Osmosis and the change in mass of plant tissue

Where does a puddle come from when you salt a cucumber? Slice one up and split the slices between two bowls. Leave one bowl of slices sitting in plain water. Sprinkle a spoonful of salt over the other. Ten minutes later, the slices in plain water are crisp and, weighed, slightly heavier. The salted slices have gone limp, sitting in a puddle that was not there before. Same cucumber, same knife, same ten minutes. The only difference is the salt, and the salt did not turn into a puddle. Water follows a rule of its own, and that rule explains the puddle.

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution, through a partially permeable membrane. A solution is something dissolved in water, such as sugar or salt. Dilute means a little is dissolved in it, and concentrated means a lot is. A partially permeable membrane lets small molecules such as water through, and holds back bigger ones such as sugar. The cell membrane is one of these. So water can cross it, while the sugar stays on its own side, too big for the gaps. Sugar solution on one side, pure water on the other: which way does water move? Into the sugar solution. Pure water is the more dilute side, so water crosses towards the more concentrated side. The reason is crowding. The dilute side has more water molecules crowded into it, so more of them drift across than drift back. That gives the definition three parts: water, from a dilute solution to a more concentrated one, through a partially permeable membrane. This sentence sounds right. In osmosis, water moves from a high concentration to a low concentration. It borrows its wording straight from diffusion. What is wrong with saying it that way for osmosis? Concentration on its own means how much is dissolved, the solute. Read that way, the sentence sends water towards less solute, which is backwards. One examiner's report on a Foundation paper explains that in chemistry concentration refers to the amount of solute in a given volume of solution so the expression high concentration on its own is taken to mean a region of more solute. Therefore, stating water moves from an area of high concentration to low concentration is not correct. The fix is to say dilute and more concentrated, every time. Water moves from a dilute solution to a more concentrated solution, through a partially permeable membrane. Water potential is another name you may meet for this. Water moves from a higher water potential to a lower water potential, and you only need the term, never a calculation.

A piece of plant tissue in a solution follows the same rule. Its cell contents are a solution too, held inside each cell membrane. So the first thing to settle, every time, is whether the solution outside is more or less concentrated than the cell contents. If the solution is more dilute than the cell contents, water moves into the cells by osmosis. The tissue gains mass and goes firm. The contents press out against the cell wall, the stiff outer layer of a plant cell, and the wall holds them in. Now the other way round. This time the solution is more concentrated than the cell contents. What happens to the water now, and to the tissue's mass? Water moves out of the cells by osmosis. The tissue loses mass and goes soft. The cell contents shrink away from the wall, while the wall itself keeps its shape. And if the solution matches the cell contents, there is no net movement of water, so the mass does not change. A firm cell is called turgid, and a limp one flaccid. A cell whose contents have pulled away from the wall is plasmolysed. Those labels are handy, but an answer still needs the osmosis explanation behind them. One examiner's report on a Foundation paper records that most candidates appreciated that the potato cylinder would gain mass because water moves into the potato cells by osmosis. Another report, on a Higher paper about eggs in a solution, records the step that goes missing. A number of students omitted reference to the nature of the membrane. The habit that fixes it: end every osmosis answer by naming the partially permeable membrane, the third part of the definition. Now back to the cucumber slices with salt on them. So where did the puddle under the salted slices come from? From inside the slices. The salt dissolved in the film of water on each slice, making a solution far more concentrated than the cell contents, so water moved out by osmosis. The salt never went in. It stayed on top, and the puddle is the water that came out.

