B1.5B1. Characteristics of Living Organisms and Cells
Osmosis — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 15 parts Lesson 6 of 14 1 quiz
Lesson summary
B1.5 Osmosis — COMBINED SCIENCE notes: Define osmosis and describe how it is investigated using potato tissue and dialysis tubing.Solvent — many substances dissolve in water, which makes it essential for life.Transport — dissolved substances move around organisms, in the xylem an…
- Cell membrane
- High water potential
- Low water potential
- Direction in terms of potential
- Dialysis (Visking) tubing
- Partially permeable membrane
Section 1 of 15Water as a solvent
Solvent
- Solvent many substances dissolve in water, which makes it essential for life.
Transport
- Transport dissolved substances move around organisms, in the xylem and phloem of plants and as dissolved food molecules in the blood.
Digestion
- Digestion digested food molecules in the gut can only reach the cells because they dissolve in water.
Excretion
- Excretion toxic substances such as urea and excess salts dissolve in water, so they are removed in urine.
Cytoplasm
- Cytoplasm water is an important part of the cytoplasm, where metabolic reactions happen.
Section 2 of 15What osmosis is
Osmosis
Osmosis the diffusion of water molecules from a dilute solution (low solute concentration) to a more concentrated solution (high solute concentration) across a partially permeable membrane.
Direction
- Direction water moves down its concentration gradient, from where there is more water to where there is less.
Section 3 of 15The partially permeable membrane
- Cell membrane
- all cells are surrounded by a partially permeable membrane.
- It lets small molecules such as water through but not larger solute molecules.
Section 4 of 15Water potential
Water potential
High water potential
- High water potential a dilute solution with a low solute concentration.
Low water potential
- Low water potential a concentrated solution with a high solute concentration.
Direction in terms of potential
- Direction in terms of potential water moves from a higher water potential to a lower water potential through a partially permeable membrane.
Section 5 of 15Osmosis in potato tissue
1
Cut cylinders of potato or radish of equal size.
2
Weigh each cylinder.
3
Place each in distilled water or a sucrose solution of increasing concentration.
4
Leave for 20 to 30 minutes.
5
Remove the cylinders and dry them to remove excess liquid.
6
Reweigh them and record the change in mass.
Section 6 of 15Interpreting the potato results
| Result | Meaning |
|---|---|
| Mass gain | Water moved in, and the solution was more dilute than the tissue |
| Mass loss | Water moved out, and the solution was more concentrated than the tissue |
| No change | No net movement, because the concentrations were equal |
- No change
- water still moves in both directions but equally.
Section 7 of 15Osmosis with dialysis tubing
1Dialysis (Visking) tubing
- Dialysis (Visking) tubing a non-living partially permeable membrane made from cellulose.
- Its pores let small molecules such as water and glucose pass but stop large molecules such as sucrose.
2Method
- Method fill a section of tubing with concentrated sucrose solution and suspend it in a tube of water.
- Water moves into the tubing by osmosis, so the water level outside falls.
Section 8 of 15More from the specification
More from the specification
Water Potential Principles in Osmosis
- Osmosis is the net diffusion of water molecules from a region of higher water potential to a region of lower water potential (concentrated solution) across a selectively permeable membrane.
- (dilute solution)
Osmotic Behaviour of Animal Cells
- Lacking rigid cell walls, animal erythrocytes swell and undergo osmotic lysis in pure hypotonic water,
- and crenate through dehydration when exposed to hypertonic saline solutions.
Plant Cell Turgor and Plasmolysis
- Hypotonic water uptake drives internal hydrostatic pressure against plant cell walls to generate turgor support;
- in hypertonic media, water efflux causes protoplast shrinkage away from the cell wall (plasmolysis).
Active transport
- Movement of particles against a concentration gradient using energy from respiration.
Carbon dioxide
- Gas that turns limewater cloudy.
Section 9 of 15Plant cells in dilute solutions
Water uptake
- Water uptake plant cells in a solution of higher water potential, such as distilled water, take in water by osmosis.
Vacuole
- Vacuole it gets bigger and pushes the cell membrane against the cell wall.
Turgid
- Turgid the cell becomes rigid and firm, and turgor pressure rises.
