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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Osmosis and Water Movement
Osmosis is one of the most important processes in living organisms. It's how plants get water to their leaves, how your cells stay hydrated and even how some sea creatures survive in salty water. Let's dive into this fascinating biological process!
Key Definitions:
Imagine you have two solutions separated by a partially permeable membrane. One side has more dissolved sugar (solute) than the other. Water molecules will naturally move from the area with less sugar (higher water concentration) to the area with more sugar (lower water concentration). This movement continues until the concentrations become equal - or until something stops it!
Osmosis is driven by differences in water potential. Water always moves from an area of higher water potential to an area of lower water potential. Pure water has the highest possible water potential (0 kPa). When solutes are added, the water potential decreases (becomes negative). The more negative the water potential, the more water will be drawn in!
The concentration of solutions compared to cells determines how water will move. This is crucial for understanding what happens to cells in different environments.
When a cell is placed in an isotonic solution, the concentration of solutes inside and outside the cell is the same. Water moves in and out at equal rates, so there's no net movement. The cell maintains its normal size and shape.
Example: 0.9% saline solution for human blood cells.
When a cell is placed in a hypotonic solution, there's a higher concentration of water outside than inside the cell. Water rushes into the cell by osmosis, causing it to swell.
Example: Plant cells become turgid (firm), animal cells may burst (lyse).
When a cell is placed in a hypertonic solution, there's a lower concentration of water outside than inside the cell. Water rushes out of the cell by osmosis, causing it to shrink.
Example: Plant cells become plasmolysed, animal cells become crenated (shrivelled).
Plants rely on osmosis for their survival and structure. Unlike animal cells, plant cells have a rigid cell wall that affects what happens during osmosis.
Plant cells behave differently from animal cells because they have a cell wall. This strong outer layer prevents the cell from bursting when water enters by osmosis, but it doesn't stop water from leaving.
When a plant cell is placed in a hypotonic solution (like pure water), water enters by osmosis. The cell swells, but the cell wall prevents it from bursting. Instead, the cell becomes turgid (firm) as the cell membrane pushes against the cell wall. This turgidity provides structural support for plants - it's why non-woody plants stand upright!
When a plant cell is placed in a hypertonic solution (like strong salt water), water leaves the cell by osmosis. The cell membrane shrinks away from the cell wall - a process called plasmolysis. The cell becomes flaccid and the plant may wilt. If plasmolysis continues for too long, the plant may die.
For thousands of years, humans have used salt and sugar to preserve food. But how does it work? When foods like meat or fruit are covered in salt or sugar, they create a hypertonic environment around any bacteria or fungi present. Water leaves these microorganisms by osmosis, causing them to dehydrate and die, preventing food spoilage. This is why jam (high sugar) and salted fish can last much longer than fresh food!
Animal cells don't have cell walls, which means they respond differently to osmotic conditions compared to plant cells.
Without a cell wall, animal cells are more vulnerable to extreme osmotic conditions. This is why maintaining the right balance of water and solutes in body fluids is so important.
When an animal cell is placed in a hypotonic solution (like pure water), water enters by osmosis. Without a cell wall to provide resistance, the cell swells and may eventually burst - a process called haemolysis or lysis. This is why intravenous (IV) fluids must be carefully balanced to match the body's osmotic conditions.
When an animal cell is placed in a hypertonic solution (like strong salt water), water leaves the cell by osmosis. The cell shrinks and becomes wrinkled or crenated. This is why your fingers get wrinkly after spending too long in the bath - your skin cells are losing water!
Osmosis isn't just a theoretical concept - it has many practical applications in biology and everyday life.
As we saw in our case study, osmosis helps preserve food by dehydrating microorganisms. Pickling, salting and making jam all use this principle.
IV fluids must be isotonic to blood to prevent cell damage. Dialysis machines use osmosis to remove waste products from the blood of people with kidney failure.
Reverse osmosis is used to purify water by forcing it through a membrane that traps contaminants, producing clean drinking water.
Water potential (Ψ) is measured in kilopascals (kPa) and helps us predict the direction of water movement. Pure water has a water potential of 0 kPa. When solutes are added, the water potential becomes negative.
Water potential (Ψ) = Pressure potential (Ψp) + Solute potential (Ψs)
Water always moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential.
A common experiment to demonstrate osmosis involves potato cylinders placed in different concentrations of sugar solution. When placed in a hypotonic solution, the potato gains mass as water enters by osmosis. When placed in a hypertonic solution, the potato loses mass as water leaves by osmosis. By measuring the change in mass, you can determine the concentration at which the potato neither gains nor loses mass - this is when the solution is isotonic to the potato cells!
Osmosis plays vital roles in many biological processes that keep organisms alive and functioning.
Plants use osmosis to absorb water from the soil through their roots. Root hair cells have a lower water potential than the soil solution, so water moves in by osmosis. This water then moves up through the plant in xylem vessels, eventually reaching the leaves where it's used for photosynthesis or lost through transpiration.
Animals that live in extreme environments have special adaptations to deal with osmotic challenges. Freshwater fish constantly take in water by osmosis, so they produce dilute urine and rarely drink. Marine fish lose water by osmosis to the salty seawater, so they drink seawater and produce concentrated urine. Some organisms, like tardigrades, can survive extreme dehydration by replacing water in their cells with special sugars that preserve their cell structures.
Osmosis is a fundamental process that affects all living organisms. It's responsible for water uptake in plants, maintaining cell volume in animals and countless other biological processes. Understanding osmosis helps us explain everything from why plants wilt without water to why drinking seawater is dangerous for humans. By controlling the movement of water, osmosis helps maintain the delicate balance needed for life to function properly.
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