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The People's Share · Look Again

Quiz 10 · Part II: The Cell and the Organism

The Membrane

What crosses the boundary, which way, and at what cost

The Guide

A boundary that chooses

A cell's membrane is not a wall and not a sieve. Some things cross it freely, some cross only with help, and some do not cross at all. Biologists call this being selectively permeable, which means the membrane lets some things through and keeps others out.

Diffusion: things spread out

Particles in a liquid or a gas are always moving, and they bump around at random. The result of all that random bumping is that particles drift from where they are crowded toward where they are not, until they are evenly spread. That drift is called diffusion. It costs no energy, because nothing is pushing. The particles spread out on their own.

Open a bottle of vinegar at one end of a room and eventually the whole room smells of it. Nobody moved the vinegar.

Osmosis: the water version, and the step people skip

Osmosis is diffusion of water across a membrane. It is the same idea, with one difficulty that catches nearly everyone: when you look at a glass of salty water, the thing you notice is the salt, so you track the salt. But in osmosis the salt is usually the thing that cannot cross. The water crosses. So you have to watch the one you were not watching.

Here is the procedure. Three steps, and the second is the one that gets skipped.

  1. Look at both sides and ask which has more dissolved in it — more salt, more sugar, more of anything.
  2. Water moves toward that side. Toward the crowded side, not away from it. This runs against most people's first instinct, which is that water should flow away from where things are crowded.
  3. So that side gains water and rises; the other side loses water and falls.
Water moving across a membrane toward the more crowded side Two containers, each divided down the middle by a membrane. In the first, labeled at the start, the left side holds four dissolved particles and the right side holds twelve, and the water level is the same on both sides. In the second, labeled a while later, the particle counts are unchanged but the water level on the right has risen and the level on the left has fallen. A gold arrow points from the left side to the right side, through the membrane. At the start More dissolved on the right. A while later The right side gained water. Water moves toward the side with more dissolved in it.

Why the water and not the salt. The dots in that drawing never move. They cannot — they are too large for this membrane. Only the water can cross, so only the water does. The levels change because water piled up on one side, not because anything was pushed there.

Active transport: going the hard way

Sometimes a cell needs something it already has plenty of, and has to pull more in from outside where there is very little. That is uphill — the opposite of what diffusion would do on its own — and moving something uphill costs energy. This is active transport, and it is the only one of the three that has a price.

The test of which is which is not what is moving or how fast. It is the direction. Ask which way the substance itself is moving. From where there is a lot of it to where there is little, it moves on its own and costs nothing: that is diffusion, and osmosis is the same thing for water. Into the side where there is already more of it, it is going uphill, and that costs energy: that is active transport.

Worked Examples

Two questions, worked through

Example 1. A dried raisin is left in a cup of plain water overnight. In the morning it is plump. What happened?

Run the three steps. Which side has more dissolved in it? Inside the raisin — it is concentrated fruit sugar. Outside is plain water with almost nothing in it. So water moves toward the raisin, and the raisin gains water and swells.

Example 2. A spoonful of sugar is dropped into a cup of tea and left without stirring. An hour later the tea is sweet all through. Is that osmosis?

Check for a membrane. There isn't one — the sugar and the tea are in the same cup with nothing between them. The sugar spread from where it was crowded, at the bottom, to where it was not, which is diffusion.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 4 One cell placed in each of three different liquids Three containers. In beaker 1 the liquid outside is pure water and the cell inside is drawn large and swollen. In beaker 2 the liquid outside is salty water and the cell inside is drawn small and shrunken. In beaker 3 the liquid outside has the same amount dissolved in it as the cell does, and the cell is drawn at its normal size. Beaker 1 Pure water outside Beaker 2 Salty water outside Beaker 3 Same amount dissolved as inside The same cell in each beaker, twenty minutes later.

1.The cell in beaker 1 sits in pure water. Which way did water move, and what followed?

2.The cell in beaker 2 has shrunk. What does that tell you about the liquid around it?

3.The cell in beaker 3 did not change size. Why not?

4.A gardener waters a plant with very salty water. What happens to the root cells?

Passage A — questions 5 to 7

Oxygen is plentiful in the blood and scarce inside a working muscle cell. Oxygen crosses the cell membrane from the blood into the cell. This costs the cell nothing.

Potassium is scarce in the fluid outside the cell and plentiful inside it. The cell nevertheless pulls more potassium in, holding it against the direction it would otherwise drift. Doing this uses up part of the cell's energy supply.

5.Which of the two movements described is diffusion?

6.Which movement costs the cell energy, and why?

7.If the cell ran out of energy, what would happen first?

Passage B — questions 8 to 10

A cook slices a cucumber into a bowl and sprinkles salt over the slices. Within twenty minutes a pool of liquid has collected in the bottom of the bowl, and the slices have gone limp.