Section 10 of 15Importance of turgor
Support
- Support firm cells give support and strength, so the plant stands upright.
Leaves
- Leaves turgid cells hold the leaves out to catch sunlight.
Cell wall
- Cell wall the pressure of the wall stops too much water entering and prevents the cell from bursting.
Section 11 of 15Plant cells in concentrated solutions
Water loss
- Water loss in a solution of lower water potential, water moves out of the cells by osmosis.
Flaccid
- Flaccid the cells become soft and limp.
Plasmolysis
- Plasmolysis under the microscope the cell membrane may have pulled away from the cell wall.
Wilting
- Wilting if plants do not receive enough water, the cells cannot stay turgid and the plant wilts.
Section 12 of 15Animal cells in different solutions
No cell wall
- No cell wall animal cells have no supporting cell wall, so the effects are more severe.
Strong sugar solution
- Strong sugar solution the cell loses water by osmosis and shrivels, which is crenation.
Distilled water
- Distilled water the cell gains water and has no cell wall to create turgor pressure.
- It keeps taking water until the membrane stretches too far and the cell bursts.
Section 13 of 15More from the specification
More from the specification
Definition and Direction of Osmosis
- Osmosis is the net diffusion of water molecules from a region of higher water potential to a region of lower water potential (concentrated solution) across a partially permeable membrane.
- (dilute solution)
Dialysis Tubing as a Partially Permeable Model
- In laboratory experiments, synthetic dialysis tubing models cell membranes,
- allowing small water and glucose molecules to diffuse through microscopic pores while retaining large macromolecules like starch.
Water Potential Effects on Plant and Animal Tissue
- When plant tissue cylinders are placed in pure water, cells absorb water by osmosis and become turgid;
- in concentrated sucrose solutions, water leaves the tissue,
- causing plasmolysis and a decrease in tissue mass.
Active Transport vs. Passive Osmosis and Diffusion
- Unlike passive diffusion and osmosis which occur down concentration gradients without metabolic energy, active transport requires cellular energy from ATP to move solutes against a concentration gradient via membrane carrier proteins.
Solvent
- Liquid in which a solute dissolves.
Solute
- Substance that dissolves in a solvent.
Active transport
- Movement of particles against a concentration gradient using energy from respiration.
Carbon dioxide
- Gas that turns limewater cloudy.
Dialysis tubing
- A non-living partially permeable membrane.
Diffusion
- Movement of particles from a region of high amount to a region of low amount.
Molecule
- Two or more atoms joined together chemically.
Tissue
- A group of similar cells working together to do one job.
Organism
- A whole living thing made of organ systems.
Gradient
- Slope of a graph line.
Section 14 of 15Key terms
| Term | Meaning |
|---|---|
| Osmosis | The diffusion of water through a partially permeable membrane from dilute to concentrated solution. |
| Partially permeable membrane | A membrane that lets small molecules through but not large ones. |
| Solvent | A liquid in which substances dissolve. |
| Solute | A substance dissolved in a solvent. |
| Water potential | A measure of the tendency of water to move, highest in dilute solutions. |
| Dialysis tubing | A non-living partially permeable membrane. |
| Term | Meaning |
|---|---|
| Turgid | Firm and swollen with water. |
| Flaccid | Soft and limp after losing water. |
| Plasmolysis | The cell membrane pulling away from the cell wall. |
| Crenation | Shrivelling of an animal cell that has lost water. |
| Turgor pressure | The pressure of the cell contents against the cell wall. |
| Wilting | Drooping of a plant when its cells lose turgor. |
Section 15 of 15Exam technique
1
Saying osmosis moves solute — osmosis is the movement of water.
2
Saying water moves from concentrated to dilute — it moves from dilute to concentrated.
3
Forgetting the partially permeable membrane — osmosis needs one.
4
Thinking no mass change means no water movement — movement is equal in both directions.
1
Saying turgid cells are the same as plasmolysed cells — turgid cells are full of water and plasmolysed cells have lost it.
2
Thinking animal cells become turgid — animal cells have no wall, so they burst.
3
Thinking water moves from concentrated to dilute — it moves from dilute to concentrated.
4
Saying plasmolysis means the cell wall moves — the membrane pulls away from the wall.
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