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Osmosis and Excretion: From a Vacuole to a Kidney

Osmosis is the movement of water through a membrane toward the side with more dissolved substance. Animals must control their water and throw out waste. The tool for this changed over time: contractile vacuole, flame cells, nephridia, then kidneys.

๐ŸŽฌ Step-by-step story

  1. Osmosis. A membrane splits the box. Pure water is on the left, salty water on the right. Blue water bits slip through the membrane toward the salty side. Yellow salt cannot cross.
  2. An amoeba lives in fresh water. Its inside is saltier than the pond, so water keeps coming in. The contractile vacuole fills up and then squeezes the extra water out.
  3. A flatworm has flame cells. Tiny hairs in each one flicker like a flame and push waste water down a tube to a pore in the skin.
  4. An earthworm has a pair of nephridia in almost every segment. A funnel takes waste from the body cavity and a tube lets it out of the body.
  5. Vertebrates have kidneys. Blood is filtered. Water and sugar the body needs go back into the blood. The rest, urine, goes to the bladder.
  6. Free play: raise the salt on the right. More salt, more water pulled across, and the water level climbs on that side.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

๐Ÿค” Common doubts, cleared

Does salt move in osmosis?

No. The membrane blocks the salt. Only the water moves, toward the salty side.

Why does the amoeba vacuole keep filling?

Osmosis keeps pushing water into the cell because the cell is saltier than fresh water. The vacuole pumps it out again.

What do flame cells do?

They collect fluid from the body and push it along tubes with beating hairs to a pore. They mostly control water.

How is a nephridium different from a flame cell?

A nephridium has an open funnel from the body cavity and is found in every segment. A flame cell ends blind.

Where does the water go back to the blood in a kidney?

Along the tube, useful water, sugar and salts are taken back into the nearby blood vessel. What is left is urine.

What if both sides have the same amount of salt?

Then water moves equally both ways, and the levels stay equal. Set the slider to zero to see.

What is osmosis?

A selectively permeable membrane lets water pass but blocks bigger dissolved things like salt or sugar. If the two sides have different amounts of dissolved substance, water moves through the membrane to the side with more dissolved substance (less free water). This movement of water is osmosis. It needs no energy from the cell and stops only when both sides match, or when pressure pushes back.

In the 3D, more salt on the right means more water is pulled over, and the level there rises.

Why animals must control water and waste

Cells make waste: ammonia, urea or uric acid from protein breakdown, plus extra salts and water. These can poison the body if they build up. Excretion removes these wastes. Osmoregulation keeps the right amount of water and salts inside.

Animals in fresh water get flooded by osmosis and must pump water out. Animals in the sea tend to lose water and must throw out salt. Land animals must save water.

Evolution of the excretory system

  1. Contractile vacuole (amoeba, paramecium in fresh water): a small bubble that fills with extra water and bursts it out. It mostly controls water.
  2. Flame cells, or protonephridia (flatworms): blind-ended tubes with a tuft of beating hairs (cilia). They collect fluid and push it out through pores. They manage water; much ammonia also leaves by diffusion through the skin.
  3. Nephridia (earthworm and other annelids): a coiled tube in each segment. A ciliated funnel opens into the body cavity, and the tube leaves by a pore. They remove waste and save useful salts.
  4. Malpighian tubules (insects): thin tubes attached to the gut that take waste from the blood. They save water, which suits land.
  5. Kidneys (vertebrates): millions of tiny filters called nephrons. Blood is filtered under pressure, the body takes back what it needs (water, sugar, salts) and the rest becomes urine.

Each step handles more water and waste, with finer control. Roundworms have simple canals and special large cells, also on this path.

Waste types and the habitat

Ammonia is very poisonous and needs a lot of water to wash away. Water animals (fish, amoeba) can release it straight into the water. Urea is less poisonous, so mammals and amphibians can store it for a while and use less water. Uric acid is a paste or crystal that needs almost no water, which suits birds, reptiles and insects, and helps saving water inside an egg.

Try it

In the 3D: go to step 6, set salt to 0 and watch the levels, then slowly raise it. Predict which side gets higher.

At home: cut two potato strips of the same size. Put one in plain water and one in very salty water for an hour. Which one is firm and which is floppy? Which way did the water go?

Key formulas and definitions

Worked examples

1. A beaker has a membrane in the middle. Left: pure water. Right: 5% salt. Which way does water move?

Toward the right, the side with more dissolved salt. The salt cannot cross the membrane, so the right side gains water.

2. Why does a freshwater amoeba need a contractile vacuole, but a sea amoeba hardly does?

Pond water is much less salty than the cell, so water keeps entering by osmosis and must be pumped out. Sea water is about as salty as the cell, so there is little extra water.

3. Name the excretory organ of earthworm, flatworm and insect.

Earthworm: nephridia. Flatworm: flame cells (protonephridia). Insect: Malpighian tubules.

4. Why can birds save so much water?

They make uric acid, a semi-solid waste that needs almost no water to remove.

Common mistakes

Practice quiz

1. In osmosis, water moves toward:
2. The contractile vacuole mainly removes:
3. Flame cells are found in:
4. Which animal has nephridia in its segments?
5. Which waste needs the least water to remove?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is the difference between osmosis and diffusion?

Diffusion is any substance spreading from more to less. Osmosis is only about water crossing a selectively permeable membrane, toward the side with more dissolved substance.

Do all animals have kidneys?

No. Kidneys are found in vertebrates. Simpler animals use contractile vacuoles, flame cells, nephridia or Malpighian tubules.

Why does a freshwater fish not swell up?

Its gills and kidneys work to push out the extra water as plenty of dilute urine, and gills take in salts.

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