A second batch of slices from the same cucumber is left in a bowl of plain water. Those slices stay firm.

8.Where did the liquid in the first bowl come from?

9.Why did the second batch stay firm?

10.Salting was used to preserve meat and fish long before anyone knew what a cell was. Why does it work?

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10    Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. D — water moved in and the cell swelled. Step one: which side has more dissolved in it? The cell does — pure water has nothing. Step two: water moves toward that side, so into the cell. Step three: the cell gains water and swells, which is how it is drawn. C has salt crossing the membrane. In this drawing there is no salt outside to cross, and in most osmosis questions the dissolved material is exactly the thing that cannot get through. Watch the water.
2. A — more dissolved in it than the cell. This runs the procedure backward. The cell lost water, so water moved out, so water moved toward the outside, so the outside had more dissolved in it. The label agrees: salty water. B is the answer you get by keeping the rule in mind but pointing it the wrong way. If you are unsure which direction you have got, check it against beaker 1, where the answer is obvious, and see whether your rule gives the right result there.
3. C — water moved both ways at the same rate. Water is always crossing in both directions. What changes the cell's size is whether more goes one way than the other. Here the two sides match, so the two flows match, and nothing changes on the outside even though a great deal is happening. A imagines the membrane switching off. It does not. "No change" and "nothing happening" are different things, and this is one of the harder ideas in the quiz.
4. B — the cells lose water and the plant wilts. Salty water outside means more dissolved outside than in, so water leaves the root cells. The large vacuole from Quiz 9 empties, and the pressure that was holding the plant up goes with it. A has salt entering and swelling the cell, which is the same reversal as item 1's option C. Watering a plant with seawater kills it, and this is why: the salt does not poison the plant; it pulls the water out of its cells.
5. C — oxygen into the cell. Plentiful outside, scarce inside, moves from crowded to less crowded, costs nothing. That is diffusion in one sentence. The passage hands you the test in its last words for each paragraph: "costs the cell nothing" and "uses up part of the cell's energy supply." When a passage states the deciding fact plainly, use it rather than reasoning around it.
6. A — potassium, because it moves toward the crowd. Potassium is already plentiful inside and the cell pulls in more. That is uphill, and uphill costs energy. This is active transport. B treats crossing the membrane as the expensive part. It is not — oxygen crosses the same membrane for free. The price is charged for the direction, not for the crossing.
7. D — potassium would stop being pulled in. Only one of the two movements is paid for. Cut off the payment and that one stops; the free one carries on. A stops the wrong movement. Diffusion needs no energy from the cell, so a cell with no energy left still lets oxygen in. It is worth sitting with that: a dead cell still diffuses.
8. B — from inside the cucumber cells. Salt on the outside makes the outside the crowded side, so water leaves the cells and collects in the bowl. The slices go limp for the same reason the plant wilts in item 4. A is what it looks like, since the salt does disappear into the liquid. But dry salt does not become a pool. The volume came out of the cucumber, which is why the slices shrink as the pool grows.
9. A — less dissolved outside, so water moved in. The same cucumber, the opposite surroundings, the opposite result. Water entered the cells and pressed outward, and that pressure is what firmness is. B is A with the comparison reversed, and both options are long and similar on purpose, because on the test the difference between the right and wrong answer is often one word — here, "less" against "more."
10. C — salt draws water out of bacteria. A bacterium on a salted ham is a cell in beaker 2. Water leaves it, and a cell without water cannot grow or divide. People worked this out by trial over thousands of years without knowing any of the words in this quiz. D imagines salt as a barrier, which is the most reasonable wrong answer available. It is not sealing anything. It is changing which side of the membrane the water wants to be on.

Your Score

What the number means

8 to 10Solid. Check topic 10 on your map and go on to Quiz 11, on photosynthesis.
6 or 7Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewerWorth another pass. Work the three steps out loud on each beaker in turn — which side has more dissolved, which way the water goes, what happens to the size.

Misses on 1, 2, 4, 8, or 9 nearly always come from the same place: the rule was remembered and pointed the wrong way. Check yourself against beaker 1, where the answer is plain, before trusting your direction on a harder one. Misses on 5, 6, or 7 mean diffusion and active transport are still blurred, and the difference is only ever direction and cost. A miss on 3 is different in kind — it is the idea that no change on the outside can mean a great deal happening on the inside.

The plate for this quiz

The gallery image is a plate of red blood cells, photographed with an electron microscope. Healthy cells are smooth discs with a dip in the middle. Placed in salty fluid the same cells go spiky and shrunken, and placed in pure water they swell into spheres and burst — which is why a saline drip has to be mixed to match the blood it is going into, and why plain water was never an option.

Electron micrographs of blood cells are held in several public collections, including the National Institutes of Health image library, where the images are in the public domain. The one we want shows normal and shrunken cells side by side.