OsmosisCambridge IGCSE Biology: Flashcards

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Define osmosis.

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Define osmosis.: Osmosis is the net movement of water molecules from higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane.
What is a partially permeable membrane?: A partially permeable membrane allows small molecules (such as water) to pass through, but prevents the passage of larger molecules (such as dissolved solutes).
Define water potential.: Water potential is the tendency of water molecules to move from one region to another. Pure water has the highest water potential, and the water potential decreases as solute concentration increases.
Explain why water moves by osmosis across a partially permeable membrane.: Water molecules move randomly in all directions. More water molecules move from the dilute solution (higher water potential) to the concentrated solution (lower water potential) than in the reverse direction, creating a net movement of water.
Define turgor pressure.: Turgor pressure is the pressure exerted by the cell sap inside a plant cell pressing outwards against the cell wall and cell membrane.
What does it mean if a plant cell is turgid?: A turgid plant cell is firm and rigid. It has taken in water by osmosis, so the vacuole is full and exerts maximum turgor pressure against the cell wall.
Define plasmolysis.: Plasmolysis is the loss of water from a plant cell by osmosis when the cell is placed in a concentrated solution, causing the cytoplasm and cell membrane to shrink away from the cell wall.
What is a flaccid plant cell?: A flaccid plant cell is one that has lost water by osmosis and has reduced turgor pressure. The cell membrane is no longer pressed tightly against the cell wall.
Describe what happens to a plant tissue immersed in a concentrated solution.: Water moves out of the plant cells by osmosis. The cells become plasmolysed, losing turgor pressure. The tissue becomes limp and wilted as the cells are no longer supported by turgor pressure.
Describe what happens to a plant tissue immersed in a dilute solution.: Water moves into the plant cells by osmosis. The cells become turgid as the vacuole fills with water and turgor pressure increases. The tissue becomes firm and is supported by the pressure of water against the cell walls.
How does water act as a solvent in digestion?: Water dissolves nutrients and other digestive products during digestion, allowing them to be absorbed into the bloodstream and transported around the body.
Explain the role of water in excretion.: Water is a solvent that allows waste products (such as urea and excess salts) to dissolve and be transported in the blood to the kidneys, where they are excreted as urine.
Explain the role of water in transport.: Water acts as a solvent in the blood and in plant transport tissues (xylem and phloem), allowing dissolved substances such as glucose, ions, and oxygen to be carried around organisms.
Why is it important for plants to maintain turgor pressure?: Turgor pressure provides support and rigidity to plant tissues, keeping the plant upright and firm. Without adequate turgor pressure, plants become limp and wilted, affecting their ability to photosynthesise and function.
Explain the importance of water potential in the uptake and loss of water by organisms.: Water potential differences drive the osmotic movement of water into and out of cells. Organisms maintain their water balance by regulating water potential gradients across cell membranes, allowing them to absorb water when needed and excrete excess water.