Crossing the Membrane

Leave one slice of cucumber in salty water and another in plain distilled water overnight. The salty slice gets lighter and turns soft and floppy, while the other slice gets heavier. That happened in a filmed Open University demonstration. Something crossed the edge of the cucumber’s cells, in opposite directions in the two dishes. The same question explains a stranger fact: drinking seawater leaves a person more dehydrated, not less.

Explore: put a cell in salt water

In the osmosis chamber you place a plant cell or a red blood cell in a solution and set how salty it is. Arrows show which way water moves on balance, and the cell swells, holds its size or shrinks. A second control switches on a protein pump that moves a substance from lower to higher concentration and counts the ATP it spends. The speeds in the model are for display, not measured rates.

Try this.

1. Start with a red blood cell in a solution exactly as salty as the cell. Predict whether water stops moving. Then watch the arrows.
2. Make the outside much less salty. Predict what the red blood cell will do. Then switch to a plant cell, keep the same solution, and compare.
3. Make the outside much saltier than the cell. Predict which way the arrows point and what happens to each kind of cell.
4. Turn on the pump. Predict whether it can keep working if the ATP runs out. Then test it.

Explain: a thin, choosy boundary

Every cell has a cell membrane, the thin boundary that separates the inside of the cell from its surroundings.

The membrane is built from phospholipids. Each phospholipid has a head that is attracted to water and two fatty tails that are not. In water, phospholipids line up in two layers. The heads face the watery outside and the watery inside of the cell. The tails point inward, toward each other, away from water.

Proteins sit in this double layer, and many cross it from side to side. The parts can drift sideways, so the membrane is flexible, not a wall.

Because of this build, the membrane is selectively permeable: it lets some substances cross easily and others hardly at all. Small molecules such as oxygen slip through the fatty middle. Charged ions such as sodium and chloride are almost entirely blocked and cross mainly through proteins.

The membrane does three jobs.

• Transport. It controls which materials enter and leave the cell, and how.
• Communication. Receptor proteins on the surface bind signal molecules from outside, such as hormones, and set off a response inside the cell.
• Recognition. Chains of sugar attached to surface proteins and lipids act like name tags. They help cells tell their own kind from others, which your immune system relies on.

A screen door is one analogy: it lets air in and keeps flies out. The analogy breaks down because a screen has fixed holes, while a membrane chooses by size, charge and its proteins.

Explain: moving downhill with no energy

A concentration gradient is a difference in how much of a substance is dissolved in one place compared with another. Molecules move at random all the time, so more cross from the crowded side than come back.

Passive transport is movement across a membrane from higher to lower concentration that needs no energy from the cell. It has three forms.

• Diffusion is the spreading of a substance from where it is more concentrated to where it is less concentrated.
• Facilitated diffusion is diffusion through a membrane protein. Channel proteins form water-friendly tunnels, and carrier proteins grab a molecule and change shape to pass it across. Glucose enters many of your cells by carriers, still moving from high to low.
• Osmosis is the net movement of water across a selectively permeable membrane. Much of it goes through protein water channels.

Here is the rule that trips people up: water moves on balance toward the side with more dissolved solute (a substance dissolved in the water, such as salt or sugar), because that side has less free water. Three words compare a solution with the inside of a cell.

A plant cell has a stiff wall outside its membrane, and water flowing in makes the cell press against it. That push is turgor pressure, and it holds up the leaves and stems of soft, nonwoody plants. In salty water, the cells lose water and turgor, so lettuce wilts and the cucumber slice goes floppy. A red blood cell has no wall, so in pure water it can swell until it bursts.

Explain: paying to move uphill

Active transport is movement across a membrane that uses energy from the cell, often from ATP, and it can move a substance against its concentration gradient. ATP is the molecule that carries usable energy around the cell, as you saw in the respiration lesson.

The best-known pump is the sodium-potassium pump in animal cells. Each cycle, it uses ATP to move three sodium ions out and two potassium ions in. Both trips go uphill: sodium is already higher outside, and potassium is already higher inside. Without ATP, the pump stops.

Loads too big for any channel or pump travel in vesicles, small bubbles of membrane. This also needs energy.

• Endocytosis brings material into the cell. The membrane folds inward around it, and the pocket pinches off as a vesicle inside the cell. Some white blood cells engulf bacteria this way.
• Exocytosis releases material from the cell. A vesicle joins the cell membrane and spills its contents outside. The organelles lesson follows these vesicles.

Seawater holds about 35 grams of dissolved salts in each kilogram, according to NOAA. That is far saltier than your body fluids, which have about the same pull on water as a 0.9% salt solution. NOAA explains that getting rid of the extra salt takes more water than the seawater supplied, so the body loses water overall.

Elaborate: weighing cucumber slices

In the Open University demonstration, one peeled cucumber slice went into water with added kitchen salt and one into distilled water. Each was blotted and weighed over time. Percent change in mass is:

percent change in mass = 100 × (final mass − starting mass) ÷ starting mass

Source: Janet Sumner, Open University (OpenLearn) filmed demonstration and transcript, accessed October 2026; no date is given. Masses were read to the whole gram, the salt concentration was not measured, and there was one slice for each liquid. Percent changes are calculated from the reported masses.

1. Describe the trend for each slice over time.
2. The distilled-water slice gained 8 grams overnight and the salty slice lost 5 grams. Why is percent change a fairer way to compare them than grams?
3. Can these data tell you the salt concentration at which a slice would neither gain nor lose mass? Explain what is missing.

The Chesapeake Bay also spans a range of salt in nature. The Chesapeake Bay Program describes its head and tidal rivers as below 0.5 parts per thousand of salt and its mouth as averaging 25 to 30 parts per thousand.

Design problem

Plan an investigation to find the sugar concentration at which potato pieces neither gain nor lose mass. Name your variables, constants, control and sample size, and say how you will stay safe.

Evaluate: check your thinking

The salty water was hypertonic to the cucumber cells, so on balance water left them by osmosis. The cells lost turgor pressure, the slice got lighter, and it went soft. In distilled water the opposite happened.

Science practices you used: making predictions and testing them with a model, calculating percent change from measured masses, reading a data table and naming its limits, and planning a fair test with a control, repeated pieces and a mean.