Osmosis and Diffusion Semi-permeable Membrane: allows the movement of select substances through the membrane A cell membrane is a semi-permeable membrane made of phospholipids. Because the head is polar it can form hydrogen bonds with water – this makes it “water loving” or hydrophilic. (philic = loving) The tail is nonpolar, so it does not form hydrogen bonds, therefore it is “water fearing” or hydrophobic. (phobic = fearing) Because the tails of phospholipids are hydrophobic, they turn away from water. In the bilayer of a cell membrane all tails point towards the middle of the layer while the heads are on the outside. This way the heads are always in contact with the aqueous solution, while the tails are not. Movement of Substances Across the Cell Membrane Concentration: mass of the solute in a solution (mass/volume or g/mL3) Concentration Gradient: difference in the concentration of two different solutions. Solute: material that is dissolved or held in another substance in solution Ex. In salt-water, salt is the solute Solvent: material which holds the other in solution (it does the “dissolving”) Ex. In salt-water, water is the solvent

Active Transport – Movement of a substance across a membrane against the concentration gradient (from low concentration to high concentration). Active transport the requires the use of ATP (energy) Passive Transport – Movement of a substance across a membrane down the concentration gradient (from high concentration to low concentration). Passive transport doe NOT required the use of ATP (energy) Diffusion – type of passive transport in which solute particles move from areas of high concentration to areas of low concentration. A has a higher solute concentration than B. The solution with a higher SOLUTE concentration is called Hypotonic B has a lower solute concentration than A. The solution with the lower SOLUTE concentration is called Hypertonic Therefore, B is hypertonic in relation to A. A is hypotonic is relation to B. In this example the concentration of solute in solution A equals the concentration of solute in solution B. There is no concentration gradient. This is called Isotonic. (“iso”= same) Note: Particles do NOT stop moving when solutions become isotonic, but movement is of solute is equal back and forth across the membrane.

Facilitated Diffusion – movement of molecules down the concentration gradient with the help of a carrier protein or ion channel. This does NOT use ATP.

Osmosis: movement of water molecules through a semi-permeable membrane. Note: the water level changed, but not the amount of solute in each solution. A is Hypotonic in relation to B before osmosis occurs. In the “After” illustration, the solutions are isotonic.

Ion Channel – allows ions to pass through the cell membrane. Ion channels are usually specific to one ion. Ions will move down the concentration gradient.

Carrier Protein – protein imbedded in the cell membrane. Binds to specific polar molecule, and “carries” molecule across the membrane. The molecule is then released. Carrier proteins are specific to one molecule or closely related molecules. Movement will occur down the concentration gradient.

Gas exchange in plants – Diffusion and Osmosis through stomata

In plant cells, osmosis is important as it controls how water moves from the roots to the leaves of the tree. Water is absorbed in the roots and must travel up to the leaves where photosynthesis occurs. Diffusion of the gases CO2 and O2 is also necessary for photosynthesis to occur.

Remember: Photosynthesis – the process by which plants make food using sunlight, carbon dioxide and water uses the following formula:

(Sunlight) 6H2O + 6CO2 àààà C6H12O6 + 6O2

(glucose)

Where do these reactants come from?

Water Water is brought up from the roots utilizing osmosis (movement of the water from a higher concentration in the roots to a lower concentration in the leaves) and hydrogen bonding between water molecules that allows capillary action to occur.

Carbon Dioxide Carbon Dioxide must be brought in from the air. This is done through small pores located on the leaves of plants. These pores are called stomata and are responsible for gas exchange in plants. CO2 is brought in through the pores and O2 (a byproduct of photosynthesis) is released. Both of these gases move through passive transport, going from a higher concentration to a lower concentration.

Risks associated with gas exchange for a plant While gas exchange is necessary for a plant to obtain CO2 and release O2, whenever a plant opens it stomata (or pores in the leaves) it is at risk for losing water as well. This loss of water is called transpiration. What determines how great the risk is?

Temperature: higher temperature = increased transpiration Humidity: lower humidity = increased transpiration Wind: increased wind = increased transpiration

Would you expect to see a greater number of stomata on plants native to a hot/dry climate or a moderate/humid climate? The cooler temperature and increased humidity would decrease the risk associated with having a large number of stomata. Having a sufficient number of stomata is important because it allows adequate gas exchange for the highest level of photosynthetic efficiency. How plants control gas exchange The stomata on a plant’s leaves are surrounded by a pair of guard cells. These kidney bean shaped cells open and close over the opening of the stomata to allow gas exchange when conditions are acceptable for the plant. Guard cells have very large vacuoles for water storage. When water is plentiful in the plant, and opening the stomata would not put the plant at risk for dehydration, these vacuoles are full and have a high turgur pressure, causing the guard cells to swell and open. When sufficient water is not available the guard cells will become flaccid and close, thus preventing water loss through the stomata.

Guard Cell

Stomata

Closed guard cells