water entered by osmosis.
Potato cores in a series of sucrose or salt solutions: weigh, soak, blot and reweigh, then calculate percentage change in mass:

% change = (final mass − initial mass) ÷ initial mass × 100%

Percentage change (not just grams) is used because the cores don't start at exactly the same mass. The concentration where the change is zero is isotonic with the potato cells.

Worked example

Example: Finding the isotonic concentration of potato cells

Potato cores were soaked in sucrose solutions for 2 hours (simulated data).

0.0 mol/L: 5.12 g → 6.04 g
0.2 mol/L: 4.95 g → 5.30 g
0.4 mol/L: 5.06 g → 4.91 g
0.6 mol/L: 5.20 g → 4.58 g
0.8 mol/L: 4.88 g → 3.95 g

Calculate the % change in mass for each, graph the results, and estimate the sucrose concentration that is isotonic with the potato cells.

0.0 mol/L: (6.04 − 5.12) ÷ 5.12 × 100% = 0.92 ÷ 5.12 × 100% = +18.0%
0.2 mol/L: 0.35 ÷ 4.95 × 100% = +7.1%
0.4 mol/L: −0.15 ÷ 5.06 × 100% = −3.0%
0.6 mol/L: −0.62 ÷ 5.20 × 100% = −11.9%
0.8 mol/L: −0.93 ÷ 4.88 × 100% = −19.1%
The change crosses zero between 0.2 (+7.1%) and 0.4 mol/L (−3.0%). Interpolate along a straight line: 0.2 + 0.2 × 7.1 ÷ (7.1 + 3.0) = 0.2 + 0.2 × 0.703 = 0.34 mol/L.
Interpretation: below 0.34 mol/L the solutions were hypotonic (cores gained water); above it they were hypertonic (cores lost water).

% changes: +18.0, +7.1, −3.0, −11.9, −19.1. The isotonic concentration is about 0.34 mol/L sucrose.

Percentage change in mass of potato cores (simulated data) against sucrose concentration. The line crosses zero at about 0.34 mol/L, the concentration isotonic with the potato cells.

Just for fun

What did the red blood cell say when it was dropped into pure water?

"I'm so full of it, I could burst." (Hypotonic: it literally could.)

Hands-on activity

Plan and conduct: potato strips in salt water (B2.2)

Grown-up needed for cutting the potato.

You need: 1 large potato, a knife and cutting board, a ruler, a kitchen scale (ideally reading to 0.1 g, but 1 g works if pieces are large), table salt, a measuring cup and teaspoon, 5 identical glasses or jars, distilled water (sold in grocery stores) or tap water, paper towel, masking tape and a marker, and a timer.

Plan first (in writing): question, hypothesis with a reason, independent variable (salt concentration), dependent variable (% change in mass, and length), controlled variables, and a data table.
Make 200 mL of each solution: 0, 1, 2, 3 and 5 g of salt per 100 mL of water (that is 0, 2, 4, 6 and 10 g in 200 mL). Stir until dissolved and label the glasses.
Cut 10 potato strips of the same size, about 6 cm × 1 cm × 1 cm, with no skin (2 per glass). Blot each, measure its length to the nearest millimetre, and weigh each pair together.
Put 2 strips in each glass, fully covered. Wait 2 hours (or overnight in the fridge for bigger changes).
Remove the strips, blot them gently the same way, and remeasure mass and length. Bend each strip gently and note whether it is firm or floppy.
Calculate % change in mass for each glass, graph it against salt concentration, and estimate the isotonic salt concentration. Write a short report with sources of error.

What to look for: strips in plain water gain mass and feel stiff (turgid); strips in 3% and 5% salt lose mass and become floppy (water left by osmosis; cells are plasmolyzed). The isotonic point for potato is usually at a low salt concentration, likely around 1% or a little less, but your graph decides. A good evaluation mentions blotting technique, scale resolution (A1.13: a 1 g scale on a 20 g sample means ±5%), and strips that weren't identical.

Your turn

What makes diffusion, osmosis and facilitated diffusion all "passive"?

Define osmosis precisely.

A red blood cell is placed in a 2% salt solution. Is the solution hypotonic, isotonic or hypertonic to the cell (about 0.9%)? What happens?

Why don't plant cells burst in pure water?

How is a channel protein different from a carrier protein?

List three factors that increase the rate of diffusion.

More practice: warm-up

In which direction do substances move by diffusion?

What is plasmolysis?

What are aquaporins?

What does "isotonic" mean?

Does facilitated diffusion use ATP?

More practice: on your own

A potato core goes from 4.20 g to 4.62 g. Calculate the % change in mass.

Explain why road salt can harm plants beside a highway even when the soil is wet.

In the dialysis-tubing experiment, the iodine diffused in but the starch did not diffuse out. Explain.

The rate of glucose uptake into red blood cells levels off at high glucose concentrations, but O₂ uptake keeps increasing with O₂ concentration. Explain the difference.

Why are percentage changes used instead of changes in grams when comparing potato cores?

Challenge

Using the example data, a student says the isotonic concentration is exactly 0.34 mol/L. Give two reasons this estimate is uncertain and suggest how to improve it.

A person who drinks large amounts of seawater becomes dehydrated. Use osmosis to explain why.

Explain why many freshwater protists have contractile vacuoles but most marine protists don't.

Check your understanding

Quiz

Osmosis is the movement of:

An animal cell placed in a hypotonic solution will:

Which statement about facilitated diffusion is correct?

A potato core changes from 5.00 g to 4.60 g. Its % change in mass is:

The rate of transport by a carrier protein levels off at high concentration because:

A plant cell in a hypertonic solution becomes:

For parents and teachers

Big idea: This lesson covers the passive transport part of B3.6 (diffusion, facilitated diffusion) and B2.2 (an investigation of movement across a membrane, such as salt water and distilled water on potato, which is the ministry's own example). It also practises choosing instruments (A1.2), recording and calculating data with appropriate precision (A1.5, A1.6, A1.13) and evaluating error (A1.8).

How to help: The potato activity needs an adult to cut the strips. Ask your teen to write their plan before starting, then compare the results with their hypothesis. Check the % change calculations together: the most common error is dividing by the final mass instead of the initial mass.

Common difficulties:

Saying water moves "toward the lower concentration." Water moves toward the higher solute concentration.
Confusing hypotonic and hypertonic. Memory aid: hypotonic means "under" (less solute outside), so water goes in and the cell swells like a hippo.
Thinking facilitated diffusion uses energy because it uses proteins. It doesn't; it is passive.