Eduqas Biology for A Level Year 1 & AS: Student Bk

Knowledge check Water enters root hair cells by osmosis. Calculate the solute potential ( ψ S ) of the root hair cell, when there is no net movement of water. The solute potential of the soil water is –100 kPa and the pressure potential ( ψ P ) inside the root hair cell is +200 kPa. Use the formula ψ = ψ S + ψ P . Show your working and units. 18 Knowledge check Link the processes 1–4 with the following statements A–D. Some statements may be applicable to more than one process. 1. Diffusion. 2. Facilitated diffusion. 3. Osmosis. 4. Active transport. A. Does not take place in the presence of cyanide. B. Does not require cell energy. C. A special form of diffusion involving water molecules. D. Movement involves membrane proteins. 19 Study point Animal and plant cells react differently in hypertonic and hypotonic solutions, as animal cells have no cell wall. When the cell can take in no more water, it is turgid . Turgor is important to plants, especially young seedlings, because it provides support, maintains their shape and holds them upright. Turgid and plasmolysed cells Osmosis and animal cells An animal cell has no cell wall and so pressure potential does not have to be considered. The water potential is therefore the same as the solute potential, i.e. ψ = ψ S . The diagram shows that if red blood cells are in distilled water, water enters by osmosis and, without a cell wall, they burst. This is haemolysis . If red blood cells are placed in concentrated salt solution, water leaves the cells and they shrink, becoming ‘crenated’. Osmosis in animal cells Blue arrow shows movement of water by osmosis. Fig 37 nucleus cellulose cell wall ψ = 0 ψ p = – ψ s ψ p = 0 ψ = ψ s ψ p = 0 ψ > ψ s cytoplasm cytoplasm pushed against cell wall cytoplasm begining to pull away from cell wall cytoplasm completely pulled away from cell wall Turgid Plasmolysed incipient plasmolysis Fig 39 pure water or dilute solution (hypotonic to cell) solution with same concentration as red cell (isotonic) concentrated solution (hypertonic to cell) cell bursts normal red cell cell shrinks CORE 1.3 Cell membranes and transport 61