Lecture 40 May 7 2003 R. Jones Chapters 5.14-5.17, 32, and 36.13-36.17

1. Specialized thickenings in the cell walls of the guard cells allow the cells to move relative to each other forming a pore or stoma. The movement of the guard cells is brought about by the movement of water into and out of the guard cells by osmosis (Figure 32.4).

2. Osmosis is a physical process, i.e. osmosis occurs in living and non-living systems (Figure 5.15). Defined as the movement of water across a semi-permeable membrane from a region where the water concentration is high to a region where the water concentration is low. The presence of dissolved solutes such as sugars and ions reduce the water concentration, therefore another way to define osmosis is to say that water moves from a region where the solute concentration is low to where the solute concentration is high. A semi-permeable membrane is one that allows only movement of water.

3. Terms that you should know in the context of osmosis and living cells are plasmolysis, hemolysis, turgor pressure, tonicity- isotonic, hypertonic and hypotonic.

4. Back to guard cells. Stomates of most plants open during the early morning and close at dusk. Guard cells have chloroplasts whereas other epidermal cells do not. When the guard cell begins to photosynthesize in the early morning CO₂ is consumed and the pH of the guard cell rises (less acidic, because CO₂ dissolved in water gives a weak acid called carbonic acid).The rising pH of the guard cell causes ion channel pores in the membrane of the guard cell to open allowing potassium ions (K⁺) to enter the cell. These ions lower the water concentration of the cell (i.e. raise the solute concentration or tonicity of the cell) causing water to enter by osmosis. This causes the cell to move away from each other causing the stomates to open. At night the opposite occurs. Photosynthesis in guard cells stops because of lowered light intensity, CO₂ fixation is reduced causing CO₂ concentrations in the cell to rise (note that respiration producing CO₂ is unchanged), cell pH is lowered and ion channel pores in the membrane that permit outward flow of ions to open causing solute concentration to drop and water to leave. Loss of water reduces turgor (hydrostatic pressure) and cells collapse and close stomate.

5. Opening and closing of the stomates is also regulated in plants by a hormone called abscisic acid (ABA). This hormone is made in roots and the amounts of the hormone change according to the water status of the soil. As soil dries ABA is made, this travels to the guard cell, causing the stomates to close rapidly, thus conserving water. What must be remembered is that when stomates are closed CO₂ can not be taken up and therefore plants cannot photosynthesize thus crop yield declines. It's all about the water!

6. Water moves up the plant from soil to the atmosphere by a process called evapotranspiration. When stomates are open water vapor escapes from the leaf and this initiates a cascade of events that results in the drawing of water up the dead tracheary elements, the tracheids and vessels of the xylem (Figure 32.3). This evapotranspiration process is clearly strong enough to pull water up trees that are taller than 300 feet.

7. Movement of sugars though the phloem of plants is simply by osmosis. The phloem cells are living and accumulate sugars and other organic solutes such as amino acids where they are made. During the day in leaves sugars are made by photosynthesis, they are transported to the phloem where they accumulate (a process called sugar loading) and this causes osmosis to occur into the sugar-rich phloem cells (Figure 32.5). This pushes sugars to other parts of the plant where the sugars are used, i.e., in roots where they may be converted to starch. Unloading of sugars in the roots lowers the sugar concentration causing water to flow out of the phloem.

8.The Nitrogen Cycle. Nitrogen fixation is a key process in keeping the nitrogen content of the soil high in natural ecosystems. Although N is very abundant (80% of atmospheric gas is N₂) N is generally the most limiting element in agricultural productivity and farmers must constantly add nitrogen fertilizers to soil (Figure 36.15). Modern agriculture has exacerbated the problem because it short-circuits the N Cycle by harvesting plants and not allowing animal wastes to be returned to the land.

9. Only prokaryotes can convert N₂ gas to a form of nitrogen usable by plants. Bacteria in the soil or living in association with plants roots in nodules convert N₂ to ammonia. Other bacteria convert ammonia to nitrate. Plants take up nitrate and convert nitrate to ammonium ion which then gets incorporated into amino acids and N-containing bases. Animals can not incorporate inorganic N (nitrate or ammonium) into organic compounds. Among eukaryotes only plants and fungi can do this. Animals lack the ability to metabolize nitrate/nitrite and to incorporate ammonium into amino acids. Animals also lack the ability to make 8 of the 20 amino acids incorporated into proteins and these must be obtained by eating plants (see section 21.16). These 8 amino acids are known as essential amino acids.