IGCSE Double Award Extended Coordinated Science Biology 2.2 - Movement In and Out of Cells Diffusion You need to know that diffusion is: the net movement of molecules from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement. Particles are said to diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. Diffusion is how molecules of oxygen and carbon dioxide can move between the alveoli and the blood (Unit B6) And also how molecules of digested nutrients move from the intestines into the blood (Unit B4) Diffusion in gases is quicker than in liquids because the molecules are more free to move around, there are more space between the molecules and are moving much faster. Osmosis You need to know that osmosis is the diffusion of water molecules through a partially permeable membrane (semi-permeable membrane) from a region of high water potential to low water potential. A partially permeable membrane allows small molecules like water to pass through, but not large dissolved solutes. Water potential is a property of a solution that describes how free the water molecules are. If there are a lot of solutes dissolved, the water potential is low. If the solution is very dilute, the water potential is high. (Think of it as concentration of water in the solution). Pure water has the highest possible water potential (zero) You need to know how to describe and compare solutions in terms of water potential using the terms, These terms are all used to compare solutions to one another. - Hypertonic: More solute concentration (low water potential) - Isotonic: Equal solute concentration - Hypotonic: Less solute concentration (high water potential) So through osmosis, water will move from a hypotonic solution to hypertonic solution. Effects of Osmosis You need to know how osmosis can affect a cell, both animal and plant, when they are places in different solutions. Animal Cell The animal cell membrane is a partially permeable membrane, so it will allow osmosis of water. You need to know that if an animal cell is placed in a: - - Hypotonic solution : water will move into the cell, because the water potential higher outside the cell. - The cell will burst (lysis) - because animal cells do not have a strong outer support. - This is why cells have to store solutes like glucose as non-dissolvable glycogen, because if there was a lot of dissolved glucose in the cell, the cells would burst. Isotonic solution : water potential in and out of the cell is equal (at equilibrium), no net osmosis. - Nothing would happen. This is the ideal environment for cells. Hypertonic solution : water will move out of the cell, b ecause the water potential is higher in the cell. - The cell will shrivel up, because it is losing water to the environment. Plant Cell In addition to the cell membrane, plant cells have a cell wall, which is fully permeable to water. The direction of osmosis is the same as an animal cell, but the extra support from the cell wall gives a plant cell different results. You need to know that if a plant cell is placed in a: - - Hypotonic solution : water will move into the cell, because the water potential higher outside the cell. - The cell will go turgid - the cell wall prevents bursting. Turgid cells are strong and supportive. - Plant cells try to be in hypotonic environments by making the sap vacuole very hypertonic. Isotonic solution :the water potential in and out of the cell is in equilibrium so there is no net osmosis. - In a plant cell, this is called flaccid, the cells are slightly soft because it is not full of water. Hypertonic solution : water will move out of the cell, b ecause the water potential is higher in the cell. - The cell becomes plasmolysed, meaning the cell membrane shrivels and detaches from the cell wall. Benefits of Osmosis You need to know be able to describe and explain how plants cells use osmosis to their advantage. As mentioned above, plant cells like to be in hypotonic environments so they can be turgid. Turgid cells are useful in stems and leaves so that they can be strong enough to support the plant. Osmosis is more important in the uptake of water in root hair cells. You need to know that as mentioned in Unit B2.1, the root hair cell stores a lot of solutes in the sap vacuole, making the sap vacuole very hypertonic to the soil water. The soil water, with very high water potential, will be forced into the cell. This is a method by which the root hair cells maximise water uptake. The syllabus says you should be able to, (SO check if you can): - Define diffusion Describe the importance of diffusion of gases and solutes and of water as a solvent. Define osmosis Describe the importance of osmosis in the uptake of water by plants, and its effects on plant and animal cells. Describe and explain the importance of a water potential gradient in the uptake of water by plants.
