The drawing above shows Brownian motion, kinetic energy and DIFFUSION. BROWNIAN MOTION is the random motion of molecules which are able to move as a result of their inherent KINETIC ENERGY. DIFFUSION: Is the NET movement of molecules of a substance from a region of higher concentration to a region of lower concentration of that substance DOWN A CONCENTRATION GRADIENT.
DYNAMIC EQUILIBRIUM is observed. The ether molecules are still moving but equally in all directions. There is no longer a concentration gradient and so diffusion stops (even though the molecules continue to move)
We learn about the impermeable wall in xylem tissue and we know that the cellulose cell wall is permeable. A semi-permeable membrane is the cell membrane and all the membranes surrounding the organelles and the nucleus. It has tiny pores which allow only small molecules like water, carbon dioxide and wate through but restricts the passage of larger molecules like glucose. A semi-permeable membrane is part of our explanation of osmosis.
DEFINE OSMOSIS This is the NET MOVEMENT of WATER MOLECULES from the region of HIGHER WATER CONCENTRATION to a region of LOWER WATER CONCTRATION through a SEMI-PERMEABLE MEMBRANE. (Note: you can also say from a region of HIGHER KINETIC ENERGY TO A REGION OF LOWER KINETIC ENERGY ; or FROM A REGION OF HIGHER WATER POTENTIAL TO A REGION OF LOWER WATER POTENTIAL )
The experiment is controlled. The next slide shows it.
Here the hollow contains water, eliminating a water concentration gradient. Thus a control eliminates the factor being measured in the experiment (osmosis due to a concentration gradient). If osmosis takes place in the control then we may conclude that osmosis does not occur down a concentration gradient.
Here in the experiment the hollow fills, showing that osmosis occurs down a concentration gradient. In the control, nothing happens confirming the experiment.
Water enters the above cell by endosmosis. The cell stars to swell up as its volume increases.
Two forces/pressures are seen as a result of endosmosis, an outward TURGOR PRESSURE as a result of the increase in volume and WALL PRESSURE, where the wall resists this increase in volume. The two forces are opposite – Turgor outward and Wall pressure inward. When they are equal, osmosis stops and we say the cell is TURGID
When a cell is placed in a hypertonic solution (low water concentration) exosmosis out f the cell occurs. The volume of the cytoplasm decreases and it pulls away from the cell wall. The cell is now plasmolysed and becomes FLACCID (SOFT). This occurs in plants in dry conditions and the whole plant WILTS
These are the answers to the questions in the notes on page 4
TYPES OF MEMBRANES Type of membrane Size of pores in membrane Particles which can pass through membrane Examples of membranes Impermeable membrane No pores None Lignin (secondary cell wall); waterproof Permeable membrane Large pores All molecules which normally move through living cells Cellulose cell wall (primary cell wall in most plant cells) Semi-permeable membrane (or selectively permeable, or differentially permeable Sub-microscopic pores Water; carbon dioxide; oxygen Cell membrane; tonoplast; all membranes surrounding organelles
Semi-permeable membrane SEMI-PERMEABLE MEMBRANE PURE WATER High water concentration GLUCOSE SOLUTION Low water concentration
OSMOSIS <ul><li>This is the NET MOVEMENT of WATER MOLECULES from the region of HIGHER WATER CONCENTRATION to a region of LOWER WATER CONCENTRATION through a SEMI-PERMEABLE MEMBRANE. </li></ul>
PLASMOLYSIS CELL WALL CELL MEMBRANE TONOPLAST VACUOLE Low water concentration (hypertonic) High water concentration (Hypotonic)
Questions <ul><li>PLASMOLYSIS: </li></ul><ul><li>Cell B placed in a solution with a low water concentration. </li></ul><ul><li>Exosmosis causes cell contents to shrink, and cytoplasm pulls way from the wall. </li></ul><ul><li>Cell becomes soft and flaccid </li></ul>W: CYTOPLASM X: CELL WALL Y: CELL MEMBRANE Z: VACUOLE
ACTIVE TRANSPORT <ul><li>The movement of molecules of a substance against the concentration gradient (Low to High), </li></ul><ul><li>This requires energy provided by a cell’s mitochondrion. </li></ul>