Osmosis changes how much a piece of tissue weighs, and that change is given as a percentage. Pieces of tissue start at different masses, so grams alone cannot be compared fairly. A percentage compares each piece against its own starting mass. Percentage change in mass is the final mass take away the starting mass, divided by the starting mass, times one hundred. A plus sign means a gain, so water went in. A minus sign means a loss, so water came out. Cylinder P of carrot starts at two point five nought grams. After forty minutes in a sugar solution, it weighs two point nought five grams. First, the change, on its own line. Two point nought five take away two point five nought is minus nought point four five grams. Then divide by the starting mass. Minus nought point four five, divided by two point five nought, is minus nought point one eight. Then times one hundred. P's percentage change in mass is minus eighteen per cent. Minus means P lost mass, so water moved out of the carrot cells. P's solution was more concentrated than the cell contents. Now cylinder Q, which breaks that pattern. It starts at one point six nought grams, and it gained nought point two four grams. Find its percentage change in mass. Try it on paper before I show you the answer. Plus fifteen per cent. The change was handed to you already, so there is nothing to take away this time. Nought point two four divided by one point six nought is nought point one five. Times one hundred is fifteen, and it is a gain, so the sign is plus. Q gained mass, so water moved in by osmosis. Q's solution was more dilute than the cell contents. Here's a third cylinder, R, which changed by plus one per cent. Which of P, Q and R sat in the solution closest to its cell contents? R. Its change is the smallest, so there was almost no net movement of water, and its solution almost matched the cell contents. Now a student's working for P. They divide nought point four five by two point nought five, and write plus twenty-two per cent. What two slips has the student made in that working? The two slips: they divided by the final mass, not the starting mass, and they dropped the minus sign, so a loss reads as a gain.

Under a microscope, osmosis shows up with nothing weighed at all. Cells from the thin skin of a red onion sit in tap water, their red contents filling each cell right up to the wall. A strong sugar solution is drawn under the coverslip. Ten minutes later, every red blob has shrunk and pulled away from the wall, and the wall has kept its shape. One examiner's report on a Higher paper describes a question about plant cells in a salt solution. The applied nature of this osmosis question challenged many candidates. Candidates either scored well for recognising that osmosis had occurred and successfully describing it or did not score any marks, often because they thought the chloroplasts moved in response to sunlight. The way to avoid that is one question, asked first: is the solution more or less concentrated than what is inside the cells? That question means osmosis. A student writes: the red pigment moved to the middle of the cell to get away from the sugar. Explain step by step what really happened in the sugar solution. The sugar solution is more concentrated than what is inside the cells, so water moves out of them by osmosis. It passes through the partially permeable cell membrane, which lets water out and holds the sugar back. The contents lose water and shrink. The cell wall is rigid and does not shrink, so the contents pull away from it. Nothing moved to get away from anything; water left. A piece of that onion tissue would have lost mass. Check the three parts: water, from dilute to more concentrated, through a partially permeable membrane. All three are there.

Three ways across a membrane now sit side by side, with osmosis between the other two. Diffusion moves small particles such as oxygen down the concentration gradient, with or without a membrane, and the cell spends no energy on it. Osmosis moves only water, from a dilute solution to a more concentrated one, and it always needs a partially permeable membrane. Active transport moves substances such as mineral ions against the concentration gradient, across a cell membrane, using energy released by respiration. Now a case you have not met. A root cell sits in soil that has become very salty. Water leaving that root cell: diffusion, osmosis or active transport? Osmosis. Only water is moving, from the dilute cell contents to the more concentrated soil water, through the partially permeable cell membrane.

Three to finish, and the cucumber comes back for the last one. What are the three parts of the definition of osmosis? Osmosis is water moving from a dilute solution to a more concentrated solution, through a partially permeable membrane. Now a slice of potato goes from five grams to four. What is its percentage change? Minus twenty per cent. The change is minus one gram, divided by the starting five grams, times one hundred. Here's the cucumber: why do slices left in plain water get a little heavier? Plain water is more dilute than the cell contents, so water moves in by osmosis, through the partially permeable membrane. And the salted slices lost their water into the puddle, while the salt stayed on top.

If osmosis and the percentage change feel steady now, thumb this one up. This one's yours, and it can come off your list. If it hasn't settled yet, save it, give it a day, and look again. The required practical on osmosis and the mass of plant tissue has its own video, where the method is taught in full.

Next in the chapter: "Exchange surfaces and the surface area to volume ratio". Hopefully that's next in your feed, but if not, search for ScholaFly B I oh four, oh three.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.